Recent work on the drag experienced by an entrained particle in relative motion through a fluid is reviewed. It is shown that fluid turbulence, acceleration, particle shape and orientation, and particle‐fluid mass transfer can all have a significant effect on the value of the drag force, particularly when the non‐idealities cause a change in the flow pattern around the particle. Conditions under which the drag force can be predicted with certainty are still very limited, and some potentially valuable directions for future research are suggested.

The behavior of circular cylinders moving singly through water under the influence of gravity was studied with a motion picture camera over a range of particle. Reynolds number extending from 70 to 2400. The terminal velocity was determined for each particle and its drag coefficient was evaluated. At Re‐greater than 300, and in some cases as low as 80, the particle acquired a secondary motion, consisting of an angular oscillation about its mean orientation and a periodic lateral deviation from its mean path of fall. A resultant dependence of the drag coefficient on particle density was found to occur. A theoretical method of predicting the period of particle oscillation was developed from consideration of variations in the location of the front‐surface centre of pressure.

A theoretical study is reported on the use of d.c. and r.f. plasma jets as chemical reactors for the processing of minerals in the form of a fine powder. The temperature and flow fields of the jet are calculated by solving the integral boundary layer equations. Single particles trajectories are obtained by solving the Basset-Odar equations. A multi-particle model is then developed for a feed of known particle size and injection velocity distributions under low loading conditions. Calculators are made on the thermal decomposition of molybdenum disulphide ( 5 to 30-micron equivalent diameter). The parameters investigated are the free jet velocity, the mean injection velocity, and thereactor ambient conditions. The results are presented as the probebility density distributions of the gas loading, particle temperature, and conversion at different levels downstream of the nozzle. he use of high temperature plasmas for the pro-

Axial and radial profiles of temperature and velocity of an argon plasma jet in open air, were measured by a calorimetrie probe, at temperatures up to 11 600 0 K and velocities up to 410 mIs. The radial profiles were shown to be Gaussian.Velocities and decelerations of elose-sized microspheres (30 -140~m dia) in the plasma jet were measured by a high-speed streak camera. A maximum partiele deceleration of 8 000 g was observed.Experimental drag coefficients were found to be about 30% greater than standard curve values. This increase was in accord with the computer predictions of the value of the Basset history term in the full equation of motion.

Drag coefficients of aerodynamically smooth spheres having a density variation of from 0.252 to 1.91 g./cc. and a diameter variation from 1.56 to 3.21 mm. were obtained for acceleration rates varying from 103.5 ftJsec.2 to -30 ftJsec.2 and for relative intensities of up to 45%. The particle-to-Eulerian macroscale ratios varied from 0.50 to 0.16, and the diameter-to-Eulerion microscale ratios varied from 10 to 2.The drag coefficients were found to be a function of the particle Reynolds number and of the relative intensity but not of the acceleration and relative macro-and-microscale variations.A transition theory for the system investigated is presented, which predicts that the product of the critical Reynolds number and the square of the relative intensity should be a constant;it is supported by the experimental results obtained.The theoretical analysis of the momentum transfer occurring in multiparticle dilute phase solids-gas flow systems has been hindered by an inability to estimate the fluid drag forces which act on the individual particles owing to their movement relative to the fluid. The fluid drag force is related to the relative motion by the use of a coefficient of drag defined by __ L. B. Torobin is with Esso Research and Engineering Company, Linden, New Jersey.Reliable drag coefficients have been evaluated only for bodies in steady motion with respect to turbulent-free flows, and these conditions are not encountered in most solids-gas systems.The drag coefficients indirectly obtained from pressure-drop data ( I , 2, 3 ) are invalidated by the assumptions made in their calculation. More direct methods in which the velocities of individual particles have been measured ( 4 , 5, 6,7,8,9 ) suffer in that the fluid velocity and turbulence parameters along the particle trajectory were not determined.To completely characterize a momentum transfer situation in a turbulent fluid the turbulent parameters