124507620
Effects of Fuel Lewis Number on Localised Forced Ignition of Globally Stoichiometric Stratified Mixtures: a Numerical Investigation
The influences of fuel Lewis number LeF on localised forced ignition of globally stoichiometric stratified mixtures have been analysed using three-dimensional compressible Direct Numerical Simulations (DNS) for cases with LeF ranging from 0.8 to 1.2. The globally stoichiometric stratified mixtures with different values of root-mean-square (rms) equivalence ratio fluctuation (i.e. ϕ′= 0.2, 0.4 and 0.6) and the Taylor micro-scale lϕ of equivalence ratio ϕ variation (i.e. lϕ/lf= 2.1, 5.5 and 8.3 with lf being the Zel’dovich flame thickness of the stoichiometric laminar premixed flame) have been considered for different initial rms values of turbulent velocity u′. A pseudo-spectral method is used to initialise the equivalence ratio variation following a presumed bi-modal distribution for prescribed values of ϕ′ and lϕ/lf for global mean equivalence ratio 〈ϕ〉=1.0. The localised ignition is accounted for by a source term in the energy transport equation that deposits energy for a stipulated time interval. It has been observed that the maximum values of temperature and the fuel reaction rate magnitude increase with decreasing LeF during the period of external energy deposition. The initial values of LeF, u′/Sb(ϕ=1), ϕ′ and lϕ/lf have been found to have significant effects on the extent of burning of the stratified mixtures following localised ignition. For a given value of u′/Sb(ϕ=1), the extent of burning decreases with increasing LeF. An increase in u′ leads to a monotonic reduction in the burned gas mass for all values of LeF in all stratified mixture cases but an opposite trend is observed for the LeF=0.8 homogeneous mixture. It has been found that an increase in ϕ′ has adverse effects on the burned gas mass, whereas the effects of lϕ/lf on the extent of burning are non-monotonic and dependent on ϕ′ and LeF. Detailed physical explanations have been provided for the observed LeF, u′/Sb(ϕ=1), ϕ′ and lϕ/lf dependences.
95552476
The influence of flow parameters on minimum ignition energy and quenching distance
Experiments have been carried out on the effects of pressure, velocity, mixture strength, turbulence intensity and turbulence scale on minimum ignition energy and quenching distance. Tests were conducted at room temperature in a specially designed closed-circuit tunnel in which a fan was used to drive propane/air mixtures at subatmospheric pressures through a 9 cm square working section at velocities up to 50 m/sec. Performated located at the upstream end of the woring section provided near-isotropic turbulence in the ignition zone ranging from 1 to 22 percent in intensity, with values of turbulence scale up to 0.8 cm. Ignition was effected using capacitance sparks whose energy and duration could be varied independently. The results of these tests showed that rectangular, arc-type sparks of 60 μsec duration gave lower than previously reported values of ignition energy for both stagnant and flowing mixtures. It was found that both quenching distance and minimum ignition energy increased with (a) increase in velocity, (b) reduction in pressure, (c) departures from stoichiometric fuel/air ratio, and (d) increase in turbulence intensity. Increase in turbulence scale either raised or lowered ignition enegy, depending on the level of tubulence intensity. Equations based on an idealized model of the ignition process satisfactorily predicted all the experimental data on minimum ignition energy.
95437923
Ignition and flame quenching of flowing heterogeneous fuel-air mixtures
Abstract A model is described for the ignition of heterogeneous fuel-air mixtures that assumes that mixing rates and chemical kinetics are infinitely fast and that the sole criterion for successful ignition is an adequate concentration of fuel vapor in the ignition zone. From analysis of the relevant heat and mass-transfer processes involved, expressions for quenching distance and minimum ignition energy are derived that have general application to both quiescent and flowing mixtures. In the present investigation attention is focused on flowing mixtures, and minimum ignition energies are measured over wide ranges of pressure, velocity, turbulence intensity, equivalence ratio, mean drop size, and fuel volatility. The results obtained show very satisfactory agreement with corresponding predicted values, thus confirming the basic premise of the model.
97991908
A general model of spark ignition for gaseous and liquid fuel-air mixtures
In a previous publication a model for the spark ignition of heterogeneous fuel-air mixtures is described which assumes that chemical reaction rates are infinitely fast, and that the sole criterion for successful ignition is an adequate concentration of fuel vapor in the ignition zone. In the present study this model is extended to include (1) the effects of finite chemical reaction rates, which are known to be significant for well-atomized fuels at low pressures and low equivalence ratios, and (2) the presence of fuel vapor in the mixture flowing into the ignition zone. Thus, it now has general application to both quiescent and flowing mixtures of air with either gaseous, liquid or evaporated fuel, or any combination of these fuels. The general validity of the new model is demonstrated by a close level of agreement between theoretical predictions of minimum ignition energy and the corresponding experimental values obtained over wide ranges of pressure, velocity, equivalence ratio, mean fuel drop size and fuel volatility. As no experimental data appeared to exist on minimum ignition energies in heterogeneous mixtures of liquid fuel drops and air in the presence of fuel vapor, some new experiments were undertaken to procure such data for both quiescent and flowing mixtures. The results obtained from this program agree closely with the corresponding predicted values, thereby providing additional support for the new model.
