MODELING OF A THIN-LIQUID FALLING-FILM IN ABSORPTION COOLING SYSTEMS BY Kamilu Moradeyo ODUNFA B.Sc. (Hons) Mechanical Engineering (Ibadan) M. Sc. Mechanical Engineering (Ibadan) M. Phil. Mechanical Engineering (Ibadan) (Matriculation Number: 44113) A thesis in the Department of MECHANICAL ENGINEERING Submitted to Faculty of Technology in partial fulfillment of the requirement for the Degree of DOCTOR OF PHILOSOPHY of the UNIVERSITY OF IBADAN Mechanical Engineering Department, University of Ibadan, Ibadan. MARCH 2012 ABSTRACT Absorption refrigeration systems are generally characterized by low Coefficient of Performance (COP). Absorption enhancement is an effective way of improving the COP of refrigeration systems. Literature is sparse on the use of magnetic field for the enhancement of absorption refrigeration systems despite its cheapness and environmental friendliness as compared with other enhancement methods. Although the method has recently been employed on ammonia solution, its influence on lithium bromide (LiBr) and lithium chloride (LiCl) solutions is yet to be fully studied. In this study a numerical model for the magnetic field enhancement of the absorption cooling-system using LiBr and LiCl solutions was developed and evaluated. The flow within the film thickness to the absorber wall was considered as a two- dimensional steady laminar flow. A Finite Difference model was developed based on conservation of mass, momentum, energy equations and mass transport relationship. The model was validated using data from the literature on ammonia solution. Standard -3 parameters including absorber wall length (1 m), film thickness (10 m), magnetic field -6 -2 2 vacuum permeability (1.257 x 10 kgmA s ), magnetic mass susceptibilities and magnetic induction intensities were used for LiBr and LiCl solutions’ modeling. Changes in their concentrations, both in the direction of falling film and across its thickness, were investigated. Data were analysed using descriptive statistics and Student’s t-test (p=0.05). The concentration distribution for ammonia solution within the film thickness was not significantly different from results in the literature. For the magnetic induction range of 0.0 and 3.0 Tesla, the concentration distribution of LiBr solution in the direction of falling film was between 54.9% and 60.0%, while that of LiCl solution ranged between 39.9% - 45.0%. Meanwhile, across the film thickness and for the same range of magnetic induction of 0.0 and 3.0 Tesla, the concentration distribution for LiBr solution was between 0.0 and 0.19 and those of LiCl solution were between 0.0 and 0.13. The concentration of LiBr solution increased from 0.0 to 4.7 and 0.0 to 21.7 when magnetic induction was increased from 0.0 to 1.4 and 0.0 to 3.0 Tesla, respectively. Similarly for LiCl solution, increased values of 0.0 to 3.3 and 0.0 to 15.5 were obtained when magnetic induction was increased from 0.0 to 1.4 and 0.0 to 3.0 Tesla, respectively. In both cases, it implies higher cooling effect. Relative to 0.0 Tesla, the COP of LiBr and LiCl solutions ii absorption refrigeration systems was increased by 0.1% when magnetic induction was 1.4 Tesla, while increment of 0.3% and 0.2% respectively were obtained when magnetic induction was 3.0 Tesla. The percentage increments in COP of LiBr solution were not significantly different from that of the LiCl solution. Magnetic field enhanced the absorption performance in the lithium bromide and lithium chloride solutions; hence can be used in typical absorption refrigeration systems. Keywords: Magnetic field, Refrigeration, Thin-liquid Falling-film, Absorption cooling- Systems Word count: 457 iii DEDICATION To all Who fear God and acknowledge the power of His might, The doer of His word Through our Lord Jesus Christ This work Is dedicated iv ACKNOWLEDGEMENT To God be the glory honor and power for His mercy and kindness who out of His marvelous light discovered and set me free from the power of darkness and oppression. He clothed my nakedness, opened my eyes and established my going academically, professionally and in every other ramification of my life. God has been so wonderful to me, He gives me life, strength and sound health even throughout the first frustrating seven years of my imprisonment and entanglement in my first aborted M. Phil degree programme. He did not leave or forsakes me in my successful M. Phil degree programme even up to this end, indeed He is a shelter in time of storm. I must appreciate my supervisor Prof. R.O Fagbenle for being so kind to me. In my state of academic hopelessness, he assured me of greater hope. For allowing himself to be used of the Lord to gather my battered and shattered academic life, I say a big thank you Sir. In deed you are a builder of life, especially for the young ones, you are also a template of a good Professor; an excellent father academically, professionally and in every other facet of human life, I prophesy onto your life Sir, it shall be well with you and your family. You will continue to be a vessel of honor in God’s mansion. Throughout your life you will not sorrow, the joy of the Lord will continue to be your strength. I am grateful to Prof. A. E Oluleye, the present Dean of the Faculty of Technology, University of Ibadan. He is such a loving and caring father, a template of a Dean in the citadel of learning. He enthusiastically took interest in the progress and development of young ones myself inclusive. Words can not be enough to express my gratitude to you for this. I can only pray that God in His infinite mercies reward you with v blessings. The Lord will satisfy you with long life and sound health, you will live to eat the fruits of your labour. I must equally express my gratitude to entire staff of the Department of Mechanical Engineering, University of Ibadan, whose love and concern to see me to the end of this programme was intoxicating. I must specifically thank the immediate and the present head of the Department Dr. O. Oluwole and Dr. M.O Oyewola respectively for helping me throughout, your constant encouragement on this programme has been of tremendous help. God will continue to bless you all. My specific appreciation also goes to Dr. Dare, Dr. T.A.O Salau, Engr. R. Abu, Dr. Ismail, Dr. I.F, Odesola and Dr. Fadare for their supports in one form or the other. To Dr.(Mrs) F,O Akintayo of Civil Engineering Department, I say thank you. The Dean of Postgraduate School Prof. Olorunnisola, you are highly appreciated Sir, Dr. Victor Oladokun and all other heads of the Departments in the Faculty of Technology, I appreciate you all, God will continue to bless you. I specifically express my appreciation to the past Faculty Sub-Dean Post graduate Dr. Falade of the Department of Food Technology, University of Ibadan. He is such a loving and caring colleague, he enthusiastically took interest in my research work. Words can not be enough to express my gratitude to you for your assistance and support. I can only pray that God in His infinite mercies will reward you with more blessings. The Lord will satisfy you with long life and sound health, you will live to eat the fruits of your labour. Immediate past and present Faculty Sub-Deans Postgraduate Dr. Ogunjuyigbe and Dr. Ewemoje, you are all highly appreciated. My special appreciation also goes to the present Sub-Dean Postgraduate School Dr. J.O Babalola of Chemistry Department and vi immediate past ASUU Chairman, University of Ibadan, Dr Ademola Aremu; May the Lord God continue to lift you all higher and higher. I must thank specially my loving wife, an Estate Surveyor per excellent Mrs. Victoria Oluwafunmike Odunfa (a.k.a Peace). Indeed you have demonstrated your name by allowing yourself to be the channel of the peace of the Lord to my life. I thank you for your moral support and continued endurance. You have been a vessel of honor in God’s mansion, so you will continue to be, I pray that God will continually make you a light to many generations. I am grateful to my God-given children, Victoria, Esther and Victor; in deed you are children of signs and wonder to me. The Lord will build around you the wall of fire that will continually separate you from the dangers of both day and night. You will live to continue to declare the glory of God in the land of the living. I must also appreciate Ojo Temitope O, my able research and engineering assistant, you have been so wonderful and helpful to me in this work, May the Lord God grant you your heart desire. I am also grateful to the present President of the Gospel Fellowship Incorporated Bro. Abel Aro for his care and concern, and his spiritual support throughout the period of this programme, I say thank you Sir. I am appreciative to the University for the Improved Internet Facilities that have made it possible to have a better grip of the subject of study. There are many others whom I may not be able to mention, but have contributed in one way or the other towards the success of this work. I am grateful to all and wish you success in your endeavours. K.M Odunfa vii TABLE OF CONTENTS ABSTRACT .................................................................................................................. ii-iii DEDICATION.................................................................................................................. iv ACKNOWLEDGEMENT.... .……………………………………………………….v-vii TABLE OF CONTENT……………………….…......................................................... ix LIST OF FIGURES ..................................................................................................... x-xx LIST OF TABLES ................................................................................................... xix-xxi LIST OF SYMBOLS .................................................................................................... xxii CERTIFICATION ....................................................................................................... xxiii CHAPTER ONE: INTRODUCTION ............................................................................ 1 1.1: General Background ................................................................................................ 1 1.2:Statement of the problem .......................................................................................... 5 1.3: Justification /Objectives………………………………………………………......6 CHAPTER TWO: LITERATURE REVIEW ................................................................ 7 2.1: Introduction ............................................................................................................. 7 2.1.1: Cooling Technology ............................................................................................ 7 2.1.2: Advantages and disadvantages of vapour compression and Absorption refrigeration………………………………………………………………………7 2.1.3: Vapour compression System ............................................................................... 8 2.1.3: Vapour compression cycle ................................................................................... 9 2.1.4: Absorption System ............................................................................................ 11 2.1.5: Principle of Absorption System ......................................................................... 11 2.2: Working fluids/Refrigerants .................................................................................. 14 2.2.1: Various designs of Absorption Refrigeration Cycles………..…………………14 2.2.2: Practical Absorption Cycle…………………………………..…………………25 2.4: Numerical Modeling ........................................................................................ 26-31 CHAPTER THREE: METHODOLOGY ................................................................... 32 3.1: Assumptions ........................................................................................................ 32 3.2: Governing equations ............................................................................................. 32 viii 3.3: Finite Difference formulation of the governing equations…………….…….. .. 36 3.4: Formulation of the Magnetic field enhanced finite difference model of the governing equations..…………………………………………………………....39 3.5: Boundary conditions ……………………………………………………..……..39 3.6: Solution method………………….……………………………………...…..…..43 3.6.1:Computer rogramming…………………………………………………………43 3.6.2: Main Program …………………………………………………………………44 3.6.3: Subroutines…………………………………………………………………….45 3.6.4:Main Program Flow chart....................................................................................45 3.6.5: Subroutine solution flow charts…………………………………………....46-47 3.7: Data…………………………………………………………………………..48-50 CHAPTER FOUR: RESULTS AND DISCUSSION.................................................51 4.1: General remark on the result presentation.........................................................51 4.2: Numerical Results in the direction of falling film ......................................51- 52 4.3: Tabular and Graphical representation of the literature and present work's result…..