Mixed convection with flow reversal in the thermal entrance region of horizontal and vertical pipes

Abstract Numerical analyses have been conducted for combined free and forced laminar convection flow at low Peclet numbers in the thermal entrance region of horizontal and vertical pipes. Effects of both buoyancy force and axial conduction on the hydrodynamic and heat transfer characteristics are systematically investigated, and numerical results are extensively presented for velocity and temperature profiles, distributions of the Nusselt number and wall shear stress in both horizontal and vertical pipes. In a horizontal pipe, the secondary flow pattern has already developed at the beginning of heating and a reverse flow occurs near the pipe top as the axial conduction and buoyancy effects become large. Consequently, the local Nusselt number at the pipe top and then the circumferential average Nusselt number decrease with increasing secondary flow. The regime of reverse flow is clearly identified in the Pe−Ra coordinates. In a vertical pipe, flow reversals are observed at the pipe center in the heating case and near the wall in the cooling case at relatively high | Gr Re |. The regime of reverse flow is identified for both heating and cooling cases in the Pe−| Gr Re | coordinates.