An analytical model for the prediction of the dynamic response of premixed flames stabilized on a heat-conducting perforated plate

The dynamic response of a premixed flame stabilized on a heat-conducting perforated plate depends critically on their coupled thermal interaction. The objective of this paper is to develop an analytical model to capture this coupling. The model predicts the mean flame base standoff distance; the flame base area, curvature and speed; and the burner plate temperature given the operating conditions; the mean velocity, temperature and equivalence ratio of the reactants; thermal conductivity and the perforation ratio of the burner. This coupled model is combined with our flame transfer function (FTF) model to predict the dynamic response of the flame to velocity perturbations. We show that modeling the thermal coupling between the flame and the burner, while accounting for the two-dimensionality of the former, is critical to predicting the dynamic response characteristics such as the overshoot in the gain curve (resonant condition) and the phase delay. Good agreement with the numerical and experimental results is demonstrated over a range of conditions.     

Identification of flame transfer functions in the presence of intrinsic thermoacoustic feedback and noise

The Large Eddy Simulation/System Identification (LES/SI) approach is a general and efficient numerical method for deducing a Flame Transfer Function (FTF) from the LES of turbulent reacting flow. The method may be summarised as follows: a simulated flame is forced with a broadband excitation signal. The resulting fluctuations of the reference velocity and of the global heat release rate are post-processed via SI techniques in order to estimate a low-order model of the flame dynamics. The FTF is readily deduced from the low-order model. The SI method most frequently applied in aero- and thermo-acoustics has been Wiener–Hopf Inversion (WHI). This method is known to yield biased estimates in situations with feedback, thus it was assumed that non-reflective boundary conditions are required to generate accurate results with the LES/SI approach. Recent research has shown that the FTF is part of the so-called Intrinsic ThermoAcoustic (ITA) feedback loop. Hence, identifying an FTF from a compressible LES is always a closed-loop problem, and consequently one should expect that the WHI would yield biased results. However, several studies proved that WHI results compare favourably with validation data. To resolve this apparent contradiction, a variety of identification methods are compared against each other, including models designed for closed-loop identification. In agreement with theory, we show that the estimate given by WHI does not converge to the actual FTF. Fortunately, the error made is small if excitation amplitudes can be set such that the signal-to-noise ratio is large, but not large enough to trigger nonlinear flame dynamics. Furthermore, we conclude that non-reflective boundary conditions are not essentially necessary to apply the LES/SI approach.     

Identification and analysis of instability in non-premixed swirling flames using LES

Large eddy simulations (LES) of turbulent non-premixed swirling flames based on the Sydney swirl burner experiments under different flame characteristics are used to uncover the underlying instability modes responsible for the centre jet precession and large scale recirculation zone. The selected flame series known as SMH flames have a fuel mixture of methane-hydrogen (50:50 by volume). The LES solves the governing equations on a structured Cartesian grid using a finite volume method, with turbulence and combustion modelling based on the localised dynamic Smagorinsky model and the steady laminar flamelet model respectively. The LES results are validated against experimental measurements and overall the LES yields good qualitative and quantitative agreement with the experimental observations. Analysis showed that the LES predicted two types of instability modes near fuel jet region and bluff body stabilised recirculation zone region. The mode I instability defined as cyclic precession of a centre jet is identified using the time periodicity of the centre jet in flames SMH1 and SMH2 and the mode II instability defined as cyclic expansion and collapse of the recirculation zone is identified using the time periodicity of the recirculation zone in flame SMH3. Finally frequency spectra obtained from the LES are found to be in good agreement with the experimentally observed precession frequencies.     

Experimental evaluation of DC electric field effect on the thermoacoustic behaviour of flat premixed flames

One promising approach to eliminate thermoacoustic instabilities in combustion appliances is the use of adaptive control of the flame/burner acoustic transfer function (TF). Application of a DC electric field (EF) as a spatially distributed, easily and quickly adjustable and low-energy method to affect the flame behaviour may be considered as a possible actuation method to control the flame TF. Experimental evaluation of such a possibility is the main goal of the present study. The effect of a DC EF on the acoustic TF of premixed flat burner-surface stabilized flames is studied systematically as a function of the following parameters: flow velocity, equivalence ratio, applied voltage and burner geometry. It is established that the response of the flame TF on the DC EF can be characterized as a TF shift towards higher frequencies. The mechanism of TF alteration is related to the decrease of the flame stand-off distance and the related increase of the burner deck surface temperature. From a practical point of view, the efficiency of the EF control of the flat flame TF is restricted to a relatively narrow frequency range around the position of the TF resonance peak. To get insight into the physics of the EF–flame interaction, the method of [1] to measure the EF effect on the adiabatic flame speed is improved and the measurement range is extended. The new measurements allow a revision of previous results and allow an explanation for the ambiguity in the old measurements.     

