High heat flux flow boiling in silicon multi-microchannels - Part III: Saturated critical heat flux of R236fa and two-phase pressure drops

Abstract

New experimental critical heat flux results for saturated boiling conditions have been obtained for R236fa flowing in a silicon multi-microchannel heat sink composed of 67 parallel channels, 223 μm wide, 680 μm high and with 80 μm thick fins separating the channels. The microchannel length was 20 mm. The footprint critical heat fluxes measured varied from 112 to 250 W/cm2 and the wall critical heat fluxes from 21.9 to 52.2 W/cm2 for mass velocities from 276 to 992 kg/m2s. When increasing the mass velocity, the wall critical heat flux was observed to increase. The inlet saturation temperatures (20.31 ≤ Tsat,in ≤ 34.27 °C) and the inlet subcoolings (0.4 ≤ Δ Tsub ≤ 15.3 K) were found to have a negligible influence on the saturated CHF. The best methods for predicting the data were those of Wojtan et al. [L. Wojtan, R. Revellin, J. R. Thome, Investigation of critical heat flux in single, uniformly heated microchannels, Exp. Therm. Fluid Sci. 30 (2006) 765-774] and Revellin and Thome [R. Revellin, J. R. Thome, A theoretical model for the prediction of the critical heat flux in heated microchannels, Int. J. Heat Mass Transfer 50 (in press)]. They both predict the experimental CHF results with a mean absolute error of around 9%. Using the critical vapour quality, an annular-to-dryout transition is also proposed as a limit in a diabatic microscale flow pattern map. Pressure drop measurements were measured and analysed, showing that the homogeneous model could correctly predict the observed trends. © 2008 Elsevier Ltd. All rights reserved.