Heat transfer enhancement by porous substrate addition on the inner wall of a tubular heat exchanger

A study of flow regime and heat transfer in an annular heat exchanger partially filled with a porous medium is presented in this work. Constant heat flux and constant wall temperature boundary conditions on the inner cylinder are considered, while the outer cylinder is assumed adiabatic. The study is for both the thermal entry region and the thermally fully developed region. The flow in the porous region is modelled either by the Darcy-Brinkman equation for which an exact solution is developed or by the Darcy-Brinkman-Forchheimer equation in order to take into account inertial effects. For this case a numerical solution based on a control volume method is discussed. The results emphasize the effect of the porous layer attached to the inner cylinder on the thermal development length and heat transfer rate. It is shown that the porous substrate reduces the thermal entry length. When the effective thermal conductivity of the saturated porous medium is of the order of the fluid thermal conductivity, the local Nusselt does not vary monotonically with the thickness of the substrate. However, the use of a porous matrix always leads to an increase in the heat transfer rate provided its thermophysical properties and thickness are well chosen. © Elsevier.