55750219
A transition on minimum ignition energy for lean turbulent methane combustion in flamelet and distributed regimes
Minimum ignition energy (MIE) of lean methane–air mixtures is quantitatively measured using a high-power pulse generator which can vary ignition energies of a spark-electrode in the central position of a large fan-stirred cruciform burner. The burner equipped with a pair of counter-rotating fans and perforated plates can be used to generate isotropic turbulence having a very wide range of turbulent intensities (u 0 ) up to 8 m/s with negligible mean velocities. Observations of ignition, flame kernel development, and subsequent flame propagation in the central uniform region of the burner are recorded by a CMOS high-speed camera (5000 frames/s), showing distributed-like flames of very dispersive and fragmental structures with filiform edges for the first time. A complete MIE data set of lean methane–air mixtures at the equivalence ratio / = 0.6 as a function of u 0 /SL is obtained, where SL is the laminar burning velocity. It is found that there is a transition on values of MIE due to different modes of combustion. Before the transition, MIE only increases gradually with u 0 /SL. Across the transition when u 0 / SL > 24 corresponding to the commonly defined turbulent Karlovitz number Ka =( u 0 /SL) 2 (ReT) � 0.5 >8 , MIE increases abruptly, where ReT is the turbulent Reynolds number based on the integral length scale of turbulence. This transitional value of Ka is much greater than the Klimov–Williams criterion (Ka = 1). Since values of MIE under different levels of turbulence should be relevant to the size of
96625799
Applications of direct numerical simulation to premixed turbulent combustion
This paper describes recent progress in the field of Direct Numerical Simulations (DNS) for premixed turbulent combustion. A classification of Computational Fluid Dynamics techniques used in reacting flows is first proposed. Numerical methods and problems specific to DNS of combustion like grid resolution or boundary conditions treatment are discussed. The presentation of DNS results is done in the framework of flamelet modeling. The domain of validity of the flamelet assumptions is studied with simulations of flame vortex interactions. The local structure of flamelets in premixed turbulent combustion is investigated using two and three-dimensional DNS with simple chemistry models. Effects of complex chemistry are considered through recent two-dimensional DNS performed with realistic chemical schemes. Flame surface densities, flame strain and curvature statistics obtained from various three dimensional simulations are given and implications for modeling are presented. DNS is also exploited to deal with some specific problems like turbulent flame ignition or flame-wall interactions. This review is concluded with a discussion of current limitations and a list of future challenges.
122664026
Effects of Turbulence on Self-sustained Combustion in Premixed Flame Kernels: A Direct Numerical Simulation (DNS) Study
The effects of mean flame radius and turbulence on self-sustained combustion of turbulent premixed spherical flames in decaying turbulence have been investigated using three-dimensional direct numerical simulations (DNS) with single step Arrhenius chemistry. Several flame kernels with different initial radius or initial turbulent field have been studied for identical conditions of thermo-chemistry. It has been found that for very small kernel radius the mean displacement speed may become negative leading ultimately to extinction of the flame kernel. A mean negative displacement speed is shown to signify a physical situation where heat transfer from the kernel overcomes the heat release due to combustion. This mechanism is further enhanced by turbulent transport and, based on simulations with different initial turbulent velocity fields, it has been found that self-sustained combustion is adversely affected by higher turbulent velocity fluctuation magnitude and integral length scale. A scaling analysis is performed to estimate the critical radius for self-sustained combustion in premixed flame kernels in a turbulent environment. The scaling analysis is found to be in good agreement with the results of the simulations.
94433404
EFFECTS OF TURBULENCE ON SPARK IGNITION IN INHOMOGENEOUS MIXTURES: A DIRECT NUMERICAL SIMULATION (DNS) STUDY
Spark ignition in inhomogeneous mixtures is numerically studied using three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The thermal effect of the spark is represented by a Gaussian power distribution in the energy transport equation. Success of the spark ignition and subsequent self-sustained flame propagation is shown to be heavily dependent on the turbulent velocity fluctuation. It has been found that high levels of turbulent velocity fluctuation have detrimental effects on the spark ignition event, which ultimately may lead to misfire. It is observed that the flame shows a tribrachial structure following successful ignition. The local flame propagation speed may attain values equal to several times the laminar burning velocity of an unstrained premixed stoichiometric flame S L , but may also be negative, in agreement with previous findings. The mean flame propagation speed decreases with increasing turbulent intensity.
122016487
Direct Numerical Simulations of Localised Forced Ignition in Turbulent Mixing Layers: The Effects of Mixture Fraction and Its Gradient
The effects of mixture fraction value ξ and the magnitude of its gradient |∇ξ| at the ignitor location on the localised forced ignition of turbulent mixing layers under decaying turbulence is studied based on three-dimensional compressible Direct Numerical Simulations (DNS) with simplified chemistry. The localised ignition is accounted for by a spatial Gaussian power distribution in the energy transport equation, which deposits energy over a prescribed period of time. In successful ignitions, it is observed that the flame shows a tribrachial structure. The reaction rate is found to be greater in the fuel rich side than in stoichiometric and fuel-lean mixtures. Placing the ignitor at a fuel-lean region may initiate ignition, but extinction may eventually occur if the diffusion of heat from the hot gas kernel overcomes the heat release due to combustion. It is demonstrated that ignition in the fuel lean region may fail for an energy input for which self-sustained combustion has been achieved in the cases of igniting at stoichiometric and fuel-rich locations. It is also found that the fuel reaction rate magnitude is negatively correlated with density-weighted scalar dissipation rate in the most reactive region. An increase in the initial mixture fraction gradient at the ignition centre for the ignitor placed at stoichiometric mixture decreases the spreading of the burned region along the stoichiometric mixture fraction isosurface. By contrast, the mass of the burned region increases with an increase in the initial mixture fraction gradient at the ignition location, as for a given ignition kernel size the thinner mixing layer includes more fuel-rich mixture, which eventually makes the overall burning rate greater than that compared to a thicker mixing layer where relatively a smaller amount of fuel-rich mixture is engulfed within the hot gas kernel.
