Buoyancy effects on heat transfer to air in laminar flow

Herein is reported the effect of combined free and forced convective heat transfer to air flowing through a uniformly heated horizontal pipe in laminar regime. Experimentally determined velocity and temperature profiles in the vertical as well as horizontal directions along the pipe diameters at the entrance and exit of the 'effective test section' under the influence of buoyancy effects are found to be significantly different from the usual parabolic forms. The present experimental data (1) extend the validity of Morton's equation up to ReRa = 10000, (2) do not lend support to the equation of Faris and Viskanta and (3) indicate higher values than those that could be obtained from the correlation of Morcos and Bergles. Due to buoyancy effects, the improvement in the average Nusselt number is nearly 100% of the value for pure forced convection in the ReRa range from 2000 to 20000 where there is a lack of experimental data in the available literature.