Forced-Convection Water-Vapor Transfer across the Boundary Layer on the Flat Leaf Model of Small Dimensions

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  • HASEBA Tetsuya
    Laboratory of Agricultural Meteorology, College of Agriculture, Ehime University

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  • 比較的小さい面の強制対流水蒸気輸送係数
  • ヒカクテキ チイサイメン ノ キョウセイ タイリュウ スイジョウキ ユソウ ケ

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Abstract

Evaporation rates from isothermal wet surfaces of circular and square plates were measured in laminar air-flow parallel to the surface, in order to clarify the dimension dependence of water-vapor transfer coefficients on the flat leaf of small dimensions for forced-convection.<br>Below a critical Reynolds number of about 2×104, the average forced-convection transfer coefficient is proportional to the 0.5 power of wind speed.<br>The average transfer coefficients for circular and square models of larger dimensions with a diameter/side-length above 5 to 7cm agree with the predicted values from the “approximate boundary layer theory”. For smaller models with dimensions below 5 to 7cm, however, the observed coefficients are larger than the predicted. The ratio of the observed coefficient to the predicted one (α) becomes larger as the model dimension (l, cm) decreases. A relation between them leads to α≅1.2l-0.09.<br>Some experiments on evaporation from the square wet surfaces whose leading edges made part of that of a larger rectangular plate showed that a relative increase in the transfer coefficient could be partly caused by the turbulent transport of vapor around the model edges.

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