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1.
Detonation development inside spark ignition engines can result in the so called super-knock with extremely high pressure oscillation above 200?atm. In this study, numerical simulations of autoignitive reaction front propagation in hydrogen/air mixtures are conducted and the detonation development regime is investigated. A hot spot with linear temperature distribution is used to induce autoignitive reaction front propagation. With the change of temperature gradient or hot spot size, three typical autoignition reaction front modes are identified: supersonic reaction front; detonation development and subsonic reaction front. The effects of initial pressure, initial temperature, fuel type and equivalence ratio on detonation development regime are examined. It is found that the detonation development regime strongly depends on mixture composition (fuel and equivalence ratio) and thermal conditions (initial pressure and temperature). Therefore, to achieve the quantitative prediction of super-knock in engines, we need use the detonation development regime for specific fuel at specific initial temperature, initial pressure, and equivalence ratio. 相似文献
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High-speed schlieren visualizations show that a composite reaction-shock cluster structure is formed in the last flame acceleration stage prior to detonation transition for ethylene/oxygen mixture in a narrow channel. The composite structure is bounded by a normal shock at the leading edge of the structure, and series of parallel oblique shocks interweave with reaction front on the other end in the cluster. Propagating velocity of the reaction front at the inception of the cluster is ~ 45–50% of Chapman-Jouguet detonation velocity of the mixture. Reaction front accelerates rapidly after the formation of the reaction-shock cluster, and run into detonation in tens of microseconds except for very lean mixtures. The angle between the parallel oblique shocks in the cluster and the side wall, defined as ω-angle, is found to be constant for a specific mixture as the reaction wave propagates. Dependence of ω-angle on mixture equivalence ratio and channel size are investigated in the study. Analysis shows that DDT distance is linearly proportional to ω-angle, and an empirical correlation is derived. 相似文献
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Brent A. Rankin Joshua R. Codoni John L. Hoke Frederick R. Schauer 《Proceedings of the Combustion Institute》2019,37(3):3479-3486
The structure of detonation waves propagating through the annular channel of an optically accessible non-premixed rotating detonation engine (RDE) are investigated using mid-infrared imaging. The RDE is operated on hydrogen–air mixtures for a range of air mass flow rates and equivalence ratios. Instantaneous images of the radiation intensity from water vapor are acquired using a mid-infrared camera and a band-pass filter (2.890?±?0.033?µm). The instantaneous mid-infrared images reveal the stochastic nature of the detonation wave structure, position and angle of oblique and reflected shock waves, presence of shear layer separating products from the previous and current cycles, and extent of mixing between the reactants and products in the reactant fill zone in front of the detonation wave. The images show negligible signal directly in front of the detonation waves suggesting that there is minimal mixing between the reactants and products from the previous cycle ahead of the detonation wave for most operating conditions. The mid-infrared images provide insights useful for improving fundamental understanding of the detonation structure in RDEs and benchmark data for evaluating modeling and simulation results of RDEs. 相似文献
