MIXED CONVECTION LAMINAR-FLOW AND HEAT-TRANSFER OF LIQUIDS IN ISOTHERMAL HORIZONTAL CIRCULAR DUCTS

Numerical analysis of thermally developing and simultaneously developi ng mixed convection flow and heat transfer with variable viscosity in an isothermal horizontal tube has been carried out. Parametric computa tions were performed to investigate the effect of inlet Prandtl number , inlet Rayleigh number, wall-to-inlet temperature difference, and inl et axial velocity profile on the Nusselt numbers and apparent friction factors for both heating and cooling conditions. The results indicate that the effect of variable viscosity is more pronounced on the frict ion factor than on Nusselt numbers. in addition, the effect of inlet R ayleigh number and inlet velocity profile on the Nusselt numbers and f riction factors exists only in the near-inlet region. A parameter foun d by scaling analysis was used to empirically correlate the. computed Nusselt number and the friction factor data and the available experime ntal Nusselt number data for both thermally developing and simultaneou sly developing flow and heat transfer. The developed correlations are more accurate, have wider ranges of applicability than those available in the literature, ana should be of much use to designers.