Natural convection heat transfer and fluid flow around a thick hollow vertical cylinder suspended in air: A numerical approach

The present study reports numerical investigation on natural convection heat transfer and fluid flow from a thick hollow vertical cylinder suspended in air within laminar regime for a Rayleigh number (Ra) of 104 to 108. Numerical simulations have been performed by changing the length to outer diameter ratio (L/Do) of the cylinder from 0.2 to 20 and thickness ratio (Di/Do) from 0.5 to 0.9. Variation of average Nusselt number (Nu) and local Nusselt number with the above parameters has been elucidated graphically. Subsequently, the influences of Ra, L/Do, Di/Do on mass-flow rate and velocity profile inside the cylinder has also been investigated. Temperature contour and velocity vector plot around the cylinder have been represented pictorially to describe the thermal and velocity field around the vertical thick hollow cylinder. It has been observed that the average Nu of a thick hollow cylinder increases with L/Do, attains a peak and then decreases to a minimum and then again increases with L/Do. This phenomenon is very well observed at high Ra of 106 compared to lower Ra of 104. The Nu of the inner surface is found to be always lower compared to the outer surface and as L/Do increases beyond 8 the inner surface Nu falls drastically at high Ra compared to lower Ra. It has also been observed that the non-dimensional mass-flow rate due to buoyancy driven flow inside the thick hollow cylinder increases with L/Do, attains a peak and then decreases for all Ra. Cooling curve and a correlation for Nusselt number as a function of geometrical and thermal parameter have been presented which are useful for industrial purposes.