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Effects of tube diameter and equivalence ratio on reaction front propagations of ethylene/oxygen mixtures in capillary tubes were experimentally analyzed using high speed cinematography. The inner diameters of the tubes investigated were 0.5, 1, 2 and 3 mm. The flame was ignited at the center of the 1.5 m long smooth tube under ambient pressure and temperature before propagated towards the exits in the opposite directions. A total of five reaction propagation scenarios, including deflagration-to-detonation transition followed by steady detonation wave transmission (DDT/C–J detonation), oscillating flame, steady deflagration, galloping detonation and quenching flame, were identified. DDT/C–J detonation mode was observed for all tubes for equivalence ratios in the vicinity of stoichiometry. The velocity for the steady detonation wave propagation was approximately Chapman–Jouguet velocity for 1, 2, and 3 mm I.D. tubes; however, a velocity deficit of 5% was found for the case in 0.5 mm I.D. tube. For leaner mixtures, an oscillating flame mode was found for tubes with diameters of 1 to 3 mm, and the reaction front travelled in a steady deflagrative flame mode with velocities around 2–3 m/s when the mixture equivalence ratio becomes even leaner. Galloping detonation wave propagation was the dominant mode for the fuel lean regime in the 0.5 mm I.D. tube. For rich mixtures beyond the detonation limits, a fast flame followed by flame quenching was observed. 相似文献
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《Combustion Theory and Modelling》2013,17(4):745-770
The propagation of one-dimensional detonations in hydrogen–air mixtures is investigated numerically by solving the one-dimensional Euler equations with detailed finite-rate chemistry. The numerical method is based on a second-order spatially accurate total-variation-diminishing scheme and a point implicit time marching algorithm. The hydrogen–air combustion is modelled with a 9-species, 19-step reaction mechanism. A multi-level, dynamically adaptive grid is utilized, in order to resolve the structure of the detonation. Parametric studies for an equivalence ratio range of 0.4–2.0, initial pressure range of 0.2–0.8 bar and different degrees of detonation overdrive demonstrate that the detonation is unstable for low degrees of overdrive, but the dynamics of wave propagation varies with fuel–air equivalence ratio and pressure. For equivalence ratios less than approximately 1.2 and for all pressures, the detonation exhibits a short-period oscillatory mode, characterized by high-frequency, low-amplitude waves. Richer mixtures exhibit a period-doubled bifurcation that depends on the initial pressure. Parametric studies over a degree of overdrive range of 1.0–1.2 for stoichiometric mixtures at 0.42 bar initial pressure indicate that stable detonation wave propagation is obtained at the high end of this range. For degrees of overdrive close to one, the detonation wave exhibits a low-frequency mode characterized by large fluctuations in the detonation wave speed. The McVey–Toong short-period wave-interaction theory is in qualitative agreement with the numerical simulations; however, the frequencies obtained from their theory are much higher, especially for near-stoichiometric mixtures at high pressure. Modification of this theory to account for the finite heat-release time significantly improves agreement with the numerically computed frequency over the entire equivalence ratio and pressure ranges. 相似文献
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建立三维的铝粉-空气两相爆轰计算模型,采用时-空守恒元解元(CE/SE)方法求解,并开发了悬浮铝粉尘爆轰的三维数值模拟程序.基于消息传递接口(MPI)技术实现了程序的并行化设计.通过对激波管问题以及爆轰管中铝粉-空气两相爆轰实验的模拟验证程序的可靠性.对拐角空间中左侧浓度为368 g·m-3的铝粉-空气混合物两相爆轰及其在拐角空间右侧和下方空气域内形成的冲击波和温压效应开展数值模拟,获得复杂空间内爆轰波或冲击波的传播、反射以及绕射过程.结果表明:两相爆轰在离铝粉尘区域2 m远的空气域内产生的后效冲击波能达到2.66 MPa的固壁反射压力,火球燃烧范围会超出初始铝粉尘区域约0.8 m,并且造成初始铝粉尘区域附近1.5 m范围内空气的温度高达1 600 K.模拟程序可用于铝粉尘爆轰的后效研究,对工业安全及其防护具有指导意义. 相似文献
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在长为32.4 m、内径为0.199 m的大型长直水平管道中,对铝粉-空气两相流的燃烧转爆轰(DDT)过程及爆轰波结构进行了实验研究。对铝粉-空气混合物弱点火条件下DDT过程不同阶段的特征进行了分析,实验结果显示混合物经历了缓慢反应压缩阶段、压缩波加速冲击波形成阶段、冲击反应过渡阶段、冲击反应向过压爆轰过渡阶段和爆轰阶段,得到了混合物各阶段的DDT参数,由此进一步分析了DDT浓度的上、下限。在1.4 m爆轰测试段的4个截面的环向上各均匀安装8个传感器,对爆轰波结果进行测试,并对铝粉-空气混合物爆轰波的单头结构进行了分析。 相似文献
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Qiaofeng Xie Bing Wang Haocheng Wen Wei He Piotr Wolanski 《Proceedings of the Combustion Institute》2019,37(3):3425-3432
