Conjugate heat transfer effects on flow boiling in microchannels

Abstract

• Heat distributes nonuniformly around the perimeter of microchannels in evaporators. • In single-phase flow, low aspect-ratio channels yield best heat transfer. • In two-phase flow, the Nusselt number trends vs. aspect-ratio are mixed. • High aspect-ratio channels exhibit larger thermal resistance and higher evaporator temperatures. • Conjugate heat transfer should be included in design models for evaporators. 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 µm, 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 ϵ = 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 ≃ 4 and T b − T s a t ≃ 9 K when ϵ = 4 , to Nu ≃ 6 and T b − T s a t ≃ 2 K when ϵ = 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 aspect-ratio 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 ϵ = 0.5 –2 the Nusselt number appears weakly dependent on ϵ . 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 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.