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MIXED CONVECTION TO AIR IN A VERTICAL PIPE

Brian P. Axcell
Nuclear Engineering Research Group Manchester School of Engineering, University of Manchester, Manchester M13 5PL

W. B. Hall
Nuclear Engineering Laboratories, University of Manchester, U.K.

Abstract

Turbulent flows in which the "forced" and "free" components of convection act in opposition (e.g., a downward heated or an upward cooled flow in a vertical pipe) frequently give a higher heat transfer coefficient than would obtain with either component acting alone. This is in contrast to the behaviour of non-turbulent flows, in which such a situation leads to a reduction in the heat transfer coefficient. It is believed that this behaviour is a consequence of the changes in turbulence production caused by the modification of the shear stress distribution in the fluid by buoyancy forces.

The experiments described in the paper were made on a 0.613 m diameter pipe, 36 m in height. Heat transfer to a downward flow of air (2.104 < Re < 1.3.105) was measured; velocity and temperature distribution were also obtained. The results are compared with a correlation proposed by Fewster and Jackson, and also with a numerical solution based on a simple "one-equation" model of turbulence.

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