Numerical Analysis of Transient Conjugated Downward Laminar Mixed Convection in a Vertical Pipe

Abstract

Transient conjugate conduction and laminar mixed convection for downward flow in a vertical pipe with a uniformly heated zone, preceded and followed by adiabatic zones, is investigated numerically using the finite-volume method (SIMPLE algorithm). The effects of the wall-to-fluid diffusivity ratio A and the Grashof number on the transient development of the recirculation cell and of the axial evolution of the interfacial heat flux, as well as of the friction coefficient, are analyzed in this work. The numerical results show, with elapsing time, an important widening of the recirculation cell in the upstream section, resulting in a redistribution of a significant portion of the applied heat flux in this section. Additionally, it is also found that in the whole transient period, the upstream widening of the recirculation cell and consequently the upstream redistribution of the applied heat flux slow down as the ratio A decreases.