119594587
Effects of Fuel Lewis Number on Localised Forced Ignition of Turbulent Mixing Layers
The influences of fuel Lewis number LeF on localised forced ignition of inhomogeneous mixtures are analysed using three-dimensional compressible Direct Numerical Simulations (DNS) of turbulent mixing layers for LeF = 0.8, 1.0 and 1.2 and a range of different root-mean-square turbulent velocity fluctuation u′ values. For all LeF cases a tribrachial flame has been observed in case of successful ignition. However, the lean premixed branch tends to merge with the diffusion flame on the stoichiometric mixture fraction isosurface at later stages of the flame evolution. It has been observed that the maximum values of temperature and reaction rate increase with decreasing LeF during the period of external energy addition. Moreover, LeF is found to have a significant effect on the behaviours of mean temperature and fuel reaction rate magnitude conditional on mixture fraction values. It is also found that reaction rate and mixture fraction gradient magnitude $\vert \nabla \xi \vert $ are negatively correlated at the most reactive region for all values of LeF explored. The probability of finding high values of $\vert \nabla \xi \vert $ increases with increasing LeF. For a given value of u′, the extent of burning decreases with increasing LeF. A moderate increase in u′ gives rise to an increase in the extent of burning for LeF = 0.8 and 1.0, which starts to decrease with further increases in u′. For LeF = 1.2, the extent of burning decreases monotonically with increasing u′. The extent of edge flame propagation on the stoichiometric mixture fraction ξ = ξst isosurface is characterised by the probability of finding burned gas on this isosurface, which decreases with increasing u′ and LeF. It has been found that it is easier to obtain self-sustained combustion following localised forced ignition in case of inhomogeneous mixtures than that in the case of homogeneous mixtures with the same energy input, energy deposition duration when the ignition centre is placed at the stoichiometric mixture. The difficultly to sustain combustion unaided by external energy addition in homogeneous mixture is particularly prevalent in the case of LeF = 1.2.
94536098
Direct numerical simulations of turbulent flame expansion in fine sprays
Direct numerical simulations of expanding flame kernels following localized ignition in decaying turbulence with the fuel in the form of a fine mist have been performed to identify the effects of the spray parameters on the possibility of self-sustained combustion. Simulations show that the flame kernel may quench due to fuel starvation in the gaseous phase if the droplets are large or if their 
number is insufficient to result in significant heat release to allow for self-sustained flame propagation for the given turbulent environment. The reaction proceeds in a large range of equivalence ratios due to the random location of the droplets relative to the igniter location that causes a wide range of mixture fractions to develop through pre-evaporation in the unreacted gas and through evaporation in the preheat zone of the propagating flame. The resulting flame exhibits both premixed and non-premixed 
characteristics.
97197780
Influence of scalar dissipation on flame success in turbulent sprays with spark ignition
Abstract A Direct Numerical Simulation (DNS) study is performed to determine a quantitative indicator of imminent global extinction in spray flames ignited by a spark. The cases under consideration have Group Combustion numbers sufficiently small that each droplet has an individual flame form around it, which subsequently merge. The structure of the flames is examined, including identification of non-premixed behaviour in the core of the flame and premixed flame fronts except in the presence of droplets, which cause strong non-premixed behaviour. The reaction progress variable c is studied and its dissipation rate is identified as being a key indicator of whether a flame will globally extinguish after being ignited by the spark. Specifically, immediately after the spark is deactivated, the volume containing the end of the flame front and hot products is studied in detail with respect to c. For successful flames, it is observed that regions of zero dissipation of c were predominantly restricted to the highest reaction progress variable ( c > 0.98 ) , with zero probability within the range 0.95 c 0.98 and low probability within 0.9 c 0.95 . In contrast, cases which subsequently extinguished had substantial probability of zero dissipation for 0.95 c 0.98 . This region was a secondary structure separate from the main flame kernel that was unable to evaporate sufficient liquid to create a self-sustaining flame and therefore contributed to the subsequent quenching of the flame. In the successfully-burning case under consideration, this region was part of the main flame structure. The low reaction rate contributed to a thickened flame structure near the hot core, which reduced the heat transfer to the flame front and prevented effective evaporation and preheating of the fluid ahead of the flame front. Calculation of the conditional probability of c for its dissipation rate being zero could provide a quantitative measure to determine whether a flame is likely to extinguish within a relatively short timeframe. This is equivalent to detecting that, for every value of 0.9 c 1 , there are volumes of significant size where the value of c is uniform. Note that a successful flame must have a volume of substantial size with c = 1 . From a practical perspective, if each individual flame kernel is monitored, then extinction is imminent if secondary structures of incomplete reactions are present when the spark ceases adding energy.
96992301
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.
97806397
Measurements of ignition probability in turbulent non-premixed counterflow flames
Spark ignition of turbulent counterflow non-premixed flames has been examined in terms of the ignition probability Pign and evolving flame structure. It was found that in the case of successful ignition, the flame spread as an edge flame with a large scatter in its radial position between individual realizations at the same time from the spark. Pign decreased with bulk velocity, which suggests that the local strain rate can be detrimental to ignition so that, even with the strongest spark tested, ignition could not be achieved at a bulk velocity about 90% of the extinction velocity. Pign was greater than zero even in regions well into the fuel and air streams where the mixture fraction fluctuations were virtually zero, giving zero probability of finding flammable mixture at the spark location. This observation was attributed to non-local effects, for example the convection of the spark heat to the flammable region. The data can assist the validation of advanced combustion models.
