Flame Kernel Growth Research Articles

Influence of flow and ignition fluctuations on cycle-to-cycle variations in early flame kernel growth

Engine Cycle-to-Cycle Variations (CCVs) constitute a restriction for numerous concepts aimed at improving fuel consumption in spark-ignited engines. In consequence, engine CCV modeling has become an active research topic. Nevertheless, few fundamental studies have thus far examined the individual influences of the key parameters. In the present study, we performed 2D Direct Numerical Simulations (DNSs) of early flame kernel growth with semi-detailed chemistry solving to identify the controlling parameters of heat release fluctuations that are relevant to engine CCVs. Our objective was to assess how certain turbulence and flame parameters – namely turbulent flow motion pattern, turbulence intensity, integral length scale, mixture strength and initial kernel size – influence combustion variability. Two extreme mixture conditions were selected (stable operation at stoichiometry and unstable operation at the lean flammability limit), and computation conditions were derived from actual low load engine operating points. Results provide a classification of the key parameters, indicating initial kernel size and turbulent flow motion pattern as the main contributors. Additionally, DNSs highlighted the crucial role of mixture strength in the occurrence of local flame quenching and in the related amplification of heat release fluctuations.

Comparative investigations of flame kernel development in a laser ignited hydrogen–air mixture and methane–air mixture

Concerns about energy availability and pollutant emissions, such as oxides of nitrogen and particulates have driven concerted efforts toward the design of next generation internal combustion engines, capable of using newer fuels, delivering higher efficiencies and lowering emissions. Among various new engine designs and concepts, laser ignition is one of the promising approaches to attain these objectives.In this research, experimental investigations are carried out to study the laser ignition of hydrogen–air mixtures, which are compared with the baseline data of laser ignited methane–air mixtures. Experiments were carried out in a high pressure constant volume combustion chamber (CVCC) to investigate the flame kernel development and visualization, and flame speeds. The initial operating conditions of the experiments were set to 373 K chamber temperature and 10 bar chamber filling pressure. The flame kernel growth was investigated for different hydrogen–air mixture strengths ranging from λ = 2.0–5.0 and compared with the baseline of methane–air mixtures of λ = 1.2. A Q-switched Nd: YAG laser at 1064 nm wavelength with pulse duration of 6–9 ns was used for ignition by generating laser plasma inside the CVCC. The results indicate that the phenomenon of micro-explosions in the wave-front of hydrogen–air mixtures lead to significantly different flame front evolution and higher flame speeds compared to methane–air mixtures.

Enhancement of flame development by microwave-assisted spark ignition in constant volume combustion chamber

The enhancement of laminar flame development using microwave-assisted spark ignition has been investigated for methane–air mixtures at a range of initial pressures and equivalence ratios in a 1.45l constant volume combustion chamber. Microwave enhancement was evaluated on the basis of several parameters including flame development time (FDT) (time for 0–10% of total net heat release), flame rise time (FRT) (time for 10–90% of total net heat release), total net heat release, flame kernel growth rate, flame kernel size, and ignitability limit extension. Compared to a capacitive discharge spark, microwave-assisted spark ignition extended the lean and rich ignition limits at all pressures investigated (1.08–7.22bar). The addition of microwaves to a capacitive discharge spark reduced FDT and increased the flame kernel size for all equivalence ratios tested and resulted in increases in the spatial flame speed for sufficiently lean flames. Flame enhancement is believed to be caused by (1) a non-thermal chemical kinetic enhancement from energy deposition to free electrons in the flame front and (2) induced flame wrinkling from excitation of flame (plasma) instability. The enhancement of flame development by microwaves diminishes as the initial pressure of the mixture increases, with negligible flame enhancement observed above 3bar.