Experimental low-stretch gaseous diffusion flames in buoyancy-induced flowfields

The present study experimentally investigates the structure and instabilities associated with extremely low-stretch (1 s⁻¹) gaseous diffusion flames. Ultra-low-stretch flames are established in normal gravity by bottom burning of a methane/nitrogen mixture discharged from a porous spherically symmetric burner of large radius of curvature. OH-PLIF and IR imaging techniques are used to characterize the reaction zone and the burner surface temperature, respectively. A flame stability diagram mapping the response of the ultra-low-stretch diffusion flame to varying fuel injection rate and nitrogen dilution is explored. In this diagram, two main boundaries are identified. These boundaries separate the stability diagram into three regions: sooting flame, non-sooting flame, and extinction. Two distinct extinction mechanisms are noted. For low fuel injection rates, flame extinction is caused by heat loss to the burner surface. For relatively high injection rates, at which the heat loss to burner surface is negligible, flame radiative heat loss is the dominant extinction mechanism. There also exists a critical inert dilution level beyond which the flame cannot be sustained. The existence of multi-dimensional flame phenomena near the extinction limits is also identified. Various multi-dimensional flame patterns are observed, and their evolutions are studied using direct chemiluminescence and OH-PLIF imaging. The results demonstrate the usefulness of the present burner configuration for the study of low-stretch gaseous diffusion flames.     

Impact of swirl and bluff-body on the transfer function of premixed flames

The frequency response of three lean methane/air flames submitted to flowrate perturbations is analyzed for flames featuring the same equivalence ratio and thermal power, but a different stabilization mechanism. The first flame is stabilized by a central bluff body without swirl, the second one by the same bluff body with the addition of swirl and the last one only by swirl without central insert. In the two last cases, the swirl level is roughly the same. These three flames feature different shapes and heat release distributions, but their Flame Transfer Function (FTF) feature about the same phase lag at low frequencies. The gain of the FTF also shows the same behavior for the flame stabilized by the central insert without swirl and the one fully aerodynamically stabilized by swirl. Shedding of vortical structures from the injector nozzle that grow and rollup the flame tip controls the FTF of these flames. The flame stabilized by the swirler-plus-bluff-body system features a peculiar response with a large drop of the FTF gain around a frequency at which large swirl number oscillations are observed. Velocity measurements in cold flow conditions reveal a strong reduction of the size of the vortical structures shed from the injector lip at this forcing condition. The flame stabilized aerodynamically only by swirl and the one stabilized by the bluff body without swirl do not exhibit any FTF gain drop at low frequencies. In the former case, large swirl number oscillations are still identified, but large vortical structures shed from the nozzle also persist at the same forcing frequency in the cold flow response. These different flame responses are found to be intimately related to the dynamics of the internal recirculation region, which response strongly differs depending upon the injector used to stabilize the flame.     

A LES/PDF simulator on block-structured meshes

A block-structured mesh large-eddy simulation (LES)/probability density function (PDF) simulator is developed within the OpenFOAM framework for computational modelling of complex turbulent reacting flows. The LES/PDF solver is a hybrid solution methodology consisting of (i) a finite-volume (FV) method for solving the filtered mass and momentum equations (LES solver), and (ii) a Lagrangian particle-based Monte Carlo algorithm (PDF solver) for solving the modelled transport equation of the filtered joint PDF of compositions. Both the LES and the PDF methods are developed and combined to form a hybrid LES/PDF simulator entirely within the OpenFOAM framework. The in situ adaptive tabulation method [S.B. Pope, Computationally efficient implementation of combustion chemistry using in situ adaptive tabulation, Combust. Theory Model. 1 (1997), pp. 41–63; L. Lu, S.R. Lantz, Z. Ren, and B.S. Pope, Computationally efficient implementation of combustion chemistry in parallel PDF calculations, J. Comput. Phys. 228 (2009), pp. 5490–5525] is incorporated into the new LES/PDF solver for efficient computations of combustion chemistry with detailed reaction kinetics. The method is designed to utilise a block-structured mesh and can readily be extended to unstructured grids. The three-stage velocity interpolation method of Zhang and Haworth [A general mass consistency algorithm for hybrid particle/finite-volume PDF methods, J. Comput. Phys. 194 (2004), pp. 156–193] is adapted to interpolate the LES velocity field onto particle locations accurately and to enforce the consistency between LES and PDF fields at the numerical solution level. The hybrid algorithm is fully parallelised using the conventional domain decomposition approach. A detailed examination of the effects of each stage and the overall performance of the velocity interpolation algorithm is performed. Accurate coupling of the LES and PDF solvers is demonstrated using the one-way coupling methodology. Then the fully two-way coupled LES/PDF solver is successfully applied to simulate the Sandia Flame-D, and a turbulent non-swirling premixed flame and a turbulent swirling stratified flame from the Cambridge turbulent stratified flame series [M.S. Sweeney, S. Hochgreb, M.J. Dunn, and R.S. Barlow, The structure of turbulent stratified and premixed methane/air flames I: Non-swirling flows, Combust. Flame 159 (2012), pp. 2896–2911; M.S. Sweeney, S. Hochgreb, M.J. Dunn, and R.S. Barlow, The structure of turbulent stratified and premixed methane/air flames II: Swirling flows, Combust. Flame 159 (2012), pp. 2912–2929]. It is found that the LES/PDF method is very robust and the results are in good agreement with the experimental data for both flames.     

