介观尺度下活性炭微粒的光镊捕捉、点火和扩散燃烧特性研究

为探索介观尺度下固体燃料微粒的燃烧现象,本文提出采用光镊工具对活性炭微粒进行捕捉、悬浮、定位,再通过激光点燃,研究其着火及扩散燃烧特性.介观尺度燃烧室中,光镊捕捉7.0μm活性炭微粒的最低捕捉功率为3.2 mW,捕捉速率范围为103.7—70.0μm/s;活性炭微粒在静止气流中的最低点火功率为3.2 mW,颗粒的等效粒径、周长、面积和圆形度对最低点火功率影响甚微,点火延迟时间约48 ms,提高点火功率,点火延迟时间缩短,最小点火延迟时间小于6 ms;活性炭在着火后先发生无焰燃烧,紧接着发生有焰燃烧,无焰燃烧的扩散燃烧速率满足粒径平方直线规律,其燃烧速率范围为15.0—8.0μm/s;有焰燃烧的火焰面积和强度随燃烧时间发生闪烁,其闪烁频率约29.1 Hz.对于粒径为3.0μm的活性炭微粒,从加热到完全燃烧殆尽所需时间约0.648 s.结果表明:对于聚焦后的高能激光束点燃活性炭微粒的着火属于联合着火模式,在挥发份析出之前,活性炭非均相着火而发生无焰燃烧,挥发份析出后被点燃发生均相着火,火焰面始终保持圆形.     

激光引爆炸药和冲击加载铝板的数值研究

本文用一维反应流体力学程序SSS模拟研究PETN炸药的激光引爆过程。分别讨论了激光波形、窗口厚度的影响,认为激光功率密度在10~6～10~8W/cm^2时,其引爆机制主要为热机制;炸药受热膨胀是激光能量损耗的主要因素之一。此外,本文还讨论了激光引爆后,爆轰波与铝靶的相互作用。     

利用OH自由基特征发射谱测量正庚烷的点火延迟时间

在化学激波管中利用反射激波进行点火,采用OH自由基在306.4nm处特征发射谱线强度的急剧变化标志燃料的着火,由光谱单色仪、光电倍增管、压力传感器和示波器组成测量系统,测量了正庚烷/氧气的点火延迟时间,点火压力(1.0±0.1)和(0.75±0.05)atm,点火温度1 170～1 730K,当量比1.0,得到了在此实验条件下正庚烷/氧气点火延迟时间随温度变化的关系式。研究结果表明正庚烷/氧气点火延迟时间随温度的增加呈指数减小,点火压力为0.75atm时,随着点火温度的增加,点火延迟时间的变化率要小于1.0atm条件时。实验结果为建立正庚烷燃烧反应动力学模型,验证正庚烷燃烧反应机理提供了实验依据。     

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.     

不同重力环境下辐射加热材料表面着火特性分析

研究外界辐射加热下，不同重力环境中热薄燃料的着火特性．探讨了重力、环境氧浓度、环境压力及外界辐射强度对着火的影响．结果表明，随着重力的变化，存在不同的着火机制．在微重力和在高的环境氧浓度中，材料的着火延迟时间变短．压力减小，着火延迟时间增大．随着辐射强度的增大，着火延迟时间变小．     

光纤传输激光驱动飞片实验研究

构建了一种基于光纤传输高功率激光的飞片发射系统,并测试了飞片速度.飞片膜层为三明治结构:铝烧蚀层、氧化铝隔离层和铝飞片产生层.飞片膜层采用磁控溅射技术沉积在玻璃衬底上,总厚度为5.5 μm.激光辐照铝膜层产生高温高压等离子体,驱动剩余膜层产生高速飞片,速度达数km/s.同时,实验研究了光纤传能系统的输出激光空间分布特性和传输激光能量容量,它们决定了飞片的平面性和最大速度.光纤端面损伤是限制光纤传输激光能量容量的关键因素,光纤端面通过精密机械抛光和激光预处理可以获得理想的抗激光损伤能力.采用基于光纤阵列探针的时间序列测试技术获得了飞片的平均速度,并评估了飞片的平面性.采用搭建的基于光纤传输高功率激光的飞片发射系统获得了速度达1.7 km/s、直径接近1 mm的高速飞片. 关键词： 激光驱动飞片 激光辐照 光纤阵列探针 激光等离子体     

