Oscillatory thermocapillary convection in open cylindrical annuli. Part 1. Experiments under microgravity

We report results from microgravity experiments on thermocapillary convection in open annuli with outer radius R-0 = 40 mm and inner radius R-i = 20 mm of various aspect ratios Ar. The measurements are from more than 230 equilibrated states in the Ar-Marangoni-number space. We found time-independent and oscillatory states and report some selected oscillation and Fourier spectra from thermocouple measurements. We measured the critical temperature difference DeltaT(c) for the onset of temperature oscillations in the range 1 less than or equal to Ar less than or equal to 8. We report supercritical oscillation periods and attribute the oscillations in the larger Ar range to hydrothermal waves. This conclusion is supported by the values of the oscillation periods and of the critical Marangoni numbers in that Ar range. The hydrothermal waves exhibit an internal corotating multicellular pattern. For the smaller Ar and near the threshold we report m-fold temperature patterns on the free surface with m decreasing for decreasing Ar. At 4DeltaT(c) these patterns become very irregular. Most of the findings are in accordance with the numerical results reported in Part 2 (Sim et al. (2003)). The experimental DeltaT(c) are higher and the experimental periods tau(c) are smaller than the numerical values for Biot number Bi = 0. However, analysis of the experimental free-surface thermal boundary conditions shows that there was heat input to the free surface. Good agreement with numerical results for DeltaT(c) and tau(c) is obtained with Bi not equal 0 (heat input).