Flame Displacement Speed Research Articles

1009 ルイス数の異なるDNSデータを用いた火炎変位速度の実験解析手法に関する評価(燃焼・エンジンシステムII)

1009 ルイス数の異なるDNSデータを用いた火炎変位速度の実験解析手法に関する評価(燃焼・エンジンシステムII)

The blow-off mechanism of a bluff-body stabilized laminar premixed flame

The objective of this work is to investigate the dynamics leading to blow-off of a laminar premixed flame stabilized on a confined bluff-body using high fidelity numerical simulations. We used unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. The flame–wall interaction between the hot reactants and the heat conducting bluff-body was accurately captured by incorporating the conjugate heat exchange between them. Simulations showed a shear-layer stabilized flame just downstream of the bluff-body, with a recirculation zone formed by the products of combustion. The flame was negatively stretched along its entire length, primarily dominated by the normal component of the strain. Blow-off was approached by decreasing the mixture equivalence ratio, at a fixed Reynolds number, of the incoming flow. A flame is stable (does not undergo blow-off) when (1) flame displacement speed is equal to the flow speed and (2) the gradient of the flame displacement speed normal to its surface is higher than the gradient of the flow speed along the same direction. As the equivalence ratio is reduced, the difference between the former and the latter shrinks until the dynamic stability condition (2) is violated, leading to blow-off. Blow-off initiates at a location where this is first violated along the flame. Our results showed that this location was far downstream from the flame anchoring zone, near the end of the recirculation zone. Blow-off started by flame pinching separating the flame into an upstream moving (carried within the recirculation zone) and a downstream convecting (detached from the recirculation zone) flame piece. Within the range of operating conditions investigated, the conjugate heat exchange with the bluff-body had no impact on the flame blow-off.

Consistent definitions of “Flame Displacement Speed” and “Markstein Length” for premixed flame propagation

The definition of the flame displacement speed (FDS), often used to characterize the dynamical properties of premixed flames, is generally ambiguous because, except for a steadily propagating planar flame, the mass flow rate through the combustion region varies with distance through the flame and one is therefore faced with the difficulty of choosing a proper iso-surface to represent the flame surface. A directly related issue is the determination of the proportionality coefficient in the linear flame speed-flame stretch relation of weakly-stretched flames, known as the Markstein length, which depends strongly on the location inside the flame zone where it is measured or calculated. The objective of the present study is to identify an iso-surface and thereby a definition of the FDS that is well conditioned and less prone to uncertainties, and a consistent and unambiguous expression for the Markstein length. With a selected isotherm to represent the flame surface, the two most common definitions of the FDS are based either on the energy equation with the temperature as the progress variable, or on the kinematic characteristics of the surface (the propagation speed relative to the flow). In this study we examine the spherical flame geometry, a setup that provides an independent determination of the FDS that is not contingent upon an arbitrary selection of the flame surface and thus permits a proper evaluation of the two FDS definitions. A large number of simulations of premixed spherical propane/air flames with equivalence ratio ranging from 0.8 to 1.4 were carried out at various temperatures and pressures using both global single-step and detailed reaction schemes. Outwardly propagating spherical (or cylindrical) flames and inwardly propagating stationary spherical flames were examined. The dependence of the flame speed and flame temperature on stretch, and the corresponding Markstein length were identified for different isotherms selected to represent the flame surface, and the results were carefully compared to the asymptotic theory of weakly-stretched flames. The excellent agreement between theory and simulations provides a clear explanation and quantification of the differences found between the trends in the flame speed-flame stretch relation and the corresponding Markstein lengths, exhibited when the FDS was calculated based on an isotherm in the burned or unburned sides of the flame. We show that the proper isotherm for the evaluation of the FDS which is well-conditioned and properly accounts for the physics must be sufficiently close to the burned side of the flame. This choice is less prone to uncertainty, as the slope of the flame speed-flame stretch relation when reaching the burned side of the flame becomes less dependent on the selected iso-value. On the other hand, the choice of the fresh combustible mixture temperature as a reference location for the calculation of the FDS, and the corresponding unburned Markstein length, is ill conditioned and should be avoided.

