Characterizing effect of particle size on flow boiling in sintered porous-microchannels

Flow boiling in porous microchannels draws extensive attention recently owing to its great potential in high heat flux applications. Subcooled flow boiling of deionized and degassed water was carried out to characterize transport performance. Porous microchannels were sintered from six-sized dendritic-like copper particles: 10, 30, 50, 90, 120 and 150 μm with the same layer thickness of 400 μm. The dimensions of 23 parallel porous microchannels are 600 μm × 1200 μm × 2800 μm (width × depth × length). The inlet subcooling degree and mass flux was maintained at 40 K and 142 kg/m2·s, respectively. Experimental results show that the particle size has great effects on the heat transfer coefficient (HTC) and Critical heat flux (CHF) of flow boiling in porous microchannels. Medium particle size (90 μm or 120 μm) remains higher performance in HTC and CHF. In heat fluxes between 60∼160 W/cm2, average HTCs of both PM-90 and PM-120 could reach almost 120 kW/(m2·K) or so. The visualization study reveals that the underlying heat transfer mechanism in porous microchannels is dominated by the nucleate boiling in low heat flux and then, by the thin film evaporation from moderate to high heat flux. Moreover, two boiling crisis phenomena have been observed: (a) in microchannels sintered from optimum size particles, the rewetting-dryout cycle has been well established and sustained without being interrupted by occasionally explosive boiling events, indicating capillary limits; (b) in non-optimum samples, high wall superheat in the rear part usually leads to explosive boiling persistently, which interrupts periodic oscillation modes with heat flux exceeding to moderate level and results in CHF conditions. An optimal ratio of bottom wall thickness and particle diameter, δ/d, is found in the range of 3∼5 for the flow boiling in sintered porous microchannels.