…………………………………………….…,………….…..52-61 4.4: Numerical Results in the direction of falling film (X) for LiBr and LiCl solutions and their Coefficients of Performance….………………………..63-130 4.5: Numerical Results in the direction of the film thickness……..…………133-187 CHAPTER FIVE: CONCLUSION AND RECOMMENDATION..........................188 5.1 Conclusions........................................................................................................188 5.2: Recomendations…………………………………………………………189-190 REFERENCES………………………………………………………………......191-196 APPENDICES..………………………………………………………………………..197 APPENDIX A: DEVELOPMENT OF THE MODEL EQUATIONS……………198-203 APPENDIX B: COMPUTER PROGRAM FOR THE MODEL ……………......204-234 APPENDIX C:COMPUTER PROGRAM RESULTS….......................................235-266 APPENDIX D: GAUSSIAN ELIMINATION..……………………..…….….. …267-268 ix LIST OF FIGURES Page Figure 2a: Vapour Compression Chiller-three stage Compressor ……….........................9 Figure.2b: Basic Vapour Compression cycle- Single Stage……………………..……....10 Figure 2c: Basic Vapour Compression cycle-Two Stage compressure and a flash chamber......................................................................................................................10 Figure 2d: Typical two stage Absorption Chiller ……………………………………….11 Figure 2e: An Intermittent Absorption Cycle……………………………………………13 Figure 2f: A continuous absorption refrigeration cycle composes of two processes…....13 Figure 2g: A Single–effect LiBr/water absorption refrigeration system with a solution heat exchanger (HX)…………………………………….……….…………….15 Figure 2h: Absorption heat transformer Cycle…………………………………………..16 Figure 2i: A double – effect water/LiBr absorption cycle……………………………….16 Figure 2j: A double – effect absorption cycle operates with two pressure levels………..17 Figure 2k: A triple – effect absorption cycle operates at 4 pressure levels……………...17 Figure 2l: An Absorption Cycle with heat transference from high to low temperature in the generator…………………...………………………………………..18 Figure 2m: The cycle with absorber heat recovery uses heat of absorption of preheat the outgoing stream from the absorber to the generator……………………..18 Figure 2n: A half – effect absorption cycle is a combination of two single – effect cycles but working at different pressure levels………...……………………….19 Figure 2o: Combined vapour absorption/compression heat pump………………………19 Figure 2p: Configured Double effect absorption –compression cycle as a heat pump….20 Figure 2q: A combined cycle proposed by Caccoila et al.(86)………………………….20 Figure 2r: A resorption cycle proposed by Altenkirch uses two solution circuits………21 Figure 2s: Solar driven dual cycle absorption employs to different working fluids i.e NH3/water and water/LiBr………………………………………………………21 Figure 2t: A modified double effect combined ejector – absorption refrigeration cycle..22 Figure 2u: A combined ejector/absorption system using DMETEG/R22 and DMETEG/R21 as working fluids……………………………………………….22 Figure 2v: A combined ejector/absorption proposed by Aphornratana and Eames…….23 x Figure 2w: A combined cycle proposed by Eames and Wu (93)……………………….23 Figure 2x: An osmotic membrane absorption cycle…………………………………….24 Figure 2y: The diagram shows a bubble pump in the generator module……………….24 Figure 2zi: Lithium bromide-water single-stage absorption Refrigeration cycle……....25 Figure 2zii:Lithium bromide-water two-stage absorption Refrigeration cycle…………25 Figure 3.1a: 2-d representation of a thin-liquid falling-film……………………….……35 Figure 3.1b: Finite difference diagram…………………………………………….……37 Figure 3.2: Temperature boundary conditions analysis…………………………………41 Figure 3.3: Concentration boundary conditions analysis…………………………….… 42 Figure 3.4a: Main program flow-chart.............................................................................45 Figure 3.4b:Subroutine solution flow-chart......................................................................46 Figure 3.4c: Subroutine Velocity flow-chart……………………………………………47 Figure 4.1: Literature (NH3-H2O ) velocity distribution in the film Comparison @ 0.0, 1.4 and 3.0Tesla……………………………………..……………55 Figure 4.1.1: Present work (NH3-H2O) Velocity Distribution in the Film Comparison @ 0.0, 1.4 and 3.0Tesla ..……………………………………………………………...55 Figure 4.1.2: Literature (NH3-H2O ) Temperature distribution in the film Comparison @ 0.0, 1.4 and 3.0Tesla……………………………………..……………58 Figure 4.1.3: Present work (NH3-H2O) Temp.distribution in the Film Comparison @ 0.0, 1.4 and 3.0Tesla ..……………………………………………………………...58 Figure 4.1.4: Literature (NH3-H2O ) Concentration distribution in the film Comparison @ 0.0, 1.4 and 3.0Tesla……………………………………..……………61 Figure 4.1.5: Present work (NH3-H2O) Conc. distribution in the Film Comparison @ 0.0, 1.4 and 3.0Tesla ..……………………………………………………………...61 Figure 4.1.6: LiBr-H2O Velocity Distribution in the Film Comparison @ 0.0, 1.4 and 3.0 Tesla……..………….………………………………………………………….64 Figure 4.1.7: LiBr-H2O Velocity Changes within the Film in X-Direction from 0.0 to 1.4Tesla……………………………………………………………………65 Figure 4.1.8 : LiBr-H2O % Velocity Changes within the Film in X-Direction from 0.0 to 1.4Tesla……………………………………………………………………66 Figure 4.1.9: LiBr-H2O Velocity Changes within the Film in X-Direction xi from 0.0 to 3.0 Tesla…..……………………………………………………………….66 Figure 4.2: LiBr-H2O % Velocity Changes within the Film in X-direction from 0.0 to 3.0Tesla……………………………………………………………………67 Figure 4.2.1: LiBr-H2O Vel. Distribution in the Film comparison @ the Interface....... 68 Figure 4.2.2: LiBr-H2O Velocity Changes at the Interface in X-direction from 0.0 to .1.4Tesla….………………………………………………………………...70 Figure 4.2.3: LiBr-H2O % Velocity Changes at the Interface in X-Direction from 0.0 to1.4Tesla…………………………………………………………………….70 Figure 4.2.4: LiBr-H2O velocity Changes at the Interface in X-Direction from 0.0 & 3.0Tesla….…………………………………………………………………71 Figure 4.2.5: LiBr-H2O % velocity Changes at the Interface in X-Direction from 0.0 to 3.0Tesla…..……………………………………………………………….71 Figure 4.2.6: LiBr-H2O Temp.Distribution in the Film Comparison @ 0.0Tesla……..73 Figure 4.2.7: LiBr-H2O % Deviation in Temperature Distribution in the Film Comparison @ 0.0Tesla……………………………………………………………….73 Figure 4.2.8: LiBr-H2O Temp. Distribution in the Film @ 1.4 Tesla (Pre.Result)……74 Figure 4.2.9: LiBr-H2O Temp. Distribution in the Film @ 3.0 Tesla(Pre.Result)……..74 Figure 4.3: LiBr-H2O Temperature Distribution in the Thin-Liquid Smooth Falling film (Bulk) @ 0.0,1.4 and 3.0Tesla…………………………….……………...75 Figure 4.3.1 LiBr-H2O Temperature Distribution at the Interface Comparison @ 0.0 Tesla…….……………………………………………………………………...78 Figure 4.3.2: LiBr-H2O % deviation in Temperature distribution at the interface Comparison@ 0.0Tesla………………………………………………………………..78 Figure 4.3.3: LiBr-H2O Temperature Distribution at the Interface @ 1.4 Tesla……...79 Figure 4.3.4: LiBr-H2O Temperature Distribution at the Interface @ 3.0Tesla………79 Figure 4.3.5: LiBr-H2O Temperature Distribution at the Interface Comparison @ 0.0, 1.4 and 3.0Tesla……………………………………………….……………...80 Figure 4.3.6: LiBr-H2O Conc. Distribution in the Film Comparison @ 0.0Tesla……82 Figure 4.3.7: LiBr-H2O % Deviation in Concentration Distribution in the Film. Comparison @ 0.0Tesla……………………………………………………………....82 Figure 4.3.8: LiBr-H2O Concentration Distribution in the Film @ 1.4Tesla………..83 xii Figure 4.3.9: LiBr-H2O Concentration Distribution in the Film @ 3.0Tesla…………83 Figure 4.4: LiBr-H2O Concentration Changes within the Film in X-Direction from 0.0 and 1.4 Tesla………………………………………………………………..85 Figure 4.4.1: LiBr-H2O % Concentration Changes within the Film in X-direction @ 0.0 and 1.4Tesla…………………………………………………...85 Figure 4.4.2: LiBr-H2O Concentration Changes within the Film in X-direction from 0.0 & 3.0 Tesla…………………………………………………….86 Figure 4.4.3: LiBr-H2O % Concentration Changes within the Film in X-Direction @ 0.0 & 3.0Tesla………………………………………………………..86 Figure 4.4.4: LiBr-H2O Concentration Distribution at the Interface Comparison @ 0.0Tesla……………………………………..………………………..90 Figure 4.4.5: LiBr-H2O % Deviation in Concentration Distribution at the Interface Comparison @ 0.0Tesla….………………………………………….90 Figure 4.4.6: LiBr-H2O Concentration Distribution at the Interface @ 1.4Tesla (present Result)……………………………………..………………………91 Figure 4.4.7: LiBr-H2O Concentration Distribution at the Interface @ 3.0Tesla (Present Result)……………………………………………..………………91 Figure 4.4.8: LiBr-H2O Concentration Changes at the interface in X-Direction from 0.0 to 1.4 Tesla……………………………………………...…….93 Figure 4.4.9: LiBr-H2O % Concentration Changes at the interface in X-direction from 0.0 to 1.4Tesla……………………………………………………..93 Figure 4.5: LiBr-H2O Concentration Changes at the interface in X-direction @ 0.0 & 3.0 Tesla………………………………………………………94 Figure 4.5.1: LiBr-H2O % Concentration Changes at the interface in X-Direction from 0.0 to3.0Tesla……………………………………..…………..94 Figure 4.5.2: LiBr-H2O Concentration Distribution at interface X-direction @ 0.0, 1.4 & 3.0 Tesla………………………………………………….95 Figure 4.5.3: LiBr-H2O Concentration Distribution in the Bulk at X-direction @ 0.0, 1.4 & 3.0 Tesla…………………………………………………..95 Figure 4.5.3a: LiCl-H2O velocity distribution in the film comparison @ 0.0, 1.4 and 3.0Tesla………………………………………………………………………….97 xiii Figure 4.5.4: LiCl-H2O Temp. Distribution in the Film Comparison @ 0.0Tesla.….98 Figure 4.5.5: LiCl-H2O % Deviation in Temp. Distribution in the Film Comparison @ 0.0Tesla…………………..………………………………………….99 Figure 4.5.6: LiCl-H2O Temp.Distribution in the Film @ 1.4 Tesla (Pres Result) .. 99 Figure 4.5.7: LiCl-H2O Temp.Distribution in the Film @ 3.0 Tesla(Pres Result)…100 Figure 4.5.8: LiCl-H2O Temp. Distribution in the Thin-Liquid Smooth Falling film (Bulk) @ 0.0,1.4 and 3.0Tesla….………………………………………..………….100 Figure 4.5.9: LiCl-H2O Conc. Distribution in the Film Comparison @ 0.0Tesla…...102 Figure 4.6: LiCl-H2O % Deviation in Concentration Distribution in the Film Comparison @ 0.0Tesla……………………………………………………………..102 Figure 4.6.1: LiCl-H2O Concentration Distribution in the Film @ 1.4Tesla……….103 Figure 4.6.2: LiCl-H2O Concentration Distribution in the Film @ 3.0Tesla………..103 Figure 4.6.3: LiCl-H2O Concentration Distribution in the Film Comparison @ 0.0, 1.4 and 3.0 Tesla………………………………………………..104 Figure 4.6.4: LiCl-H2O Vel. Distribution in the Film comparison @ the Interface…106 Figure 4.6.5: LiCl-H2O Temp.Distribution at the Interface Comparison@ 0.0Tesla..110 Figure 4.6.6: LiCl-H2O % deviation in Temperature distribution at the interface Comparison@ 0.0Tesla…………………………………………………………........110 Figure 4.6.7: LiCl-H2O Temperature Distribution at the Interface @ 1.4 Tesla….....111 Figure 4.6.8: LiCl-H2O Temperature Distribution at the Interface @ 3.0Tesla……..111 Figure 4.6.9: LiCl-H2O Temp. Distri. at the interface @ 0.0,1.4 and 3.0Tesla105….112 Figure 4.7: LiCl-H2O Concentration Distribution at the Interface Comparison @ 0.0Tesla…………………………………………………………….115 Figure 4.7.1: LiCl-H2O % Deviation in Concentration Distribution at the Interface Comparison @ 0.0Tesla………………………………………………115 Figure 4.7.2: LiCl-H2O Concentration Distribution at the Interface @ 1.4Tesla (present Result)……………………………………………………… 116 Figure 4.7.3: LiCl-H2O Concentration Distribution at the Interface @ 3.0Tesla (Present Result)…………………………………………………………. 116 Figure 4.7.4: LiCl-H2O Concentration Distribution at the Interface @ 0.0 1.4 and 3.0Tesla…………………………………………………………....117 xiv Figure 4.7.5: LiCl-H2O Vel. Changes within the Film in X-Direction from 0.0 to 1.4 Tesla………………………………………………………………119 Figure 4.7.6: LiCl-H2O % Velocity Changes within the Film in X-Direction from 0.0 to 1.4Tesla…………………………………………………119 Figure 4.7.7: LiCl-H2O Vel. Changes within the Film in X-Direction from 0.0 to 3.0 Tesla…………………………………………………………….120 Figure 4.7.8: LiCl-H2O % Velocity Changes within the Film in X-direction from 0.0 to 3.0Tesla………………………………………………………………120 Figure 4.7.9: LiCl-H2O Velocity Changes at the Interface in X-direction from 0.0 to 1.4Tesla………………………………………………………………122 Figure 4.8: LiCl-H2O % Velocity Changes at the Interface in X-Direction from 0.0 to1.4Tesla……………………………………………………………….122 Figure 4.8.1: LiCl-H2O velocity Changes at the Interface in X-Direction from 0.0 & 3.0Tesla………………………………………………………………123 Figure 4.8.2: LiCl-H2O % velocity Changes at the Interface in X-Direction from 0.0 to 3.0Tesla………..