The curvature Markstein length and the definition of flame displacement speed for stationary spherical flames

The separate effects of flame stretch and flame curvature are investigated in the present study considering a stationary spherical flame configuration that experiences no stretch but has a finite curvature. A theoretical expression for the flame displacement speed, derived from asymptotic analysis, is compared with numerical simulations exhibiting very good agreement between the two. The notion of the curvature Markstein length is introduced, a coefficient which is independent from diffusion and chemical reaction effects, in contrast to the Markstein length associated with total flame stretch. It is shown that its value depends strongly on the iso-surface chosen to define the flame, exhibiting large variations in the preheat zone and more insensitive behavior close to the burned side.

Ignition probability of a partially premixed burner using LES

To comply with stringent pollutant emission regulation, low-emission aeronautical gas turbines rely on lean premixed combustion. Such technology raises the issue of ensuring a reliable ignition since the combustor operates closer to blow-off. Ignition is however known as a stochastic phenomenon, associated to various sources of system variability. These variabilities and their impact on the success or failure of ignition are still not fully understood. In this paper, Large Eddy Simulation (LES) of laser ignition sequences in an academic swirled turbulent partially premixed burner are performed to obtain statistical information at three selected ignition locations representative of the various ignition scenarios observed. The velocity and mixing fields are first validated against measurements to eliminate uncertainty associated with the non-reacting flow. LES is then shown to recover the ignition statistical behavior and probability for the selected ignition locations. Moreover, LES analysis allows to identify the various mechanisms that drive ignition failure or success. Statistics of flame displacement speed are used to demonstrate the effect of curvature and stretch in regions of intense turbulence and the impact of partial premixing on the ignition process.

Turbulence–flame interaction and fractal characteristics of H2–air premixed flame under pressure rising condition

Direct numerical simulation (DNS) of turbulent premixed flames in a constant volume vessel is conducted to investigate turbulence–flame interaction and fractal characteristics under pressure rising condition. DNS is conducted for hydrogen–air mixture with decaying homogeneous isotropic turbulence of which the Reynolds number based on Taylor micro scale is 97.1. The detailed kinetic mechanism including 12 reactive species and 27 elementary reactions is used to represent the hydrogen–air flames. Even for the pressure rising process, the local heat release rate, the flame curvature and the tangential strain rate can be scaled by the maximum laminar heat release rate corresponding to instantaneous mean pressure, the Kolmogorov length scale and the ratio of the turbulent intensity to the Taylor micro scale in the unburned mixture. These facts show a rapid response of flame characteristics to the pressure change. In the propagation process, flame displacement speed is very high compared to open flames due to the expansion of the burned gas. The compression of the unburned mixture due to the high displacement speed increases pressure in the vessel and induces slow decaying or increasing in the turbulent Reynolds number even for no mean flow. It is clarified that the fractal dimension of the flame surface does not show any dependence on pressure increase. The inner cutoff obtained in the pressure rising condition is in good agreement with the expression proposed in our previous study (Shim et al., 2011) [6] where the ratio of the diameter of coherent fine scale eddy, which is a universal fine scale structure of turbulence, to the laminar flame thickness is used as an important parameter.

Early flame propagation in a spark-ignition engine measured with quasi 4D-diagnostics

This paper presents the first results toward experimentally resolving the local three-dimensional (3D) flame propagation and turbulence–chemistry interaction in a spark-ignition engine using temporally resolved multi-planar laser diagnostics. The experimental method utilizes simultaneous dual-plane laser induced fluorescence (LIF) of OH and stereoscopic PIV (SPIV) to locally resolve 3D flame displacement speed during the early flame development when less than 5% of the mass has been consumed. OH-LIF is used to track the reaction-zone position and flame normal direction in 3D space, while SPIV measures the convection of the identified flame contours. Based on the vectorial difference of the 3D convection and absolute propagation of the reaction-zone, the 3D displacement speed (sT) is calculated. An instantaneous flame realization shows a large dynamic range of local sT and local flow transport, while also revealing the importance to resolve these quantities in 3D. Several flame-flow configurations are shown along the flame surface and each uniquely defined the local flame transport along the individual flame realization. A detailed uncertainty and sensitivity analysis is performed, confirming the validity of the sT distribution resolved for the methodology and operating conditions. A discussion on the different mechanisms leading to the large distribution of sT for the given operations is included and testifies to the complex nature of the in-cylinder flame development at this early stage. The limitations of the presented methodology are discussed particularly in the need for improved spatial resolution and additional volumetric information. The merits and limitations of the presented work provides an improved understanding of what is further needed to better resolve local 3D flame transport in engines for both experimental and numerical methodologies.

