The linear stability of mixed convection in a vertical annulus

The linear stability characteristics of non-isothermal flow in vertical annuli has been determined for two geometries. The analysis demonstrates that a fully developed mixed-convection flow in a vertical annulus is unstable in certain regions of an appropriate parameter space. Consequently, parallel countercurrent flows, predicted by previous numerical models and commonly used by engineers, are often physically unrealizable and can be observed experimentally only in special circumstances. In addition, it is found that the most unstable disturbances are often asymmetric in the parameter range of practical interest. The instability behaviour was also found to depend on whether the inner or outer cylinder was heated.