Effect of multi-bend geometry on deflagration to detonation transition of a hydrocarbon-air mixture in tubes

We present a numerical investigation of gaseous deflagration-to-detonation transition (DDT) triggered by a shock in a multi-bend geometry. The ethylene-air mixture filled rigid tube with obstacles is considered for understanding the effects of complex confinement and initial flame size on DDT. Our calculations show generation of hot spots by flame and strong shock interactions, and flame propagation is either restrained or accelerated due to the wall obstacles of both straight and bent tubes. The effect of initial flame size on DDT in complex confinement geometry is analyzed as well as the hot spot formation on promoting shock–flame interaction, leading to a full detonation.     

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 0.55-0.75S_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.     

Understanding ignition processes in spray-guided gasoline engines using high-speed imaging and the extended spark-ignition model SparkCIMM: Part B: Importance of molecular fuel properties in early flame front propagation

Recent high-speed imaging of ignition processes in spray-guided gasoline engines has motivated the development of the physically-based spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray/vaporization model and a model for fully developed turbulent flame front propagation. Previously, both SparkCIMM and high-speed optical imaging data have shown that, in spray-guided engines, the spark plasma channel is stretched and wrinkled by the local turbulence, excessive stretching results in spark re-strikes, large variations occur in turbulence intensity and local equivalence ratio along the spark channel, and ignition occurs in localized regions along the spark channel (based upon a Karlovitz-number criteria).In this paper, SparkCIMM is enhanced by: (1) an extended flamelet model to predict localized ignition spots along the spark plasma channel, (2) a detailed chemical mechanism for gasoline surrogate oxidation, and (3) a formulation of early flame kernel propagation based on the G-equation theory that includes detailed chemistry and a local enthalpy flamelet model to consider turbulent enthalpy fluctuations. In agreement with new experimental data from broadband spark and hot soot luminosity imaging, the model establishes that ignition prefers to occur in fuel-rich regions along the spark channel. In this highly-turbulent highly-stratified environment, these ignition spots burn as quasi-laminar flame kernels. In this paper, the laminar burning velocities and flame thicknesses of these kernels are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelets over a wide range of stoichiometry and exhaust gas dilution. The criteria for flame propagation include chemical (cross-over temperature based) and turbulence (Karlovitz-number based) effects. Numerical simulations using ignition models of different physical complexity demonstrate the significance of turbulent mixture fraction and enthalpy fluctuations in the prediction of early flame front propagation. A third paper on SparkCIMM (companion paper to this one) focuses on the importance of molecular fuel properties and flame curvature on early flame propagation and compares computed flame propagation with high speed combustion imaging and computed heat release rates with cylinder pressure analysis.The goals of SparkCIMM development are to (a) enhance our fundamental understanding of ignition and combustion processes in highly-turbulent highly-stratified engine conditions, (b) incorporate that understanding into a physically-based submodel for RANS engine calculations that can be reliably used without modification for a wide range of conditions (i.e., homogeneous or stratified, low or high turbulence, low or high dilution), and (c) provide a submodel that can be incorporated into a future LES model for physically-based modeling of cycle-to-cycle variability in engines.     

Ignition of turbulent non-premixed flames

The initiation of turbulent non-premixed combustion of gaseous fuels through autoignition and through spark ignition is reviewed, motivated by the increasing relevance of these phenomena for new combustion technologies. The fundamentals of the associated turbulent-chemistry interactions are emphasized. Background information from corresponding laminar flow problems, relevant turbulent combustion modelling approaches, and the ignition of turbulent sprays are included. For both autoignition and spark ignition, examination of the reaction zones in mixture fraction space is revealing. We review experimental and numerical data on the stochastic nature of the emergence of autoignition kernels and of the creation of kernels and subsequent flame establishment following spark ignition, aiming to reveal the particular facet of the turbulence causing the stochasticity. In contrast to fully-fledged turbulent combustion where the effects of turbulence on the reaction are reasonably well-established, at least qualitatively, here the turbulence can cause trends that are not straightforward.     

