Study Of Combustion Instabilities Research Articles

Experimental study of combustion instability and emission characteristics of ethanol/ammonia co-firing swirl flame

Ammonia and ethanol are promising carbon–neutral alternative fuels that can be produced through renewable energy sources. This study investigates the combustion instability and emission characteristics of ethanol/ammonia co-firing flame with ammonia heat fraction up to 40% in a swirl burner. Three different ammonia blending strategies are studied: premixed addition, root injection, and middle injection. The co-firing flame exhibited intense combustion instability under certain operating conditions with a pulsating frequency of about 150 Hz. The instability intensity weakens as the ammonia ratio increases. The pulsating pressure and OH* chemiluminescence intensity first increase and then decrease with the equivalence ratio, reaching a peak value near Ф=0.35. Adding a small amount of ammonia will cause CO and NOx emissions to increase simultaneously, but when αNH3 > 0.15, CO and NOx emissions will decrease. When the Ф increases, NOx emissions of premixed addition gradually decrease, while the NOx emissions of root injection and middle injection do not change significantly. The unburned NH3 significantly decreases with increasing Ф, and ammonia combustion is more efficient with the middle injection method.

Large Eddy Simulation and Dynamic Mode Decomposition of Supersonic Combustion Instability in a Strut-Based Scramjet Combustor

Supersonic combustion instability studies are crucial for the future maturation of scramjet engines. In the present paper, the supersonic combustion instability in a strut-based scramjet combustor is investigated through large eddy simulation and dynamic mode decomposition. The results show significant pressure oscillation in the strut-based scramjet combustor when the air parameters at the combustor inlet and the fuel parameters at the injector outlet are under certain conditions, and these pressure oscillation situations correspond to supersonic combustion instability. The oscillations have multiple dominant frequencies, including relatively low frequency of 2984 Hz, high frequency of 62,180 Hz, and very high frequency of 110,562 Hz. Large pressure oscillations in the strut-based scramjet combustor are closely related to wake instability, shear layer instability, shear layer and wave interactions, and combustion. Reducing the air total temperature at the combustor inlet can attenuate the pressure oscillations, and reducing the fuel flow rate at the injector outlet can also attenuate the pressure oscillations.

Effect of Outlet Boundary Condition on the Acoustic Modeshape and Flame Dynamics of a Partially Premixed Swirl Stabilized Combustor

Abstract Industrial gas turbines commonly use lean premixed swirl stabilized combustors that are prone to thermo-acoustic instabilities. Combustor testing involves several steps ranging from atmospheric to high-pressure conditions. An open outlet boundary condition is maintained for atmospheric tests commonly, whereas high-pressure testing involves complex exit boundary conditions, which change the reflection coefficient and can affect the nature of instability. Current work aims at studying how the change in outlet boundary affects the nature of instability due to changes in the exit reflection coefficient and acoustic mode shape inside the combustor. A laboratory-scale industrial swirl burner that uses partially premixed methane and air at atmospheric conditions is analyzed for this purpose. A constant area contraction ratio of 6.5:1 is maintained at the exit of the combustor while varying the inlet Reynolds number at a constant global equivalence ratio. Flame dynamics based on conditional phase averaged OH* chemiluminescence images and spatial Rayleigh index maps were used to compare different flow rate and exit boundary cases. The outlet contraction affects both the frequency and amplitude of the dominant thermoacoustic mode. The orifice plate at the exit reduces the outlet reflection coefficient leading to a change in acoustic mode shape inside the combustor. Overall, the instability amplitude is reduced considerably for cases with an orifice plates at the exit boundary compared to open exit boundary cases. The results show the importance of defining outlet boundary conditions as a parameter in combustion instability studies. Care should be taken while comparing and interpreting results from different facilities where outlet boundary condition is different.

A compressible two-phase flow framework for Large Eddy Simulations of liquid-propellant rocket engines