……………………………………….123 Figure 4.8.3: LiCl-H2O Temperature Distribution within the Bulk @ 0.0, 1.4 and 3.0Tesla……………………………………………………..125 Figure 4.8.4: LiCl-H2O Temp. Distribution at the interface @ 0.0, 1.4 and 3.0Tesla..........................................................................................126 Figure 4.8.5: LiCl-H2O Conc. Changes within the Film in X-Direction from 0.0 and 1.4 Tesla……………………………………………………………128 Figure 4.8.6: LiCl-H2O % Concentration Changes within the Film in X-direction @ 0.0 and 1.4Tesla……………………………… …………128 Figure 4.8.7: LiCl-H2O Concentration Changes within the Film in X-direction from 0.0 & 3.0 Tesla…………………………………………129 Figure 4.8.8: LiCl-H2O % Concentration Changes within the Film in X-Direction @ 0.0 & 3.0Tesla…………………..……………………129 Figure 4.8.9: LiCl-H2O Concentration Changes at the interface in X-Direction from 0.0 to 1.4 Tesla…………………………………..…………131 Figure 4.9: LiCl-H2O % Concentration Changes at the xv interface in X-direction from 0.0 to 1.4Tesla………………………..……………131 Figure 4.9.1: LiCl-H2O Concentration Changes at the interface in X-direction @ 0.0 & 3.0 Tesla………………………..………………………132 Figure 4.9.2: LiCl-H2O % Concentration Changes at the interface in X-Direction from 0.0 t0 3.0Tesla……………………..……………………....132 Figure 4.9.3: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at 0.25m level……………………………..138 Figure 4.9.4: LiBr-H2O% Velocity Changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ =0.25m level………………… ……….138 Figure 4.9.5: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at 0.25m level………………………………..139 Figure 4.9.6: LiBr-H2O %Velocity Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ =0.25m…………………………..…139 Figure 4.9.7: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.25m level……………………..140 Figure 4.9.8: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at 0.50m level………………….………….141 Figure 4.9.9: LiBr-H2O %Velocity Changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ =0.50m level………………………...142 Figure 4.10: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at 0.50m level……………………………..142 Figure 4.10.1: LiBr-H2O% Velocity Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ =0.50m……………………………...143 Figure 4.10.2: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.50m level………………………143 Figure 4.10.3: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at 0.75m level………………………………145 Figure 4.10.4: LiBr-H2O %Velocity Changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ =0.75m level…………………….......145 Figure 4.10.5: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at 0.75m level………………………………146 xvi Figure 4.10.6: LiBr-H2O %Velocity Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ =0.75m…………………………….146 Figure 4.10.7: LiBr-H2O Velocity distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.75m level…………………….147 Figure 4.10.8: LiBr-H2O Temperature distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.25m level……………….....…148 Figure 4.10.9: LiBr-H2O Temperature distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.50m level……………………150 Figure 4.11: LiBr-H2O Temperature distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.75m level…………..………….151 Figure 4.11.1: LiBr-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at X=0.25m level …………………………153 Figure 4.11.2: LiBr-H2O% concentration changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ X=0.25m level……………………...153 Figure 4.11.3: LiBr-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at X= 0.25m level………………………....154 Figure 4.11.4: LiBr-H2O % concentration Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ X= 0.25m level……………………..154 Figure 4.11.5: LiBr-H2O concentration distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at X= 0.25m level…………………155 Figure 4.11.6: LiBr-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at X=0.50m level …………………………156 Figure 4.11.7: LiBr-H2O % concentration changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ X=0.50m level………………………157 Figure 4.11.8: LiBr-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at X= 0.50m level……………………..…..157 Figure 4.11.9: LiBr-H2O % concentration Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ X= 0.50m level…………………….158 Figure 4.12: LiBr-H2O concentration distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at X= 0.50m level………………...158 Figure 4.12.1: LiBr-H2O concentration distribution in the Direction xvii of Film Thickness@ 0.0 &1.4Tesla at X=0.75m level ………………………..160 Figure 4.12.2: LiBr-H2O % concentration changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ X=0.75m level……………………..160 Figure 4.12.3: LiBr-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at X= 0.75m level……………………..…161 Figure 4.12.4: LiBr-H2O % concentration Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ X= 0.75m level…..……………….161 Figure 4.12.5: LiBr-H2O concentration distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at X= 0.75m level……………….162 Figure 4.12.6: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at 0.25m level…………………………...163 Figure 4.12.7: LiCl-H2O% Velocity Changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ =0.25m level…..………..………..164 Figure 4.12.8: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at 0.25m level…………………………..164 Figure 4.12.9: LiCl-H2O %Velocity Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ =0.25m……………………………165 Figure 4.13: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.25m level……………….…...165 Figure 4.13.1: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at 0.50m level…………………………...167 Figure 4.13.2: LiCl-H2O %Velocity Changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ =0.50m level……………………….167 Figure 4.13.3: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at 0.50m level…………………………….168 Figure 4.13.4: LiCl-H2O% Velocity Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ =0.50m….…………………………168 Figure 4.13.5: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.50m level…………………….169 Figure 4.13.6: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at 0.75m level…………………………….170 xviii Figure 4.13.7: LiCl-H2O %Velocity Changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ =0.75m level………………………171 Figure 4.13.8: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at 0.75m level……………………………171 Figure 4.13.9: LiCl-H2O %Velocity Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ =0.75m……………………………172 Figure 4.14: LiCl-H2O Velocity distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.75m level……………………172 Figure 4.14.1: LiCl-H2O Temperature distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.25m level……………….……174 Figure 4.14.2: LiCl-H2O Temperature distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.50m level…………..…….….175 Figure 4.14.3: LiCl-H2O Temperature distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at 0.75m level……………..….….177 Figure 4.14.4: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at X=0.25m level ……………………….178 Figure 4.14.5: LiCl-H2O% concentration changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ X=0.25m level……………………..179 Figure 4.14.6: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at X= 0.25m level………………………...179 Figure 4.14.7: LiCl-H2O % concentration Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ X= 0.25m level…………………….180 Figure 4.14.8: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at X= 0.25m level…...……………180 Figure 4.14.9: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at X=0.50m level ………………………………...182 Figure 4.15: LiCl-H2O % concentration changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ X=0.50m level……………………………..182 Figure 4.15.1: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at X= 0.50m level………………………………...183 Figure 4.15.2: LiCl-H2O % concentration Changes in the Direction of Film xix Thickness from 0.0 to 3.0Tesla @ X= 0.50m level…………………………….183 Figure 4.15.3: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at X= 0.50m level…………………………184 Figure 4.15.4: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &1.4Tesla at X=0.75m level ………………………………..185 Figure 4.15.5: LiCl-H2O % concentration changes in the Direction of Film Thickness from 0.0 to 1.4Tesla @ X=0.75m level……………………………..186 Figure 4.15.6: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0 &3.0Tesla at X= 0.75m level………………………………...186 Figure 4.15.7: LiCl-H2O % concentration Changes in the Direction of Film Thickness from 0.0 to 3.0Tesla @ X= 0.75m level…………………………….187 Figure 4.15.8: LiCl-H2O concentration distribution in the Direction of Film Thickness @ 0.0, 1.4 and 3.0 Tesla at X= 0.75m level…………………………187 xx LIST OF TABLES Table 3.1: Generaral Data …………………………………………………………...….48 Table 3.2: NH3-H2O Data …………………………………………………………...….48 Table 3.3: LiBr-H2O Data …………………………………………………………...….49 Table 3.3: LiCl-H2O Data………………………………………………………….........50 Table 4.1: NH3-H2O Velocity Distribution in the film Comparison ................………...52 Table 4.1.1: NH3-H2O Velocity Deviation Analysis……................................................53 Table 4.1.2: NH3-H2O Velocity changes within the film in X-direction…….................54 Table 4.1.3: NH3-H2O Velocity T-Test Analysis……..…………………………….......54 Table 4.1.4: NH3-H2O Temp. Distribution in the film Comparison ................………...56 Table 4.1.5: NH3-H2O Temp. Deviation Analysis……....................................................57 Table 4.1.6: NH3-H2O Temp. T-Test Analysis……..………………………………......57 Table 4.1.7: NH3-H2O Conc. Distribution in the film Comparison ................………...59 Table 4.1.8: NH3-H2O Conc. Deviation Analysis……....................................................60 Table 4.1.9: NH3-H2O Conc. T-Test Analysis……..………………………….……......60 Table 4.2: LiBr-H2O Velocity Distribution in the film Comparison………..………… .63 Table 4.2.1: LiBr-H2O Velocity changes within the film inX-direction………………...64 Table 4.2.2: LiBr-H2O Velocity distribution in the film in X-direction @ interface…………………………………………….………………...67 Table 4.2.3: LiBr-H2O Velocity changes within the film in X-direction @ interface…………………………………………….………………...69 Table 4.2.4: LiBr-H2O Temperature Distribution in the film Comparison………….…..72 Table 4.2.5: LiBr-H2O Temperature Distribution at the interface Comparison…….….. 75 Table 4.2.6: LiBr-H2O Temp. Deviation Analysis in the bulk and at the Interface…….76 Table 4.2.7: LiBr-H2O Temp. Deviation Analysis in the bulk and at the Interface….....77 Table 4.2.8: LiBr-H2O Concentration Distribution in the film Comparison…………….80 Table 4.2.9: LiBr-H2O Concentration Changes within the film in X-direction………....84 Table 4.3: LiBr-H2O Concentration Distribution at the interface Comparison…………87 Table 4.3.1:LiBr-H2O Conc. Deviation Analysis in the bulk and at the Interface……...88 xxi Table 4.3.2:LiBr-H2O Conc. Deviation Analysis in the bulk and at the Interface……...89 Table 4.3.3: LiBr-H2O Concentration Changes at the interface in X-direction…………92 Table 4.3.4: LiCl-H2O Velocity Distribution in the film Comparison…..............………96 Table 4.3.5: LiCl-H2O Temperature Distribution in the film Comparison………….…..97 Table 4.3.6: LiCl-H2O Concentration Distribution in the film Comparison………….. 101 Table 4.3.7: LiCl-H2O Velocity Distribution at the interface Comparison….................105 Table 4.3.8: LiCl-H2O Temperature Distribution at the interface Comparison………..107 Table 4.3.9: LiCl-H2O Temp. Deviation Analysis in the bulk and at the Interface…...108 Table 4.4: LiCl-H2O Temp. Deviation Analysis in the bulk and at the Interface…......109 Table 4.4.1: LiCl-H2O Concentration Distribution at the interface Comparison………112 Table 4.4.2: LiCl-H2O Conc.Deviation Analysis in the bulk and at the Interface…….113 Table 4.4.3: LiCl-H2O Conc.Deviation Analysis in the bulk and at the Interface…….114 Table 4.4.4: LiCl-H2O Velocity Changes within the film in X-direction…...................118 Table 4.4.5: LiCl-H2O Velocity Changes within the film in X-direction @ the interface .…………………………………………………………………….....121 Table 4.4.6: LiCl-H2O Temperature Changes within the film in X-direction…………124 Table 4.4.7: LiCl-H2O Temperature Changes at the interface Comparison……………125 Table 4.4.8: LiCl-H2O Concentration Changes within the film in X-direction……..