| With the implementation and advancement of the policy of emission peak and carbon neutrality in China,improving energy efficiency and reducing emission are attractive issues at present.There is a large amount of small-scale waste heat from industrial production and our daily life,which is released into the environment,such as exhaust heat from vehicles and vessels,waste heat of small-scale industrial processes,and so on.If there is a demand for cooling capacity,ammonia-based absorption refrigeration technology can be used to recover the small-scale waste heat to provide cooling capacities and improve energy efficiency.Theoretical cycle analysis and experimental performance assessment on exhaust-heat-driven small-scale absorption refrigeration systems using ammonia-based working fluids are studied in this thesis.1)A theoretical mathematical model of the exhaust-heat-driven absorption refrigeration system is built.The working fluids selected in the theoretical cycle analysis are NH3/LiNO3,NH3/Na SCN and NH3/LiNO3+H2O.The comprehensive theoretical analysis of the absorption refrigeration system is conducted and evaluated.The influence of external operating conditions and heat and mass transfer performance of the components on the theoretical cycle performance are discussed.A comparison is made on the results of the theoretical analysis of three working fluids.The theoretical cycle analysis provides a reference for the experimental research.2)A prototype of exhaust-heat-driven absorption refrigeration system is designed and built.A gas-liquid finned-tube generator is designed and used to replace the common generator of pool boiling.The start-up steps in the experiments,the real-time dynamic changes of operating parameters and the test of steady-state operations are presented and explained.It is proved that the experimental results of the prototype of exhaust-heat-driven absorption refrigeration system have high accuracy and credibility.3)A comprehensive experimental assessment is conducted on the prototype of exhaust-heat-driven absorption refrigeration system under varying operating conditions,operating with binary working fluids（NH3/LiNO3 and NH3/Na SCN）.And a comparison is made on the experimental performance of two kinds of ammonia/salt systems.The experimental results show that,the performance of NH3/Na SCN system is significantly better than that of NH3/LiNO3 system under most operaion conditions,but the NH3/LiNO3 system can reach lower evaporating temperatures.There exist large deviations between the experimental results and the original theoretical analysis results.After the modification and improvement of the theoretical cycle analysis according to the the experimental results,the deviations of COP before and after modification decreased from 16.9%and 4.6%to 4.0%and 3.8%,respectively;the deviations of mass fraction ranges before and after modification decreased from 8.9%and 30.2%to 3.8%and 3.0%,respectively.4)An experimental assessment is carried out on the prototype operating with ternary working fluids,NH3/LiNO3+H2O.Four kinds of NH3/LiNO3+H2O solutions with different water component proportions are selected as working fluids which are No.1（5.0%）<No.2（9.7%）<No.3（14.4%）<No.4（19.3%）.The increasing water component proportions of the working fluids can significantly improve the performance.The performance of the system is significantly improved by using the working fluids with larger mass fraction of water component proportions.Compared with NH3/LiNO3 system,the mass fraction ranges of No.3 and No.4 system is improved by 24.8%and 26.4%,respectively.By summarizing the experimental performance results of binary and ternary working fluids,the results show that,NH3/Na SCN system is more suitable for the operating conditions with high evaporating temperatures and normal generating temperatures;NH3/LiNO3 system is more suitable for the operating conditions with low evaporating temperatures,but its performance is not good under most operating conditions;NH3/LiNO3+H2O system with large water component proportions has better performance and lower driven temperatures,which can operate normally under a wide operating temperature range and complex operating conditions.5)The performance of the heat-transfer devices in the exhaust-heat-driven absorption refrigeration system is analyzed and calculated.The flow boiling heat transfer mechanisms in the generator tubes are estimated and analyzed,and the flow boiling heat transfer coefficients of different working fluids in the generator tubes under varying conditions are calculated.The overall flow boiling process in the generator tubes is influenced by both nucleate boiling mechanism and convective boiling mechanism,in which the convective boiling mechanism is dominant;the mass-transfer resistance also makes great difference on the flow boiling process in the generator tubes.The heat-transfer performance of NH3/Na SCN and NH3/LiNO3+H2O is better than that of NH3/LiNO3.And the heat-transfer performance of different NH3/LiNO3+H2O working fluids improves with the increase of water component proportions.The reason for the different heat-transfer performance is that,different working fluids have different viscosity and thermal conductivity.The research of this thesis provides an important theoretical and experimental reference for the selection and application of binary NH3/salt working fluids and ternary NH3/LiNO3+H2O working fluids in absorption refrigeration system.The thesis also provides a feasible scheme for the application of absorption refrigeration system in the field of recovery of exhaust waste heat.And it is a valuable reference for the miniaturization of absorption refrigeration system and the optimization,development and application of absorption refrigeration technology. |
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