98535093
Effects of residual burnt gas heterogeneity on early flame propagation and on cyclic variability in spark-ignited engines
Abstract This paper investigates the effects of Residual Burnt Gas (RBG) heterogeneity on premixed early flame propagation. The aim is to quantify the contribution of RBG spatial fluctuations on Cycle-to-Cycle Variations (CCVs) observed in Spark-Ignited (SI) engines under stoichiometric operation. We performed Direct Numerical Simulations (DNSs) of turbulent flame propagation with a semi-detailed kinetic mechanism for iso-octane (29 species and 48 reactions). To obtain relevant results for SI engines, aerothermochemical conditions (pressure, temperature, turbulence characteristics) representative of low-load engine operation were extracted from experimental data and engine Large Eddy Simulation computations. The main findings are as follows: (1) RBG heterogeneity combines two opposing effects. Hot temperature pockets tend to locally increase the laminar flame speed, while diluted mixtures slow down flame propagation. (2) RBG heterogeneities locally enhance flame propagation because of the dominant effect of temperature over dilution. (3) RBG heterogeneities are able to create additional flame wrinkling even without the introduction of sensible enthalpy fluctuations. (4) DNS results contain valuable information to further our understanding of previously misinterpreted experimental observations in engines [Sztenderowicz and Heywood, SAE paper 902142, 1990]. The influence of RBG heterogeneities on the flame development process is minor compared to their contribution on heat release fluctuations and therefore on engine CCVs as a result of the non-linear chemical response to local mixture conditions.
96813108
Localised forced ignition of globally stoichiometric stratified mixtures: A numerical investigation
Localised forced ignition of globally stoichiometric stratified mixtures (i.e. < φ > =1.0) has been analysed here based on direct numerical simulations for different initial values of velocity and equivalence ratio fluctuations (i.e. u′ and φ′), and the Taylor micro-scale lφ of equivalence ratio φ variation. The localised ignition is accounted for by a source term in the energy transport equation which deposits energy over a stipulated time interval. It has been found that combustion takes place predominantly under premixed mode in the case of successful ignition. The initial values of φ′ and lφ have been found to have significant effects on the extent of burning of stratified mixtures following localised ignition. It has been found that an increase in u′(φ′) has adverse effects on the burned gas mass, whereas the effects of lφ on the extent of burning are non-monotonic and dependent on φ′. Detailed physical explanations have been provided for the observed u′, φ′ and lφ dependences on the extent of burning in stratified mixtures.
95231182
Numerical study of initiation of a combustion wave by an ignition kernel
Abstract The response of an infinite fuel system to the sudden deposition of energy into a spherically symmetric ignition kernel is studied. Using a highly idealized fuel chemistry model, the governing differential equations are solved numerically. Nondimensional parameters based on the size and temperature of the ignition kernel are developed and used to determine the boundary between decay of the ignition kernel and initiation of a combustion wave. It is shown that for ignition to occur, the radius of the hot gas kernel generated by the initial input of energy must exceed a thickness characteristic of a propagating combustion wave. The effect of changing the value of the Lewis number for a fixed energy input is investigated and is found to change the boundary between decay and wave propagation. Results are presented which show the effects of serveral of the model parameters on the position and velocity of the combustion wave as it develops.
93285991
Critical conditions for spherical flame initiation in mixtures with high Lewis numbers
A theoretical and numerical study was conducted to clarify the physical mechanism controlling the flame initiation in mixtures with high Lewis numbers. The analysis was based on the effects of curvature and heat addition on flame structure. It was found that the conditions for the existence of a self-sustained expanding spherical flame determine the critical power and energy of the ignition source for successful initiation. A new criterion was established to predict both the critical power and energy.(Some figures in this article are in colour only in the electronic version; see www.iop.org)
121666060
Direct numerical simulations of the turbulent mixing of a passive scalar
The evolution of scalar fields, of different initial integral length scales, in statistically stationary, homogeneous, isotropic turbulence is studied. The initial scalar fields conform, approximately, to ‘‘double‐delta function’’ probability density functions (pdf ’s). The initial scalar‐to‐velocity integral length‐scale ratio is found to influence the rate of the subsequent evolution of the scalar fields, in accord with experimental observations of Warhaft and Lumley [J. Fluid Mech. 88, 659 (1978)]. On the other hand, the pdf of the scalar is found to evolve in a similar fashion for all the scalar fields studied; and, as expected, it tends to a Gaussian. The pdf of the logarithm of the scalar‐dissipation rate reaches an approximately Gaussian self‐similar state. The scalar‐dissipation spectrum function also becomes self‐similar. The evolution of the conditional scalar‐dissipation rate is also studied. The consequences of these results for closure models for the scalar pdf equation are discussed.
98222470
Turbulent flame propagation in partially premixed combustion
Stratified spark-ignition engines that use direct fuel injection into the combustion chamber feature both small- and large-scale spatial variations in unburned mixture composition. In these configurations, the spark-ignited turbulent flame propagates into a mixture with variable equivalence ratio. Under lean-rich conditions, the reaction zone can be described as a staged combustion system with a first stage corresponding to a propagating premixed flame front followed by a second stage corresponding to multiple post-diffusion flames that burn the remaining excess fuel and excess oxidizer. Direct numerical simulations (DNS) are used in the present study to bring basic information onto the effects of partial premixing both on the turbulent flame topology and the overall mean reaction rate and to determine if and how flamelet combustion models should be modified to account for these effects. The numerical configuration corresponds to three-dimensional partially premixed flames propagating into a temporally decaying turbulent flow. The simulations use different degrees of scalar inhomogeneity around mean stoichiometric conditions. It is found that while partial premixing has a negligible contribution to the premixed flame surface wrinkling, it has a net negative impact on the overall mean premixed reaction rate. This effect is related to the reduced values of mean flamelet mass burning rate per unit flame surface area observed in partially premixed configurations compared to perfectly premixed flames.
