缸内直接喷射式汽油机点火稳定性的数值分析

缸内直接喷射式汽油机的一个显著特点是依靠火花塞点燃喷入缸内的汽油油束。由于缸内混合气浓度极不均匀，所以其点火及火焰传播过程与普通均质燃烧式发动机有很大的不同。火焰核心的稳定形成及初始火焰发展对缸内的整个燃烧过程有极其重要的影响。本文利用二维两相混合模型模拟喷雾过程，利用一个详细的准维模型模拟火花塞的点火过程，并采用特殊处理方法使两个子模型相匹配，计算了缸内直接喷射式汽油机从喷雾到形成稳定火核的全过程，分析了多种因素对点火稳定性的影响，尤其是对涡流比、点火时刻和喷油定时之间的适当配合进行了模拟分析。计算结果对优化实验有明显的指导作用。     

摩托车汽油机的爆燃预测研究

结合燃烧模型，湍流火焰传播模型以及化学动力学模型，建立了摩托车四冲程汽油机双区准维燃烧模型。运用该模型模拟燃烧过程，并进行爆燃预测。用此模型CUB100摩托车汽油机进行了计算，预测了CUB100提高压缩比后爆燃的发生。     

汽油机燃烧过程模拟计算及爆震预测

将计算焰前反应的化学反应动力学模型与燃烧模型及湍流火焰传播模型相结合,建立了含有碳氢燃料焰前反应的简易化学动力学模型的汽油机双区燃烧模型。用该模型能较好地模拟汽油机燃烧过程,并能实现爆震预测,研究影响爆震发生的诸多因素。采用此模型对４９２ Ｑ Ａ 汽油机进行模拟计算的结果与试验结果能较好地吻合,证明了该模型的可行性。     

用紊流卷吸燃烧模型计算研究点火时刻紊流参数对燃烧循环的影响

利用准维紊流卷吸燃烧模型计算研究了点火时刻气缸内紊流参数的变化对汽油机燃烧循环变动的影响。结果表明，点火时刻紊流强度u′及紊流长度积分标尺L的变动对汽油机燃烧循环变动均有很大影响，增大u′或L均有利于加快燃烧速度及火焰传播速度，缩短火焰发展期；u′或L的变动加大，燃烧循环变动也随之加大，其中，u′对燃烧循环变动的影响尤其大。     

Spark ignition and the early stages of turbulent flame propagation

A hydrodynamic mathematical model is presented, without chemical kinetics, for the ignition of gaseous mixtures. Solution schemes are discussed and computed results presented for the initial shock wave propagation and the slower spreading of the thermal wave from the narrow spark discharge channel.Different spark power-time profiles are analyzed and it is clear that a central problem is the slow spread of energy from the discharge channel to ignite a critical volume. An initial high energy input aids this process. Ignition delay times are presented, based upon thermal ignition criteria. Minimum ignition energy is increased for turbulent mixtures because of the effects of flame straining.Ignition experiments are reported for a fan-stirred bomb. Spark power profiles and turbulent velocities were controlled and CH emission intensities measured. Ignition and flame development were traced from high speed schieren photographs. Three regimes are delineated: localized ignition, spark assisted flame propagation, and normal propagation.     

Flame propagation in stratified hydrogen–air mixtures: Spark placement effects

Two-dimensional laminar flame simulations of a forced-ignition event in an initially quiescent mixing layer of hydrogen and air have been carried out at atmospheric pressure using detailed chemistry and effective binary diffusion coefficients. Since control of the ignition location is known to be critical in direct-injection spark-ignition engines, this study primarily investigates the effect of initial spark placement within the flammability limits of hydrogen–air. Displacement and stabilization speeds of the propagating flame fronts have been computed along isocontours of water vapor representing 10% and 25% of the downstream equilibrium concentration. Following the period of spark energy addition the flame kernel is observed to develop into tribrachial flames that subsequently propagate along the stoichiometric line. For all cases of successful ignition, transient spark effects are observed to dissipate within 0.18 ms. Subsequent structure and propagation speed of the flame are not influenced by the transient development phase.     