Effect of the density of an ammonium perchlorate-ultradispersed aluminum composition on the threshold of its ignition by a millisecond-long laser pulse

Pyrotechnical ammonium perchlorate-ultradispersed aluminum compositions were ignited in air with 1.06-μm 3.5-ms-long laser pulses. The reflection coefficients, ignition energy threshold, and ignition delay were measured at various densities of the samples. The peculiarities of the ignition and burning of this composition were examined.     

Ignition mechanisms of pulse detonator initiated scramjet cavity

A fueled cavity in a supersonic crossflow was ignited via a pulse detonator (PD) producing detonation waves that were then decoupled to produce varying degrees of shock-flame separation at the exit of the PD tube. This decoupling allowed for observation of the cavity ignition mechanism, and the key parameters required for successful cavity ignition were identified. Measurements were made using high-frame-rate OH Planar Laser-Induced Fluorescence (PLIF) and schlieren and chemiluminescence imaging. It was shown that the entrainment of high-temperature intermediate species into the forward region of the cavity, immediately behind the step, is the principal criterion for cavity ignition. Both coupled and slightly decoupled detonation cases induced significant OH shedding into the step region, leading to ignition and flame stabilization within the cavity. At conditions where OH shedding into the step region did not occur, cavity ignition was not observed. In coupled and slightly decoupled cases, there is more shedding of OH behind the step due to the greater disturbances created in the flowfield. As the degree of detonation decoupling increases, there is less shedding of OH and therefore a lower likelihood of ignition. Additionally, the time required for cavity combustion to reach its steady-state condition varied with the degree of decoupling of the detonation. Coupled detonation cases were shown to be more disruptive to the cavity and thus required more time to reach steady state than the decoupled cases.     

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 kinetics and dynamics study on the auto-ignition of dimethyl ether at low temperatures and low pressures

Though the combustion chemistry of dimethyl ether (DME) has been widely investigated over the past decades, there remains a dearth of ignition data that examines the low-temperature, low-pressure chemistry of DME. In this study, DME/‘air’ mixtures at various equivalence ratios from lean (0.5) to extremely rich (5.0) were ignited behind reflected shock waves at a fixed pressure (3.0 atm) over the temperature range 625–1200 K. The ignition behavior is different from that at high-pressures, with a repeatable ignition delay time fall-off feature observed experimentally in the temperature transition zone from the negative temperature coefficient (NTC) regime to the high-temperature regime. This could not be reproduced using available kinetic mechanisms as conventionally homogeneous ignition simulations. The fall-off behavior shows strong equivalence ratio dependence and disappears completely at an equivalence ratio of 5.0. A local ignition kernel postulate was implemented numerically to quantifiably examine the inhomogeneous premature ignition. At low temperature, no pre-ignition occurs in the mixture. A conspicuous discrepancy was observed between the measurements and constrained UV simulations at temperatures beyond the NTC regime. A third O₂ addition reaction sub-set was incorporated into AramcoMech 3.0, together with related species thermochemistry calculated using the G3/G4/CBS-APNO compound method, to explore the low-temperature deviation. The new reaction class does not influence the model predictions in IDTs, but the updated thermochemistry does. Sensitivity analyses indicate that the decomposition of hydroperoxy-methylformate plays a critical role in improving the low-temperature oxidation mechanism of DME but unfortunately, the thermal rate coefficient has never been previously investigated. Further experimental and theoretical endeavors are required to attain holistic quantitative chemical kinetics based on our understanding of the low-temperature chemistry of DME.     

