Studies on the Leaf Temperature (2)

On the Heat Transfer from the Leaf of Citrus in Laminar Flow

Abstract

The temperature of and the transpiration from Citrus leaf were simultaneously measured in relation to solar radiation and wind velocity in laminar flow parallel to the leaf surface.
Fig. 1 and 2 show the temperature departure of the leaf from the surrounding air increases in linear proportion to increasing difference between solar radiation absorbed by the leaf and the transpiration heat, when wind velocity is constant.
The lower the wind velocity, the larger is the temperature effect.
Fig. 3 shows the transpiration rate is linearly proportional to solar radiation at a constant wind velocity.
The heat flow from the leaf to air can be calculated as the residual term of the heat economy equation (15).
Fig. 5 shows the relation between logarithm of the heat transfer coefficient and of wind velocity. Then, it is found that the former increases in perfectly linear proportion to the latter and the proportional constant is 0.509±0.009.
This result coincides perfectly with E. Pohlhausen's theoretical formula of heat transfer coefficient on a flat plate in laminar flow.
Therefore, the measured values of heat flow from the leaf do also coincide sufficiently with theoretical values of heat flow from elliptical flat plate resembled to the leaf, as shown in Fig. 6.