Aiding and opposing mixed convection heat transfer in a vertical tube: Loss of boundary condition at different Grashof numbers

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 about 1000 to 30,000 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 data corroborate earlier studies that found the way in which Nu is reduced is not identical for aiding and opposing flows. Loss of boundary condition significantly affects normal trends in the data at low Reynolds numbers and is caused by a temperature profile in the wall interacting with low driving force temperature difference.