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 Dh=229 µm, a uniform base heat flux of qb=100 kW/m2, and by varying the channel width-to-height aspect-ratio and channel fin thickness in the range ϵ=0.25–4 and Wf=Dh/8−Dh, 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≃4 and Tb−Tsat≃9K when ϵ=4, to Nu≃6 and Tb−Tsat≃2K when ϵ=0.25, for same fin thickness Wf=Dh/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 aspect-ratio are non-monotonic and exhibit a marked dependence on the channel fin thickness. For small fin thicknesses, Wf=Dh/8 and Wf=Dh/4, an overall ascending trend of Nu for increasing aspect-ratios is apparent, although in the narrower range ϵ=0.5–2 the Nusselt number appears weakly dependent on ϵ. For thicker fins, Wf=Dh/2 and Wf=Dh, the Nusselt number decreases slightly when increasing the aspect-ratio in the range ϵ=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 ϵ<1 seem to promote lower evaporator temperatures than higher aspect-ratio conduits.