Prediction of autoignition in a lifted methane/air flame using an unsteady flamelet/progress variable model

An unsteady flamelet/progress variable (UFPV) model has been developed for the prediction of autoignition in turbulent lifted flames. The model is a consistent extension to the steady flamelet/progress variable (SFPV) approach, and employs an unsteady flamelet formulation to describe the transient evolution of all thermochemical quantities during the flame ignition process. In this UFPV model, all thermochemical quantities are parameterized by mixture fraction, reaction progress parameter, and stoichiometric scalar dissipation rate, eliminating the explicit dependence on a flamelet time scale. An a priori study is performed to analyze critical modeling assumptions that are associated with the population of the flamelet state space.For application to LES, the UFPV model is combined with a presumed PDF closure to account for subgrid contributions of mixture fraction and reaction progress variable. The model was applied in LES of a lifted methane/air flame. Additional calculations were performed to quantify the interaction between turbulence and chemistry a posteriori. Simulation results obtained from these calculations are compared with experimental data. Compared to the SFPV results, the unsteady flamelet/progress variable model captures the autoignition process, and good agreement with measurements is obtained for mixture fraction, temperature, and species mass fractions. From the analysis of scatter data and mixture fraction-conditional results it is shown that the turbulence/chemistry interaction delays the ignition process towards lower values of scalar dissipation rate, and a significantly larger region in the flamelet state space is occupied during the ignition process.     

火花点火式天然气发动机燃烧模拟计算及爆燃预测

本文提出了一种利用化学反应动力学模型与燃烧模型及紊流火焰传播模型建立的点火式天然气发动机双区燃烧模型 ,用该模型能较好地模拟点火式天然气发动机燃烧过程 ,并能实现爆燃预测及研究其发生的诸多因素。采用此模型对CA6 10 2汽油机改装为点火式天然气发动机 (以下简称为“CNG发动机”)进行模拟计算的结果与试验结果能较好地吻合 ,证明了该模型的可行性。     

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.     

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.     

Large eddy simulation of forced ignition of an annular bluff-body burner

The optimization of the ignition process is a crucial issue in the design of many combustion systems. Large eddy simulation (LES) of a conical shaped bluff-body turbulent nonpremixed burner has been performed to study the impact of spark location on ignition success. This burner was experimentally investigated by Ahmed et al. [Combust. Flame 151 (2007) 366–385]. The present work focuses on the case without swirl, for which detailed measurements are available. First, cold-flow measurements of velocities and mixture fractions are compared with their LES counterparts, to assess the prediction capabilities of simulations in terms of flow and turbulent mixing. Time histories of velocities and mixture fractions are recorded at selected spots, to probe the resolved probability density function (pdf) of flow variables, in an attempt to reproduce, from the knowledge of LES-resolved instantaneous flow conditions, the experimentally observed reasons for success or failure of spark ignition. A flammability map is also constructed from the resolved mixture fraction pdf and compared with its experimental counterpart. LES of forced ignition is then performed using flamelet fully detailed tabulated chemistry combined with presumed pdfs. Various scenarios of flame kernel development are analyzed and correlated with typical flow conditions observed in this burner. The correlations between, velocities and mixture fraction values at the sparking time and the success or failure of ignition, are then further discussed and analyzed.     

采用考虑详细化学反应机理的火焰面模型模拟湍流扩散火焰

采用详细的甲烷氧化化学反应动力学机理(GRI-Mech3．0)对不同拉伸率条件下的拉伸层流扩散火焰面结构进行了数值计算，建立了一个包含一系列拉伸层流火焰面结构的火焰面数据库．将这些层流火焰面结构和美国Sandia国家实验室测得的湍流扩散火焰(FlameD)的平均火焰结构进行了对比，发现层流火焰面所覆盖的范围基本包含了所考虑的湍流火焰中不同位置的平均火焰结构，这表明火焰面模型是合理的．然后，采用火焰面模型对该湍流扩散火焰进行了数值模拟并和实验数据进行了比较，考察了火焰面模型的精确程度和模拟深度.     

