Computer-based analysis of steady-state and transient heat transfer of small-sized leaves by free and mixed convection

In order to study convective heat transfer of small leaves, the steady-state and transient heat flux of small leaf-shaped model structures (area of one side = 1730 mm(2)) were studied under zero and low (= 100 mm s(-1)) wind velocities by using a computer simulation method, The results show that: (1) distinct temperature gradients of several degrees develop over the surface of the model objects during free and mixed convection; and (2) the shape of the objects and onset of low wind velocities has a considerable effect on the resulting temperature pattern and on the time constant tau. Small leaves can thus show a temperature distribution which is far from uniform under zero and low wind conditions, The approach leads, however, to higher leaf temperatures than would be attained by 'real' leaves under identical conditions, because heat transfer by transpiration is neglected, The results demonstrate the fundamental importance of a completely controlled environment when measuring heat dissipation by free convection. As slight air breezes alter the temperature of leaves significantly, the existence of purely free convection appears to be questionable in the case of outdoor conditions. Contrary to the prognoses yielded by standard approximations, no quantitative effect of buoyancy on heat transfer under the considered conditions could be detected for small-sized leaf shapes.