Shear stress and particle removal measurements of a round turbulent air jet impinging normally upon a planar wall

Abstract

When a round jet of air impinges normally upon a wall, it imposes a shear stress parallel to the wall in all directions from the impingement point. Particle removal from that surface is assumed to be mainly due to the imposed shear stress. This shear stress has been difficult to measure directly and has, in the past, been inferred from particle removal rates. Here we make a fundamental measurement of the mean shear stress imposed upon a planar wall by a normally-impinging turbulent air jet using the technique of oil-film interferometry. The resulting shear–stress distribution is then compared with measured removal rates of latex microspheres from a planar glass surface as a function of the radial distance from jet impingement normalized by the height of the nozzle exit above the surface. The particle removal experiments are carried out with sparse (few collisions) particle distributions. These experiments show that the efficiency of particle removal is directly but not linearly related to the imposed shear stress. A distinct shear stress threshold was found, below which little or no particle removal occurred.