The enhancement of continuously rotating detonation in oxygen-enriched air was demonstrated in an annular rotating detonation combustor (RDC) under a diffusive supply of hydrogen and an oxidizer. Experimental tests were performed to reveal the effects of oxygen volume fraction, mass flow rate, and equivalent ratio on the propagation of continuously rotating detonation wave (CRDW). It is observed that an increase in the air mass flow rate from 25 g/s to 225 g/s causes an increase in the propagation velocity of the stable CRDW in the RDC. For an oxygen volume fraction up to 35%, the difference between the propagation velocity of detonation and the theoretical Chapman–Jouguet value is less than 5%. Under the chemical stoichiometric ratio condition for air, the CRDW is stabilized when the air mass flow rate reaches 185 g/s. However, stabilized CRDW is observed even when the air flow rate is only 45 g/s under the presence of 30% or 35% oxygen. Increase in the oxygen volume fraction leads to an extension of the rich/lean limit for generating a stable CRDW. This study aims to provide guidance for the modulation of continuously rotating detonation. 相似文献
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为了研究涡轮导向器对旋转爆轰波传播特性的影响,以氢气为燃料,空气为氧化剂,在不同当量比下开展了实验研究.基于高频压力传感器及静态压力传感器的信号,详细分析了带涡轮导向器的旋转爆轰燃烧室的工作模式以及涡轮导向器对非均匀不稳定爆轰产物的影响.实验结果表明:在当量比较低时,爆轰燃烧室以快速爆燃模式工作;逐渐增大当量比,爆轰燃烧室开始以不稳定旋转爆轰模式工作;继续增大当量比,爆轰燃烧室以稳定旋转爆轰模式工作,且旋转爆轰波的传播速度和稳定性均随当量比的增大逐渐提高.爆轰波下游的斜激波与涡轮导向器相互作用,涡轮导向器对压力振荡的幅值具有明显的抑制作用,但对压力振荡频率的影响较小.随着当量比的增大,涡轮导向器上下游的静压均同时增大,经过涡轮导向器的作用,涡轮下游静压明显降低. 相似文献
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Jens Prager Habib N. Najm Dimitris A. Goussis 《Proceedings of the Combustion Institute》2009,32(1):509-517
An automated procedure has been previously developed to generate simplified skeletal reaction mechanisms for the combustion of n-heptane/air mixtures at equivalence ratios between 0.5 and 2.0 and different pressures. The algorithm is based on a Computational Singular Perturbation (CSP)-generated database of importance indices computed from homogeneous n-heptane/air ignition solutions. In this paper, we examine the accuracy of these simplified mechanisms when they are used for modeling laminar n-heptane/air premixed flames. The objective is to evaluate the accuracy of the simplified models when transport processes lead to local mixture compositions that are not necessarily part of the comprehensive homogeneous ignition databases. The detailed mechanism was developed by Curran et al. and involves 560 species and 2538 reactions. The smallest skeletal mechanism considered consists of 66 species and 326 reactions. We show that these skeletal mechanisms yield good agreement with the detailed model for premixed n-heptane flames, over a wide range of equivalence ratios and pressures, for global flame properties. They also exhibit good accuracy in predicting certain elements of internal flame structure, especially the profiles of temperature and major chemical species. On the other hand, we find larger errors in the concentrations of many minor/radical species, particularly in the region where low-temperature chemistry plays a significant role. We also observe that the low-temperature chemistry of n-heptane can play an important role at very lean or very rich mixtures, reaching these limits first at high pressure. This has implications to numerical simulations of non-premixed flames where these lean and rich regions occur naturally. 相似文献
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An experimental investigation of the propagation mechanism of critical deflagration waves that lead to the onset of detonation 总被引:3,自引:0,他引:3
