Natural Convection Heat Transfer in Heated Vertical Tubes

Experimental and theoretical investigation of natural convection heat transfer in heated vertical tubes dissipating heat from the internal surface is presented. The tubes are open-ended and circular in cross section. The test section is electrically heated by imposing the circumferentially and axially constant wall heat flux. Heat transfer experiment is carried out for four different channels of different length but having same diameter and thickness. Studies are also carried out on intensified channels with discrete rings of rectangular section having different thickness and spacing. A systematic experimental database for the local steady state natural convection heat transfer behaviour is obtained. The effects of L/D ratio, ring thickness, ring spacing and wall heating conditions on the local steady-state heat transfer phenomena are studied. A correlation is proposed for average Nusselt number as a function of modified Rayleigh number and another correlation is also developed for modified Rayleigh number as a function of modified Reynolds number. It is observed that the correlations predict the data with reasonable accuracy.