強制対流による葉形湿面からの蒸発

気流に対する面の傾きが葉面水蒸気輸送に及ぼす効果

抄録

Averge coefficients of water-vapor transfer by forced convection from a single and both wet surfaces of leafshaped flat plates inclined to an air-flow were experimentally obtained. These results may be valid for analyses of the mass transfer from leaves inclined to the wind-direction or fluttering leaves within a plant canopy.
Leaf-models of thin elliptic plates with diameters of 5cm and 10cm were placed in the test section of an Eiffel-type wind-tunnel so as to set the short diameter in the direction of air-stream. The amount of evaporation from the wet surface of a plate was measured by weighing-method, in the ranges of wind velocities between 0.5 and 7m/s and the angles of the plate to air-flow between 0° and 90°, under the conditions of constant air-temperature and relative humidity.
Since the plates were not heated, the temperature of the surface, except for the vicinities of the leading and trailing edges of the plate was nearly uniform so that the variation of the water-vapor concentration over the surface can be neglected.
Average vapor-transfer coefficients (D_f, cm/s) were calculated by the following equations: for the plate with a single side wetted, D_f=w/ΔC-D_n, for the plate with both sides wetted, D_f=2ΔC-D_n, where, w is mean rate of evaporation in g/cm²s, and ΔC is the difference in vapor concentration between the surface and the air outside the boundary-layer over the plate in g/cm³. D_n is the transfer coffeicient due to the effect of free convection and calculated after Grashof number and so on.
The results obtained are as follows:
1) The average forced-convection transfer-coefficients of the plate parallel to the air-flow, for a single side and both sides wetted, were proportional to the 0.5-th power of wind velocity and the values agreed well with the theoretical ones for laminar boundary-layer over the plate.
2) For average transfer coefficients of the single wetted surface facing windward, it was confirmed that the 0.5-th power law for the wind velocity was applied in the whole range of the inclination-angles, and the proportional constant increased gradually with increasing angle of incidence. The value of the proportional constant of the transfer-coefficient for a plate perpendicular to the flow was about 1.2 times as large as that for a parallel plate.
3) For a single surface facing leeward, except for the angles between 10° and 20°, the average transfer-coefficients were apparently proportional to the 0.5-th power of wind velocity. An increasing incidence-angle over 30° diminished the proportional constant to about 80% of that for the parallel plate.
In the range of the incidence angles from 10° to 20°, the average transfer-coefficient was approximately proportional to the power between 0.6 and 0.7 of wind velocity.
4) The character of the average transfer-coefficient of the plate with both sides wetted showed the averaged character of those for respective single sides wetted.
Except for the angles between 10° and 20°, an apparent exponent of the wind velocity for the average transfer-coefficient was 0.5, and the change in prorortional constant did not appeared for the change in the inclination-angles.
In the range of the angles between 10° and 20°, the average transfer-coefficients were correlated with wind velocity by the exponent of about 0.6.