Heat transfer and pressure drops during high heat flux flow boiling of low-GWP refrigerants in 200 μm-wide multi-microchannels

Thermal management of electronics is continuously challenged by increasing heat fluxes to be dissipated at minimum power consumption. This study investigated the pressure drops and the heat transfer coefficient of two low-GWP refrigerants, R1234yf and R1234ze(E), during flow boiling at high heat fluxes. The test geometry comprised 25 parallel microchannels, 200 µm wide, 1200 µm deep and 1 cm long, nominally. Inlet orifices were used to stabilize the flow. A parametric analysis involving the mass flux and heat flux was conducted for nominal saturation temperatures of 30.5 and 40.5, resulting in a maximum confinement number N_Conf of 2.16 for R1234yf and 2.52 for R1234ze(E). As the applied heat flux was increased from low to near-critical values, the two-phase heat transfer transitioned from a heat-flux-dominated mode to a convection-dominated one. The flow finally approached dryout, which led to the critical heat flux upon further heating. The experimental results were analysed through a regression with relevant non-dimensional groups, in particular the confinement number,N_Conf , the equivalent wall Biot number, Bi, the boiling number, Bo, the convection number, Co, and the Weber number, We. It was shown that Bo and Bi had a major influence in the heat-flux-dominated region, while Co and N_Conf determined the performance in the convective region, mainly due to the establishment of an intermittent annular flow. The total channel pressure drops were evaluated through the experimental measurement of the pressure drops in the inlet-outlet manifold, and the orifices. Due to the short channel length and the high heat fluxes, the pressure drops were dominated by the momentum change contribution, which accounted for at least 60% of the total pressure drops in the lowest case. The flow-wise heat transfer coefficients and the channel pressure drops were compared with the predictions of existing correlations, showing generally a fair agreement.