Control of diffusing flow at low Reynolds numbers with annular wall jets

Abstract

During the course of experiments to determine the influence of coflowing streams of air on the lift-off height of a methane jet flame, it was necessary to achieve a smooth axial decay of the velocity field in a conical diffuser. It was also crucial that the mean axial velocity profile was flat in the central region of the flow. These criteria proved to be quite difficult to meet, even with diffuser half-angles limited to 12.6°, partly because of the low Reynolds numbers involved, which were in the range 780-3500 based on the inlet pipe diameter.

A satisfactory solution, following Duggins, proved to be the injection of high-velocity air along the wall from a narrow annulus around the inlet. For diffusers with half-angles of 3.5, 7.5 and 12.6°, the optimum rates of injection were found to be independent of the half-angle but dependent on the Reynolds number and on whether a gauze mesh was present at the inlet. The efficiency of diffusion reached an optimum value of 60 per cent at the highest Reynolds numbers, and this was achieved both with and without the mesh.