基于快速压缩机的超级爆震研究

使用高速摄影可视化方法研究了快速压缩机内异辛烷空气混合气在压缩终点点火后的燃烧及爆震过程,分析了不同末端混合气自燃模式与发动机内的爆震形式之间的关系,研究了点火位置和热力条件对末端混合气燃烧模式的影响。结果明确了发动机的超级爆震是缸内产生爆轰的结果,点火位置对末端混合气燃烧模式的影响较弱,但混合气的能量密度对燃烧模式有重要影响。     

机油液滴诱发早燃的快速压缩机试验研究

早燃是在火花点火内燃机中的一种非正常燃烧现象,容易诱发超级爆震并破坏发动机。本文在快速压缩机中引入机油液滴,研究了机油液滴对异辛烷可燃混合气着火特性的影响,同步采集了压力数据和燃烧图像。试验结果表明,高温高压异辛烷可燃混合气中的机油液滴能够自燃并点燃周围混合气,其火焰传播特征与火花点火接近。采用3种市场常用发动机机油引入燃烧室均出现了自燃现象,提高压缩终点压力会使机油液滴的着火延迟时间缩短。     

火花点燃式发动机内燃烧不稳定性研究

为了发展高效、清洁的汽车发动机,需要进一步增大缸内压缩比。而压缩比的提高,使得敲缸现象无法规避。对于火花点火发动机,敲缸机理解释普遍接受的观点是末端混气自燃,但这种自燃如何能产生敲缸的机理尚不十分明确。为此,本研究以形成敲缸现象的最基本要素,即中心火花引燃的火焰面、末端壁面高压高温自燃区、两侧可反射压力波的壁面为物理模型,并基于详细化学反应机理和一维可压缩反应流模型,分别在热壁提前点火和末端混合气自燃模式下模拟得到氧燃料火花点燃式发动机的敲缸现象。本文分析了两种不同模式产生初始压力波的不同物理机制,并发现导致初始压力波被放大继而产生强敲缸现象的压力波和火焰的耦合作用机理。这样的耦合作用,使得火焰传播由扩散控制,转变为在敲缸发生时由扩散和压力波共同控制。     

H_2/Air旋转爆震发动机起爆实验研究

为揭示旋转爆震发动机的点火特性,采用普通火花塞和高能火花塞作为发动机点火装置,对以氢气/空气为反应物的旋转爆震发动机进行了实验研究,结合高频压力测量与高速摄影结果分析了旋转爆震波的建立过程,并通过一系列点火实验得到了发动机的稳定工作范围。研究结果表明,两种点火方式均能成功起爆发动机,点火产生的燃烧波通过火焰加速与DDT过程形成旋转爆震波,增大点火能量能够大幅缩短旋转爆震波建立时间。发动机共有三种工作模式,稳定工作范围随燃料质量流量的增加而扩大,且不同工作模式会随反应物当量比的变化而相互转换。     

火焰加速产生压力锋面机理分析

基于定容燃烧弹研究预混层流火焰穿过孔板之后的加速过程。通过纹影法,观测到火焰之前压力锋面形成过程。分析火焰传播速率、火焰前锋结构变化、火焰加速对末端气体的影响以及从计算流体力学的角度分析了压力锋面的形成过程。结果表明,层流火焰穿过孔板会明显加速,火焰锋面之前形成清晰的压力锋面,这导致末端气体温度、压力升高,发生自燃,为小型化汽油机爆震研究提供借鉴。     

煤油氧气脉冲爆震火箭发动机爆震特性

脉冲爆震火箭发动机(PDRE)是一种利用脉冲式爆震波产生高温、高压燃气发出的冲量来产生推力的推进系统.与常规液体火箭发动机相比,脉冲爆震火箭发动机具有更高的性能,并且结构更简单.本文以航空煤油为燃料、氧气为氧化剂、压缩氮气为隔离气体,并利用电磁阀控制燃料、氧化剂和隔离气体的间歇式供给.利用低的点火能量(50mJ),在内径50mm,长度1.1m的爆震管内进行了大量的多循环爆震试验,研究煤油氧气电磁阀脉冲爆震火箭发动机的爆震波特性.研究结果为进一步研究气液两相多次爆震燃烧机理提供了依据,为研制工程应用的PDRE提供理论和实践基础.     

