Buoyancy suppression in gases at high temperatures

The computational fluid dynamics code FLUENT was used to study Rayleigh instability at large temperature differences in a sealed gas-filled enclosure with a cold top wall and a heated bottom wall (Bénard problem). Both steady state and transient calculations were performed. Instability boundaries depending on the geometry, temperature, and pressure were defined that showed the system tended to become more unstable when the hot-wall temperature increased beyond a certain level, a result of the dampening effect of gas viscosity at higher temperatures. Results also showed that the eventual system stability depended on the final pressure reached at steady state, regardless of how fast the bottom-wall temperature was ramped up to minimize time spent in the unstable region of fluid motion. It was shown that the final system state can differ depending on whether results are obtained via a steady-state or transient calculation, demonstrating that the history of the flow structure development and corresponding temperature fields in this type of system has a profound effect on the final state. Finally, changes in the slope of the pressure-versus-time curve were found to be good indicators of flow pattern changes, and can be a convenient experimental tool for diagnosing the expected changes in flow behavior in such systems.