Mixing of inclined dense jets in stationary ambient

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: θo = (15°, 30°, 38°, 45°, 52°, & 60°) 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 θo > 15°. The dimensionless maximum rise height Zmax/(FrD) is independent of source conditions for Fr ≥ 25, and varies from 0.44 for θo = 15° to 2.08 for θo = 60°. Dilution measurements at terminal rise show the difference in dilution is small for θo = 38°–60° and the asymptotic dilution constant is St/Fr = 0.45. The impact dilution Si is also not sensitive to jet angle for θo = 38°–60° and can be expressed as Si/Fr = 1.06 for Fr ≥ 20.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 10–15% 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.

► We present steady-state trajectory and dilution measurements of dense jets for a broad range of jet angles and discharge Fr.
► The flow is jet-like before terminal rise and the dilution achieved is almost constant for jet angles 38°–60°.
► An original derivation of the general formulation of entrainment coefficient for an inclined buoyant jet is presented.
► An ad hoc entrainment hypothesis is not required for dense jets and the detrainment is not related to the general formulation.
► Employing the general formulation, jet integral models under-predict trajectory by 10–15% and associated dilution by 30%.