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

Abstract

We report results from microgravity experiments on thermocapillary convection in open annuli with outer radius $R_{o}{\,=\,}40\,\hbox{mm}$ and inner radius $R_{i}{\,=\,}20\,\hbox{mm}$ 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 $\uDelta T^{\hspace*{1pt}c}$ for the onset of temperature oscillations in the range $1{\,\le\,}Ar{\,\le\,}8$. 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 $4\uDelta T^{\hspace*{1pt}c}$ 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 $\uDelta T^{\hspace*{1pt}c}$ 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 $\uDelta T^{\hspace*{1pt}c}$ and $\tau^{\hspace*{1pt}c}$ is obtained with $Bi{\,\ne\,}0$ (heat input).