Singing flames - The coupling of acoustics and combustion

An overview of the physics of the interaction between sound and flames is presented. The flame dynamics are investigated through experiment and computer modelling. The complementary rôles of wave analysis, low-order models for unsteady combustion, Computational Fluid Dynamics and experiment are illustrated by examples of oscillations in gas turbine combustors and in generic premixed ducted flames. The potential for 'antisound' and passive acoustic absorbers to eliminate the instability is also discussed.

Investigation of extinction in unsteady flames in turbulent combustion by 2D-LIF of OH radials and flamelet analysis

An experimental and theoretical investigation of premixed turbulent combustion in an engine simulator is presented. The distribution of hydroxyl radicals formed in the combustion of propane/air mixtures was visualized by 2D-LIF and used to monitor the progress of the combustion process. For stoichiometric mixtures, images showed a continuous wrinkled flame front, while in lean (λ=1.5) mixtures, local flame extinction was observed as discontinuities in the reaction zone. A bright active reaction zone was still observed in flame inlets and closed concave structures. The effects of self-absorption and of collisional quenching on the fluorescence signal are considered and appear to have only a minor net influence on the shape and width of the flame front. The images are evaluated and interpreted in terms of the Lewis number effect and the laminar flamelet model. Analysis was performed by determining the contour lines of the images (specifically, the ratios of average maximum to equilibrium OH concentration) and comparing with corresponding ratios from unstrained flame simulations. The results show that although the degree of turbulence is not high enough for straining effects to be important, flamelet curvature does play a significant role in the combustion of lean mixtures; this is manifested by a mean effective flame velocity that is less than the laminar burning velocity. © 1991 Combustion Institute.

Model-based thermoacoustic oscillation control via ℋ ∞ loop-shaping and integral quadratic constraints

Lean premixed prevaporized (LPP) technology has been widely used in the new generation of gas turbines in which reduced emissions are a priority. However, such combustion systems are susceptible to the damage of self-excited oscillations. Feedback control provide a way of preventing such dynamic stabilities. A flame dynamics assumption is proposed for a recently developed unsteady heat release model, the robust design technique, ℋ ∞ loop-shaping, is applied for the controller design and the performance of the controller is confirmed by simulations of the closed-loop system. The Integral Quadratic Constraints(IQC) method is employed to prove the stability of the closed-loop system. ©2010 IEEE.

Fundamentals and Challenges

An improved understanding of lean fuel turbulent premixed flames must play a central role in the fundamental science of these new concepts.

Scalar dissipation rate approach

An improved understanding of lean fuel turbulent premixed flames must play a central role in the fundamental science of these new concepts.

Future directions in modelling

An improved understanding of lean fuel turbulent premixed flames must play a central role in the fundamental science of these new concepts.

On the stabilisation of turbulent lifted flames

This study explores the stabilisation mechanisms of turbulent lifted flames by examining the scalar dissipation rate (SDR) of both passive and reactive scalars and their cross dissipation (CDR) in the stabilisation region. DNS results of a laboratory scale hydrogen turbulent lifted flame has been used for this analysis. Various definitions of the flame leading edge (FLE) has been compared and differences are illustrated. Time and spatial averaged statistic of SDR and CDR were examined. It was found that the averaged SDR for the mixture fraction at FLE was well below the reference quenching value for stoichiometric mixture. The averaged SDR for the progress variable is in the same order of the unstrained premixed laminar flame value. It was observed that the averaged CDR changed from negative to weakly positive at FLE. The change in sign was explained by a change in the relative alignment of the gradients of mixture fraction and progress variable. It was thus evident that the CDR was a good marker for stabilisation region and an important quantity in stabilisation mechanism.

Effects of flow-geometry and Damkohler number on turbulence-flame interaction

DNS of turbulent hydrogen-air premixed flame is conducted for freely propagating and V-flames, using complex chemical kinetics. The results are analysed to study the influence of flame configuration on the turbulence-scalar interaction, which is critical for the scalar gradient generation process. The result suggests that this interaction process is not influenced by the flame configuration and the flame normal is found to predominantly align with the most extensive strain in the region of intense heat release.

