Thermocapillary flow instabilities in an annulus under microgravity - Results of the experiment magia

Abstract

We investigated thermocapillary flow in an annular gap with outer heated container of radius R 1 = 40 mm and inner cooled cylinder of R 2 = 20 mm and with an adjustable height h, 2.5 ≤ h ≤ 20 mm. The gap was filled flat up to the rim with the 0.65 cSt silicone oil hexamethyldisiloxane (Prandtl number Pr = 6.7). The temperature differences △T, 0 K ≤ △T ≤ 40 K between outer and inner wall generated thermocapillary flow in the free upper surface and various flow structures have been observed under microgravity. We identified hydrothermal waves for small hand more complicated oscillations for larger h. For small h and small △T the multiroll structure was visible via IR-images of the free surface: concentric steady convection rolls with the same sense of rotation, embedded into the main thermocapillary roll. We measured the critical Marangoni number Ma c for the transition to time-dependent flow in the aspect ratio range A = h/(R 1 - R 2 ), 0.125 ≤ A ≤ 1, where it was virtually constant Ma c ≈ 5 10 4 . We report and discuss a steady temperature asymmetry, changing its direction from time to time, as recorded by the IR-camera. This symmetry breaking is most probably due to slowly changing residual acceleration in the satellite. © 2002 COSPAR. Published by Elsevier Ltd. All rights reserved.