Analysis of turbulent premixed flame structure using simultaneous PIV-OH PLIF

In the present study, we used a simultaneous PIV-OH PLIF measurement to acquire the strain rate and the chemical intensity and suggested a new combustion phase diagram. This simultaneous measurement was used to analyze the flame structure and to classify the combustion regimes of the opposed impinging jet combustor according to the change of the orifice diameters at the pre-chambers. The shear strain rates were obtained from the velocity measurement by PIV to represent flow characteristics and the OH radical intensities were obtained from OH PUF to indicate the flame characteristics. When the strain rate and OH intensity at each point of the measurement zones are plotted at the strain rate-chemical intensity diagram, the distribution of each case showed the characteristics of each flame regime. The change of combustor condition made different distribution in the combustion phase diagram. As the orifice diameter of the pre-chamber decreases, well-mixed turbulent flames are produced and the combustion phase is moved from the moderated turbulence regime to the thickened reaction regime.     

LES-CMC simulations of a turbulent bluff-body flame

The large Eddy simulations (LES)-conditional moment closure (CMC) method with detailed chemistry is applied to a bluff-body stabilized flame. Computations of the velocity and mixture fraction fields show good agreement with the experiments. Temperature and major species are well-predicted throughout the flame with the exception of the flow regions in the outer shear layer close to the nozzle where the pure mixing between hot recirculating products and fresh oxidizer cannot be captured. LES-CMC generally improves on results obtained with RANS-CMC and on LES that uses one representative flamelet to model the dependence of reactive species on mixture fraction. Simulated CO mass fractions are generally in good agreement with the experimental data although a 10% overprediction can be found at downstream positions. NO predictions show a distinct improvement over the flamelet approach, however, simulations overpredict NO mass fractions at all downstream locations due to an overprediction of temperature close to the nozzle. The potential of LES-CMC to predict unsteady finite rate effects is demonstrated by the prediction of endothermic—or “flame cooling”—regions close to the neck of the recirculation zone that favours ethylene production via the methane fuel decomposition channel.     

Investigation of conditional source-term estimation applied to a non-premixed turbulent flame

A turbulent combustion model, Conditional Source-term Estimation (CSE) is applied to a non-premixed turbulent jet methane flame. The conditional chemical source terms are determined on the basis of first order closure and the conditional averaged species concentrations are obtained by inverting an integral equation. The Tikhonov method is implemented for regularisation. Detailed chemistry is tabulated using the trajectory generated low-dimensional manifold method. Radiation due to the gaseous species is included. Reynolds Averaged Navier–Stokes calculations are performed using two different turbulence models. The objectives of the paper are (i) assessment of the impact of the main numerical parameters in CSE and (ii) comparison of the CSE numerical predictions with available experimental data and results from previous simulations for the selected flame. The number of CSE domains and the number of points in each CSE domain are shown to have a significant impact on the results if not selected appropriately. The present CSE calculations always converge to unique and stable predictions. The corrected k–ε model yields mixture fraction profiles in good agreement with the experimental data values for axial locations in the first half of the flame. Farther downstream, the RNG k–ε model performs better. Overall, the current predictions for the mixture fraction are in good agreement with the experimental data. The predicted temperatures using CSE and the k–ε turbulence model with a modified value of C_ε1 = 1.47 are found to be in very good agreement with the experimental data. Further, the current CSE results are of comparable quality with previous simulations using the flamelet model and conditional moment closure. Future work may include further investigation on optimal determination of the regularisation parameter and alternative regularisation techniques, soot modelling within the CSE formulation, and improved formulation of radiation.     

On the turbulent propagation of a flame in a closed volume

Regularities in the mutual variation of the velocity of propagation and electrical conductivity of a flame are investigated experimentally for turbulent combustion in a closed volume. An estimate proposed as a result of analysis of experimental data describes analytically the dependence of the velocity of propagation of the flame on the signal amplitude at ionization probes. In our opinion, the singularity observed on the approximate curve describing the dependence of the flame propagation velocity on the signal amplitude at ionization probes is responsible for the competition between generation and destruction of charged particles in the flame and corresponds to the conditions of quasi-stationary concentrations. The proposed method and established experimental facts can be used in the development of methods for diagnosing local intensity of burning in combustion chambers.     