点火能对气体爆炸过程中薄壁管道应变规律的实验研究（英文）北大核心CSCD

点火能量是管道内可燃气体爆炸的重要影响因素。实验采取末端闭口的管道,利用传感器以及数据采集系统测定各测点处的压力及压力波作用对管壁的动态响应,研究不同点火能作用下管道内甲烷-空气预混气体的火焰传播特性,并对管壁的动态应变进行了初步分析。结果表明,点火能量越大,爆炸反应程度越剧烈,管道内最大爆炸压力就越大,管壁的最大动态应变也越大,并且动态应变信号和压力波信号呈现较好的一致性。研究结果为预防管道内可燃气体爆炸事故提供了理论基础。     

点火能对气体爆炸过程中薄壁管道应变规律的实验研究（英文）

点火能量是管道内可燃气体爆炸的重要影响因素。实验采取末端闭口的管道,利用传感器以及数据采集系统测定各测点处的压力及压力波作用对管壁的动态响应,研究不同点火能作用下管道内甲烷-空气预混气体的火焰传播特性,并对管壁的动态应变进行了初步分析。结果表明,点火能量越大,爆炸反应程度越剧烈,管道内最大爆炸压力就越大,管壁的最大动态应变也越大,并且动态应变信号和压力波信号呈现较好的一致性。研究结果为预防管道内可燃气体爆炸事故提供了理论基础。     

一种高频脉冲爆震火箭发动机排气及推力研究

为研究一种高频脉冲爆震火箭发动机(Pulse Detonation Rocket Engine,简称PDRE)的排气及推力特性,开展了相关实验和数值模拟研究。利用高速摄影拍摄了PDRE高频工作下的排气阴影图和尾焰图。构建了PDRE单次工作过程的数值模型,得到了发动机的排气过程和推力特性。结果表明:PDRE在靠近出口端点火,形成向封闭端传播的爆震波。回爆波向出口传播,形成第一次排气;在封闭端反射的压力波传出爆震管,形成第二次排气;两次排气贡献的冲量大小相近。     

燃烧过程测量、模拟及爆震燃烧技术应用专题序言

爆震燃烧是基本的燃烧模态之一具有极高的强度和极快的燃烧速率以其构建热力循环并将其应用于先进动力装置具有区别于爆燃燃烧过程的显著优势是人们始终追求的梦幻动力.然而也恰恰由于爆震燃烧过程剧烈因此在受限空间内形成稳定、可控、高频率的爆震波成为挑战.在工程上发展稳定的爆震燃烧动力装置需要解决可靠点火起爆、燃料/氧化剂快速掺混、爆震波稳定传播等关键技术难题.近些年连续旋转爆震取得了长足的进展.连续旋转爆震仅需单次成功点火在环形或者圆柱形燃烧室内形成周向传播的爆震波从而使得不断充入燃烧室的未燃混合气快速起爆.基于连续旋转爆震的火箭发动机、冲压发动机甚至连续旋转爆震涡轮发动机都取得了创造性的成绩.尽管在工程化应用的征途上仍然需要解决诸多爆震物理基础问题和关键技术但人们孜孜以求的决心和动力从未泯灭也必将带来颠覆性技术的诞生.     

Effects of pre-spark heat release on engine knock limit

It is well known that spark ignited engine efficiency is limited by end gas autoignition, commonly known as knock. This study focuses on a recently discovered phenomena, pre-spark heat release (PSHR) due to low-temperature chemistry, and its impact on knock behavior. Boosted operating conditions are more common as engines are downsizing and downspeeding in efforts to increase fuel economy and prone to PSHR. Experiments were prone at fixed fueling and air fuel ratio for a range of intake temperature that spanned the threshold for PSHR. It was found that when PSHR occurred, the knock-limited combustion phasing was insensitive to intake temperature; higher intake temperatures did not require retarded timings as it is usual. Inspection of the temperature–pressure history overlaid on ignition delay contours allow the results to be explained. The temperature rise from the low-temperature reactions moves the end gas state into the negative temperature coefficient (NTC) region, which terminates the heat release reactions. The end gas then resides in the long ignition delay peninsula, which inhibits knock.     

