Temperature and velocity fields due to surface tension driven flow

Abstract

Abstract. Temperature and velocity fields near an air-bubble in silicon-oil under a heated horizontal wall were investigated. The T* studies were made with silicon oils of different viscosities so that a t wide range of Marangoni numbers was encountered. Schlieren interferograms were taken to analyse the temperature field. For the u axisymmetric problem the Abel integral equation was solved numerically by using a coefficient procedure. From the recorded temx perature distributions isotherms, radial temperature lines and the y temperature along the bubble surface were determined graphically, z At low Prandtl-and high Marangoni-numbers an oscillatory instaz~ bility was observed. The flow field was investigated with neutralz* 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.

Heat transfer near an isolated hemispherical gas bubble: The combined influence of thermocapillarity and buoyancy

“…The influence of the temperature gradient, oil viscosity and bubble shape was analysed. Contrary to the experimental observation in [20], thermocapillarity was seen to be active along the entire contour of the bubble and was dominant over the buoyancy-driven convection in the immediate vicinity of the bubble surface. Weak secondary counter-rotating vortices beneath the primary vortices were observed due to the interaction of surface tension and gravity forces.…”

“…Indeed, the fluid velocity increases with increasing Ma. However, with increasing Ma the primary vortex is squeezed and pushed closer towards the bubble cap, a phenomenon also observed by Raake et al [20]. Interestingly, the stagnation points also move toward the bubble with increasing Ma, and an elongation of both vortices is observed.…”

“…This vortex is caused by the thermocapillary effect and acts to re-circulate cold fluid from the lower regions of the test cell toward the heated wall. These vortices have been observed experimentally by several authors [4,7,9,20].…”

“…The opposing influences provoke a counter-rotating sheardriven secondary vortex as depicted in Figure 8. This behaviour has been observed in [20,22,32]. From the figure it is evident that with increasing gravitational acceleration, and therefore increasing buoyancy effect, the thermocapillary effect becomes progressively more confined to a region close to the bubble.…”

“…The silicone oil is seeded with hollow glass spheres of mean diameter in the range of 9-13μm, and a density of 1100 +/-50 kg/m 3 . These seeding particles are recommended for oil and water flows [28], and have been used in similar studies [20]. Depending on the flow velocity and the factor of magnification of the camera lens, the delay of the laser pulses is chosen such that adequate displacement of the particle images on the CCD camera is obtained.…”