Turbulent Jet Mixing Enhancement and Control Using Self-Excited Nozzles

[+] Author and Article Information
Uri Vandsburger

Department of Mechanical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061

Yiqing Yuan

Department of Powertrain Virtual Simulation, Advanced Vehicle Engineering, CIMS 484-01-13, DaimlerChrysler Corporation, Auburn Hills, MI 48326-2757

J. Fluids Eng 129(7), 842-851 (Jan 03, 2007) (10 pages) doi:10.1115/1.2745840 History: Received September 13, 2006; Revised January 03, 2007

A new self-excited jet methodology was developed for the mixing enhancement of jet fluid with its surrounding, quiescent, stagnant, or coflowing fluid. The nozzles, of a square or rectangular cross section, featured two flexible side walls that could go into aerodynamically-induced vibration. The mixing of nozzle fluid was measured using planar laser-induced fluorescence (PLIF) from acetone seeded into the nozzle fluid. Overall, the self-excited jet showed enhanced mixing with the ambient fluid; for example, at 390Hz excitation a mixing rate enhancement of 400% at xD=4 and 200% at xD=20 over the unexcited jet. The mixing rate was sensitive to the excitation frequency, increasing by 60% with the frequency changing from 200 to 390Hz (corresponding to a Strouhal number from 0.052 to 0.1). It was also observed that the mixing rate increased with the coflow velocity. To explain the observed mixing enhancement, the flow field was studied in detail using four-element hot wire probes. This led to the observation of two pairs of counter rotating large-scale streamwise vortices as the dominant structures in the excited flow. Shedding right from the nozzle exit, these inviscid vortices provided a rapid transport of the momentum and mass between the jet and the surrounding fluid at a length scale comparable to half-nozzle diameter. Moreover, the excited jet gained as much as six times the turbulent kinetic energy at the nozzle exit over the unexcited jet. Most of the turbulent kinetic energy is concentrated within five diameters from the nozzle exit, distributed across the entire jet width, explaining the increased mixing in the near field.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Schematic of the self-excited nozzle

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Figure 2

Test tunnel and the coordinate system

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Figure 3

PLIF experimental setup for nozzle fluid concentration measurements

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Figure 4

Domains for the definition of the mixing index

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Figure 5

Concentration maps for a square nozzle; Re=17,000, St=0.12; (a) Unexcited, λ=0; (b). Excited, λ=0; (c). Unexcited, λ=0.3; (d). Excited, λ=0.3.

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Figure 6

Mixing index of a self-excited square nozzle for various excitation frequencies and velocity ratios; Re=31,000

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Figure 7

Effect of nozzle aspect ratio on mixing index; Re=17,000, St=0.12

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Figure 8

Mean axial velocity and turbulent intensity profiles across a natural square jet at x∕D=0.2

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Figure 9

End-on velocity vector maps of the excited 1:4.72 jet; phase=0deg, Re=10,000, λ=0.5, St=0.19

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Figure 10

Velocity vector maps of the excited 1:4.72 jet at 0deg phase of cycle; Re=10,000, λ=0.5, St=0.19

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Figure 11

Schematic of the flow entrainment induced by excitation near the nozzle exit and roll-up of the large-scale streamwise vortices

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Figure 12

Turbulent kinetic energy maps of the excited 1:4.72 jet; Re=10,000, λ=0.5, St=0.19

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Figure 13

Power spectra of the excited 1:4.72 jet along its centerline; Re=10,000, λ=0.5, St=0.19

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Figure 14

Period-averaged power spectra of the excited 1:4.72 jet along its centerline; Re=10,000, λ=0.5

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Figure 15

Streamwise variation of the spectral component at the excitation frequency of the excited 1:4.72 jet along its centerline; Re=10,000, λ=0.5, St=0.19, fe=475Hz



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