The effect of hydraulic diameter on flow boiling within single rectangular microchannels and comparison of heat sink configuration of a single and multiple microchannels

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter D_h, whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of D_h = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different D_h, the applied heat and mass flux values varied between 20 to 200 kW⁄m² and 150 to 2400 kg⁄m²s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nu_glob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with D_h of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter D_h of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance RR_l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the RR_l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms.