The Effects Of Adsorbent Particle Diameter On Combined Heat And Mass Transfer Performance Of A Silica Gel-Water Adsorption Refrigeration System

Because of the scarcity of natural source and the deterioration of ecological environment,the solid adsorption refrigeration has been widely studied by global scholars for its energy-saving and environmental protection.The adsorption refrigeration system has a lot of features such as simple structure,free of noise,no crystallization and corrosion,besides,it can be driven by lower grades energy and green environmental protection.But the special constructure of adsorbent porous media causes the discontinuity of the heat and mass transfer performance in the adsorption bed,thus the system capability becomes weaker,hindering the generlization and application of such a potencial refrigeration.The adsorbent particle diameter is an influential parameter in the adsorptive performance,heat and mass transfer and system capability of an adsorption chiller.Although it has been clarified that the adsorbent particle diameter affects the system capacity of an adsorption cooling system,most of them focus on the integral properties of the system instead of the heat and mass transfer mechanism.What is worse,experts tend to research the influence of the adsorbent grain size itself separated with some other important factors and lose sight of the combined effect.Therefore,the paper investigated the effects of particle diameter concerned with porosity and heating temperature on the heat and mass transfer as well as the system performance for different physical properties and operating conditions.At the beginning,the paper analyses the cycle characteristics of solid adsorption cooling system in theory,and a transient two-dimensional numerical model of the anular adsorbent bed as well as evaporator and condenser models are developed and solved by numerical simulation with the help of Fluent software,which is verified by the exprimental data refering to other literature.Then,the heat and mass transfer performance has been discussed from the aspects of the integral coefficient of heat transfer,thermal contact resistance and the resistance of the mass transfer.The results show that:at the begining of the desorption process,the average temperature of the adsorbent bed increased rapidly and then turns slowly until the temperature comes close to the balanced temperature at the end of the desorption precess;for smaller particle diameter,different particle diameter has a significant influence on the heat and transfer performance of the adsorbent bed,and the smaller the particle diameter is,the better the performance will be;with the increase of the particle diameter,the SCP of the cooling system raised in the earlier and then declined,but the SCP remains constantly.Furthermore,the paper has studied the combined effects of the particle diameter and the total porosity in the heat and mass transfer and system performance and summarised as follows:the particle diameter has a stronger influence on the system performance for larger total porosity,besides,the smaller particle diameter behaves better;while the particle diameter is smaller,a larger porosity could enhance the system performance,but the conclusion turns oppsitely under in the case of a larger particle diameter;it would be wise to chose a samller particle diameter matching with a larger porosity in order to intensify the heat and mass transfer effect.In addition,the effect of particle size on the system performance for different heating temperature of thermal fluid has been investigated for optimum performance of the adsorption system.It is found that:the heating temperature of thermal fluid shows weaker influernce on the system performance under the circumstances of larger particle size;with the higher heating temperature of thermal fluid,the effects of particle diameter becomes more and more obviously;the optimum particle diameter,which is corressponded to the maximum SCP,is slightly shifted to larger values fot higher heating temperature.