Experimental Investigation And Theoretical Calculation Of Carbon Dioxide Throttle Refrigeration In Microchannels

Microchannel technology is widely used in the field of electronic equipment,automotive air conditioning,aerospace and biomedical engineering,etc.Micro Joule-Thomson(J-T)cryocooler,as a miniature refrigerator using the Joule-Thomson effect of actual gas,usually includes a recuperative heat exchanger,throttling device,and evaporation chamber.A new type of microchannel J-T cryocooler fabricated by microchannel technology is composed of multilayer microchannels.The utility model has the advantages of small size,compact structure and good cooling effect.In this paper,characteristics of fluid flow and heat transfer in rectangular microchannels are analyzed theoretically and experimentally by using carbon dioxide gas as the working fluid.1)Based on the theory of gas molecule,it is convenient to differentiate the flow regimes in function of Knudsen number(Kn),with the classification as continuum flow,slip flow,transition flow and free molecular flow.Although tinged with empiricism,it is usually accepted.The rarefaction effects of carbon dioxide gas in a rectangular microchannel is calculated.The results show that the flow is in the continuous flow region.2)According to the empirical correlations of friction factor in the microchannel,the theoretical calculation of pressure loss of carbon dioxide gas in microchannels is carried out,including major losses and minor losses.The results show that the pressure loss in the microchannel mainly exists in the high and low pressure heat transfer sections and throttle section,while the minor losses are less.Compared with the conventional channel,the pressure loss of fluid flow in the microchannel are obviously larger,which reflects the flow characteristics of gas in microchannels.3)Based on the integral Joule–Thomson effect of carbon dioxide,the temperature differences before and after throttling is calculated under different working conditions.The results show that the temperature drop in the heat exchanger is mainly in the throttle section.4)The experimental system in this paper is an open refrigeration system.Two microchannel heat exchangers are studied under different working conditions.The lowest cooling temperature is about-48.4?,which can meet the requirement of refrigeration to some extent.The experimental results show that under the same temperature,the higher the inlet pressure,the lower the refrigeration temperature;under the same pressure,the cooling temperature decrease when reduce the inlet temperature.As a result of the increase of the number and width of the microchannles for the second type cooler,the flow rate of gas is increased,and the cooling effect is achieved at lower pressure.In this paper,the research on the characteristics of fluid flow and heat transfer in microchannel J-T cryocooler has made some achievements.The main theoretical and experimental results can provide a reference for the design of microchannel J-T cryocooler.