Large eddy simulation of piloting effects on turbulent swirling flames

Pilot flames, created by additional injectors of pure fuel, are often used in turbulent burners to enhance flame stabilization and reduce combustion instabilities. The exact mechanisms through which these additional rich zones modify the flame anchoring location and the combustion dynamics are often difficult to identify, especially when they include unsteady hydrodynamic motion. This study presents Large Eddy Simulations (LES) of the reacting flow within a large-scale gas turbine burner for two different cases of piloting, where either 2 or 6% of the total methane used in the burner is injected through additional pilot flame lines. For each case, LES shows how the pilot fuel injection affects both flame stabilization and flame stability. The 6% case leads to a stable flame and limited hydrodynamic perturbations in the initial flame zone. The 2% case is less stable, with a small-lift-off of the flame and a Precessing Vortex Core (PVC) in the cold stabilization zone. This PVC traps some of the lean cold gases issuing from the pilot passage stream, changes the flame stabilization point and induces instability.     

Combustion dynamics of inverted conical flames

An inverted conical flame anchored on a central bluff-body in an unconfined burner configuration features a distinctive acoustic response. This configuration typifies more complex situations in which the thermo-acoustic instability is driven by the interaction of a flame with a convective vorticity mode. The axisymmetric geometry investigated in this article features a shear region between the reactive jet and the surrounding atmosphere. It exhibits self-sustained oscillations for certain operating conditions involving a powerful flame collapse phenomenon with sudden annihilation of flame surface area. This is caused by a strong interaction between the flame and vortices created in the outer jet shear layer, a process which determines the amplitude of heat release fluctuation and its time delay with respect to incident velocity perturbations. This process also generates an acoustic field that excites the burner and synchronizes the vortex shedding mechanism. The transfer functions between the velocity signal at the burner outlet and heat release are obtained experimentally for a set of flow velocities fluctuations levels. It is found that heat release fluctuations are a strong function of the incoming velocity perturbation amplitude and that the time delay between these two quantities is mainly determined by the convection of the large scale vortices formed in the jet shear layer. A model is formulated, which suitably describes the observed instabilities.     

Computational and experimental study of oxygen-enhanced axisymmetric laminar methane flames

Three axisymmetric laminar coflow diffusion flames, one of which is a nitrogen-diluted methane/air flame (the ‘base case’) and the other two of which consist of nitrogen-diluted methane vs. pure oxygen, are examined both computationally and experimentally. Computationally, the local rectangular refinement method is used to solve the fully coupled nonlinear conservation equations on solution-adaptive grids. The model includes C2 chemistry (GRI 2.11 and GRI 3.0 chemical mechanisms), detailed transport, and optically thin radiation. Because two of the flames are attached to the burner, thermal boundary conditions at the burner surface are constructed from smoothed functional fits to temperature measurements. Experimentally, Raman scattering is used to measure temperature and major species concentrations as functions of the radial coordinate at various axial positions. As compared to the base case flame, which is lifted, the two oxygen-enhanced flames are shorter, hotter, and attached to the burner. Computational and experimental flame lengths show excellent agreement, as do the maximum centreline temperatures. For each flame, radial profiles of temperature and major species also show excellent agreement between computations and experiments, when plotted at fixed values of a dimensionless axial coordinate. Computational results indicate peak NO levels in the oxygen-enhanced flames to be very high. The majority of the NO in these flames is shown to be produced via the thermal route, whereas prompt NO dominates for the base case flame.     

