甲基环己烷燃烧反应特性的光谱研究

利用激波管实验装置由反射激波点火,在点火温度1 164～1 566 K,点火压力1.03～1.99 atm,燃料浓度为1.0%,当量比为1.0的条件下,用光谱单色仪、光电倍增管、压力传感器和示波器等组成测试系统,测量了甲基环己烷燃烧过程中主要中间产物OH,CH和C2自由基特征光辐射随时间的连续变化,并测得了甲基环己烷/氧气/氩气的点火延迟时间。通过对测量结果的分析,初步认识了甲基环己烷燃烧反应中几个主要中间产物的光辐射特性及其反映出的甲基环己烷燃烧反应特性。实验所测点火延迟时间与已报道的实验结果和燃烧反应机理预测结果符合较好。本文实验结果为构建和验证甲基环己烷燃烧反应机理提供了实验依据。     

正癸烷燃烧反应中OH,CH和C2自由基的瞬态发射光谱

采用ICCD瞬态光谱测量系统和加热激波管,在点火压力2.0atm,点火温度1 100～1 600K,当量比1.0,燃料摩尔分数1.0%条件下,实时测得了正癸烷/氧气/氩气燃烧过程的瞬态发射光谱,光谱范围200～850nm。结果显示燃烧过程中主要发射光谱带归属于小分子中间产物OH,CH和C2自由基,光谱强度的变化反映了燃烧过程中三种自由基浓度的变化历程;正癸烷燃烧过程中光谱强度峰值之比大于同为链烷烃的正庚烷相应OH/CH峰强度之比,揭示出两种链烃燃烧反应机理有较大差异。实验还获得了正癸烷燃烧过程中能显示谱带转动结构的CH和C2高分辨特征发射光谱。实验结果对了解正癸烷燃烧性质和验证正癸烷燃烧反应机理很有意义。     

正庚烷燃烧反应中间自由基的光谱测量

采用ICCD瞬态光谱探测系统和化学激波管,在点火温度1 408K,点火压力2.0atm,燃料摩尔分数1.0%,当量比1.0的条件下,拍摄了正庚烷燃烧过程中不同时刻的瞬态发射光谱,光谱曝光时间6μs,拍谱范围200～850nm。确认了在所拍光谱范围内主要是OH,CH和C2自由基的特征辐射光谱,表明小自由基OH,CH和C2是正庚烷燃烧过程中重要的反应中间产物。所拍时间分辨光谱显示,在正庚烷燃烧反应中,OH,CH和C2自由基一出现很快就达到其浓度峰值,但CH和C2自由基随着反应的进行迅速减少至消失,OH自由基持续的时间却长很多。实验结果为了解正庚烷燃烧反应微观过程和验证其燃烧反应机理提供了实验依据。     

瞬态发射光谱法研究正十二烷燃烧反应特性

采用加热激波管和增强型CCD瞬态光谱测量系统,在波长范围200~900nm,点火压力4.0atm,点火温度(1 200~1 300)K,当量比0.5、1.0和2.0的条件下,实时测得了正十二烷/空气和正十二烷/氧气/氩气燃烧过程的瞬态发射光谱.结果表明:燃烧过程在此波段内的主要发射光谱带归属于反应中间产物OH、CH和C2自由基;在不同当量比条件下,燃烧过程中OH(306.4nm)/CH(431.4nm)/C2(516.4nm)的光谱强度显著不同,贫油情形有利于OH自由基生成,富油情形有利于C2自由基生成;浴气的不同会导致燃料燃烧温度的不同,从而引起燃料燃烧发射光谱的不同.所测燃烧反应自由基的时间分辩光谱直观反映出正十二烷燃烧过程中重要中间产物OH、CH和C2的变化情况.研究结果有助于认识正十二烷燃烧反应特性和验证其燃烧反应机理.     

