Abstract
This study conducts an experimental investigation and visualization for convective flow boiling heat transfer of methanol/water mixtures in parallel diverging multichannels with a hydraulic diameter of 468.5μm. In order to reduce flow reversal in the two-phase region, a tree-like inlet distributor with a sudden expansion structure in front of each channel is designed. The MEMS technology is applied to fabricate the microchannel. The effect of molar fractions and mass fluxes on flow patterns, heat transfer, and critical heat flux are studied. Four distinct two-phase flow patterns can be identified, namely, (a) bubbly-elongated slug flow, (b) annular flow, (c) film breakup, and (d) dryout. Moreover, flow pattern maps on the planes of wall superheat or wall heat flux versus molar fractions, separately, are established based on the data of this study. For low to medium heat fluxes, the dominant flow patterns are elongated slug flow and annular flow, which indicates that the evaporation of a thin liquid film is the major mechanism of heat transfer for convective flow boiling of methanol/water mixtures in a microchannel. For these two-phase flow patterns, the heat transfer coefficient generally increases with an increase in heat flux. The liquid film breakup is the dominant flow pattern for high heat fluxes. For these particular flow pattern, the heat transfer coefficient decreases with an increase in heat flux. Moreover, the temperature span of film breakup region has a widest range for xm = 0.3, suggesting that the most important factor of the liquid film breakup be the agitation of the surface tension on the liquid-vapor surface. A molar fraction of 0.3 presents the highest CHF in the present study. The present study also reveals that an increase in the mass flux may result in the better performance for heat transfer in both single and two phase regions.