Flame Wrinkling Research Articles

Study on flame-vortex interaction in a spark ignition engine fueled with methane/carbon dioxide gases

A numerical study on the in-cylinder flame-vortex interaction of gaseous spark ignited engine fueled with methane/carbon dioxide is carried out by means of large-eddy method. Evolution of in-cylinder turbulence in charge phase and flame-vortex interaction during combustion process is analyzed in great detail. It's found out that the large scale coherent structures are transformed into homogeneous small scale vortexes during the intake and compression stroke. The strong vortex cores are generated by interaction between flame and in-cylinder background turbulence. Those generated vortex cores wrinkle flame surface and augment turbulent flame speed. The contra-rotation between the two vortexes of vortex-pair in the unburned area results in the appearance of large scale flame wrinkles, which is because the vortex-pair movement leads to the local entrainment and hence stretchs of the flame surface. With the increase of volume fraction of carbon dioxide in the gases, the turbulent flame speed is decreased, the effect of vortex pair on the flame structure is weakened, and the level of the flame wrinkling is decreased correspondingly.

Direct numerical simulation of the turbulent premixed flame propagation with radiation effects

This paper investigates the effects of turbulence on radiative heat transfer in premixed flame propagation using a combination of direct numerical simulation (DNS) and the discrete ordinates method (DOM). The DNS code has been validated and established in previous research, and the newly written DOM code is validated by solving a sample radiative heat transfer problem. The DOM code is explicitly combined with the DNS code using the 3rd-order Runge–Kutta scheme. In the numerical experiments, an initially flat laminar premixed flame interacts with imposed turbulent fluctuations and evolves over time. A remarkable increase in the temperature self-correlation factor <T4>/<T>4 is observed in the middle of the reaction. Higher u′ conditions induced faster and more intensive flame wrinkle evolution. However, flow turbulence reduces the radiative heat loss in planar premixed flame due to the flame curvature effects.

Modeling the Response of Turbulent Flames to Harmonic Forcing

ABSTRACTThis article analyzes the response of a turbulent, premixed flame to harmonic forcing. This problem has been worked extensively for laminar flames, and the key parameters influencing the flame transfer function are well understood. For turbulent flames, several prior studies have utilized a “quasi-laminar” approach, by utilizing the time-averaged flame position and ensemble-averaged disturbance field, as inputs to what is otherwise identical to the laminar problem. More generally, the manner in which turbulent flames respond to harmonic disturbances is not amenable to analytical solutions because of the nonlinear interactions between stochastic flow disturbances and harmonic flame wrinkling. We utilize a turbulent burning velocity closure proposed by Shin and Lieuwen (2013), who showed that the ensemble-averaged turbulent burning rate for a harmonically forced flame is proportional to the ensemble-averaged flame curvature. Shin and Lieuwen (2013) previously used it to analyze the ensemble-averaged space-time flame wrinkle characteristics. Here, we extend these results to analyze the spatial variation of ensemble-averaged flame surface area and burning rate and then compare these results to computations. These results show that, for low stochastic forcing amplitudes, wrinkling of the front exerts quantitative differences between those predicted by a quasi-laminar and the actual flame response (e.g., reducing peak values of the flame transfer function and eliminating nodes), but does not change the key qualitative features. While this result needs to be considered for strongly turbulent flames, it does suggest why good agreement has been observed between quasi-laminar approaches and experimental data for harmonically excited, turbulent flames. Two results for model problems showing the linearized flame transfer functions are also presented, which explicitly demonstrate qualitative turbulence effects on harmonically excited flames.

An analysis of flame instabilities for hydrogen–air mixtures based on Sivashinsky equation

In this paper flame instabilities are analyzed utilizing the Sivashinsky equation in order to derive the flame wrinkling factor. This is a synthetic variable representing the excess of flame surface which is obtained for a wide range of hydrogen concentrations, considering the Darrieus–Landau and the Thermo-Diffusive instabilities, and also taking into account the effect of acceleration. Additionally, the time for the development of the cellularity is also analyzed. The study is carried out for a wide range of hydrogen–air mixtures as well as for a large domain of accelerations. Models representing both the wrinkling factor and the time of development of the instabilities are obtained.

