Experimental study on transition to oscillatory thermocapillary flow in a low Prandtl number liquid bridge

Abstract

The transition behavior from stationary to oscillatory thermocapillary flow was experimentally studied with a liquid bridge of molten tin (Prandtl number Pr=0.01) of 1.5 mm in radius. The experimental difficulties such as preventing oxidation of the melt surface and detecting a small amplitude of surface temperature fluctuation without any external disturbances on the flow field were successfully solved by employing a high vacuum condition, ion etching to remove the oxide film on the surface and a high resolution radiation thermometer. A temperature fluctuation was first observed at a Marangoni number Ma of 43.3. Frequency and amplitude of the fluctuation were 0.08 Hz and 0.3 K, respectively, which is significantly larger than temperature resolution of the radiation thermometer used. The temperature oscillation developed with increasing temperature difference imposed between the upper and lower ends of the liquid column and then observed a standing wave of 0.42 Hz with very strong amplitude at a Marangoni number of 91. The flow transition was directly proved by a surface flow visualization experiment. Tin-oxide particles, which were pre-mixed with molten tin, were observed to repeat oscillatory movement along the circumferential direction with 0.5 s interval after the onset of the oscillation. Those results are the first and clear experimental evidence for the transition to oscillatory flow in a low Pr fluid by a non-contact diagnostic, which has never been observed yet. Validity of the experimentally determined critical Marangoni number and the frequency of the standing wave were discussed by comparing with a numerical result.