…127 Table 4.4.9: LiCl-H2O Concentration Changes at the interface in X-direction……….130 Table 4.5: Percentage increment of coefficient of Performance (COP) @ 1.4 Tesla…136 Table 4.5.1: Percentage increment of coefficient of Performance (COP) @ 3.0 Tesla.136 Table 4.5.2: LiBr & LiCl COP increments at 3.0 Tesla T-Test analysis…………….136 Table 4.5.3: LiBr-H20: Velocity Changes in the direction of film thickness ( ) @ X=0.25m…………………………………………………………,,..137 Table 4.5.4: LiBr-H20: Velocity Changes in the direction of film thickness ( ) @ X=0.50m…………………………………………………………….140 Table 4.5.5: LiBr-H20: Velocity Changes in the direction of film thickness ( ) @ X=0.75m……………………………………………………….144 Table 4.5.6: LiBr-H20: Temperature Changes in the direction of film thickness ( ) @ X=0.25m……………………………………………………….147 Table 4.5.7: LiBr-H20: Temperature Changes in the direction of xxii film thickness ( ) @ X=0.50m……………………………………………………….149 Table 4.5.8: LiBr-H20: Temperature Changes in the direction of film thickness ( ) @ X=0.75m……………………………………………………….150 Table 4.5.9: LiBr-H20: Concentration Changes in the direction of film thickness ( ) @ X=0.25m……………………………………………………….152 Table 4.6: LiBr-H20: Concentration Changes in the direction of film thickness ( ) @ X=0.50m………………………………………………………155 Table 4.6.1: LiBr-H20: Concentration Changes in the direction of film thickness ( ) @ X=0.75m………………………………………………………159 Table 4.6.2: LiCl-H20: Velocity Changes in the direction of film thickness ( ) @ X=0.25m………………………………………………………162 Table 4.6.3: LiCl-H20: Velocity Changes in the direction of film thickness ( ) @ X=0.50m………………………………………………………166 Table 4.6.4: LiCl-H20: Velocity Changes in the direction of film thickness ( ) @ X=0.75m……………………………………………………….169 Table 4.6.5: LiCl-H20: Temperature Changes in the direction of film thickness ( ) @ X=0.25m….………………………………………………..173 Table 4.6.6: LiCl-H20: Temperature Changes in the direction of film thickness ( ) @ X=0.50m…………………………………………………...174 Table 4.6.7: LiCl-H20: Temperature Changes in the direction of film thickness ( ) @ X=0.75m…………………………………………………...176 Table 4.6.8: LiCl-H20: Concentration Changes in the direction of film thickness ( ) @ X=0.25m…………………………………………………...177 Table 4.6.9: LiCl-H20: Concentration Changes in the direction of film thickness ( ) @ X=0.50m….………………………………………..………181 Table 4.7: LiCl-H20: Concentration Changes in the direction of film thickness ( ) @ X=0.75m…………………………………………………...184 xxiii LIST OF SYMBOLS Unit μ = film dynamic viscosity……………………………………… kg/m/s υo = mean velocity………………………………………………. m/s 2 α = thermal diffusivity……..……………………………………… m /s k = thermal conductivity of fluid ……………................................. W/m K 3 ρ = liquid density………………………………………………….. kg/m 2 D = species diffusivity………………… …………………………. m /s -1 β = cubic expansivity of fluid …………………………………….. K 0 Tw = dimensional wall temperature………………………………… C 0 Tin = inlet refrigerant temperature…………………………………. C Cin = initial absorbent concentration ………….............................. % Ceq = equilibrium absorbent concentration……………………….. % 2 g = gravity………………………………………………………. m/s h0 = mean film thickness….……………………………………… m 2 ν = kinematic viscosity of fluid…………………………………. m /s Ha = heat of absorption…………………………………………... kJ/kg Pv = absorbent vapour pressure……………………………......... mm. Hg Ref = film Reynolds number Ґ = film mass flow rate xxiv CERTIFICATION I certify that this research work was carried out by Mr. K.M. Odunfa in the Department of Mechanical Engineering, University of Ibadan, Ibadan, Nigeria. ______________ Research Supervisor, Prof. R. O. Fagbenle. FNSE B.S.M.E. (Urbana-Champaign), M.S.M.E. (Iowa State), PhD (Urbana-Champaign), R.Engr. Professor Department of Mechanical Engineering, University of Ibadan, Ibadan, Nigeria. xxv CHAPTER ONE INTRODUCTION 1.1 General Background In recent years, the terms energy conservation and environmental safety have become a thing of global concern due to the increasing energy prices, energy security and environmental impact of energy prospecting, processing and utilization. In time past, not much emphasis was placed on the issue as it is now. The worldwide attention to Climate Change phenomenon and its impacts which have been conclusively linked to fossil energy use has prompted the emergence of the new technologies in many areas of global economy, such as in cooling system development sector. Cooling system basically may be divided into two categories; Vapour compression system and sorption system. Sorption system is further sub-divided into absorption and adsorption systems. Vapour compression system involves the use of a compressor for the compression process; An absorption system is simply the replacement of the traditional compression with a thermo chemical fluid lifting process. In other words it is the mixture of a gas in a liquid, the two fluids present a strong affinity to form a solution, while adsorption is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or, more rarely, a liquid (adsorbent), forming a molecular or atomic film (the adsorbate). In the manufacturing of cooling machine/system, for the increasing interest in the efficient use of energy at minimum environmental cost has necessitated the increased demand for absorption refrigeration systems driven by waste heat or solar thermal energy instead of conventional systems driven by fossil-energy derived or conventional. The current imbalance of energy demand and supply coupled with the environmental degradation in many developing countries such as Nigeria has further increased the urgent need for highly efficient and sustainable energy technologies. In the absorption process, heat and mass transfer usually take place within a thin-liquid falling-film. Heat and mass transfer in thin-liquid falling film absorption process has received the attention of many researchers over the years especially in the last two decades. This is as a result of its wider application in many modern devices such as absorption air-conditioners, absorption chillers, absorption heat pumps etc Yang and Wood (1992). Absorption enhancement is another aspect in this area that has also attracted the attention of researchers. Absorption enhancement is an effective way to improve the performance of absorption refrigeration systems. Generally, 1 there are three kinds of methods in absorption enhancement (Kim et al.,2006).The first kind falls under the category of mechanical methods, which improve the performance by modifying the shape, surface and structure of the heat transfer tubes (Chen et al.,2006). The second kind comprise chemical methods which involve the addition of surfactant in the absorbent while the third kind is the addition of nano-particles in the absorbing solution e.g Cu, CuO and Al2O3 nano-particles added into ammonia-water solution (Kim et al., 2007a and 2007b), Fe and Carbon nano-tubes (CNT) in lithium bromide-water solution (Yong Tae Kang et al.,2007). Research on nano-fluids / nano-particles in absorbent are categorized into five groups(1) stability analysis and experiments; (2)property measurement such as thermal conductivity and viscosity;(3)convective and boiling heat transfer;(4)mass transfer in binary nanofluids; and (5)theoretical analysis and model development. However, the effect of magnetic field on absorption refrigeration system is seldom mentioned in the literature apart from its established influence on the absorption process in ammonia vapour into ammonia-water solution absorption refrigeration system (Niu et al., 2006). The magnetic field may therefore also have certain influence on the absorption process in other absorption refrigeration systems such as water vapour into lithium bromide-water solution and water vapour into lithium chloride-water solution absorption system. In previous studies without any form of enhancement, Andberg (1982), Grossman (1983) and Andberg and Vliet (1983) have employed the modeling technique. Modeling is categorized into two namely (i) Numerical and (ii) Experimental. Numerical methods have been used over many decades and continue to be developed to effectively tackle many engineering problems. Some of these numerical methods include Finite Element method, finite difference method, boundary element method, Monte Carlo technique and Vortex Method. These methods may be categorized into two: probabilistic approach and deterministic approach. Probabilistic methods generally make constant recourse to random numbers, and Monte Carlo and Vortex element techniques fall into this category while Finite Element, Finite difference and boundary element techniques are deterministic in nature. In absorption process modeling either unenhanced or enhanced, experimental modeling has been extensively employed, while the numerical modeling of the problem has been rather difficult due to the presence of waves in the falling liquid film. However, the smooth film absorption assumption has allowed successful modeling of the problem even though without any 2 form of enhancement. One of the earliest of such models was developed by G. Grossman (1983), later followed by other researchers such as Andberg and Vliet (1983) and Yang and Wood, B.D (1992). In the area of numerical modeling of enhanced absorption system, Xiao Feng Niu et al. (2006) established the mathematical model for magnetic field enhanced absorption process for ammonia-water solution on a falling-film. The changes in physical properties of ammonia-water solution in absorption, the variation of falling film and the convection in the direction of thickness of liquid film were considered in the modeling; Distribution of some parameters in falling–film absorption, such as velocity, temperature and concentration; in the application of magnetic field was obtained. Numerical results obtained show that magnetic field can improve the performance of ammonia-water falling-film absorption, and the absorption strengthening effect increases with the enhancement of magnetic induction intensity. The strengthening effect is limited within the magnetic field intensity of 0-3 T, but there are trends of increasing strengthening effect in stronger magnetic fields. In both unenhanced and enhanced absorption system studies, several working fluids have been investigated, some of which are Lithium bromide-water (LiBr-H2O), Lithium-Chloride Water (LiCl-H2O) and Ammonia-water (NH3- H2O) all of which are popularly used in single-stage and advanced absorption air- conditioning/heat pump technology. Xiao Feng Niu et al. (2009) experimentally studied the effect of external magnetic field on falling film absorption for ammonia-water system. The study established the following findings: (i) An external magnetic field acting in the same direction as falling film has an enhancing effect on absorption of ammonia-water process, and the absorption enhancement is more greater in stronger magnetic field. When external magnetic field with the same direction as the falling film is exerted, several absorption variables, including the concentration of the ammonia-water solution after absorption, the outlet temperature of the cooling water, the absorption heat and absorption mass in the external magnetic field, are higher than those without external magnetic field exerted. Moreover, the four absorption variables increase with the increase in magnetic induction intensity, (ii) Not all magnetic fields can enhance the ammonia- water absorption process. When an external magnetic field acting against the direction of falling film is exerted, the absorption variables in magnetic field are all smaller than those in conventional absorption without magnetic field exerted. The magnetic field opposing the 3 direction of the falling film weakens the absorption of ammonia-water (iii) The absorption can be more intense if the external magnetic field is combined with optimal operating conditions. Experimental results show that the changes in the outlet cooling water temperature, absorption heat and absorption mass with and without external magnetic field exerted, are larger when the inlet solution concentration is lower. Larger cooling water flow rate, lower cooling water temperature and smaller solution flow rate are beneficial to absorption in magnetic field as they do in conventional absorption. The finite difference method has been applied much more than the finite element method in the analysis of absorption/adsorption systems. The finite element method sometimes described as a versatile and powerful numerical method is a piecewise approximation method. It approximates a problem described by a system of differential equations by a number of algebraic equations relating a finite number of variables (unknowns). These algebraic equations are then solved on the digital computer (for large systems) using an appropriate solution technique. The fundamental concept of the method is that any continuous field quantity (e.g. temperature and concentration) in a continuum can be approximated by a discrete model composed of a set of piecewise continuous parameter functions defined over a finite number of sub-domains. These sub-domains which are referred to as finite elements are connected at discrete points called nodes. The parameter functions must satisfy specific compatibility and completeness requirements. In engineering problems, either probabilistic or deterministic methods could be used depending on the degree of accuracy required of the solution in comparison with experimentally data or analytic solution. Absorption process enhancement under magnetic field apart for its established effect on ammonia-water absorption process (Xiao Feng Niu et al.