Shock tube studies of thermal radiation of diesel-spray combustion under a range of spray conditions

A tailored interface shock tube and an over-tailored interface shock tube were used to measure the thermal energy radiated during diesel-spray combustion of light oil, α-methylnaphthalene and cetane by changing the injection pressure. The ignition delay of methanol and the thermal radiation were also measured. Experiments were performed in a steel shock tube with a 7 m low-pressure section filled with air and a 6 m high-pressure section. Pre-compressed fuel was injected through a throttle nozzle into air behind a reflected shock wave. Monochromatic emissive power and the power emitted across all infrared wavelengths were measured with IR-detectors set along the central axis of the tube. Time-dependent radii where soot particles radiated were also determined, and the results were as follows. For diesel spray combustion with high injection pressures (from 10 to 80 MPa), the thermal radiation energy of light oil per injection increased with injection pressure from 10 to 30 MPa. The energy was about 2% of the heat of combustion of light oil at P _inj = about 30 MPa. At injection pressure above 30 MPa the thermal radiation decreased with increasing injection pressure. This profile agreed well with the combustion duration, the flame length, the maximum amount of soot in the flame, the time-integrated soot volume and the time-integrated flame volume. The ignition delay of light oil was observed to decrease monotonically with increasing fuel injection pressure. For diesel spray combustion of methanol, the thermal radiation including that due to the gas phase was 1% of the combustion heat at maximum, and usually lower than 1%. The thermal radiation due to soot was lower than 0.05% of the combustion heat. The ignition delays were larger (about 50%) than those of light oil. However, these differences were within experimental error.

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An abridged version of this paper was presented at the 18th Int. Symposium on Shock Waves at Sendai, Japan during July 21 to 26, 1991 and at the 19th Int. Symposium on Shock Waves at Marseille, France during July 26 to 30, 1993.     

Effects of turbulence enhancement on combustion process using a double injection strategy in direct-injection spark-ignition (DISI) gasoline engines

Direct-injection spark-ignition (DISI) gasoline engines have been spotlighted due to their high thermal efficiency. Increase in the compression ratio that result from the heat absorption effect of fuel vaporization induces higher thermal efficiency than found in port fuel injection (PFI) engines. Since fuel is injected at the cylinder directly, various fuel injection strategies can be used. In this study, turbulent intensity was improved by a double injection strategy while maintaining mixture homogeneity. To analyze the turbulence enhancement effects using the double injection strategy, a side fuel injected, homogeneous-charge-type DISI gasoline engine with a multi-hole-type injector was utilized. The spray model was evaluated using experimental data for various injection pressures and the combustion model was evaluated for varied ignition timing. First and second injection timing was swept by 20 degree interval. The turbulent kinetic energy and mixture inhomogeneity index were mapped. First injection at the middle of the intake stroke and second injection early in the compression stroke showed improved turbulent characteristics that did not significantly decrease with mixture homogeneity. A double injection case that showed improved turbulent intensity while maintaining an adequate level of mixture homogeneity and another double injection case that showed significantly improved turbulent intensity with a remarkable decrease in mixture homogeneity were considered for combustion simulation. We found that the improved turbulent intensity increased the flame propagation speed. Also, the mixture homogeneity affected the pressure rise rate.     

基于光学诊断的某新型含铝推进剂燃烧特性分析

为探究某新型含铝固体推进剂燃烧特性和规律,在模拟固体发动机的高压条件下,采用可调功率激光器结合高速摄影、发射光谱等光学诊断技术对该新型含铝固体推进剂开展了系统的点火及燃烧过程研究。通过对该推进剂的点火延迟、退移速率、燃烧温度以及团聚物颗粒尺寸的定量测量和分析,明确了该推进剂的点火延迟量级;证实此推进剂的退移速率严格遵循Summerfield燃速公式;判断出其最高燃烧温度高于3 300 K,且随压力增大而升高;通过对燃烧过程中发光凝聚相产物面积的量化分析得出推进剂产物中团聚物粒径尺寸受环境参数的影响规律。     

Determination of ignition delay times of different hydrocarbons in a new high pressure shock tube

The impending scarcity of fossil fuel in the future requires continued development in hydrocarbon combustion research. Biofuels offer a promising alternative to traditional fossil fuel-based combustion. To optimize engine design for biofuels, adequate combustion characteristics for new fuels have to be known. In this study, a new high pressure stainless steel shock tube for measuring ignition delay times is presented. When compared with other shock tubes for investigating ignition delays, the new tube provides superior maximum working pressures and geometric properties. Shock tube performance is determined by reference experiments with air as driven gas. These experiments allow to determine the available test time and the influence of shock attenuation. Owing to the large inner diameter of the shock tube, shock attenuation is <1% as it is typical for low pressure shock tubes. However, contrast to typical low pressure shock tubes, non-diluted fuel–air mixtures at high pressures can be investigated in the new shock tube due to the high allowable working pressure. First experiments concerning the ignition delay time have been performed with methane and n-heptane. The results of these experiments show a good agreement to literature data. As a first biofuel ethanol has been investigated at elevated pressures up to 40 bar.     

