THE BOUNDARY LAYER IN THE CONVERGING NOZZLE OF A SWIRL ATOMIZER

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Abstract
Swirl atomizers are used in agricultural spraying machinery and in oil-fired furnaces. The swirl is produced by leading in the liquid tangentially at the outside of a chamber and allowing it to spray out through a central orifice of small diameter. The liquid is forced towards the centre by high pressure in the supply pipe. When a liquid which is not swirling approaches an orifice through a converging chamber, it converges from all points of a cross-section, only decreasing in velocity in the boundary layer near the wall. When the liquid is swirling, however, the radial pressure gradient, which necessarily accompanies the swirling motion and holds the liquid particles in their circular paths, acts on the retarded boundary layer, driving it along the surface of the chamber towards the orifice. Thus a condition can arise in which practically the whole of the outflow from the orifice is fed by a boundary-layer current close to the surface of the swirl chamber. The study of boundary layers containing transverse as well as longitudinal components of velocity is of interest apart from its practical importance, and one case has been investigated completely, namely, the boundary layer close to a rotating disk. The case in which the solid surface is at rest and the liquid rotating, or swirling in a free vertex, is much more complicated, and the present analysis is only an approximate one based on Pohlhausen's use of the momentum integrals through the boundary layer. The growth of the boundary layer from the edge of a cone immersed in a fluid swirling in a free vertex is discussed, and the velocity with which the fluid in the layer flows towards the vertex is calculated.

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