Abstract
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 Dh 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 Dh = 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 Dh, 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 Nuglob 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 Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh 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 R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯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.
Highlights
IntroductionThe rapid development of micro-electronics and high-power density electronics has resulted in a continuous reduction in the channel size in microchannel evaporators, making heat removal even more challenging for the researchers and designers due to the limited surface area
The results from the simulations examining the effect of hydraulic diameter considering a single rectangular microchannel are presented in the following paragraphs
80 kW/m2 ) and constant volumetric flow rate of V = 9.1611 ×10−9 m3 /s with varying mass fluxes (150, 267, 600, 2400 kg/m2 ), which correspond to hydraulic diameters of 200, 150, 100, and 50 μm, respectively, will be presented
Summary
The rapid development of micro-electronics and high-power density electronics has resulted in a continuous reduction in the channel size in microchannel evaporators, making heat removal even more challenging for the researchers and designers due to the limited surface area This transition has made necessary the classification between microchannels, minichannels, and conventional tubes. Literature reveals that previous experimental works who tried to identify and differentiate the transition limits between conventional tubes and micro-scale channels, in most cases, disagree in their findings Some of these studies have concluded that the hydraulic diameter range for microchannels should be between 10 and 200 μm [1,2] while some other studies state that this range should be between 1 and 1000 μm [3,4,5]. Such separation is only conventional, and we will conservatively consider only small channels below 200 μm
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
