| Acting the role of fatal heat and mass transfer device in a absorption refrigeration system,absorber causes great effects on system performance due to its structure and working principle.Falling film absorber is a certain type of non-adiabatic absorber which proceeds absorption process through the direct contact between strong solution surface and refrigerant vapor.The falling film absorber gains the superiority of large mass transfer area,compact structure,relatively excellent heat transfer performance and continuous refrigerant flow.It is commonly believed that reducing thickness of falling film is an efficient way to enhance absorption performance of falling film absorbers and increase system efficiency furthermore.In the present study,a novel micro-scale falling film absorber is proposed which takes the advantage of pressure difference in absorption refrigeration system with ammonia-salt working pair.In the absorber,micro-scale falling film is ejected along with the inner wall of vertical tubes by pressure-drop and velocity-raise process.For the sake of comprehensive and profound investigation of micro-scale falling film absorption enhancement mechanism,theoretical as well as experimental study have been proceeded.That research work includes:(1)CFD simulation method combined with numerical modeling has been applied to analyze flow field characteristics of the micro-scale falling film produced by micro-channel of ammonia-lithium nitrate and ammonia sodium thiocyanate working pairs.Factors which cause effects on micro-scale liquid film absorption process such as micro-channel sizes have been compared and analyzed.Through these numerical results,smaller micro-channel sizes,as well as higher falling film velocities are able to enhance mass and heat diffusion inside falling film.The more homogeneous gradient of concentration and temperature distributions are capable of strengthening heat and mass transfer during the absorption process.Theoretical results reveal the enhancements of ammonia-salt micro-scale liquid film,providing theory support for the following experimental research in the meanwhile.(2)Complete absorption refrigeration experimental table has been established for the sake of experimental study of absorption characteristics in traditional absorber with stainless steel structured packing for ammonia-salt working pair.Relationships between various working conditions and absorption features in the absorber with stainless steel structured packing have been fully studied as well as comparison between ammonia-lithium nitrate and ammonia-sodium thiocyanate working pair.Dimensionless correlations of Sherwood number for the mass transfer process of the two ammonia-salt working pairs have also been induced.(3)Experimental study of absorption characteristics in the novel micor-scale falling film absorber for ammonia-salt working pair have been proceeded.Relations between absorption characteristics and working conditions have been obtained,as well as Dimensionless correlations of Sherwood number for the mass transfer process.Comparison has been made to analyze the difference of absorption features between absorber with stainless steel structured packing and micro-scale falling film absorber for the two kinds of ammonia-salt working pairs.The results show that micro-scale falling film absorber gains the absorption rate 10 times than that of absorber with structured packing.Compared with traditional absorber with structured packing,micro-scale falling film absorber are capable of improving absorption ratio and absorption perfection.The micro-scale falling film can also improve system COP under lower driving temperatures,which contributes to the availability of low-grade energy sources.Strong absorption intensity can be formed through small sized micro-channel.Evaporating temperature at-27.7 ℃ under a relatively high system COP can be otainted through small sized micro-channel.The experimental and theoretical results are in agreement which indeed proves the enhancement caused by micro-scale falling film.In this study,ammonia-salt working pairs not only own a higher absorption rate but also can create evaporating temperatures below zero at COP greater than 0.4.Moreover,the novel micro-scale falling film absorber gains a compact structure and is easy to be manufactured.With extensions of a number of falling film tubes,mass flow rates can be adjusted to meet various needs.It has good augment ability and contributes to miniaturizations of absorption refrigeration systems. |
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