Combustion Instabilities In Gas Turbines Research Articles

Effect of Hydrogen Enrichment on Transfer Matrices of Fully and Technically Premixed Swirled Flames

Abstract Knowledge of flame responses to acoustic perturbations is of utmost importance to predict thermoacoustic instabilities in gas turbine combustors. However, measuring transfer functions linking acoustic quantities upstream and downstream of flames are very challenging in practical systems and these measurements can significantly deviate from state-of-the-art models. Moreover, there is a lack of studies investigating the effect of hydrogen enrichment on the response of natural gas (NG) flames. In this work, measurements of flame transfer matrices (FTMs) of turbulent H2/NG flames in an atmospheric combustor featuring an axial swirler burner have been performed, allowing us to unravel the transition between FTM in fully premixed (FP) and in technically premixed (TP) conditions. Furthermore, imaging of OH* chemiluminescence and OH-planar laser induced fluorescence are obtained for characterizing the topology of the flame for varying H2 fraction and mixing conditions. Transfer matrices are measured using the multimicrophone method for H2 fractions ranging from 12% to 43% in power. Afterward, the flame transfer functions (FTFs), which linearly relate the coherent fluctuations of the heat release rate to the acoustic velocity oscillations, are obtained from the FTM by using the Rankine–Hugoniot jump conditions across the flame. Using the OH* chemiluminescence intensity as a surrogate for the heat release rate, the FTF based on this optical measurement is also extracted and compared to the one exclusively obtained with the multimicrophone method. As expected, the two different methods are in very good agreement for the FP case and significantly differ for the TP case. Indeed, chemiluminescence fluctuations cannot be directly linked to heat release rate fluctuations when the acoustic forcing induces equivalence ratio fluctuations at the flame, making the optical method unusable for TP configurations. We also show that the two methods agree in the high end of the explored excitation frequency range and we provide an explanation to this intriguing finding. Moreover, we investigate the sensitivity of the FTM measurement to the estimate of the speed of sound in the rig in FP conditions. Finally, the measured FTFs are fitted with FTF models based on multiple distributed time delays. This allows us to explain the frequency dependence and the hydrogen fraction dependence of the gain and the phase in FP and TP conditions.

Control of Azimuthal Combustion Instability Through the Injector Mounting Surface of Annular Combustors

A three-dimensional combustion instability model is presented to study the effects of the wall impedance distributions of annular combustors on thermoacoustic unstable modes, consequently seeking an effective passive methodology to suppress azimuthal combustion instability. The eigenfunction is obtained by use of Green’s function in an enclosed space, whereas the scattered disturbances at flames, the perforated screen where the injectors are installed, and the side perforated liner are characterized by the monopoles, respectively. Results show that compared to the side perforated liner with the restriction of radial size, the impedances over the injector mounting surface provide more acoustic energy dissipation to suppress the azimuthal combustion instabilities. Two patterns of impedance distribution, uniform and radial nonuniform, over the injector mounting surface are then investigated through the parametric variations and associated acoustic energy analysis. A stable combustion system can be realized in combination with homogeny or difference between the spatial impedance distributions and associated optimum perforated parameters, such as perforation ratio, Mach number of the mean bias flow, and aperture radius. It allows mitigating possible design limitations with the appropriate parameter combinations. The present investigation thereby offers an alternative model and methodology for suppression of azimuthal combustion instabilities in gas turbines.

Water-Cooled Fast-Response Wall-Static Pressure Probes for Harsh Combustor Environment Applications

The present paper discusses the design methodology for a water-cooled fast-response wall-static pressure probe intended for measurements in the combustion chamber of gas turbines, as well as the results from a series of tests performed with the prototype of the probe mounted in the primary zone of the combustion chamber of a turboshaft engine test rig. The first part of the present paper reviews the design methodology of a water-cooled fast-response wall static pressure probe intended for measurements of combustion noise and instabilities in gas turbine combustors, describing the optimization of measurement performance in terms of frequency bandwidth ([0 – 40kHz]) as well as the design of the cooling layout ensuring the sensor’s integrity within a harsh environment. The second part of the paper focuses on the analysis of the results obtained from trial tests performed using the probe prototype mounted in the primary zone of the combustion chamber of a helicopter engine test rig. The strengths and strong potential for further growth of the current probe design are highlighted, while its shortcomings and (short-term) solutions to these shortcomings are also discussed.

