冷起动工况柴油/煤油燃料喷雾燃烧特性研究

本文利用流动式定容燃烧弹结合米散射法、直拍法对低温冷起动工况下柴油/煤油混合燃料喷雾燃烧特性进行可视化测量。结果表明:柴油/煤油混合燃料液相喷雾贯穿距离随着煤油掺混比、温度和密度的增加呈现减小趋势。液相喷雾锥角随着煤油掺混比和环境密度的增加分别呈现减小和增大的趋势,而对环境温度的变化不敏感。不同环境温度、密度下,混合燃料滞燃期、火焰浮起长度、蓝色火焰占总体火焰强度比例随着煤油掺混比的增加呈现先减小后增加的变化趋势。此外,环境温度和密度的增加均使得着火滞燃期显著缩短。     

直喷柴油机火焰浮起长度的数值模拟研究

采用三维CFD模型模拟了直喷柴油机缸内喷雾燃烧过程,模拟缸压曲线得到了实验的验证.通过高温区与喷嘴之间的稳定距离来确定柴油机火焰浮起长度,研究在不同进气条件下火焰浮起长度的变化情况.该模型成功地预测了火焰浮起长度随着初始进气压力的增大而减少,随着进气温度的升高出现先增大后减少的趋势.同时模拟了在不同EGR率下柴油机缸内燃烧情况,发现火焰浮起长度和燃料着火延迟时间都随EGR率的增加而增大.     

燃料敏感性对内燃机部分预混燃烧(PPC)光谱特性的影响研究

在一台光学发动机上,利用火焰高速成像技术和自发光光谱分析法,研究了燃料敏感性(S)为0和6时对发动机缸内火焰发展和燃烧发光光谱的影响。试验过程中,通过改变喷油时刻 (SOI=-25,-15和-5°CA ATDC) 使燃烧模式从部分预混燃烧过渡到传统柴油燃烧模式。通过使用正庚烷、异辛烷、乙醇混合燃料来改变燃料敏感性。结果表明,在PPC模式下(-25°CA ATDC),火焰发展过程是从近壁面区域开始着火,而后向燃烧室中心发展,即存在类似火焰传播过程,同时在燃烧室下部未燃区域也形成新的着火自燃点。敏感性对燃烧相位影响较大,对缸内燃烧火焰发展历程影响较小;高敏感性燃料OH和CH带状光谱出现的时刻推迟,表明高敏感性燃料高温反应过程推迟,且光谱强度更低,表明碳烟辐射强度减弱。在PPC到CDC之间的过渡区域(-15°CA ATDC),燃烧火焰发光更亮,燃烧反应速率比-25°CA ATDC时刻的反应速率更快。高、低敏感性燃料对缸压放热率的影响规律与-25°CA ATDC相近,此时的燃烧反应更剧烈,放热率更高,碳烟出现时刻更早。该喷油时刻下的光谱强度高于PPC模式下的光谱强度,说明此时的CO氧化反应与碳烟辐射更强。在CDC模式下(-5°CA ATDC),由于使用的燃料活性较低,燃烧放热时刻过于推迟,放热量很小,缸内燃烧压力低,因此燃料敏感性对缸压和放热率的影响不明显,但从燃烧着火图像中可以看到高敏感性燃料的火焰出现时刻较低敏感性燃料推迟。低敏感性燃料的燃烧初期蓝色火焰首先出现在燃烧室中心,着火火焰出现时刻更早,之后蓝色火焰从中心向周围扩散,呈现火焰传播为主导的燃烧过程;燃烧后期,局部混合气过浓区导致亮黄色火焰面积逐渐增大并向周围扩散。高敏感性燃料的火焰发展趋势与低敏感性燃料类似,黄色火焰的亮度与面积更小。尽管高、低敏感性燃料的OH和CH带状光谱的出现时间相近,但高敏感性燃料的光谱强度仍更低。综合分析,火焰发展结构与自发光光谱特征主要受喷油时刻的影响,燃料的敏感性主要影响着火时刻和火焰自发光光谱强度,且高敏感性燃料的光谱强度更低。     