Large eddy simulation of a growing turbulent premixed flame kernel using a dynamic flame surface density model

The relevant description of a flame kernel growth in homogeneous isotropic turbulence environment is a key ingredient for the simulation of combustion in spark-ignited internal combustion engine. A small flame kernel burns at an increasing rate as it grows and becomes progressively more and more wrinkled by the prevailing turbulence. Large eddy simulation (LES) models based on algebraic expressions for the turbulent flame speed, the flame surface density or the flame wrinkling factor cannot reproduce such a situation because they assume an equilibrium between turbulence and flame wrinkling which is generally not reached during the simulation as the flame is laminar at early stages. A possible solution is to solve an additional balance equation for the flame surface density or for the wrinkling factor but a promising alternative is to implement a dynamic model where the wrinkling factor, measuring the subgrid scale flame surface, is automatically determined along the computation from the known instantaneous resolved flame front. A Germano-like identity expresses the conservation of the total flame surface at filter and test filter scales to automatically adjust the parameter of a fractal-like wrinkling factor model from the instantaneous resolved flame front characteristics. This model is tested against the experimental data of Renou et al. (2000) [33] to reproduce the temporal evolution of the flame kernel growth. For a range of ignition parameters, preliminary results are in good agreements with experimental data and show that the model parameter evolves with time and depends on operating conditions, validating the dynamic procedure.

Optical investigations of the early combustion phase in spark ignition boosted engines

An improved understanding of the thermo-fluid dynamic phenomena that occur during the combustion process in boosted spark ignition engines is necessary for future developments in these engines. In particular, increased understanding of flame kernel evolution is fundamental since the initial growth of the flame contributes significantly to cycle-to-cycle variation in engine performance and emissions. In this work, flame inception and early stages of the combustion process were investigated in an optically accessible single-cylinder ported fuel injection engine. This engine was equipped with the cylinder head of new generation spark ignition turbocharged engine with the same characteristics as the research engine. Cycle-resolved visualization was applied to follow the flame propagation from the spark ignition to the flame kernel growth. A retrieval procedure for the optical data was realized to obtain information about the flame radius evolution. Natural emission spectroscopy in the ultraviolet–visible wavelength range allowed the detection of the chemical markers of the combustion process such as CH, OH, and HCO radicals, and formaldehyde molecules. In-cylinder optical investigations were correlated with engine parameter measurements obtained by conventional methods.

A spark ignition model for large eddy simulation based on an FSD transport equation (ISSIM-LES)

The aim of this study is to develop a spark ignition model for LES of aero-engines and piston engines. This model called ISSIM-LES (Imposed Stretch Spark Ignition Model) is based on the same electrical circuit description as AKTIM [1] and flame kernel initialisation as AKTIMeuler [2]. The main difference with AKTIMeuler resides in the description of the flame kernel growth which is now solely modelled thanks to a modified flame surface density (FSD) equation. This new model allows multi-ignition description without any ad hoc adaptation since the different ignitions are directly controlled by the flame surface density (FSD) equation, which is modified during early flame ignition. This model is applied to flame kernel growth in isotropic homogeneous turbulence in a fan-stirred bomb for various equivalence ratios and turbulence levels [3]. Discussions and analysis focus on the sensitivity of the results to the transition from the SGS ignition kernel to the fully resolved turbulent flame, on the comparison between ISSIM-LES and AKTIMeuler, on the effect of stretch on the flame kernel growth and finally, on the sensitivity of the early flame kernel development to the initial turbulent velocity field.

LES study of cycle-to-cycle variations in a spark ignition engine

Multi-cycle Large Eddy Simulation (LES) of flow and combustion in a single cylinder engine set-up are presented in order to validate the ability of this technique to reproduce cyclic combustion variability in a spark ignition engine. The simulated engine operating point is part of a larger experimental database acquired at IFP and specifically designed to validate LES techniques in terms of cycle-to-cycle prediction. To characterize individual cycles, the database combines acquisitions of pressure and temperature in different locations of the engine set-up with optical diagnostics in the intake pipe and the cylinder. This new experimental database combined with LES is a powerful tool to study cycle-to-cycle variations (CCV). The operating points include points with low and high CCV levels. The LES includes the whole engine set-up, and covers 25 consecutive four-stroke engine cycles for a reference operating point with low CCV. The results show that LES is able to reproduce the experimental findings, and illustrate how LES can help explain the sources of CCV. In the present case it is shown that CCV are essentially due to velocity fluctuations at the spark plug, which induce variations of the early flame kernel growth and of the overall combustion duration.

