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

Abstract

An experimental study of the effect of ventilation on the rate of evaporation of small water drops falling at terminal velocity in air has been carried out in a wind tunnel where water drops could he suspended freely in the tunnel airstream. For Reynolds numbers NRe≳2 it was found that the Sherwood number NSh was linearly related to NRe1/2NI, in a manner which could best be expressed by the equation 1/2 Nsh=0.78 + 0.308 Nsc1/2NRe1/2, where NSc is the Schmidt number. For NRe≲ the Sherwood number was found to smoothly approach 1/2 NSh=1 at NRe=0, with d(1/2NSh/d(NSc1/2)=0 according to the relationship 1/2 NSh=1.00 + 0.108 (NSc1/2NRe1/2)2. The experimental results are discussed in the light of previous experimental and theoretical work reported in the literature. Excellent agreement was found between our experimental results and the numerical results of Woo and Hamielec presented in an accompanying article. Two tables are given that easily allow computing the evaporation rates for cloud drops and small rain drops of sizes between 20 and 600μ radius for a wide range of atmospheric temperatures from 0–40C and relative humidities from 0–100%. As an example, the evaporation rate of drops failing in a NACA standard atmosphere is calculated and compared to similar computations reported in the literature.