Natural-convection characteristics of a horizontally-based vertical rectangular fin-array in the presence of a shroud

The steady-state natural convective cooling of horizontally-based, vertical rectangular fins, when in close proximity to an adiabatic horizontal shroud, situated adjacent to and above the horizontal fin-tips, was investigated experimentally. The resuls are of significance for the designers of electronic arrays, the components of which should be maintained at temperatures less than 65°C in order to ensure operational reliability. The optimal fin separation, which corresponds with the maximum rate of heat loss from the fin-array, has been deduced for various combinations of fin protrusions and distances of the shroud above the vertical fins, when the fins' base was maintained at a uniform temperature of 40 ± 0·5°C above that of the environment (21·0 ± 0·5°C). For a constant temperature-difference between the fin-base and the environment, lower optimal fin separations and higher steady-state heat-dissipation rates ensued when the shroud clearance to the fin height ratio was increased from zero to unity. Increasing the fin protrusion above the horizontal base also resulted in higher heat-dissipation rates from the fin-array. However, the fin-array with maximum shroud clearance is a much more favourable configuration (e.g. with respect to requiring less material) for achieving heat-transfer enhancement, than the fin array which employs large fin protrusions. For an open-ended duct of approximately the dimensions considered in this project (i.e. of maximum rectangular section 240 mm x 180 mm), the fins should protrude to less than half the internal height of the duct in order that a high convective performance of the fins is achieved. There is an optimal value of the shroud clearance to fin height ratio which exceeds unity for each specific operation, i.e. the exact optimal ratio being dependent upon the geometry and temperatures involved. Average Nusselt numbers, evaluated from the experimental data, are correlated non-dimensionally with respect to the fin-array geometry and the Grashof number. This correlation indicates that variations of the shroud clearance to fin height ratio produce only marginal variations in the average Nusselt number.