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Water Absorption BINARY FLUID HEAT AND MASS EXCHANGE AT THE MICROSCALES IN INTERNAL AND EXTERNAL AMMONIA- WATER ABSORPTION A Dissertation Presented to The Academic Faculty by Ananda Krishna Nagavarapu In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in Mechanical Engineering Georgia Institute of Technology December 2012 BINARY FLUID HEAT AND MASS EXCHANGE AT THE MICROSCALES IN INTERNAL AND EXTERNAL AMMONIA- WATER ABSORPTION Approved by: Dr. Srinivas Garimella , Adviser Dr. William Koros George W. Woodruff School of School of Chemical and Biomolecular Mechanical Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Sheldon Jeter Dr. Thomas Fuller George W. Woodruff School of School of Chemical and Biomolecular Mechanical Engineering Engineering Georgia Institute of Technology Georgia Institute of Technology Dr. Samuel Graham George W. Woodruff School of Mechanical Engineering Georgia Institute of Technology Date Approved: August 7, 2012 To my family for their unconditional love and unwavering support ACKNOWLEDGEMENTS I would like to express my deepest gratitude to my adviser and mentor, Dr. Srinivas Garimella, for his guidance, support, and encouragement throughout my graduate education. Without the many long hours he devoted over several years, this work would not have been possible. I have always deeply valued his advice and hope this to be the beginning of a long association. I would also like to thank all the current and past members of the Sustainable Thermal Systems Laboratory for their friendship and support. I would specially like to thank Christopher Keinath, Jared Delahanty, and Drs. Matthew Determan, Sangsoo Lee, and Lalit Bohra for their help and guidance in the development and operation of the various test facilities. Discussions and collaborations with them have often resulted in valuable insights that have helped address the various challenges faced throughout this work. I would also like to thank Michael Garrabrant and Stone Mountain Technologies Incorporated for their valuable inputs during the course of this study. Finally, I would like to thank my Ph.D. committee members, Drs. Sheldon Jeter, Samuel Graham, William Koros, and Thomas Fuller for their valuable suggestions and patient perusal of my dissertation. iv TABLE OF CONTENTS Page ACKNOWLEDGEMENTS iv LIST OF TABLES xii LIST OF FIGURES xv NOMENCLATURE xxi SUMMARY xxviii CHAPTER 1. Introduction 1 1.1. Absorption heat pump 2 1.2. Absorber 4 1.3. Research issues 6 1.4. Scope of present research 8 1.5. Dissertation organization 9 2. Literature review 12 2.1. Falling-film mode absorption 12 2.1.1. Analytical and numerical studies 12 2.1.2. Experimental studies 27 2.1.3. Miniaturization studies 40 2.1.4. Summary of falling-film absorption studies 47 2.2. Bubble and convective mode absorption 49 2.2.1. Analytical and numerical studies 50 2.2.2. Experimental studies 57 v 2.2.3. Miniaturization studies 69 2.2.4. Summary of bubble and convective mode absorption 74 2.3. Comparative studies 76 2.4. Research needs 80 2.5. Research objectives 83 3. Experiments: falling-film absorption 108 3.1. Test facility 109 3.2. Microchannel falling-film absorber 115 3.2.1. Absorber outer shell 115 3.2.2. Microchannel tube array 116 3.2.3. Absorber assembly and coupling fluid 117 3.3. Instrumentation and data acquisition system 120 3.3.1. Instrumentation 120 3.3.2. Data acquisition system 121 3.4. Test facility operational features 124 3.5. Experimental procedures 124 3.5.1. Leak testing and charging 124 3.5.2. Safety precautions 126 3.5.3. Experimental procedures 127 3.6. Test matrix 131 4. Falling-film absorption heat and mass transfer analysis 134 4.1. System level data analysis 135 4.1.1. Concentration range 139 vi 4.2. Absorber analysis 140 4.2.1. Absorber heat duty calculations 141 4.2.2. Absorber segmental analysis 143 4.3. Results and discussion 166 4.3.1. Overall absorber component 166 4.3.2. Solution heat transfer coefficient 170 4.4. Summary 180 5. Modeling of local heat and mass transfer rates 181 5.1. Droplet formation, growth, and break up 184 5.1.1. Primary droplet 186 5.1.2. Droplet spacing and frequency 189 5.1.3. Droplet surface area 190 5.2. Film formation and spreading 193 5.2.1. Feed region 195 5.2.2. Spread region 197 5.2.3. Average transfer area 199 5.3. Hydrodynamic model results and discussion 203 5.3.1. Droplet characteristics 203 5.3.2. Film characteristics 205 5.4. Heat and mass transfer modeling 210 5.4.1. Solution pool 210 5.4.2. Droplet absorption 216 5.4.3. Film absorption 221 vii 5.4.4. Overall heat and mass transfer 224 5.5. Definition of dimensionless numbers 231 5.6. Heat