OSCILLATORY CONVECTION DUE TO COMBINED BUOYANCY AND THERMOCAPILLARITY

An experimental study is performed on oscillatory convection due to co mbined thermocapillarity and buoyancy in open cylindrical containers, The fluid in the containers is heated by a heating rod placed along th e centerline of the containers, Various container diameters, ranging f rom 0.39 to 3.1 cm, are used to change the effect of buoyancy on stead y and oscillatory convection. The container aspect ratio (height/radiu s) ranges from 0.75 to 2 and the heater diameter is 10% of the contain er diameter in all cases, The test fluids are 1- and 2-cS silicone oil s (Prandtl number = 13 and 27 at 25 degrees C). The velocity field is studied by how visualization, whereas the temperature field is measure d by a thermocouple probe and thermography, The onset of oscillations and the oscillation frequency are investigated under various condition s, It is found that the basic flow structure is mainly unicellular and thermocapillarity is important for the oscillations in all of the tes ts. When thermocapillarity is dominant, the Marangoni number is not su fficient to describe the onset conditions. When buoyancy is not neglig ible, it tends to squeeze the primary cell by thermal stratification a nd the dynamic Bond number is the parameter characterizing the transit ion.