A fundamental function of the coefficient of heat/mass transfer across a boundary layer on a plant leaf for forced convection was evaluated in relation to the leaf dimension. The average transfer coefficients on a model-leaf were obtained from the measurements of water evaporation from the surface of flat rectangular plates with various lengths and widths from 1 to 32cm. The surface of the model-leaf was set in parallel with the air flow in a wind tunnel.
The average coefficients were proportional to the square root of the wind speed in a laminar region. For plates with relatively small lengths in the direction of the flow, compared to their transverse widths, the observed coefficients were in good agreement with the theoretical ones. For plates with lengths more than a critical value, however, the observed coefficients were larger than the theoretical ones. In a width-fixed model-leaf, the ratio of the observed coefficient to the theoretical one increased with an increase in length, whereas in a length-fixed leaf it decreased with increasing width.
A realistic transfer coefficient for forced convection was formulated as a function of the length and width of model-leaf and wind speed.