Heat transfer of combined forced and natural convection from horizontal cylinders to air

Experimental investigations have been carried out for combined convective flows of air induced around uniformly heated, horizontal cylinders. Three cases of aiding, opposing and cross flows were treated. The experiments covered the ranges of the Reynolds and modified Reyleigh numbers as; Red=50-900 and Rad* = 5×104-3×106. The flow fields around the cylinders were visualized with smoke. The results showed that separation points of forced flow gradually shift from the sides to the upper edge of the cylinder with increasing wall heat fluxes. The local heat transfer coefficients of the cylinders were also measured. Although the local coefficients show complex variations with the forced flow velocities and the wall heat fluxes, the overall coefficients become higher than those estimated from the pure forced and natural convections throughout the cases of aiding, opposing and cross flows. Moreover, it was found that the overall Nusselt numbers and the separation points are well predicted by the non-dimensional parameter (Grd*/NudRed2).