Mixed convection in a vertical double pipe heat exchanger

A numerical study of steady state, simultaneously developing, laminar mixed convection in a vertical double pipe heat exchanger has been conducted for upward parallel flow. The model is elliptic and takes into account conduction in the solid walls as well as dissipation in the two streams. The viscosity and the density of the fluids depend on the temperature while all other thermophysical properties are constant. Results have been calculated for fixed inlet temperatures, a Richardson number equal to 1 for the annular space and three different values in the cylinder (4.85, 1 and 0.34). Flow reversal occurs in the warm fluid for Ric⩾1 and affects significantly the thermal and hydrodynamic fields. In particular, the temperature of the wall separating the two fluids varies considerably in the flow direction despite its high conductivity. The results include temperature and velocity profiles at different cross sections as well as the axial evolution of bulk and wall temperatures, Nusselt numbers and friction factors in the cylinder and the annular region. The effect of Ric on the asymptotic values of the Nusselt numbers is analyzed.