( 2006), (2009)) as earlier mentioned, magnetic field effect is seldom used and to the best of the knowledge of the researcher, it has not been used in either lithium bromide-water or lithium chloride-water absorption system. This present work therefore investigates the effect of a magnetic field enhanced absorption process on a smooth thin-liquid falling-film in a cooling system using lithium bromide-water (LiBr-H2O) and (LiCl- H2O) refrigerants/absorbents combinations. A model of the problem is first developed from the resulting differential equations and solved using the finite difference method. Such an investigation would reveal sections of the absorber that 4 need to be redesigned and its material re-specified, e.g for optimal efficiency of refrigerant absorption by the absorbent. Statement of the Problem In the manufacturing of modern absorption devices such as absorption air-conditioners, chillers and heat pumps, the most popularly used refrigerant/absorbent combination or working fluids are Lithium-Water Bromide (LiBr-H2O), Lithium Chloride-Water (LiCl-H2O), Ammonia- water (NH3–H2O), Pentafluoroethane (HFC-125) and Trifluoroethane (HFC-143a). Due to the zero ozone depletion potential (ODP) and zero global warming potential (GWP) of NH3-H2O and H2O-LiBr) Fagbenle et al. (1994), these two refrigerant/absorbent combination are relatively environmentally friendly. Ammonia-water solution is however incompatible with copper, the popular tubing material for transporting refrigerants, for this reason, it is less popularly used than the other working fluids. The other working fluids such as Halogenated Chlorofluorocarbons (HCFCs), and Azeotropic mixtures (R-500 and R-502) that were popularly used in the past have been declared non-environmentally friendly following the Montreal Protocol and subsequent London and Copenhagen meetings of 1990 and 1992, Fagbenle et al. (1994). This is because of their high Ozone Depletion Potential (ODP) and high Global Warming Potential (GWP) coupled with their roles in the green house effect (either directly through the fluids used or indirectly through the energy consumption in systems using fossil-energy) and as such they are no more in use. Numerical modeling of the absorption process on a thin-liquid falling-film generally has been difficult due to complication arising from the presence of waves. The Odunfa (2008), approached this issue by considering waves as a second order effect, thereby, appropriating the flow as a smooth falling-film. The degree to which ignoring the second-order effect affects the results by using a thin-liquid falling-film approximation to the absorption process was investigated in his work using lithium bromide-water and lithium chloride-water as working fluids. The results obtained then compared well with the existing experimental results. This research work thus aims at further investigating and establishing the influence of the magnetic field on the absorption process in a smooth thin-liquid falling-film for a cooling system using (LiBr-H2O) and (LiCl-H2O) refrigerants/absorbents. 5 Justification Energy conservation and environmental concerns are increasingly attracting global attentions, due to the increasing energy prices and environmental impact of energy prospecting, processing and utilization. As such conventional refrigeration systems are gradually giving way to newer technologies such as absorption cooling systems. Since the major concern in absorption refrigeration systems hinges largely on the energy conservation, environmental issues and efficient cooling system which the present work is to be addressed, hence the justification for this work. Aims and Objectives The present work aims and objectives are (i) to develop and evaluate a numerical model for the magnetic field enhancement of the absorption cooling systems using lithium bromide (LiBr) and lithium chloride (LiCl) water solution. (ii) to establish absorption system‟s performance improvement with increment in magnetic induction intensity enhancement on the two investigated fluids and (iii) to establish the enhancement of the Coefficient of Performance (COP) of both fluids with magnetic field. Expected practical application of this work includes the design of efficient absorption plant components such as absorbers, evaporators, condensers and generators. Also in this study, key factors such as refrigerant/absorbent combination‟s parameters and film velocity which influence the improvement level in the absorption performance under magnetic field enhancement would be established. 6 CHAPTER TWO LITERATURE REVIEW 2.1: Introduction The production of cold temperature has application in many fields of human endeavour, e.g. for preservation of perishable products, in the food processing industry, in the air conditioning sector and for the preservation of pharmaceutical products. Based on the cooling temperature requirement, the applications of the absorption systems/machines can be broadly o o classified into three categories; air-conditioning (8-15 C) for space cooling, refrigeration (0-8 C) o for food and vaccine storage and freezing (< 0 C) for ice making purposes. 2.1.1: Cooling Technology Cooling technology is classified into two broad systems; vapour compression and absorption cooling systems. Based on the cooling temperature requirement, either of the two classifications can be further broken down into the three categories mentioned above. 2.1.2: Advantages and Disadvantages of Vapour Compression and Absorption Refrigeration i. The amount of power required: The compressor of the vapor compression cycle requires large amount of power for its operation and it increases as the size of the refrigeration system increases. In case of the vapor absorption refrigeration system, the pump power required to circulate the absorber-absorbent fluids is relatively very small. ii. Running cost: The vapor compression refrigeration system can run only on electric power, and they require large amount of power. In case of the absorption refrigeration system only small pump is required whose electric power consumption is generally quite low. Thus the running cost of the absorption refrigeration system is much smaller than for the vapor compression system. iii. Foundations required and noise: The compressor of the vapour compression system is operated at very high speeds and with consequent vibrations and noise. It also requires very strong foundation to withstand the vibrations. In the absorption refrigeration system, there are no 7 major moving parts hence they do not vibrate and make no noise. The absorption refrigeration system operates silently with no vibration iv. Maintenance: The compressor is a key component of the compression cycle having a number of moving parts. Thus the compression system generally requires a lot maintenance attention. In the absorption refrigeration system the only moving part is the small pump that fails rarely making the system to be robust and requiring little or no maintenance. v. Type of refrigerant used and its cost: The refrigerant used by the absorption refrigeration system is environmentally friendly; easily and cheaply available. In the case of the vapour compression refrigeration system, halocarbons that are been used as the refrigerants are not environmentally friendly and also very expensive. vi. Leakage of the refrigerant: In the absorption refrigeration system leakage of the refrigerant seldom occurs while the refrigerant itself is relatively inexpensive. Thus refrigerant recharging costs is minimal. In vapour compression systems, leakages of the refrigerant occurs quite often requiring regular refrigerant recharge of the system usually at a relatively high cost. vii. Greenhouse effect: Most of the halocarbon refrigerants used in the compression refrigeration system has a high GWP. Following the Montreal Protocol, their use has to stop completely by the year 2020. In the absorption refrigeration system, refrigerants have very low or zero GWP and they are not in any exclusion list. 2.1.3: Vapour compression System Fig. 2a is a typical vapour compression system (Chiller) York model. The following major components can be identified in the figure: compressor, condenser coil, evaporator and fans. Other minor components such as filter drier, oil line and other accessories are also shown in the figure. 8 Fig.2a. Vapour Compression Chiller-three stage Compressor 2.1.4: Vapour compression cycle Fig 2b shows the typical schematic diagram of a single stage compression system containing a compressor, a condenser, an evaporator and an expansion valve. The two- stage compression system shown in fig 2c contains a flash chamber, and two compressors instead of one as in the single-stage system. 9 Condenser Compressor Evaporator Expansion Valve Fig.2b. Basic Vapour Compression cycle- Single Stage Condenser Compressor Compressor Flash Chamber Evaporator Fig.2c. Basic Vapour Compression cycle - Two-stage Compressor and a flash chamber 10 2.1.5: Absorption System Fig. 2d is a typical two-stage absorption system (Chiller). It contains two generators (G1& G2), two condensers (C1& C2), an absorber, and an evaporator. Other components such as pump, an expansion valves and solution heat exchanger (SHE) are also in the figure. The line diagram of the two-stage absorption refrigeration cycle appears in fig 2f. Fig. 2d.Typical two-stage Absorption Chiller 2.1.6: Principle of operation of Absorption system The working fluid in an absorption refrigeration system is a binary solution consisting of refrigerant and absorbent. In the fig. 2e below, two evacuated vessels are connected to each 11 other. The left vessel contains liquid refrigerant while the right vessel contains a binary solution of absorbent/refrigerant. The solution in the right vessel will absorb refrigerant vapour from the left vessel causing pressure to reduce. While the refrigerant vapour is being absorbed, the temperature of the remaining refrigerant will reduce as a result of its vaporization. This causes a refrigeration effect to occur inside the left vessel. At the same time, solution inside the right vessel becomes more dilute because of the higher content of refrigeration absorbed. This is called the “absorption process”. Normally, the absorption process is an exothermic process, therefore it must reject heat out to the surrounding in order to maintain its absorption capability. Whenever the solution cannot continue with the absorption process because of saturation of the refrigerant, the refrigerant must be separated out from the diluted solution. Heat is normally the key for this separation process. It is applied to the right vessel in order to dry the refrigerant from the solution as shown in fig. 2e(b) below. The refrigerant vapour will be condensed by transferring heat to the surroundings. With these processes, the refrigeration effect can be produced by using heat energy. However, the cooling effect cannot be produced continuously as the process cannot be done simultaneously. Therefore, an absorption refrigeration cycle is a combination of these two processes as shown in fig. 2f. As the separation process occurs at a higher pressure than the absorption process, a circulation pump is required to circulate the solution. Coefficient of performance of an absorption refrigeration system is obtained from; cooling capacity obtained at evaporator COP = heat input for the generator  work input for the pump The work input for the pump is negligible relative to the heat input at the generator; therefore, the pump work is often neglected in the evaluation of the COP. 12 Refrigerant Solution Refrigerant Solution QI QI QL QH (a) (b) Fig. 2e: An intermittent Absorption Cycle . (a) Absorption process occurs in right vessel causing cooling effect in the other, (b) Refrigerant separation process occurs refrigerant separation process generator condenser QH QI absorber evaporator QI QL absorption process Fig. 2f: A continuous absorption refrigeration cycle composes of two processes mentioned in the earlier figure. 