Modelling and multi mode control of thermoacoustic instabilities

• Modellierung und Multimodenregelung von thermoakustischen Instabilitäten

Shariati, Sadaf; Abel, Dirk (Thesis advisor); Moeck, Jonas (Thesis advisor)

Aachen (2017, 2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

This research study focuses on modelling, simulating, and multi mode control of the thermoacoustic instabilities in combustion chambers of modern gas turbines. The chamber is excited acoustically by a loudspeaker mounted underneath the flame zone. The chamber is identified in cold flow condition as well as both steady and unsteady combustion. The stability analysis based on the identification results shows that premixed flame becomes unstable at fuel to air ratio larger than 0.8. The frequency response obtained from the identification results shows that the Helmholtz mode is the dominant unstable mode of the chamber which couples with the flame and forms thermoacoustic instability. The analytical model is developed in Matlab based on the 1D acoustic equations and the resulting wave equation with the nonlinear flame transfer function. The loudspeaker is identified experimentally and modelled by Multi Microphone Method (MMM) approach. The main purpose of modelling the combustion chamber is to predict the onset of the thermoacoustic instabilities when a control parameter such as equivalence ratio or fuel flow is altered. In the analytical model developed in this work, the delay factor in the flame transfer function influences the interaction between the incoming acoustic wave and the flame heat release, affects the thermoacoustic stability. The loudspeaker is an electromechanical system coupled with the pressure acoustic inside the tube in which each subsystem is governed by different sets of differential equations. Hence, multiphysics modelling is required to model this system. For further stability analysis, a finite element model (FEM) is developed in Comsol environment and the results are compared to the results obtained from the analytical model developed in Matlab. The reason for using FEM is its high accuracy and convenience in modelling bulk resonance such as Helmholtz mode. On the other hand, the analytical model developed in Matlab is linearised and used in model based control and for control prototyping. Based on the obtained reduced order model, several model based control approaches are developed and compared. The experimental results show that the dual mode controller with a Pade approximation as local stabilizer, is the most robust controller and works in a wider range of OPs. Finally, two different model based constrained approaches are developed to stabilize a Rijke tube with repetitively pulsing plasma discharge: a dual mode controller and a dynamic matrix controller. The two concepts are evaluated by using both simulation and the real setup.