A Wind Tunnel Investigation of the Rate of Evaporation of Large Water Drops Falling at Terminal Velocity in Air

Abstract

An experimental study of the effect of ventilation on the rate of evaporation of millimeter sized water drops failing at terminal velocity in air has been carried out in a wind tunnel where drops were suspended freely in the tunnel air stream. It was found that for drops in the size range 1150 µm≤a0≤2500 µm, the mean ventilation coefficient f̄vf̄h could be expressed as f=(0.78±0.02)+(0.308±0.010)X, where X=N&frac13Sc,v N½Re. Previously, we showed that this relation holds for drops in the size range 60 µm≤a0≤400 µm. Taken together, our present and previous data suggest that with reasonable accuracy f̄=0.78+0.308X, for 1.4≤X≤51.4 (60 µm≤a0≤2500 µm). For 0≤X≤1.4 (0≤a≤60 µm), one may use our previous result f=1.00+0.108 X2. To illustrate how the present data may be applied, we computed the distance which is required for a water drop to travel from cloud base through a NACA Standard Atmosphere of various relative humidities, in order to reach the earth's surface with a given size.