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-stat e and transient heat flux of small leaf-shaped model structures (area of on e 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 surfac e 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 o n the resulting temperature pattern and on the time constant tau. Small lea ves can thus show a temperature distribution which is far from uniform unde r zero and low wind conditions, The approach leads, however, to higher leaf temperatures than would be attained by 'real' leaves under identical condi tions, because heat transfer by transpiration is neglected, The results dem onstrate the fundamental importance of a completely controlled environment when measuring heat dissipation by free convection. As slight air breezes a lter the temperature of leaves significantly, the existence of purely free convection appears to be questionable in the case of outdoor conditions. Co ntrary 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.