Zone conditional assessment of flame-generated turbulence with DNS database of a turbulent premixed flame

Zone conditional two-fluid equations are derived and validated against a DNS database for a turbulent premixed flame. The conditional statistics of major flow variables are investigated to understand the mechanism of flame-generated turbulence. The flow field in the burned region shows substantially increased, highly anisotropic turbulence to conserve mass through a flamelet surface. The transverse component may be larger than the axial component for a distributed pdf of the flamelet orientation angle in the middle of the flame brush. The opposite occurs due to redistribution of turbulent kinetic energy and flamelet orientation mostly normal with respect to the mean flow at the end of the flame brush. The major source or sink terms of turbulent kinetic energy are the interfacial transfer by the mean reaction rate and the work terms induced by fluctuating pressure and velocity on the flame surface. Ad hoc modeling of some interfacial terms may be required for further application of the two-fluid model for modeling turbulence in turbulent premixed combustion simulations.     

LES of a premixed jet flame DNS using a strained flamelet model

Turbulent premixed flames in the thin and broken reaction zones regimes are difficult to model with Large Eddy Simulation (LES) because turbulence strongly perturbs subfilter scale flame structures. This study addresses the difficulty by proposing a strained flamelet model for LES of high Karlovitz number flames. The proposed model extends a previously developed premixed flamelet approach to account for turbulence’s perturbation of subfilter premixed flame structures. The model describes combustion processes by solving strained premixed flamelets, tabulating the results in terms of a progress variable and a hydrogen radical, and invoking a presumed PDF framework to account for subfilter physics. The model is validated using two dimensional laminar flame studies, and is then tested by performing an LES of a premixed slot-jet direct numerical simulation (DNS). In the premixed regime diagram this slot-jet is found at the edge of the broken reaction zones regime. Comparisons of the DNS, the strained flamelet model LES, and an unstrained flamelet model LES confirm that turbulence perturbs flame structure to leading order effect, and that the use of an unstrained flamelet LES model under-predicts flame height. It is shown that the strained flamelet model captures the physics characterizing interactions of mixing and chemistry in highly turbulent regimes.     

Modelling cycle to cycle variations in an SI engine with detailed chemical kinetics

This paper presents experimental results and a new computational model that investigate cycle to cycle variations (CCV) in a spark ignition (SI) engine. An established stochastic reactor model (SRM) previously used to examine homogeneous charge compression ignition (HCCI) combustion has been extended by spark initiation, flame propagation and flame termination sub-models in order to simulate combustion in SI engines. The model contains a detailed chemical mechanism but relatively short computation times are achieved. The flame front is assumed to be spherical and centred at the spark location, and a pent roof and piston bowl geometry are accounted for. The model is validated by simulating the pressure profile and emissions from an iso-octane fuelled single cylinder research engine that showed low CCV. The effects of key parameters are investigated. Experimental results that show cycle to cycle fluctuations in a four-cylinder naturally aspirated gasoline fuelled SI engine are presented. The model is then coupled with GT-Power, a one-dimensional engine simulation tool, which is used to simulate the breathing events during a multi-cycle simulation. This allows an investigation of the cyclic fluctuations in peak pressure. The source and magnitude of nitric oxide (NO) emissions produced by different cycles are then investigated. It was found that faster burning cycles result in increased NO emissions compared with cycles that have a slower rate of combustion and that more is produced in the early stages of combustion compared with later in the cycle. The majority of NO was produced via the thermal mechanism just after combustion begins.     

