Thermoacustic instabilities in gas turbine burners : new diagnostic methodology

Abstract

Gas turbine engines for power generation and propulsion applications have traditionally used diffusion-flame combustors because of their reliable performance and reasonable stability characteristics. Unfortunately, this type of combustor usually produces unacceptably high levels of thermal NOx. The increasingly strict regulation for pollutant emissions has recently led engine manufacturers to develop combustors that meet various regulatory requirements. New concepts for combustion technology have been introduced to the Gas Turbine industry, including lean-premixed combustion. Lean-premixed combustion appears to be the most promising technology for practical systems at the present time. In lean-premixed combustion, the fuel and air are premixed upstream of the combustor to avoid the formation of stoichiometric regions. The combustion zone is operated with excess air to reduce the flame temperature; consequently, thermal NOx is virtually eliminated. Unsteady flow oscillations, also referred to as combustion instability, however, have emerged as a common problem, and hindered the development of lean-premixed combustors. These oscillations may reach sufficient amplitudes to interfere with engine operation, and in extreme cases, lead to failure of the system due to excessive structural vibration and heat transfer to the chamber. The associated pressure oscillations and possibly enhanced heat transfer can lead to a deterioration in the system performance, and may be sufficiently intense to cause structural damage. Combustion instability remains a critical issue limiting the development of low-emission, leanpremixed Gas Turbine combustion systems. Combustion oscillations are not limited to gas turbine engines. They have been observed in the development of virtually all propulsion systems, including liquid rocket engines. The system usually operates near the lean blowout limit, then a small perturbation in the equivalence ratio may produce a significant variation in heat release, which, if it resonates with the chamber acoustic wave, can result in large excursions of combustion oscillations. The phenomenon may be defined as the unsteady motions in a dynamic system capable of sustaining large oscillations over a broad range of frequencies. The work is organized in two parts: an extensive bibliographic review of combustion instabilities and the motivation of this work in part 1; and the study about a new diagnostic methodology for thermoacoustic instabilities detection and future control in part 2. The part 1 is important because of lack of books that describe clearly and exhaustively the complex phenomenology of thermoacoustic instabilities, however the experts of thermoacoustic instabilities can skip directly from it to part 2 where is possible to find the improvements with respect to the state of the art. The part 1 is organized into five chapters. The chapter 1 gives a general background about modern Gas Turbine engine and shows the need of new sensors for measurements systems. The chapter 2 explains the characteristic of combustion noise, investigates the mechanisms driving combustion instabilities, the causes of instabilities and damping processes, and then provides a comprehensive review of the advances made in thermo-acoustic instabilities. The physics of combustion oscillations, most commonly caused by a coupling between acoustic waves and unsteady heat release, are discussed, and the concept of using feedback control to interrupt these interactions is introduced. The chapter 3 provides a survey of recent progress in passive and active control of combustion instabilities. The objective is to optimize combustor operations, monitor the process and alleviate instabilities and their severe consequences. It contains a review of some facets of combustion and focuses on the sensors that take or could take part to combustion control solutions. The chapter 4 provides a critical review of analytical and numerical models and criteria forcombustion instability. The chapter 5 shows an application of instability control to real heavy duty Gas Turbine of Vx4.3A Siemens-Ansaldo. It shows the final purpose of the studies about thermoacoustic instability and one of the industrial fields where the new technique ODC proposed here could be easily applied. The part 2 is organized into three chapters. The chapter 6 explains the conventional optical techniques for concentration of chemical species, velocity measurements and temperatures, and then it gives a theoretical approach for thermal emission of flames. The chapter 7 proposes a new diagnostic technique named ODC (Optical Diagnostics of Combustion) developed and patented in ENEA and it shows an experimental approach for radiative emission of flames. Radiant energy spectra have the same dynamics of Turbulence (macroscale, inertial and dissipative ranges; slope= - 5/3) and reveal Chemical Kinetics high frequency effects. The chapter also explains why Radiant Energy shows Turbulence dynamics. The chapter 8 shows the results and gives an experimental proof that real-time information of combustion instabilities and mean velocity component can be performed by analyzing Radiant Energy captured by means of photo-diodes (ODC). It investigates the use of flame Radiant Energy signal: due to its relation with both Turbulence and Chemical Kinetics, it may reveal the state of a flame and the eventual instability precursor events. It shows experimental analysis of turbulent premixed combustion by means of ODC to laboratory and industrial burners. A collection of radiative emissions by several flames is analyzed in this chapter. Such flames have been obtained in a number of burners in a large range thermal power (i.e., 3 kWt- 1MWt), either premixed or not, fed with different fuels (i.e., methane, hydrogen, oil) and with different physical states (i.e., gas and liquid fuel), while radiative emission is collected by means of a photo-diode. The purpose of this chapter is to show that the usefulness of measurements of Radiant Energy emission from flames can be enhanced by focusing on a spatially limited region, by means of the auto-correlation and cross-correlation of signals from two points. Finally it shows the fulfilment of a test facility for real burner in ENEA Research Center, i.e., COMET-HP (COMbustion Experimental Tests in High Pressure), 1 MWt premixed CH4/Air and gives several experimental tests. Concluding, a new optical instrument based on photo-diodes, called ODC, has been applied for thermoacoustic instability analysis. ODC provides information at very low cost and in real time about turbulent and chemical scales. Lastly five attachments conclude the PhD thesis. The attachment A shows Lighthill’s theory and acoustic analogy whereby the governing equations of motion of the fluid are coerced into a form reminiscent of the wave equation of “classical” (i.e. linear) acoustics. The attachment B provides a brief explanation of random signals, e.g. random vibrations, and spectra analysis, according to which a periodic function can be broken down into its harmonic components. The attachment C presents an analytical treatment of acoustic signals. The attachment D gives a theoretical approach for thermodynamic equations. Turbulent combustion is a multi-scale problem where complexity lies in the interaction between fluid dynamics and chemistry. This attachment provides a theoretical understanding of some of the scale physics in turbulent reacting flows. The attachment E provides laws for thermal radiation. At high temperatures thermal radiation is the main mode of heat transfer.