Velocity Characteristics of a Confined Coaxial Jet

Abstract

The velocity characteristics of a turbulent, confined, coaxial-jet flow have been determined by measurement and by the solution of conservation equations in differential form. The ratios of maximum annulus to pipe velocity were 3 and 1 and, in both cases, the profiles were fully developed in the exit plane. The geometric arrangement corresponded to a model furnace and the investigation was undertaken to provide information, for isothermal flow, relevant to furnace flows. The measurements were obtained with a hot-wire anemometer and include distributions of the axial mean velocity and the components of the Reynolds-stress tensor. They show, for example, that the larger velocity ratio results in a larger region of recirculation, larger velocity gradients and larger turbulence intensities in the mixing region and downstream of the region of reverse flow. Numerical solutions of the time-averaged forms of the equations of conservation of mass and momentum, together with equations for turbulence energy and dissipation rate, provided results which are in close agreement with the measurements except in regions of more than one major component of the velocity-gradient tensor where they are substantially in error. The reasons for the discrepancies and the consequential value of the calculation method are discussed.