Air flow generated by thermal convection was used in a low-speed wind tunnel to evaluate leaf boundary-layer transfer coefficients. The 20cm long test section had a 25cm×25cm square cross-section over which a stable laminar air flow with turbulence intensities of less than 0.02 was produced under a range of flow speed from 10 to 130cm·sec-1.
By applying this wind tunnel, water-vapor transfer coefficients were evaluated for heated leaf models with various temperature differences between the model surface and the air. Transfer coefficients were also evaluated for non-heated leaf models of various lengths and widths both between 1 and 16cm. From the experimental results, the effect of buoyancy on the coefficient was investigated.
Transfer coefficients obtained from the present experiments were larger than the theoretical laminar forced-convection ones. The ratio of the transfer coefficients to the theoretical forced-convection ones was represented as a function of the aspect ratio, the Grashof and Reynolds numbers.
It is concluded that, in light winds, the free-convection over the leaf surface and the advection around the edge produce a larger boundary-layer transfer than the pure forced-convection one.