发射光谱法研究甲基环己烷燃烧时激发态自由基时间历程

采用三组单色仪探测系统,测量了甲基环己烷在高温反射激波作用下瞬态燃烧反应过程中三种激发态自由基OH*,CH*和C*₂的特征光辐射,得到了激发态自由基时间历程和光辐射相对强度随温度的变化规律。反射激波温度1 200～1 700 K,激波压力1.5 atm,甲基环己烷摩尔分数0.1%,当量比1.0。在点火燃烧初始阶段三种自由基几乎同时产生,自由基持续时间随着温度的升高而变短。相同温度下CH*和OH*自由基持续时间大于C*₂自由基,在1 400 K以下C*₂自由基发光消失。OH*和CH*自由基发光强度在T<1 400 K时对温度变化不敏感,而在T>1 400 K时CH*自由基峰值随温度快速增长,C*₂和OH*峰值随温度增大比较平缓。将实验结果和化学反应机理模拟结果进行了对比,实验获得的OH*自由基时间历程在低温时和机理预测结果吻合较好,但在高温时有一定差异。CH*自由基时间历程在高温与机理结果吻合较好,在低温时机理预测结果CH*自由基持续时间要长于实验结果。实验测得的结果为含激发态物种化学反应动力学机理的验证和优化提供了依据。     

超声速预混可燃气流的点火与燃烧

在激波风洞一激波管组合设备上开展了碳氢燃料超声速预混可燃气流的点火与燃烧实验研究。实验结果表明:利用激波对燃料进行预热,并以高温燃气作为引导火焰,可以有效缩短汽油空气超声速可燃混气的点火延迟时间,使之缩短到 0.2 ms以下。利用纹影照片对超声速燃烧流场结构作出了分析;研究了超声速预混可燃气流的温度以及当量比对超声速燃烧流场结构、点火与火焰传播特性的影响。     

JP-10点火延时的激波管研究

在预加热激波管上测定了JP-10的点火延时时间．采用高精度真空仪直接测定注入激波管中JP-10蒸气压力，获得了JP-10气相浓度，解决了高碳数碳氢燃料点火延时激波管实验时管壁吸附影响燃料气相浓度确定的困难．采用压力传感器、单色仪和光电倍增管记录得到了完整的点火过程引起的压力变化和OH或CH自由基发射强度变化．自由基发射信号作为诊断点火发生的手段．当实验压力为151?556 kPa，温度为1000?2120 K，JP-10摩尔百分比为0.1%?0.55%，化学当量比为0.25、0.5、1.0、2.0时，获得了点火延时时间与实验温度、JP-10浓度、O2浓度的依赖关系，结果还表明，高温区和低温区呈现出不同的依赖关系．     

正庚烷裂解及乙醇的影响

论文研究乙醇对正庚烷氧化过程的影响.采用同步辐射方法测量当量比为1.0时正庚烷/氧气/氩气,以及正庚烷/乙醇/氧气/氩气低压层流预混火焰中主要成分的浓度.研究结果表明,添加乙醇改变了正庚烷向己烷裂解的路线,促进了庚烷直接裂解成丁烯基的趋向,但丁烷、戊烷都仍是正庚烷的主要裂解产物.添加乙醇,火焰中环已二烯、环戊二烯浓度增加,而且出现了乙烯醇、丁醇.研究结果可作为乙醇燃烧特性进一步研究的参考依据.     

激光消光法测量甲苯高温裂解的碳烟产率

建立了碳氢燃料在反射激波作用下高温裂解碳烟生成的检测系统,利用激光消光法测量了甲苯/氩气在高温条件下裂解生成碳烟的产率。实验条件：甲苯摩尔浓度0.25%和0.5%,压力约2和4 atm,温度1 630～2 273 K。获得了碳烟产率随温度、压力和燃料浓度的变化规律。碳烟产率随温度变化呈高斯分布,随着压力或浓度的增大,碳烟产率增大,碳烟产率最大达55%。产率的峰值温度随压力变化不大,但甲苯摩尔浓度从0.25%增大到0.5%时,峰值温度从1 852变为1 921 K。对比了压力为4 atm,燃料摩尔浓度为0.5%的甲基环己烷和甲苯的碳烟产率,甲基环己烷裂解碳烟产率峰值对应的温度为2 045 K,比甲苯约高135 K,但其最大碳烟产率仅有甲苯的1/8。结果为研究发动机内碳烟颗粒物排放及碳烟形成机理提供了实验依据。     

1-庚烯/空气的高压层流火焰传播研究

本文使用定容圆柱形燃烧弹,在初始温度373 K和初始压力1、2、5、10 atm的条件下,对当量比从0.7到1.5的1-庚烯/空气混合物的层流火焰传播进行了研究.利用记录的纹影图像处理得到层流火焰传播速度和马克斯坦长度.基于先前报道的1-己烯燃烧反应动力学模型,发展了1-庚烯的模型.该模型验证了本工作测量的1-庚烯层流火焰传播速度数据及文献中的1-庚烯着火延迟时间数据.通过开展敏感性分析和路径分析,帮助理解了1-庚烯在不同压力下的高温化学及其对层流火焰传播的影响.另外,比较了1-庚烯/空气和先前报道的正庚烷/空气的层流火焰传播.由于更强的放热性及反应活性,1-庚烯/空气的层流火焰传播速度在绝大多数条件下均快于正庚烷/空气的结果.     

