In order to understand the characteristics of double-diffusive capillary convection in a shallow annular pool with the capillary ratio of -1

a series of three-dimensional numerical simulations were performed. The working fluid is the toluene/n-hexane mixture with the Prandtl number of 5.54 and Lewis number of 25.8. The inner cylinder is maintained at low temperature and solutal concentration while the outer cylinder is fixed at high temperature and solutal concentration. Results indicate that the double-diffusive capillary convection is steady and axisymmetric at a small thermal capillary Reynolds number. With the increase of thermal capillary Reynolds number

the flow will destabilize into unsteady flow. For the aspect ratio of 0.15

the critical thermal Reynolds number is 231.6 and the dimensionless critical frequency is 10.6. When the capillary Reynolds number exceeds the critical value

there orderly appear the two-dimensional and three-dimensional periodical oscillatory convections. Furthermore

periodical oscillatory convection

subcritical unsteady convection and three- dimensional steady convection are discovered with the variation of aspect ratio.