This study numerically investigates the effect of porous blocks thickness on laminar mixed convection in a horizontal annulus. With attached four porous blocks on the inner cylinder, steady 3D laminar mixed convection is presented for the fully developed region of horizontal concentric annuli. Results are presented for two values of porous blocks thickness and a range of the values of the Grashoff number, Darcy number and the conductivity ratio between the porous medium and the fluid. Results are presented in the form of contours plots of the streamlines and for the temperature isotherms, and in terms of the overall heat transfer coefficients and friction factor.
The average Nusselt number increases significantly with an increase of the thickness of porous blocks. With the use of the four porous blocks, the friction factor is consequently increased compared with the situation without porous blocks. The decrease of the Darcy number leads to an increase of the friction factor. If the fully fluid case is taken as a reference, the use of porous blocks is justified only when the ratio of the average Nusselt number to the friction factor is enhanced.