Experimental study of the effect of noncondensables on buoyancy-thermocapillary convection in a volatile silicone oil

The convective flow in a layer of volatile silicone oil confined to a sealed cavity with a transverse aspect ratio of 3.2 was visualized using particle pathlines and quantified by particle-image velocimetry at dynamic Bond numbers estimated to be of order unity and laboratory Marangoni numbers as great as 3400. The effect of noncondensables (i.e., air) was studied by comparing convection in the liquid layer below a vapor space dominated by air at atmospheric pressure with that under a vapor space consisting mostly of silicone-oil vapor (and 13 mol% air) under otherwise identical conditions. The results for convection under air at atmospheric conditions are in qualitative agreement with previous studies, and clarify the time-dependent flow observed at high Marangoni numbers. Comparison with the results under mostly silicone-oil vapor suggest that noncondensables have a significant effect on convection in the liquid layers, and that this effect appears to be due to transport in the vapor (vs. liquid) phase. The results also suggest that air, even at partial pressures of O(10 2 Pa), has a significant effect on this transport, and that the fluid in the vapor space should be modeled as a binary mixture in most cases of practical interest.