Experimental study and modeling of shock tube ignition delay times for hydrogen-oxygen-argon mixtures at low temperatures

Recent literature has indicated that experimental shock tube ignition delay times for hydrogen combustion at low-temperature conditions may deviate significantly from those predicted by current detailed kinetic models. The source of this difference is uncertain. In the current study, the effects of shock tube facility-dependent gasdynamics and localized pre-ignition energy release are explored by measuring and simulating hydrogen-oxygen ignition delay times. Shock tube hydrogen-oxygen ignition delay time data were taken behind reflected shock waves at temperatures between 908 to 1118 K and pressures between 3.0 and 3.7 atm for two test mixtures: 4% H₂, 2% O₂, balance Ar, and 15% H₂, 18% O₂, balance Ar. The experimental ignition delay times at temperatures below 980 K are found to be shorter than those predicted by current mechanisms when the normal idealized constant volume (V) and internal energy (E) assumptions are employed. However, if non-ideal effects associated with facility performance and energy release are included in the modeling (using CHEMSHOCK, a new model which couples the experimental pressure trace with the constant V, E assumptions), the predicted ignition times more closely follow the experimental data. Applying the new CHEMSHOCK model to current experimental data allows refinement of the reaction rate for H + O₂ + Ar ↔ HO₂ + Ar, a key reaction in determining the hydrogen-oxygen ignition delay time in the low-temperature region.     

Application of an aerosol shock tube to the measurement of diesel ignition delay times

Shock tube ignition delay times were measured for DF-2 diesel/21% O₂/argon mixtures at pressures from 2.3 to 8.0 atm, equivalence ratios from 0.3 to 1.35, and temperatures from 900 to 1300 K using a new experimental flow facility, an aerosol shock tube. The aerosol shock tube combines conventional shock tube methodology with aerosol loading of fuel-oxidizer mixtures. Significant efforts have been made to ensure that the aerosol mixtures were spatially uniform, that the incident shock wave was well-behaved, and that the post-shock conditions and mixture fractions were accurately determined. The nebulizer-generated, narrow, micron-sized aerosol size distribution permitted rapid evaporation of the fuel mixture and enabled separation of the diesel fuel evaporation and diffusion processes that occurred behind the incident shock wave from the chemical ignition processes that occurred behind the higher temperature and pressure reflected shock wave. This rapid evaporation technique enables the study of a wide range of low-vapor-pressure practical fuels and fuel surrogates without the complication of fuel cracking that can occur with heated experimental facilities. These diesel ignition delay measurements extend the temperature and pressure range of earlier flow reactor studies, provide evidence for NTC behavior in diesel fuel ignition delay times at lower temperatures, and provide an accurate data base for the development and comparison of kinetic mechanisms for diesel fuel and surrogate mixtures. Representative comparisons with several single-component diesel surrogate models are also given.     

Ignition delay times of methyl oleate and methyl linoleate behind reflected shock waves

Ignition delay times for methyl oleate (C₁₉H₃₆O₂, CAS: 112-62-9) and methyl linoleate (C₁₉H₃₄O₂, CAS: 112-63-0) were measured for the first time behind reflected shock waves, using an aerosol shock tube. The aerosol shock tube enabled study of these very-low-vapor-pressure fuels by introducing a spatially-uniform fuel aerosol/4% oxygen/argon mixture into the shock tube and employing the incident shock wave to produce complete fuel evaporation, diffusion, and mixing. Reflected shock conditions covered temperatures from 1100 to 1400 K, pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.6 to 2.4. Ignition delay times for both fuels were found to be similar over a wide range of conditions. The most notable trend in the observed ignition delay times was that the pressure and equivalence ratio scaling were a strong function of temperature, and exhibited cross-over temperatures at which there was no sensitivity to either parameter. Data were also compared to the biodiesel kinetic mechanism of Westbrook et al. (2011) [10], which underpredicts ignition delay times by about 50%. Differences between experimental and computed ignition delay times were strongly related to existing errors and uncertainties in the thermochemistry of the large methyl ester species, and when these were corrected, the kinetic simulations agreed significantly better with the experimental measurements.     

