In highly subcooled boiling, many microbubbles are emitted from coalesced bubbles on the heating surface in transition boiling and the heat flux increases higher than that of ordinary critical heat flux. This is called Microbubble Emission Boiling, MEB. The present paper introduces MEB generated in a horizontal rectangular channel and the application to high heat flux cooling technology for power electronics. According to the pressure fluctuations in the channel and the boiling sound, MEB is classified into two types which are a violent MEB and a silent MEB. In violent MEB, the heat flux increases very high and the maximum one is more than 10 MW/m~(2) for small heating surface of 10 mm X 10 mm. The heat flux increases roughly in proportion to the frequency of the pressure fluctuations in the channel. The pressure frequency is considered the frequency of liquid supply into the heating surface or solid - liquid contact. In silent MEB, microbubbles are emitted continuously from coalesced bubbles on the heating surface and the pressure fluctuation is very small. After reaching maximum heat flux, MEB turns to film boiling through terminal stage of MEB. At terminal stage of MEB, microbubble emission becomes poor, then the heating surface is covered with a thin vapor film and the boiling turns to film boiling with instantaneous temperature rise of heating surface. For large scale heating surface of 10 cm long used for IC power package, an active cooling device with dual channel and multi-nozzles are proposed to prevent the downstream section of heating surface from dry-out. The high heat flux of 5 MW/m~(2) is removed from the heating surface by the subcooled impinging jets in addition to the main channel cooling.
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