BUOYANCY EFFECT ON FORCED-CONVECTION IN THE LEAF BOUNDARY-LAYER

Mixed convection (forced convection plus free convection) in the leaf boundary layer was examined by air flow visualization and by evaluation of the boundary layer conductance at different leaf-air temperature differences (T(L)-T(A)) under low wind velocities. The visualized air flow was found to become more unstable and buoyant at higher T(L)-T(A). An ascending longitudinal plume was induced along the upper surface, and the air flow along the lower surface ascended after passing the trailing leaf edge. The air flow modified by buoyancy was considered to result in an increase in boundary layer conductance (G(A)) for mixed convection, which became higher with higher T(L)-T(A) as compared with the conductance for pure forced convection without buoyancy. This increase in G(A) appeared larger at larger Grashof number (Gr) and at smaller Reynolds number (Re). The dependences of buoyancy effect on Gr and Re were related to 'edge-effects'.