基于欧拉喷雾与火焰面/反应进度变量湍流燃烧模型的PRF燃料喷雾燃烧模拟研究

在开源计算流体软件OpenFOAM环境下,将基于欧拉方法的Σ-Y喷雾模型与非稳态火焰面/反应进度变量湍流燃烧模型相耦合,发展用于高温高压环境中液体燃料喷雾湍流燃烧高精度计算模型,分别对非燃烧和燃烧工况下的五种典型参比燃料(Primary Reference Fuel,PRF)的燃油喷射雾化与湍流燃烧过程开展数值研究。结果表明：该新型耦合模型能够准确的预测PRF燃料的喷雾和着火燃烧特性;所开发的重构数值喷雾纹影图像和燃烧OH^*图像处理方法能够很好地捕捉到试验的滞燃期、火焰浮起长度及喷雾火焰结构;研究揭示了不同比例的PRF燃料对喷雾及着火燃烧过程的影响特性,为替代燃料在发动机上的高效应用提供了理论指导。     

乙腈池火燃烧场分析与特征产物浓度的定量反演研究

乙腈广泛应用于医药、化工等领域,而乙腈属于易燃易爆化学品,其引发的火灾事故具有极大的危害。研究乙腈燃烧的温度场与浓度场、火焰辐射光谱以探究其火灾污染特性具有重要实用价值。首先采用平面激光诱导荧光技术(PLIF)与Fluent数值模拟方法,获取了5 cm尺度乙腈池火燃烧产物NO在20、 40、 60和80 s时刻的空间浓度值,并结合CFD与FDS仿真模拟获取了不同时刻下乙腈燃烧温度场与浓度场信息。其次,采用所获取的乙腈火焰温度场和浓度场数据(将火焰划分为6个热力学平衡区域),并基于HITRAN数据库内高温气体分子吸收系数与火焰总体辐射传输方程构建了乙腈火焰光谱辐射模型。再次,将所得乙腈浓度场与温度场数据代入火焰光谱辐射模型,模型模拟计算结果与相同条件下乙腈火焰光谱实测数据进行对比,以验证模型精度,然后再与Radcal模型进行精度对比。最后,利用自行构建的火焰光谱辐射模型对燃烧特征污染产物NO进行了浓度反演。结果表明：(1)5 cm尺度乙腈池火火焰温度范围为400～1 000 K,在池火上方60～80 mm区域温度较高,最高温度为945 K。(2)在20、 40、 60和80 s时刻下5 ...     

甲烷/氧气层流同轴射流扩散火焰OH*自由基的数值研究

OH*自由基是火焰中主要的激发态自由基之一,它所产生的化学发光可用于描述火焰的结构、拉伸率、氧燃当量比和热释放速率等特征信息,因此被广泛应用于火焰燃烧状态的在线诊断。以甲烷/氧气层流同轴射流扩散火焰作为研究对象,采用GRI-Mech 3.0机理结合OH*自由基生成和淬灭反应进行数值计算,对OH*自由基的二维分布特性进行研究,分析不同区域内OH*自由基的生成路径,并探讨不同氧燃当量比例和不同喷嘴出口尺寸对OH*自由基强度和分布特性的影响。模拟结果与实验研究基本吻合,表明计算模型能够准确描述火焰中OH*自由基的二维分布。结果表明：在甲烷/氧气层流同轴射流扩散火焰中,OH*自由基存在两种不同形态的分布区域,分别由反应CH+O₂=OH*+CO和H+O+M=OH*+M生成;随着氧燃当量比提高,OH*自由基的分布区域逐渐向火焰下游扩张,根据其分布形态的变化可以对火焰燃烧状况进行判断;如果OH*自由基仅分布于火焰的上游区域且呈断开形态,则说明火焰处于贫氧燃烧状态。如果OH*分布呈环状形态,则说明火焰处于富氧燃烧状态;相同氧气流量条件下,缩小喷嘴出口的环隙尺寸有助于加强燃料和氧气的化学反应程度,从而使火焰中OH*自由基的摩尔分数显著提高,增强OH*化学发光的辐射强度,提高火焰光谱诊断的准确性。     