Atomization of liquid jets is a key feature of many propulsion systems, such as jet engines, internal combustion engines or liquid-propellant rocket engines (LRE). As it controls the characteristics of the spray, atomization has a great influence on the complex interaction between phenomena such as evaporation, turbulence, acoustics and combustion. In this context, Computational Fluid Dynamics is a promising way to bring better understanding of dynamic phenomena involving atomization, such as e.g. high-frequency combustion instabilities in LRE. However the unsteady simulation of primary atomization in reactive compressible two-phase flows is very challenging, due to the variety of the spatial and temporal scales, as well as to the high density, velocity and temperature gradients which require robust and efficient numerical methods. To address this issue, a numerical strategy is proposed in this paper, which is able to describe the dynamics of the whole chain of mechanisms from the liquid injection to its atomization and combustion. Primary atomization is modeled by a coupling between a homogeneous diffuse interface model and a kinetic-based Eulerian model for the spray. This strategy is successfully applied to the unsteady simulation of an operating point of the Onera’s Mascotte test bench, representative of one coaxial injector of LRE operating under subcritical conditions. The dynamics of the liquid core is retrieved and the flame shape as well as Sauter mean diameters are in good agreement with experimental results. These results demonstrate the ability of the strategy to deal with the harsh conditions of cryogenic combustion, and provide a promising framework for future studies of combustion instabilities in LRE.

Experimental characterization of the spatiotemporal dynamics of a turbulent flame in a gas turbine model combustor using computed tomography of chemiluminescence

An experimental study was conducted based on the recently available computed tomography of chemiluminescence (CTC) technique with a swirl-stabilized gas turbine model combustor to obtain both time-resolved and three-dimensional (3D) dynamics of the turbulent flame in confined situation. The CTC system employed in the present study was designed in a cost-effective, but highly-efficient manner, which allows simultaneous recordings of six projections of the target flame with three cameras at a repetition rate of 1 kHz. Instantaneous flame topologies were successfully captured and highly-transient flame dynamics in the combustion chamber, such as local extinction and flame shape variations were also observed. A further analysis based on the instantaneous and phase-averaged 3D movements of the flame centroids suggested that the flame in the present combustor exhibited a helical but relatively stochastic propagation and a distinct azimuthal rotation around the combustor, which is in the direction of rotation imposed by the swirler to the flow. Results obtained in this work demonstrate the capability of the CTC technique to resolve the detailed spatiotemporal dynamics of turbulent flames in confined combustion systems, which is expected to be particularly helpful for the study of combustion instabilities, such as periodic combustion oscillations in fuel-lean gas turbine engines.

Book Review: Unsteady Combustor Physics

Combustion dynamics is a subject that has been studied for several decades that started due to the requirements of aerospace industries. But in recent times, instabilities that accompany techniques like lean premixed combustion adopted in land-based applications also have reinforced the interest of the combustion community in this area. Thus this area is becoming a major area of combustion research. However, working in this subject requires knowledge of a number of disciplines apart from combustion, which makes initiation into the area of combustion dynamics rather challenging for a beginner. Traditionally, in studies of combustion instabilities, the role of acoustics have received the major attention primarily because of the strong coupling that exists between acoustic disturbances and heat release rate fluctuations and the catastrophic influence that this coupling can have on the performance and even structural integrity of the combustion systems. The current research trend, however, looks beyond thermoacoustics and critically investigates the roles of various factors like flow dynamics, combustion chemistry, heat and species transport to name a few. It is in this context that the utility of the present book under review has to be assessed. The book is designed as an advanced level textbook and gives a broad overview of the various factors that play major roles in combustion dynamics. As stated by the author in the “Preface” the book requires prior knowledge of fluid mechanics and combustion but no prerequisite in acoustics. The first part of the book (Chapters 1 to 6) introduces the basic concepts of reacting flow, hydrodynamic stability and acoustic wave propagation. Starting from general introductions to the subjects, the chapters bring out topics that are directly relevant to combustion dynamics. The second part of the book (Chapters 6 to 12) deals with interactions of these three subjects that are observed in combustion dynamic problems. The first chapter gives a very concise but useful coverage of the basics of transport phenomena in reacting flows. Although primarily of recapitulative nature, the treatment International journal of spray and combustion dynamics · Volume .6 · Number . 2 .2014 – pages 221 – 223 221

Sensitivity of the Numerical Prediction of Turbulent Combustion Dynamics in the LIMOUSINE Combustor

The objective of this study is to investigate the sensitivity and accuracy of the reaction flow-field prediction for the LIMOUSINE combustor with regard to choices in computational mesh and turbulent combustion model. The LIMOUSINE combustor is a partially premixed, bluff body-stabilized natural gas combustor designed to operate at 40–80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermoacoustic instabilities is a very time-consuming process. For that reason, the meshing approach leading to accurate numerical prediction, known sensitivity, and minimized amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the computational domain. Typically, the structural mesh topology allows using much fewer grid elements compared to the unstructured grid; however, an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter, the studies are extended to combustible flows using standard available ansys-cfx combustion models. To validate the predicted variable fields of the combustor's transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under nonreacting conditions. More significant differences are observed in the mixing behavior of air and fuel flows. Here, the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow, resolved with the use of the hexahedral mesh, presents better agreement with experimental data and demands less computational effort. Finally, in the paper, the performance of the combustion model for reacting flow is presented and the main issues of the applied combustion modeling are reviewed.