94719073
Effects of compositional fluctuations on premixed flames
Fully compressible direct numerical simulation (DNS) is used to study the behaviour of a stratified turbulent premixed flame in the wrinkled flamelet regime (ReΛ = 38, u′/S L = 0.7, Da = 2.3) with reduced multi-step chemistry. A laminar premixed flame in an initially quiescent domain propagates into a turbulent flow so that the scalar and velocity fields are correctly coupled. Comparison of a stratified scenario (0.4 < φ < 1.2) with an otherwise identical homogeneous case indicates that in this regime the turbulent flame speed is increased by enhanced flame front wrinkling as a result of differential propagation speeds. The preferential propagation into more combustible mixture results in a correlation between the mixture fraction and progress variable, particularly at the leading edge of the flame brush. In front of the flame, the composition gradient preferentially aligns with the most compressive strain. Within the flame front, the progress variable gradient preferentially aligns with the most extensive turbulent strain, and in the latter portion of the flame the alignment between the mixture fraction gradient and the most compressive principal strain deteriorates. Despite this, the convenient approximation of the joint pdf by assuming statistical independence of the mixture fraction and progress variable does not introduce a significant error in the approximation of the reaction rates.
58909431
A simple one-step chemistry model for partially premixed hydrocarbon combustion
Abstract This work explores the applicability of one-step irreversible Arrhenius kinetics with unity reaction order to the numerical description of partially premixed hydrocarbon combustion. Computations of planar premixed flames are used in the selection of the three model parameters: the heat of reaction q, the activation temperature T a , and the preexponential factor B. It is seen that changes in q with equivalence ratio ϕ need to be introduced in fuel-rich combustion to describe the effect of partial fuel oxidation on the amount of heat released, leading to a universal linear variation q ( ϕ ) for ϕ > 1 for all hydrocarbons. The model also employs a variable activation temperature T a ( ϕ ) to mimic changes in the underlying chemistry in rich and very lean flames. The resulting chemistry description is able to reproduce propagation velocities of diluted and undiluted flames accurately over the whole flammability limit. Furthermore, computations of methane–air counterflow diffusion flames are used to test the proposed chemistry under nonpremixed conditions. The model not only predicts the critical strain rate at extinction accurately but also gives near-extinction flames with oxygen leakage, thereby overcoming known predictive limitations of one-step Arrhenius kinetics.
97975840
A Study of the Laminar Flame Tip and Implications for Premixed Turbulent Combustion
Abstract Flame surface curvature is a significant geometrical parameter that affects the structure and propagation of premixed laminar and turbulent flames. In this study, the flame tip of a two-dimensional laminar Bunsen burner is investigated using a quasi-one dimensional model, direct numerical simulations and experimental results. The laminar flame tip is a simple prototype of curved flamelets embedded in a turbulent flow field. It is shown that two characteristic flame speeds are necessary to give a local description of a given flamelet: the consumption speed associated with the structure of the reaction zone, and the displacement speed of the flame front relative to the unburned flow. The quasi-one dimensional model shows that three different mechanisms affect the displacement speed of a curved flame in a non-uniform flow field: a chemical mechanism associated with the expansion of the reaction zone structure, a hydrodynamic mechanism due to isothermal area modification by lateral flow divergence an...
98518006
Vorticity in unsteady premixed flames: vortex pair–premixed flame interactions under imposed body forces and various degrees of heat release and laminar flame thickness
Abstract The dynamics of vortical structures are investigated when a vortex pair propagates into a premixed flame under different imposed body forces in the direction of mean flame propagation and various degrees of heat release and laminar flame thicknesses. The direct numerical simulation assumes zero Mach number, adiabatic, simple chemistry equations, and constant diffusivities. Visual pictures of the qualitatively different behaviors of the vortical structures emerge. These range from destruction of the incoming vortex pair and flame generation of flame-attached counter-rotating (rotating in the opposite sense to the incoming vortex pair) vortical structures to amalgamation of the incoming vortex pair with flame-generated, flame-attached, co-rotating (rotating in the same sense as the incoming vortex pair) vortical structures. Understanding of the different qualitative behaviors is aided by examination of the vorticity transport equation in two dimensions. Baroclinic torque is found to scale more strongly with heat release and laminar flame thickness than dilatation. As a result, increasing values of heat release and decreasing values of laminar flame thickness significantly strengthen the intensity of the flame-attached vortical structures. For experimentally realizable values of heat release and laminar flame thickness the intensity of the flame-attached vortical structures can be significantly greater than the incoming wrinkle-inducing vortex pair, supporting baroclinic torque as a mechanism for the increase in the conditional burnt gas turbulence intensities observed experimentally (Cheng and Shepherd, 1987). With a positive mean pressure gradient from reactants to products, the pressure gradient in the unburnt gas in the flame finger formed by the incoming vortex pair is close to that in the burnt gas. For strong adverse body forces this results in streamwise velocities in the unburnt gas in the flame finger greater than in the burnt gas around the finger. This potentially creates a gradient transport mechanism for turbulent scalar fluxes through the Bray–Moss–Libby (BML) model of the turbulent scalar flux ρ u ″ i c″ = ρc(1 − c)( u ib − u iu ), where u iu and u ib are conditional mean velocities in unburnt and burnt gas, respectively.