13 2.1.7: Working fluids/Refrigerants A review of literature carried out on the refrigerants used in air-conditioning and refrigerating systems reveals that fully halogenated chlorofluoro carbon (CFCs) and halon compounds, carbon tetra chloride (CCL4), 1,1, l-trichloride ethane (Methyl Chloroform CH3CCl3) and methyl bromide (CH3Br) are the commonly used refrigerants in the industrial and commercial compression cooling systems. Following the scientific findings on the above mentioned refrigerants that they have high ozone depletion potential (ODP) and global warming potential (GWP), the 1987 Montreal Protocol was made in 1990 and amended in Copenhagen in 1992 to phase out or ban these refrigerants from usage by the year 2015. Ever since this protocol, the search for replacement chemicals has been exclusively in the developed countries, because the bulk of the production and consumption of these restricted chemicals is in these countries. Furthermore the technical expertise, research infrastructure and the direct and allied economic investment are all in these countries. Researchers in this field of study worldwide have intensified efforts towards finding alternative refrigerants to the CFC‟s which will not only be environmentally friendly, but which will also be energy efficient. A survey of absorption fluids provided by Marcriss et al.(1988) suggests that, there are some 40 refrigerant compounds and 200 absorbent compounds available. However, the most common working fluids are water lithium bromide (H2O–LiBr), water lithium Chloride (H2O–LiCl) and Ammonia–water (NH3–H2O) solutions. Heat and mass transfer analysis of cooling systems has been continuously undertaken by researchers such as Yang and Wood (1992), Muhsin et al.(2004), Fernandez et al.(2005), Xu et al.(2006) and Gu et al.(2007) in order to improve on plant and component design and efficiencies. Water lithium bromide (H2O– LiBr) and water lithium chloride (H2O-LiCl) solutions remain the most popularly used refrigerant-absorbent pairs in absorption cooling systems and as such the present work employs these two working fluids, albeit with magnetic field enhancement. 2.1.8: Various designs of absorption refrigeration cycles Pongsid Srikhirin et al. (2001) in his review of absorption refrigeration technologies established the following designs of absorption refrigeration cycles: the single-effect absorption system (Fig.2g), the absorption heat transformer (Fig.2h), the double-effect water/LiBr absorption cycle (Fig.2i), the double-effect absorption cycle operates with two pressure levels 14 (Fig.2j), a triple-effect absorption cycle operates at four pressure levels (Fig.2k), the dotted loop shows secondary fluid used for transferring heat from high the temperature section in the absorber to low temperature section in the generator (Fig.2l), The cycle with absorber heat recovery (Fig. 2m), A half – effect absorption cycle (Fig. 2n), Combined vapour absorption/compression heat pump (Fig. 2o), Double effect absorption –compression cycle, (Fig. 2p), A combined cycle proposed by Caccoila et al (Fig.2q, A resorption cycle proposed by Altenkirch (Fig. 2r), Solar driven dual cycle absorption (Fig. 2s), A modified double effect combined ejector – absorption refrigeration cycle (Fig. 2t), A combined ejector/absorption system using DMETEG/R22 and DMETEG/R21 as working fluids (Fig. 2u), A combined ejector/absorption proposed by Aphornratana and Eames (Fig. 2v), A combined cycle proposed by Eames and Wu (Fig. 2w), osmotic-membrane absorption cycle (Fig.2x), and diffusion absorption refrigeration system DAR) (Fig.2y). generator condenser QH QI HX QI absorber evaporator QL Fig.2g: A Single–effect LiBr/water absorption refrigeration system with a solution heat exchanger (HX) that helps decrease heat input at the generator 15 QH absorber evaporator QI HX generator condenser QI QI Fig.2h: Absorption heat transformer absorbs waste heat at the generator. Liquid refrigerator is pumped to the evaporator to absorb waste heat. High temperature useful heat from the absorber is heat of absorption. generator I QH HX II Condenser QI generator II HX I QI absorber evaporator QL Fig.2i: A double – effect water/LiBr absorption cycle. Heat released from the condensation of refrigerant vapour is used as heat input in generator II. This cycle is operated with 3 pressure levels i.e high, moderate and low pressure. 16 rectifier QI Condenser generator I generator II QH rectifier Heat of HX I Absorption HX II QI evaporator absorber I QI absorber II Fig. 2j: A double – effect absorption cycle operates with two pressure levels. Heat of absorption from Absorber II is supplied to the Desorber I for the refrigerant separation process. QH 1st stage generator HX I QI HX II condenser HX III QI absorber evaporator QL Fig. 2k: A triple – effect absorption cycle operates at four pressure levels. Heat of condensation from the higher – pressure stage is used for refrigerant separation in the lower pressure stage. 17 generator condenser QI secondary rectifier fluid QH condensate precooler QL absorber QL evaporator Fig. 2l: The dotted loop shows secondary fluid used for transferring heat from high temperature section in the absorber to low temperature section in the generator. QH generator condenser QI HX absorber heat recovery evaporator QL QI absorber Fig. 2m: The cycle with absorber heat recovery uses heat of absorption of preheat the outgoing stream from the absorber to the generator. 18 QI condenser generator II QH HX QI absorber II generator I HX QL evaporator absorber I QI Fig. 2n: A half – effect absorption cycle is a combination of two single – effect cycles but working at different pressure levels. Letting heat source temperature be lower than the minimum temperature is necessary for a single – effect cycle working at the same pressure level. QI resorber QH resorber QL desorber QI resorber (a) (b) Fig. 2o: Combined vapour absorption/compression heat pump. 19 absorber I absorber II QH heat of absorption compressor HX HX desorber I QI desorber II Fig. 2p: Double effect absorption –compression cycle is configured as a heat pump. Heat of absorption in the first stage will be supplied to the second stage for refrigerant separation. QH generator condenser QI HX QI absorber QL desorber Fig.2q: A combined cycle proposed by Caccoila et al.(86), employing two combinations of working fluids ie. NH4/H2O/KHO. The rectifier is absent and also the highest pressure is decreased. 20 QI resorber generator QH HX HX QL desorber absorber QI Fig. 2r: A resorption cycle proposed by Altenkirch uses two solution circuits. ammonia/ water circuit water/ LiBr circuit NH3/ Water Water/LiBr condenser generator generator generator Q Solar absorber absorber evaporator evaporator QL QI QL Fig. 2s: Solar driven dual cycle absorption employs to different working fluids i.e NH3/water and water/LiBr. Heat of absorption and condensation from NH3/water cycle are supplied to the generator of water/LiBr cycle. 21 QH generator I HX generator II HX ejector QI absorber generator QL Fig. 2t: A modified double effect combined ejector – absorption refrigeration cycle where there is no condenser included. QH generator generator QI HX condenser condenser QL QI Fig. 2u: A combined ejector/absorption system using DMETEG/R22 and DMETEG/R21 as working fluids. The strong solution in the returning leg from generator serves as primary fluid and refrigerant vapour from evaporator as second fluid. 22 ejector QH generator condenser QI HX QI absorber evaporator QL Fig. 2v: A combined ejector/absorption proposed by Aphornratana and Eames, was invented. High pressure refrigerant vapour from the generator enters the ejector as motive fluid to carry the refrigerant vapour from the evaporator. ejector Steam QH generator condenser generator HX QL absorber evaporator QL Fig. 2w: A combined cycle proposed by Eames and Wu (93). The highest solution circuit temperature ia maintained at about 18C. So the corrosion problem is alleviated. 23 QH QI condenser generator Membrane absorber QI QL condenser Fig. 2x: An osmotic membrane absorption cycle employs heat for refrigerant separation and producing pressure within the system. generator condenser QI absorber evaporator QL QH QI Fig. 2y: The diagram shows a bubble pump in the generator module. Heat input to the generator is used for both circulation of working fluid and evaporation of refrigeration. 2.1.9: Practical Absorption Cycle Fig. 2zi is a basic absorption cycle that uses a lithium bromide and water solution as an absorbent and water as refrigerant. The single-stage fundamental absorption refrigeration system (ARS) contains a generator, an absorber, a condenser, an evaporator, a solution pump, expansion valves or restrictors, and a solution heat exchanger (SHE). The flow path is shown in the figure with arrows. 24 6 Condenser Generator 5 1 Heat Out 7 Heat Exchanger Restrictor 2 Solution Restrictor Pump 4 8 Vapour 3 Evaporator 9 Spillover Absorber Low Heat Temperature Out In Fig.2zi. Lithium bromide-water single-stage absorption Refrigeration cycle 6 Condenser 2 Generator C2 G2 61 Condenser Generator G C 1 51 1 Heat Heat Exchanger Out 7 2 Solution Restrictor Restrictor Pump 4 8 Vapour 3 Evaporator 9 AbsorberSpillover Heat Out Fig. 2zii. Lithium bromide-water two-stage absorption Refrigeration cycle 25 2.3: Numerical Modeling Experimental modeling of heat and mass transfer on thin-liquid falling film in unenhanced and enhanced absorption has been extensively studied, while there are relatively few studies on numerical modeling of the problem. In experimental unenhanced absorption process modeling, Ali et al. (2002) investigated the technical feasibility of driving a lithium chloride- water solution absorption-cooling unit by a low-temperature heat source (such as solar energy using a simple flat-plate collector) for air-conditioning applications. The operating characteristics of the unit were extensively investigated and the C.O.P of the unit was found to be 19% as against the design value of 21%. Safarik et al. (2004) also carried out an experimental modeling on a solar power absorption chiller with low capacity using lithium bromide-water solution as the working fluid. The field test carried out at three sites in the summer of 2003 after the prototype test in this experimental modeling showed that the absorption chiller performed reliably and flexibly over a wide range of external conditions. Abdelmessih et al. (2005) experimentally investigated the use of non-traditional absorbent/refrigerant pairs such as ethylene glycol-water on a designed and built absorption refrigeration cycle. The investigation was successful in replacing traditional hazardous absorbent/refrigerant pairs with ethylene glycol-water pair. The working fluids chosen are safe, unlike the commercial absorbent/refrigerant pairs. Yaxiu et al. (2007) also examined experimentally a compact solar pump-free lithium bromide absorption refrigeration system equipped with a second generator, a falling-film absorber, a falling-film evaporator and an efficient luminate thermosiphon elevation tube. The experiment confirmed a 48.5% increase in the COP. Since the numerical modeling of both unenhanced and enhanced absorption process is complicated by the presence of the waves in the falling liquid-film, the smooth falling film unenhanced absorption approximation has been more popularly investigated, the earliest of such being the work of Grossman and Andberg (1983). This was even considered complicated in formulation due to the restriction of the model to the case with the inlet absorbent temperature being equal to that of the wall. In the same year Andberg and Vliet (1983) also investigated the smooth falling-film absorption under laminar flow using a different model from his research model, this was also considered quite sophisticated and somewhat too complicated in formulation. 