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.

Compact Green's Function for a Generic Rijke Burner

A theoretical examination is made of the thermo-acoustic properties of a Rijke burner of large aspect ratio rectangular cross-section. Such a generic device has been proposed by Kok et al. (2009 paper presented at the 16th International Congress on Sound & Vibration) to make canonical studies of combustion instabilities. An aeroacoustic Green's function is derived which permits the sound pressure produced by arbitrary thermal and vortex sources within the burner to be calculated by convolution. The Green's function corresponds to the potential flow sound field produced by an impulsive point source; its calculation taking account of flame-holder geometry is facilitated by use of the Schwarz-Christoffel transformation. The transformation is performed numerically to accommodate complex burner geometry and validated by comparison with an alternative procedure involving the direct numerical integration of Laplace's equation.

Break-up characteristics of gelled propellant simulants with various gelling agent contents

Gel propulsion systems have many advantages with respect to high performance, the energy management of liquid propulsion systems, storability, high density impulse, and low leakage of liquid propellants. The atomization process provides sufficient contact surface area between the gelled fuel and oxidizer jets. It is important to study how injection characteristics of gelled propellants are related with break-up and spray distribution. The break-up and mixing processes are very important in achieving maximum efficiency and necessitate the careful study of combustion instability. Gelled propellants are non-Newtonian fluids in which the viscosity is a function of the shear rate, and they have a high dynamic shear viscosity which depends on the amount of gelling agent contents. The present study has focused on the break-up process, wave development of ligament and liquid sheets formed by impinging jets with various gelling agent contents. Especially, the break-up processes of the impinging jets at the initial conditions are studied. The break-up process of like-on-like doublet impinging jets are experimentally characterized using non-Newtonian liquids which are mixed by ionized water 98.5 wt%, Carbopol 941 0.5wt% or 1.0wt%, and NaOH(concentration 10%) 1.0wt%. For the like-on-like doublet injector, the generation of a liquid sheet at the impinging point of two jets was observed. The spray shape with elliptical pattern is distributed in a perpendicular direction to the momentum vectors of the jets. Gelled propellant simulants with high viscosity jets are more stable and produce less pronounced surface waves than low viscosity jets. Generally, the break-up length decreased due to the increasing Reynolds number. However, surface waves and atomized droplets increased. Gelled propellant simulants from like-on-like doublet impinging jets have the spray shape of closed rim patterns at low pressure. Also, the rim patterns of spray have no disturbances on the spray sheet. As the injection pressure increased, rimless patterns which were composed of ligament sheets and small droplets emerged due to the effect of the aerodynamic action. Periodic wave-like structures observed from the near impingement point and atomized droplets were observed at a location further downstream.

Large Eddy Simulation of self excited azimuthal modes in annular combustors

While most academic set ups used to study combustion instabilities are limited to single burners and are submitted mainly to longitudinal acoustic modes, real gas turbines exhibit mostly azimuthal modes due to the annular shape of their chambers. This study presents a massively parallel Large Eddy Simulation (LES) of a full helicopter combustion chamber in which a self-excited azimuthal mode develops naturally. The whole chamber is computed from the diffuser outlet to the high pressure stator nozzle. LES captures this self-excited instability and results (unsteady pressure RMS and phase fields) show that it is characterized by two superimposed rotating modes with different amplitudes. These turning modes modulate the flow rate through the 15 burners and the flames oscillate back and forth in front of each burner, leading to local heat release fluctuations. LES demonstrates that the first effect of the turning modes is to induce longitudinal pulsations of the flow rates through individual burners. The transfer functions of all burners are the same and no mechanism of flame interactions between burners within the chamber is identified.