122000172
Direct numerical simulation of homogeneous turbulence in combination with premixed combustion at low Mach number modelled by the $G$-equation
We consider the direct numerical simulation (DNS) of a homogeneously turbulent flow in combination with a premixed flame. The combustion takes place in the flamelet regime which means that combustion occurs in a very thin layer, called the flame front. The position of the flame front is modelled by means of the $G$-equation, in which the flame front is represented by an isosurface $G_0$ of a scalar field $G({\bm x},t)$. The flow is described by the Navier–Stokes equations in the low-Mach-number limit, which allows for the inclusion of gas expansion due to the temperature increase by the combustion. The advantage of the low-Mach-number approximation is that efficient numerical methods, used for incompressible flows, can be applied to solve the discretized equations. The calculations are carried out in a box with homogeneous isotropic turbulence. In addition, a uniform mean velocity is imposed with a inflow boundary condition at $x\,{=}\,0$. The inflow velocity is adjusted such that the mean position of the flame is stabilized at a fixed position. This allows us to use time averaging to obtain accurate statistics, which are very difficult to obtain when the flame is allowed to propagate. In the $y$- and $z$-direction, periodic boundary conditions are applied. The numerical code has been checked with a well-known theoretical result, the so-called Darrieus–Landau instability of a thin flame front. The results show a good agreement between the computed growth rate and the theoretical value when the thickness of the flame front is much smaller than the wavelength of the disturbance. When this condition is not met, the growth rate becomes lower than the theory in agreement with the restriction under which the theory is valid. For the computations in homogeneous turbulence, the results show an increase in the turbulent flame speed with increasing turbulent intensity at the position of the flame front. This is in good agreement with experimental data and theory. The turbulent flame speed shows also an increase as a function of the heat release parameter. This is because disturbances on the flame front, induced by the turbulence, are enhanced by the Darrieus–Landau instability. The budgets of the turbulent kinetic energy and the enstrophy show that the expansion of the gas across the flame front suppresses the turbulence. At higher expansion rates, turbulence in the direction of the mean velocity increases and as a result turbulence becomes strongly anisotropic. The increase is due to two processes. The first is the influence of the Darrieus–Landau instability already mentioned. The second is the baroclinic production of vorticity owing to the flame front density and pressure gradients not being aligned.
94593698
The structure of turbulent nonpremixed flames
Flamelet theories are examined in the context of turbulent nonpremixed combustion. The criterion requiring that the reaction zone be thinner than the Kolmogoroff length scale is converted to one in which the range of mixture fraction over which reaction occurs is compared with the scalar scale for the small scale fluctuations. It is found that the criterion is violated in many of the flames of interest, particularly away from the nozzle in jet flames and in recirculating flows. Laser imaging data for scalar mixing is used to illustrate the structure of quasi-equilibrium distributed reaction (QEDR) flames at high Damkohler number. The structure is found to tend toward that of the scalar dissipation. The apparent success of flamelet theories in predicting mean concentrations of CO in hydrocarbon flames is ascribed to the metastable equilibrium of rich mixtures arising from chain terminating reactions of radicals with the fuel. Differences in flame structure for piloted jet diffusion flames near extinction of methane and CO/H 2 /N 2 fuels are discussed in terms of these fluid dynamic and kinetic insights.
97034465
DNS of spark ignition and edge flame propagation in turbulent droplet-laden mixing layers
Abstract A parametric study of forced ignition at the mixing layer between air and air carrying fine monosized fuel droplets is done through one-step chemistry direct numerical simulations to determine the influence of the size and volatility of the droplets, the spark location, the droplet-air mixing layer initial thickness and the turbulence intensity on the ignition success and the subsequent flame propagation. The propagation is analyzed in terms of edge flame displacement speed, which has not been studied before for turbulent edge spray flames. Spark ignition successfully resulted in a tribrachial flame if enough fuel vapour was available at the spark location, which occurred when the local droplet number density was high. Ignition was achieved even when the spark was offset from the spray, on the air side, due to the diffusion of heat from the spark, provided droplets evaporated rapidly. Large kernels were obtained by sparking close to the spray, since fuel was more readily available. At long times after the spark, for all flames studied, the probability density function of the displacement speed was wide, with a mean value in the range 0.55 – 0.75 S L , with S L the laminar burning velocity of a stoichiometric gaseous premixed flame. This value is close to the mean displacement speed in turbulent edge flames with gaseous fuel. The displacement speed was negatively correlated with curvature. The detrimental effect of curvature was attenuated with a large initial kernel and by increasing the thickness of the mixing layer. The mixing layer was thicker when evaporation was slow and the turbulence intensity higher. However, high turbulence intensity also distorted the kernel which could lead to high values of curvature. The edge flame reaction component increased when the maximum temperature coincided with the stoichiometric contour. The results are consistent with the limited available experimental evidence and provide insights into the processes associated with ignition of practical spray flames.
93289616
Ignition and flame quenching in flowing gaseous mixtures
The influence of pressure, velocity, turbulence intensity, turbulence scale and mixture composition on minimum ignition energy and quenching distance in flowing gaseous mixtures is examined experimentally for methane and propane fuels. In some experiments, the nitrogen in the air is replaced by various inert gases such as carbon dioxide, helium or argon, while in others the nitrogen is either partly or totally replaced by oxygen. The tests are conducted at room temperature in a 9 cm square working section through which the combustible mixture is arranged to flow at various levels of pressure, turbulence and velocity. At each test condition, the spark energy required to ignite the flowing mixture is measured for several gap widths in order to establish the optimum gap width corresponding to minimum ignition energy. From analysis of the relevant combustion and heat transfer processes involved, expressions for the prediction of quenching distance in flowing mixtures are derived. Support for the model employed in this analysis is demonstrated by a close level of agreement between theoretical predictions of quenching distance and corresponding values calculated from the experimental data on minimum ignition energy obtained over a wide range of mixture compositions and flow conditions.
