Water-Vapor Transfer across the Boundary-Layer on Rice-Leaf

Abstract

Evaporation from wet surface of an elongated plate was measured in laminar air flow, in order to clarify the convection coefficient of water-vapor transfer across the boundary layer on a leaf-blade of rice plant. The plate was shaped like a rice leaf of 30cm length in the direction of air flow and the maximum width 1cm, and a rectangular plate of the same dimensions was also used.The forced-convection transfer coefficient on a leaf shaped plate is about 1.05 times as large as that on a rectangular plate. This is smaller than the ratio between the coefficient for an elliptic plate and that for a rectangular one, which is attributable to the shape of a rice leaf-blade.When the plate is not heated, the distribution of water-vapor density over the surface along the flow direction is so slight that the effect of the distribution on the transfer can be neglected. On the other hand, when the plate is uniformly heated, the difference in water-vapor density between the surface and the ambient air varies to be proportional to the 0.1 to 0.3 power of the distance from the leading edge of the plate. The vapor density distribution leads to an increase in forced-convection transfer coefficient for a heated plate by a factor of about 1.1 over that for a non-heated one.The average coefficient of convection transfer may be represented by the sum of the coefficient for forced convection and the buoyancy term, which is given approximately by the free-convection transfer coefficient for a flat plate in the laminar range.The average coefficient of forced-convection transfer from each side surface of an elongated rectangular plate is proportional to the 0.5 power of the wind speed for every angle of the attack of air flow when wind is not too strong. Above a certain wind speed, the coefficient is proportional to the 0.8 power of the speed. For a plate in parallel with the air flow, the critical wind speed at which the transition occurs is about 8m/s. For a plate inclined to the flow, the transition on the surface facing winward occurs at a relatively high wind speed, and that on the surface facing leeward at a relatively low wind speed.For the wake side surface of a plate with an attack angle of about 5°, the forced-convection coefficient may be expressed as varying in a 0.55 power of wind speed. However, it seems to be practical to take that the coefficient is proportional to the 0.5 power of the wind speed.In the laminar range, the average forced-convection transfer coefficient for an elongated rectangular plate is considerably larger than that estimated from the laminar boundary layer theory for an isothermal infinite strip of plane surface with the same dimensions parallel to the flowing fluid.Moreover, the transfer coefficient on each side surface of an inclined flat plate is larger than that of a plate in parallel with the air flow. The ratio of the former coefficient to the latter increases with the increasing attack angle of the flow on the plate.The mean value of the transfer coefficient of both side surfaces of a flat plate is approximately equal to the geometric average of those of each side surface with the same attack angle.In the laminar flow, the average forced-convection transfer coefficient for a bent rice-leaf of 30cm length and 1cm in the maximum width may be about 5 to 6 times as large as the value obtained from the boundary layer theory for the isothermal surface of a flat-rectangular plate of the same dimensions in parallel with the air flow, mainly because of three factors such as the correction due to the dimension, the effects of non-homogeneous temperature distribution over the leaf surface, and the leaf inclination effect.