The reflection of a CJ detonation from a perforated plate is used to generate high speed deflagrations downstream in order to investigate the critical conditions that lead to the onset of detonation. Different perforated plates were used to control the turbulence in the downstream deflagration waves. Streak Schlieren photography, ionization probes and pressure transducers are used to monitor the flow field and the transition to detonation. Stoichiometric mixtures of acetylene–oxygen and propane–oxygen were tested at low initial pressures. In some cases, acetylene–oxygen was diluted with 80% argon in order to render the mixture more “stable” (i.e., more regular detonation cell structure). The results show that prior to successful detonation initiation, a deflagration is formed that propagates at about half the CJ detonation velocity of the mixture. This “critical” deflagration (which propagates at a relatively constant velocity for a certain duration prior to the onset of detonation) is comprised of a leading shock wave followed by an extended turbulent reaction zone. The critical deflagration speed is not dependent on the turbulence characteristics of the perforated plate but rather on the energetics of the mixture like a CJ detonation (i.e., the deflagration front is driven by the expansion of the combustion products). Hence, the critical deflagration is identified as a CJ deflagration. The high intensity turbulence that is required to sustain its propagation is maintained via chemical instabilities in the reaction zone due to the coupling of pressure fluctuations with the energy release. Therefore, in “unstable” mixtures, critical deflagrations can be supported for long durations, whereas in “stable” mixtures, deflagrations decay as the initial plate generated turbulence decays. The eventual onset of detonation is postulated to be a result of the amplification of pressure waves (i.e., turbulence) that leads to the formation of local explosion centers via the SWACER mechanism during the pre-detonation period. 相似文献
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《Proceedings of the Combustion Institute》2023,39(4):4771-4780
In this work, oblique detonation of n-heptane/air mixture in high-speed wedge flows is simulated by solving the reactive Euler equations with a two-dimensional (2D) configuration. This is a first attempt to model complicated hydrocarbon fuel oblique detonation waves (ODWs) with a detailed chemistry (44 species and 112 reactions). Effects of freestream equivalence ratios and velocities are considered, and the abrupt and smooth transition from oblique shock to detonation are predicted. Ignition limit, ODW characteristics, and predictability of the transition mode are discussed. Firstly, homogeneous constant-volume ignition calculations are performed for both fuel-lean and stoichiometric mixtures. The results show that the ignition delay generally increases with the wedge angle. However, a negative wedge angle dependence is observed, due to the negative temperature coefficient effects. The wedge angle range for successful ignition of n-heptane/air mixtures decreases when the wedge length is reduced. From two-dimensional simulations of stationary ODWs, the initiation length generally decreases with the freestream equivalence ratio, but the transition length exhibits weakly non-monotonic dependence. Smooth ODW typically occurs for lean conditions (equivalence ratio < 0.4). The interactions between shock/compression waves and chemical reaction inside the induction zone are also studied with the chemical explosive mode analysis. Moreover, the predictability of the shock-to-detonation transition mode is explored through quantifying the relation between ignition delay and chemical excitation time. It is demonstrated that the ignition delay (the elapsed time of the heat release rate, HRR, reaches the maximum) increases, but the excitation time (the time duration from the instant of 5% maximum HRR to that of the maximum) decreases with the freestream equivalence ratio for the three studied oncoming flow velocities. Smaller excitation time corresponds to stronger pressure waves from the ignition location behind the oblique shock. When the ratio of excitation time to ignition delay is high (e.g., > 0.5 for n-C7H16, > 0.3 for C2H2 and > 0.2 for H2, based on the existing data compilation in this work), smooth transition is more likely to occur. 相似文献