The anchoring mechanism of a bluff-body stabilized laminar premixed flame

The objective of this work is to investigate the mechanism of the laminar premixed flame anchoring near a heat-conducting bluff-body. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane–air combustion. No artificial flame anchoring boundary conditions were imposed. Simulations show a shear-layer stabilized flame just downstream of the bluff-body, with a recirculation zone formed by the products of combustion. A steel bluff-body resulted in a slightly larger recirculation zone than a ceramic bluff-body; the size of which grew as the equivalence ratio was decreased. A significant departure from the conventional two-zone flame-structure is shown in the anchoring region. In this region, the reaction zone is associated with a large negative energy convection (directed from products to reactants) resulting in a negative flame-displacement speed. It is shown that the premixed flame anchors at an immediate downstream location near the bluff-body where favorable ignition conditions are established; a region associated with (1) a sufficiently high temperature impacted by the conjugate heat exchange between the heat-conducting bluff-body and the hot reacting flow and (2) a locally maximum stoichiometry characterized by the preferential diffusion effects.

Consideration on the Flow Velocity in the Experimental Analysis of the Flame Displacement Speed Using DNS Data of Turbulent Premixed Flames with Different Lewis Numbers

The flame displacement speed is one of the major characteristics in turbulent premixed flames. The flame displacement speed is experimentally obtained from the displacement normal to the flame surface, while it is numerically evaluated by the transport equation of the flame surface. The flame displacement speeds obtained both experimentally and numerically cannot be compared directly because their definitions are different. In this study, two kinds of experimental flame displacement speeds—involving the mean inflow velocity and the local flow velocity—were simulated using the DNS data with the different Lewis numbers, and were compared with the numerical flame displacement speed. The simulated experimental flame displacement speed involving the mean inflow velocity had no correlation with the numerical flame displacement speed, while the simulated displacement speed involving the local flow velocity had a clear correlation with the numerical displacement speed in the cases of higher Lewis number than unity. The correlation coefficient of the simulated displacement speed involving the local flow velocity with the numerical displacement speed had a maximum value on the isosurface of the reaction progress variable with the maximum temperature gradient where the dilation effect of the flame is strongest.

DNS Analysis on the Indirect Relationship between the Local Burning Velocity and the Flame Displacement Speed of Turbulent Premixed Flames

The local burning velocity and the flame displacement speed are the dominant properties in the mechanism of turbulent premixed combustion. The flame displacement speed and the local burning velocity have been investigated separately, because the flame displacement speed can be used for the discussion of flame-turbulence interactions and the local burning velocity can be used for the discussion of the inner structure of turbulent premixed flames. In this study, to establish the basis for the discussion on the effects of turbulence on the inner structure of turbulent premixed flames, the indirect relationship between the flame displacement speed and the local burning velocity was investigated by the flame stretch, the flame curvature, and the tangential strain rate using DNS database with different density ratios. It was found that for the local tangential strain rate and the local flame curvature, the local burning velocity and the flame displacement speed had the opposite correlations in each density ratio case. Therefore, it is considered that the local burning velocity and the flame displacement speed have a negative correlation.

Three-dimensional flame displacement speed and flame front curvature measurements using quad-plane PIV

In this work we demonstrate the use of a quad-plane particle image velocimetry technique in order to investigate the three-dimensional behavior of several important quantities for combustion research such as the flame displacement speed and the flame front curvature. In results from a premixed methane flame stabilized in a diffuser burner, a comparison of three-dimensional and two-dimensional data is made in order to critically analyze the error of the usually performed planar measurements. It is shown that two-dimensional measurements can only give an estimate of the real situation under certain circumstances, such as with mainly spherical structures in the flame and the perfect alignment of both the flame propagation and flow direction to the measurement plane. However, in turbulent flames, this alignment can never be achieved due to fluctuations stemming from turbulence. The application of two crossed planes leads to significant improvements and good agreement with the three-dimensional quantities can be observed, although no perfect match is achieved. Flame displacement speeds ranging from −0.4sL to 4.5sL with a mean of 1.1sL were recorded, but were not correlated with the flame curvature or strain rate.

