Heat transfer by forced convection between a fluid and an immersed body implies a temperature difference and hence a density difference. The density difference gives rise to free convection. The effect of buoyant motion in the direction of the forced flow is to increase the velocity in the boundary layer and thereby the rate of heat transfer over that for pure forced convection. Buoyant motion in opposition to the forced motion reduces the velocity and the rate of heat transfer relative to pure forced or free convection. Also, assisting flows retard and opposing flows advance the point of separation of the boundary layer on immersed bodies. Hall and Price (1970) found that the rate of heat transfer in a turbulent free convection was at first decreased and then increased by a superimposed forced flow in the same direction. They attributed the decrease to the suppression of turbulence. In view of these complexities, it is apparent that the suggestion of McAdams (1954), that the higher of the rates of heat transfer for the two pure processes be used for the combined process, can be considered only as a first-order approximation. More accurate correlating equations for various regimes are recommended below.

A. Assisting convection

(a) Thin laminar boundary-layer regime

Extensive theoretical and experimental results have been obtained for aiding free and forced convection in the laminar boundary-layer regime, and many expressions have been proposed for their correlation, generally in the form

\[\label{eq1} \mbox{Nu}^{n}=\mbox{Nu}^{n}_{F}+\mbox{Nu}^{n}_{N}\tag{1}\]