Development of a reduced chemical kinetic mechanism for a gasoline surrogate for gasoline HCCI combustion

A reduced chemical kinetic mechanism consisting of 48 species and 67 reactions is developed and validated for a gasoline surrogate fuel. The surrogate fuel is modeled as a blend of iso-octane, n-heptane, and toluene. The mechanism reduction is performed using sensitivity analysis, investigation of species concentrations, and consideration of the main reaction path. Comparison between ignition delay times calculated using the proposed mechanism and those obtained from shock tube data show that the reduced mechanism can predict delay times with good accuracy at temperatures above 1000 K. The mechanism can also predict the two-stage ignition at the moment of ignition. A rapid compression machine (RCM) is designed to measure ignition delay times of gasoline and gasoline surrogates at temperatures between 890 and 1000 K. Our experimental results suggest that a new gasoline surrogate that has a different mixture ratio than previously defined surrogates is the most similar to gasoline. In addition, the reduced mechanism is validated for the RCM experimental conditions using CFD simulations.     

Ignition and combustion of isopropyl nitrate

The ignition delay times for mixtures of isopropyl nitrate (IPN) with air and argon are measured in a rapid-injection reactor at a pressure of 1 atm and in a shock tube at 2–3 atm. It is shown that the ignition delay time τ of mixtures in which heat is largely released due to oxidation by the oxygen contained in the IPN molecule is determined by the unimolecular decomposition of IPN over the entire temperature range covered (500–730 K). For mixtures in which heat is mainly produced by oxidation reactions involving air oxygen, the ignition delay time at high temperatures is controlled by secondary reactions of oxidation of the hydrocarbon moiety of the IPN molecule, leading to an increase in τ by more than an order of magnitude. Liquid IPN burns in a nitrogen atmosphere only at pressures above 40 atm, at a linear rate of ~4 mm/s. The measured flame temperatures are in close agreement with the respective values calculated using a thermodynamic code.     

A comprehensive experimental and improved kinetic modeling study on the pyrolysis and oxidation of propyne

To improve our understanding of the combustion characteristics of propyne, new experimental data for ignition delay times (IDTs), pyrolysis speciation profiles and flame speed measurements are presented in this study. IDTs for propyne ignition were obtained at equivalence ratios of 0.5, 1.0, and 2.0 in ‘air’ at pressures of 10 and 30 bar, over a wide range of temperatures (690–1460 K) using a rapid compression machine and a high-pressure shock tube. Moreover, experiments were performed in a single-pulse shock tube to study propyne pyrolysis at 2 bar pressure and in the temperature range 1000–1600 K. In addition, laminar flame speeds of propyne were studied at an unburned gas temperature of 373 K and at 1 and 2 bar for a range of equivalence ratios. A detailed chemical kinetic model is provided to describe the pyrolytic and combustion characteristics of propyne across this wide-ranging set of experimental data. This new mechanism shows significant improvements in the predictions for the IDTs, fuel pyrolysis and flame speeds for propyne compared to AramcoMech3.0. The improvement in fuel reactivity predictions in the new mechanism is due to the inclusion of the propyne + H?₂ reaction system along with ?H radical addition to the triple bonds of propyne and subsequent reactions.     

Autoignition of reacting mixtures at engine-relevant conditions using confined spherically expanding flames

Propagation of a confined spherically expanding flame induces isentropic compression that can culminate in autoignition and/or detonation under conducive thermodynamic conditions. This relatively simple technique measures a distinct ‘characteristic ignition delay time’ and complements other established approaches such as the rapid compression machine and shock tube. The present study details this methodology by examining the autoignition characteristics of dimethyl-ether/oxygen/nitrogen/helium reactive mixtures for equivalence ratios of 0.6 and 0.9, an initial temperature of 468 K, and initial pressures of 3 to 6 atm. The experimental results display the classic two-stage ignition typical of dimethyl-ether oxidation at low-temperatures with first-stage ignition occurring at approximately 3.6 times the initial pressure. To aid in the interpretation of the experimental results, two numerical models were used: a zero-dimensional batch reactor model, which accepts experimental pressure-time history and calculates the sensitivities of characteristic ignition delay times to kinetics, and a low Mach number, Lagrangian one-dimensional code that was developed to model both flame propagation and end-gas autoignition. Simulation results were shown to adequately capture the physics of unsteady flame propagation, end-gas autoignition, and the controlling reactions of the latter. It was found also that under certain conditions the behavior of first and second ignition stages could be modified by unsteady pressure effects.