Transient conjugated downward and upward laminar mixed convection heat transfer in vertical pipe, submitted partially to a constant heat flux, has been investigated computationally. The governing Navier Stockes and energy equations were solved by a developed code using the finite volume method and Boussinesq's assumptions. Solutions were obtained for (Pr, Re, Gr)=(5, 100, 5 10~5), wall-to-fluid conductivity ratio K=10, 50 and 100 and pipe-thickness to diameter ratio Δ=0.05 and 0.25. From a parametric study, transient distribution of the normalized interfacial heat flux and of the friction coefficient ratio as a function of the axial coordinates as well as transient evolution of the vector velocities were obtained and effect of K and Δ were investigated. It was found that the magnitude and the extent of upstream and downstream heating increase monotonically with an increase of Δ or K. In turn, this heat flux redistribution has affected significantly the friction coefficient ratio and velocity field in the downstream and upstream adiabatic sections respectively for the downward and upward mixed convection cases.
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