Combustion of Low-Calorific Waste Biomass Syngas

The industrial combustion chamber designed for burning low-calorific syngas from gasification of waste biomass is presented. For two different gases derived from gasification of waste wood chips and turkey feathers the non-premixed turbulent combustion in the chamber is simulated. It follows from our computations that for stable process the initial temperature of these fuels must be at least 800 K, with comparable influx of air and fuel. The numerical simulations reveal existence of the characteristic frequency of the process which is later observed in high-speed camera recordings from the industrial gasification plant where the combustion chamber operates. The analysis of NO formation and emission shows a difference between wood-derived syngas combustion, where thermal path is prominent, and feathers-derived fuel. In the latter case thermal, prompt and N₂O paths of nitric oxides formation are marginal and the dominant source of NO is fuel-bound nitrogen.     

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. Submitted as a full-length article to Flow Turbulence and Combustion.     

再生式喷雾燃烧过程中着火延迟期的影响特性

对液体药火炮中的再生式喷雾燃烧过程进行了大量的实验研究，发现了着火延迟期对再生式喷雾燃烧过程的重要影响。建立了反映着火后燃烧室内压力状态与着火延迟期间各参量之间关系的分析模型；结果表明，随着着火延迟期的延长，延迟期间液体药堆积量不断增加，进而导致着火后压力升高率急剧增大。运用零维数学模型对整个再生喷雾燃烧过程进行了模拟，模拟结果进一步证实了分析模型结论，较好地再现了正常燃烧过程和非正常燃烧过程的基本特征，阐明了着火延迟期对再生喷雾燃烧过程影响的物理本质。     

Optical investigation of the combustion behaviour inside the engine operating in HCCI mode and using alternative diesel fuel

In order to understand the effect of both the new homogeneous charge compression ignition (HCCI) combustion process and the use of biofuel, optical measurements were carried out into a transparent CR diesel engine. Rape seed methyl ester was used and tests with several injection pressures were performed. OH and HCO radical were detected and their evolutions were analyzed during the whole combustion. Moreover, soot concentration was measured by means the two colour pyrometry method. The reduction of particulate emission with biodiesel as compared to the diesel fuel was noted. Moreover, this effect resulted higher increasing the injection pressure. In the case of RME the oxidation of soot depends mainly from O₂ content of fuel and OH is responsible of the NO formation in the chamber as it was observed for NO_x exhaust emission. Moreover, it was investigated the evolution of HCO and CO into the cylinder. HCO was detected at the start of combustion. During the combustion, HCO oxidizes due to the increasing temperature and it produces CO. Both fuels have similar trend, the highest concentrations are detected for low injection pressure. This effect is more evident for the RME fuel.     

Influence of heat and mass transfer on the ignition and NOx formation in single droplet combustion

The effect of heat and mass transfer on the ignition, and in a second step on the nitrogen oxide (NO _x ) generation, of single burning droplets is examined in a numerical study. Spherical symmetry with no gravity and no forced convection is presumed; ambient temperature is set at 500 K, below the auto-ignition point. The essentials of a forced droplet ignition by an external energy source are introduced. Two methods are applied: heat introduction at a fixed radial position r and heat introduction at a fixed local equivalence ratio ϕ _r . This study’s distinctiveness compared to previous research is its focus on and its combination of partially pre-vaporized droplets and detailed chemistry, both being technically relevant in kerosene and diesel fuel combustion. The fuel of choice is n-decane (C₁₀H₂₂), and NO _x production is studied exemplarily as a representative group of pollutant emissions. The conducted simulations show a decrease of NO _x formation with an increase of the pre-vaporization rate \Uppsi. \Uppsi. This decrease is generally valid for both methods of heat introduction. However, results on flame stabilization and NO _x production reveal a high sensitivity to parameters of the ignition model. The burning behavior during the initial stages is dominated by the ignition position. Extracting heat from the exhaust gas region of burning droplets shows no impact on the flame position nor on the relative NO _x production. As a consequence, a well-founded modeling of the investigated droplet regime needs to resort to an iterative adaptation of the heat introduction parameters based on the findings of droplet burning and exhaust gas production.     

