Carbon dioxide evaporation in a single microchannel

Summary and Conclusion An experimental study of CO 2 evaporation inside a 0.8 mm-hydraulic diameter microchannel is presented in this work. The average heat transfer coefficient along the microchannel was measured and visualization of the flow regimes was conducted. A total of 67 tests were performed at saturation temperatures around 23.3 °C for one heat flux of 1800 W/(m 2 °C). Vapor qualities ranged from 0.005 to 0.88 and mass flux ranged from 58 to 235 kg/(m 2 s). An average heat transfer coefficient of 9700 W/(m 2 °C) with a standard deviation of 35% was obtained. The high data scattering did not permit identify a clear dependency of the heat transfer coefficient with the mass flux, as well as with the vapor quality, which seems to characterize nucleate boiling regime for the test conditions. The correlation for nucleate boiling proposed by Gorenflo (1993), with h o =4170 W/(m 2 °C), resulted in h=8320 W/(m 2 °C), and it was the best comparison to the experimental average result. The dryout of the flow, characterized by the sudden reduction in the heat transfer coefficient, was identified at vapor qualities around 0.85. Flow visualization results showed three flow regimes. For low vapor qualities (up to about 0.25), plug flow was predominant, while slug flow occurred at moderated vapor qualities (from about 0.25 to 0.50). Annular flow was the flow pattern for high vapor qualities (above 0.50).