Effects of subgrid scale and combustion modelling on flame structure of a turbulent premixed flame within LES and tabulated chemistry framework

The effects of combustion and SubGrid Scale (SGS) modelling on the overall flame characteristics of a turbulent premixed flame are investigated. This is achieved in terms of mean flow statistics, variances and flame surfaces. In particular, the chemical flame structure is analysed and compared. The Artificially Thickened Flame (ATF) approach coupled with the Flamelet Generated Manifolds (FGMs) and Filtered TAbulated Chemistry for LES (F-TACLES) approaches are used for this investigation. A Germano like procedure for dynamical calculation of SGS wrinkling is used which ensures the conservation of the total flame surface for both models. It turns out that using the dynamic SGS wrinkling model improves the results. Although the results of both combustion models in terms of statistics, mean and variances show very good agreement, the resolved flame surfaces hide different dynamic behaviour.     

Compressible turbulent flame speeds of highly turbulent standing flames

In this paper we present the first measurement of turbulent burning velocities of a highly turbulent compressible standing flame induced by shock-driven turbulence in a Turbulent Shock Tube. High-speed schlieren, chemiluminescence, PIV, and dynamic pressure measurements are made to quantify flame–turbulence interaction for high levels of turbulence at elevated temperatures and pressure. Distributions of turbulent velocities, vorticity and turbulent strain are provided for regions ahead and behind the standing flame. The turbulent flame speed is directly measured for the high-Mach standing turbulent flame. From measurements of the flame turbulent speed and turbulent Mach number, transition into a non-linear compressibility regime at turbulent Mach numbers above 0.4 is confirmed, and a possible mechanism for flame generated turbulence and deflagration-to-detonation transition is established.     

Resonance of a turbulent flame in a high-frequency acoustic wave

Resonance of a weakly turbulent flame in a high-frequency acoustic wave is obtained. Because of the resonance, an acoustic wave may increase noticeably the amplitude of flame wrinkles, and the respective increase in propagation velocity of the turbulent flame front becomes larger by a factor of 10-20. The effect of resonance is especially important for turbulent flames with realistic thermal expansion propagating in a closed burning chamber, which may account for considerable scattering of experimental results on turbulent flame velocity.     

Experimental and numerical characterization of a turbulent spray flame

A turbulent ethanol spray flame is characterized through quantitative experiments using laser-based imaging techniques. The data set is used to validate a numerical code for the simulation of spray combustion. The spray burner has been designed to generate a stable flame without the use of a bluff body or a pilot flame facilitating numerical simulations. The experiments include spatially-resolved measurements of droplet sizes (Mie/LIF-dropsizing and PDA), droplet velocity (PDA), liquid-phase temperature (2-color LIF temperature imaging with Rhodamine B) and gas-phase temperature (multi-line NO-LIF temperature imaging). The measurements close to the nozzle exit are used to determine the initial conditions for numerical simulations. An Eulerian–Lagrangian model including spray flamelet modeling is applied to calculate the development of the spray. Good agreement with the experimental data is found. The experimental data set and the numerical results will be published on a website to allow other groups to evaluate their experimental and/or numerical data.     

Large-eddy simulation of a bluff-body stabilised turbulent premixed flame using the transported flame surface density approach

