Drag coefficients of evaporating spheres in a turbulent air stream

A study was made to determine the effect of mass transfer on the drag coefficients of freely‐moving aerodynamically smooth spheres, accelerating in a co‐current turbulent air stream. The particles consisted of celite impregnated with liquid sulphur dioxide and varied in diameter from 0.184 to 0.989‐inch. Accurate time‐distance data were obtained with a new particle‐tracking technique which allowed the quantitative measurement of drag coefficients for relative turbulence intensities varying from 5 to 30%. The range of Reynolds numbers studied was from 2100 to 29,800, which, because of the effect of free‐stream turbulence, forms a part of the super‐critical flow regime. The results have thus been compared with the previously reported drag data for solid non‐evaporating spheres in a similar flow region. Mass transfer was found, in general, to decrease the super‐critical drag on a sphere and to reduce the influence of relative turbulence intensity. This alteration in momentum transfer is probably due to a reduction in the skin friction and to a pressure increase in the wake of an evaporating sphere.