Ignition delay measurements of four component model gasolines exploring the impacts of biofuels and aromatics

This study explores the impacts of combinations of biofuel (ethanol, isobutanol and 2-methyl furan) and aromatic (toluene) compounds in a four component fuel blend, at fixed research octane number (RON) on ignition delay measured in an advanced fuel ignition delay analyzer (AFIDA 2805). Ignition delay measurements were performed over a range of temperatures from 400 to 725 °C (673 to 998 K) and two chamber pressures of 10 and 20 bar. The four component mixtures are compared to primary reference fuels at RON values of 90 and 100. The ignition delay measurements show that as the aromatic and biofuel concentrations increased, two stage ignition behavior was suppressed, at both initial chamber pressures. But both RON 100 (isooctane) and RON 90 reference fuels showed two stage ignition behavior, as did fuel mixtures with low biofuel and aromatic content. RON 90 fuels showed stronger two stage ignition behavior than RON 100 fuels, as expected. Depending on the type of biofuel in the mixture, the ignition delay at low chamber temperatures could be far greater than for the reference fuels. In particular, for the RON 100 mixtures at either 10 or 20 bar initial chamber pressure, the ignition delay at 400 °C (673 K) for the high level blend of 2-methyl furan and toluene (30 vol% of each) exhibited an ignition delay that was 10 times longer than for neat isooctane. The results show the strong non-linear octane blending response of these three biofuel compounds, especially in concert with the kinetic antagonism that toluene is known to display in mixtures with isooctane. These results have implications for the formulation of biofuel mixtures for spark ignition and advanced compression ignition engines, where this non-linear octane blending response could be exploited to improve knock resistance, or modulate the autoignition process.     

Gas-phase spontaneous ignition of hydrocarbons

The ignition of hydrocarbons at low temperatures is experimentally studied in a rapid-mixture-injection static reactor. The ignition process was monitored using a high-speed color video camera. It was found that, at low temperatures, ignition starts in kernels, a feature also characteristic of methods for measuring the ignition delay time at high and medium temperatures (shock tube, rapid compression machine). Kernel-mode ignition is associated with gas-dynamic phenomena inherent in different techniques of heating the gas to the desired temperature. Ignition in the kernel is of chain-thermal nature. The emergence of a visible kernel can be considered the beginning of hot flame propagation. It is shown that, in the self-ignition mode, the propagation of the flame front from the initial kernel occurs by the induction mechanism, proposed by Ya.B. Zel’dovich, rather than by the diffusion-heat-conduction mechanism. Introduction of a platinum wire into the reactor produces a catalytic effect in the negative temperature coefficient region, while virtually unaffecting the ignition delay at lower temperatures.     

Autoignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous ternary mixtures

A numerical simulation of the ignition and combustion of hydrocarbon-hydrogen-air homogeneous and heterogeneous (gas-drop) ternary mixtures for three hydrocarbon fuels (n-heptane, n-decane, and n-dodecane) is for the first time performed. The simulation is carried out based on a fully validated detailed kinetic mechanism of the oxidation of n-dodecane, which includes the mechanisms of the oxidation of n-decane, n-heptane, and hydrogen as constituent parts. It is demonstrated that the addition of hydrogen to a homogeneous or heterogeneous hydrocarbon-air mixture increases the total ignition delay time at temperatures below 1050 K, i.e., hydrogen acts as an ignition inhibitor. At low temperatures, even ternary mixtures with a very high hydrogen concentration show multistage ignition, with the temperature dependence of the ignition delay time exhibiting a negative temperature coefficient region. Conversely, the addition of hydrogen to homogeneous and heterogeneous hydrocarbon-air mixtures at temperatures above 1050 K reduces the total ignition delay time, i.e., hydrogen acts as an autoignition promoter. These effects should be kept in mind when discussing the prospects for the practical use of hydrogen-containing fuel mixtures, as well as in solving the problems of fire and explosion safety.     

Gas-phase autoignition of a spherical volume of fuel in an oxidative medium

A mathematical model of the ignition of unmixed of fuel and oxidizer (a finite spherical volume of fuel surrounded by an infinite oxidizer medium) was developed. The regularities of the autoignition of this system were examined. It was demonstrated that the temperature maximum arising at the fuel-oxidizer interface propagates with increasing amplitude and velocity toward the center of the spherical volume and that the time it takes to attain the maximum temperature (below the autoignition threshold) and the ignition delay time (above the threshold) depend on the parameter δ nonmonotonically, more specifically, exhibit well-pronounced maxima     

Infrared borescopic characterization of corona and conventional ignition for lean/dilute combustion in heavy-duty natural-gas engines

