Suppression of flow boiling instabilities and augmentation of thermal performance in a T-shaped microchannel with sudden-expansion flow passages

Flow boiling in microchannels provides exceptional heat transfer performance but is often compromised by severe flow instability. This study proposes T-shaped microchannels with sudden-expansion flow passages (TMSE) to suppress such instability by controlling the vapor behavior in both vertical and streamwise directions. In the vertical direction, the vapor lift-off from the bottom corner of the upstream T-shaped channel facilitates stable liquid replenishment by preventing the reattachment of growing bubbles. In the streamwise direction, the vapor-venting effect, driven by Laplace pressure gradients induced by asymmetric meniscus curvatures, promotes forward vapor evacuation as slugs encounter the sudden-expansion region. A force balance analysis was conducted to quantitatively verify the effect of vapor venting on flow instability. TMSE was compared with plain surface, plate-fin, and T-shaped microchannels using visualization under heat fluxes from 7 to 170 kW/m² and mass fluxes of 200, 267, and 334 kg/(m²·s), with HFE-7100 as the working fluid. Plate-fin and T-shaped microchannels exhibited severe vapor backflow across the entire channel, driven by explosive bidirectional vapor growth at an outlet quality of 0.58 and a mass flux of 267 kg/(m²·s). By contrast, the TMSE delayed the vapor backflow regime up to an outlet quality of 0.72. This suppression of vapor backflow enhanced the thermal performance by facilitating stable thin-film evaporation. Consequently, the TMSE achieved a 77.3 % increase in heat transfer coefficients and a 315.7 % improvement in the thermal performance factor compared with T-shaped microchannels, while maintaining pressure-drop levels comparable to those of the plain surface.