Mixed convection and radiation from an isothermal bladed structure

Abstract

The total heat transfer from a finned or ‘bladed’ structure may be expected to rise from that of a flat geometry due to a larger area exposed to ambient air. However, this is not always the case because a trade-off can be found between convection and radiation as the bladed geometry and surface temperature are varied. Furthermore, a mixed convection regime adds complexity to the heat exchange between the surrounding air and the heated bladed structure, which is relevant to various applications such as cooling and solar thermal energy. In this study, mixed convection and radiation heat transfer were determined for a cuboid with isothermal blades protruding from one of its sides. A steady-state simulation based on a three-dimensional SST k–ω turbulence model was performed in OpenFOAM to estimate the average convection heat transfer coefficient as a function of structure orientation, convection regime and bladed geometry. A Monte Carlo ray tracing method was employed to calculate view factors for determining the radiation heat transfer coefficient. Wind tunnel experiments validated the combined numerical approach. An increasing pitch angle (starting from the vertical) gradually increased convection heat transfer until a maximum value at ~45° and then significantly decreased it, where both effects were due to a flow behaviour in which vortices between the blades formed (<45°) but then disappeared (>45°). The velocity and wall temperature of the mixed convection flow in between the dominantly natural and forced convection regimes were identified. In comparison to a flat geometry, the bladed structure increased the temperature value at which the heat transfer by convection and radiation equalised, and produced a lower total heat transfer coefficient than the flat configuration at high surface temperatures. Furthermore, flow bifurcations were observed between the blades as their length was increased. In contrast, a large number of blades led to a flow transition where most of the incoming air exited from the side apertures to a lid-driven-like convection with flow recirculation occurring within the blades, which created a thermal barrier that decreased convection heat transfer. These results support the design of bladed or finned structures that enhance or reduce heat transfer by mixed convection and/or thermal radiation.