火花点火发动机燃烧循环变动的理论研究

本文改进了一个火花点火发动机的准维计算模型，并对燃烧的循环变动进行了理论计算研究。这个模型包括点火时刻火花塞附近气流平均速度、湍流强度、气缸内残余废气系数以及缸内总的混合气质量等的循环变动的影响。将计算结果和试验结果进行了比较，证实了用这个模型可以较精确地预测燃烧的循环变动。另外，运用这个模型分别讨论了湍流强度、火焰中心位置在缸内的移动，以及残余废气系数的循环变动对不同燃烧阶段循环变动的影响程度，从而得出了一些有益的结论。     

Availability analysis of a coke oven gas fueled spark ignition engine

The current work investigates a coke oven gas fueled spark ignition (SI) engine from the perspective of the first and second laws in order to understand the energy conversion performance of fuels and achieve highly efficient utilization. A detailed energy and exergy analysis is applied to a quasi-dimensional two-zone spark ignition engine model which combines turbulence flame propagation speed model at 1500 rpm by changing gas fuel types, compression ratio, load and ignition timing. It was found that the irreversibility of methane is the maximum and that of syngas is the minimum among the three different fuels. The irreversibility in the combustion process of a coke oven gas fueled SI engine is reduced when the compression ratio or the throttle valve opening angle is increased and the ignition timing is delayed. Increasing the compression ratio and delaying the ignition timing can improve the first and second law efficiency and reduce the brake specific fuel consumption (BSFC). The power performance and fuel economy are good and the energy is also used effectively when the compression ratio is 11, the throttle angle is 90% and the ignition time is ?10° CA ATDC respectively.     

Turbulent burning velocity, burned gas distribution, and associated flame surface definition

Experimental studies of premixed, turbulent, gaseous explosion flames in a fan-stirred bomb are reported. The turbulence was uniform and isotropic, while changes in the rms turbulent velocity were achieved by changes in the speed of the fans. Central spark ignitions created mean spherical flame propagation. The spatial distributions of burned and unburned gases during the propagation were measured from the Mie scattering of tobacco smoke in a thin planar laser sheet. The plane was located just in front of the central spark gap and was generated by a copper vapor laser operating at a pulse rate of 4.5 kHz. High-speed schlieren images also were captured simultaneously.The distributions of the proportions of burned and unburned gases around circumferences were found for all radii at all stages of the explosion, and mean values of these proportions were derived as a function of the mean flame radius. The flame brush thickness increased with flame radius. The way the turbulent burning velocity is defined depends on the chosen associated flame radius. Various definitions are scrutinized and different flame radii presented, along with the associated turbulent burning velocities. Engulfment and mass turbulent burning velocities are compared. It is shown how the latter might conveniently be obtained from schlieren cine images. In a given explosion, the burning velocity increased with time and radius, as a consequence of the continual broadening of the effective spectrum of turbulence to which the flame was subjected. A decrease in the Markstein number of the mixture increased the turbulent burning velocity.     

Hybrid presumed pdf and flame surface density approaches for Large-Eddy Simulation of premixed turbulent combustion: Part 1: Formalism and simulation of a quasi-steady burner

Extended Coherent Flame Model for Large-Eddy Simulation (ECFM-LES) and Presumed Conditional Moments-Flame Prolongation of Intrisic Low Dimensional Manifolds (PCM-FPI) are some of the combustion models exploited for Large-Eddy Simulations (LES) of turbulent premixed flames. Combustion is then either modeled by tracking the flame surface density or by combining computations of flamelets with presumed probability density functions (pdf). The first approach enables to control the turbulent flame speed but models chemical kinetics in a simplified manner. The second directly accounts for detailed chemistry via the flamelet structure but the turbulent propagation speed cannot be easily estimated a priori. Simple one-dimensional tests are then performed in this study to evaluate flame velocities of PCM-FPI. A restricted operating range of this model, that enables to retrieve an evolution of the propagating speed similar to ECFM-LES predictions, is highlighted. This zone is limited in terms of filter width and sub-grid scale turbulent viscosity. An attractive alternative to both ECFM-LES and PCM-FPI approaches thus appears to be a model integrating their respective main strengths. For this purpose, two hybrid models are proposed in this paper and tested through LES of a lean-premixed turbulent swirling flame. Results in terms of statistics for temperature and mass fractions of chemical species are compared to experimental data and to previous results already obtained with PCM-FPI. The coupled models enable to properly locate the reaction zone via the flame surface density closures along with a correct prediction of the chemistry evolution in the flame front.