Mixing and flow in ducted turbulent jets

Abstract

The aerodynamic flow pattern of a confined, turbulent jet can differ profoundly from that of a free jet, most pronouncedly so in the massive recirculation of downstream gases which occurs under the operating conditions of many industrial furnaces. This paper reports on a study of furnace mixing patterns as represented in the cold by a round jet of oilfog-marked airissuing axially into a cylindrical duct fed by a uniform stream of clear air. Investigation of the fields of mean and fluctuating concentration was made possible by applying the recently developed scattered light technique of illuminating the space-point of interest with a strong light beam and measuring the intensity of 90°-scattered light. The technique has been refined to the point where the signal can be representative of a cubical volume element as small as 0.5 mm on a side. Emphasis was on the aerodynamic flow regime in which appreciable recirculation occurs. A mapping of the field of mean velocity was made with impact tubes. Velocity fluctuations were not measured as such, but indirect evidence was obtained from a mapping of the field of mean static-pressure defect in the jet. The sol-scattered-light technique itself yielded the fields of mean concentration of nozzle fluid, γ¯, and r.m.s. fluctuating concentration, (γ′2¯)12; lateral and longitudinal correlation coefficients and spatial scales of concentration fluctuations; and the wave-number spectrum of concentration fluctuations.