Thermal-hydraulic characteristics of a heat exchanger: The vertical rectangular fins being aligned parallel to the mean air-flow in the duct

Abstract

An investigation is reported of the steady-state rates of heat transfer from an array of vertical rectangular uniform duralumin fins (constructed with various inter-fin spacings, heights and lengths, all of which remain constant for each system), protruding from a horizontal rectangular base, in the presence of a horizontal, almost adiabatic shroud, situated above the horizontal fin tips. For a constant fin-base temperature, optimal fin separations, corresponding to maximum heat-transfer rates, were determined for selected values of fin height, length and ratio of shroud clearance to fin height. Higher optimal inter-fin spacings ensued as either the fin height or the length was increased. Greater fin heights produced higher heat-transfer rates from the fin array, whereas increasing the fin length resulted in lower steady-state rates of heat dissipation per unit base area. The optimal fin separation, which ensued for each specified combination of fin height and length was in the range of 4·5 10·5 mm. Of all the arrangements tested, the fin array system with no shroud clearance above the vertical fins, always gave the best performance as a forced-convective heat-exchanger, with a fixed air-mass flow-rate across it. A single non-dimensional heat-transfer correlation, whichaccounts for the effects of varying all the geometrical parameters and operating conditions of the heat exchanger, was obtained. Significant changes in the ratio of the shroud clearance above the vertical fins to the fin protuberance from the base produced only marginal variations in the average Nusselt number. For a constant air-mass flow rate, large streamwise pressure losses and relatively high heat-transfer rates resulted when the fins were narrowly spaced and there was little or no clearance above the vertical fins. The frictional characteristics of the air flow around the fin-array, when experssed by the friction factor versus Reynolds number relationship, exhibited surprisingly little dependence upon the shroud clearance to fin-height ratio.