Two-phase flow of refrigerants in 85 μm-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 12.7 mm silicon evaporator composed of 135 microchannels with 85 μm wide and 560 μm high channels separated by 46 μm wide fins. There were 35 local heaters and temperature measurements arranged in a 5 x 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 oC, inferring that the boiling process was very stable. For wall heat fluxes over 45 kW/m2, the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness.