An Experimental Study of Marangoni Convection in Confined and Volatile Binary Liquids: The Effect of Noncondensables

Abstract

Marangoni convection in a liquid layer subject to a horizontal temperature gradient due to variations in the surface tension at the liquid-vapor interface and confined in a rectangular enclosure is a basic problem of heat transfer and fluid mechanics, with applications in evaporative cooling. In general, surface tension increases as temperature decreases, giving rise to thermocapillary stresses that drive liquid coolant away from hot regions. In a volatile binary-fluid mixture, however, the two components can be chosen so that the surface tension of the mixture increases as temperature increases due to differential evaporation, giving rise to solutocapillary stresses that can drive liquid instead towards hot regions, and hence oppose the thermocapillary stresses. It is of course well-known that noncondensables suppress phase change. Yet there have been few, if any studies of how noncondensables affect the Marangoni convection of volatile binary fluids in a confined geometry, where evaporation and condensation must balance. An experimental study was therefore performed of Marangoni convection in a layer of methanol-water (MeOH-H2O) mixture confined in a sealed rectangular cuvette. The cuvette was symmetrically heated on one end and cooled on the other end using Peltier devices, giving temperature differences of ∼6 °C over a horizontal distance of 4.9 cm, and two-dimensional, two-component particle-image velocimetry (2D-2C PIV) was used to measure the velocity fields in this steady flow. The study focuses on convection in liquid layers with a depth of ∼0.3 cm (vs. a test cell height of 1 cm), and how this flow is affected by changes in the relative concentration of noncondensables (i.e., air) in the gas space above the liquid. The results suggest that changing the concentration of noncondensables, which also has a marked effect on the pressure, in the gas space, can be used to “adjust” the relative importance of solutocapillary and thermocapillary stresses.