Spray–turbulence–chemistry interactions under engine-like conditions

Turbulent multiphase combustion plays an important role in both nature (e.g., volcanos and pool fires) and industry (e.g., industrial furnaces, aeroengines, and internal combustion engines). It is a highly complex multiscale and multi-physicochemical process in which interactions between the dispersed and continuous phases, phase change, droplet collisions, evaporation, mixing, heat transfer, and chemical reactions occur simultaneously. In recent years, significant progress has been made in understanding the mechanisms of spray flames and their behaviors in combustion engines. This paper covers key and representative developments in the area of turbulent spray combustion with a focus on spray–chemistry–turbulence interactions. The effects of turbulence–chemistry, spray–turbulence, and spray–chemistry interactions on the spray process, ignition, flame stabilization and emission are comprehensively discussed at elevated pressures and temperatures. Furthermore, spray–radiation and spray flame–wall interactions, which are important to engine performance and emission characteristics, are scrutinized. Supercritical spray flames and turbulent spray flames in dual-fuel engines are also discussed. Finally, outlooks and further challenges for the research field are outlined.     

Numerical analysis of reaction–diffusion effects on species mixing rates in turbulent premixed methane–air combustion

The scalar mixing time scale, a key quantity in many turbulent combustion models, is investigated for reactive scalars in premixed combustion. Direct numerical simulations (DNS) of three-dimensional, turbulent Bunsen flames with reduced methane–air chemistry have been analyzed in the thin reaction zones regime. Previous conclusions from single step chemistry DNS studies are confirmed regarding the role of dilatation and turbulence–chemistry interactions on the progress variable dissipation rate. Compared to the progress variable, the mixing rates of intermediate species is found to be several times greater. The variation of species mixing rates are explained with reference to the structure of one-dimensional premixed laminar flames. According to this analysis, mixing rates are governed by the strong gradients which are imposed by flamelet structures at high Damköhler numbers. This suggests a modeling approach to estimate the mixing rate of individual species which can be applied, for example, in transported probability density function simulations. Flame–turbulence interactions which modify the flamelet based representation are analyzed.     

Chemical structure of autoignition in a turbulent lifted H2/N2 jet flame issuing into a vitiated coflow

In the present paper autoignition is studied as the main stabilization mechanism in turbulent lifted H₂/N₂ jet flames issuing into a vitiated hot coflow. The numerical study is performed using the joint scalar PDF approach with detailed chemistry in a two dimensional axisymmetric domain. The SSG Reynolds stress model is used as a turbulence model in the simulation. Chemical structure and characteristics of autoignition are investigated using various methods and parameters. Reaction rate analysis is made to analyze the ignition process at the flame base. The results show the occurrence of a chain branching reaction preceding thermal runaway, which boosts the chain branching process in the flame. This demonstrates the large impact of autoignition at the flame base on the stabilization of the lifted turbulent flame. Further investigation using the scatter-plots of scalars reveals the characteristics of the ignition. The relation between the behavior of temperature and of key intermediate species demonstrates the formation of OH through consumption of HO₂ at nearly isothermal conditions in a very lean-fuel mixture at the flame base. Flux analyses in the conservation equations of species are used to explore the impacts of mass transport on ignition process. Ignition is found to be mainly controlled by chemical features rather than the mixing processes near the flame base. Characteristics of autoignition are also investigated in terms of Damköhler number and progress variable.     

