Effects of thermal boundary condition on buoyancy driven transitional air flow in a vertical cylinder heated from below

A three-dimensional unsteady numerical computation was carried out here to in vestigate the effects of the thermal boundary condition on the con vection flow in a vertical, bottom heated cylinder containing air. The thermal condition at the sidewall of the cylinder is assumed to be thermally well insulated or perfectly conducting. Results were obtained for air in a ( ) cylinder of finite aspect ratio G s 2 for various thermal Rayleigh numbers. The predicted results indicate that the flow in the sidewall insulated cylinder is highly asymmetric e ven at steady state and contains multicellular vortices. The flow formation processes leading to the above structures are relati vely complicated. In the early transient two axisymmetric circular vortex rolls, one on top of another, appear. Then the rolls merge asymmetrically. In the late stage the flow deflection by the cylinder top and bottom results in a very complex flow. In the cylinder with a perfectly conducting sidewall the transition from a steady to a time dependent flow is subcritical. Howe ver, in the cylinder with an insulated sidewall the flow transition is supercritical.