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An experimental study on the vortical structures and behaviour of jets issuing from inclined coaxial nozzles

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Abstract

An experimental study on inclined coaxial jets using laser-induced fluorescence and particle image velocimetry is presented here. The Reynolds numbers of the inner primary jet and outer secondary jet were Re = 2,500 and between Re = 500 and 2,000 (based on gap size), respectively, which corresponded to secondary-to-primary jet velocity ratios (VR) of VR = 0.5–2.0. The secondary-to-primary jet area ratio was 2.25, and 45° and 60° incline-angles were studied. Flow visualizations show that relatively independent inclined primary and secondary jet vortex roll-ups were formed at VR = 0.5. At VR = 1.0, regular pairings and mergings between primary and secondary jet vortex roll-ups led to large-scale entrainment of secondary jet and ambient fluids into the primary jet column and conferred a “serpentile”-shaped outline upon it. While the “serpentile”-shaped outline continued to exist at VR = 2.0, it was a result of stronger secondary jet inner vortex roll-ups which “pinched” the primary jet column regularly. These flow behaviours are observed to intensify with an increase in the incline-angle used. Velocity measurements demonstrate that inclined coaxial nozzles promoted vectoring of the primary jet momentum towards the longer nozzle lengths when velocity-ratio and/or incline-angle were increased. Lastly, peak velocity and higher turbulence intensity levels due to augmented vortical interactions are also detected along shorter nozzle lengths.

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Abbreviations

d :

Annular gap size, d = (D 2 − D 1)/2-t w

D 1 :

Primary nozzle diameter

D 2 :

Secondary nozzle diameter

H :

Nozzle mean height

t w :

Nozzle wall thickness

u :

Streamwise velocity

u rms :

Root-mean-square streamwise velocity

U 1 :

Mean primary jet exit velocity

U 2 :

Mean secondary jet exit velocity

x :

Streamwise distance from nozzle mean height

y :

Cross-stream distance from nozzle centre

ν:

Kinematic viscosity of water

θ:

Nozzle azimuthal location

Re 1 :

Primary jet Reynolds number, Re 1 = U1 D 1/ν

Re 2 :

Secondary jet Reynolds number, Re 2 = U2d/ν

St 1 :

Inner shear region vortex formation frequency, St 1 = fD 1/U 1

St 2 :

Outer shear region vortex formation frequency, St 2 = fd/U 2

AR:

Area-ratio, AR = (D 2/D 1)2

VR:

Velocity-ratio, VR = U 2/U 1

HWA:

Hot-wire anemometry

LDA:

Laser Doppler anemometry

LIF:

Laser-induced fluorescence

PIV:

Particle image velocimetry

px:

Pixel

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Acknowledgments

The authors would like to acknowledge the support for the study by UK Engineering and Physical Science Research Council under project grant EP/F003102/1.

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Authors and Affiliations

  1. School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore, 639798, Singapore

    T. H. New

  2. School of Engineering, University of Liverpool, Liverpool, L69 3GH, United Kingdom

    E. Tsioli

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  1. T. H. New
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  2. E. Tsioli
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Correspondence to T. H. New.

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New, T.H., Tsioli, E. An experimental study on the vortical structures and behaviour of jets issuing from inclined coaxial nozzles. Exp Fluids 51, 917–932 (2011). https://doi.org/10.1007/s00348-011-1120-4

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  • DOI: https://doi.org/10.1007/s00348-011-1120-4

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