Experimental annular stratified flames characterisation stabilised by weak swirl

A burner for the investigation of lean stratified premixed flames propagating in intense isotropic turbulence has been developed. Lean pre-mixtures of methane at different equivalence ratios were divided between two concentric co-flows to obtain annular stratification. Turbulence generators were used to control the level of turbulence intensity in the oncoming flow. A third annular weakly swirling airflow provided the flame stabilisation mechanism. A fundamental characteristic was that flame stabilisation did not rely on flow recirculation. The flames were maintained at a position where the local mass flux balanced the burning rate, resulting in a freely propagating turbulent flame front. The absence of physical surfaces in the vicinity of the flame provided free access for laser diagnostics. Stereoscopic Planar Image Velocimetry (SPIV) was applied to obtain the three components of the instantaneous velocity vectors on a vertical plane above the burner at the point of flame stabilisation. The instantaneous temperature fields were determined through Laser Induced Rayleigh (LIRay) scattering. Planar Laser Induced Fluorescence (PLIF) of acetone was used to calculate the average equivalence ratio distributions. Instantaneous turbulent burning velocities were extracted from SPIV results, while flame curvature and flame thermal thickness were calculated using the instantaneous temperature fields. The PDFs of these quantities were analysed to consider the separate influence of equivalence ratio stratification and turbulence. Increased levels of turbulence resulted in the expected higher turbulent burning velocities and flame front wrinkling. Flames characterised by higher fuel gradients showed higher turbulent burning velocities. Increased fuel concentration gradients gave rise to increased flame wrinkling, particularly when associated with positive small radius of curvature.     

Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure

Partially premixed combustion is involved in many practical applications, due to partial premixing of combustible and oxidant gases before ignition, or due to local extinctions, which lead to mixing of reactants and burned gases. To investigate some features of flames in stratified flows, the stabilization processes of lifted turbulent jet flames are studied. This work offers a large database of liftoff locations of flames stabilized on turbulence-free jets for different fuels and nozzle diameters studied over their flame stability domains. Methane, propane, and ethylene flames are investigated for nozzle diameters of 2, 3, 4, and 5 mm. Blowout velocities are measured and compared with an approach based on large-scale structures of the jet. The axial and radial locations of the flame base are measured by planar laser-induced fluorescence (PLIF) of the OH radical through high sampling (at least 5000 points). From this large database the average locations of the flame base are analyzed for the fuels investigated. The pdfs exhibit an evolution of their shapes according to the region of the turbulent jet where the flame stabilizes (potential core, transition to turbulence, or fully developed turbulence regions). This dependence is probably due to the interaction of the flame with the jet structures. This is confirmed by the comparison between the amplitude of the height fluctuations and the local size of the large-scale structures deduced from particle image velocimetry measurements and self-similarity laws for velocity. The results show the flame can be carried over a distance equal to the local diameter of the jet within the region of fully developed turbulence for propane and ethylene, and over a slightly larger distance for methane.     

Turbulent hydrocarbon jet flames

To consider turbulent hydrocarbon jet flames as an ensemble of wrinkled laminar flames gives useful information about details of the combustion process by studying reactive diffusive interfaces. Combined with the conserved scalar approach, non-premixed turbulent jet flames can be computed with satisfactory accuracy.Due to heat release in the region of largest shear, orderly flow structures are by far more pronounced in hydrocarbon jet flames than in nonreacting jets. Heat release in the region of largest shear influences the development of turbulence in the initial flame region. However, the influence of density gradients on the turbulent flow field is negligible over the major part of diffusion flames.     

Numerical study on spontaneous ignition of pressurized hydrogen release through a length of tube