Premixed flame inhibition by CF3Br and C3H2F3Br (2-BTP)

The un-stretched burning velocities and Markstein lengths of premixed CH4– and C3H8–air flames with added C3H2F3Br (2-BTP) or CF3Br (Halon 1301), have been studied experimentally and numerically. For CF3Br flame inhibition, the un-stretched burning velocities, predicted using a recently updated kinetic model for CF3Br flame inhibition, were in excellent agreement with the experimental results over a range of fuel-air equivalence ratio and CF3Br loading. For C3H2F3Br flame inhibition, the un-stretched burning velocities predicted using a recently developed kinetic mechanism were in good agreement with the experimental results for most of the equivalence ratios tested; nonetheless, for very lean flames approaching the flammability limit, model predictions differed by up to 25%, even for uninhibited flames. The influence of inhibitor on the flame response to stretch and susceptibility to instabilities was examined through consideration of the measured burned gas Markstein lengths. Markstein lengths were very large, leading to large stretch effects on the flame stability after ignition, and flame wrinkling during explosion tests, greatly increasing the rate of pressure rise. The influence of stretch with regard to flame inhibitor effectiveness is discussed.

Effect of axial diffusion on the response of diffusion flames to axial flow perturbations

This paper elucidates the behavior and dynamics of non-premixed flames responding to bulk fluctuations in flow velocity. It expands previous work on this problem by consistently incorporating finite Peclet number (▪) effects, and differentiating inflow boundary and dynamical effects on the flame dynamics. For analytical tractability, prior treatments of this problem generally prescribe the inflow boundary conditions into the domain. This paper shows, however, that prescribed inflow conditions, such as a step or constant local diffusive flux boundary condition, neglect axial diffusion effects in the region where their effects are most important; i.e., in the near-burner exit region where high transverse gradients and mass burning rates control the heat release dynamics. As the burning rate of non-premixed flames are controlled by mixture fraction gradients, the influence of axial diffusion substantively influences several burning rate characteristics of the flame. In addition, these effects cause the leading edge position of the flame front to oscillate, even for infinitely fast chemistry. Even in Pe≫1 flames, axial diffusion introduces several fundamentally new features to the problem, resulting in exponentially decaying, dispersive flame wrinkle propagation with downstream distance. Also investigated here are asymptotic results for the heat release dynamics. It is shown that axial diffusion introduces a triple-zone asymptotic structure into the unsteady heat release characteristics, resulting in O(1) flame transfer function trends for St≪1 (to which an n–τ model is developed), O(1/St1/2) for intermediate Strouhal numbers, and O(1/St) for high Strouhal numbers. Finally, it is shown that the phase of the heat release response is approximately half that of a premixed flame with the same length, due to the concentration of unsteady heat release near the burner outlet, where transverse gradients are largest.

Premixed flame response to helical disturbances: Mean flame non-axisymmetry effects

A key component of the thermoacoustic instability feedback mechanism is the excitation of hydrodynamic flow disturbances by narrowband acoustic fluctuations. In earlier work, we considered the response of time-averaged axisymmetric flames to helical disturbances. That study showed that although all helical modes cause local flame wrinkling, only the axisymmetric hydrodynamic mode, m = 0, contributes to the global heat release rate fluctuations. This paper extends this work to consider time-averaged flames that are non-axisymmetric. We show that distortions of the unforced flame shape changes its receptivity to helical disturbances, as it does not allow for the perfect destructive interference seen in axisymmetric flames. This results in a non-zero global flame response contribution from helical modes. Given that helical modes often have the largest amplitudes in jet flows, particularly those with swirl, these results show that the degree of non-axisymmetry of the flame has an important influence in determining which hydrodynamic modes, axisymmetric or helical, control the global heat release response of the flame. These points have been illustrated with example calculations that show how different control parameters and the degree of time-averaged non-axisymmetry influence the global flame response. An important implication of these results relates to scaling results from simplified geometries where a round jet is placed inside a round enclosure, to more realistic ones (such as where multiple jets are placed next to each other) where the flame shape will be distorted from being perfectly circular.

