In highly sub cooled flow boiling, coalescing bubbles on the heating surface collapse to many microbubbles in the beginning of transition boiling and the heat flux increases higher than the ordinary critical heat flux. This phenomenon is called microbubble emission boiling, MEB. It is generated in sub cooled flow boiling and the maximum heat flux reaches about 1 kW/cm2(10 MW/m2) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s for a small heating surface of 10 mm x 10 mm which is placed at the bottom surface of horizontal rectangular channel. The high pressure in the channel is observed at collapse of the coalescing bubbles and it is closely related the size of coalescing bubbles. Periodic pressure waves are observed in MEB and the heat flux increases linearly in proportion to the pressure frequency. The frequency is considered the frequency of liquid-solid exchange on the heating surface. For the large sized heating surface of 50 mm length x 20 mm width, the maximum heat flux obtained is 500 W/cm 2 (5 MW/m2) at liquid subcooling of 40 K and liquid velocity of 0.5 m/s. This is considerably higher heat flux than the conventional cooling limit in power electronics. It is difficult to remove the high heat flux by MEB for a longer heating surface than 50 mm by single channel type. A model of advanced cooling device is introduced for power electronics by sub cooled flow boiling with impinging jets. The maximum cooling heat flux is 500 W/cm2 (5 MW/m2). Microbubble emission boiling is useful for a high heat flux transport technology in future power electronics' used in a fuel-cell power plant and a space facility.
High heat flux transport by microbubble emission boiling
Hoher Wärmetransport durch Mikroblasenemissionssieden
International Topical Team Workshop on Two-Phase Systems for Ground and Space Applications, 2 ; 148-150
Microgravity Science and Technology ; 19 , 3/4
2007
3 Seiten, 5 Bilder, 8 Quellen
Conference paper
English
MICROBUBBLE EMISSION BOILING : FUNDAMENTALS AND APPLICATIONS
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