Natural gas (NG) is attractive for heavy-duty (HD) engines for reasons of cost stability, emissions, and fuel security. NG requires forced ignition, but conventional gasoline-engine ignition systems are not optimized for NG and are challenged to ignite mixtures that are lean or diluted with exhaust-gas recirculation (EGR). NG ignition is particularly difficult in large-bore engines, where it is more challenging to complete combustion in the time available. High-speed infrared (IR) in-cylinder imaging and image-derived quantitative metrics were used to compare two ignition systems in terms of the early flame-kernel development and cycle-to-cycle variability (CCV) in a heavy-duty, natural-gas-fueled engine that had been modified to enable exhaust-gas recirculation and to provide optical access via borescopes. Imaging in the near IR and short-wavelength IR yielded strong signals from the water emission lines, which acted as a proxy for flame front and burned-gas regions while obviating image intensification (which can reduce spatial resolution). The ignition systems studied were a conventional system and a high-frequency corona system. The air/fuel mixtures investigated included stoichiometric without dilution and lean with EGR. The corona system produced five separate elongated, irregularly shaped, nonequilibrium-plasma streamers, leading to immediate formation of five spatially distinct wrinkled flame kernels around each streamer. Compared to the conventional spark ignition, which produces a single flame kernel that exhibits an initial laminar growth regime before wrinkling, corona ignition's early achievement of higher flame surface areas significantly shortened the ignition delay, resulting in reduced overall combustion duration and CCV for each mixture. Additionally, although the lean, dilute mixture produced higher CCV than the stoichiometric, minimally diluted mixture with both igniters, the mixtures ignited by the corona system suffered less than those ignited by the conventional system. Image-based measurements of CCV agreed with those based on in-cylinder pressure.     

Effect of steam on the single particle ignition of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions

This article investigates the effect of steam on the ignition of single particles of solid fuels in a drop tube furnace under air and simulated oxy-fuel conditions. Three solid fuels, all in the size range 125–150 µm, were used in this study; specifically, a low rank sub-bituminous Colombian coal, a low-rank/high-ash sub-bituminous Brazilian coal and a charcoal residue from black acacia. For each solid fuel, particles were burned at a constant drop tube furnace wall temperature of 1475?K, in six different mixtures of O₂/N₂/CO₂/H₂O, which allowed simulating dry and wet conventional and oxy-fuel combustion conditions. A high-speed camera was used to record the ignition process and the collected images were treated to characterize the ignition mode (either gas-phase or surface mode) and to calculate the ignition delay times. The Colombian coal particles ignite predominately in the gas-phase for all test conditions, but under simulated oxy-fuel conditions there is a decrease in the occurrence of this ignition mode; the charcoal particles experience surface ignition regardless of the test condition; and the Brazilian coal particles ignite predominately in the gas-phase when combustion occurs in mixtures of O₂/N₂/H₂O, but under simulated oxy-fuel conditions the ignition occurs predominantly on the surface. The ignition delay times for particles that ignited in the gas-phase are smaller than those that ignited on the surface, and generally the simulated oxy-fuel conditions retard the onset of both gas-phase and surface ignition. The addition of steam decreases the gas-phase and surface ignition delay times of the particles of both coals under simulated oxy-fuel conditions, but has a small impact on the gas-phase ignition delay times when the combustion occurs in mixtures of O₂/N₂/H₂O. The steam gasification reaction is likely to be responsible for the steam effect on the ignition delay times through the production of highly flammable species that promote the onset of ignition.     

