Natural-Convective Heat Exchanger with Vertical Rectangular Fins and Base: Design Criteria

Steady state rates of free-convective, conductive and radiative heat transfer from a polished, duralumin and extended-plate, heat exchanger through air have been measured. When its vertical base was maintained at 40.0 (±0.2) K above the local air temperature (=15.5 ± 0.4°C) in the laboratory, optimal values of the vertical-fin array's geometrical parameters, corresponding to achieving maximum rates of heat loss, were deduced. For a base of width 190 mm and length 250 mm, the optimal uniform separation between 3 mm thick fins rose from 9.5 to 11.1 mm as the fin protrusion was decreased from 90 to 30 mm: it was almost independent of temperature within the range considered. For a base of width 190 mm and length 500 mm, the optimal uniform thickness for fins of 60 mm protrusion rose from 1.5 to 3.6 mm as the fin separation was increased from 15 to 55 mm, the optimal fin thickness being almost invariant with respect to changes in the fins' base temperature.

No generalized quantitative recommendations emerge from the present investigation with respect to the optimal values for the fins' length or fins' protrusion. These would have to be evaluated, by making additional due allowances for the weight and cost of material involved in each proposed heat exchanger, according to each particular application envisaged. Nevertheless, it can be concluded that the use of two separated heat exchangers of the considered type, rather than a single one of similar design but whose length is the sum of the lengths of the previous two, should be considered when attempting to enhance the heat-transfer performance.