Investigation on critical heat flux of flow boiling in parallel microchannels with large aspect ratio: Experimental and theoretical analysis

Critical heat flux (CHF) for microchannel heat sink makes significant sense to industrial applications in preventing thermal safety accidents and avoiding facility burnout. Nevertheless, the fundamentals of CHF in confined microchannels are lack of clear comprehension, and related visualization experimental data about CHF also needs complement. The present work explores flow boiling CHF in parallel shallow microchannel heat sink with visualization experiments, aiming to provide new CHF data for microchannels with large aspect ratio (AR = W/H = 5). Two test configurations (S1: H = 400 μm, W = 2000 μm, S2: H = 300 μm, W = 1500 μm) and two kinds of working fluids (ethanol and acetone) are adopted. By capturing bubble behaviors with high speed photography, the characteristics and mechanism of CHF for such shallow microchannels are revealed. It is found out that the boiling curve keeps a linear growth with the wall superheating degree after the local dry-out, where the dominate heat transfer mechanism directly transits to the film boiling without experiencing the transition boiling. Meanwhile, the effect of mass flux, working fluid, and hydraulic diameter of channel on the CHF is also studied respectively. The experimental results illustrate that the CHF in the microchannels with large AR is significantly influenced by flow instability and presents a third order polynomial increasing trend with G. Besides, statistic analysis on the CHF data with some well-known correlations is conducted to find out the suitable theoretical mode for the shallow microchannels. A new CHF correlation that can especially fit for the microchannels with aspect ratio heavily deviated from 1 is proposed, where 116 data points involving 8 different working fluids and 9 types of microchannels are verified with MAE of 7.3%.