An analysis of the effect of the footprint orientation on the thermal-hydraulic performance of a microchannels heat sink during flow boiling of R245fa

The present study concerns an experimental investigation of flow boiling of R245fa in a 50 parallel rectangular 123.3 × 494.2 μm2 microchannels heat sink. Heat transfer coefficient and pressure drop results were obtained for footprint heat fluxes up to 300 kW/m2, mass velocities from 300 to 1000 kg/m2 s, liquid subcoolings at the test section inlet of 5 and 10 °C and saturation temperature of 30 °C. The heat sink performance was evaluated for its footprint area horizontally positioned (HP), its footprint area vertically aligned with the microchannels horizontally positioned (VHP) and its footprint area vertically positioned with upflow through the microchannels (VP). Average footprint heat transfer coefficients up to 30 kW/m2 °C were obtained. Moreover, for two-phase flow and fixed heat flux, the heat transfer coefficient increases with decreasing mass velocity and liquid subcooling, while the pressure drop increases with increasing mass velocity and decreasing liquid subcooling. The heat sink according to the HP orientation provides higher overall heat transfer coefficients. The highest and lowest pressure drops were observed for the VHP and VP orientations, respectively. The effect of the footprint orientation on the heat transfer coefficient increases with increasing mass velocity and liquid subcooling. Temperature oscillations with higher amplitude and frequency were observed for the VHP orientation. Images of the flow boiling process revealed that bubbles agglomerate in the lower part of the heat sink for the VHP orientation under conditions of high mass velocities. Reverse flows were observed only for HP and VHP footprint orientations. Discontinuities in the boiling curve just before the ONB have occurred simultaneously to bubble nucleation at the outlet plenum. Li and Wu [42] was the best method to predict the heat transfer coefficient data. For pressure drop, the Homogeneous Model using the two-phase viscosity given by Cicchitti et al. [46] presented the best prediction of the experimental results compare with the others predictive methods evaluated, but no one of pressure drop predictive methods evaluated in the present study was accurate enough to predict the R245fa database for flow boiling conditions.