Laminar mixed convection in inclined semicircular ducts under buoyancy-assisted and -opposed conditions

Abstract

Laminar mixed convection in inclined semicircular ducts (with the flat surface in the vertical position) is investigated theoretically in the fully-developed region, and experimentally in the developing and fully-developed regions, under buoyancy assisted and buoyancy opposed conditions. The investigation started with the numerical analysis of laminar, fully-developed flow and heat transfer. This analysis used a control-volume-based finite-difference approach in solving the governing equations. Results were obtained for the two limiting thermal boundary conditions; uniform heat input axially with uniform peripheral wail temperature (H1) and uniform heat input axially with uniform wall heat flux circumferentially (H2). These theoretical results include the axial velocity and temperature distributions, the secondary flow pattern, a map for the onset of flow reversal and data for the friction factor and Nusselt number. Next, a series of experiments for laminar water flow in the entrance region of a semicircular duct with upward and downward inclinations within +-20 were performed using the thermal boundary condition of uniform heat input axially. The experiment was designed for determining the effect of inclination (particularly the downward) on the wall temperature, the local and fully-developed Nusselt numbers, and the overall pressure drop across the test section at three Reynolds numbers (500, 1000, and 1500) and a wide range of Grashof numbers. The circumferential variation of wall temperature was found to increase with 'Gr' for all angles of inclinations. However, in the upward inclinations the experimental data showed less circumferential variation in wall temperature than that for the horizontal orientation, while for downward inclinations the circumferential variation of wall temperature was much larger than that for the upward inclinations. For the upward inclinations, the experimental values of Nusselt number were found to increase with Grashof number and the inclination angle (up to 20), while the effect of Reynolds number was found to be small. For the downward inclinations, however, Reynolds number has a strong effect on Nusselt number and the manner by which it varies with Grashof number. Two recent publications summarizing the major results of this investigation have been prepared by Busedra and Soliman [1, 2]. (Abstract shortened by UMI.)