Flow pattern transition of thermal–solutal capillary convection with the capillary ratio of −1 in a shallow annular pool

• Thermal–solutal capillary convection in annular pool is numerically simulated.
• Four kinds of flow patterns at the capillary ratio of −1 are exhibited.
• The basic characteristics of the various flow patterns are analyzed.
• Effect of the local capillary ratio on the flow pattern is discussed.

In order to understand the flow pattern transition processes, a series of three-dimensional numerical simulations for thermal–solutal capillary convection in an annular pool subjected to simultaneous radial temperature and solutal concentration gradients were conducted. The capillary ratio was fixed at −1. The working fluid was the toluene/n-hexane mixture with the Prandtl number of 5.54 and the Schmidt number of 142.8. Results show that there exists a quiescent conductive state in the liquid pool when thermocapillary Reynolds number is small. With the increase of thermocapillary Reynolds number, the flow bifurcates orderly into three different kinds of the oscillatory flows, i.e., the travelling wave, the combined travelling wave with stationary wave, and the vibrating spoke pattern. The oscillatory frequency increases monotonously for the travelling wave and the combined travelling wave with stationary wave. Furthermore, the wave number is independent of the thermocapillary Reynolds number. These multiple complicated flow patterns are due to the different distributions of the local capillary ratio along the free surface.