碳氢扩散火焰实验研究：燃烧气氛对火焰光谱与温度的影响

辐射是各种燃烧过程中热传递的主要方式。在不同的火焰中,辐射光谱分布十分复杂。在这项工作中,利用光谱仪测量了可见光(200～900 nm),近红外(900～1 700 nm)和中红外(2 500～5 000 nm)波段火焰的光谱强度,分析了空气和富氧气氛下扩散火焰的光谱特征。并基于光谱分析,定量得到了火焰中碳烟以及气体发射的辐射力,计算了火焰的温度分布。结果表明,空气燃烧中的火焰温度低于富氧燃烧中的火焰温度。在空气气氛下,火焰中的碳烟和气体均对中的热辐射起着重要作用。而在富氧气氛下,气体对于火焰热辐射更为重要。在可见光和近红外波段,由于在空气气氛下火焰中碳烟的大量形成,光谱曲线显示出了良好连续性。而富氧气氛下火焰的辐射光谱降低。在中红外波段,空气气氛下火焰的气体辐射明显弱于富氧气氛下火焰的气体辐射。     

初始条件对喷雾火焰着火影响的大涡模拟研究

本文基于LES-LEM模拟方法开展了不同环境温度、压力条件下正庚烷喷雾火焰的大涡模拟研究。首先,通过与ECN喷雾燃烧基础数据库(Engine Combustion Network,ECN)中的实验结果进行对比,发现LE-LEM方法预测的着火延迟期和火焰浮升长度与实验值非常符合。因此,基于此分析了不同发动机工况下喷雾燃烧的着火和燃烧过程。结果显示高温火核首先出现在浓混合气区域,同时,喷雾火焰中同时存在预混和非预混燃烧。计算结果显示燃烧能够使喷雾火焰加速向下游发展,但高环境压力又能抑制喷雾火焰向下游运动。     

CS2燃烧火焰光谱辐射模型的构建与特征污染产物浓度反演研究

CS₂在当今化工等领域占据了重要地位,而CS₂火灾污染事故危害性极大。通过研究CS₂燃烧火焰光谱辐射以探究其火灾污染特性极为必要。搭建了CS₂燃烧火焰光谱测试平台,采用黑体辐射源对VSR仪器进行了标定,通过多用途傅里叶变换(VSR)红外光谱辐射仪测试了5,10和20 cm三种燃烧尺度下CS₂燃烧的火焰光谱,并通过热电偶测试了整个燃烧时间段内不同燃烧时刻下的火焰温度,以及在火焰上方安装了烟气分析仪对火焰中的燃烧产物浓度进行监测。测量了CS₂整个燃烧时间段内火焰温度,以及不同燃烧时间、不同燃烧尺度下的火焰光谱、燃烧产物组分信息。测试结果表明,CS₂火焰中主要含有高温SO₂,CO₂,CO气体和空气中卷入的H₂O分子,并获取了特征污染产物SO₂的浓度。由于现有光谱仪测量分辨率有限,室内实验测量的火焰尺度有限,为了能实现火灾在线监测需要建立一个火焰光谱辐射模型来反演CS₂火灾时的污染物浓度相关信息。基于HITRAN数据库可知在2.7 μm附近为高温水蒸气的发射峰,4.2 μm附近特征峰为高温CO₂气体的发射峰,4.7 μm附近有CO微弱的发射峰,在7.4 μm附近特征峰为高温SO₂气体的发射峰,并获得了CS₂燃烧时产生的SO₂,CO₂,CO和H₂O气体在火焰燃烧相同温度下的吸收系数,通过计算得到了CS₂燃烧时产生的SO₂,CO₂,CO和H₂O混合气体的透过率与发射率,并结合气体辐射传输方程、气体吸收系数等方程,创建了CS₂燃烧的火焰光谱辐射模型。利用该光谱辐射模型反演了不同燃烧时间下特征污染产物SO₂的浓度,并与实验测得的数据进行了对比分析。结果表明,该模型精度高,可用于燃烧产物浓度的定量化反演,SO₂分子含量在燃烧时间20,40,60和80 s时的反演精度分别是89.5%,82.5%,85.6%和86.5%。为遥感反演CS₂型大尺度火灾中燃烧产物的浓度奠定基础。     

