Abstract
Laminar natural convection heat transfer from vertical hollow polygonal cylinders with a wide range of cross-sectional areas is investigated. The buoyancy-driven three-dimensional flow around hollow polygonal cylinders immersed in ambient air with equal outer and inner surface temperatures is analyzed. Results of the mean Nusselt number for the outer and inner surfaces are obtained by varying a number of key parameters that include the Rayleigh number based on the cylinder height, a non-dimensional cross-sectional area, and the number of sides of the polygon. The study shows that the heat transfer rate from the inner surface decreases as the polygon area decreases, whereas the heat transfer rate on the outer surface increases as the polygon area decreases. It has also been found that the behavior of the buoyancy-driven flow in the vicinity of the outer surface is fundamentally different than that near the inner surface. Additional details about this fundamental difference are presented in the Results and Discussion section of the paper. New correlations to calculate the average velocity at the exit surface of the cylinder inner core and the mean Nusselt number for both the outer and inner surfaces have also been developed. Also, correlations have been developed for selecting the optimal cross-sectional area for purposes of identifying the regions where the thermal and velocity boundary layers overlap within the inner core of the cylinder.