Nonstationary burning of propellants with variable surface temperature

The author examines nonstationary processes (combustion at varying pressure, quenching, and ignition) for a model propellant whose burning rate u and surface temperature t₁ depend on pressure p and initial temperature T₀. All the processes in the surface reaction zone and the gas phase are assumed inertialess. It is shown that a theory of nonstationary combustion for such a model can be constructed by analogy with the Zel'dovich theory [1, 2], in which the surface temperature of the powder is assumed fixed. The variation of burning rate with time has been investigated for small sudden pressure changes. It is shown how a sufficiently large and steep pressure drop may cause quenching of the propellant. The process of propellant ignition is subjected to a qualitative analysis.The author thanks O. I. Leipunskii, A. G. Istratov, V. B. Librovich, and A. D. Margolin for their comments and advice.     

An Experimental Investigation of the Turbulence Effect on the Combustion Propagation in a Rapid Compression Machine

The role of different combustion modes that govern the combustion propagation inside a rapid compression machine is discussed. Aiming at the control of the compression generated turbulence, the careful design of the UPMC-RCM is described. A methodology is then proposed to investigate the influence of the residual post-compression turbulence level on the visible combustion propagation process. Through the fuel type, the main parameter varied is the ratio of the ignition delay time to the characteristic decay time for the post-compression turbulence. A high-speed camera images the visible combustion. Particle Image Velocimetry provides two-dimensional velocity fields of the post-compression flow prior to ignition. The residual turbulence level is shown to influence the combustion propagation phenomenology, highlighting the coexistence of both volumetric and frontlike combustion modes for the shorter aforementioned ratios. This conclusion could contribute to an explanation of the experimental discrepancies among the ignition delays measured in different RCMs as the residual turbulence level is highly set-up dependent.     

硅烷对JP10和煤油点火特性影响的激波管研究

在JP10和煤油点火特性激波管实验的基础上，实验研究了硅烷对这两种典型高碳数碳氢燃 料点火特性的影响. 在预加热到70 C的激波管上，采用缝合运行条件获得了近7ms 的实验时间，将实验延伸至低温区. 采用气相色谱分析和高精度真空仪直接测定压力相结合 的方法，确定了燃料气相浓度，解决了高碳数碳氢燃料点火激波管实验时由于管壁吸附影响 燃料气相浓度确定的困难. 实验记录了点火过程中OH自由基发射强度变化，并作为判断点 火发生的标志. 实验温度范围880~1800K, 压力范 围0.16~0.53\,MPa. 当硅烷加入量约为燃料的10%~15%(摩尔比)， 质量比为2%~3%, 观测到明显的点火促进作用. 该研究对超燃研究中发动机设计、 燃料选择等方面具有直接的工程意义，也可用于检验燃烧化学动力学模型的合理性.     

氢气定容燃烧的实验研究

利用定容燃烧弹和高速数据采集系统对氢气定容燃烧进行实验研究,得出氢气定容燃烧压力变化过程、燃烧爆压及爆炸常数的变化规律。研究结果表明:中心点火定容燃烧的压力变化过程为:从开始的火花跳火干扰到平稳的等压燃烧,再到压力的慢速和快速增加,在燃烧的中后期会出现压力振荡;在非燃烧极限工况下,随着燃空当量比的增加燃烧爆压先增加后减小,随着初始压力的升高燃烧爆压几乎线性增加,随着温度的增加燃烧爆压和最大燃烧爆压都减小;随着燃空当量比的增加爆炸常数先增加后减小,在燃空当量比小于4.0的工况,燃烧爆炸常数随初始压力的升高而增加,而燃空当量比大于4.0的工况随着初始压力的升高而下降;在燃空当量比小于2.5时,燃烧爆炸常数随温度升高而减小,在燃空当量比大于2.5时,则正好相反。     

High-speed imaging for direct-injection gasoline engine research and development

In recent years, new laser and camera technology have enabled the development of high-speed imaging diagnostics for measurements at frame rates commensurate with the time scales of turbulent mixing, combustion, and emission formation in internal combustion engines. The ability to study the evolution of in-cylinder flow, fuel/air mixing, ignition, and combustion within individual cycles and for many consecutive cycles provides new insights into the physics and chemistry of internal combustion engine performance. Data for model development and device development are obtained with unprecedented access to the identification of random events such as cycle to cycle variation and ignition instabilities. This paper summarizes high-speed diagnostics developments with a focus on application to spark-ignition direct-injection gasoline engines. A range of optical techniques is described along with examples of applications in research and near-production engines. Measurements of in-cylinder velocities were conducted with particle image velocimetry. The spray evolution was followed with Mie scattering. Quantitative fuel distributions were recorded with laser-induced fluorescence. Fuel impingement on surfaces was quantified with refractive index matching. Combined velocity and fuel measurements were used to study ignition reliability. Chemiluminescence techniques provided insights into the evolution of the spark plasma as well as the growing flame kernel. Chemiluminescence and black body radiation imaging yielded insights into the formation and oxidation of soot.     