Mode switching of combustion instabilities associated with flame transfer function non-linearities

Studies of combustion instabilities indicate that the frequency of oscillation changes during the growth of oscillations before a limit cycle is reached. It is also found that a system can switch from one mode to another when the amplitude exceeds a certain threshold. It is shown that these features can be linked to the flame response non-linearity. The analysis is carried out by examining combustion instabilities appearing on a multipoint injection burner, placed a the end of a cylindrical manifold. This element acts like a resonator and its length can be modified easily. Experiments indicate that mode switching takes place between two resonant modes of the cavity and this is shown to be caused by the non-linearity of the flame response. The phenomenon can be analyzed by considering the non-linear evolution of the flame transfer function. It is possible to combine this information with a stability analysis of the burner, and define conditions which give rise to mode switching. It is found that this behavior is related to a change in phase difference between heat release rate fluctuations and velocity perturbations at the burner outlet. This phase shift takes place as the amplitude of oscillation increases.

Experimental Diagnostics for the Study of Combustion Instabilities in Lean Premixed Combustors

An improved understanding of the mechanisms of unstable combustion in lean premixed combustors is essential to the development of stable gas turbine combustion systems. To obtain such understanding, detailed experimental studies of the phenomenology of unstable combustion are required. A number of experimental diagnostic techniques for characterizing unstable combustion and the underlying instability mechanisms are discussed. This includes techniques based on pressure, chemiluminescence emission, infrared absorption, and laser-induced fluorescence measurements. The techniques themselves are discussed briefly; however, the primary objective is to present and discuss results illustrating how these techniques can be used to characterize the mechanisms of unstable combustion, to gain an improved understanding of unstable combustion, and to develop strategies for suppressing unstable combustion in lean premixed gas turbine combustors.

Combustion instabilities in propulsion systems

Periodic acoustic motions in combustion chambers, a phenomenon commonly known as combustion instability, were discovered in air-breathing and rocket engines at about the same time in the late 1930s. Since then, unstable oscillations have occurred in most, if not practically all, new development programs. Indeed, because of the high density of energy release in volume having relatively low losses, conditions normally favor excitation and sustenance of oscillations in any combustion chamber intended for a propulsion system. This presentation will provide an overview of combustion instabilities in three types of propulsion systems (i.e., solid rocket motors, liquid rocket engines, and air-breathing engines), with emphasis focused on the state-of-the-art understanding and future research needs. Various research issues in acoustics, fluid mechanics, and chemistry related to oscillatory combustion in practical systems will be discussed. Both passive and active control techniques will be covered. In particular, the application of contemporary CFD schemes, approximate analytical methods, and experimental diagnostic tools to combustion instability studies will be addressed in detail.

A computational study of combustion instabilities due to vortex shedding

The influence of unsteady vortex shedding on combustion instabilities is examined using simulation databases for two configurations: a dump combustor and a model afterburner equipped with a bluff-body flame holder for different operating conditions. These databases have been generated using a recently developed flame-wrinkling large eddy simulation (LES) model, described elsewhere. As a first necessary step, a detailed comparison is made between predicted and experimentally obtained statistics to quantify the accuracy of the LES and determine whether LES reproduces the observed statistical trends in the experiments. The objective of the present study is to examine these databases using visualization techniques in order to investigate the mechanisms responsible for combustion instabilities in relation to unsteady vortex phenomena. Combustion instabilities are found, experimentally as well as computationally, in both configurations and under different operating conditions. Based on the LES databases, the origin of these instabilities can be identified and attributed to key events such as vortex shedding, excitation of pressure fluctuations due to exothermicity, extinction by strain, and operating conditions. The unsteady behavior of the dump combustor is dominated by extinction due to the high strain rate, while the dynamics of the model afterburner was associated mainly with excitation of pressure fluctuations due to exothermicity and operating conditions.

Direct C2 Radical Imaging in Combustion Instabilities

Abstract The past decade has witnessed the development of various laser diagnostic techniques for combustion, whose application, particularly for flow visualization, is now widespread. Continued progress in this field is due, in part, to advances in detector technology. In this brief communication we point out that the detector sensitivity is now sufficient to permit direct time-resolved visualization of natural radical emission from turbulent flames, without the need for laser excitation. We present a visualization technique which provides both instantaneous and conditionally phase-averaged images of C2 radical emission, which we apply in the study of combustion instabilities in a model ramjet combustor.

