Mass and momentum transport in a turbulent buoyant vertical axisymmetric jet

Abstract

The entrainment and mixing processes in an axisymmetric vertical turbulent buoyant jet and its transition from a jet to a plume have been studied. The ambient fluid is of uniform density and calm except for the flow induced by the jet, and the density variations are assumed small. A systematic set of experiments was carried out to examine turbulent buoyant jet behavior over a wide range of initial jet Richardson numbers. All experiments were performed in a glass wall tank with dimensions 1.15m x 1.15m x 3.30m deep, equipped with a jet flow source and an instrument carriage to enable the velocity and concentration measurements in the entire jet flow field. The axial and radial velocity components and the concentration of a Rhodamine 6G dye were measured simultaneously at the same point of the jet flow field using a two-reference beam laser-Doppler velocimeter combined with a laser induced fluorescence measuring device. From the time signals of the axial and radial velocity components (w) and (u) and the concentration (c) of the Rhodamine 6G dye, information was obtained concerning the mean values, turbulent fluctuations and correlations between w, u and c, up to 100 jet and 80 plume diameters downstream of the jet exit. More specifically, the mean flow (including the spread rate of the mean velocity and tracer concentration profiles and distribution along the jet axis) and the turbulent structure (including the profile of turbulence intensity, turbulent mass flux of a tracer and turbulent momentum flux) were investigated as a function of distance from jet origin made dimensionless by a characteristic length scale based on jet buoyancy and momentum fluxes. The results from a detailed dimensional analysis were verified experimentally. It was determined that the turbulent flux of a tracer (or buoyancy) varied from 6-10% for jets and was 15-20% of the total for plumes. The turbulent momentum flux was found to be 15% of the local momentum flux transported by the mean flow. While the profiles of w and c and the turbulent velocity profiles are found to be much the same for both jets and plumes, the turbulence intensity profiles of the concentration take higher values in plumes than in jets. More rapid dilutions were obtained in buoyancy driven plumes than in momentum driven jets. Useful information concerning engineering applications is provided from the experimental constants derived.

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