An empirical model for the ignition of explosively dispersed aluminum particle clouds

An empirical model for the ignition of aluminum particle clouds is developed and applied to the study of particle ignition and combustion behavior resulting from explosive blast waves. This model incorporates both particle ignition time delay as well as cloud concentration effects on ignition. The total mass of aluminum that burns is found to depend on the model, with shorter ignition delay times resulting in increased burning of the cloud. After the Al particles ignite, a competition for oxidizer between the booster detonation products and Al ensues. A new mass-averaged ignition parameter is defined and is observed to serve as a useful parameter to compare cloud ignition behavior. Investigation of this variable reveals that both peak ignition as well as the time required to attain peak ignition, are sensitive to the model parameters. The peak degree of dissociation in the fireball is about 19 % and the associated energy can play a significant role on the dynamics of the problem. The peak degree of ionization is about 2.9 % and the energy associated with this is much lower than the other controlling factors. Overall, this study demonstrates that the new ignition model developed captures effects not included in other combustion models for the investigation of shock-induced ignition of aluminum particle clouds.     

Large-Eddy Simulation of Turbulent Flow Around a Finite-Height Wall-Mounted Square Cylinder Within a Thin Boundary Layer

The paper describes the results of a computational study of the auto-ignition of a fuel spray under Exhaust Gas Recirculation (EGR) conditions, a technique used to reduce the production of NO_x. Large Eddy Simulation (LES) is performed, and the stochastic field method is used for the solution of the joint sub-grid probability density function (pdf) of the chemical species and energy. The fuel spray is n-heptane, a diesel surrogate and its chemical kinetics are described by a reduced mechanism involving 22 species and 18 reaction steps. The method is applied to a constant volume combustion vessel able to reproduce EGR conditions by the ignition of a hot gas mixture previously introduced into the chamber. Once the prescribed conditions are reached the fuel is then injected. Different EGR conditions in terms of temperature and initial ambient chemical composition are simulated. The results are in good overall agreement with measurements both regarding the ignition delay times and the lift-off heights.     

Quantification of the Pre-ignition Front Propagation in DNS of Rapidly Compressed Mixture

A method to estimate the propagation speed of a three-dimensional ignition front in Direct Numerical Simulation (DNS) is discussed. The objective is to contribute to the design of advanced numerical tools for the study of sporadic pre-ignition kernels leading to violent pressure waves, which may for instance damage moving parts in engines. Estimating the speed of a propagating ignition front in three-dimensional DNS, before it is occurring, is not an easy task, because this speed scales as the inverse of the spatial gradient of the time left till ignition, which is a priori an unknown quantity. The proposed approach introduces, for every point of the DNS, an estimation of the time left till ignition, which is obtained from reactors dynamically parameterized from the time evolving DNS results. This provides a three-dimensional distribution of ignition delays at every instant in time of the DNS fields. The time evolution of the ignition speed is then computed from the space derivative of the ignition delay field. Only the pre-ignition phase is examined and the demonstration of the method is made with oversimplified chemistry, in order to apply it to an existing rapid compression machine, in which the mixture composition is homogeneous, whereas the temperature distribution is non-uniform due to heat-transfer at wall. The two expected distinct ignition regimes are reported. In the first, ignition propagates at the speed of sound, or even above, and occurs over a large portion of the combustion chamber, with a strong and sudden pressure increase. The second ignition regime is much more localized in space and with a propagation mechanism pertaining to a deflagration mode. A method is also discussed to delineate in the DNS between ignition influenced by either the constant pressure or the constant volume canonical behaviors.     