LES-pdf simulation of a spark ignited turbulent methane jet

A spark ignited turbulent jet flame is simulated using Large Eddy Simulation (LES) in conjunction with the filtered probability density function (pdf) equation approach, which is solved using the Eulerian stochastic field method. The spark energy deposition is mimicked by a gaussian distributed source term in the enthalpy equation. A dynamic model for the sub-grid stresses together with a simple gradient diffusion approximation for the scalar fluxes is adopted. The chemistry is represented by a global 4-step chemistry involving seven species. The different stages of the ignition sequence, namely the flame kernel growth, the triple flame propagation against the flow and the stabilisation were in good agreement with the experimental data.

Modeling ignition phenomena in spray-guided spark-ignited engines

An ignition model for spray-guided (SG) direct-injection gasoline engines called SparkCIMM—Spark Channel Ignition Monitoring Model—is presented in this paper. The model concept is motivated by high-speed imaging data showing complex processes for spark (formation, turbulent stretching and wrinkling, and multiple restrikes) and ignition (localized flame kernel formation and growth). Turbulent fluctuations occurring on the scale of turbulent spark-channel wrinkling (∼0.05–0.1 mm) and local ignition (∼0.1–0.5 mm) are analyzed using expressions for the sub-grid turbulent kinetic energy, eddy turnover velocity and mixture-fraction variance. Computational particles are introduced along the spark channel to monitor its motion and ignitability. Ignition is determined by a local Karlovitz-number criterion that incorporates effects of turbulence, detailed chemical kinetics, and continuous energy deposition from the spark. Wherever suitable ignition conditions are found along the elongated spark channel, a small flame kernel is launched and tracked as it grows and merges with other flame kernels to form the turbulent flame surface. When the flame surface is sufficiently large, a G-equation flamelet combustion model tracks the turbulent flame front. Both the early flame-kernel-growth and G-equation combustion models include effects of local mixture-fraction variance. Results from the SparkCIMM ignition model and G-equation combustion model are compared with experiments in an SG direct-injection engine. The model reproduces the general experimental features of spark-channel stretching, corrugation, and localized ignition; and flame front location probabilities are in good agreement.

Prediction of Combustion Parameters, Performance, and Emissions in Compressed Natural Gas and Gasoline SI Engines

The simulation of heat release, flame propagation speeds, and pollutant formation was carried out in both a turbocharged compressed natural gas (CNG) engine and a multivalve naturally aspirated bifuel engine running on either CNG or gasoline. The predictive tool used for investigation is based on an enhanced fractal geometry concept of the flame front, which is able to capture the modulation of turbulent to laminar burning speed ratio throughout the overall combustion phase without introducing flame kernel growth or burnout submodels. The prediction model was applied to a wide range of engine speeds, loads, relative air-fuel ratios, and spark advances, and the obtained results were compared to experimental data. These latter were extracted from measured in-cylinder pressure by an advanced diagnostics technique that was previously developed by the authors. The results confirmed a quite accurate prediction of burning speed even without any kind of tuning, with respect to different currently available fractal as well as nonfractal approaches for the simulation of flame-turbulence interaction. Furthermore, the computational code proved to be capable of capturing the effects of fuel composition, different combustion-chamber concepts, and operating conditions on engine performance and emissions.

