Mixing of inclined dense jets in stationary ambient

Abstract

This paper reports results of a comprehensive experimental investigation of inclined round dense jets in an otherwise stagnant fluid. The tracer concentration field is measured for six jet discharge angles θ0=(15o,30o,38o,45o,52o,&60o) and jet densimetric Froude number of Fr = 10 – 40 using the planar laser-induced fluorescence (LIF) technique; selected jet velocity measurements are made using Particle Image Velocimetry (PIV). The detailed jet mixing characteristics and turbulence properties are presented. The direct velocity measurement reveals that the mixing is jet-like until the maximum rise. Empirical correlations for the maximum jet rise height, jet dilution at maximum rise, and impact dilution are presented. Both the time-mean concentration and intermittency show that the upper jet edge spreading is similar to a positively buoyant jet; at the lower edge the buoyant instability induces significant detrainment and mass outflux for θ0>15o. The dimensionless maximum rise height Zmax/(FrD) is independent of source conditions for Fr25, and varies from 0.44 for θ0=15o to 2.08 for θ0=60o. Dilution measurements at terminal rise show the difference in dilution is small for θ0=38o60o and the asymptotic dilution constant is Si/Fr = 0.45. The impact dilution Si is also not sensitive to jet angle for θ0=38o60o and can be expressed as Si/Fr = 1.06 for Fr20. The Lagrangian jet model VISJET is used to interpret the experimental results. A detailed derivation for a general formulation of the entrainment coefficient is presented. Despite the observed detrainment, the trajectory and dilution are reasonably predicted; the maximum jet rise is generally under-predicted by 1015% and associated dilution by 30%. However, the predicted variation of jet behavior with discharge angle is in good agreement with measurements. The experimental data is also compared with predictions of alternative models that employ an ad hoc entrainment hypothesis.

Publication
Journal of Hydro-environment Research, 6(1): 9-28
Chris CK Lai
Chris CK Lai
Assistant Professor

My research interests include experimental fluid mechanics, turbulent mixing and transport, and theory and modeling of turbulence.

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