Phase separation evaporator using pin-fin-porous wall microchannels: Comprehensive upgrading of thermal-hydraulic operating performance

Abstract

Microchannel evaporator has intrinsic drawback of two-phase flow instabilities with large pressure drop that is induced from bubble blockage in microchannels. Here, we propose a new concept of phase-separation evaporator, in which flow instabilities can be completely suppressed by using gradient pin-fin-porous wall microchannels. The phase-separation concept stems from the self-driven motion of nucleated bubbles from pin-fin region to bare channel region. Vapor bubbles release from porous wall at a high frequency of ~1000 Hz that ensures bare channels not totally wrapped by vapor. Thus, ultra-stable flow and heat transfer can be achieved in a wide range of operating parameters: Tin = 81.9~92.6 °C, Pin = 110.57~219.77 kPa, G = 112~264 kg/m2s, q = 20.79~292.82 kW/m2 and xout = 0.005~0.318. Both convective evaporation in bare channels and bubble nucleation in porous walls contribute to enhanced performance. Heat transfer coefficients are not changed as the Bo numbers ranging from 6.91 × 10–5 to 7.44 × 10–4. This is because when heat flux and/or mass flux is changed, one mechanism is strengthened while the other mechanism is weakened to achieve constant overall heat transfer coefficient. This heat transfer mechanism is different from that in conventional microchannels in which nucleation mechanism dominates at small Bo while convective mechanism dominates at large Bo. With mass fluxes in the range of 112 ~264 kg/m2s, pressure drops are not changed versus mass fluxes at a given heat flux, which are caused by the thermal driving effect in porous walls. Stable heat transfer coefficients and pressure drops ensure flexible selection of running parameters. The evaporator can operate at smaller mass flow rates, as long as dry-out does not occur, it can also operate at larger flow rates without accompanying additional pressure drops.