Effects of free surface deformation and heat transfer on thermocapillary flows of high Prandtl number fluids.

摘要

The effects of free surface deformation and heat transfer on the oscillatory thermocapillary flow of high Prandtl number fluids are investigated in the liquid bridge configuration. The focus of this work is a better understanding of the physical mechanism of oscillatory thermocapillary flow of high Prandtl number fluids.; By changing the aspect ratio and/or the diameter ratio of the bridges, it is found that the onset of oscillations is highly dependent on the shape of the bridge and there exist two branches, fat and slender branches, which have different features. For a slender bridge, the critical Marangoni number (Ma_cr) predicts the onset of oscillations well. But in the fat branch, Ma_cr alone cannot determine the flow instability.; With a sensitive laser displacement meter and a high magnification flow visualization system, the free surface motions in an oscillatory thermocapillary flow are investigated. It is noticed that the free surface deforms periodically and the magnitude of the deformation is within a few microns. The oscillation amplitude changes with locations, and both the oscillation amplitude and frequency increase with increasing applied temperature difference. The existence of an organized free surface wave is shown.; The free surface heat transfer is studied by varying the cold wall temperature or changing the airflow. The air motion is also simulated numerically to determine its magnitude and to compute the free surface heat transfer. A relation between the critical Marangoni number and the free surface heat loss is found: for fat bridges, Ma_cr decreases with increasing free surface heat loss although the free surface heat loss is rather small; the free surface heat loss has no effect for slender bridges.; The above results show that the oscillation phenomenon in fat bridges of high Prandtl number fluids is not a result of linear instability, but it is a non-linear process in which dynamic free surface deformation, albeit small, play an important role. The small free surface heat loss interacts with the small free surface deformation and affects the critical conditions significantly. The oscillation mechanism involving the heat loss is discussed.