Modelling the response of a turbulent jet flame to acoustic forcing in a linearized framework using an active flame approach

This study performs a linear mean field analysis of a turbulent reacting methane-air jet flame, with the goal of predicting the response of the reacting flow to upstream acoustic actuation. Unlike previous studies, this work develops and applies an active flame approach by taking the heat release oscillations of the flame resulting from the acoustic fluctuations into account. For an active flame approach in the linear mean field analysis, a linearized combustion model is necessary. Linearizing Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) combustion models leads to closure problems, making their application in this context troublesome, whereas Reynolds-averaged Navier Stokes (RANS) combustion models prove to circumvent this problem making them suitable candidates for this purpose. The RANS combustion models are linearized around the temporal mean state variables of the turbulent jet flame, which is obtained by LES. An a priori analysis shows that a linearized RANS–Eddy Break Up (EBU) model is the best suited among all investigated combustion models for the investigated set-up and reproduces with high accuracy the fluctuations in reaction rate obtained in the LES. Furthermore, the linearized governing equations of the flow including the linearized EBU model for the reaction rate are solved for incoming acoustic perturbations. The response modes show that the reaction rate oscillations are caused by Kelvin–Helmholtz vortex rings, which perturb the jet flame. The results are in good agreement with the LES simulations in terms of the mode shapes of both reaction rate and velocity fluctuations. This study represents a basis for linear mean field analysis of turbulent flames and monolithic modelling approaches for thermoacoustic instabilities in gas turbine combustors.

Effects of Acoustic Excitation on the Combustion Instability of Hydrogen-Methane Lean Premixed Swirling Flames.

Lean premixed flames are useful for low nitrogen oxide (NOx) emissions but more prone to induce combustion instability in gas turbines. Combustion instability of a lean premixed swirling flame (LPSF) with hydrogen–methane was investigated experimentally. The effects of hydrogen addition on combustion instability with equivalence ratios 0.75–1 were investigated with acoustic frequencies (90–240 Hz) and acoustic amplitudes (the ratio of velocity fluctuation to an average velocity of 0–0.5), respectively, which are characterized by the gain and phase of the flame describing function (FDF). The evolution of vortex and the flame morphologies were observed by the particle image velocimetry (PIV), intensified charge-coupled device (ICCD), photomultiplier tube (PMT), and Cassegrain optical systems. The global and local heat release fluctuations of the LPSF were shown by CH*/OH* chemiluminescence and temperature measurements. Results show that the FDF features maximum and minimum gain values in the acoustic frequency range of 90–240 Hz and reaches local maximum peaks at 110 and 180 Hz and local minimum peaks at 160 Hz. It can also be observed that varying velocity amplitudes (0–0.5) have greater effects on the gain and phase of FDF than changing equivalence ratios (0.75–1) for lean swirling flames. Higher velocity amplitudes more effectively intensified the compression of the flame length, which enhanced the mixing of the high-burning gas and the unburned gas, and then heat release fluctuations increased. However, it is more interesting that the effects of hydrogen addition on the combustion instability of the LPSF show a completely opposite phenomenon due to acoustic frequency under all experimental conditions. The FDFs were compared at typical frequencies of 140 and 180 Hz, and it was found that combustion instability enhanced with increasing hydrogen content at 140 Hz while weakened at 180 Hz. The flow field of PIV images shows that it is related to the location and development of vortices in the flame with varying acoustic frequencies. The intensity of OH*/CH* chemiluminescence, local temperature, and heat release rate show the same changing trend with the flame morphology for two acoustic parameters with the increasing hydrogen content in the LPSF. This directly affects the compression and curvature of the LPSF and thereby changes the mixture and temperature of the combustible gas, which influence the heat release fluctuation of the LPSF.

