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Flow and mixing characteristics of swirling double-concentric jets subject to acoustic excitation

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Abstract

Characteristic flow modes, flow evolution processes, jet spread width, turbulence properties, and dispersion characteristics of swirling double-concentric jets were studied experimentally. Jet pulsations were induced by means of acoustic excitation. Streak pictures of smoke flow patterns, illuminated by a laser-light sheet, were recorded by a high-speed digital camera. A hot-wire anemometer was used to digitize instantaneous velocity instabilities in the flow. Jet spread width was obtained through a binary edge identification technique. Tracer-gas concentrations were measured for information on jet dispersions. Two characteristic flow patterns were observed: (1) synchronized vortex rings appeared in the low excitation intensity regime (the excitation intensity less than one) and (2) synchronized puffing turbulent jets appeared in the high excitation intensity regime (the excitation intensity greater than one). In the high excitation intensity regime, the “suction back” phenomenon occurred and therefore induced in-tube mixing. The jet spread width and turbulent fluctuation intensity exhibited particularly large values in the high excitation intensity regime at the excitation Strouhal numbers smaller than 0.85. At the excitation Strouhal numbers >0.85, the high-frequency effect caused significant decay of jet breakup and dispersion—the jet spread width and fluctuation intensity decreased sharply and may, at very high Strouhal numbers, asymptotically approach values almost the same as the values associated with unexcited jets. Exciting the jets at the high excitation intensity regime, the effects of puffing motion and in-tube mixing caused breakup of the jet in the near field and therefore resulted in a small Lagrangian integral time and small length scales of fluctuating eddies. This effect, in turn, caused drastic dispersion of the central jet fluids. It is possible that the excited jets can attain 90 % more improvements than the unexcited jets. We provide a domain regarding excitation intensity and Strouhal number to facilitate identification of characteristic flow modes.

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Abbreviations

A a :

Area at exit of swirling jet (= π (D 2o  − D 2)/4)

A c :

Area at exit of central jet (= π d 2/4)

B :

Blockage ratio (= D 2/D 2o ), 0.563

C CO 2 :

Concentration of carbon dioxide gas

D :

Diameter of circular disc, 30 mm

D h :

Hydraulic diameter of annular swirling jet at exit (= D o − D)

D m :

Mean diameter for calculating swirl numbers (= (D + D o)/2)

D o :

Outer diameter of annular swirling jet at exit, 40 mm

d :

Diameter of central jet at exit, 5 mm

F :

Formation number

f exc :

Excitation frequency

f j :

Frequency of flow oscillation in jet

f res :

Resonance frequency

l L :

Lagrangian integral length scale

Q a :

Volumetric flow rate of annular flow

Q c :

Volumetric flow rate of central jet

R τ :

Autocorrelation coefficient

Re a :

Reynolds number of annular flow (= u a D h/\( \upsilon \))

Re c :

Reynolds number of central jet (= u c d/\( \upsilon \))

r :

Radial coordinate, originating from center of circular disc

S :

Swirl number of annular jet

St exc :

Strouhal number based on excitation frequency f exc (= f exc d/u c)

St j :

Strouhal number of flow oscillation in jet

T :

Period of excitation

u :

Axial velocity component

u a :

Volumetric mean axial velocity of annular swirling jet at exit (= Q a /A a)

u c :

Volumetric mean axial velocity of central jet at exit (= Q c/A c)

u c0 :

Instantaneous axial velocity measured at exit of central jet

u′:

Root mean square of axial velocity fluctuations measured along central axis

\( u_{{{\text{c}}0}}^{\prime } \) :

Root mean square of axial velocity fluctuations measured at exit of central jet

w :

Azimuthal velocity component

x :

Axial coordinate, originating from the center of circular disc

α :

Phase angle of excitation waveform

τ :

Shifting time in performing convolution calculation for R τ

τ L :

Lagrangian integral timescale

\( \upsilon \) :

Kinematic viscosity of air

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Authors and Affiliations

  1. Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taipei, 10672, Taiwan, ROC

    R. F. Huang & S. R. Jufar

  2. Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, 10672, Taiwan, ROC

    C. M. Hsu

Authors
  1. R. F. Huang
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  2. S. R. Jufar
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  3. C. M. Hsu
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Correspondence to R. F. Huang.

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Huang, R.F., Jufar, S.R. & Hsu, C.M. Flow and mixing characteristics of swirling double-concentric jets subject to acoustic excitation. Exp Fluids 54, 1421 (2013). https://doi.org/10.1007/s00348-012-1421-2

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  • DOI: https://doi.org/10.1007/s00348-012-1421-2

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