Laminar and Turbulent Natural Convective Flow in a High Aspect Ratio Enclosure

摘要

The transition from laminar to turbulent natural convective flow in tall rectangular enclosures has been numerically studied. The aspect ratios of the enclosures considered are similar to those which arise in conventional window-blind systems. The flow in the enclosure has been assumed to be steady and fluid properties have been assumed constant except for the density change with temperature that gives rise to the buoyancy forces, this being dealt with using the Boussinesq approach. The solution has the Rayleigh number, the Prandtl number and the Aspect Ratio of the enclosure (height-to-width ratio) as parameters. Results have only been obtained for a Prandtl number of 0.73. Aspect ratios of between 10 and 20 and Rayleigh numbers based on enclosure height of between 105 and 1012 have been considered. Results have been obtained first by assuming that the flow under all conditions is laminar and then by obtaining results for the same conditions by incorporating a k-epsilon turbulence model that accounts for full buoyancy force effects into the solution process. The solutions were obtained using the commercial cfd solver FLUENT. By comparing the mean heat transfer rates given by the two solutions, the conditions under which turbulence effects become important can be determined.