Predicting Onset of Combustion Instability in Gas Turbines Using Fiber Optic Sensors

A data driven method based on cross-entropy derived from symbolic time series analysis of the chemiluminescence and pressure variations has recently been found to be robust and computationally efficient among existing methods for predicting instability. In this article, we report the development and testing of compact and field-deployable optical fiber-based modules for sensing chemiluminescence and pressure variations in the combustor. The time-series data obtained from the above sensor modules are used to deduce the D-Markov cross-entropy for different protocols corresponding to typical combustor operation conditions. Such cross-entropy data is compared with similar data obtained from standard high speed camera and PZT-based pressure sensors. It is found that the fiber optic sensor modules compare favorably with the conventional sensors thereby providing an exciting new pathway for field-deployable solutions to monitor the onset of thermo-acoustic instability in combustors.

Investigation of self-induced thermoacoustic instabilities in gas turbine combustors

The self-induced thermoacoustic instabilities in partially premixed combustors remain a significant challenge which has received a limited research attention. This numerical work investigates the effect of air-fuel equivalence ratio ∅ on exciting thermoacoustic instabilities and its impact on the characteristics of both the acoustic and combustion fields in a partially premixed swirl combustor. For this purpose, three sets of numerical investigations are studied. The first set is a comparison between the present numerical study and previous experimental work considering the normalized combustor wall temperature. The second set of simulations is performed to predict the behavior of the reacting and non-reacting flows with regard to pressure fluctuations, power spectral density, and the sound pressure levels. The third set of simulations considered the effect of the air-fuel equivalence ratio on the acoustics and combustion characteristics. The results reveal that at an equivalence ratio of 0.5, the sound pressure level for the non-reacting and reacting flows are 98 and 118 dB, respectively. It is concluded that the equivalence ratio plays a crucial rule on exciting combustion instabilities at various amplitudes and frequencies. The sound pressure levels increase with increasing in the air-fuel equivalence ratio due to increase the heat release rate.

Experimental and Numerical Investigation into the Propagation of Entropy Waves

Entropy waves are an important source of indirect combustion noise and potentially contribute to the generation of thermoacoustic instabilities in gas-turbine combustors. Entropy fluctuations generated by unsteady combustion are known to disperse and diffuse as they convect toward the combustor exit. In this work, the propagation of entropy waves is investigated by means of experiments in a newly developed entropy rig and numerical simulations based on the large-eddy simulation approach. Both experimental and numerical results demonstrate that the amplitude of entropy fluctuations decays as a function of wave parameters and propagation distance, and it scales well with a local Helmholtz number . A new theoretical model for the computation of the entropy transfer function suitable for inclusion in low-order models for combustion instabilities is proposed. Assessment against numerical and experimental results shows the capability of the model to give a proper representation of the decay of entropy waves in terms of both magnitude and phase of the entropy transfer function. Furthermore, by comparison with the large-eddy simulation results, it is shown that, at low Helmholtz numbers, the contribution of the differential convection to the decay of entropy waves is dominant; whereas for high values of the Helmholtz number, the turbulent mixing and diffusion also become important.

Experimental and Numerical Investigation of Thermoacoustic Sources Related to High-Frequency Instabilities

Flame dynamics related to high-frequency instabilities in gas turbine combustors are investigated using experimental observations and numerical simulations. Two different combustor types are studied, a premix swirl combustor (experiment) and a generic reheat combustor (simulation). In both cases, a very similar dynamic behaviour of the reaction zone is observed, with the appearance of transverse displacement and coherent flame wrinkling. From these observations, a model for the thermoacoustic feedback linked to transverse modes is proposed. The model splits heat release rate fluctuations into distinct contributions that are related to flame displacement and variations of the mass burning rate. The decomposition procedure is applied on the numerical data and successfully verified by comparing a reconstructed Rayleigh index with the directly computed value. It thus allows to quantify the relative importance of various feedback mechanisms for a given setup.