A premixed propane–air flame stabilised on a triangular bluff body in a model jet-engine afterburner configuration is investigated using large-eddy simulation (LES). The reaction rate source term for turbulent premixed combustion is closed using the transported flame surface density (TFSD) model. In this approach, there is no need to assume local equilibrium between the generation and destruction of subgrid FSD, as commonly done in simple algebraic closure models. Instead, the key processes that create and destroy FSD are accounted for explicitly. This allows the model to capture large-scale unsteady flame propagation in the presence of combustion instabilities, or in situations where the flame encounters progressive wrinkling with time. In this study, comprehensive validation of the numerical method is carried out. For the non-reacting flow, good agreement for both the time-averaged and root-mean-square velocity fields are obtained, and the Karman type vortex shedding behaviour seen in the experiment is well represented. For the reacting flow, two mesh configurations are used to investigate the sensitivity of the LES results to the numerical resolution. Profiles for the velocity and temperature fields exhibit good agreement with the experimental data for both the coarse and dense mesh. This demonstrates the capability of LES coupled with the TFSD approach in representing the highly unsteady premixed combustion observed in this configuration. The instantaneous flow pattern and turbulent flame behaviour are discussed, and the differences between the non-reacting and reacting flow are described through visualisation of vortical structures and their interaction with the flame. Lastly, the generation and destruction of FSD are evaluated by examining the individual terms in the FSD transport equation. Localised regions where straining, curvature and propagation are each dominant are observed, highlighting the importance of non-equilibrium effects of FSD generation and destruction in the model afterburner.     

Ethanol turbulent spray flame response to gas velocity modulation

A numerical investigation of the interaction between a spray flame and an acoustic forcing of the velocity field is presented in this paper. In combustion systems, a thermoacoustic instability is the result of a process of coupling between oscillations in heat released and acoustic waves. When liquid fuels are used, the atomisation and the evaporation process also undergo the effects of such instabilities, and the computational fluid dynamics of these complex phenomena becomes a challenging task. In this paper, an acoustic perturbation is applied to the mass flow of the gas phase at the inlet and its effect on the evaporating fuel spray and on the flame front is investigated with unsteady Reynolds averaged Navier-Stokes numerical simulations. Two flames are simulated: a partially premixed ethanol/air spray flame and a premixed pre-vaporised ethanol/air flame, with and without acoustic forcing. The frequencies used to perturb the flames are 200 and 2500 Hz, which are representative for two different regimes. Those regimes are classified based on the Strouhal number St = (D/U)f_f: at 200 Hz, St = 0.07, and at 2500 Hz, St = 0.8. The exposure of the flame to a 200 Hz signal results in a stretching of the flame which causes gas field fluctuations, a delay of the evaporation and an increase of the reaction rate. The coupling between the flame and the flow excitation is such that the flame breaks up periodically. At 2500 Hz, the evaporation rate increases but the response of the gas field is weak and the flame is more stable. The presence of droplets does not play a crucial role at 2500 Hz, as shown by a comparison of the discrete flame function in the case of spray and pre-vaporised flame. At low Strouhal number, the forced response of the pre-vaporised flame is much higher compared to that of the spray flame.     

Interaction of a Gaussian acoustic wave with a turbulent non-premixed flame

A Direct Numerical Simulation (DNS) of a turbulent non-premixed flame interacting with a Gaussian acoustic wave is carried out in this work. This numerical simulation takes into account detailed transport phenomena including the Soret effect as well as complete chemical kinetics on a very fine mesh. Turbulent non-premixed flame calculations are carried out both with and without an acoustic wave and results are recorded at the same time. By a simple difference it is then possible to obtain the influence of the acoustic wave/turbulent flame interaction. Using an extension of the non-linear Rayleigh criterion to a system with many species and elementary reactions, the obtained results can be further analysed. The initially planar acoustic wave develops strong perturbations along its transverse direction because of the interaction process, even at very early times. The amplitude of the pressure perturbation presents locally high positive as well as negative values, demonstrating the importance of focussing/defocussing effects and local amplification (resp. damping) phenomena. In the same way, the heat release rate is locally modified (either increased or decreased) during the interaction process. Finally, the presented Rayleigh criterion is used to identify regions where local amplification (respectively damping) takes place. Both amplification and damping zones coexist directly close to each other inside the reaction zone. The observed, resulting global effect is thus based on an average over highly varying local conditions within the flame front, leading to a smoothing effect. The complexity of the coupling procedure leading to this global wave amplification or damping is demonstrated by the present analysis.     

