Drag coefficient of small spherical particles.

An experimental study was conducted to determine the drag coefficient of small, inert, spherical particles accelerating in a laminar, nonreacting, incompressible continuum flow. The Reynolds number range covered in the study was from 200 to 1700, and particle sizes ranged from 150 to 450/1. Glass beads, HP 295 ball powder, and sapphire balls were used. The convective flow behind the shock wave in a shock tube was used to accelerate the particles. The diameter and temporal displacement of a single particle were determined by use of a rotating drum camera in conjunction with an oscillating light source. Using the photographic data, the particle density, the shock speed, the initial temperature and pressure, and the normal shock relations, the particle drag coefficient was calculated. Experiments with HP 295 ball powder produced drag results that were as much as 85% higher than the standard drag coefficient curve. This increase was found to be dependent on the relative Mach number of the flow about the particle even though the Mach number was less than 0.3. The glass beads yielded similar results but to a lesser extent. The sapphire balls produced the best results, yielding the lowest drag values, less scatter, and little sensitivity to Mach number. Many possible influences were considered to explain the foregoing anomalous results. Photomicrographs showed that the ball powder was very and that the sapphire balls were the most smooth. It is concluded that the particles in solid rocket nozzles are rough and hence their drag coefficients are much higher than would normally be the case.