Investigation of self excited and acoustically forced combustion instabilities with laser diagnostics

Combustion oscillations in gas turbines can result in serious damage. One method used to predict such oscillations is to analyze the combustor acoustics using a simple linear model. Such a model requires a flame transfer function to describe the response of the heat release to flow perturbations inside the combustor. This paper reports on the application of Planar Laser Induced Fluorescence (PLIF) of OH radicals to analyze the response of a lean premixed flame to oncoming flow perturbations. Both self-excited oscillations and low amplitude forced oscillations at various frequencies are investigated in an atmospheric pressure model combustor rig. In order to visualize fluctuations of local fuel distribution, acetone-PLIF was also applied in non-reacting and acoustically forced flows at oscillation frequencies of 200 Hz and 510 Hz, respectively. OH-PLIF images were acquired over a range of operating parameters. The results presented in this paper originate from data sets acquired at fixed phase angles during the oscillation cycle. Comparative experiments in self excited and forced acoustic oscillations show that the flame and the combustion intensity develop similarly throughout the pressure cycle in both cases. Although the peak fluorescence intensities differ between self excited and the forced instabilities, there is a clear correspondence in the observed frequency and phase information from the two cases. This result encourages a comparison of the OH-PLIF and the acetone-PLIF results. Quantitative measurements of the equivalence ratio in specific areas of the measurement plane offer insight on the complex phenomena coupling acoustic perturbations, i.e. flow velocity fluctuations, to fluctuations in fuel distribution and combustion intensity, ultimately resulting in self excited combustion oscillations.

Model Refinement for the Active Control of Thermoacoustic Instability

Active control has been shown as a feasible technology for suppressing thermoacoustic instability in continuous combustion systems, and the control strategy design is substantially dependent on the reliability of the flame model. In this paper, refinement of G-equation flame model for the dynamics of lean premixed combustion is investigated. Precisely, the dynamics between the flame speed S_u and equivalence ratio phi are proposed based on numerical calculations and physical explanations. Finally, the developed model is tested on one set of experimental data.

Scalar and its dissipation in the near field of turbulent lifted jet flame

In this study various scalar dissipation rates and their modelling in the context of partially premixed flame are investigated. A DNS dataset of the near field of a turbulent hydrogen lifted jet flame is processed to analyse the mixture fraction and progress variable dissipation rates and their cross dissipation rate at several axial positions. It is found that the classical model for the passive scalar dissipation rate ε{lunate}̃ZZ gives good agreement with the DNS, while models developed based on premixed flames for the reactive scalar dissipation rate ε{lunate}̃cc only qualitatively capture the correct trend. The cross dissipation rate ε{lunate}̃cZ is mostly negative and can be reasonably approximated at downstream positions once ε{lunate}̃ZZ and ε{lunate}̃cc are known, although the sign cannot be determined. This approach gives better results than one employing a constant ratio of turbulent timescale and the scalar covariance c'Z'̃. The statistics of scalar gradients are further examined and lognormal distributions are shown to be very good approximations for the passive scalar and acceptable for the reactive scalar. The correlation between the two gradients increases downstream as the partially premixed flame in the near field evolves ultimately to a diffusion flame in the far field. A bivariate lognormal distribution is tested and found to be a reasonable approximation for the joint PDF of the two scalar gradients. © 2011 The Combustion Institute.

The internal structure of igniting turbulent sprays as revealed by complex chemistry DNS

A parametric study of spark ignition in a uniform monodisperse turbulent spray is performed with complex chemistry three-dimensional Direct Numerical Simulations in order to improve the understanding of the structure of the ignition kernel. The heat produced by the kernel increases with the amount of fuel evaporated inside the spark volume. Moreover, the heat sink by evaporation is initially higher than the heat release and can have a negative effect on ignition. With the sprays investigated, heat release occurs over a large range of mixture fractions, being high within the nominal flammability limits and finite but low below the lean flammability limit. The burning of very lean regions is attributed to the diffusion of heat and species from regions of high heat release, and from the spark, to lean regions. Two modes of spray ignition are reported. With a relatively dilute spray, nominally flammable material exists only near the droplets. Reaction zones are created locally near the droplets and have a non-premixed character. They spread from droplet to droplet through a very lean interdroplet spacing. With a dense spray, the hot spark region is rich due to substantial evaporation but the cold region remains lean. In between, a large surface of flammable material is generated by evaporation. Ignition occurs there and a large reaction zone propagates from the rich burned region to the cold lean region. This flame is wrinkled due to the stratified mixture fraction field and evaporative cooling. In the dilute spray, the reaction front curvature pdf contains high values associated with single droplet combustion, while in the dense spray, the curvature is lower and closer to the curvature associated with gaseous fuel ignition kernels. © 2011 The Combustion Institute.