Heat transfer and flame stabilization of laminar premixed flames anchored to a heat-flux burner

Measurement of the burning velocity of unstretched laminar hydrogen/air premixed flames suffers from large uncertainties owing to the highly diffusive nature of hydrogen that can give rise to flame instability. This paper reports on a numerical study of the structures and stability of laminar premixed CH4/O2/CO2 flames and H2/O2/N2 flames anchored to a heat-flux burner using a high-order numerical method with detailed chemical kinetic mechanisms and detailed transport properties. The aim is to elucidate the effect of the flow and temperature inhomogeneity generated by the burner plate holes on flame structures and burning velocity. Heat transfer flux between the burner plate and the surrounding gaseous mixture is investigated under various standoff distances and burner plate temperatures. The burning velocity and the detailed flow, temperature and species distributions in flames with a zero net heat flux between the flames and the burner plate are analyzed. It is found that for the methane flames, when the standoff distance is sufficiently small, the burner can essentially suppress the intrinsic flame instability, but the plate holes can give rise to flame wrinkles of the size of the holes. At high standoff distances, the non-uniformity of the flow from the burner plate holes has a minor effect on the flame surface wrinkling; however, large-scale cellular structures can appear on the flame surface due to intrinsic flame instability. For the studied methane flames the effect of non-uniformity of the flow from the burner plate holes on the burning velocity is fairly small. For the studied hydrogen flames the burner plate could not totally suppress the intrinsic flame instability. The intrinsic flame instability can give rise to a significant increase in the flame surface area and mean burning velocity, with more than 25% increase in the burning velocity.

Analysis of dynamic models for large eddy simulations of turbulent premixed combustion

Dynamic procedures to automatically determine flame wrinkling factors from known resolved fields in large eddy simulations of turbulent premixed combustion are investigated from a priori tests processing a DNS database of a turbulent swirled flame. These flame wrinkling factors measure the ratio of total to resolved flame surfaces in the filtering volume and enter directly or indirectly into various flamelet combustion models through the sub-grid scale turbulent flame speed. They are usually modeled by algebraic expressions derived assuming equilibrium between turbulence motions and flame dynamics, a situation generally not reached during early stages of flame developments. Dynamic models then appear as a promising alternative to flame wrinkling factor or flame surface density balance equations to handle out-of-equilibrium situations. Attention is paid to three key requirements: (i) the correct prediction of propagating laminar flame fronts; (ii) the replacement of the averaging volume introduced to determine resolved and test-filtered flame wrinkling factors by a Gaussian operator easier to implement on unstructured meshes and/or massively parallel machines; (iii) the use of a local model parameter, evolving both in space and time. The two first requirements suggest basing the procedure on flame surface conservation instead of on chemical reaction rates. The saturated form of the Charlette et al. efficiency function [1], ΞΔ=(Δ/δl)β, where Δ is the filter width and δl the flame thickness, is found to be very well suited to dynamic determination of the model parameter β, easy to implement and very robust in practice, as confirmed by preliminary a posteriori tests.

Statistical Behavior of Scalar Dissipation Rate in Head-On Quenching of Turbulent Premixed Flames: A Direct Numerical Simulation Analysis

ABSTRACTThe statistical behavior of the scalar dissipation rate (SDR) and its transport in the context of Reynolds averaged Navier Stokes (RANS) simulations has been analyzed for head-on quenching of turbulent premixed flames using a three-dimensional simplified chemistry based direct numerical simulations (DNS) database. The flame quenching statistics have been analyzed in terms of wall Peclet number (i.e., non-dimensional distance of the flame in the wall normal direction) and non-dimensional wall heat flux magnitude . It has been found that flame wrinkling induces fluctuations of wall heat flux in turbulent cases and the magnitude of the maximum wall heat flux increases with increasing root-mean-square turbulent velocity .The closure of mean reaction rate using the SDR of reaction progress variable in the near wall region has been assessed based on a priori analysis of DNS data. It has been demonstrated that the existing SDR-based reaction rate closure does not work satisfactorily near the wall. A modification to this existing closure has been proposed, which is found to satisfactorily predict the mean reaction rate of reaction progress variable in the near wall region and approaches the existing closure away from the wall. The wall effects on the unclosed terms of the SDR transport equation have been analyzed and the order of magnitude estimates of the leading order contributors to the SDR transport have been utilized to modify an existing algebraic SDR closure to account for the near wall effects. A priori DNS analysis suggests that the proposed modification to the aforementioned SDR closure provides satisfactory prediction both away from and near to the wall.