Interactions between Propagating Flame Fronts and Obstacles in an Explosion Chamber with a H/L Ratio of 0.86

높이/길이 비가 0.86을 가지는 폭발챔버 내에서 전파하는 화염과 서로 다른 형태의 다중 장애물 사이의 상호작용을 조사하기 위해 폭발실험을 수행하였다. 챔버 내에 장애물 형태는 삼각기둥, 사각기둥 및 원통형으로 변화시켰으며, 장애물의 blockage ratio는 0.43으로 하였다. 전파하는 화염과 장애물 형태에 따른 상호작용을 조사하기 위해 고속카메라를 사용하였으며, 고속카메라로 얻어진 화염 이미지로부터 장애물 주위의 국부 화염속도 및 그 화염속도의 확률밀도함수를 계산하였다. 실험결과, 장애물 형태가 삼각형이었을 때 국부 화염속도가 가장 높게, 원형에서는 가장 낮게 나타났으며, 장애물 후류에서의 국부 화염속도는 장애물 형태에 큰 의존성을 가지는 것으로 나타났다. 또한, 전파하는 화염이 장애물 후류의 미연소가스의 유동과 상호작용할 때 국부 화염속도의 확률밀도함수는 화염속도가 높은 방향으로 광범위하게 분포되는 것으로 나타났다. Experimental studies were carried out to investigate the interactions between the propagating flame fronts and different multiple obstacles within an explosion chamber. The explosion chamber is 600 mm in height, <TEX>$700{\times}700mm^2$</TEX> in cross-section and has a H/L value of 0.86. Three different multiple obstacles with the blockage ratio of 0.43 were replaced within the chamber. The results showed that relatively higher local flame displacement speed was observed with the triangular obstacle while the lower was observed with the circular one. It was found that the local flame displacement speeds behind the obstacle were largely dependent on the obstacle types. It was also found that as the flame interacted with the flow field generated behind the obstacle, the probability density functions(PDFs) of the local flame displacement speed were extensively distributed toward higher speeds.

G061055 乱流予混合火炎のDNSデータを用いた火炎変位速度の実験的解析手法に関する検討([G061-05] 熱工学部門,一般セッション(5) : ふく射,燃焼(1))

The relation of the flame displacement speeds of turbulent premixed flame obtained by both of an experimental analysis method and a numerical calculation was researched using the three-dimensional DNS database of the turbulent premixed flame. The two-dimensional flame displacement speed is experimentally evaluated by a laser sheet imaging approach. In this study, firstly, the flame displacement speeds were obtained by the numerical calculation using both the three-dimensional set and the two-dimensional component of DNS data, and were compared. It was found that negative flame displacement speed appeared only in the two-dimensional calculation. Secondly, the flame displacement speed was evaluated by the analysis method imitated experimental imaging approach, and was compared with the two-dimensional flame displacement speed obtained by the numerical calculation. As a result, it was found that the statistics of the flame displacement speed differed depending on the way to consider the local velocity in experimental analyses.

Numerical Investigation of a Flame Response to the Fuel Concentration Oscillation in Stagnating Laminar Premixed Methane/Air Flames

Responses of the premixed methane/air mixture flames under equivalence ratio oscillations were numerically investigated assuming axi-symmetric stagnation flow fields. The flame motion was numerically investigated at three different oscillation frequencies (10, 20 and 50 Hz) and with three oscillation cases namely: lean case, rich case and lean rich crossover case. Methane/air mixture with equivalence ratio oscillation was issued from the burner exit with 1.0 m/s uniform velocity profile. The effects of frequency and amplitude of the equivalence ratio oscillation were discussed. The amplitude of the equivalence ratio oscillation is attenuated between the burner exit and the upstream edge of the preheat zone. The attenuation is much significant for higher frequency. The amplitude of the flame temperature oscillation attenuates following the attenuation of the equivalence ratio oscillation. The flame location makes the closed cycle around the flame location of correspond equivalence ratio in the steady state condition. The formation of the cycle can be explained by the back support effect. It was further demonstrated that, the back support effect influences the dynamic response of the flame location, in that, the direction of the cycles of the dynamic response in the lean case and the rich case are different. Furthermore, the time variation of the flame location plays a significant effect to the flame displacement speed.

