Growth of diffusion-controlled vapour bubbles at a wall in a known temperature gradient

Abstract

Previous studies of individual vapour bubbles growing at a wall into initially stagnant liquid have been extended by introducing controlled initial temperature profiles in the liquid. The simplest such cases were studied first (zero gravity, bulk liquid at saturation temperature), using dimensional analysis and comparing with previous experiments and analysis in an even simpler situation, i.e. initially uniform temperature. Various patterns of behaviour can be seen, which are largely in accordance with expectation, but can now be quantified. In the absence of gravity, bubbles depart from the wall at times and sizes which can be predicted, at least for bubbles within our range of experimental conditions. That range was extended as far as possible, and corresponds broadly to nucleate boiling at atmospheric pressure or below. For saturation boiling (i.e. T bulk = T sat), one particularly simple and universal result is a relation between time and diameter (or volume) at departure. The relation is independent of growth rate and of thermal boundary layer thickness, as it is simply D D t 2 3 D ≃ 1.5 σ ρ 1 3 or V D t 2 D ≃ 2.0 σ ρ . Since σ/ρ does not vary very greatly for normal fluids, this relation is only weakly dependent on the fluid. The well-established phenomenon of bubbles departing ‘against gravity’ (i.e. in negative or non-buoyant gravity, as from a downward-facing surface), is now observed for the first time under controlled conditions, and the conditions which favour such behaviour have been identified. These are principally thin thermal boundary layer and strong subcooling. One unexpected finding is that positive (buoyant) gravity can, in some cases, cause a bubble to depart with larger size than if gravity were absent.