Opposing Free and Forced Convective Heat Transfer to Turbulent Air Flow in a Verticle Pipe

Abstract

<p>The interaction between the opposing free and forced convection has been studied, by first defining the limits of the onset of flow reversal, through flow visualisation experiments involving the cooling of upward flowing hot water. From dynamic similarity considerations, these experiments were used to define the operating conditions, for experiments involving the heating of downward flowing air, to ensure unidirectional flow. Radial measurements of the temperature and velocity were made, in a 24 in. long, 8.054 in. diameter test section, with fully developed turbulent air flow entering its inlet. The inlet Reynolds numbers ranged from 7500 to 37500, the test section wall temperature was maintained constant for any run; the temperature range covered was 122ºF to 890ºF. Some temperature profiles exhibited unusual behaviour and this indirectly indicated flow reversal in situations where it was expected from the dynamic similarity considerations.</p> <p>The partial differential equations for the conservation of mass, momentum and energy were solved numerically by an implicit finite difference formulation. Good agreement between the predictions and the observations was obtained for negligible buoyancy force; while deviations increased with an increase in the relative magnitude of the buoyancy force. During the various attempts to resolve this discrepancy, it was felt that a change in the turbulence structure might have occurred. The effective conductivity profiles, calculated from the measured temperature profiles indicated an increase in the turbulent activity with increasing free-forced convection interaction. Heat transfer rates were found to be 215% higher than those obtained from pure forced convection considerations, for Gr/Re² ~ 0.90.</p>