Mechanisms of stabilization and blowoff of a premixed flame downstream of a heat-conducting perforated plate

The objective of this work is to investigate the flame stabilization mechanism and the conditions leading to the blowoff of a laminar premixed flame anchored downstream of a heat-conducting perforated-plate/multi-hole burner, with overall nearly adiabatic conditions. We use unsteady, fully resolved, two-dimensional simulations with detailed chemical kinetics and species transport for methane-air combustion. Results show a bell-shaped flame stabilizing above the burner plate hole, with a U-shaped section anchored between neighboring holes. The base of the positively curved U-shaped section of the flame is positioned near the stagnation point, at a location where the flame displacement speed is equal to the flow speed. This location is determined by the combined effect of heat loss and flame stretch on the flame displacement speed. As the mass flow rate of the reactants is increased, the flame displacement speed at this location varies non-monotonically. As the inlet velocity is increased, the recirculation zone grows slowly, the flame moves downstream, and the heat loss to the burner decreases, strengthening the flame and increasing its displacement speed. As the inlet velocity is raised, the stagnation point moves downstream, and the flame length grows to accommodate the reactants mass flow. Concomitantly, the radius of curvature of the flame base decreases until it reaches an almost constant value, comparable to the flame thickness. While the heat loss decreases, the higher flame curvature dominates thereby reducing the displacement speed of the flame base. For a stable flame, the gradient of the flame base displacement speed normal to the flame is higher than the gradient of the flow speed along the same direction, leading to dynamic stability. As inlet velocity is raised further, the former decreases while the latter increases until the stability condition is violated, leading to blowoff. The flame speed during blow off is determined by the feedback between the growing recirculation zone and the cooling burner plate.

Measurement of laminar burning velocity and Markstein length relative to fresh gases using a new postprocessing procedure: Application to laminar spherical flames for methane, ethanol and isooctane/air mixtures

The purpose of this study is to present a new tool for extracting the laminar burning velocity in the case of spherically outward expanding flames. This new procedure makes it possible to determine the laminar burning velocity directly based on the flame displacement speed and the global fresh gas velocity near the preheat zone of the flame front. It therefore presents a very interesting alternative to the standard method (commonly used in the literature), which is based on the flame front displacement and the ratio of unburned and burned gas densities. The influence of external flame stretching on the burning velocity can be characterized and the Markstein length relative to the unburned gases (i.e., fresh gases) can be deduced by using this new tool. Contrary to the standard procedure, the unstretched laminar burning velocity is determined directly without using the fuel mixture properties. The temporal evolution of the flame front is visualized by high-speed laser tomography and the algorithm, based on a tomographic image correlation method, makes it possible to accurately measure the fresh gas velocity near the preheat zone of the flame front. The measurements of laminar flame speeds are carried out in a high-pressure and high-temperature constant-volume vessel over a wide range of equivalence ratios for methane, ethanol, and isooctane/air mixtures. To validate the experimental facility and the postprocessing of the flame images, fresh gas velocities and unstretched laminar burning velocities, as well as Markstein lengths relative to burned and unburned gases, are presented and compared with experimental and numerical results of the literature for methane/air flames. New results concerning ethanol/air and isooctane/air flames are presented for various experimental conditions (373 K, equivalence ratios range 0.7–1.5, pressure range 0.1–5 MPa).

Comparison of 2D and 3D density-weighted displacement speed statistics and implications for laser based measurements of flame displacement speed using direct numerical simulation data

