Experimental investigations into flow boiling heat transfer in ribbed micro-channels with rectangular reentrant cavities

Reentrant cavities are fabricated at the side walls of ribs and bottom walls of grooves to enhance the flow boiling heat transfer in rectangular-ribbed micro-channels. To reveal the geometrical effect of reentrant cavities, two different reentrant cavities, i.e. RRMB-1 (ribbed micro-channel with narrow reentrant cavities) and RRMB-2 (ribbed micro-channel with wide reentrant cavities), are designed and the RM micro-channel (i.e. smooth ribbed micro-channel without reentrant cavities) is manufactured as the baseline case. The flow boiling heat transfer performance in the micro-channels is experimentally investigated at four mass fluxes and a range of heat fluxes. The flow regimes are visualized during the flow boiling process. The influences of reentrant cavities on the heat transfer coefficient, critical heat flux, pressure drop, outlet temperature fluctuation and performance evaluation criterion of three different micro-channels are analyzed and compared. The results showed that the capillary effect in the reentrant cavities results in a 19.5% increase in the heat transfer coefficient, and a 23.4% increase in the critical heat flux for the RRMB-1 at a low mass flux. The pressure drop in the RRMB-1 microchannel is slightly higher than that in the RM micro-channel in the single-phase region, while it is opposite at the high heat flux condition. The critical heat flux in the RRMB-2 micro-channel is lower than the RRMB-1 microchannel by 34.5%, but the pressure drop in the RRMB-2 micro-channel is higher than the RRMB-1 micro-channel by 50.3% in the two-phase region.