Abstract
Laminar convection in vertical systems is affected by buoyancy so that it becomes a combination of natural and forced convection. In downflow the buoyancy forces oppose the main flow. The problem has previously been studied for flows in vertical circular tubes and is here extended to flows in vertical annuli with uniformly heated inner surfaces. This configuration retains the simplicity desirable for both analysis and experiment, but also represents quite closely certain practical systems such as flow in nuclear reactors in which cylindrical components are placed axially in vertical coolant channels.
Buoyancy forces opposing the main flow cause the fluid to be slowed down near the heat transfer surface until it is eventually reversed. In simple theory this flow reversal is associated with a decrease in heat transfer performance.
An experimental study has been performed in a vertical annulus of ¾ in inner and 2¼ in outer diameter using water. Flow visualization by dye injection indicates that while small reversed flows may occur in a stable form, higher buoyancy forces give rise to radial components of the flow. These radial components cause the reversed flow regime to become unsteady and the Nusselt numbers to be higher than those predicted by laminar convection analysis.
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