Autoignition and detonation development from a hot spot inside a closed chamber: Effects of end wall reflection

The advancement of highly boosted internal combustion engines (ICEs) with high thermal efficiency is mainly constrained by knock and super-knock, respectively, due to the end gas autoignition and detonation development. The pressure wave propagation and reflection in a small confined space may strongly interact with local end gas autoignition, leading to combustion characteristics different from those in a large chamber or open space. The present study investigates the transient autoignition process in an iso-octane/air mixture inside a closed chamber under engine-relevant conditions. The emphasis is given to the assessment of effects of the pressure wave-wall reflection and the mechanism of extremely strong pressure oscillation typical for super-knock. It is found that the hot spot induced autoignition in a closed chamber can be greatly affected by shock/pressure wave reflection from the end wall. Different autoignition modes respectively from the hot spot and the end wall reflection are identified. A non-dimensional parameter quantifying the interplay between different length and time scales is introduced, which helps to identify different autoignition regimes including detonation development near the end wall. It is shown that detonation development from the hot spot may cause super-knock with devastating pressure oscillation. However, the detonation development from the end wall can hardly produce pressure oscillation strong enough for the super-knock. The obtained results provide a fundamental insight into the knocking mechanism in engines under highly boosted conditions.     

Investigation of the end-gas autoignition process in natural gas engines and evaluation of the methane number index

Engine knock and misfire are barriers to pathways leading to high-efficiency Spark-Ignited (SI) Natural Gas (NG) engines. The general tendency to knock is highly dependent on engine operating conditions and the fuel reactivity. The problem is further complicated by the wide range of chemical reactivity in pipeline quality NG, represented by the Methane Number (MN) (65< MN<95). Understanding the underlying phenomena responsible for engine knock can support the development of predictive tools capable of identifying knock onset/intensity as well as a fuel's propensity to knock, allowing engine manufacturers to expand the knock envelope and design more efficient/robust SI NG engines. Additionally, there is an opportunity for increased efficiency by controlling levels of end-gas autoignition if this can be predicted and controlled. This work focuses on the development of a novel methodology to understand/predict a fuel's propensity to knock. This methodology is based on the charge fraction undergoing autoignition, namely fractional end-gas autoignition (F-EGAI), and was developed based on first order laminar flame speeds and ignition delay analysis combined with a 0-D homogeneous batch reactor model. This methodology proved to be suitable to predict a fuel's propensity to knock, even under conditions when light knock was observed. The simple modeling approach was used to explain the results from a series of MN tests with multiple NG compositions exhibiting a wide range of reactivity compositions and providing insight on why fuels of very different chemical compositions can have the same MN. Lastly, a CFD model was developed was used to confirm the methodology capability and provide further insights in the physical and chemical phenomena behind end gas autoignition.     

Effects of NOx addition on autoignition and detonation development in DME/air under engine-relevant conditions

Exhaust gas recirculation (EGR) technology can be used in internal combustion engines to reduce NOx emission and improve fuel economy. However, it also affects the end-gas autoignition and engine knock since NOx in EGR can promote ignition. In this study, effects of NOx addition on autoignition and detonation development in dimethyl ether (DME)/air mixture under engine-relevant conditions are investigated. Numerical simulation considering both low-temperature and high-temperature chemistry is conducted. First the kinetic effects of NOx addition on the negative temperature coefficient (NTC) regime are assessed and interpreted. It is found that NOx addition greatly promotes both low-temperature and high-temperature ignition stages mainly through increasing OH production. Then the autoignitive reaction front propagation induced by either local NO accumulation or a cold spot within NTC regime with different amounts of NO addition is investigated. For the first time, supersonic autoignition modes including detonation induced by local NO accumulations are identified. This indicates that local accumulation of NOx in end gas might induce super-knock in engines with EGR. A new parameter quantifying the ratio of sound speed to average reaction front propagation speed is introduced to identify the regimes for different autoignition modes. Compared to the traditional counterpart parameter used in previous studies, this new parameter is more suitable since it yields a detonation development regime in a C-shaped curve which is almost unaffected by the initial conditions. The results in this study may provide fundamental insights into knocking mechanism in engines using EGR technology.     

