Numerical Study On Thermocapillary Convection Of High Prandtl Number Fluid

Convective motion induced by local variations of surface tension along a liquid free surface is called Marangoni convection. Although such variations can be caused by differences in temperature or composition, our main interest here is temperature-induced convection, which is often called thermocapillary convection. In a terrestrial environment, Marangoni convection is usually overshadowed by buoyancy-driven flow. One of the purposes of the space environment utilization is the processing of a new material, because in the reduced gravity environment of space, buoyancy is greatly reduced and Marangoni convection could become very important. In applications such as crystal growth from melts and two-phase flow with heat transfer, Marangoni flow is known to play an important role. Since the floating-zone crystal growth process is considered to be a promising method of obtaining high quality crystals in microgravity, Marangoni convection in floating-zone melts is an important subject.One important feature of Marangoni convection in the liquid bridge configuration is a transition from steady to oscillatory flow. Since oscillations have significant implications to crystal growth, it is important to understand how and when the transition occurs. The mechanism for low Prandtl number fluids is well understood, but that for high Prandtl fluids is still being debated. In the high Prandtl range, although the effects of time-depending g-jitter, the heating rate or constant temperature difference have subjected to numerous experimental and numerical investigations, few of them have taken the dynamic free surface deformation and ambient air effects into consideration.In the present paper, the numerical simulation has been conducted to investigate the mechanism of flow structure, temperature field and oscillation status in a liquid bridge of high Prandtl number fluid under reduced gravity, microgravity and horizontal g-jitter conditions by taking both the dynamic free surface deformation and ambient air effects into consideration. The Navier-Stokes equations coupled with the energy conservation equation are solved on a staggered grid, and the free surface deformation is captured by introducing the mass conserving level set approach, which allows us to adopt larger density and viscosity ratios between the liquid bridge and the ambient gas-phase.In the present work, three main studies can be concluded as follows:1. Studies on thermocapillary convection of high Prandtl number fluid in zero-gravity environment;2. Effects of different horizontal g-jitter and gravity levels on the flow structure, temperature distribution and dynamic free surface deformation have been investigated;3. Oscillation of thermocapillary convection. The corresponding conclusions can be drawn from the present work:1. A pair of vortexes is generated due to the thermocapillary convection in the liquid bridge initially; and the recirculating flow generates a radial convection, which tends to make the bulk fluid temperature distribution rather uniform near the free surface;2. It is represented that the amplitude of the horizontal surface vibration decreases gradually, and the thermocapillary convection inside the liquid bridge starts to turn into a steady state after the initial period;3. For the case of2cSt silicone oil, when the temperature difference exceeds the critical one Tc, oscillation inside the liquid bridge occurs. The critical Marangoni number we obtained is Mac=43960corresponding to Tc=49.87℃...