Global stretch effects in premixed turbulent combustion

Effects of large-scale stretching of a premixed turbulent flame by a spatially non-uniform mean flow on the burning velocity are studied in the simple case of an expanding spherical flame. Assuming that the mean flame structure is self-similar during flame kernel growth, an analytical expression is obtained for the difference in the burning velocities of statistically one-dimensional, planar and spherical flames, all other things being equal. In the limit case of a weakly curved flame, the expression is simplified to characterize the linear response of turbulent burning velocity to the curvature of the mean flame brush by introducing a “turbulent” Markstein number via analogy with the laminar case. The two aforementioned expressions (non-linear for highly curved flames and linear for weakly curved ones) are tested against the experimental data recently obtained in the Leeds fan-stirred bomb. When the ratio of the mean flame brush thickness to the flame radius is markedly less than unity, the linear expression and the turbulent Markstein number work well. When this ratio is comparable with unity, non-linear effects play a substantial role and the non-linear expression should be used to predict the experimental data. The analysis shows that the global stretching and development of the expanding spherical turbulent flame substantially affect turbulent burning velocity. In particular, the linear growth of the observed flame speed with time, well documented in the Leeds bomb, is explained. A method of processing the dependencies of flame radius on time is proposed to evaluate the fully developed unperturbed turbulent burning velocity.

Spark ignition of lifted turbulent jet flames

This paper presents experiments on ignition and subsequent edge flame propagation in turbulent nonpremixed methane jets in air. The spark position, energy, duration, electrode diameter and gap, and the jet velocity and air premixing of the fuel stream are examined to study their effects on the ignition probability defined as successful flame establishment. The flame is visualized by a high-speed camera and planar laser-induced fluorescence of OH. It was found that after an initially spherical shape, the flame took a cylindrical shape with a propagating edge upstream. The probability of successful ignition increases with high spark energy, thin electrode diameter and wide gap, but decreases with increasing dilution of the jet with air. The flame kernel growth rate is high when the ignition probability is high for all parameters, except for jet velocity. Increasing the jet velocity decreases the ignition probability at all locations. The average flame position as a function of time from the spark was measured and the data were used to estimate a net propagation speed, which then resulted in an estimate of the average edge flame speed relative to the incoming flow. This was about 3 to 6 laminar burning velocities of a stoichiometric mixture. The measurements can assist theoretical models for the probability of ignition of nonpremixed flames and for edge flame propagation in turbulent inhomogeneous mixtures, both of which determine the success of ignition in practical combustion systems.

A fractal flame-wrinkling large eddy simulation model for premixed turbulent combustion

Combustion plays an important role in a wide variety of industrial applications, such as gas-turbines, furnaces, spark-ignition engines, and various air-breathing engines. The ability to predict and understand the behavior of reacting flows in practical devices is fundamental to improved combustors with higher efficiency and reduced levels of emissions. At present, large eddy simulation is considered the most promising approach for premixed combustion modeling since the large-scale energy containing flow structures are resolved on the grid. However, the typically thin reaction zone cannot be resolved. To overcome this difficulty flamelet models, in which the reaction is assumed to take place in thin layers, wrinkled by the turbulence can sometimes be used. In these models, the turbulent flame speed can be represented as the product of the laminar flame speed, S u, corrected for the effects of stretch (strain and curvature) and the flame-wrinkling, Ξ. In this study, we propose to model Ξ using fractal theory. This model requires sub-models for the fractal dimension, and the inner and outer cut-offs—the latter being set by the grid. A model is proposed for the inner cut-off, whereas an empirical parameterization is used to provide the fractal dimension. The proposed model is applied to flame kernel growth in homogeneous isotropic turbulence in a fan-stirred bomb and to a lean premixed flame in a plane symmetric dump combustor. Good qualitative and quantitative agreement with experimental data were obtained for the proposed model in both cases. Comparison with other well-known turbulent flame speed closure models shows that the proposed model behaves at least as good, or even better, than the reference models.

