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Active Control of Combustion Instabilities

Göransson, Anders (2001) In MSc Theses
Department of Automatic Control
Abstract
Modern gas turbines use lean premixed combustion to achieve the best compromise between emissions and efficiency. This type of combustion is very sensitive to thermoacoustic instabilities causing high pressure sound waves. One reason for these intense pressure fluctuations is the flame is acting as source of sound, that is placed in an acoustic resonator accumulating acoustic energy. Due to the reflection of sound at the ends standing waves occur with very high amplitudes. The characteristics of these standing waves are determined by the design of the combustor and the speed of sound through the gas. They basically correspond to harmonics of an organ-pipe and define the instability frequency. The high acoustic pressure can lead to higher... (More)
Modern gas turbines use lean premixed combustion to achieve the best compromise between emissions and efficiency. This type of combustion is very sensitive to thermoacoustic instabilities causing high pressure sound waves. One reason for these intense pressure fluctuations is the flame is acting as source of sound, that is placed in an acoustic resonator accumulating acoustic energy. Due to the reflection of sound at the ends standing waves occur with very high amplitudes. The characteristics of these standing waves are determined by the design of the combustor and the speed of sound through the gas. They basically correspond to harmonics of an organ-pipe and define the instability frequency. The high acoustic pressure can lead to higher emissions and structural damage. <p>Suppressing the acoustic fluctuations is critical for efficient and long lasting use of a gas turbine and this thesis focuses on an active control strategy. The basic idea is to excite external sound waves in the <br> combustion system, achieving sound canceling. A feedback control loop is applied where the controller is connected to a valve modulating the fuel to the burner. The varying fuel flow excites sound waves through the thermoacoustic coupling of heat release and acoustic pressure. By measuring the acoustic pressure in the combustor with a watercooled microphone, amplitude and phase can be registered. This pressure signal is used by the controller that determines which sound waves to be excited in the combustion chamber. The controllers were derived by using a mathematically complex control strategy called H-infinity optimization. They were optimized based on an analytical acoustic model and then tested on a combustion test-rig. The results of the optimal controllers were very encouraging since the acoustic <br> pressure was reduced to a fraction of the non-controlled system. </p> <p>Certain characteristics of the optimized controllers were observed and a simple controller overtaking only the characteristics around the instability frequency was designed. This type of controller not based on a mathematical model, with parameters obtained using a simple system identification technique at the <br> instability frequency, also efficiently reduced the acoustic pressure without increasing any pollutants. <br> </p> (Less)
Please use this url to cite or link to this publication:
author
Göransson, Anders
supervisor
organization
year
type
H3 - Professional qualifications (4 Years - )
subject
publication/series
MSc Theses
report number
TFRT-5664
ISSN
0280-5316
language
English
id
8848238
date added to LUP
2016-03-20 11:25:28
date last changed
2016-03-20 11:25:28
@misc{8848238,
  abstract     = {Modern gas turbines use lean premixed combustion to achieve the best compromise between emissions and efficiency. This type of combustion is very sensitive to thermoacoustic instabilities causing high pressure sound waves. One reason for these intense pressure fluctuations is the flame is acting as source of sound, that is placed in an acoustic resonator accumulating acoustic energy. Due to the reflection of sound at the ends standing waves occur with very high amplitudes. The characteristics of these standing waves are determined by the design of the combustor and the speed of sound through the gas. They basically correspond to harmonics of an organ-pipe and define the instability frequency. The high acoustic pressure can lead to higher emissions and structural damage. <p>Suppressing the acoustic fluctuations is critical for efficient and long lasting use of a gas turbine and this thesis focuses on an active control strategy. The basic idea is to excite external sound waves in the <br> combustion system, achieving sound canceling. A feedback control loop is applied where the controller is connected to a valve modulating the fuel to the burner. The varying fuel flow excites sound waves through the thermoacoustic coupling of heat release and acoustic pressure. By measuring the acoustic pressure in the combustor with a watercooled microphone, amplitude and phase can be registered. This pressure signal is used by the controller that determines which sound waves to be excited in the combustion chamber. The controllers were derived by using a mathematically complex control strategy called H-infinity optimization. They were optimized based on an analytical acoustic model and then tested on a combustion test-rig. The results of the optimal controllers were very encouraging since the acoustic <br> pressure was reduced to a fraction of the non-controlled system. </p> <p>Certain characteristics of the optimized controllers were observed and a simple controller overtaking only the characteristics around the instability frequency was designed. This type of controller not based on a mathematical model, with parameters obtained using a simple system identification technique at the <br> instability frequency, also efficiently reduced the acoustic pressure without increasing any pollutants. <br> </p>},
  author       = {Göransson, Anders},
  issn         = {0280-5316},
  language     = {eng},
  note         = {Student Paper},
  series       = {MSc Theses},
  title        = {Active Control of Combustion Instabilities},
  year         = {2001},
}