Convective Cooling at Low Airspeeds and the Shapes of Broad Leaves

Circular, abstractly lobed, and leaf-shaped flat copper plates were heated in a very low-speed wind tunnel. Surface temperature distributions of the plates were matched to those of real leaves. With the centres of the plates 15 °C above ambient temperature, their heat dissipation was measured at wind velocities of < 1, 10, and 30 cm s_1 from below and laterally with horizontal and vertical plate orientations. Even very slight forced-air movements markedly increased heat dissipation in this range of mixed free and forced convection. Lobed plates were more effective dissipators than circles, with the greatest differences occurring where flow was normal to the plate. Circular plates dissipated about one-fourth more heat when vertical than when horizontal in still air (free convection). By contrast, dissipation was essen tially independent of orientation for extensively lobed models. Under some circumstances, maximum dissipation occurred with lobed plates oblique to a forced air stream. Physical explanations and biological implications of these results are discussed.