The issue of spontaneous ignition of highly pressurized hydrogen release is of important safety concern, e.g. in the assessment of risk and design of safety measures. This paper reports on recent numerical investigation of this phenomenon through releases via a length of tube. This mimics a potential accidental scenario involving release through instrument line. The implicit large eddy simulation (ILES) approach was used with the 5th-order weighted essentially non-oscillatory (WENO) scheme. A mixture-averaged multi-component approach was used for accurate calculation of molecular transport. The thin flame was resolved with fine grid resolution and the autoignition and combustion chemistry were accounted for using a 21-step kinetic scheme.The numerical study revealed that the finite rupture process of the initial pressure boundary plays an important role in the spontaneous ignition. The rupture process induces significant turbulent mixing at the contact region via shock reflections and interactions. The predicted leading shock velocity inside the tube increases during the early stages of the release and then stabilizes at a nearly constant value which is higher than that predicted by one-dimensional analysis. The air behind the leading shock is shock-heated and mixes with the released hydrogen in the contact region. Ignition is firstly initiated inside the tube and then a partially premixed flame is developed. Significant amount of shock-heated air and well developed partially premixed flames are two major factors providing potential energy to overcome the strong under-expansion and flow divergence following spouting from the tube.Parametric studies were also conducted to investigate the effect of rupture time, release pressure, tube length and diameter on the likelihood of spontaneous ignition. It was found that a slower rupture time and a lower release pressure will lead to increases in ignition delay time and hence reduces the likelihood of spontaneous ignition. If the tube length is smaller than a certain value, even though ignition could take place inside the tube, the flame is unlikely to be sufficiently strong to overcome under-expansion and flow divergence after spouting from the tube and hence is likely to be quenched.     

LES–ODT study of turbulent premixed interacting flames

The LES–ODT model is implemented for the study of twin turbulent premixed flames in decaying isotropic turbulence. The approach is based on the coupling of large-eddy simulation (LES) for mass and momentum with a fixed 3D lattice of 1D fine-grained solutions based on the one-dimensional turbulence (ODT) model. The ODT solutions for momentum and reactive scalars are designed to capture subgrid scale physics that is not captured by LES. The LES–ODT formulation is capable of capturing important fine-scale processes, such as flame–flame interactions, which play an important role in flame shortening in turbulent premixed flames, and the role of preferential diffusion on curved flames’ structures.     

Investigations of swirl flames in a gas turbine model combustor: II. Turbulence–chemistry interactions

The thermochemical states of three swirling CH₄/air diffusion flames, stabilized in a gas turbine model combustor, were investigated using laser Raman scattering. The flames were operated at different thermal powers and air/fuel ratios and exhibited different flame behavior with respect to flame instabilities. They had previously been characterized with respect to their flame structures, velocity fields, and mean values of temperature, major species concentrations, and mixture fraction. The single-pulse multispecies measurements presented in this article revealed very rapid mixing of fuel and air, accompanied by strong effects of turbulence–chemistry interactions in the form of local flame extinction and ignition delay. Flame stabilization is accomplished mainly by hot and relatively fuel-rich combustion products, which are transported back to the flame root within an inner recirculation zone. The flames are not attached to the fuel nozzle, and are stabilized approximately 10 mm above the fuel nozzle, where fuel and air are partially premixed before ignition. The mixing and reaction progress in this area are discussed in detail. The flames are short (<50 mm), especially that exhibiting thermoacoustic oscillations, and reach a thermochemical state close to adiabatic equilibrium at the flame tip. The main goals of this article are to outline results that yield deeper insight into the combustion of gas turbine flames and to establish an experimental database for the validation of numerical models.     

玉米粉火焰在开口垂直管道中的传播

以对粉尘云状态参数的定量测定为基础,对玉米粉尘火焰在开口垂直管道中向上传播的过程进行了实验研究.在情形A中,火焰从管道的封闭端向开口端传播,在情形B中,从开口端向封闭端传播.实验中,观察到两种粉尘火焰,即湍流火焰和层流火焰,火焰形态转变对应的点火延迟时间约等于1.1 s,即粉尘云湍流运动强度为10cm/s.情形A中,层流火焰的传播出现周期性振荡现象,湍流火焰在传播过程中不断加速;情形B中,两种火焰都匀速传播,湍流火焰传播速度明显大于层流火焰.在所考察的实验条件下,粉尘浓度对于玉米粉尘火焰传播速度的影响不大.     

