Marangoni heat transfer in subcooled nucleate pool boiling

The surface tension driven flow in the liquid vicinity of gas bubbles on a heated wall and its contribution to the heat transfer are investigated experimentally in a configuration where surface tension force and buoyancy forces oppose one another. This liquid flow caused by the temperature gradient along the interface is called thermocapillary or thermal Marangoni convection. The studies were made with silicone oils of different viscosities so that a wide range of dimensionless numbers were encountered. The velocity fields are determined from the motion of carbon particles in the meridian plane of the bubble. The influence of the temperature gradient, the oil viscosity, and the bubble shape on the profiles along the interface and in the direction normal to the interface is analyzed. The temperature field is determined by holographic interferometry. For the axisymmetric problem, the interferograms are evaluated by solving the Abel-integral equation. From the isotherms, the temperature distribution along the bubble surface and in the liquid beneath the bubble is measured. To quantify the contribution of thermocapillarity to the heat transfer, the heat flux transferred by thermocapillarity is measured. A heat exchange law giving the increase in heat flux due to Marangoni convection in comparison to themore » conductive regime is proposed.« less

Experimental analysis of the heat transfer induced by thermocapillary convection around a bubble

Thermocapillary convection is studied experimentally using particle-image-velocimetry for flow visualization and analysis This method offers the advantage of measuring the entire flow field (velocity field, streamlines etc) in a selected plane within the fluid at a given instant of time in contrast to point by point methods like laser-Doppler-velocimetry (LDV) The paper describes the method and presents quantitative results for different Marangoni numbers

Particle-image-velocimetry applied to thermocapillary convection

In this article, we study steady and oscillatory thermocapillary and natural convective flows generated by a bubble on a heated solid surface. The dynamic characteristics of the time-dependent convection are captured using a combined numerical-experimental approach. The index of refraction fringe distribution patterns constructed numerically by taking an inverse Abel transform of the computed temperature fields are compared directly to the experimental Wollaston prism (WP) interferograms for both steady-state and oscillatory convection. The agreement between numerical predictions and experimental measurements is excellent in all cases. It is shown that below the critical Marangoni number, steady-state conditions are attainable. With increasing Ma, there is a complete transition from steady state up to a final nonperiodic fluctuating flow regime through several complicated symmetric and asymmetric oscillatory states. The most prevalent oscillatory mode corresponds to a symmetric up and down fluctuation of ...

Steady and oscillatory thermocapillary convection generated by a bubble

Heater size and heater aspect ratio effects on subcooled pool boiling heat transfer in low-g

Temperature and velocity fields near an air-bubble in silicon-oil under a heated horizontal wall were investigated. The studies were made with silicon oils of different viscosities so that a wide range of Marangoni numbers was encountered. Schlieren interferograms were taken to analyse the temperature field. For the axisymmetric problem the Abel integral equation was solved numerically by using a coefficient procedure. From the recorded temperature distributions isotherms, radial temperature lines and the temperature along the bubble surface were determined graphically. At low Prandtl- and high Marangoni-numbers an oscillatory instability was observed. The flow field was investigated with neutral-buoyant light scattering glass spheres by observation of the meridian plane of the bubble with a thin light-sheet. The convective mechanism of the flow was recorded photographically by taking pictures with various exposure times. Thus a qualitative and quantitative description of the flow was possible.

Temperature and velocity fields due to surface tension driven flow

Many experimental investigations and numerical analyses on the thermocapillary flow in the surroundings of hemispherical bubbles under a horizontal heated wall have been carried out, mainly to clarify the role of this convection in boiling heat transfer (McGrew et al. 1966, Gaddis 1968, Larkin 1970, Kao & Kenning 1972 and Huplik & Raithby 1972). Recently the interest on the thermocapillary flow in the vicinity of bubbles attached to a heated or a cooled solid wall has increased, particulary in context with microgravity applications, e.g. problems of chemical engineering, material sciences and liquid management in the space-laboratory and spacecraft.

Ch. H. Chun

Papers

Temperature and velocity fields due to surface tension driven flow

Steady and Oscillating Convection Phenomena Caused by an Air Bubble Beneath a Heated Wall