Experimental And Numerical Investigation On R32Flow Boiling Characteristics In Micro-channels

Considering the balance of environmental characteristics, safety, theoretic cycleperformance and market availability, R32would be one of the promising refrigerantsalternatived to replace R22. With decreasing of channel dimension, the flow boiling heattransfer characteristics of R32in micro-channels would be significantly different fromthe conventional channels. As a result, the research on R32flow boiling characteristicsin micro-channels would have the important theoretical significance and practical value.The main work of this paper is as follows:Experiment: R32flow boiling characteristics in a smooth micro-tube with innerdiameter of2mm and a multiport extruded tube with micro-fin and inner diameter of1.7mm were investigated. The experimental conditions were100~400kg/(m2·s) massflux,10~40kW/m2heat flux,10~20oC saturation temperature,0.05~0.98quality.Results showed that the flow boiling characteristics in smooth the two micro-channelswere similar: Heat transfer coefficient increases with increasing mass flux, heat flux andsaturation temperature.The critical steam quality decreases with increasing mass flux,heat flux and saturation temperature. The pressure drop increases with increasing massflux, heat flux, quality but decreases with increasing saturation temperature. The heattransfer coefficient and pressure drop in multiport extrude with micro-fin are40.51%and48.3%larger than in smooth micro-tube respectively on average.Correlation: Confinement number Co, which reflects the effect of channeldimension on flow boiling, was introduced into the heat transfer coefficient andpressure drop correlation. The new heat transfer coefficient correlations for smoothmicro-tube before and after critical quality were developed. The new pressure dropcorrelation was developed based on Lockhart-Martinelli pressure drop correlation. Thenew heat transfer coefficient could correlate93%the existing experimental data and89.5%the present experimental data were within±20%errors. The new pressure dropcould correlate85%the existing experimental data and all the present experimental datawere within±20%errors.Simulation: A three-dimensional steady simulation, using Mixture multiphase flowmodel and Hertz–Knudsen phase change model by FLUENT, was conducted toinvestigate the flow boiling heat transfer of R32in a2mm smooth micro-tube. The results showed that the effect of gravity on two phase distribution weakens withincreasing mass flux, and the nucleate boiling near the wall strengthens with increasingheat flux. The average deviations of heat transfer coefficient and pressure drop betweennumerical and experimental results are11.3%and–1.1%, respectively.