In a recent study, a light sheet imaging approach has been proposed (Hartung et al., J. Appl. Phys. B 96 (2009) 843–862) which permits measurement of a quantity S d ∗ 2 D , which is the two-dimensional projection of the actual density-weighted displacement speed S d ∗ for turbulent premixed flames. Here the statistics of S d ∗ and S d ∗ 2 D are compared using a direct numerical simulation database of statistically planar turbulent premixed flames. It is found that the probability density functions (pdfs) of S d ∗ 2 D approximate the pdfs of S d ∗ satisfactorily for small values of root-mean-square turbulent velocity fluctuation u′, though the S d ∗ 2 D pdfs are wider than the S d ∗ pdfs. Although the agreement between the pdfs and the standard-deviations of S d ∗ 2 D and S d ∗ deteriorate with increasing u′, the mean values of S d ∗ 2 D correspond closely with the mean values of S d ∗ for all cases considered here. The pdfs of two-dimensional curvature κ m 2 D and the two-dimensional tangential-diffusion component of density-weighted displacement speed S t ∗ 2 D are found to be narrower than their three-dimensional counterparts (i.e. κ m and S t ∗ respectively). It has been found that the pdfs, mean and standard-deviation of π / 2 × κ m 2 D and π / 2 × S t ∗ 2 D faithfully capture the pdfs, mean and standard-deviation of the corresponding three-dimensional counterparts, κ m and S t ∗ respectively. The combination of wider S d ∗ 2 D pdfs in comparison to S d ∗ pdfs, and narrower S t ∗ 2 D pdfs in comparison to S t ∗ pdfs, leads to wider S r ∗ + S n ∗ 2 D = S d ∗ 2 D - S t ∗ 2 D pdfs than the pdfs of combined reaction and normal-diffusion components of density-weighted displacement speed S r ∗ + S n ∗ . This is reflected in the higher value of standard-deviation of S r ∗ + S n ∗ 2 D , than that of its three-dimensional counterpart S r ∗ + S n ∗ . However, the mean values of S r ∗ + S n ∗ 2 D remain close to the mean values of S r ∗ + S n ∗ . The loss of perfect correlation between two and three-dimensional quantities leads to important qualitative differences between the S r ∗ + S n ∗ 2 D - κ m 2 D and S r ∗ + S n ∗ - κ m , and between the S d ∗ 2 D - κ m 2 D and S d ∗ - κ m correlations. For unity Lewis number flames, the S d ∗ - κ m correlation remains strongly negative, whereas a weak correlation is observed between S d ∗ 2 D and κ m 2 D . The study demonstrates the strengths and limitations of the predictive capabilities of the planar imaging techniques in the context of the measurement of density-weighted displacement speed, which are important for detailed model development or validation based on experimental data.

Numerical Investigation of Edge Flame Propagation Behavior in an Igniting Turbulent Planar Jet

The effects of strain rate and curvature on the edge flame propagation characteristics in an igniting turbulent coflowing planar jet are studied based on 3-dimensional compressible Direct Numerical Simulation (DNS) with a modified single-step Arrhenius chemistry. In the present configuration the high-speed jet fluid is considered to be fuel-rich, whereas the slow-moving coflowing fluid is taken to be fuel-lean. Consistent with previous work with DNS without mean flow and shear, the resulting flame from localized forced ignition at the jet exhibits predominantly premixed edge flame structure where premixed flames are formed on both the fuel-rich and fuel-lean sides and the intersection between these two branches propagate on the stoichiometric mixture fraction isosurface. The edge flame propagation behavior has been studied in terms of the statistics of the edge flame displacement speed S d , which refers to the speed at which the fuel mass fraction Y F isosurface moves normal to itself, relative to an initially coincident material surface at the intersection between the fuel-rich and fuel-lean premixed flames on the stoichiometric mixture fraction isosurface. The probability density function of the density-weighted edge flame displacement speed shows nonzero probability of finding negative values of at later stages of self-sustained flame propagation. The mean value of decreases after the energy deposition is switched off but eventually settles to a value that does not change appreciably with time. The is found to be predominantly negatively correlated with curvature but the correlation between and tangential strain rate shows both positive and negative correlating trends with the positive correlating trend dominant at later stages of flame propagation. It has been found that strain rate and curvature dependences of |∇Y F | have significant influences on the statistical behavior of in response to strain rate and curvature. The observed strain rate and curvature dependences of have been explained in detail in terms of statistical behaviors of the reaction, normal diffusion, and the tangential diffusion components of (i.e., , , and ).

Numerical study on dynamics of local flame elements in turbulent jet premixed flames

Direct numerical simulations (DNSs) of turbulent jet premixed flames have been conducted to investigate dynamics of local flame elements. Behaviors of turbulent jet flames are discussed by considering characteristics of local flame elements such as flame displacement speed, local turbulent burning velocity, curvature and tangential strain rate. The peak of probability density function (pdf) of the local turbulent burning velocity locates at laminar burning velocity, whereas the maximum turbulent burning velocity reaches to about 5 times laminar one and the minimum burning velocity is negative. The result indicates that dynamics of turbulent flame cannot be explained by the flamelet concept. The concept of flame stretch is evaluated by growth rate of the flame area, which is a function of flame displacement speed, flame curvature and tangential strain rate. In the concept of flame stretch, the time-averaged growth rate must be zero because flame front should stay in a certain area statistically. However, in DNS, the mean growth rate is not zero, which suggests that flame does not exist as a statistically steady state. The present results indicate a possibility of the existence of flame elements which cannot be explained by the flamelet concept and the concept of flame stretch.

DNS of spark ignition and edge flame propagation in turbulent droplet-laden mixing layers

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 0.55–0.75SL, with SL 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.