Augmentation of Natural Convection Heat Transfer In Vertical Cylindrical Annulus.(Dept.M)

Abstract

In the present work, the natural convection heat transfer through vertical cylindrical annular tubes was experimentally investigated the inner cylinder is heated electrically with constant heat flux, while the outer one is subjected to ambient air. Rayleigh number was varied between 80 and 1500 throughout this investigation Aspect ratio of 50 and radius ratio of 1 5 were used. The annular gap was provided with helical-wire-eoil rings wound around the outer surface of the inner cylinder as augmentation devices. The effects of coil pitch, ring pitch, coil diameter and wire diameter were investigated from the results of the present investigation it is concluded that the heal transfer rate increases with increase of coil diameter and wire diameter and decrease of ring pitch, while it has a peak value at coil pitch to gap thickness nearly equal 0.125. It is eoncluded (rom the experimental results that the heat transferred through the annular gap is not only by convection but also by radiation and conduction. The helical-wire-coil rings are found to enhance heat transfer by a factor of 8.25. A general eorrelation is presented in order 10 help in the design of such devices