Onset of microbubble emission boiling at reduced pressure using a confined vessel for subcooled pool boiling

• Subcooled pool boiling in a confined vessel was investigated at reduced pressure. • Three types of copper surfaces were used for heat transfer surfaces. • Microbubble emission boiling (MEB) occurred with water at reduced pressure. • No onset of MEB was observed with a flat, solid-copper heat-spreader. Boiling heat transfer (BHT) attracts attention for application to next-generation electronics cooling owing to its high heat transfer coefficient. In practical use, the shrinking of cooling devices using BHT is needed. Therefore, BHT in a confined space is widely studied. A liquid chamber (LC), like a vapor chamber, is a cooling device that uses BHT within a compact vessel and operates at reduced system pressure. Typically, LCs are operated at temperatures near the saturated temperature of a working fluid. Subcooled pool boiling, however, is a method to increase the critical heat flux (CHF). Moreover, microbubble emission boiling (MEB), which is observed at a high liquid subcooling, is a promising technology for achieving high heat flux cooling, exceeding the ordinary CHF. There are many studies of subcooled pool boiling with MEB in large vessels at atmospheric pressure. However, the MEB characteristics in a confined space at reduced pressure are also important to realization of next-generation cooling devices. To reveal the BHT characteristics at high liquid subcooling and in a confined space, subcooled pool boiling is investigated with water at reduced pressure using a confined vessel. Three water-contacting heat transfer surface configurations are investigated: a small-diameter flat surface, a thin and solid heat-spreader plate, and a finned surface. It is shown that the onset of MEB in subcooled pool boiling is not observed using a heat spreader at reduced pressure. The fabricated LC with the 9-mm-diameter heat transfer surface connected to the finned copper surface has succeeded in removing a maximum heat flux of 311 W/cm 2 at the maximum temperature of 94 °C