柴油掺混正丁醇对火焰发展及光谱的影响

正丁醇是一种很有前景的柴油替代燃料,针对缸内火焰发展和燃烧中间产物的自发光光谱开展研究,有助于深入理解柴油掺混正丁醇混合燃料对柴油机燃烧过程的影响规律。因此,在一台光学发动机上,利用火焰高速成像技术和自发光光谱分析法,研究纯柴油与柴油掺混不同比例正丁醇后对发动机缸内火焰发展和自发光光谱的影响。试验过程中,光学发动机转速为1 200 r·min~(-1),喷油压力为600 bar,进气加热到398 K,使上止点附近达到约900 K温度。纯柴油、柴油掺混20%正丁醇燃料和柴油掺混40%正丁醇燃料分别用D100, DB20和DB40表示,三种燃料在每个着火循环喷入的油量分别为17.5, 18.7和19.2 mg,从而保证发动机输出功相同。试验结果表明:冷却水温不变时,喷油时刻推迟,滞燃期缩短,初始火核形成时刻推迟,蓝色预混火焰比例减小;喷油时刻不变时,提高冷却水温度,滞燃期缩短,初始火核形成时刻提前,蓝色预混火焰比例减小。随着正丁醇掺混比例增加,呈现局部混合气率先着火的特征且着火时刻推迟,蓝色预混火焰比例增加,火焰亮度降低,火焰亮度从大到小依次为:D100DB20DB40。D100燃料随喷油推迟,整体光谱的峰值向长波方向移动,碳烟辐射增强, OH谱带的光强峰值先增大后减小, OH和CH_2O谱带出现的时刻推迟,表明高温和低温反应时刻推迟;喷油时刻不变时,提高冷却水温,整体光谱的光强增加, OH和CH_2O谱带的出现时刻提前,表明高温和低温反应时刻提前。掺混正丁醇后的DB40燃料随喷油推迟,光谱的整体光强增加, OH和CH_2O谱带的光强峰值提高,表明推迟喷油对DB40燃料也是有助于促进高温和低温反应。DB40燃料光谱的整体光强低于D100燃料,其OH和CH_2O的谱带出现的时刻迟于D100燃料,表明掺混正丁醇后燃料的高温和低温反应时刻都相对D100燃料推迟。SOI-15、冷却水温95℃工况下, D100燃料的谱线经过2℃A就呈现出了类似碳烟黑体辐射谱的特征,而DB40燃料先呈现出CO氧化连续谱的特征,经过15℃A才呈现碳烟黑体辐射谱的特征。     

Development and validation of an n-dodecane skeletal mechanism for spray combustion applications

n-Dodecane is a promising surrogate fuel for diesel engine study because its physicochemical properties are similar to those of the practical diesel fuels. In the present study, a skeletal mechanism for n-dodecane with 105 species and 420 reactions was developed for spray combustion simulations. The reduction starts from the most recent detailed mechanism for n-alkanes consisting of 2755 species and 11,173 reactions developed by the Lawrence Livermore National Laboratory. An algorithm combining direct relation graph with expert knowledge (DRGX) and sensitivity analysis was employed for the present skeletal reduction. The skeletal mechanism was first extensively validated in 0-D and 1-D combustion systems, including auto-ignition, jet stirred reactor (JSR), laminar premixed flame and counter flow diffusion flame. Then it was coupled with well-established spray models and further validated in 3-D turbulent spray combustion simulations under engine-like conditions. These simulations were compared with the recent experiments with n-dodecane as a surrogate for diesel fuels. It can be seen that combustion characteristics such as ignition delay and flame lift-off length were well captured by the skeletal mechanism, particularly under conditions with high ambient temperatures. Simulations also captured the transient flame development phenomenon fairly well. The results further show that ignition delay may not be the only factor controlling the stabilisation of the present flames since a good match in ignition delay does not necessarily result in improved flame lift-off length prediction.     

