Tongran Qin

CONVECTION, EVAPORATION, AND CONDENSATION OF SIMPLE AND BINARY FLUIDS IN CONFINED GEOMETRIES

Rayleigh-Benard and Marangoni convection in a layer of a homogeneous fluid with a free surface in the absence of phase change is a classic (and extensively studied) problem of fluid mechanics. Phase change has a major effect on the convection problem. Most notably, significant latent heat generated at the free surface as a result of phase change can dramatically alter the interfacial temperature, and hence, the thermocapillary stresses. Furthermore, differential evaporation in binary fluids can lead to considerable variation in the concentration field, producing solutocapillarity stresses, which can compete with thermocapillarity and buoyancy.This talk describes numerical studies of convection in alcohol and alcohol-water mixtures due to a horizontal temperature gradient in the presence of phase change. We illustrate how the composition of the liquid and the presence of non-condensable gases (e.g., air) can be used to alter the balance of the dominant forces. In particular, by adding or removing air from the test cell, the direction of the flow can be reversed by emphasizing either the thermocapillary or the solutocapillary stresses.© 2012 ASME

The effect of phase change on stability of convective flow in a layer of volatile liquid driven by a horizontal temperature gradient

Buoyancy-thermocapillary convection in a layer of volatile liquid driven by a horizontal temperature gradient has been studied extensively in experiment and numerics. Recent studies have shown that the composition of the gas phase, which is typically a mixture of vapour and air, has a noticeable effect on the critical Marangoni number describing the onset of convection as well as on the observed convection pattern. Specifically, as the total pressure or, equivalently, the average concentration of air is decreased, the threshold of the instability leading to the emergence of convective rolls is found to increase rather significantly. We present a linear stability analysis of the problem which shows that this trend can be readily understood by considering the transport of heat and vapour through the gas phase. In particular, we show that transport in the gas phase has a noticeable effect even at atmospheric conditions, when phase change is greatly suppressed.

THE EFFECT OF NONCONDENSABLES ON THE BUOYANCY-THERMOCAPILLARY CONVECTION IN CONFINED AND VOLATILE FLUIDS

Convection in confined layers of volatile liquids has been studied extensively under atmospheric conditions. Recent experimental results [1] have shown that removing most of the air from a sealed cavity significantly alters the flow structure and, in particular, suppresses transitions between the different convection patterns found at atmospheric conditions. Yet, at the same time, this has almost no effect on the flow speeds in the liquid layer. To understand these results, we have formulated and numerically implemented a detailed transport model that accounts for mass and heat transport in both phases as well as the phase change at the interface. Surprisingly, the numerical simulations show that noncondensables have a large effect on buoyancy-thermocapillary flow at concentrations even as low as 1%, i.e., much lower than those achieved in experiment.Copyright