Combining liquid inertia and evaporation momentum forces to achieve flow boiling inversion and performance enhancement in asymmetric Dual V-groove microchannels

Abstract

Increasing critical heat flux (CHF) and heat transfer coefficient (HTC), and reducing pressure drop (Δp) are highly desirable during flow boiling. An asymmetric Dual-V groove microchannels geometry was developed and combined with a tapered open manifold to achieve significant performance enhancements in all three aspects while reporting boiling inversion for the first time in flow boiling. Evaporation momentum force is utilized to direct bubbles along a central region of the doubly-finned structure, while inertia force is utilized to create a two-phase flow in the microchannel flow passages formed by the fins. The microchannel passages are contoured as adjoining pairs of asymmetric Dual-V grooves. Bubbles nucleate at the corners of the V-grooves and the evaporation momentum force modulates their traverse over the inclined surfaces of the microchannels. High-speed video images reveal that the growing bubbles flow rapidly over the microchannel walls and emerge into the microgap region a certain distance away from their respective nucleation sites. This leads to a self-feeding mechanism that causes boiling inversion in which wall superheat drops with increase in heat flux. Early departure from the nucleation site before fully growing improves the CHF and the bubble traverse normal to flow direction improves HTC. Furthermore, the momentum of the growing bubbles in the tapered open microgap configuration reduces the pressure drop. Using water as working fluid, we have reached a heat dissipation of 508.1 W/cm2 without reaching CHF at a wall superheat of 12.3 °C with a pressure drop of only 3 kPa. The resulting HTC was 412.2 kW/(m2 K).