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NUMERICAL INVESTIGATION OF MARANGONI CONVECTION AROUND A BUBBLE

DOI: 10.1615/ICHMT.2008.CHT.1300
16 pages

Seamus M. O'Shaughnessy
Dept. of Mechanical & Manufacturing Engineering, The University of Dublin, Trinity College, College Green, Dublin 2, Ireland

Abstrakt

The liquid motion generated by surface tension variation, termed the thermocapillary or Marangoni effect, and its contribution to boiling heat transfer is not yet well understood. This convection has previously been overlooked as a significant mode of heat transfer, because under terrestrial conditions, it is superimposed by the strong buoyancy convection, making it difficult to obtain quantitative experimental results. The thermocapillary flow caused by a bubble situated under a wall immersed in a liquid layer subject to a stable temperature gradient is therefore investigated numerically. The accurate description of the flow driven by surface tension gradients along the liquid-vapour interface required the solution of the nonlinear equations of free-surface hydrodynamics. For this problem, the procedure involved solving simultaneously the coupled equations of fluid mechanics and heat transfer using the finite difference numerical technique. The model is set up so that the thermocapillary and gravitational forces, or buoyancy, act to oppose one another. A hemispherical 1.0 millimetre radius bubble is placed on a heated wall in a liquid layer of depth 5.0 millimetres. From experimental data, it is known that the flow structure is symmetrical about the vertical bubble axis at low Marangoni numbers, so an axisymmetric geometry was used. In order to analyse the resulting flow field and in particular the wall heat transfer, the influence of Marangoni and Rayleigh numbers was investigated. To study the affect of Marangoni number, simulations were carried out with conditions Pr = 250, Ra = 0.68 and Marangoni numbers in the range 100 ≤ Ma ≤ 600. Similar tests were conducted for Ma = 600, Pr = 250, and 0 ≤ Ra ≤ 1.38. For the ranges tested, it was found that the Marangoni caused no significant change in the flow pattern, instead influencing the strength of the primary vortex and intensity of the jet-like flow. An increase in Rayleigh number modifies the flow pattern by inhibiting the jet flow from the bubble apex and reducing the vertical distance of the primary vortex. These results are in good agreement with those seen in previous numerical studies. As expected, an increase in Marangoni number also causes an increase in heat transfer. Increasing the Rayleigh has a detrimental effect on wall heat transfer.

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