and mass transfer results and discussion 232 5.6.1. Solution pool 232 5.6.2. Droplet 233 5.6.3. Film 236 5.6.4. Heat transfer coefficient and Nusselt number 239 5.7. Nusselt number correlation development 245 5.7.1. Parametric evaluation of film Nusselt number correlation 247 5.7.2. Comparison of film Nusselt number correlation with literature 251 5.8. Summary 261 6. Microscale forced-convective absorption 264 6.1. Microscale forced-convective absorption concept 264 6.2. Segmental heat and mass transfer model 269 6.2.1. Solution and vapor, bulk and interface conditions 272 6.2.2. Vapor mass transfer 272 6.2.3. Overall heat and mass transfer rates 274 6.3. Absorber test sections 278 6.3.1. Design conditions 278 6.3.2. Design constraints 279 6.3.3. Fabrication techniques 281 6.3.4. Representative absorber test sections 284 6.4. Segmental heat and mass transfer model predictions 289 viii 6.4.1. Overall results 289 6.4.2. Segmental results 291 6.5. Summary 301 7. Demonstration of absorption cooling using microscale geometries 302 7.1. Microchannel system component 302 7.1.1. 2.5 kW cooling capacity chiller 303 7.1.2. 2 kW cooling capacity chiller 309 7.2. Breadboard test facility 320 7.2.1. Coupling fluid loops 320 7.2.2. Solution loop 323 7.2.3. Instrumentation and data acquisition 325 7.3. Experimental procedures 330 7.3.1. Component and system leak testing, and system charging 330 7.3.2. Safety precautions 331 7.3.3. Experimental procedures 331 7.4. Absorber testing 332 7.4.1. System-level analysis 332 7.4.2. Microscale absorber analysis 335 7.5. Discussion of convective absorber performance 348 7.6. Summary 353 8. Comparative assessment of falling-film and forced-convective absorption 354 8.1. Design conditions and constraints 355 8.1.1. Absorber operating conditions 355 ix 8.1.2. Design constraints 355 8.2. Absorber designs and variants 357 8.2.1. Microchannel falling-film absorbers 358 8.2.2. Microscale forced-convective absorbers 362 8.3. Comparative assessment 368 8.3.1. Heat and mass transfer characteristics 368 8.3.2. Fabrication and packaging aspects 374 8.4. Summary 378 9. Conclusions and recommendations 380 9.1. Conclusion 380 9.2. Recommendations 386 9.2.1. Local level measurements 386 9.2.2. Flow visualization studies 388 9.2.3. Computational treatments 390 9.2.4. System level studies 390 9.2.5. Microscale component fabrication 391 APPENDIX A. Ammonia-water mixture properties 392 A.1. Thermodynamic properties 392 A.2. Transport properties 394 A.2.1. Vapor phase 394 A.2.2. Liquid phase 396 B. Uncertainty calculations 399 x C. Droplet and film transfer areas 410 C.1. Droplet instantaneous and average surface area 410 C.1.1. Hemispherical section 410 C.1.2. Conical section 412 C.2. Film instantaneous and average surface area 414 C.2.1. Triangular region 414 C.2.2. Trapezoidal region 415 D. Segmental heat and mass transfer model sample calculations for microscale forced- convective absorption 417 E. Pressure drop estimation in microscale forced-convective absorbers 430 E.1. Pressure drop model 430 E.2. Estimation of channel blockage 433 REFERENCES 436 xi LIST OF TABLES Page Table 2.1: Summary of studies on falling-film absorption 85 Table 2.2: Summary of studies on bubble and convective mode absorption 99 Table 2.3: Summary of comparative studies 107 Table 3.1: Summary of heat exchangers in the falling-film absorption test facility 114 Table 3.2: Microchannel falling-film absorber geometry 118 Table 3.3: Summary of instrumentation in the falling-film absorption test facility 122 Table 3.4: Falling-film absorption test matrix 133 Table 4.1: Summary of range of concentrations investigated 140 Table 4.2: Summary of energy balances on the absorber, condenser, and evaporator 143 Table 4.3: Summary of calculated uncertainties 166 Table 4.4: Summary of relevant heat transfer studies 177 Table 5.1: Summary of droplet spacing and number of droplets 203 Table 5.2: Summary of droplet formation times 204 Table 5.3: Summary of droplet volume and surface area characteristics 205 Table 5.4: Summary of film thickness characteristics 208 Table 5.5: Summary of film transfer area characteristics 209 Table 5.6: Summary of droplet and film mass transfer characteristics 224 Table 5.7: Summary of solution pool heat and mass transfer 233 Table 5.8: Summary of droplet heat transfer 234 Table 5.9: Summary of droplet mass transfer 235 Table 5.10: Summary of film mass transfer 236 xii Table 5.11: Summary of absorption in each region 237 Table 5.12: Variation of film characteristics with solution concentration 249 Table 5.13: Summary of accuracy of prediction of Bohra et al.
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