Autoignition of varied cetane number fuels at low temperatures

The reactivity of six kerosene based control fuels, specifically formulated for cetane number variation, are investigated by measuring ignition delay time in a heated rapid compression machine. Cetane numbers vary from 30 to 55 (increment of 5) while holding other properties relatively constant by adjusting chemical group composition. Main cetane variation was controlled through the distribution of normal alkanes and isoalkanes, which was fine-tuned using additives. Other fuel properties such as density, viscosity, H/C ratio, etc. were balanced using cyclic compounds and aromatics. Fuels were tested in the RCM at compressed pressures of P_c=?10 and 20?bar, equivalence ratios of ??=?0.25, 0.5 and 1.0, in the low to intermediate temperature range (620?K?≤?T_c?≤?730?K). Relations between cetane number and ignition delay time have been evaluated at multiple test conditions, and further analysis on multistage ignition has been conducted. Ignition delay times of fuels with higher cetane numbers are shorter at these temperatures for most conditions. First stage ignition delay time measurements have been observed to be relatively insensitive to P_c, ?, and fuel type, while deviations in overall ignition delay times are mainly attributed to second stage ignition delay time, impacted by variations in the first stage temperature. Control fuels of this type offer an opportunity to be used in practical experiments to determine the impact of cetane number on combustion dynamics.     

激光驱动飞片加速特征分析

在激光驱动飞片研究中,飞片的加速特征是需要认识的关键问题之一。设计了强激光作用金属膜驱动飞片实验,采用聚偏氟乙烯（PVDF）压电薄膜测量了飞片到达不同距离的时间,计算得到飞片速度和加速度,分析激光能量对飞片加速性能的影响。基于Gurney能理论,建立了激光驱动飞片速度的计算模型,根据实验结果获得了激光能量损失系数和有效吸收系数,分析了激光能量和膜体厚度对飞片速度的影响。实验结果表明:不同激光能量下飞片的加速特征基本相似,激光能量变化对飞片的加速时间影响较小; 激光能量较大的情况下,膜体厚度对飞片最大速度、能量耦合系数的影响更显著; 当膜体超过一定厚度时,能量耦合系数不再增加。     

Ignition of dimethyl ether/air mixtures by hot particles: Impact of low temperature chemical reactions

Understanding and characterizing ignition of flammable mixtures by hot particles is important for assessing and reducing the risk of accidental ignition and explosion in industry and aviation. Recently, many studies have been conducted for ignition of gaseous mixtures by hot particles. However, the effects of low-temperature chemistry (LTC) on ignition by hot particles received little attention. LTC plays an important role in the ignition of most hydrocarbon fuels and may induce cool flames. The present study aims to numerically assess the effects of LTC on ignition by the hot particles. We consider the transient ignition processes induced by a hot spherical particle in quiescent and flowing stoichiometric dimethyl ether/air mixtures. 1D and 2D simulations, respectively, are conducted for the ignition process by hot-particles in quiescent and flowing mixtures. A detailed kinetic model including both LTC and high-temperature chemistry (HTC) is used in simulations. The results exhibit a premixed cool flame to be first initiated by the hot particle. Then a double-flame structure with both premixed cool and hot flames is observed at certain conditions. At zero or low inlet flow velocities, the hot flame catches up and merges with the leading cool flame. At high inlet flow velocities, the hot flame cannot be initiated due to the short residence time and large convective loss of heat and radicals. Comparing the results with and without considering LTC confirms that LTC accelerates substantially ignition via HTC in a certain range of hot particle temperatures. The mechanism of ignition promotion by LTC is interpreted by analyzing the radical pool produced by the LTC and HTC surrounding the hot particle. Moreover, the influence of inlet flow velocity on ignition by hot particles is assessed. Non-monotonic change of ignition delay time with flow velocity is observed and discussed.     

乙烯着火延迟的激波管测量与分子碰撞理论分析

在75%和96%两个不同的稀释度下,测量了乙烯/氧气/氩气混合气的着火延迟时间,实验当量比为1,压力为1.3-3.0 atm,温度为1092-1743 K.实验结果表明,着火延迟时间的对数与温度倒数呈良好的线性关系,在两个稀释度下,着火延迟时间随着温度增加而减少.通过回归分析,得到了乙烯着火关联式.计算得乙烯着火延迟在96%稀释度时是75%稀释度的5倍.采用分子硬球碰撞模型,计算了不同稀释度下,乙烯与氧分子的碰撞次数,在96%稀释度下,乙烯与氧气分子碰撞次数为1.53×10²⁹/（s·cm³）,而在75%稀释度下,该碰撞次数增加为5.99×10³⁰/（s·cm³）,约为前者40倍,而着火延迟时间的差异在两条件下仅为5倍的关系,可能由于位阻因子的影响所致.