Clouds of liquid and solid spheres accelerating in airstreams were studied over a range of airstream pressure, temperature, and velocity conditions. Diameter and velocity data for individual droplets and solid spheres in the clouds were obtained with a high-speed camera developed at the NACA Lewis laboratory. From these data, linear accelerations of spheres (20 to 120 microns in diam.) were determined, and instantaneous drag coefficients for unsteady momentum transfer were calculated. The drag coefficients CD for droplets (isooctane, water, and trichloroethylene) and solid spheres (magnesium and calcium silicide) were found to correlate Reynolds number Re- as given by the empirical expression CD = 27/Re0.84, for 6 < Re < 400. When acceleration rates were low, the unsteady-state drag coefficients were in agreement with steady-state values from previous investigations. From this expression for drag coefficient, an equation relating distance and time was derived for calculating trajectories of solid spheres. In the case of droplets, a graphical method was used to relate droplet diameter to distance when evaporation rates were high. When evaporation rates were low, solid-sphere trajectory equations were found applicable.
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