Systematic experiments on turbulent premixed Bunsen flames including turbulent flux measurements

The existence of good experimental data for turbulent premixed flames is of interest for the development and validation of numerical models. In this paper special focus is laid on the systematic variation of either the fuel-air ratio with fixed flow rate or varied flow and turbulence with fixed composition. In total, 15 different turbulent Bunsen flames are investigated. With the planar conditioned particle image velocimetry (CPIV) technique, simultaneous access is given to the flow, turbulence, and flame position data, using PIV for flow and turbulence and the density jump at the instantaneous flame front for reaction progress variable and density. This allows the fast determination of a large amount of statistical data such as Reynolds- and Favre-averaged mean reaction progress and velocities, which are needed for the density-weighted sets of equations used in the numerical codes. Additionally, conditioned and unconditioned mean velocities and velocity fluctuations can be determined and the turbulent flux terms of the reaction progress variable can be measured directly. Thus, a comprehensive data set is presented, which can be used for validation studies. The measured turbulent flux is compared with two models, the gradient diffusion assumption and a relation proposed by Veynante et al. that accounts for the competition between gas-dynamic expansion and turbulent mixing. While the former approach fails, the latter shows reasonable agreement for the radial flux and also accounts for the trend observed for the varied flow or flame conditions. The axial turbulent flux near the tip of the flame is not fully resolved.     

Effect of free stream turbulence on NOx and soot formation in turbulent diffusion CH4-air flames

A two-dimensional axisymmetric RANS numerical model was solved to investigate the effect of increasing the turbulence intensity of the air stream on the NOx and soot formation in turbulent methane diffusion flames. The turbulence–combustion interaction in the flame field was modelled in a k − ε/EDM framework, while the NO and soot concentrations were predicted through implementing the extended Zildovich mechanism and two transport equations model, respectively. The predicted spatial temperature gradients showed acceptable agreement with published experimental measurements. It was found that the increase of free stream turbulence intensity of the air supply results in a significant reduction in the NO formation of the flame. Such phenomenon is discussed by depicting the spatial distribution of the NO concentration in the flame. An observable reduction of the soot formation was also found to be associated with the increase of inlet turbulence intensity of air stream.     

Effects of strain rate,turbulence, reactant stoichiometry and heat losses on the interaction of turbulent premixed flames with stoichiometric counterflowing combustion products

Intense strain, turbulence, heat transfer, and mixing with combustion products can affect premixed flames in practical combustion devices. These effects are systematically studied in turbulent premixed CH₄/N₂/O₂ flames using a reactant versus product counterflow system and independently varying bulk strain rate, turbulent Reynolds number, equivalence ratio of the reactant mixture, and temperature of the stoichiometric counterflowing combustion products. The flow field and the turbulent flames are investigated using particle image velocimetry (PIV) measurements and laser-induced fluorescence (LIF) imaging of OH. The OH-LIF images are used to identify the interface between the counterflowing streams, referred to here as the gas mixing layer interface (GMLI). The flame response for different flow conditions is compared in terms of the probability of localized extinction along the GMLI, the turbulent flame brush thickness, and flame position relative to the GMLI, by using an OH-LIF-based progress variable. The probability of localized extinction at the GMLI increases as the separation between the turbulent flame brush and the GMLI decreases. Flame fronts in the vicinity of the GMLI are more likely to extinguish as a result of heat losses, dilution of the reaction zone by the product stream, and large local strain rates. A higher probability of localized extinction at the GMLI is induced by either a larger bulk strain rate or a slower flame speed. As the turbulent Reynolds number increases, the corresponding increase in turbulent flame brush thickness enhances the interactions of the flame fronts with the GMLI. Heat losses are substantially less significant for cases in which the turbulent flame brush is sufficiently separated from the GMLI. For flames in close proximity to the GMLI, the effects of the product stream on the flame front differ for lean and rich reactant mixtures. These disparities are attributed in part to differences in the ignitibility of the reactant mixtures by the hot product stream.     

