Analysis of particle movement in the velocity layer on rotating surfaces

Abstract

Experiments on rotating in the air cones with vertex angle β=120∘ and the flat disc (i.e. a cone with β=180∘) show that for frequencies Ω ≥ 150 rpm projections of the particles' trajectory on a horizontal plane are close to a logarithmic spiral. The analysis of this approximating function and the results of the physical modeling, which helped to determine the coefficient dependence of the particles’ near-surface resistance CD* when moving in the viscous fluid (air), demonstrated that the particles do neither slide nor roll on the surface of the rotary heat exchangers. The average distance between a single particle and the surface is less than but comparable to the thickness of the stationary near-surface velocity layer of the viscous fluid, the thickness of which is at least 10 times greater than the size of the particles. When subjected to the gravity field and the centrifugal force, particles are “jumping” on the surface, the height of these “jumps” being approximately equal to the displacement thickness of the velocity layer. The movement of the particles alongside the normal to the surface as well as their unidirectional rotation under the influence of the velocity gradient in the velocity layer lead to the mass exchange between the layers of the viscous fluid; consequently they intensify the heat abstraction from the hot surface and its transfer to the air and the particles. These factors lead to the reduction of the heating time of the particles and the increase the quality of the flash-products.