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Research Papers: Fundamental Issues and Canonical Flows

Influence of Geometry on Starting Vortex and Ejector Performance

[+] Author and Article Information
Fei Zheng, Andrey V. Kuznetsov

Department of Mechanical & Aerospace Engineering,  North Carolina State University, Raleigh, NC 27695

William L. Roberts1

Department of Mechanical & Aerospace Engineering,  North Carolina State University, Raleigh, NC 27695

Daniel E. Paxson

 NASA Glenn Research Center, 21000 Brookpark Road, Cleveland, OH 44135

1

Corresponding author.

J. Fluids Eng 133(5), 051204 (Jun 07, 2011) (8 pages) doi:10.1115/1.4004082 History: Received June 03, 2010; Revised April 18, 2011; Published June 07, 2011; Online June 07, 2011

For many propulsion devices, the thrust may be augmented considerably by adding a passive ejector, and these devices are especially attractive for unsteady propulsion systems such as pulse detonation engines and pulsejets. Starting vortices from these unsteady devices dominate the flowfield and control to a great extent the level of the thrust augmentation. Therefore, it is of fundamental interest to understand the geometric influences on the starting vortex and how these manifest themselves in augmenter/ejector performance. An unsteady Reynolds averaged Navier–Stokes calculation was used to study the physics of a starting vortex generated at the exit of a pulsed jet and its interaction with an ejector. A 50 cm long pulsejet (typical hobby scale, allowing comparison with experimental data) with a circular exit was modeled as the resonant driving source and used to suggest an optimal ejector geometry and relative position. Computed limit-cycle thrust augmentation values compared favorably to experimentally obtained values for the same ejector geometries. Results suggest that the optimal diameter of the ejector is related to its relative position, dictated by the trajectory of the vortex toroid. The effect of the length of the ejector (which determines the natural frequency of the ejector, related to the acoustic processes occurring in the ejector) on overall performance was also investigated and shown to be less important than the ejector diameter.

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Copyright © 2011 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Mass flow through the ejector

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Figure 2

Pulsejet and vortex generated by the jet

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Figure 3

Mesh generation for combined pulsejet and ejector simulations

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Figure 4

Axial force on the outside surface of the ejector

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Figure 5

Geometrical parameters of the ejector

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Figure 6

Pressure distribution generated by the vortex ring. Upper picture shows pressure, lower picture shows vorticity.

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Figure 7

(a) Moving averages of chamber pressure and various thrust sources for the pulsejet with the base case ejector. (b) Instantaneous chamber pressure over several cycles.

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Figure 8

(a) Pressure downstream of the exit of the pulsejet and in the ejector. (b) Pressure downstream of the pulsejet exit. (c) Thrust on the ejector during one pulsejet cycle. Pictures in (a) corresponding to horizontal position in (c): A(5.16), B(5.2), C(5.22), D(5.24), E(5.26), F(5.28), G(5.4), H(5.52), I(5.6), J(5.72).

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Figure 9

Solid line: trajectory of vortex ring core in absence of the ejector. Symbols: position of the leading edge of the ejector for the simulated cases.

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Figure 10

Thrust augmentation as a function of D/d

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Figure 11

Thrust augmentation as a function of x/d

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Figure 12

Pressure oscillations in the ejector

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Figure 13

Thrust augmentation as a function of L/d

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