Evaporative cooling, which takes advantage of the large latent heats associated with phase change, is a leading technology for removing high heat fluxes over small areas. Although it is well-known that noncondensables have a significant effect on condensation heat transfer, there are very few studies of buoyancy-thermocapillary convection for the situation most relevant to evaporative cooling devices: convection in a volatile liquid layer under its vapor in the near-absence of noncondensables confined to a sealed geometry.
This thesis therefore experimentally studies convection driven by a horizontal temperature gradient in a ~0.3 cm deep liquid layer confined in a sealed rectangular test cell. Velocity field in a volatile simple (i.e., single-component) silicone oil, hexamethyldisiloxane, was measured by two-dimensional two-component (2D-2C) particle-image velocimetry (PIV). The effects of varying parameters such as the applied temperature difference and the composition of the gas phase were investigated. These experimental results are compared with numerical simulations that also consider vapor-phase transport and incorporate thermocapillary effects. Our preliminary results demonstrate that noncondensables have a significant effect on liquid-phase flow, suppressing phase change.
We will next study convection in a binary water-methanol (H2O-MeOH) mixture. This binary mixture has solutocapillary stresses that push liquid towards hot regions, because the more volatile component (MeOH) has a surface tension that is less than that of the less volatile component (H2O). Differential evaporation will then lead to higher H2O fractions, and hence higher surface tension of the mixture, in regions of higher temperature. These studies will therefore consider the effect of varying both the liquid-phase and gas-phase compositions (as well temperature difference) by varying the relative fractions of the two components and the amount of noncondensables, respectively. Although our initial visualizations demonstrate that solutocapillarity can be used to push liquid towards hot regions, these solutocapillary stresses depend strongly on the liquid-phase and gas-phase compositions.