Experimental studies of Marangoni convection with buoyancy in simple and binary fluids

Abstract

The flow in a layer of volatile fluid driven by a horizontal temperature gradient is a fundamental transport model for numerous evaporative passive cooling applications. When a thin film of a volatile liquid is subject to a horizontal temperature gradient, changes in the surface tension at the free surface lead to Marangoni stresses that drive the flow. In a thicker liquid layer, the flow is also affected by buoyancy. This thesis describes experimental studies of convection driven by a combined action of Marangoni stresses and buoyancy in simple and binary volatile liquid layers confined in a sealed rectangular cavity heated at one end and cooled at the other. Experiments with varying concentrations of noncondensables (i.e., air) ca were performed to investigate their effect on the phase change and heat and mass transport. In the simple liquid, thermocapillary stresses drive the liquid near the free surface away from the heated end. Varying ca is shown to strongly affect the stability of this buoyancy-thermocapillary flow for Marangoni numbers Ma = 290 - 3600 and dynamic Bond numbers BoD = 0.56 - 0.82: removing air suppresses transition to multicellular and unsteady flow. The results are compared with numerical simulations and linear stability analysis. In the binary liquid considered here, a methanol-water (MeOH-H2O) mixture, solutocapillary stresses drive the flow near the free surface towards the heated end. Four distinct flow regimes are identified for this complex flow driven by thermocapillarity, solutocapillarity, and buoyancy, and are summarized in a flow regime map as a function of ca and the liquid composition (MeOH concentration). At low ca, solutocapillary effects are strong enough to drive the liquid near the free surface towards the heated end over the entire liquid layer, suggesting that binary-fluid coolants could significantly reduce film dryout.