26 Yang and Wood (1992) investigated a numerical model of the absorption process on a smooth liquid falling-film in lithium bromide water and a lithium chloride water solution system. Yang et al. (1992) developed a numerical absorption model of a simple smooth-film of LiCl-H2O and LiBr-H2O systems with Reynolds number of 2.7, 27 and 100 using finite difference solution approach. The solutions obtained were similar to the results of the earlier work of Grossman et al. (1983) and generally agreed with available experimental data. Argiriou et al. (2004) conducted a numerical investigation on a prototype low capacity solar assisted lithium bromide absorption heat pump coupled with a sub-floor system using the commercial simulator known as TRNSYS. The results indicated that the estimated energy savings against a conventional cooling system using a compression type heat pump was in the range of 20-27%. Xu et al. (2006) simulated an absorption process in an advanced energy storage system. The latest of the modeling in this area was on a two-stage absorption chiller driven at two-temperature levels using thermodynamic modeling technique by Gustavo et al. (2007). The study established that 0 the machine can operate in summer as a double-stage chiller driven by heat at 170 C from 0 natural gas, as a single-stage chiller driven by heat at 90 C from solar energy, or simultaneously in combined mode at both temperatures. It also established the capability of operating in winter 0 in “double-lift” mode for heating with a driving heat at 170 C from natural gas. Ghaddar et al. (1996) modeled solar lithium bromide absorption system performance in Beirut using a simulated computer program. The results shows that for each ton of refrigeration it 2 is required to have a minimum collector area of 23.3m with an optimal water storage tank capacity ranging from 1000 to 1500 litres for the system to operate solely on solar energy for about seven hours a day. The energy use in cooling was also found to be of function of solar collector area and storage tank capacity. Based on the economic assessment performed on the current cost of conventional cooling system, it was also found that the solar cooling system is marginally competitive only when combined with domestic water heating. Bruno et al (2004) modeled Ammonia-water-sodium hydroxide mixtures absorption refrigeration plant using a commercial process simulator “Aspen Plus 2003”. It was found that the system performance is notably increased (lower driving temperature and higher COP). Fernandez et al. (2005) also modeled an absorption processes taking place in Ammonia-water absorption system using finite difference approach. The simultaneous heat and mass transfer set 27 of nonlinear differential equations were solved using the above mentioned approach. The results established the expected typical range of values of xvb < z < ∞.or – ∞ < z < xLb and xLb < z < xvi for mass transfer against temperature variations in different components of the plant such as absorber and evaporator where x, z, b, L, i and v are defined as ammonia molar concentration, ammonia to net molar flux transferred ratio, bulk conditions, liquid, liquid-vapour interface and vapour respectively. Icksoo et al. (2006) developed a water-lithium bromide absorption process model over a horizontal tube using finite difference approach. The model predicts that significant absorption takes place in the drop formation regime with a considerable variation of temperature and mass fraction. Xu et al. (2006) simulated aqueous lithium bromide (LiBr-H2O) advanced energy storage system using finite difference method. The result predicts the dynamic characteristics and performance of the system, including the temperature and concentration of the working fluid, the mass and energy in the storage tanks, the compressor intake mass or volume flow rate, discharge pressure, compression ratio, power and consumption work, the heat loads of heat exchanger devices in the system and so on. The result also indicated that the integrated coefficient of performance (COPint) of the system was 3.09 and 3.26 respectively as against the expected value of 3.0 under the two storage strategies, while the isentropic efficiency of water vapour compressor was set as 0.6. These results were found to be very helpful in understanding and evaluating the system as well as for system design, operation and control. Gustavo et al. (2007) studied a two-stage LiBr-water absorption chiller driven at two temperature levels using thermodynamic modeling technique. The study established that the 0 machine can operate in summer as a double-stage chiller driven by heat at 170 C from natural 0 gas, as a single-stage chiller driven by heat at 90 C from solar energy, or simultaneously in combined mode at both temperatures. It also established the capability of operating in winter in 0 “double-lift” mode for heating with a driving heat at 170 C from natural gas. Balghouthi et al. (2007) conducted both experimental and numerical modeling on solar water-lithium bromide 0 0 absorption air conditioning in Tunisian climatic conditions (36 Latitude and 10 longitude, 2 400cal/cm day average solar irradiation, and 3700h/year and 350 total insolation period and sunny days per year respectively) using the TRNSYS and „EES‟ Engineering Equation Solver programs in the study. The model established that absorption solar air-conditioning system was suitable under Tunisian conditions. Despite the initial high cost and almost zero maintenance 28 cost, this system could help to minimize fossil fuel-based energy use, reduce electricity demand on the national grid especially at peak demand periods in summer and eliminate the use of CFCs. In experimental enhanced absorption process study, Wen-long Cheng et al. (2003) investigated experimentally the effect of additive on falling film absorption of water vapour in aqueous LiBr. The experimental results showed that small amounts of additive can enhance the heat transfer of absorption process significantly, and the enhancement degree is influenced by additive concentration and Reynolds number. Based on a dimensionless analysis of the Navier- Stokes equations applied to the falling film absorption process, a new dimensionless parameter, surface renewal number Rn was introduced, and a semi-empirical equation of enhancement factor of additive was obtained, which shows that the enhancement effect of additive on Nusset number of absorption process is determined by the absorption Marangoni number Ma, the surface Marangoni number MaA, the surface renewal number Rn, the adsorption number ƞ, and the Reynolds number Re. It was proved that the semi-empirical equation can agree with the experimental results well by introduction of the parameters related to surface tension into the equation. The study concluded as follows: i.There is an optimum additive concentration in which the enhancement effect of additive is strongest, ii.The Marangoni number Ma, the surface Marangoni number MaA, and the surface renewal number Rn enlarge the enhancement of the heat transfer during absorption, iii. The adsorption number ƞ reduces the heat transfer of absorption, and iv. The enhancement factor decreases as the Reynolds number increases. Yong Tae Kang et al. (2006) in their experimental study also obtained the following results: (i). The vapour absorption rate increases with increasing solution mass flow rate and the concentration of Fe nanoparticles and CNT. The effect of coolant mass flow rate on the vapour absorption rate is not significant under the experimental conditions, (ii). The heat transfer rate increases with increasing the solution mass flow rate while it is not much affected by the concentration of nanoparticles, (iii). The mass transfer enhancement is much more significant than the heat transfer enhancement in the binary nanofluids with Fe nanoparticles and CNT, and (iv). The mass transfer enhancement from the CNT (average 2.16 for 0.01 wt % and average 2.48 for 0.1 wt %) becomes higher than that from the Fe nanoparticles (average 1.71 for 0.01 wt % and average 1.90 for 0.1 wt %). Therefore, the CNT is a better candidate than Fe nanoparticles for performance enhancement in H2O/LiBr absorption system. 29 In numerical enhanced absorption refrigeration study, Staicovici et al. (2005) modeled water-lithium bromide absorption/generation processes in a Marangoni Convection (applied practical method by the thermal absorption technology in the past decades to significantly improve the absorption process) Cell using the Two-Point theory (TPT) of mass and heat transfer. The model established the capability of (TPT) approach in the Marangoni convection assisted water-lithium bromide absorption process following the successful modeling of the ammonia-water absorption process. It also confirms Marangoni convection basic mechanism explanation in the case of the water-lithium bromide medium. Xohar et al. (2005) investigated the influence of diffusion in the ammonia-water diffusion absorption refrigeration (DAR) cycle configuration on the system performance using a computer simulator known as „EES‟ Engineering Equation Solver. The result reveals that DAR cycle without condensate sub-cooling shows higher COP of 14-20% compared with the DAR cycle with the condensate sub-cooling, 0 but it occurs at higher evaporator temperature of about 15 C. Niu et al. (2006) performed a numerical analysis of falling film ammonia-water absorption in a magnetic field using a computer program known as TDMA due to the tri- diagonal matrix formation of the equations after discretization. It was found that when the magnetic induction intensity at the solution‟s inlet was 3Tesla (T), the increment in concentration of ammonia-water solution at outlet was 1.3% and the absorbability increased by 5.9%. The COP of the absorption refrigeration system increased by 4.7% and the decrement in circulation ratio was 8.3%. This establishes a positive effect on the ammonia-water falling film absorption to some degree. From the above review, it is evident that the finite difference approach has received much attention in the existing literature thereby establishing its popularity and reliability. In addition the review also established some works done in enhanced absorption refrigeration study most especially using additives or nano-fluids / nano-particles in both ammonia-water and water- lithium bromide (H2O-LiBr) solution. However to the best of knowledge of the researcher, only magnetic field enhanced ammonia-water falling film absorption has been worked upon while none on this type of enhancement has been done on either water-lithium bromide (H2O-LiBr) or water-lithium chloride (H2O-LiCl) pairs. This present work therefore uses the finite difference method in an application of the falling film magnetic field enhanced absorption model to the 2-D flow over a vertical flat surface. The two working fluids to be investigated under this magnetic 30 field enhancement are lithium chloride-water (LiCl-H2O) and lithium bromide-water (LiBr-H2O) pairs. 31 CHAPTER THREE METHODOLOGY 3.1 Assumptions In developing the governing equations for this flow modeling the absorption process in a smooth thin-liquid film, the following assumptions are made. i. The flow is a fully developed steady laminar flow as shown in fig. 3.1a hence velocity (v) in Y-direction is zero ii. The fluid properties are constant and not varying with temperature and concentration. iii. The mass rate of vapour absorbed is very small compared to the solution flow rate such that the film thickness and flow velocities can be treated as constant. iv. Heat transfer in the vapor phase is negligible. v. Vapor pressure equilibrium exists between the vapour and liquid at the interface. vi. The Peclet numbers are large enough such that the diffusion in the flow direction can be neglected. vii. Diffusion thermal effects are negligible. viii. The magnetic induction intensity descends linearly along the flow of falling-film. ix. The shear stress at the liquid–vapor interface is negligible The model coordinate system is as shown in Fig. 3.1a Yang and Wood (1992) where U is the velocity in the film in X-direction. 3.2: Governing equations Niu et al. (2006) in his study of magnetic field enhancement effect on absorption process of ammonia-water solution, the first of its kind, utilized model governing equations (1a) through (1e). The study was done on non-smooth thin-liquid falling film. The equations are as follows; the heat transfer energy equation (1a), continuity equation (1b), momentum equation (1c) and quality or mass transfer equation (1e). 32 T T   T  C pu  C  p v   (1a) x y y  y   u v   0 (1b) x y u u   u  u  v     g  mag (1c) x y y  y        u  v  D m (1d) x y y  y   Where  mag (Li, G.D (1999)) in the above equation is the magnetic force which the falling-film solution experienced per unit volume. It is in the direction of downward vertically.  B 2  mag  (1e) where  is the magnetic mass susceptibility of l.0 either Lithium bromide and or Lithium chloride water solution, B is the magnetic induction intensity, l is the length of falling-film flows, and 0 is the vacuum‟s permeability. Odunfa (2008) utilized the following heat transfer, mass transfer, continuity equation and the velocity field equation in smooth thin-liquid falling film (Bird et al (1960) and Yang and Wood (1992)). The study was done on a smooth thin-liquid falling film. The equations are as follows; the energy conservation equation (2a), continuity equation (2b), conservation of linear momentum equation Bird et al (1960) (2c) and quality or mass transfer equation (2d). 