Design Criterion for the Heat-Transfer Coefficient in Opposing Flow, Mixed Convection Heat Transfer in a Vertical Tube

Mixed convection heat transfer in a vertical tube with opposing flow (downflow heating) was studied experimentally for Reynolds numbers ranging from about 1000 to 30 000 at constant Grashof numbers ranging about 1 1 / 2 orders of magnitude under constant wall temperature (CWT) conditions. Three correlations developed for opposing mixed convection flows in vertical conduits predicted the data reasonably well, except near and into the asymptote region for which these equations were not designed. A critical Reynolds number is developed here, above which these equations can be used for design purposes regardless of the boundary condition. Below Re crit, the correlations, the asymptote equation should be used for the CWT boundary condition, which is more prevalent in process situations than the uniform heat flux (UHF) boundary condition.