Air Flow Modulation for Refined Control of the Combustion Dynamics Using a Novel Actuator

A specific actuator able to modulate the air feed of a gas a burner at a given frequency and amplitude is presented. The Combustion Department at the Institute for Thermal Turbomachinery and Machine Dynamics at the Graz University of Technology has experience on the study of combustion instabilities in gas turbines using a flow excitor. The stability of an industrial burner is tested at elevated pressure and temperature conditions in the frame of the NEWAC project. For practical matters of operation among which the possibility to induce progressively a perturbation when the flame conditions are all set, the need was expressed to design, construct and validate a flexible actuator able to set an air flow modulation at a given frequency and at a desired amplitude level, with the possibility during operation to let these two factors vary in a given range independently from each other. This device should operate within the 0–1 kHz range and 0%–20% amplitude range at steady-state, during transients, or follow a specific time sequence. It should be robust and sustain elevated pressures. The objective is to bring a perturbation in the flow to which the combustor will respond, or not. For elevated levels of pulsation, it can simulate the presence of vortex-driven combustion instabilities. It can also act as a real-time actuator able to respond in frequency and in phase to actively damp a “natural” combustion instability. Other issues are a better and quicker mixing due to the enhanced turbulence level, and pushing forward the blow out limits at lean conditions with controlled injection dynamics. The basic construction is the one of a siren, with an elevated pressure side where the air is throttled, and a low pressure outlet where the resulting sonic jet is sheared by a rotating wheel. A mechanism allows to let vary the surface of interaction between the wheel and the jet. Two electromotors driven by Labview set both frequency and amplitude levels. This contribution describes the actuator’s principles, design, operation range and the results of the characterization campaign.

Geometric and number effect on damping capacity of Helmholtz resonators in a model chamber

An acoustic cavity was selected as a stabilization device to control high-frequency combustion instabilities in gas turbines or liquid rocket engine combustors, and the acoustic damping capacity of the acoustic cavity was investigated for various geometric configurations under atmospheric non-reacting conditions. The tuning frequency of the acoustic cavity and the acoustic responses of a model chamber with a single acoustic cavity were studied first. Damping capacity was initially quantified through the frequency width of two split modes and the amplitude-damped ratio. The results showed that the cavity with the largest orifice area or the shortest orifice length was the most effective in acoustic damping of the harmful resonant mode. The effect of the number of cavities on acoustic damping capacity was also studied. Damping capacity was improved by increasing the number of cavities. For a better evaluation of acoustic damping capacity, two quantified parameters; the acoustic absorption, meaning the damping efficiency, and acoustic conductance, meaning the acoustic power loss, were introduced. The case was observed that has had insufficient loss of acoustic power in spite of having the highest absorption efficiency. As a result, fine geometric tuning for the acoustic cavity is required for the sufficient passive control. Also, the choice of the number of cavities is important to optimize the damping efficiency and absolute damping loss in consideration of the restriction of the cavity volume.

Low pressure premixed CH4/air flames with forced periodic mixture fraction oscillations: experimental approach

An experimental setup for the generation and investigation of periodic equivalence ratio oscillations in laminar premixed flames is presented. A special low-pressure burner was developed which generates stable flames in a wide pressure range down to 20 mbar and provides the possibility of rapid mixture fraction variations. The technical realization of the mixture fraction variations and the characteristics of the burner are described. 1D laser Raman scattering was applied to determine the temperature and concentration profiles of the major species through the flame front in correlation to the phase-angle of the periodic oscillation. OH* chemiluminescence was detected to qualitatively analyze the response of the flame to mixture fraction variations by changing shape and position. Exemplary results from a flame at p=69 mbar, forced at a frequency of 10 Hz, are shown and discussed. The experiments are part of a cooperative research project including the development of kinetic models and numerical simulation tools with the aim of a better understanding and prediction of periodic combustion instabilities in gas turbines. The focus of the current paper lies on the presentation of the experimental realization and the measuring techniques.

