Mixed heat convection in vertical pipe flow submitted to variable flow rate.

In this paper, we propose a numerical study on the mixed heat convection of a laminar flow inside a vertical pipe. A flow rate step is imposed at the inlet of the tube and a heat transfer coefficient is used to calculate the external heat convection. Both cases of positive and negative steps are considered. A strong difference in the thermal and the dynamic behaviour of the flow is observed between these two cases. Moreover, a non-dimensional study on the influence of some parameters like the thermal buoyancy coefficient, characterized by Ri(infinity) Re-infinity, and the external heat transfer, characterized by Bi*/Pe (generalized Biot/Peclet numbers), has shown that, in the case of laminar mixed convection, their influence on both the inner flow and the wall heat transfer is great. (C) Elsevier, Paris.