Influence of NOx chemistry on the prediction of natural gas end-gas autoignition in CFD engine simulations

Natural gas (NG) represents a promising low-cost/low-emission alternative to diesel fuel when used in high-efficiency internal combustion engines. Advanced combustion strategies utilizing high EGR rates and controlled end-gas autoignition can be implemented with NG to achieve diesel-like efficiencies; however, to support the design of these next-generation NG ICEs, computational tools, including single- and multi-dimensional simulation packages will need to account for the complex chemistry that can occur between the reactive species found in EGR (including NOx) and the fuel. Research has shown that NOx plays an important role in the promotion/inhibition of large hydrocarbon autoignition and when accounted for in CFD engine simulations, can significantly improve the prediction of end-gas autoignition for these fuels. However, reduced NOx-enabled NG mechanisms for use in CFD engine simulations are lacking, and as a result, the influence of NOx chemistry on NG engine operation remains unknown. Here, we analyze the effects of NOx chemistry on the prediction of NG/oxidizer/EGR autoignition and generate a reduced mechanism of a suitable size to be used in engine simulations. Results indicate that NG ignition is sensitive to NOx chemistry, where it was observed that the addition of EGR, which included NOx, promoted NG autoignition. The modified mechanism captured well all trends and closely matched experimentally measured ignition delay times for a wide range of EGR rates and NG compositions. The importance of C2-C3 chemistry is noted, especially for wet NG compositions containing high fractions of ethane and propane. Finally, when utilized in CFD simulations of a Cooperative Fuels Research (CFR) engine, the new reduced mechanism was able to predict the knock onset crank angle (KOCA) to within one crank angle degree of experimental data, a significant improvement compared to previous simulations without NOx chemistry.     

燃料射流与空气协流混合中的自点火预测研究

预混燃烧室燃料与空气混合过程中出现的自点火会引起回火与挂火,烧毁燃料喷嘴。针对这一问题,利用实验台模拟贫燃燃烧室预混过程,燃料射流与预热后的空气协流同向喷入石英管预混段中,研究自点火现象。本文结合机器学习和物理规律分析,开展湍流混合过程的自点火预测研究。基于二元逻辑回归建立了机器学习模型,模型的特征由分析影响自点火的物理规律得到,训练和校验模型所需的数据由燃料射流-空气协流的自点火实验获得。结果显示,机器学习方法能快速、准确地预测混合过程中自点火的发生和火焰类型,并揭示其关键影响因素。与传统的数值计算方法相比,机器学习方法预测自点火所需的时间仅为传统数值模拟方法的几千分之一。     

Detonation development in PRF/air mixtures under engine-relevant conditions

The development of advanced boosted internal combustion engines (ICEs) is constrained by super-knock which is closely associated with end gas autoignition and detonation development. The present study numerically investigates the transient autoignition and detonation development processes under engine-relevant conditions for primary reference fuel (PRF) consisting of n-heptane and isooctane. The effects of PRF composition are systematically examined. By considering the transient local sound speed rather than its initial value, a new non-dimensional parameter is proposed to assess the transient chemical-acoustic interaction and to quantify the autoignition modes. Two detonation sub-modes, normal and over-driven detonation, are identified and the corresponding mechanisms are interpreted. For the over-driven detonation, there exist two developing regimes with weak/strong chemical-acoustic coupling and slow/rapid pressure enhancement. It is found that the maximum pressure caused by autoignition decreases with the blending ratio of isooctane, mainly due to the increase in excitation time. Besides, the strongest detonation induced by hot spot usually occurs within the over-driven detonation sub-regime. Its condition can be well quantified by the new non-dimensional parameter proposed in work and its strength is determined by the ratio of hot spot acoustic time to excitation time. The deviation of transient autoignition front propagation from prediction based on homogenous ignition is mainly attributed to the non-uniform compression effect caused by gradually enhanced pressure wave, while the influence of heat conduction and mass diffusion is negligible. The initial expansion stage dominating the induction period of local autoignition is greatly influenced by the compression of pressure wave. Therefore, the continuously enhanced pressure wave non-uniformly changes the local ignition delay (i.e. reduces its spatial gradient) within the hot spot and thereby accelerates the autoignition front propagation. The relationship among the parameters quantifying the detonation propensity is assessed and interpreted. The present study provides helpful understanding of detonation development under engine conditions.     