124494957
Effects of Energy Deposition Characteristics on Localised Forced Ignition of Homogeneous Mixtures
The effects of the characteristic width of the energy deposition profile and the duration of energy deposition by the ignitor on localised forced ignition of stoichiometric and fuel-lean homogeneous mixtures have been analysed using simplified chemistry three-dimensional compressible Direct Numerical Simulation (DNS) for different values of root-mean-square turbulent velocity fluctuation. The localised forced ignition is modelled using a source term in the energy transport equation, which deposits energy in a Gaussian manner from the centre of the ignitor over a stipulated period of time. It has been shown that the width of ignition energy deposition and the duration over which ignition energy is deposited have significant influences on the success of ignition and subsequent flame propagation. An increase in the width of ignition energy deposition (duration of energy deposition) for a given amount of ignition energy has been found to have a detrimental effect on the ignition event, which may ultimately lead to misfire. Moreover, an increase in u′ gives rise to augmented heat transfer rate from the hot gas kernel, which in turn leads to a reduction in the extent of overall burning for both stoichiometric and fuel-lean homogeneous mixtures but the detrimental effects of high values of u′ on localised ignition are particularly prevalent for fuel-lean mixtures.
123132262
Boundary conditions for direct simulations of compressible viscous flows
Abstract Procedures to define boundary conditions for Navier-Stokes equations are discussed. A new formulation using characteristic wave relations through boundaries is derived for the Euler equations and generalized to the Navier-Stokes equations. The emphasis is on deriving boundary conditions compatible with modern non-dissipative algorithms used for direct simulations of turbulent flows. These methods have very low dispersion errors and require precise boundary conditions to avoid numerical instabilities and to control spurious wave reflections at the computational boundaries. The present formulation is an attempt to provide such conditions. Reflecting and non-reflecting boundary condition treatments are presented. Examples of practical implementations for inlet and outlet boundaries as well as slip and no-slip walls are presented. The method applies to subsonic and supersonic flows. It is compared with a reference method based on extrapolation and partial use of Riemann invariants. Test cases described include a ducted shear layer, vortices propagating through boundaries, and Poiseuille flow. Although no mathematical proof of well-posedness is given, the method uses the correct number of boundary conditions required for well-posedness of the Navier-Stokes equations and the examples reveal that it provides a significant improvement over the reference method.
118851466
Numerical experiments in homogeneous turbulence
The direct simulation methods developed by Orszag and Patternson (1972) for isotropic turbulence were extended to homogeneous turbulence in an incompressible fluid subjected to uniform deformation or rotation. The results of simulations for irrotational strain (plane and axisymmetric), shear, rotation, and relaxation toward isotropy following axisymmetric strain are compared with linear theory and experimental data. Emphasis is placed on the shear flow because of its importance and because of the availability of accurate and detailed experimental data. The computed results are used to assess the accuracy of two popular models used in the closure of the Reynolds-stress equations. Data from a variety of the computed fields and the details of the numerical methods used in the simulation are also presented.
54786050
A numerical study on flame stability at the transition point of jet diffusion flames
Flame stability of a fuel jet diffusion flame was studied numerically by using finite rate chemistry. The flow is time dependent and plane two-dimensional, and the chemical reaction is described by simplified, overall one-step kinetics. The variable parameters are the jet Reynolds number, P Rc o and Damkohler number, Da o : and three types of flame stability behavior were observed depending on values of these parameters. The first one is the local and occasional extinction at the transition point from a laminar to a turbulent flame. When Re 0 is kept at a rather high value and Da 0 is decreased, local extinction at the transition point begins to occur at a certain critical value. The occasional extinction is caused at the instant when the local scalar dissipation rate in the reaction zone becomes too large, producing a rupture in the reaction zone layer. The rupture is quickly connected again to recover the continuous reaction zone layer. As Da 0 is decreased further, however, the frequency of rupture increases, and at another critical value, complete extinction is produced at the transition point, leaving a short, residual rim flame immediately downtream of the injector. This is the second type of flame stability. As Da 0 is decreased further, the third and final stability characteristic is observed: the blow-off of the whole flame from the injector rim. When the flme is extinguished completely at the transition point, most of the injected fuel flows downstream as a fuel jet entraining the surounding air to produce a lifted, turbulent diffusion flame in the downstream flow. This study of the structure of the flame has shown that it is actually an ensemble of instantaneous local premixed, diffusion., and partially premixed flames.
98149160
Ignition probabilities in turbulent mixing flows
The paper describes experiments on ignitability carried out in a turbulent jet of natural gas. The turbulence properties of this jet, in terms of its mean and fluctuating velocity and concentration parameters have been extensively measured using laser Raman spectroscopy, and in the 17th Combustion Symposium the authors presented a theory in which the turbulent concentration parameters were used to derive a flammability factor and predict ignition probability. By using a single spark source this theory has been tested and results are presented which demonstrate the validity of the theory and define the flammable boundary of the jet. The results confirm that in a turbulent flow it is not meaningful to consider just the mean concentration in order to define the flammability or probability of ignition. However, in the absence of intermittency, this probability will approach zero when the mean falls below about half the lower flammable limit.
6308615
Ignition Characteristics Of Turbulent Jet Flows
Releases of flammable materials from chemical plant are a hazard since their dispersion down to safe levels only occurs at some distance from their source. Within this distance, accidental ignition can give rise to fires or explosions. This paper describes the development of computational fluid dynamic models capable of predicting the ignition hazards presented by turbulent releases of flammable materials. A mathematical model, based on solutions of the fluid flow equations, is described, with a prescribed probability density function (pdf) approach used to determine the distribution of conserved scalar quantities. Three distributions are explored, and their ability to simulate experimental data in jet and wake flows, in isolation of fluid flow calculations, is tested. A composite pdf (consisting of β- and δ- functions, and a power law distribution for the viscous super-layer) demonstrates excellent agreement with data, particularly in those regions of a flow that contain significant intermittency Coupling of this pdf with flow field calculations yields reasonable predictions of scalar pdfs in a methane jet, with probabilities of ignition implied from measured and predicted pdfs being in close accord. The results demonstrate that prescribed pdf methods are likely to be capable of predicting ignition probabilities in many practical flows.
