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

Abstract

The convective flow in a layer of volatile silicone oil with a kinematic viscosity of 0.65 cSt 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 3600. The effect of noncondensables (i.e., air) was studied by comparing convection in the liquid layer below a vapor space at pressures ranging from 4.8 kPa to 101 kPa, corresponding to air molar fractions ranging from 14% to 96%, respectively, and silicone-oil vapor, under otherwise identical conditions. The results for convection at 101 kPa are in qualitative agreement with previous studies, and clarify the time-dependent flow observed at high Marangoni numbers. The results show that decreasing the relative air concentration increases the critical Marangoni numbers for transition between different flow states, even though the air concentration does not appear to affect the speeds near the interface. Linear stability analysis shows that transitions are suppressed due to the latent heat generated or absorbed at the interface due to the enhancement of phase change. Furthermore, the experimental results suggest that air, even at relative concentrations as small as 14%, or partial pressures of O(102 Pa), has a significant effect on the vapor flow, and that the fluid in the vapor space should be modeled as a binary mixture in many cases of practical interest.