Effects of temperature and equivalence ratio on the ignition of n-heptane fuel spray in turbulent flow

Direct numerical simulations were performed to study the autoignition process of n-heptane fuel spray in a turbulent field. For the solution of the carrier gas fluid, the Eulerian method is employed, while for the fuel droplets, the Lagrangian method is used. Droplets are initialized at random locations in a two-dimensional isotropic turbulent field. A chemistry mechanism for n-heptane with 44 species and 112 reactions was adopted to describe the chemical reactions. Three cases with the same initial global equivalence ratio (0.5) and different initial gas phase temperatures (1100, 1200, and 1300 K) were simulated. In addition, two cases with initial global equivalence ratios of 1.0 and 1.5 and initial temperature 1300 K were simulated to examine the effect of equivalence ratio. Evolution of temperature, species mass fraction, reaction rate, and the joint PDF of temperature and equivalence ratio are presented. Effects of the initial gas temperature and equivalence ratio on vaporization and ignition are discussed. A correlation was derived relating ignition delay times to temperature and equivalence ratio. It was confirmed that with the increase of initial temperature, the autoignition occurs earlier. With the increase of the initial equivalence ratio, however, autoignition occurs later due to a larger decrease in gas phase temperature caused by fuel droplet evaporation. The results obtained in this study are expected to be constructive in understanding fuel spray combustion, such as that in homogeneous charge compression ignition systems.     

Detonation development in hydrogen/air mixtures inside a closed chamber: role of a cold wall

Detonation development from a hot spot has been extensively studied, where ignition occurs earlier than that in the surrounding mixtures. It has also been reported that a cool spot can induce detonation for large hydrocarbon fuels with Negative Temperature Coefficient (NTC) behavior, since ignition could happen earlier at lower temperatures. In this work we find that even for hydrogen/air mixtures without NTC behaviors, a cold wall can still initiate and promote detonation. End-wall reflection of the pressure wave and wall heat loss introduce an exothermic center outside the boundary layer, and then autoignitive reaction fronts on both sides may evolve into detonation waves. The right branch can be further strengthened by appropriate temperature gradient near the cold wall, and exhibits different dynamics at various initial conditions. The small excitation time and the large diffusivity of hydrogen provide the possibility for detonation development within the limited space between the autoignition kernel and the cold wall. Moreover, detonation may also develop near the flame front, which may or may not co-exist with detonation waves from the cold wall. Correspondingly, wall heat flux evolution exhibits different responses to detailed dynamic structures. Finally, we propose a regime diagram describing different combustion modes including normal flame, autoignition, and detonation from the wall and/or the reaction front. The boundary of normal flame regime qualitatively agrees with the prediction by the Livengood-Wu Integral method, while the detonation development from both the end wall and the reaction front observes Zel'dovich mechanism. Compared to hydrocarbons, hydrogen is resistant to knock onset but it is more prone to superknock development. The latter mode becomes more destructive in the presence of wall heat loss. This study isolates and identifies the role of wall heat loss on a potential mechanism for superknock development in hydrogen-fueled spark-ignition engines.     

Two-stage autoignition and combustion mode evolution in boundary layer flows above a cold flat plate

Boundary layers are omnipresent in fundamental kinetic experimental facilities and practical combustion engines, which can cause ambiguity and misleading results in kinetic target acquisition and even abnormal engine combustion. In this paper, using n-heptane as a representative large hydrocarbon fuel exhibiting pronounced low-temperature chemistry (LTC), two-dimensional numerical simulation is conducted to resolve the transient autoignition phenomena affected by a boundary layer. We focus on the ignition characteristics and the subsequent combustion mode evolution of a hot combustible mixture flowing over a colder flat plate in an isobaric environment. For cases with autoignition occurring within the boundary layer, similarity is observed in the first-stage ignition as manifested by a constant temperature at all locations. The first-stage ignition is found to be rarely affected by heat and radical loss within the boundary layer. While for the main ignition event, an obvious dependence of ignition process on boundary layer thickness is identified, where the thermal-chemical process exhibits similarity at locations with similar boundary layer thickness, and the main ignition tends to first occur within the boundary layer at the domain end and generates a C-shape reaction front. It is found that sequential spontaneous autoignition is the dominant subsequent combustion mode at high-pressure conditions. At low to intermediate pressures, auto-ignition assisted flame propagation is nevertheless the dominant mode for combustion evolution. This research identifies novel features of autoignition and the subsequent combustion mode evolution affected by a cold, fully developed boundary layer, and provides useful guidance to the interpretation of abnormal combustion and combustion mode evolution in boundary layer flows.