The relative effects of fuel concentration, residual-gas fraction, gas motion, spark energy and heat losses to the electrodes on flame-kernel development in a lean-burn spark ignition engine

The potential of lean combustion for the reduction in exhaust emissions and fuel consumption in spark ignition engines has long been established. However, the operating range of lean-burn spark ignition engines is limited by the level of cyclic variability in the early-flame development stage that typically corresponds to the 0-5 per cent mass fraction burned duration. In the current study, the cyclic variations in early flame development were investigated in an optical stratified-charge spark ignition engine at conditions close to stoichiometry [air-to-fuel ratio (A = 15] and to the lean limit of stable operation (A/F = 22). Flame images were acquired through either a pentroof window (‘tumble plane’ of view) or the piston crown (‘swirl plane’ of view) and these were processed to calculate the intra-cycle flame-kernel radius evolution. In order to quantify the relative effects of local fuel concentration, gas motion, spark-energy release and heat losses to the electrodes on the flame-kernel growth rate, a zero-dimensional flame-kernel growth model, in conjunction with a one-dimensional spark ignition model, was employed. Comparison of the calculated flame-radius evolutions with the experimental data suggested that a variation in A/F around the spark plug of Φ(A/F) ≈ 4 or, in terms of equivalence ratio Φ, a variation in Φ ≈ 0.15 at most was large enough to account for 100 per cent of the observed cyclic variability in flame-kernel radius. A variation in the residual-gas fraction of about 20 per cent around the mean was found to account for up to 30 per cent of the variability in flame-kernel radius at the timing of 5 per cent mass fraction burned. The individual effect of 20 per cent variations in the ‘mean’ in-cylinder velocity at the spark plug at ignition timing was found to account for no more than 20 per cent of the measured cyclic variability in flame kernel radius. An individual effect of similar level was attributed to the heat losses to the electrodes, and a much lower effect than that, namely 5-10 per cent at most, could be attributed to cyclic variations in spark-energy release traces. Even when the variations in mean velocity were coupled to both the effects of heat losses to the electrodes and spark-energy release variations, the combined effect was found to account for no more than 40-50 per cent of the observed cyclic variability in flame-kernel radius at the 5 per cent mass-fraction burned timing of 40 ° crank angle after ignition timing.

Analysis of flame kernel development with Schlieren and laser deflection in a constant volume combustion chamber

The purpose of this study is to investigate the relation between ignition systems (or energy) and flame kernel development. In this research, three different kinds of ignition systems and several different kinds of spark plug electrodes are designed and manufactured. The flame propagation velocity in a constant volume combustion chamber is measured by both a laser deflection method using an He-Ne laser and a Schlieren method using a high speed digital camera. In order to control the ignition energy, the dwell time is varied by a spark time controller. The results indicate that, when the ignition energy is increased by extending the dwell time, flame kernel growth accelerates. As the electrode gap width is increased, the breakdown energy is also increased, which stimulates the initial kernel development. The materials, diameter and shape of the electrode affect the discharged energy, the energy transfer efficiency and heat losses and, thus, these also affect the flame kernel development. The average difference in measurement of the flame velocity by the laser deflection method and Schlieren method is only 1.73 per cent. The laser deflection method is found to be preferable because it is more effective and employs simpler means for the analysis of flame kernel development.

Investigation of pre-mixed flame-kernel/vortex interactions via high-speed imaging

To reduce cycle-to-cycle variations in SI-IC engines, knowledge of early flame kernel growth in a turbulent flow field is required. Understanding the interaction between a flame kernel and a vortex is an important fundamental step toward this goal. This paper presents high-speed movies of combustion luminosity during the interaction of a laminar vortex with a spark-generated pre-mixed flame kernel in a quiescent combustion chamber. The resulting time evolution of the perturbed flame kernel shows that laminar vortices of various sizes and vortex strengths can increase the kernel growth rate by at least a factor of 3 and significantly increase combustion reaction rates by involving additional highly curved and stretched flame fronts.

