Concerning the magnitude of the maximum heat flux and the mechanisms of superintensive bubble boiling

Abstract

We have developed a stabilizer of the temperature of a thermoresistor wire electric heater based on a PID controller. Using this stabilizer, we investigated heat exchange of subcooled water in pool boiling. We found that on stabilization of the heater temperature up to that of the subcooled water, transition from convection to the regime of bubble boiling and vice versa occurs spontaneously and is accompanied by a jumpwise change in heat transfer. It is shown that in the regime of stable bubble boiling, the law of heat transfer is independent of the liquid temperature and the heater diameter and that the maximum heat loading may attain 50 MW/m 2, which is much above the values cited earlier in the literature. Based on the results obtained, a mechanism of implementation of bubble boiling for the regimes of a constant heat flux and a constant temperature is suggested. The assumption is made that the regime of heterogeneous vapour generation is possible only in the case of the heater constant temperature. In the regime of a stabilized heat flux on the heater, the spatially inhomogeneous regime of heat transfer is established. This regime represents a spatially distributed combination of three regimes: convective heat transfer, homogeneous boiling, manifesting itself in periodic boiling-up of overheated layers of the liquid near the surface and an unstable regime of heterogeneous vapour generation.