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为研究反应物当量比对旋转爆震波传播过程的影响,在圆盘形旋转爆震发动机上进行H2/air的旋转爆震实验研究,并统计分析了当量比对爆震波传播模态及参数的影响规律.实验结果表明,固定质量流率,同一种传播模态下,随着当量比的增大,爆震波的压力峰值及传播速度增大,且旋转爆震波的传播过程更加稳定.不同质量流率条件下,当量比对传播模态的影响规律不同.空气质量流率小于100 g/s时,旋转爆震波皆以单波模态传播.空气质量流率大于150 g/s时,随着当量比的增大,旋转爆震波的传播模态由单波模态向双波模态转变,再转变为不对称双波模态,最后又回到单波模态.并且在不对称双波模态中发现了低频振荡现象,振荡频率约为300 Hz.质量流率继续增大,燃烧室中发现了同向三波传播模态.随着质量流率的增加,双波模态的当量比下限降低,不对称双波模态的当量比上限增大,而双波与不对称双波模态的分界线受质量流率的影响较小. 相似文献
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M. AhmadiM. Saffar Avval T. YousefiM. Goharkhah B. NasrM. Ashjaee 《Optics and Lasers in Engineering》2011,49(7):859-865
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. 相似文献
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Flame acceleration and the transition to detonation of stoichiometric ethylene/oxygen in microscale tubes 总被引:8,自引:0,他引:8
Ming-hsun Wu M.P. Burke S.F. Son R.A. Yetter 《Proceedings of the Combustion Institute》2007,31(2):2429-2436
Flame propagation in capillary tubes with smooth circular cross-sections and diameters of 0.5, 1.0, and 2.0 mm are investigated using high-speed photography. Flames were found to propagate and accelerate to detonation speed in stoichiometric ethylene and oxygen mixtures initially at room temperature in all three tube diameters. Ignition occurs at the midpoint along the length of the tube. We observe for the first time transition to detonation in micro-tubes. Detonation was observed with both spark and hot-wire ignition. Tubes with larger diameters take longer to transition to detonation. In fact, transition distance scales with the diameter in our 1.0 and 2.0 mm cases with spark ignition. Flame structures are observed for various stages of the process. Three types of flame propagation modes were observed in the 0.5 mm tube with spark ignition: (a) acceleration to Chapman–Jouguet (CJ) detonation speed followed by constant CJ wave propagation, (b) acceleration to CJ speed, followed by the detonation wave failure, and (c) flame acceleration to a constant speed below the CJ speed of approximately 1600 m/s. The current detonation mechanism observed in capillary tubes is applicable to predetonators for pulsed detonation, micro propulsion devices, safety issues, and addresses fundamental issues raised by recent theoretical and numerical analyses. 相似文献
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Robert D. Stachler Joseph K. Lefkowitz Joshua S. Heyne Scott D. Stouffer Timothy M. Ombrello Joseph D. Miller 《Proceedings of the Combustion Institute》2019,37(4):5039-5046
Understanding of ignition processes is central to design for reliable and safe aerospace combustor systems. Ignition is influenced by many factors including combustor geometry, flow conditions, fuel composition, turbulence intensity, ignition source, and energy deposition method. A toroidal jet-stirred reactor (TJSR) utilizes bulk fluid motion, presence of recirculation zones, a bulk residence time, and turbulence intensities which emulate characteristics relevant to cavity stabilized and swirl stabilized combustors. In this work, a TJSR was used to quantify ignitability and time-to-ignition of premixed ethylene and air. The effects of inlet temperature, residence time, and reactivity were studied on forced ignition processes. Experimental conditions ranged from residence times of 15–35?ms, mixture temperatures of 340–450?K, and equivalence ratios of 0.5–1 using capacitive spark-discharge ignition. The minimum equivalence ratio for ignition (MER), or the equivalence ratio at 50% probability, shows an inverse relationship with mixture temperature and residence time. Prior theory of real engine combustor performance for lean light off, proposed by Ballal and Lefebvre, was compared to the MER and displayed similar trends to the model. Spatially integrated OH* chemiluminescence was used to measure time to ignition within the reactor. Reduction in ignitibility was experienced as the time-to-ignition approached the residence time stressing the importance of device flow time scales in relation to kernel growth dynamics and ignition probability. 相似文献