Experimental investigation on microbubble emission boiling heat transfer

Microbubble emission boiling (MEB) is a phenomenon that the heat flux increases more rapidly compared to the critical heat flux (CHF) with a little increase of the heating surface superheat. A subcooled pool boiling experimental setup was built up to investigate the bubble behaviors of MEB under the atmospheric condition at different liquid subcooling with the help of high-speed video camera. The heating element of the experimental setup is a copper block with its upper part of a 10 mm diameter cylinder. The experimental results show that, in MEB regime, an unsteady vapor film spreads on the heating surface. Numerous microbubbles are emitted into subcooled liquid continuously with an extremely high speed from the interface. Along with the rise of heat flux, the oscillation period of vapor film along its thickness gets shorter and the maximum thickness also becomes smaller. The microbubble diameter under higher power heating is much smaller than that under low heating flux. The maximum heat flux in the experiment is about 9 MW/m² at 60 K subcooling. (authors)