Les moyens necessaires a l'etude fondamentale des instabilites de combustion d'un moteur biliquide

(Necessary means for a fundamental study of combustion instabilities in a liquid propellant engine).—Within the framework of Ariane launch vehicle studies, ONERA has entered upon a fundamental investigation of combustion instabilities, that should lead to a digital program simulating the unsteady behaviour of a liquid propellant engine with radial injection, the nominal working pressure of which is near the critical pressure of propellant. The first step of this study was to define the necessary methods and means that must be worked out to characterize the various physical processes taking place in the engine, in order to achieve this object. The droplet size and velocity distributions in space and time (characteristic of injection-atomization) is the upper boundary condition of the problem. It will be empirically described from parametric test results, so a flexible and valid measurement method must be found, the exploitation of which has to be automatized. The droplet evolution within the chamber (characteristic of combustion) depends on environmental conditions, especially forced convection due to radial injection geometry. It will be analyzed with the help of a two-dimenstional single droplet combustion model, that must stay valid near the propellant critical pressure; thus the propellant state law, and its thermodynamic and transport properties, have to be known in this area. Gas flow description, droplet trajectography in a turbulent-environment and mass, momentum and energy transport consideration (characteristic of aerodynamics) will be handled in a progressive way. Each of intermediate models will be checked, then validated, with the help of suitable test results, some of them being fit to be later used as a theoretical support for the exploitation of complementary atomization or combustion experiments. The final model that is reached constitutes the synthesis of the whole study. ONERA has taken advantage of the helpful assistance of external research centers, in particular amid the French University.

Experimental determination of three-dimensional liquid rocket nozzleadmittances.

The three-dimensional nozzle admittance, an important parameter in combustion instability studies, was experimentally measured for several nozzle configurations. The admittance values were obtained using a modification of the classical impedance tube technique. The modified impedance tube method measures the admittance of a duct termination in the presence of one-dimensional mean flow and three-dimensional oscillations. Values of the nozzle admittance were obtained from pressure amplitude measurements taken at discrete points along the length of the tube. To determine the effects of nozzle geometry, nozzles were tested with half-angles of 15, 30, and 45 deg and entrance Mach numbers of 0.08, 0.16, and 0.20. The admittance results are presented as functions of nondimensional frequency for mixed first tangential-longitudinal modes. These results are compared with available theoretical predictions, and good agreement between theory and experiment is shown.

Combustion instability studies with plastic propellant

The eifect of compositional changes on the stability of combustion of plastic propellent, based upon ammonium perchlorate and polyisobutene, is investigated using a 2-in.-diam T burner for the frequencies 0.7-4.0 kHz at 1000 psig. Oxidizer particle size can have a pronounced effect both on the acoustic response and on the maximum pressure amplitude which can be sustained in the T burner, whereas changes in the oxidizer-fuel ratio have little effect. Titanium dioxide, usually added to plastic propellent to stabilize combustion, is shown to have a dual effect: the acoustic response is reduced and the gas phase particle damping is promoted. Finally, the effect of lithium fluoride and copper chromate on combustion instability is examined. An inverse correlation, which is frequency dependent, is observed between propellent burning rate and both the acoustic response and the maximum pressure amplitude sustained in the T burner.

Comparison of combustion instabilities found in various types of combustion chambers

Combustion instabilities in motors and furnaces are contrasted under three separate headings: System instabilities involve, in an essential way, interactions between processes occuring in the combustion chamber and processes occurring in at least one other component of the system. Combustion chamber instabilities consist of unsteady phenomena that are localized within the combustion chamber. Intrinsic instabilities are inherent in the reactants themselves and would be observed if combustion were to occur in the absence of any external influences. The second category, combustion-chamber instabilities, is again divided into three subcategories: Acoustic instabilities involve, in an essential way, the propagation of acoustic waves in the combustion chamber. Shock instabilities are characterized by the presence of steep-fronted finite-amplitude shock or detonation waves in the combustion chamber. Fluid-dynamic instabilities are associated with the establisment of special kinds of flow patterns, such as vortices, in the chamber. Experimental and theoretical aspects of each of these types of instabilities are discussed separately. Characteristics of each type of instability are described, and the kinds of chambers in which each has been observed are indicated. Attention is given to surface-burning systems, such as solid-propellant rockets and hybrid rocket motors, and also to volume-burning systems, such as liquid-propellant rocket engines and both gas-fired and oil-fired industrial burners. Experimental procedures for investigating each type of instability are reviewed along with selected experimental results, and the character and state of development of the theoretical methods that have been used for describing each type of instability are compared. The review is an attempt to bring about discussions between specialists who study combustion instabilities in different systems. Classification according to the type of instability rather than the type of system can emphasize the similarities and differences that exist among combustion instability problems encountered in different systems. One resulting observation is that examples of system instabilities and of acoustic instabilities are common to all kinds of combustion systems.