Turbulent transition of thermocapillary flow induced by water evaporation

Abstract

Water has been examined for thermocapillary convection while maintained just outside the mouth of a stainless-steel, conical funnel where it evaporated at different but steady rates. Evaporation at a series of controlled rates was produced by reducing the pressure in the vapor-phase to different but constant values while maintaining the temperature of the water a few millimeters below the interface at 3.56+/-0.03 degrees C in each case. Since water has its maximum density at 4 degrees C, these conditions ensured there would be no buoyancy-driven convection. The measured temperature profile along the liquid-vapor interface was found to be approximately axisymmetric and parabolic with its minimum on the center line and maximum at the periphery. The thermocapillary flow rate was determined in two ways: (1) It was calculated from the interfacial temperature gradient measured along the interface. (2) The deflection of a 12.7-microm-diameter, cantilevered probe inserted into the flow was measured and the liquid velocity required to give that deflection determined. The values determined by the two methods agree reasonably. As the vapor-phase pressure was reduced, the thermocapillary flow rate increased until a limiting value was reached. When the pressure was reduced further, certain of the variable relations underwent a bifurcation and the power spectrum of the probe displacement indicated it was a periodic function with frequency locking. These results suggest that thermocapillary flow plays an important role in the energy transport near the interface of evaporating water. In particular, it appears that the subinterface, uniform-temperature layer, reported in earlier studies, results from the mixing produced by the thermocapillary flow. The Stefan boundary condition is often applied to determine the energy flux to an interface where phase change is occurring; however, when there is strong convective flow parallel to the interface, the normal Stefan condition does not give an adequate description of the energy transport.

A microscale model for thin-film evaporation in capillary wick structures

“…Marangoni convection, which arises due to a surface tension gradient at the non-isothermal interface, has been shown to increase the heat transfer rates significantly in many systems [13][14][15]. At low superheats (< 5K), however, it is observed that Marangoni convection does not significantly affect …”

“…This causes better mixing and increased heat transfer rates. Marangoni convection under large temperature differences has been shown to have a significant enhancement effect on heat transfer [13,14,15]. Khrustalev and Faghri [16] developed a mathematical model of the evaporating liquid-vapor meniscus in a capillary slot.…”

Experimental investigation of steady buoyant-thermocapillary convection near an evaporating meniscus

“…31, a spherical coordinate system is used with origin at the center of the meniscus ͑see Fig. 9͒, and a shear stress balance at the interface is performed.…”

3D visualization of convection patterns in lab-on-chip with open microfluidic outlet

“…In (Zhang et al 2007) a thin film of an aqueous binary mixture in a large compartment was examined by the shadow-graph method to show the generation of convection patterns due to vertical temperature gradients. A vertical micro-channel is filled with water at a low temperature, incompatible with cell function, and the deflection of a cantilever is used to measure the thermocapillary flow rate dependent on the vapor-phase pressure (Ward and Duan 2004). Evaporation-induced toroidal convection flows of non-biocompatible solutions (ethanol, methanol, acetone, silicone oil, etc.…”