DYNAMICS OF VAPOR BUBBLES IN NUCLEATE BOILING

This paper considers the behaviour of a vapour bubble formed at a nucl eation site on a heated horizontal wall. This bubble is modeled as a s pherical segment which is separated from the wall by a microlayer of i ntervening liquid. The liquid is presumed to be at rest at great dista nces from the bubble. In order to avoid unwarranted assumptions about forces acting on the bubble which are specific to all known models of bubble growth and detachment, we derive equations that govern bubble b ehaviour in a rigorous way from the variational equation that describe s mechanical energy conservation for the whole system, which includes both the bubble and the liquid. The variational equation leads to a se t of two mutually independent strongly nonlinear equations which gover n bubble expansion and the motion of its centre of mass. Because these equations contain an extra unknown variable (the bubble vapour pressu re), a supplementary equation that defines bubble vapour temperature m ust be formulated with allowance made for heat transfer to the bubble both from the bulk of the surrounding liquid and through the microlaye r. The most important conclusion of this paper consists in the fact th at surface tension effects result in an effective force that tends to transform the bubble into a sphere, thereby facilitating bubble detach ment. This conclusion absolutely nullifies the generally, however erro neously, held belief that this effective force presses the bubble to t he wall. By way of example, we consider the evolution of bubbles whose growth is thermally controlled. Our analysis provides quite a natural explanation for a number of repeatedly observed phenomena, such as th e influence of gravity and surface tension on bubble growth rate and t he dependence of bubble detachment size on thermophysical parameters. Copyright (C) 1996 Elsevier Science Ltd.