Turbulent heat transfer in the separated, reattached, and redevelopment regions of a circular tube

Experiments were performed to determine the effect of flow separation on the heat-transfer characteristics of a turbulent pipe flow. The flow separation was induced by an orifice situated at the inlet of an electrically heated circular tube. The degree of flow separation was varied by employing orifices of various bore diameters. Water was the working fluid. The Reynolds number and the Prandtl number, respectively, ranged from 10,000 to 130,000 and from 3 to 6. The measurements show that the local heat-transfer coefficients in the separated, reattached, and redevelopment regions are several times as large as those for a fully developed flow. For instance, at the point of reattachment, the coefficients were 3 to 9 times greater than the corresponding fully developed values. In general, the increase of the heat-transfer coefficient owing to flow separation is accentuated as the Reynolds number decreases. The point of flow reattachment, which corresponds to a maximum in the distribution of the heat-transfer coefficient, was found to occur from 1.25 to 2.5 pipe dia from the onset of separation.