Natural alternations in subcooled pool boiling heat transfer

Subcooled boiling processes are known to be responsible for flow instabilities as well as for pressure fluctuations in boilers. In order to predict these phenomena, the specific details have been investigated which govern the growth and recondensation processes of vapor bubbles generated in a superheated boundary layer. Experiments have been conducted under pool boiling conditions with a vertical circular copper plate and degassed water at low pressures (0.1 to 0.4 bar). The subcooling has been varied from 10 to 60 K. With decreasing pressure, relative regular alternations are increasingly more pronounced between single phase convection, large bubbles at a single nucleation site and populations of small bubbles each accompanied by a more or less effective heat transfer. The contributions of these different boiling modes to transfer the given constant heat flux depend on the subcooling of the bulk fluid. A subcooling larger than 25 K can lead to an alternation of single phase convection and many small bubbles which we call swarm-bubbles. The start of the swarm-bubble boiling mode seems always to be the result of a secondary nucleation process initiated by the collapse of a big initial bubble accompanied by an audible "bump". A criterion is applied which identifies the collapse as mainly inertia controlled. The natural alternation in heat transfer can be explained as an alternation between two instationary processes with developing boundary layers each leading to its own disappearance.