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.
Drag coefficients of aerodynamically smooth spheres varying in diameter from 0.0625 to 1.004 in. and in density from 0.195 to 7.80 g./cc. were obtained at acceleration rates ranging from 120 to -30 ft./q.sec. The particles were subjected to relative turbulence intensities of 7 to 35% and to ratios of Eulerian macroscale to particle diameter of about 0.4 to 5.Quantitative measurement of particle drag coefficients was made possible by the use of a new particle tracing technique which permits the resolution of time to the nearest tenth of a millisecond. The resulting data extend farther into the supercritical flow regime than any other measurements previously reported.The variation in drag coefficient with Reynolds number indicates a continuous alteration in the flow pattern around a sphere in this region. The effect of turbulence is, essentially, to increase supercritical drag, although this effect was found to diminish with increasing Reynolds number. Possible mechanisms for the effects of Reynolds number and turbulence on the particle drag coefficient are suggested.Practical engineering applications involving the intimate contact between solid particles and a continuous fluid medium are being developed at a rapidly growing rate. These systems, in general, play an important role in a large variety of industrial operations, such as pneumatic conveying, transport reactions, spray processes, flash drying, air pollution control, and the design of solidsburning rocket engines. Although empirical relationships have been derived to deteimine the rates of heat, mass, and momentum transfer in solids-gas flow under certain conditions, the basic principles underlying the behavior of particulate systems are still, to a large extent, imperfectly understood. In particular, the effects of a number of important factors on the motion of a particle conveyed by a moving fluid still remain to be assessed. One such effect is the influence of the level of free-stream turbulence on the fluid resistance to particle motion. B O U N D A R Y LAYER T R A N S I T I O N CAUSED B Y TURBULENCEFree-stream turbulence is known to exert a significant influence on the momentum transfer from a particle by altering the flow field around it. The best known of these alterations is the transition in the attached boundary layer from laminar to turbulent flow. This action is responsible for the major effect of free-stream turbulence, namely the lowering of the critical Reynolds number, defined by convention as that value of N R e at which the declining steeply sloped portion of the C D -N R~ curve intersects the C D value of 0.3 ( 1 ) . By promoting transition, the essential effect of an increase in the intensity of free-stream turbulence is to influence the position of boundary layer separation. The latter determines the size of the wake which, in turn, has a dominating influence on the drag of a sphere. ing spherical particles into a cocurrent turbulent stream and measuring their subsequent time-distance history, they were able to show that the critical R...
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.
This paper reviews a number of experimental studies concerned with the influence of turbulence on heat and mass transfer from a particle in a fluid medium. Heat and mass transfer rates, as affected by free‐stream turbulence, undoubtedly reflect changes in the ambient flow structure around a body. Attempts have been made, therefore, to relate the established effects of turbulence on the various parts of the flow field around a particle to the variations in heat and mass transfer rates observed for investigations conducted under different conditions. A review has also been made of the effects of periodic oscillations on heat and mass transfer rates in an attempt to provide greater insight into the more complex effects of turbulence.
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