Temperature profiles and fluctuations in combined free- and forced-convection flows

Abstract

To explain the effects of buoyancy forces on the rate of heat transfer to air at low velocities, under conditions where they either aid or oppose the forced flow, temperature fluctuations were measured in a 4·42-in. dia. by 102-in. long stainless steel tube over a Reynolds number range from 270 to 6900. Nominal wall temperatures of 100°, 500° and 950°, were used. For a fully-developed type of turbulent flow (which could be obtained at much lower Reynolds numbers than the critical value for isothermal flow) the temperature fluctuations were quite similar to reported measurements of velocity and temperature fluctuations in pure forced flows. The direction of the effects of the buoyancy forces on the temperature fluctuations agreed with the predictions of a published theoretical analysis. A relation was developed between the velocity—temperature correlation coefficients in the axial and radial directions which explained the effects of the buoyancy forces on the rate of heat transfer.