An experimental study of buoyancy-Marangoni convection in confined and volatile binary fluids

Abstract

Marangoni convection in a volatile liquid film subject to a horizontal temperature gradient confined in a rectangular cavity is a basic problem of heat transfer and fluid mechanics with applications in evaporative cooling. The Marangoni stresses that drive this flow are due to surface tension gradients at the liquid–vapor interface. The thermocapillary stresses due to variations in surface tension with temperature for the most part drive the liquid away from hot regions. A volatile binary-fluid mixture can, however, be designed so that the solutocapillary stresses due to variations in surface tension with concentration drive the liquid instead towards hot regions, and hence oppose thermocapillary stresses. In thicker liquid layers, the flow is also affected by buoyancy. This work details an experimental study of buoyancy-Marangoni convection in a ∼0.31cm deep layer of methanol–water (MeOH–H2O) in a sealed rectangular cuvette driven by a temperature difference of ∼6°C over a horizontal distance of 4.9cm. Particle pathline visualizations and particle-image velocimetry (PIV) were used to study this flow and determine how the liquid composition, quantified in terms of the MeOH concentration CM, and noncondensables (i.e., air), quantified by the concentration of air in the gas space above the liquid ca, affect the flow. Solutocapillary effects are strong enough to drive the liquid near the free surface towards the heated end over the entire horizontal extent of the liquid layer at low ca (i.e., ca<6%), suggesting that binary-fluid coolants could significantly reduce film dryout. This flow, driven by thermocapillarity, solutocapillarity and buoyancy, is classified into four distinct flow regimes that are summarized in a cavs. CM flow regime map.