Some approximate theoretical expressions have been derived to show the effect of flow conditions on heat transfer between flat surfaces and fluids flowing parallel to therm Special comparison was made between the case of uniform parallel flow and that of radial flow parallel to the surface. It was predicted that the heat transfer in uniform parallel flow would be higher than in outward radial flow, other conditions being the same. Two groups of experiments were carried out. In the first group air was flowing between two parallel plates in an outward radial direction. One of the plates was electrically heated. No such heating was supplied to the second plate, but it was insulated against external radiation. Eight different gaps between the plates were tested, and the average coefficients of heat transfer were obtained. The effect of the gap on the heat transfer was obtained for both laminar and turbulent flows. All the data of the different gaps were brought together by introducing equations containing the Nusselt and Reynolds numbers and the gap-to-length ratio. Comparison was also made between the present results and the theoretical and experimental results of other workers on heat transfer from flat surfaces to air in uniform parallel flow, little or no other information on radial flow being available. The effect of flow conditions was discussed. In the second group of experiments the unheated plate was rotated. Four different speeds of rotation and four different gaps were tested. The effect of this rotation on both the flow pattern and heat transfer was studied. The experimental results were compared with analytical ones and agreement was generally fair. Finally, from the calculated length of path of the air particles, an expression was found for the effect of the disc rotation on heat transfer
