Research Papers

Pulsed Control of Thermoacoustic Instabilities: Analysis and Experiment

[+] Author and Article Information
J. Matthew Carson

1112 Donelea Lane Northwest, Concord, NC 28027mcars@vt.edu

William T. Baumann

Department of Electrical Engineering, Virginia Tech, 302 Whittemore Hall, Blacksburg, VA 24061baumann@vt.edu

William R. Saunders

 Adaptive Technologies, Inc., 2020 Kraft Drive, Suite 3040, Blacksburg, VA 24060will@adaptivetechinc.com

J. Dyn. Sys., Meas., Control 131(4), 041007 (May 18, 2009) (7 pages) doi:10.1115/1.3117192 History: Received September 16, 2007; Revised January 28, 2009; Published May 18, 2009

Thermoacoustic instabilities in combustors have been suppressed using phase-shift algorithms pulsing an on-off actuator at the limit cycle frequency (flc) or at the subharmonics of flc. It has been suggested that control at a subharmonic rate may extend the actuator lifetime and possibly require less actuator bandwidth. This paper examines the mechanism of subharmonic control in order to clarify the principles of operation and subsequently identify potential advantages for combustion control. Theoretical and experimental arguments show that there must be a Fourier component of the subharmonic control signal at flc in order to stabilize the limit cycling behavior. It is also demonstrated that the magnitude of that Fourier component must be equivalent to the signal magnitude for a linear phase-shift controller that operates directly at flc. The concept of variable-subharmonic control is introduced whereby the actuator is pulsed at the instability frequency to initially stabilize the system and then is pulsed at a subharmonic frequency to maintain stability. These results imply that an actuator used for subharmonic control cannot be effective unless its bandwidth spans the instability frequency. The advantage of reduced cycling may still be realized but will require higher control authority to produce the same effect as an actuator pulsed at the instability frequency.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 2

Pulse generator block diagram

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Figure 3

Typical pulse train

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Figure 4

Describing function with fixed-amplitude pulse heights

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Figure 5

Describing function with two intersections

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Figure 6

Depiction of pole movement in the complex plane as the control gain is increased

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Figure 7

Describing function with proportional height pulses

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Figure 8

Control algorithm illustration

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Figure 9

Test system layout

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Figure 10

Linear phase shifter hysteresis curve

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Figure 1

Combustion block diagram

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Figure 11

Actual versus expected ratios for half-harmonic signals

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Figure 12

Actual versus expected ratios for third-harmonic signals

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Figure 13

Describing function plots superimposed on hysteresis curve

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Figure 14

Instability level for fixed-pulse height controller

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Figure 15

Gain of fixed-pulse height controller

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Figure 16

Results with variable-subharmonic forcing



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