Comparison of aiding and opposing mixed convection heat transfer in a vertical tube with Grashof number variation

Mixed convection heat transfer in a vertical tube with aiding and opposing flows (upflow and downflow heating) was studied experimentally for Reynolds numbers ranging from 700 to 25,000 at constant Grashof number under constant wall temperature (CWT) conditions. The Grashof number was varied independently by adjusting steam pressure in the jacket of a vertical, double-pipe heat exchanger. The mixed convection region existed at Re between about 4,000 and 10,000 (this range was Gr dependent). As the Grashof number was reduced in this Re range, the Nusselt number was also reduced. The data show that the way in which Nu is reduced is not identical for aiding and opposing flows. Comparison of the present data with literature on the basis of buoyancy parameter show good agreement, except for the asymptote region.