Analytic prediction of unconfined boundary layer flashback limits in premixed hydrogen–air flames

Flame flashback is a major challenge in premixed combustion. Hence, the prediction of the minimum flow velocity to prevent boundary layer flashback is of high technical interest. This paper presents an analytic approach to predicting boundary layer flashback limits for channel and tube burners. The model reflects the experimentally observed flashback mechanism and consists of a local and global analysis. Based on the local analysis, the flow velocity at flashback initiation is obtained depending on flame angle and local turbulent burning velocity. The local turbulent burning velocity is calculated in accordance with a predictive model for boundary layer flashback limits of duct-confined flames presented by the authors in an earlier publication. This ensures consistency of both models. The flame angle of the stable flame near flashback conditions can be obtained by various methods. In this study, an approach based on global mass conservation is applied and is validated using Mie-scattering images from a channel burner test rig at ambient conditions. The predicted flashback limits are compared to experimental results and to literature data from preheated tube burner experiments. Finally, a method for including the effect of burner exit temperature is demonstrated and used to explain the discrepancies in flashback limits obtained from different burner configurations reported in the literature.     

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.     

对流速度对燃料液滴火焰形态及燃烧的影响

考虑气相非稳态及液滴内部环流,建立运动液滴非稳态蒸发燃烧模型.模型采用动网格方法精确追踪液滴表面位置,采用守恒方程组更新液滴表面边界条件.根据单步全局化学反应机理,仿真研究正庚烷燃料液滴在不同对流速度下的火焰形态及燃烧.结果表明:运动液滴内部环流使液滴内部低温区向环流中心移动.当液滴运动速度大于某临界值后,火焰形态由包覆火焰转变为尾迹火焰.包覆火焰的富燃区范围、高温区范围及燃烧速率明显较尾迹火焰大;包覆火焰的液滴表面温度及表面蒸发流率分布也明显不同于尾迹火焰.     

An investigation of oscillating instabilities in deflagrations with edges

We examine the Lewis-number-greater-than-1 stability of a deflagration sitting on a porous-plug burner with an inert coflow. The flame edges generated by the coflow influence the stability, and this influence is examined. Very wide flames display the same stability characteristics as unbounded flames (flames sans edges), but for moderately wide flames the instability is suppressed. A new two-dimensional instability can occur for narrow flames. There is a range of mass fluxes for which a monotonic decrease in burner (flame) width generates a transition from unstable flames to stable flames, to unstable flames, to quenching. The insertion of a cold probe into the combustion field can stabilize an unstable flame or destabilize a stable flame, depending on the point of insertion.     

Capturing multi-regime combustion in turbulent flames with a virtual chemistry approach

Multiple flame regimes are encountered in industrial combustion chambers, where premixed, stratified and non-premixed flame regions may coexist. To obtain a predictive tool for pollutant formation predictions, chemical flame modeling must take into account the influence of such complex flame structure. The objective of this article is to apply and compare two reduced chemistry models on both laminar and turbulent multi-regime flame configurations in order to analyze their capabilities in predicting flame structure and CO formation. The challenged approaches are (i) a premixed flamelet-based tabulated chemistry method, whose thermochemical variables are parameterized by a mixture fraction and a progress variable, and (ii) a virtual chemical scheme which has been optimized to retrieve the properties of canonical premixed and non-premixed 1-D laminar flames. The methods are first applied to compute a series of laminar partially-premixed methane-air counterflow flames. Results are compared to detailed chemistry simulations. Both approaches reproduced the thermal flame structure but only the virtual chemistry captures the CO formation in all ranges of equivalence ratio from stoichiometry premixed flame to pure non-premixed flame. Finally, the two chemical models combined with the Thickened Flame model for LES are challenged on a piloted turbulent jet flame with inhomogeneous inlet, the Sydney inhomogeneous burner. Mean and RMS of temperature and CO mass fraction radial profiles are compared to available experimental data. Scatter data in mixture fraction space and Wasserstein metric of numerical and experimental data are also studied. The analyses confirm again that the virtual chemistry approach is able to account for the impact of multi-regime turbulent combustion on the CO formation.     

Stability characteristics and flowfields of turbulent non-premixed swirling flames

A simple, yet representative, burner geometry is used for the investigation of highly swirling turbulent unconfined, non-premixed, flames of natural gas. The burner configuration comprises a ceramic faced bluff-body with a central fuel jet. The bluff-body is surrounded by an annulus that delivers a swirling primary flow of air. The entire burner assembly is housed in a wind tunnel providing a secondary co-flowing stream of air. This hybrid bluff-body/swirl burner configuration stabilizes complex turbulent flames not unlike those found in practical combustors, yet is amenable to modelling because of its well-defined boundary conditions. Full stability characteristics including blow-off limits and comprehensive maps of flame shapes are presented for swirling flames of three different fuel mixtures: compressed natural gas (CNG), CNG–air (1:2 by volume) and CNG–H₂ (1:1 by volume).

