Intensification of convective heat transfer by means of artificial surface roughness

Artificial roughness is considered as passive means of intensifying convective heat transfer from a wall to a turbulent stream, intensification in this case being a consequence of a boundary layer with lower thermal resistance rather than of a larger effective surface area. The mechanism is analyzed on the basis of velocity profiles through the laminar sublayer, the transitional interlayer, and the logarithmic turbulent core. The velocity profiles are obtained from the solution of the corresponding equation of one-dimensional heat transfer, taking into account total as well as molecular heat conduction. Theoretical and experimental data on plates and pipes with cooling air, water, or transformer oil were evaluated in terms of the Nusselt number as function of both the Reynolds number and the Prandtl number and with the velocity of the stream normalized to a characteristic parameter with the dimension of velocity: square root of the ratio of shearing stress to density. In the case of pipes, inside and outside cooling were considered. The results indicate how the roughness characteristics, namely surface profile and asperity height, and the Prandtl number at fixed values of the Reynolds number can be optimized for maximum heat transfer intensity.