Lewis number and Markstein length effects on turbulent expanding flames in a spherical vessel

The turbulent combustion of three different lean fuel–air mixtures has been investigated in a spherical vessel running at different turbulent intensities. Those mixtures were selected to present almost the same unstretched laminar flame speed but various Markstein lengths and Lewis numbers which are relevant parameters to describe flame-stretch interactions. Mie-scattering tomography has been used to measure a global flame stretch and an equivalent flame propagation speed. Results prove that mixtures have same ranking in terms of flame stretch sensitivity as in laminar regime and show the importance of taking into account this parameter in modeling and to evaluate fuel performance in terms of burning speed in Spark-Ignition engine. Flame wrinkling and curvature measurement were also carried out and the wrinkling effect on the flame propagation has been identified.

Optimized Reduced Chemistry and Molecular Transport for Large Eddy Simulation of Partially Premixed Combustion in a Gas Turbine

A methodology is discussed to automatically determine the parameters of closed budget equations for chemical species mass fractions and energy, in order to simulate spatially filtered flames as required in large eddy simulation (LES). The method accounts for the effects of LES filtering on chemistry and transport by simultaneously optimizing, for a reduced number of species, the Arrhenius reaction rates and a correction to mixture-averaged molecular diffusion coefficients. The objective is to match, for a given filter size, spatially filtered canonical one-dimensional flames simulated with detailed chemistry solutions. This approach is designed for quite well-resolved LES, in which most of the unresolved fluctuations result from flame thickening due to spatial filtering, thus featuring weak levels of sub-grid scale flame wrinkling. Methane-air partially premixed combustion is addressed. A four-step reduced reaction mechanism involving seven species is developed along with mass and heat molecular transport properties. The optimization is performed at atmospheric pressure and at 3 bar, for ranges of fresh gas temperatures [300–650 K] and equivalence ratios [0.4–1.2]. Comparisons with the filtered detailed chemistry solution of a planar propagating front show that the laminar flame speed, the adiabatic flame temperature, the species profiles in the reaction zone, and the flow chemical composition and temperature at equilibrium are adequately predicted. The new sub-grid scale modeling approach is then applied to three-dimensional LES of an industrial gas turbine burner. Good agreement is found between the quantities predicted with LES and experimental data, in terms of flow and flame dynamics, axial velocities, averaged temperatures, and some major species concentrations. Results are also improved compared to previous simulations of the same burner.

Multiple Cycle LES Simulations of a Direct Injection Natural Gas Engine

In this work the flow field evolution, mixture formation and combustion process in an engine with methane Direct Injection (DI) is investigated using Large Eddy Simulations. The supersonic methane injection is modeled according to Muller et al. (2013) and combustion by a level set approach. The flame propagation showed to be dependent on the grid resolution. Higher grid resolutions have two opposing effects: first the fraction of unresolved turbulence is reduced, which decrease the flame speed and second flame wrinkling is increased resulting in faster flame propagation. For the observed setup the wrinkling effect was stronger. The average in-cylinder pressure traces as well as the cyclic variability thereof were compared to experimental data and very good agreement was found. During the supersonic gaseous injection the turbulence level in the cylinder is significantly increased, which dissipates quickly and thus has only a minor effect on the flame propagation. The introduced momentum showed a larger impact, since it enhances the tumble motion resulting in increased turbulence levels as the tumble decays shortly before ignition. During DI the cyclic differences in the tumble motion are preserved, but the impact on the average tumble level results in changing relative differences of the cyclic turbulence levels at ignition timing. Thus an injection direction supporting the tumble flows is expected to reduce the Cycle-to-Cycle Variations (CCV), while a reduction of the tumble strength could increase the CCV level. Compared to the fluctuations in the turbulence levels, the cyclic variability of the equivalence ratio at the injection location with DI showed a minor effect on the simulated CCVs.