Effect of temporal pulsations of a turbulent flow on the flame velocity

Recently, a hypothesis has been proposed that temporal pulsations of a turbulent flow may explain the reduction and even saturation of the flame velocity at large turbulent intensities. However, the study was limited to very anisotropic flows. In this paper, we investigate the effect under more general conditions, which use the dependence of the velocity in the mean direction of propagation. Analytical formulae for the flame velocity in a time-dependent turbulent flow are obtained at low turbulent intensity and generalized to high intensity by using the renormalization method. These results are compared to numerical simulations. We show that the temporal pulsations do not lead to saturation of the flame velocity at high turbulent intensity, even if some effects appear at lower root-mean-square velocity.     

Monte Carlo PDF modelling of a turbulent natural-gas diffusion flame

A piloted turbulent natural-gas diffusion flame is investigated numerically using a 2D elliptic Monte Carlo algorithm to solve for the joint probability density function (PDF) of velocity and composition. Results from simulations are compared to detailed experimental data: measurements of temperature statistics, data on mean velocity and turbulence characteristics and data on OH. Conserved-scalar/constrained-equilibrium chemistry calculations were performed using three different models for scalar micro-mixing: the interaction by exchange with the mean (IEM) model, a coalescence/dispersion (C/D) model and a mapping closure model. All three models yield good agreement with the experimental data for the mean temperature. Temperature standard deviation and PDF shapes are generally predicted well by the C/D and mapping closure models, whereas the IEM model gives qualitatively incorrect results in parts of the domain. It is concluded that the choice of micro-mixing model can have a strong influence on the quality of the predictions. The same flame was also simulated using reduced chemical kinetics obtained from the intrinsic low-dimensional manifold (ILDM) approach. Comparison with the constrained-equilibrium results shows that the shape of the OH concentration profiles is recovered better in the ILDM simulation, and that the ILDM reduced chemical kinetics can correctly predict super-equilibrium OH.     

Premixed turbulent flame front structure investigation by Rayleigh scattering in the thin reaction zone regime

Premixed turbulent flames of methane–air and propane–air stabilized on a bunsen type burner were studied using planar Rayleigh scattering and particle image velocimetry. The fuel–air equivalence ratio range was from lean 0.6 to stoichiometric for methane flames, and from 0.7 to stoichiometric for propane flames. The non-dimensional turbulence rms velocity, u′/S_L, covered a range from 3 to 24, corresponding to conditions of corrugated flamelets and thin reaction zones regimes. Flame front thickness increased slightly with increasing non-dimensional turbulence rms velocity in both methane and propane flames, although the flame thickening was more prominent in propane flames. The probability density function of curvature showed a Gaussian-like distribution at all turbulence intensities in both methane and propane flames, at all sections of the flame.The value of the term Dκ, the product of molecular diffusivity evaluated at reaction zone conditions and the flame front curvature, has been shown to be smaller than the magnitude of the laminar burning velocity. This finding questions the validity of extending the level set formulation, developed for corrugated flames region, into the thin reaction zone regime by increasing the local flame propagation by adding the term Dκ to laminar burning velocity.     

A numerical study on the formation of diffusion flame islands in a turbulent hydrogen jet lifted flame

This paper presents a numerical study on the formation of diffusion flame islands in a hydrogen jet lifted flame. A real size hydrogen jet lifted flame is numerically simulated by the DNS approach over a period of about 0.5 ms. The diameter of hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The lifted flame is composed of a stable leading edge flame, a vigorously turbulent inner rich premixed flame, and a number of outer diffusion flame islands. The relatively long-term observation makes it possible to understand in detail the time-dependent flame behavior in rather large time scales, which are as large as the time scale of the leading edge flame unsteadiness. From the observation, the following three findings are obtained concerning the formation of diffusion flame islands. (1) A thin oxygen diffusion layer is developed along the outer boundary of the lifted flame, where the diffusion flame islands burn in a rather flat shape. (2) When a diffusion flame island comes into contact with the fluctuating inner rich premixed flame, combustion is intensified due to an increase in the hydrogen supply by molecular diffusion. This process also works for the production of the diffusion flame islands in the oxygen diffusion layer. (3) When a large unburned gas volume penetrates into the leading edge flame, the structure of the leading edge flame changes. In this transformation process, a diffusion flame island comes near the leading edge flame. The local deficiency of oxygen plays an important role in this production process.