Experimental Study On The Effects Of Prandtl Number And Surface Evaporation On Thermocapillary-buoyancy Convection

Generally,the surface tension of working fluids could decrease with the increase of the temperature.When the temperature of the free surface is not uniform,the surface tension gradient appears,which drives the fluid flow in the liquid layer.The surface tension gradient caused by the uneven surface temperature is called thermocapillary force,and the flow driven by thermocapillary force is called thermocapillary convection.When the buoyancy effect in the liquid layer could not be neglected,both of them could be coupled to form thermocapillary-buoyancy convection.Thermocapillary–buoyancy convection is widely existed in film process,phase change,heat transfer,crystal growth,alloy solidification and other industrial production,and has an important impact on the actual process and product quality.In this thesis,the influence of Prandtl number on thermocapillary-buoyancy convection in Czochralski configuration and the effect of surface evaporation on thermocapillary-buoyancy convection in an annular pool are explored experimentally.The effects of Prandtl number,liquid depth and the surface evaporation rate on critical value of flow destabilization are discussed,respectively.The evolution of the dissipative structure on the free surface after the flow destabilization and the corresponding three-dimensional oscillation characteristics are also presented.The main results are as follows:Firstly,the coupling effect of Prandtl number and thermocapillary buoyancyconvection in Czochralski configuration subjected to a radical temperature difference is studied.It was found that with the increase of buoyancy effect and Prandtl number,the critical Marangoni number would increase,which could effectively suppress the flow instability.In the deep liquid layer,the influence of Prandtl number on the critical Marangoni number is more profound compared to that in the shallow liquid layer.When the aspect ratio of liquid pool is small and the Marangoni number exceeds one certain threshold,with the increase of the Marangoni number,the dissipative structure on the free surface gradually transits from concentric multi-roll structure to hydrothermal waves.High Prandtl number and buoyancy effect both inhibit the hydrothermal waves and promote the appearance of concentric multi-roll structure;when the aspect ratio is large,for the small Prandtl number fluids,the flow patterns on the free surface transit from bud-shaped structure to straight spokes with the increase of liquid depths,and the spoke number could decrease gradually with the increase of Marangoni number.When Prandtl number is large,there is no bud-shaped structure on the free surface.In addition,when considering the effect of surface evaporation,it was found that the increase of the average temperature,Marangoni number and the liquid layer depth could accelerate the evaporation rate on the free surface.However,due to the weak buoyancy effect,the variation of the liquid depth has a slight effect on the average evaporation rate,and the surface evaporation plays an active role in suppressing the flow instability.When the flow destabilizes and the surface evaporation is relatively weak,with the increase of the evaporation rate,the hydrothermal waves on the free surface become blurred and finally disappear.The corresponding temperature oscillatory amplitude and the fundamental frequency would decrease.At the same time,the surface fluctuation(physical waves)and temperature oscillation(hydrothermal waves)have the same oscillation frequency.When the surface evaporation is strong,the temperature oscillation amplitude decreases greatly,and the surface fluctuation becomes disordered and chaotic due to the effect of vapor recoil.