Experimental and theoretical investigation of natural convection heat transfer in heated vertical tubes

Abstract

newlineExperimental 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. newlineA 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. It is observed that average heat transfer rate from the internal surfaces of a heated vertical pipe increases with providing discrete rings. Average heat transfer rate also increases with increasing the thickness of the rings up to a certain limit, beyond which it decreases. Similar observations for the average heat transfer rate are also found for the number of rings which shows that average heat transfer rate increases with increasing the number of rings i.e. reducing the spacing between the rings up to a certain value of spacing, but further reduction in spacing, reduces the heat transfer rate from the internal wall to air. newlineThe theoretical results such asaxial variations of local wall temperatures, temperature profiles at the channel exit for different heat fluxes and for different L/D ratio, temperature profiles at the channel exit for tubes with internal rings for different heat fluxes and L/D ratio are compared with the experimental data and it was found to be in good agreement with each other.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.