Comparison of correlations and experiment in opposing flow, mixed convection heat transfer in a vertical tube with Grashof number variation

Mixed convection heat transfer in a vertical tube with opposing flow (downflow heating) was studied experimentally for Reynolds numbers ranging from about 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. In the turbulent region at Re > 10 000 and in the asymptotic region at Re < 4000, the data agreed well with published correlations that omit buoyancy contribution. The 'mixed' convection region existed at Re between 4000 and 10 000. As the Grashof number was reduced in this Re range, the Nusselt number was also reduced. The Jackson and Fewster correlation and the Swanson and Catton correlation developed for opposing mixed convection flows in vertical conduits both predicted the data quite well, except near and into the asymptote region, for which these equations were not designed.