98719635
AN EXPERIMENTAL STUDY OF FREELY PROPAGATING TURBULENT PROPANE/AIR FLAMES IN STRATIFIED INHOMOGENEOUS MIXTURES
Experiments of freely propagating turbulent flames in nonhomogeneous propane/air mixtures are performed for various initial distributions of heterogeneities and compared with homogeneous cases. For various flame propagation times, laser tomography and planar laser-induced fluorescence on acetone are performed simultaneously to characterize the flame-front structure as well as the local mixture fraction in front of the flame. The two-dimensional scalar fields are characterized by a mean stratified mixture fraction field, symmetrical around ignition, and by a fluctuation field. The scalar scales are determined from the spatial autocorrelation coefficients and are compared with the turbulence scales, because the turbulence grid is responsible for the mixing. From the flame contours, global parameters (flame radii and wrinkling ratio) and local curvature distributions are also studied to evaluate the contribution of heterogeneities on the topology of the turbulent flame front. At the early stages of flame propagation, the flame structure is weakly affected by the local heterogeneities. Then, at higher propagation times, the heterogeneities tend to smooth the wrinkling produced by the turbulent flow field. A characteristic scale of flame curvature is then compared with the integral length scale of the flow field, L T , and scalar scale L Z . For heterogeneous conditions, the flame front presents small wrinkles with positive and negative curvatures equally distributed all around the flame with a characteristic size strongly depending on L Z .
96871513
Numerical simulation of turbulent propane–air combustion with nonhomogeneous reactants
Abstract High-resolution two-dimensional numerical simulations have been performed for premixed turbulent propane–air flames propagating into regions of nonhomogeneous reactant stoichiometry. Simulations include complex chemical kinetics, realistic molecular transport, and fully resolved hydrodynamics (no turbulence model). Aerothermochemical conditions (pressure, temperature, stoichiometry, and turbulence velocity scale) approach those in an automotive gasoline direct-injection (GDI) engine at a low-speed, part-load operating condition. Salient findings are as follows: (1) There is no leakage of the primary fuel (propane) behind an initial thin premixed heat-release zone. This “primary premixed flame” can be described using a monotonic progress variable and laminar premixed flamelet concepts. (2) For the conditions simulated, differences in global heat release and flame area (length) between homogeneous and nonhomogeneous reactants having the same overall stoichiometry are small. (3) Beyond three-to-four flame thicknesses behind the primary flame, practically all hydrocarbon fuel has broken down into CO and H2. (4) The rate of heat release in the “secondary reaction zone” behind the primary premixed flame is governed by turbulent mixing and the kinetics of CO2 production. Mixture-fraction-conditioned secondary heat release, CO, and CO2 production rates are qualitatively similar to results from a first-order conditional moment closure (CMC) model; CMC gives poor results for H2, H2O, and radical species. Description of the secondary heat release using steady laminar diffusion flamelet concepts is problematic. (5) Of the chemical species considered, HCO mass fraction or the product of CH2O and OH mass fractions correlates best with local heat-release rate [1] . (6) Computational considerations demand modifications to chemical mechanisms involving C3H7 and CH3CO. Specific changes are proposed to strike a satisfactory balance between accuracy and computational efficiency over a broad range of reactant stoichiometry.
98041735
Statistical Analysis of Displacement Speed in Turbulent Stratified Flames: A Direct Numerical Simulation Study
Statistically planar turbulent stratified flames under different initial root mean square turbulent velocity fluctuation u′ levels are simulated for global equivalence ratios  = 1.0 and  = 0.7 using 3-dimensional compressible direct numerical simulations (DNS) with a modified single-step Arrhenius-type chemistry. For the simulations of stratified flames, a sinusoidal variation of equivalence ratio is introduced in the unburned reactants ahead of the flame. The simulation parameters are chosen in such a manner that the combustion situation belongs to the thin reaction zones regime in all cases. The DNS data has been used to analyze the statistical behavior of the displacement speed S d and its components (i.e., the reaction rate component S r , the normal molecular diffusion component S n , the tangential diffusion component S t , and the component arising due to reactant inhomogeneity S ξ) and their curvature and tangential strain rate dependence. The mean behavior of displacement speed S d and its local strain rate and curvature dependences have been analyzed in detail in terms of the statistical behaviors of the various components of displacement speed (i.e., S r , S n , S t , and S ξ). It has been observed that the contribution of S d due to reactant inhomogeneity S ξ is negligible in comparison to the other components and therefore S d is most significantly affected by the contribution of (S r + S n + S t ). The displacement speed S d statistics are presented in terms of its marginal probability density functions (PDFs) as well as its joint PDFs with curvature and tangential strain rate. It has been observed that S d shows a nonlinear negative correlation with curvature but the correlation strength decreases with increasing u′. The tangential strain rate response of S d has also been found to be dependent upon the turbulence velocity fluctuation level u′ with weak correlation in higher initial u′ cases and a modest positive correlation in the lower u′ cases. It has been found that the marginal PDFs of S r and S n in stratified flames show significant differences in comparison to the corresponding PDFs obtained for perfectly premixed flames with the same values of global equivalence ratio and u′. However, the local curvature and strain rate dependences of S d and its components are found to be qualitatively similar to the corresponding statistical behaviors obtained for perfectly premixed flames with the same values of global equivalence ratio and u′. It has been demonstrated that the curvature dependence of S d has important implications in the context of modeling the curvature stretch term in the Flame Surface Density transport equation.