Modeling of flame lift-off length in diesel low-temperature combustion with multi-dimensional CFD based on the flame surface density and extinction concept

Low-Temperature Combustion (LTC) is becoming a promising technology for simultaneously reducing soot and NOx emissions from diesel engines. LTC regimes are evaluated by the flame lift-off length – the distance from the injector orifice to the location of hydroxyl luminescence closest to the injector in the flame jet. Various works have been dedicated to successful simulations of lifted flames of a diesel jet by use of various combustion modeling approaches. In this work, flame surface density and flamelet concepts were used to model the diesel lift-off length under LTC conditions. Numerical studies have been performed with the ECFM3Z model, n-Heptane and diesel fuels to determine the flame lift-off length and its correlation with soot formation under quiescent conditions. The numerical results showed good agreement with experimental data, which were obtained from an optically accessible constant volume chamber and presented at the Engine Combustion Network (ECN) of Sandia National Laboratories. It was shown that at a certain distance downstream from the injector orifice, stoichiometric scalar dissipation rate matched the extinction scalar dissipation rate. This computed extinction scalar dissipation rate correlated well with the flame lift-off length. For the range of conditions investigated, adequate quantitative agreement was obtained with the experimental measurements of lift-off length under various ambient gas O₂ concentrations, ambient gas temperatures, ambient gas densities and fuel injection pressures. The results showed that the computed lift-off length values for most of the conditions lay in a reasonable range within the quasi-steady lift-off length values obtained from experiments. However, at ambient temperatures lower than 1000 K, the lift-off length values were under-predicted by the numerical analysis. This may be due to the use of the droplet evaporation model as it is believed that evaporation has a strong effect on the lift-off length.     

甲烷/富氧扩散火焰燃烧区域的分层特性研究

本文对甲烷／富氧扩散火焰燃烧区域的分层特性进行了数值模拟和实验研究,结果表明氧化剂中氧浓度的增加加剧了火焰的分层现象(黄焰层与蓝焰层),使蓝色火焰变厚,并且使NOx生成大量增加;火焰面上的速度梯度主要影响黄色火焰厚度,蓝色火焰随着速度梯度的增加而减小, NOx生成也随之较少。对比温度及火焰结构还表明,研究中所采用的数值模拟方法可以正确地预测对向流扩散火焰特性。     

Simultaneous rainbow schlieren deflectometry and OH* chemiluminescence imaging of a diesel spray flame in constant pressure flow rig

The physical and chemical phenomena that take place during fuel injection, entrainment and fuel-air mixing, cool-flame and ignition reaction, and combustion in diesel sprays still require extensive study. Global parameters such as liquid and vapor jet penetration lengths and spreading rates render useful yet still limited information. Understanding of the temporal evolution of the spray as it progresses through various steps is needed to develop advanced clean combustion modes and high-fidelity predictive models with sufficient accuracy. In this study, high-speed rainbow schlieren deflectometry (RSD) and OH* chemiluminescence are used to simultaneously image fuel-air mixing, cool-flame reactions, ignition, flame propagation and stabilization, and combustion in a transient diesel-like flame. A constant pressure flow rig (CPFR) is used to conduct multiple injections in quick succession to obtain a statistically relevant dataset. n-heptane was injected at nominal supply pressure of 1000 bar from a single-hole diesel injector into ambient at pressure of 30 bar and temperature of 800 K. About 500 injections were performed and analyzed to reveal structural features of non-reacting and reacting regions of the spray, quantify jet penetration and spreading rates, and study cool-flame behavior, ignition, flame propagation and stabilization at lift-off length, and combustion at upstream and downstream locations.     

Temperature measurement of a premixed radially symmetric methane flame jet using the Mach-Zehnder Interferometry

The temperature field of a premixed methane symmetric laminar flame jet is visualized by studying the interferograms of the flame, using the Mach-Zehnder Interferometry. Two kinds of oxidizers are chosen for combustion: industrially pure oxygen and oxygen-enriched air. The flame is chosen to be both lean, and rich. For the lean oxygen-enriched combustion (OEC), the equivalence ratio was held constant at 0.5, and the oxygen enrichment was adjusted to 0.5 and 0.6, and for rich OEC, equivalence ratio is chosen to be 1.2 while the oxygen enrichment was 0.7 and 0.8. For methane/oxygen combustion, the equivalence ratio varied from 0.35 to 0.55 for the lean flame, and 1.3 and 1.7 for the rich flame. Attempt was made to keep the Reynolds number unchanged at 500, for OEC, and 1000, for methane/oxygen flame. In the present study a non-contact method is successfully developed to measure the temperature field of a premixed radially symmetric laminar methane flame jet. The effect of oxygen enrichment and equivalence ratio on temperature field is also investigated and depicted.     