Experimental Determination of the Burning Velocity of Mixtures of n-Heptane and Toluene in Engine-like Conditions

Although the burning velocities of fuel-air mixtures have been extensively studied at room temperature and pressure, there is relatively little experimental information available for elevated temperatures and pressures (the so-called engine like conditions). Therefore, the main aim of the present work is to generate accurate experimental burning velocities valid over a range of high unburned gas temperatures and pressures of a variety of mixtures of n-heptane and toluene, varying its proportion by 25% in volume each time. Two experimental combustion facilities have been used and their results compared. One facility consists of a constant volume cylindrical bomb in which Schlieren images can be recorded and used to calculate the flame front development. The second facility is a spherical combustion bomb with centred ignition in which burning velocities are calculated from pressure records by means of a two-zone model. In order to check that the pressure method is reliable, experiments with n-heptane at room temperature and pressure for different equivalence ratios carried out in the spherical combustion bomb were compared with the ones obtained at the same conditions in the cylindrical vessel equipped with the Schlieren technique. Once the validity has been checked, extensive experiments have been carried out for widely varying initial conditions of pressure between 0.3 and 0.7?MPa, temperature between 363?K and 453?K and equivalence ratios from 0.8 to 1.1. Over the ranges studied, by removing the influence of the ignition energy at the earliest stages of combustion and the quenching effects at the later ones, the burning velocities are fitted by a correlation of type $ Cc=Cc_{r}\cdot (T_{ub}/T_{r})^{\varepsilon }\cdot (P/P_{r})^{\beta } $ , where Cc _r , ?? and ?? depend on the equivalence ratio. The ranges of validity of the correlations obtained cover from 370?K to 700?K, from 0.3?MPa to 4.5?MPa, and from 0.8 to 1.1 equivalence ratio. A comparison with previous predicted values is also given.     

An experimental study of the combustion characteristics in SCCI and CAI based on direct-injection gasoline engine

Emissions remain a critical issue affecting engine design and operation, while energy conservation is becoming increasingly important. One approach to favorably address these issues is to achieve homogeneous charge combustion and stratified charge combustion at lower peak temperatures with a variable compression ratio, a variable intake temperature and a trapped rate of the EGR using NVO (negative valve overlap). This experiment was attempted to investigate the origins of these lower temperature auto-ignition phenomena with SCCI and CAI using gasoline fuel. In case of SCCI, the combustion and emission characteristics of gasoline-fueled stratified-charge compression ignition (SCCI) engine according to intake temperature and compression ratio was examined. We investigated the effects of air–fuel ratio, residual EGR rate and injection timing on the CAI combustion area. In addition, the effect of injection timing on combustion factors such as the start of combustion, its duration and its heat release rate was also investigated.     

Effects of Spray and Turbulence Modelling on the Mixing and Combustion Characteristics of an n-heptane Spray Flame Simulated with Dynamic Adaptive Chemistry

Accurate modelling of spray combustion process is essential for efficiency improvement and emissions reduction in practical combustion engines. In this work, both unsteady Reynolds-averaged Navier-Stokes (URANS) simulations and large eddy simulations (LES) are performed to investigate the effects of spray and turbulence modelling on the mixing and combustion characteristics of an n-heptane spray flame in a constant volume chamber at realistic conditions. The non-reacting spray process is first simulated with URANS to investigate the effects of entrainment gas-jet model on the penetration characteristics and fuel vapor distributions. It is found that the droplet motion near the nozzle has significant influence on the fuel vapor distribution, while the liquid penetration length is controlled by the evaporation process and insensitive to gas-jet model. For the case considered, both URANS with the gas-jet model and large eddy simulations can properly predict the vapor penetration. For the combustion characteristics, it is found that LES yields better predictions in the global combustion characteristics. The URANS with gas jet model yields a comparable flame length and lift-off-length (LOL) to LES, but results in a larger ignition delay time compared to the experimental data. Another focus of this work is to qualify the convergence characteristics of the dynamic adaptive chemistry (DAC) method in these transient combustion simulations, where DAC is applied to reduce the mechanism locally and on-the-fly to accelerate chemistry calculations. The instantaneous flame structures and global combustion characteristics such as ignition delay time, flame lift-off length and emissions are compared between simulations with and without DAC. For URANS, good agreements are observed both on instantaneous flame structures and global characteristics. For LES, it is shown that the errors incurred by DAC are small for scatter distributions in composition space and global combustion characteristics, while they may significantly affect instantaneous flame structures in physical space. The study reveals that for DAC application in transient simulations, global or statistic information should be used to assess the accuracy, such as manifolds in composition space, conditional quantities and global combustion characteristics. For the cases investigated, a speed-up factor of more than two is achieved by DAC with a 92-species skeletal mechanism with less than 0.2 % and 3.0 % discrepancy in ignition delay and LOL, respectively.