Enhanced pool boiling heat transfer mechanisms for selectively sintered open microchannels

摘要

Pool boiling performance can be enhanced significantly by generating separate liquid-vapor pathways through selectively coating different regions of a heat transfer surface. In this paper, heat transfer mechanisms are explored by depositing porous coatings on an open microchannel with channel width 762 μm, channel depth 400 μm and fin width 200 μm. Three surfaces were fabricated with sintered porous coatings on: (ⅰ) entire microchannel surface (sintered-throughout), (ⅱ) only the fin tops (sintered-fin-top), and (ⅲ) only the channel walls (sintered-channel). Their pool boiling performance with degassed water at atmospheric pressure was experimentally obtained. A critical heat flux (CHF) of 313 W/cm~2 at a wall superheat of 7.5 ℃ was obtained for a sintered-throughout surface with a 2.4 fold enhancement in CHF over a plain chip. Highest heat transfer coefficient (HTC) of 565 kW/m~2 ℃ was obtained for this surface which translated into a 6.5 fold enhancement when compared to a plain surface. Three enhancement mechanisms were identified: (ⅰ) Area Augmented Enhanced Nucleation, (ⅱ) Bubble Induced Liquid Jet Enhancement - Type-1 and (ⅲ) Bubble Induced Liquid Jet Enhancement - Type-2. These mechanisms were responsible for the enhancement in HTC and CHF for sintered-throughout, sintered-fin-tops and sintered-channels, respectively. Although the current testing indicated the sintered-throughout surface to provide the highest CHF and HTC enhancement for the selected microchannel dimensions, changing the microchannel dimensions is expected to influence the relative merits of these configurations as the liquid and vapor flow mechanisms are influenced in fundamentally different ways.