Conjugate heat transfer effects on flow boiling in microchannels

This article presents a computational study of saturated flow boiling in non-circular microchannels. The unit channel of a multi-microchannel evaporator, consisting of the fluidic channel and surrounding evaporator walls, is simulated and the conjugate heat transfer problem is solved. Simulations are performed using OpenFOAM v2106 and the built-in geometric Volume Of Fluid method, augmented with self-developed libraries to include liquid-vapour phase-change and improve the surface tension force calculation. A systematic study is conducted by employing water at atmospheric pressure, a channel hydraulic diameter of D-h = 229 mu m, a uniform base heat flux of q(b) = 100 kW/m(2), and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range epsilon = 0.25-4 and W-f = D-h/8 - D-h, respectively. The effects of conjugate heat transfer and channel aspect-ratio on the bubble and evaporative film dynamics, heat transfer, and evaporator temperature are investigated in detail. This study reveals that, when the flow is single-phase, higher Nusselt numbers and lower evaporator base temperatures are achieved for smaller channel aspect-ratios, from Nu similar or equal to 4 and T-b - T-sat similar or equal to 9 K when epsilon = 4, to Nu similar or equal to 6 and T-b - T-sat similar or equal to 2 K when epsilon = 0.25, for same fin thickness W-f = D-h/8. In the two-phase flow regime, Nusselt numbers in the range Nu = 12 - 36 are achieved. The trends of the Nusselt number versus the aspectratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, W-f = D-h/8 and W-f = D-h/4, an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range epsilon = 0.5-2 the Nusselt number appears weakly dependent on epsilon. For thicker fins, W-f = D-h/2 and W-f = D-h, the Nusselt number decreases slightly when increasing the aspect-ratio in the range epsilon = 0.5-2, although this trend is not monotonic when considering the entire range of aspect-ratios investigated. Nonetheless, due to conjugate heat transfer, Nusselt numbers and evaporator base temperatures follow different trends when varying the aspect-ratio, and channels with epsilon < 1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits. (C) 2022 The Author(s). Published by Elsevier Ltd.