Burning velocity and statistical flame front structure of turbulent premixed flames at high pressure up to 1.0 MPa

Statistical flame front structure of turbulent premixed flames at high pressure up to 1.0MPa was measured on a nozzle-type Bunsen burner with OH-PLIF technique. Turbulent burning velocity, flame surface density and flame brush thickness, as well as the local curvature and radius of curvature were derived from the experimental OH-PLIF images. Turbulence–flame interaction was analyzed based on the geometric parameters combined with laminar flame properties and turbulence length scales. Results show that the flame wrinkles at high pressure are dominated by small scale cusps superimposed with large scale flame branches which is a general characteristic of the turbulent premixed flames at high pressure. ST/SL increases remarkably with u′/SL and the influence of elevated pressure on ST/SL is significant. This is mainly due to the increase of flame front area caused by the turbulence wrinkling. Flame surface density significantly increases with the increase of pressure indicating that there is a large amount of fine cusps and small wrinkles in the flame front at high pressure. This would be due to the enhancement of the flame instability represented by effective Lewis number Leeff and flame intrinsic instability scale li. With the increase of turbulence intensity, the Σ at high pressure increases while slightly decreases at normal pressure. The most frequent length scale of the flame front moves to smaller value and the possibility increases with the increase of u′/SL for all pressures. The effect of flame intrinsic instability on finer flame front at high pressure is mainly on the formation of a large number of convex structures which enlarge the effective contact surface between flame front and unburned reactants, resulting in the increase of ST/SL.

Sound Generation from Swirling, Premixed Flames Excited by Helical Flow Disturbances

This article describes an analysis of the sound generated by swirling premixed flames when excited by helical disturbances, i.e., where the flow fluctuations have an azimuthal dependence of the form and denotes the helical mode number. These results illustrate the sensitivity of local and global sound generation characteristics to mode number, , swirl strength, and dimensionless frequency. An important result is the demonstration that specific mode numbers dominate particular aspects of the flame response. Specifically, particular helical modes dominate the magnitude of local flame wrinkling, , global heat release, , and sound generation, , and are generally not the same. For example, the helical mode that dominates the flame sound radiation, , is a function of flame compactness ratio, , while and are not. For compact, axisymmetric flames, , the axisymmetric, mode dominates the far-field sound radiation, both locally and in terms of global sound power. For and for , other helical modes dominate, generally those modes with closer to . Finally, the overall sound power scales as in the limit, while all modes scale equally as in the limit.

Investigation on rapid consumption of fine scale unburned mixture islands in turbulent flame via 10 kHz simultaneous CH–OH PLIF and SPIV

A system based on high repetition rate simultaneous CH–OH planar laser induced fluorescence (PLIF) and stereoscopic particle image velocimetry (SPIV) measurement has been developed. The high-speed simultaneous measurement system has an acquisition rate of 10kHz and its measurement duration time exceeds 1.0s. The developed system is applied to a methane–air turbulent jet premixed flame in order to investigate the flame and flow dynamics of the turbulent combustion field. The obtained simultaneous CH and OH PLIF images well represent the turbulent flame dynamics in shear flow. Under highly turbulent conditions, the number of flame wrinkles increases and the flame front shows a very complex geometry. Since large scale vortical structures develop due to Kelvin–Helmholtz instability, large scale unburned mixture islands are often created and ejected into the downstream. Around the flame tip under the condition of high Reynolds number (U0=20m/s, Reλ=257), the formation of fine scale unburned mixture islands is observed and its contribution to enhance the turbulent burning velocity is investigated by statistically analyzing the local consumption rate. The consumption rate is estimated from the OH and CH PLIF images on the basis of the spherical and pillar assumptions. The most probable consumption rates obtained from all adopted methods are much higher than the laminar burning velocity (0.39m/s). The importance of a high repetition rate CH acquisition as a flame marker is also discussed.

