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M. Kawaji1, S. Simic1, G. Psofogiannakis1, and S. Yoda2
1Dept. of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario M5S 3E5, Canada
2National Space Development Agency of Japan, 2-1-1 Sengen, Tsukuba, 305-8505, Japan
Marangoni convection experiments have been conducted with acetone (Pr = 4.3) and methanol (Pr = 6.8) liquid bridges to investigate the flow structures and temperature fields during transition from steady to oscillatory convection in a half floating zone. In particular, the effects of liquid evaporation on the onset of oscillatory flow were investigated by performing the experiments under different evaporation rates. The effects of horizontal vibrations with small amplitudes were also investigated to understand the magnitude of free surface oscillation induced by such vibrations. In all of the experiments, a test section with 7.0 mm diameter disks was constructed to enable reduction of the evaporation rate by a factor of 4 to 5 by providing a tight seal between the inside and outside of a quartz tube fitted around the liquid bridge. The liquid bridge height was changed from 2.0 mm to 4.5 mm, covering aspect ratios from 0.55 up to 1.3.
For acetone under reduced evaporation rates, the transition from steady to oscillatory convection was found to occur at disk temperature differences of 0.5 ~ 1.1 K, much less than 1.6 ~ 2.3 K for the partially open system. This difference in the critical temperature difference and corresponding critical Marangoni number data clearly showed the stabilizing effect of surface evaporation. The surface temperature profile for the partially open system also showed temperature inversion above the lower disk, where a cold region with a temperature below the cold disk temperature existed, as predicted by the linear stability analysis. On the other hand, the evaporation effect was found to be quite small for methanol, and the critical Marangoni numbers obtained agreed with the previous data for NaNO3 with a similar Prandtl number. For both acetone and methanol, the critical Marangoni numbers obtained for an aspect ratio of unity agreed with the predictions of a linear stability analysis, which also correctly predicted the stabilization effect of surface evaporation and an increase in the critical Marangoni number.
The effects of forced vibration on the stability of a liquid bridge surface was also investigated by subjecting the acetone liquid bridge of 7.0 mm diameter to horizontal vibrations under an isothermal condition. The acceleration level was kept at 5 mG but the vibration frequency and amplitude were varied in the experiments. The response of the liquid bridge surface was recorded by a video camera and the surface motion was analyzed for different volume and aspect ratios. The results indicated existence of resonance frequencies at which the surface oscillates with large amplitudes, and a strong effect of the volume ratio on the oscillation amplitude. The maximum surface oscillation amplitude as high as 250 mm was recorded at applied vibration frequencies of 4 and 12 Hz. The resonance frequencies at which the liquid bridge vibrated with large amplitudes decreased linearly with the height of the liquid bridge.
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