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Pool Boiling on Small Heat Dissipating Elements in Water at Subatmospheric Pressure

The results of an experimental investigation of pool boiling of water at subatmospheric pressures from small horizontal heat sources are reported. The heat sources are upward-facing copper surfaces submerged in a laterally-confined, finite pool of liquid. The saturated pool boiling heat transfer characteristics and the critical heat flux (CHF) condition were determined in the experiments. Low pressure boiling of saturated water provides a means of removing heat at high heat flux levels while maintaining low surface temperatures. However, at heat flux levels up to about 60 2 W/cm the frequency of bubble departure at low pressure is much less than the frequency of bubble departure at higher pressure (atmospheric). With low pressure boiling, only one or two very large bubbles form cyclically on the small heated surface, during the boiling process. This intermittent process may result in large, undesirable temperature oscillations at the heated surface for low pressure boiling. High-frequency surface temperature measurements were obtained in this study which indicate the waiting time between bubbles and the cyclic temperature variation. The waiting time results of this investigation are compared to a simplified heat transfer model and experimental results of previous studies. The nucleate boiling and critical heat flux results indicate that the heater size may have a significant effect on the performance. At low pressures, the few bubbles that form have a departure diameter comparable to the size of the heated surface tested. The apparent effects of heated surface size and pressure on the CHF condition are also discussed. This is a preprint of a paper that will be presented at the 1991 ASME National Heat Transfer Conference, Minneapolis, Minnesota, July 28-31, 1991. Copyright  1991 ASME