Actuation System for Active Control

Actuation technology plays a crucial role in the product development of active instability control systems in gas turbines. Flow modulating devices are usually used to actively control combustion instability. Two common devices for this purpose are fuel injectors and loudspeakers. While the former adds or reduces the mass flow (which alters the heat release), the latter adds additional velocity which affects the heat release and acoustics [173]. Many studies on the use of these actuation systems in instability closed-loop control have been reported [174 - 176].

2.6.3.1 Fuel Injectors

The fuel injection system is made up of the fluidic and electromechanical parts. The electromechanical part produces an electromechanical field which pushes a spring– loaded poppet. The movement of the poppet controls the aperture of the injector which modulates the flow rate [177] [178]. In this case, the transfer function between the voltage to the poppet position and the mass flow rate from the injector is derived from the Bernoulli, Continuity and Kirchhoff voltage laws [179].

CHAPTER 2: LITERATURE REVIEW

2.6.3.2 Loud Speakers

The actuation system of a loud speaker converts electrical energy into acoustic energy. Thus the actuator dynamics is prescribed by a transfer function between the voltage into the speaker and the acceleration of the loud speaker diaphragm, which is given as

𝑮_𝒍(𝒔) = 𝒌𝟏𝒔𝟐

𝒔𝟐_{+𝟐𝜻𝒍𝝎𝒍𝒔 + 𝝎} 𝒍

𝟐 (2.45)

where bl, kl, k1 ml represent the friction, stiffness, calibrating gain and the mass

property respectively. The effect of this actuation on the combustion system is thus modeled [180]. Table 2.1 gives some published work on the used of fuel and loudspeaker as actuators [181-185] [175][176] [33]. The main challenge now is to accurately capture the interacting mechanism of the flow, flame and the wave fields using a simple and effective actuating technique.

Table 2.1: Fuel and loudspeaker actuators in combustion instability active control.

2.7 Summary

Combustion processes of practical systems such as gas turbines play crucial roles in the enhancement of thermal performance, reduced emissions and better operability. Factors such as fuel-air mixture and injection, fuel-air composition, flow structures, flame dynamics etc. are used to classify flames. One of these classifications is lean premixed flame with high thermal performance and low emission, but with a high susceptibility to combustion disturbances such as blowoff, flashback, autoignition and combustion instabilities. In swirling flows, the coherent flow structures produce flow rotation with inherent flow pressure gradient which causes flow reversal resulting in vortex-induced CRZ. This region of high flame stretch enhances the recirculation of hot gases with cold flow, reduced NOx formation, anchors and stabilizes the flame. A very important parameter of this flow structure is the swirl number (S) which relates

S/N Reference Actuator Sensor Experiment/ModelController Type

1 Dines(1983) Loudspeaker CH* E Phase - Shift

2 Heckl (1988) Loudspeaker P' E Phase - Shift

3 Lang et al. (1987) Loudspeaker P' E/M Phase - Shift

4 Poinsot et al. (1979) Loudspeaker P' E Phase - Shift

5 Blonbou et al. (2000) Loudspeaker P' , OH E Neural Network

6 Murugappan et al. (2003) Loudspeaker P' E LQG-LTR

7 Hathou et al. (1998) Loudspeaker P' M LQR

CHAPTER 2: LITERATURE REVIEW

the axial flux of tangential momentum to the axial momentum flux. A change in this number alters the flow structure with huge impact on the flame and pressure fields. The flame also responds to oscillations in the chamber. The flame behaviour is quantified in terms of flame model which relates the heat release response to inlet velocity fluctuations. In the linear regime, the flame model holds for small oscillation amplitudes but lacks the capability to capture some dynamical features such as limit cycle, mode shape, mode switching, hysterias etc. In order to evaluate these dynamical features, the flame model is extended to the nonlinear regime called the Flame Describing Function (FDF) expressed in terms of Gain and Phase of the heat release fluctuation for different modulating frequencies and oscillation amplitudes. Combustion instabilities caused by a coupling between the unsteady heat release and the dynamic pressure is also influenced by fuel blends, due to changes in the heat release rate. The major challenges currently facing combustion experts are adequate knowledge of the precise operating conditions that cause combustion instabilities, accurate prediction and control of the instability modes. At the moment, there is a limited knowledge of the contributions of each of these factors as well as their combined effect on combustion instabilities. There is a curiosity that the combined effects of these factors could be a potential tool for the control of the destructive oscillation. This study, therefore, focuses on the characterization of the effect of each of these factors on the flow - flame field and their combined effects on the acoustic field.

CHAPTER 3: EXPERIMENTAL DESIGN, SETUP AND MEASUREMENT TECHNIQUES

3 CHAPTER THREE:

EXPERIMENTAL DESIGN, SETUP AND

MEASUREMENT TECHNIQUES

“All life is an experiment. The more experiments you make the better” --Ralph Emerson

This chapter presents the experimental procedures and measurement techniques of the parameters under investigation, as well as a Low Order combustion simulation used in validating the experimental results. Section 3.1 describes the design, manufacture and setup of the combustor, section 3.2 focuses on the measurement techniques and the instrumentation used for the experimental study, section 3.3 briefly describes a Low Order combustion software (OSCILOS) utilised in this study, and finally, section 3.4 summarises the chapter.