Low-order modelling of ducted flames with temporally varying equivalence ratio in realistic geometries

The interaction between unsteady heat release and acoustic pressure oscillations in gas turbines results in self-excited combustion oscillations which can potentially be strong enough to cause significant structural damage to the combustor. Correctly predicting the interaction of these processes, and anticipating the onset of these oscillations can be difficult. In recent years much research effort has focused on the response of premixed flames to velocity and equivalence ratio perturbations. In this paper, we develop a flame model based on the socalled G-Equation, which captures the kinematic evolution of the flame surfaces, under the assumptions of axisymmetry, and ignoring vorticity and compressibility. This builds on previous work by Dowling [1], Schuller et al. [2], Cho & Lieuwen [3], among many others, and extends the model to a realistic geometry, with two intersecting flame surfaces within a non-uniform velocity field. The inputs to the model are the free-stream velocity perturbations, and the associated equivalence ratio perturbations. The model also proposes a time-delay calculation wherein the time delay for the fuel convection varies both spatially and temporally. The flame response from this model was compared with experiments conducted by Balachandran [4, 5], and found to show promising agreement with experimental forced case. To address the primary industrial interest of predicting self-excited limit cycles, the model has then been linked with an acoustic network model to simulate the closed-loop interaction between the combustion and acoustic processes. This has been done both linearly and nonlinearly. The nonlinear analysis is achieved by applying a describing function analysis in the frequency domain to predict the limit cycle, and also through a time domain simulation. In the latter case, the acoustic field is assumed to remain linear, with the nonlinearity in the response of the combustion to flow and equivalence ratio perturbations. A transfer function from unsteady heat release to unsteady pressure is obtained from a linear acoustic network model, and the corresponding Green function is used to provide the input to the flame model as it evolves in the time domain. The predicted unstable frequency and limit cycle are in good agreement with experiment, demonstrating the potential of this approach to predict instabilities, and as a test bench for developing control strategies. Copyright © 2011 by ASME.

Effects of nonuniform reactant stoichiometry on combustion instability

This paper analyzes the forced response of swirl-stabilized lean-premixed flames to acoustic forcing in a laboratory-scale stratified burner. The double-swirler, double-channel annular burner was specially designed to generate acoustic velocity oscillations and radial fuel stratification at the inlet of the combustion chamber. Temporal oscillations of equivalence ratio along the axial direction are dissipated over a long distance, and therefore the effects of time-varying fuel/air ratio on the flame response are not considered. Simultaneous measurements of inlet velocity and heat release rate oscillations were made using a hot wire anemometer and photomultiplier tubes with narrowband OH*/CH* interference filters. Time-averaged CH* chemiluminescence intensities were measured using an intensified CCD camera. Results show that flame stabilization mechanisms vary depending on stratification ratio for a constant global equivalence ratio. For a uniformly premixed condition, an enveloped M-shaped flame is observed. For stratified conditions, however, a dihedral V-flame and a detached flame are developed for outer stream and inner stream fuel enrichment cases, respectively. Flame transfer function (FTF) measurement results indicate that a V-shaped flame tends to damp incident flow oscillations, while a detached flame acts as a strong amplifier relative to the uniformly premixed condition. The phase difference of FTF increases in the presence of stratification. More importantly, the dynamic characteristics obtained from the forced stratified flame measurements are well correlated with unsteady flame behavior under limit-cycle pressure oscillations. The results presented in this paper provide insight into the impact of nonuniform reactant stoichiometry on combustion instabilities, which has not been well explored to date. Copyright © 2011 by ASME.

A priori assessment of algebraic flame surface density models in the context of large eddy simulation for nonunity lewis number flames in the thin reaction zones regime

The performance of algebraic flame surface density (FSD) models has been assessed for flames with nonunity Lewis number (Le) in the thin reaction zones regime, using a direct numerical simulation (DNS) database of freely propagating turbulent premixed flames with Le ranging from 0.34 to 1.2. The focus is on algebraic FSD models based on a power-law approach, and the effects of Lewis number on the fractal dimension D and inner cut-off scale η i have been studied in detail. It has been found that D is strongly affected by Lewis number and increases significantly with decreasing Le. By contrast, η i remains close to the laminar flame thermal thickness for all values of Le considered here. A parameterisation of D is proposed such that the effects of Lewis number are explicitly accounted for. The new parameterisation is used to propose a new algebraic model for FSD. The performance of the new model is assessed with respect to results for the generalised FSD obtained from explicitly LES-filtered DNS data. It has been found that the performance of the most existing models deteriorates with decreasing Lewis number, while the newly proposed model is found to perform as well or better than the most existing algebraic models for FSD. © 2012 Mohit Katragadda et al.