Flame kernel formation and propagation modelling in spark ignition engines

One-dimensional, time-dependent numerical simulations of flame kernel growth produced by a spark in a combustible gasoline-air mixture are presented. This model accounts for the fundamental properties of the ignition system, the combustible mixture and flow fleld. Submodels representing the cylinder pressure and temperature, heat transfer to the cylinder wall and spark plug electrodes or ambient air, effects of the residual gas and so on. In addition, to research the effects of ignition energy, two kinds of ignition systems are used. The results show that the increased breakdown energy enlarges the plasma radius, which derives faster development of the initial flame kernel. The heat transfers to the electrodes and the cylinder wall also affect the growth of the kernel. These results appear to be coherent with previous experimental visualizations. The simulations are validated by different engine running conditions, for example air-fuel ratio, spark advance, engine speed and intake manifold pressure.

Flame growth and wrinkling in a turbulent flow

High-speed planar laser-induced fluorescence (PLIF) and 3-D large eddy simulations (LES) are used to study turbulent flame kernel growth, wrinkling and the formation of separated flame pockets in methane/air mixtures. Turbulence was effected by a set of rotary fans situated in a cylindrical enclosure. Flame wrinkling was followed on sequential 2-D OH images captured at kHz repetition rates. Under stoichiometric conditions and low turbulence levels the flame kernel remains singly connected and close to spherical in shape. By increasing turbulence or reducing the stoichiometry of the mixture the formation of separated pockets could be observed and studied. The mechanisms behind these phenomena are investigated qualitatively by LES of a level-set G-equation describing the flame surface propagation in turbulent flows.

Measurements and large eddy simulations of turbulent premixed flame kernel growth

A combined experimental and large eddy simulation (LES) study of flame kernel growth in isotropic, homogenous turbulence has been carried out. LES calculations using the combustion methodology of Weller were compared with experimental measurements from a fan-stirred bomb for iso-octane and propane air mixtures at various turbulence intensities and pressures. For the purpose of model validation, the mean radius evolution was compared with experimental measurements, obtained from Schlieren photographs. Initially, a small laminar flame kernel was produced that burned at an increasing rate as it grew and was wrinkled by the turbulent flow field. It was also observed that at atmospheric pressure propaneair flames demonstrated less variability between experimental realizations than did iso-octane air flames High-variability combustion events were associated with the convection of the flame kernel away from the spark plug during ignition. Good agreement between experiments and calculations was obtaiend for the full range of conditions investigated in this study, and the LES results were able to reproduce some of the observed variability between experimental realizations as a result of turbulent interactions with the small kernel during ignition. These results provided further validation of the combustion model, though the simple ignitiion treatment did not reproduce the full range of ignition variability due to strain sensitivity.

Effect of unsteady stretch on spark-ignited flame kernel survival

The chemistry–turbulence interaction remains one of the most important topics in combustion research. The ignition of premixed reactants in a highly turbulent environment is fundamentally coupled to this chemistry–turbulence interaction. The spark-ignition (SI) internal combustion (IC) engine relies on the ability of the flame kernel to survive the high-strain-rate, unsteady environment of a turbulent flowfield and successfully transition into a fully developed flame to operate cleanly and efficiently. If certain length and velocity scales within the turbulence spectrum are found to promote flame kernel growth, then by tailoring the flow passages and aerodynamics of the intake valves, piston, and combustion chamber, it may be possible to increase the efficiency and reduce the emissions of SI IC engines. This paper describes a novel experimental investigation of a spark-generated flame kernel interacting with a single vortex toroid with well-defined length and velocity scales. This experiment measured the ability of a vortex to quench a growing kernel in a very lean methane–air mixture at atmospheric pressure. The absence of superequilibrium OH concentrations, qualitatively determined by planar laser-induced fluorescence (PLIF), was used as in indicator of quenching. It was found that larger eddies are more effective at globally quenching the flamefront, requiring a lower strength, when compared to vortices with a smaller characteristic length. At the globally quenching condition, the maturity of the kernel was then increased incrementally until the vortex was no longer able to completely strain out the kernel. The result of this was surprising in that the larger vortices had a much narrower range of kernel maturity for which the vortex could still quench the growing kernel.