Experiments on instabilities of thermocapillary convection in shallow annular liquid layers

Abstract

An experimental study of thermocapillary convection of silicone oil (the Prandtl number Pr ≈ 18) was conducted in a shallow annular cavity with outer radius Ro = 55 mm and inner radius Ri = 15 or 27.5 mm for liquid heights H in the range 1–3 mm. The liquid layer is heated from the outer cylindrical wall and cooled at the inner wall. The experiments were performed over a wide range of Marangoni numbers, 400 < Ma < 10000. The measurement of surface temperatures and observation of instability structures were made by using an IR thermography technique. As Ma exceeded a critical value, hydrothermal waves were observed for thin liquid layers, H ⩽ 2.5 mm, of both the inner radii. These waves are characterized by curved spoke patterns propagating in the azimuthal direction. For thicker layers, H > 2.5 mm, multicell patterns were dominant. At low Ma, the multicell patterns were steady, toroidal rolls. As Ma was increased, the rolls began to rotate around the centre of the cavity and unsteady longitudinal rolls appeared. In the range 2.5 ⩽ H ⩽ 2.8 mm, hydrothermal waves and multicell coexisted. The transition map is presented in terms of Ma and Bod (the dynamic Bond number).