Two-phase flow of refrigerants in 85μm-wide multi-microchannels: Part II - Heat transfer with 35 local heaters

This article is the second part of a study on flow boiling of R236fa and R245fa. This part presents the heat transfer coefficients obtained in a 12.7mm silicon evaporator composed of 135 microchannels with 85μm wide and 560μm high channels separated by 46μm wide fins. There were 35 local heaters and temperature measurements arranged in a 5×7 array. The heat transfer results were uniform in the lateral direction to the flow (attributable to the inlet restriction) and a function of the heat flux, vapor quality and mass flux. The steady-state standard deviation of the local base temperature was less than 0.2°C, inferring that the boiling process was very stable. For wall heat fluxes over 45kW/m2, the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of Thome et al. (2004) was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness. © 2011 Elsevier Inc.