Occurrence Conditions of Sustainable Minute Bubble Emission Boiling for High Heat Flux Cooling

In order to investigate the conditions causing minute bubble emission boiling, critical heat flux experiments were conducted in the pool boiling system with different thermal capacities of heat transfer surfaces and method of heating. Stable minute bubble emission boiling was observed for a 10 mm-thick copper cylinder heated by thermal radiation. The critical heat flux obtained was 6.0 MW/m2. When the heat flux exceeded above approximately 3 MW/m2, a large vapor bubble formed on the heat transfer surface, then was condensed immediately and dispersed into minute bubbles. A 4 mm-thick silicon carbide heat transfer surface burnouted at 1.9 MW/m2. When the heat transfer surface has a small thermal capacity such as the metal foil, commonly used in CHF experiments, a temporary loss of heat removal due to large bubble formation will cause a rapid temperature increase and will result in burnout. Minute bubble emission boiling could occur stably when the time constant determined by the heating method and thermal capacity exceeds the time required for a large bubble to condense in the subcooled fluid.