The origin of thermocapillary convection in subcooled nucleate pool boiling

The significance of noncondensibles as the origin of thermocapillary convection in subcooled pool boiling is demonstrated in this paper. The temperature gradients along the bubble interface inducing thermocapillary convection are caused by saturation pressure gradients due to a nonuniform accumulation of noncondensable gas along the interface. On the vapour side, the noncondensable gas inhibits the condensation of vapour. The corresponding increase of the saturation pressure gradient and temperature gradient, respectively, forces thermocapillary convection on the liquid side of the bubble. A bubble in subcooled liquid can grow or shrink according to the heat and mass transfer at its top, and even a steady-state mass flow through the bubble can be maintained. By a combined analytical and numerical approach the various parameters of this complex system are studied. Furthermore, it is demonstrated that even small amounts of noncondensable gas are sufficient to induce thermocapillary flow. This flow, in turn, inhibits bubble detachment.