Detailed heat transfer, temperature, and velocity data are reported for the turbulent flow downstream of an abrupt increase in tube diameter (2.5:1) in which the downstream tube is maintained at a uniform elevated temperature. The heat transfer experiments cover downstream Reynolds numbers ranging from 4300 to 44,500, the flow being fully developed at the exit of the small tube (i.e., the abrupt expansion step). Maximum local heat transfer coefficients are proportional to the upstream Reynolds number to the power 2/3 with the location of the maximum moving upstream slightly as the Reynolds number is increased. Heat transfer data at a Reynolds number of 17,300 are supplemented by velocity and temperature profiles, which are especially informative. They bring out clearly that the viscous sublayer, despite the thinning that is believed to occur in the vicinity of the reattachment point, still provides the major resistance to heat transfer. The correct prediction of the Nusselt–Reynolds number relation in reattaching flows is thus crucially dependent on the variation of turbulent diffusivity in the “buffer” region of the flow.

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