Flame Leading Edge and Flow Dynamics in a Swirling, Lifted Flame

Flames in high swirl flow fields with vortex breakdown often stabilize aerodynamically in front of interior flow stagnation points. In contrast to shear layer stabilized flames with a nearly fixed, well-defined flame attachment point, the leading edge of aerodynamically stabilized flames can move around substantially as a result of both the inherent dynamics of the vortex breakdown region and externally imposed oscillations. Motion of this flame stabilization point relative to the flow field may have an important dynamical role during combustion instabilities, as it creates flame front wrinkles and heat release fluctuations. For example, a prior study has shown that nonlinear dynamics of the flame response at high forcing amplitudes were related to these leading edge dynamics. This heat release mechanism exists alongside other flame wrinkling processes, arising from such processes as shear layer rollup and swirl fluctuations. This article describes an experimental investigation of flow forcing effects on the dynamics of the leading edge of a swirl stabilized flame. Flame and flow dynamics were characterized using high-speed particle image velocimetry (PIV) and CH* chemiluminescence imaging. A range of forcing conditions was achieved by varying the forcing frequency, amplitude, and acoustic field symmetry. These results show that the flame leading edge motion is dominated by the natural flow instabilities, particularly the precession of the recirculation zone around the centerline. The fluctuations in leading edge motion at the excitation frequency are much smaller than these natural motions, but are still on the order of the fluid particle displacement associated with the external excitation, indicating that they exhibit an important influence on local flame wrinkling and heat release. Flame response modeling shows that the global, spatially integrated heat release response is controlled by three factors—vortical disturbances, acoustic flow disturbances, and flame leading edge motion. The vortical flow motions dominate the flame response, and the flame leading edge motion is a minor contributor to the overall heat release response. This is an important result, as it shows that the significant motions of the flame leading edge actually have little dynamical significance for understanding the spatially integrated, forced response of the flame.

Analyzing Three-Dimensional Multiple Shock-Flame Interactions in a Constant-Volume Combustion Channel

Advanced numerical techniques are required to accurately analyze turbulent combustion at a minimal computational cost. In the current study, the use of automatic mesh refinement based on velocity and temperature curvatures was used. In order to speed up kinetic calculations, computational cells with similar temperatures, pressures, and composition were grouped as “zones” and kinetics were solved once per zone per time step. These techniques, previously validated for internal combustion engines, are applied to study premixed combustion in a constant-volume combustor. Innovative post-processing techniques are suggested to estimate the flame surface area and local flame thickness. Pressure, heat release, vorticity, Mach number, and normal strains on the flame were evaluated to understand the mechanisms behind flame surface area increase and wrinkling, and enhanced fuel burning rates, following interactions with shock waves. Predicted in-channel pressures are validated against experimental measurements.

Unsteady vortex breakdown in an atmospheric swirl stabilised combustor. Part 1: Chamber behaviour

This paper presents the behaviour of three very different and unique flame and flow structures within an atmospheric swirl-stabilised dump combustor supplied with a lean premixed mixture of methane and air. The reactant flow was artificially perturbed with frequencies of 100Hz, 200Hz, and 400Hz. Phase average behaviour and temporal dynamics were characterised using phase locked high speed CH chemiluminescence and high speed stereo particle imaging. The interaction between the flame and flow field, in particular the internal recirculation zone of the vortex breakdown, was determined to be responsible for differences observed in behaviour at the three forcing frequencies. The 100Hz perturbation frequency displayed simple oscillatory motion. Higher perturbation frequencies of 200Hz and 400Hz gave rise to a second toroidal vortex ring which formed within the internal recirculation zone adjacent to the inner shear layer. This caused additional out of phase modulation of the heat release rate and flame area. Twin counter rotating vorticity structures attached to the annulus were formed as a result of the chamber geometry. The oscillating inlet flow and oscillating reversed flow region of the inner recirculation zone caused oscillations in vorticity magnitude which were responsible for flame wrinkling and stretch effects upon the flame front. Vorticity within the shear layers was found to be the source of harmonic frequency generation of the imposed perturbation frequencies. The data is presented in detail to facilitate CFD model comparisons, particularly LES.

The influence of combustion SGS submodels on the resolved flame propagation. Application to the LES of the Cambridge stratified flames

In Large Eddy Simulation (LES) of premixed and stratified combustion, the overall prediction of the flame consumption speed depends on various subgrid scale (SGS) submodels such as the flame wrinkling, the fuel stratification or heat losses. The objective of this study is to investigate the LES sensitivity to the submodeling strategies. Different heat losses and SGS flame wrinkling models are presented in the context of the Filtered TAbulated Chemistry for LES (F-TACLES) formulation. LES of the non-adiabatic non-swirling bluff-body stabilized Cambridge flames (SwB burner) are presented. In this complex configuration, both flame brush and flow dynamics are influenced by flame consumption speed submodels. First, accounting for heat losses impacts the prediction of both velocity and temperature of the inner recirculation zone (IRZ). Second, model constants involved into SGS wrinkling submodels have a great impact on the mean flame brush position. The non-adiabatic formulation combined with a dynamic estimation of the SGS wrinkling model constant appears to be a very attractive approach and gives a very good prediction of both the mean flame location and the IRZ flow dynamics.