Abstract:
Owing to its importance in industrial applications, many studies investigated mixed convective flow through vertical tubes. These studies usually considered tubes with relatively large diameters and short lengths. The result was that the flow was developing rather than fully developed, which affects the heat transfer characteristics and free convection effects. The purpose of this study was therefore to consider fully developed upward and downward flow at very low laminar Reynolds numbers in forced convection conditions, and to investigate the heat transfer characteristics when assisting and opposing flow became significant due to the low fluid velocities. Experiments and numerical simulations were conducted for vertically upward and downward flow between Reynolds numbers of 180 and 2300 at three different heat fluxes of 1, 1.5 and 2 kW/m2. Water was used as the test fluid and the Prandtl numbers varied between 3 and 7. The inner-tube diameter was 5.1 mm and the heated length-to-diameter ratio was 886. It was found that as the Reynolds number was decreased below 250 and 600 for upward and downward flow, respectively, free convection effects were no longer suppressed by the velocity of the fluid and assisting and opposing flow became significant. The Nusselt numbers of upward flow were always greater than for downward flow. Increasing free convection effects caused both the upward and downward results to shift further from the forced convection Nusselt numbers. Correlations were developed to determine the Nusselt numbers for assisting and opposing laminar flow.