It is found that with increased fuel flow, flame blow-off mode may change with swirl number, S_g. At low swirl, the flame remains stable at the base but blows off in the neck region further downstream. At higher swirl numbers, the flames peel off completely from the burner's base. Swirling CNG–air flames are distinct in that they only undergo base blow-off. In the low range of swirl number, increasing S_g causes limited improvement in the blow-off limits of the flames investigated and (for a few cases) can even lead to some deterioration over a small intermediate range of S_g. It is only above a certain threshold of swirl that significant improvements in blow-off limits appear. Six flames are selected for further detailed flowfield and composition measurements and these differ in the combination of swirl number, primary axial velocity through the annulus, U_s, and bulk fuel jet velocity, U_j. Only velocity field measurements are presented in this paper. A number of flow features are resolved in these flames, which resemble those already associated with non-reacting swirling flows of equivalent swirl obtained with the present burner configuration. Additionally, asymmetric flowfields inherent to some flames are revealed where the fluidic centreline of the flow (defined in the two-dimensional (U–W velocity pair) velocity field by the ?ω? = 0 tangential velocity contour), meanders strongly on either side of the geometric centreline downstream by about one bluff-body diameter. Flow structures revealed by the velocity data are correlated to flame shapes to yield a better understanding of how the velocity field influences the flames physical characteristics.     

Measurements of three-dimensional mean flame surface area ratio in turbulent premixed Bunsen flames

A data processing scheme with particular emphasis on proper flame contour smoothing is developed and applied to measure the three-dimensional mean flame surface area ratio in turbulent premixed flames. The scheme is based on the two-sheet imaging technique such that the mean flame surface area ratio is an average within a window covering a finite section of the turbulent flame brush. This is in contrast to the crossed-plane tomograph technique which applies only to a line. Two sets of Bunsen flames have been investigated in this work with the turbulent Reynolds number up to 4000 and the Damköhler number ranging from less than unity to close to 10. The results show that three-dimensional effects are substantial. The measured three-dimensional mean flame surface area ratio correlates well with a formula similar to the Zimont model for turbulent burning velocity but with different model constants. Also, the mean flame surface area ratio displays a weak dependency on turbulence intensity but a strong positive dependency on the turbulence integral length scale.     

Direct prediction of laminar burning velocity using an adapted annular stepwise diverging tube

In a combustion study, laminar burning velocities of premixed flames are essential data. A number of experimental results for various fuels have been reported. Although there are many reliable experimental methods that provide precise burning velocities, these methods are not suitable for in situ monitoring in the field. In response to this limitation, a method using an annular diverging tube (ADT) was introduced in the previous study, and its feasibility was verified. In this study, an advanced technique using an annular stepwise diverging tube (ASDT) was introduced to enhance flow uniformity and to formulate flatter flames in the majority of experimental conditions. The configuration of the burner was gradually improved based on their flame propagation characteristics. An adapted burner configuration was developed and used to predict the laminar burning velocities of methane, propane, and DME. Two types of measuring methods were compared: a transient method, which was similar to the previous study, and a static method, which was used for a direct prediction of burning velocity. The static method predicts similar burning velocities when compared to the transient method. The results of both methods were sufficiently similar to the other’s previous results. Quenching distances could also be directly predicted using this adapted ASDT system. Therefore, the static method may be the easiest and fastest method to predict burning velocity, and quenching distance at the instant. This method can be applied to the energy field and aid in the understanding of flame characteristics in narrow combustion spaces.     

A numerical study on extinction behaviour of laminar micro-diffusion flames

We conducted a numerical study on the fluid dynamic, thermal and chemical structures of laminar methane–air micro flames established under quiescent atmospheric conditions. The micro flame is defined as a flame on the order of one millimetre or less established at the exit of a vertically-aligned straight tube. The numerical model consists of convective–diffusive heat and mass transport with a one-step, irreversible, exothermic reaction with selected kinetics constants validated for near-extinction analyses. Calculations conducted under the burner rim temperature 300 K and the adiabatic burner wall showed that there is the minimum burner diameter for the micro flame to exist. The Damköhler number (the ratio of the diffusive transport time to the chemical time) was used to explain why a flame with a height of less than a few hundred microns is not able to exist under the adiabatic burner wall condition. We also conducted scaling analysis to explain the difference in extinction characteristics caused by different burner wall conditions. This study also discussed the difference in governing mechanisms between micro flames and microgravity flames, both of which exhibit similar spherical flame shape.