Combustion Dynamics and Instabilities: Elementary Coupling and Driving Mechanisms

Elementary processes that can be involved in the development of combustion instabilities in gas turbine combustors are described. The premixed mode of combustion is considered more specie cally because it is used in most advanced gas turbine systems. The processes envisaged portray the combustion dynamics of real systems, but they are analyzed in simple laboratory cone gurations. Among the many possible interactions, the most relevant mechanisms are those that generate e uctuations in heat release or induce pressure perturbations. Some typical paths are highlighted to help in the understanding of the multiple links that can exist between elementary processes. Processes involving acoustic/e ame coupling, unsteady strain rates, e ame response to inhomogeneities, interactions of e ames with boundaries, and e ame/vortex interactions are specie cally examined. For each process, a driving or a coupling path is proposed relating heat release e uctuations to acoustic variables in certain cases or leading from acoustic variables to heat release e uctuations in other cases. Stress is also put on characteristic time lags, which are key parameters in the triggering and development of instabilities. Well-controlled experiments illustrate the many possibilities and can serve to guide the modeling effort and to validate computational tools for combustion dynamics.

Flow forcing techniques for numerical simulation of combustion instabilities

Investigation of combustion instabilities in gas turbine combustors require the knowledge of flame transfer functions. Those can be obtained by experimental measurement or by Large Eddy Simulations (LES). Because calculations are usually limited to a portion of the whole combustor, boundary conditions are of crucial importance. It is common practice to inject acoustic perturbations for the flame transfer function measurement in form of velocity perturbations ( u′( t)). We present an alternative method based on a characteristic treatment of the Euler Equations. It consists of injecting sound waves traveling into the computational inlet while letting outgoing waves leave the domain without reflection. This method has several advantages concerning the study of flame transfer functions compared to injecting velocity perturbations. Both techniques are compared for cases where analytical solutions may be derived (a duct without flame and a planar laminar flame) and for one case where a CFD code is necessary (a laminar Bunsen-type flame).

LES of Chemical and Acoustic Forcing of a Premixed Dump Combustor

This paper describes the first steps in the development of a large eddy simulation (LES) code able to compute combustion instabilities in gas turbines. This code was used to compute the forcing of an experimentally investigated premixed dump combustor. It is shown that the main effect of acoustic waves entering the combustion chamber is to create large vortices and unsteady heat release when these vortices burn. Another effect of waves entering the combustor is to modulate the fuel and air flow rates produced by the feeding lines. In this case the equivalence ratio of the mixture entering the combustor may also vary. This was investigated in a “chemical effect” simulation where the inlet equivalence ratio fluctuates but the total flow rate remains constant. For perturbations from stoichiometric burning, this mechanism was shown to induce less destabilizing effects than the purely aerodynamical mechanism due to vortex formation and combustion. It is shown that the LES methodology developed is able to reproduce the experimentally observed phase shift between acoustic excitation and total reaction rate in the chamber.

Modelling the Nonlinear Dynamics of Combustion Instabilities in Gas Turbines Towards Active Control

A main obstacle to the use of premixed flames in gas turbine combustors is the occurrence of combustion instabilities. In this paper, we propose a model for the description of the coupling between the acoustic field and the heat release process. While the gasdynamics is modelled by means of transcendental transfer functions, the flame description, worked out on physical basis, is a nonlinear lumped parameter system. The developed model is intended for active control, and it accounts for the limit-cycle phenomenon.