Experimental characterization of spark ignited ammonia combustion under elevated oxygen concentrations

Due to its nature as a carbon free fuel and carrying hydrogen energy ammonia has received a lot of attention recently to be used as an alternative to fossil fuel in gas turbine and internal combustion engines. However, several barriers such as long ignition delay, slow flame speed, and low reactivity need to be overcome before its practical applications in engines. One potential approach to improve the ignition can be achieved by using oxygen enriched combustion. In this study, oxygen-enriched combustion of ammonia is tested in a constant volume combustion chamber to understand its combustion characteristics like flame velocity and heat release rates. With the help of high speed Schlieren imaging, an ammonia-oxygen flame is studied inside the combustion chamber. The influence of a wide range of oxygen concentrations from 15 to 40% are tested along with equivalence ratios ranging from 0.9 to 1.15. Ammonia when ignited at an oxygen concentration of 40% with an equivalence ratio of $\varphi =$ 1.1 at 10 bar has a maximum flame velocity of 112.7 cm/s. Reduced oxygen concentration also negatively affects the flame velocity, introducing instabilities and causing the flame to develop asymmetrically due to buoyancy effects inside the combustion chamber. Heat release rate (HRR) curves show that increasing the oxygen concentration from 21 to 35% of the mixture can help reduce the ignition delays. Peak HRR data shows increased sensitivity to air fuel ratios with increased oxygen concentrations in the ambient gas. HRR also shows an overall positive dependence on the oxygen concentration in the ambient gas.     

气化炉内柴油火焰光谱辐射特性及CH*二维辐射特性研究

火焰的自发辐射光谱与火焰的结构、温度分布等燃烧特征参数密切相关。对激发态自由基辐射的辐射强度与二维分布进行研究,可清晰地反映火焰燃烧状态而不对火焰产生扰动。基于多喷嘴对置式气流床气化实验平台,利用光纤光谱仪和配置CCD相机的高温内窥镜,对柴油扩散火焰的辐射光谱及CH*辐射二维分布特性进行研究。考察了当量比和撞击作用对火焰辐射光谱和CH*辐射分布的影响。结果表明,柴油火焰在306.47及309.12 nm处存在OH*辐射特征峰,在431.42 nm处存在CH*辐射特征峰,且存在明显的碱金属原子Na*(589.45 nm),K*(766.91和770.06 nm)发射光谱。此外,由于柴油不完全燃烧生成大量碳黑,在辐射光谱的可见光波段产生了强烈的连续黑体辐射。火焰中的黑体辐射对CH*辐射特征峰的检测存在干扰,且当量比越低时背景辐射越强,对自由基特征峰检测干扰越大。基于普朗克定律利用插值法可扣除430 nm附近波段背景辐射。柴油火焰中CH*辐射峰值随当量比的增加单调减小,CH*辐射等值线沿火焰发展方向依次出现三峰状、双峰状及单峰状,最终收缩为以反应核心区为中心的圆核。随着当量比的提高,出现各个形状的CH*辐射强度阈值不断降低,火焰主反应区面积减小且向下游移动,当量比增加到1.0附近时,理论上柴油完全燃烧,CH*辐射强度显著降低,贫燃火焰的CH*辐射强度及分布区域几乎稳定不变。利用CH*辐射强度值判定火焰举升长度,对于单喷嘴射流火焰,火焰举升长度随当量比的增加经历了显著增加后小幅下降的过程。相同当量比时两喷嘴撞击火焰CH*辐射强度峰值始终高于单喷嘴射流火焰对应值;火焰举升长度随当量比的增加小幅增加。火焰撞击的约束作用使得火焰举升长度不易随着当量比变化发生较大波动,燃烧更加稳定。这为定量判断火焰燃烧状态提供了一种直观、有效的方法,同时为柴油燃烧的化学动力学研究提供了实验依据。