33 T T   T  C  pu  C p v    (2a) x y y  y  u v   0 (2b) x y  23 y  y   U  V    0 2   (2c) 2  h0  h   0         u  v  D m (2d) x y y  y   A closer inspection of the utilized set of equations in the two cases mentioned above i.e non smooth falling-film Niu et al. (2006) and smooth thin-liquid falling-film Odunfa (2008) reveals that the equations in both cases are similar, only with the exception of equation (1c) of the first case and equation (2c) in the latter one. The first case non-smooth falling-film but enhanced magnetically, while the second case is smooth falling-film, but not enhanced magnetically. The present work is on a smooth thin-liquid falling-film which is to be enhanced magnetically. Towards achieving this, a magnetic field enhanced velocity field model equation in smooth thin liquid falling-film, using mass transport relationship was developed as shown in appendix A. This developed enhanced velocity model equation (xxi) in the appendix A was used to replace equation (2c), thus the model set of magnetic enhanced velocity, heat and mass transfer equations on a smooth thin-liquid falling-film corresponding to the coordinate system shown in fig. 3.1a will now be: T  2T u  = 0 (3a) x y 2 u   0 (3b) x  2u v B2  3 0  g   0 (3c) y 2 h20 l0   2 u  Dm  0 (3d) x y 2 34 The final developed model magnetic enhanced velocity field, heat and mass transfer equations on a smooth thin-liquid falling-film in a cooling system are:  2u v B2  3 0  g   0 (4a) y 2 h20 l0 T  2T u  = 0 (4b) x y 2   2 u  Dm  0 (4c) x y 2 where  is the magnetic mass susceptibility, B is the magnetic induction intensity, l is the length of falling-film flows, 0 is the vacuum permeability, T is temperature,  is concentration (absorbent),  is thermal diffusivity, Dm is species diffusivity, V0 is the average velocity within the film thickness and h0 is the film thickness y h0 u p v g x Fig. 3.1a: 2-d representation of a thin-liquid falling-film 35 Boundary Conditions At x = 0; u = uin , T = Tin and   equil - (5a) At y = 0; (non permeable wall);  u = 0, T  Tw ,  0 - (5b) y T  At y  h0 ;  K  Dm H a   equil (T, Pv) (5c) y y  u  At the vapour-liquid interface, y  h ;  0  0, Pv  Ts , s   const.  y  y0 Where H a = Heat of absorption, Tw = Wall temperature Pv = Vapour pressure and equil(T , Pv ) = equilibrium concentration at the interface temperature and ambient vapour pressure. 3.3: Finite Difference formulation of the governing equations The general finite difference formulation or approximation of the first derivative of a function F(x, y) with respect to x is given as dF F(x  x, y)  F(x, y)  - (6a) and dx x 2 F(x  x  x, y)  F(x  x, y) F(x  x, y)  F(x, y) d F ( d dF  ( )  x x - (6b) 2 dx dx dx x Similarly the first derivative of a function F(x, y) with respect to y is also given as dF F(x, y  y)  F(x, y)  - (7a) and dy y 36 F(x, y  y  y)  F(x, y  y) F(x, y  y)  F(x, y) 2  d F d dF y y  ( )  - (7b) 2 dy dy dy y i, j+l y i-l,j i,j i+1,j i,j-1 x Fig. 3.1b: Finite difference diagram Using a central finite difference approximation in fig. 3.1b then equation (6b) become 2 d F F(x  x, y)  2F(x, y)  F(x  x, y) (8a) 2 2 dx x F  2F  F i1, j i, j i1, j = (8b) 2 x Similarly equation (7b) become d 2 F F(x,y  y)  2F(x, y)  F(x, y  y)  (9a) dy 2 y 2 F  2F  F i, j1 i, j i, j1 = (9b) 2 y Relating the above derived equation to the heat transfer equation (1) F  F F  2F  F T 2T i1, j i, j i, j1 i, j i, j1 U  = U -  (10) x y2 x 2y If x = y = h 37 T 2T U  = U F   2 i1, j  Fi, j h -  F  2F  F  0 (10b) x y  i, j1 i, j i, j1 For F T i, j i, j T 2T U  = U T   i1, j Ti, j h  Ti, j1  2TI ,J Ti, j1  0 10c x y2 Therefore U T T  i1, j i, j h  Ti, j1  2T T I ,J i, j1  0 - 10d Similarly for the same laminar flow, relating the derived equations (8a – 9b) to the mass transfer equation (3d)   2 Fi1, j  Fi, j Fi, j1  2Fi, j  Fi, j1 u  Dm = u  D2 m (11a) x y x y 2 again if x = y = h   2 u  Dm = uF   i1, j  Fi, j  Dm Fi, j1  2Fi, j  Fi, j1 - (11b) x y 2 Fi, j   i, j   2 u  D = u   m 2 i1, j i, j  Dm i, j1  2i, j i, j1 = 0 - (11c) x y  u   i1, j i, j  Dm i, j1  2i, j i, j1 = 0 - (11d) Equations (12a and 12b) depict the basic general finite difference equations for heat and mass transfer. uT T  i1, j i, j  Ti, j1  2Ti, j T i, j1 = 0 - (12a) ui1, j  i, j  D  m i, j1  2i, j i, j1 = 0 - (12b) 38 3.4: Formulation of the magnetic field enhanced finite difference model of the governing equations The thin liquid falling-film thickness is far too small compared to the entire length of the absorber wall. The film thickness is similar to the boundary layer thickness in heat transfer analysis, therefore the above general finite difference formulation of the governing equations cannot be applied for the internal thin film regime. Therefore the required model finite difference formulation of the governing equations will be as follows: For the model, x is not equaly , therefore  2u v  2 Fi, j1  2Fi, j F0 i, j1 v0  2  3  g   [ ] 3  g   0 (13a) y 2 h2 l0 y 2 h2 l0 0 0 For Fi, j  ui, j 2 2 u v  2 3v y  2  3 0  g   (u 0 2 2 2 2 i, j1  2ui, j  ui, j1)   gy  y  0 y h l h20 l0 0 0 3v y 2  2 Therefore, (u 0 2 2i, j1  2ui, j  ui, j1)   gy  y  0 (13b) h2 l0 0 T  2T  2u  = u[y ] F  F   [x ]F  2F  F   0 x y 2  i1, j i, j  i, j1 i, j i, j1 (13c) For F T i, j i, j T  2T 2 u  = u [y ] T   i1, j Ti, j  Ti, j1  2Ti, j Ti, j1 x  0 (13d) x y 2 2 Therefore u [y ] T T i1, j i, j Ti, j1  2Ti, j T i, j1 x  0 - (14a)   2 u  Dm = uy 2 Fi1, j  Fi, j  Dm xFi, j1  2Fi, j  Fi, j1 (14b) x y 2 Fi, j   i, j 39   2 2 u  Dm = uy    2 i1, j i, j  Dm i, j1  2i, j i, j1 x = 0 (15a) x y 2 Therefore uy i1, j    i, j  Dm i, j1  2i, j i, j1 x = 0 - (15b) Equations (12b) and (13b); the finite difference model equations satisfied at each node in the interior face of the domain R for heat and mass transfer. 3v y 2  2 (ui, j1  2ui, j  ui, j1)  0  gy 2  y 2  0 - (16a) h2 l0 0 uy2 T   i1, j Ti, j  Ti, j1  2Ti, j Ti, j1 x  0 - (16b) uy2    i1, j i, j  Dm i, j1  2i, j  i, j1 x = 0 - (16c) 3.5: Boundary conditions At the boundary, usually the parameters such as temperature and concentration are known (Dirichlet conditions) or the boundary is considered to be perfectly insulated (Newmann or Adiabatic conditions). Insulated boundaries are handled by developing boundary element/nodal equations. In this model, Newmann or Adiabatic boundary condition was used along the absorber wall. Figure 3.2 shows a half element in the model lying on an insulated left boundary of the smooth film thickness portion of the absorber. The net heat flowing into this element must be equal to zero when considering steady-state condition in the film. In the boundary shown in Fig 3.2, the quantity of heat flowing into the three faces of the half element in time dt is given by the following equations: 40 x dq ydq i+1 3 1 dq dq 1 2 i dq 2 dq i-1 3 x y Fig. 3.2 Temperature boundary conditions analysis From fig. 3.2, the heat balance equation for the half element is dq1 + dq2 +dq3 = 0 (17) The complete boundary difference equations (xxiii to xxvi) in this study appearing in Appendix A was applied, thus (Left) T i1, j + 2T i, j1 + Ti-1,j - 4T i , j = 0 (18a) (Right) T i1, j + 2T i, j1+ T i1, j - 4T i , j = 0 (18b) (Upper) T i, j1 + 2T i 1, j + T i, j1 - 4T i , j = 0 (18c) (Lower) T i, j1 + 2T i1, j + T i, j1 - 4T i , j = 0 (18d) Thus, in obtaining a solution for the steady state heat flow in the smooth film absorption medium, Eq. (17) must hold at every interior grid point and Eqs. (18a) through (18d) must be 41 satisfied at any appropriate known or insulated boundaries. Gaussian elimination methods or Gauss-Seidel iteration method could be employed to solve the equations accurately for the number of grid-points required. For the steady state mass flow in the smooth film-absorption medium shown in figure 3.3, similar approach was employed to develop boundaries equations for the unknown or insulated boundaries as shown in the figure. x dm ydm 3 1 i+1 dm dm1 2 i dm 2 dm3 i-1 x y Fig. 3.3 Concentration boundary conditions analysis The net mass flowing into this element must be equal to zero when considering steady-state condition in the liquid-film. The quantity of mass flowing into the three faces of the half-element in time dt are given by the following equations. From fig. 3.3 the mass balance equation for the half-element is dm1 + dm2 + dm3 = 0 (19) 42 The complete boundary difference equations (xxviii to xxxi) in this study appearing in Appendix A was applied, thus (Left) i1, j  2i, j1 i1, j  4i, j  0 (20a) (Right) i1, j  2i, j1 i1, j  4i, j  0 (20b) (Upper) i, j1  2i1, j i, j1  4i, j  0 (20c) (Lower) i, j1  2i1, j i, j1  4i, j  0 (20d) Thus, in obtaining a solution for the steady state mass flow in the smooth film absorption medium, Equation (19) must hold at every interior grid point and Eqs. (20a) through (20d) must be satisfied at any appropriate known or insulated boundaries. Equations (17) and (19) are for the interior grid points in the smooth film absorption medium while equations (18a) through (18d) and equations (20a) through (20d) are for the boundary conditions in heat and mass transfer respectively. The development of these boundary equations took due cognisance of the inlet conditions to the absorber, absorber wall and the given condition in the thin-liquid falling-film/gas or vapour interface. The two immediate adjacent nodes to each of the corner nodes of the medium are taken into consideration in the program to determine the unknowns at the corner nodes. 3.6: Solution Method As earlier mentioned, there are many solution techniques available that are being used for solving the global finite difference matrix equations 12b and 13b coupled with the boundary equations 15 and 20 generated. Two of the most widely used methods are Gauss-Seidel iteration and Gaussian elimination method as modified by Paynes and Iron and reported by Okon, (1990).The modified Gaussian elimination method employed is given in Appendix D. 3.7: Computer Programming The finite difference method is used to solve the governing equations (1) & (2) as given in the form of finite difference equations (16a) & (16b). The solution technique used is Gaussian 43 elimination scheme as modified by Paynes and Iron and reported by Okon, (1990) on the digital computer. The computer program and the subroutines are written in FORTRAN 90 language. Main Program The program shown in Appendix A solves equations (16a) & (16b) using modified Gaussian elimination scheme. The flow chart is shown in Fig. 3.4a. This main program utilizes two (2) different subroutines. These subroutines are written to execute various steps involved in applying the finite difference scheme. The problem data are introduced into the program in the “data block”, where the input parameters can be easily modified to suit any case study. The main program, after generating the global matrix, calls subroutine “solution1”, before calling subroutine “solution2”. After calling a subroutine solution the results were finally “displayed. Subroutines These are sub-programs written to execute various steps involved in applying the finite difference method using Gaussian elimination scheme. They are called by the main program. Below are the subroutines employed to execute various steps in the main program; Solution1 & Solution 2 These are the core sub-routines, they also perform similar functions. These subroutines perform their functions after the implementation of the boundary conditions in the global domain. Solution1 generates the temperature profile of the domain, while solution2 takes care of concentration profile within the domain. DATA There is a well-known cliché in the computer world-GIGO an acronym to mean “Garbage in Garbage out” i.e. the quality of the output is no better than the quality of the input. In order to obtain high quality output of temperature and concentration profiles in the domain, the input data utilized was obtained from the literature as was used for the available experimental and numerical modelings. The data utilized from the literature are as shown in Table 3.1(General data), Table 3.2 (LiBr – H2O) and Table 3.3 (LiCl – H2O) respectively. 44 Fig. 3.4a: Main Program Flow Chart START READ DIMENSIONS AND MATERIALS PROPERTIES GAUSS-SIEDEL SCHEME TO SOLVE THE GENERATED GLOBAL MATRIX. CALL SOLUTIONS X STOP & END 45 Fig. 3.4b: Subroutine Solution Flow Chart No Is X N< NMAX Yes ? Assign initial temp . Print out iteration number &c onc . and number of points at Yes distribution which accuracy check fails Is K=0 Call Subroutine ? Velocity Y Print out tem.p & conc. in the film thickness No N=0 RETURN Print out iteration number and number of points at K=0 which accuracy check fails No DO nJ=1, 5 DO nI=2,12 No Is J= 5 ? 2 2 Is No T  T c  c J=1 Yes u   & u D .m ? x y 2 x 2y Yes No Is I = IB Yes ? Is I = IA T Dm HYes c c k T  a ,   ? y k y y Dm Ha y UTEMP=UT No Left UTEMP=UT UCONC=UC UCONC=UC Right c T Tw ,  0 .y DIFF=UTEMP-U(I,J) DIFF=UCONC-C(I,J) Is | DIFF|