Insights into non-adiabatic-equilibrium flame temperatures during millimeter-size vortex/flame interactions

Previous experimental and numerical studies have demonstrated that local flame temperatures can significantly increase above or decrease below the adiabatic-equilibrium flame temperature during millimeter-size vortex/flame interactions. Such large excursions in temperature are not observed in centimeter-size vortex/flame interactions. To identify the physical mechanisms responsible for these super- or sub-adiabatic-equilibrium flame temperatures, numerical studies have been conducted for millimeter-size vortex/flame interactions in a hydrogen-air, opposing-jet diffusion flame. Contrary to expectations, preferential diffusion between H₂ and O₂ and geometrical curvature are not responsible for these variations in local flame temperature. This was demonstrated through simulations made by forcing the diffusion coefficients of H₂ and O₂ to be equal and thereby eliminating preferential diffusion. Propagation of flame into small (∼1 mm) vortices suggested that the amount of reactant carried by such a small vortex is not sufficient to feed the flame with fresh reactant during the entire vortex/flame interaction process. Various numerical experiments showed that the reactant-limiting characteristics associated with the millimeter-size vortices and the local Lewis number (not preferential diffusion) are responsible for the generation of flame temperature that is different from the adiabatic-equilibrium value. The reactant-deficient nature of the millimeter-size vortices forces the combustion products to be entrained into the vortex. While a greater-than-unity Lewis number results in pre-heating of the reactant through the product entrainment, a less-than-unity Lewis number causes cooling of the reactant. Contrary to this behavior, a centimeter-size large vortex wraps and maintains the flame around its outer perimeter by feeding the flame with fresh reactant throughout the interaction process, thereby rendering the flame unaffected by the Lewis number. Since turbulent flames generally involve interactions with small-size vortices, the physical mechanisms described here should be considered when developing mathematical models for turbulent flames.     

Spark ignition of turbulent recirculating non-premixed gas and spray flames: A model for predicting ignition probability

A model that synthesizes previous knowledge from experiments and simulations on spark ignition of gas and liquid-fuelled non-premixed recirculating flames has been developed. Attention is focused on the flame expansion process and the overall filling of the combustor volume with flame. The model is meant to provide a quick assessment of the ignition behaviour of a combustor. It uses information from the flow patterns before ignition and calculates possible trajectories that a flame emanating from a spark may experience. The calculation of these trajectories includes flame extinction to capture the experimentally-observed flame quenching, mixture fraction fluctuations to capture the non-premixed nature of the flame, convection by the mean and the random turbulent flow to capture the probabilistic nature of the flame evolution, and uses recent results on the laminar burning velocity in sprays. The model is applied to gas and spray flames and the calculated ignition probability distributions and the timescale of complete ignition agree reasonably well with experiment. The results of the model provide insights into spark ignition processes in complicated flow patterns.     

Ignition kernel formation and lift-off behaviour of jet-in-hot-coflow flames

The stabilisation region of turbulent non-premixed flames of natural gas mixtures burning in a hot and diluted coflow is studied by recording the flame luminescence with an intensified high-speed camera. The flame base is found to behave fundamentally differently from that of a conventional lifted jet flame in a cold air coflow. Whereas the latter flame has a sharp interface that moves up and down, ignition kernels are continuously being formed in the jet-in-hot-coflow flames, growing in size while being convected downstream. To study the lift-off height effectively given these highly variable flame structures, a new definition of lift-off height is introduced. An important parameter determining lift-off height is the mean ignition frequency density in the flame stabilisation region. An increase in coflow temperature and the addition of small quantities of higher alkanes both increase ignition frequencies, and decrease the distance between the jet exit and the location where the first ignition kernels appear. Both mechanisms lower the lift-off height. An increase in jet Reynolds number initially leads to a significant decrease of the location where ignition first occurs. Higher jet Reynolds numbers (above 5000) do not strongly alter the location of first ignition but hamper the growth of flame pockets and reduce ignition frequencies in flames with lower coflow temperatures, leading to larger lift-off heights.