Theoretical Calculation on the Piloted Ignition of PMMA

This study presents the comparison of the experimental results and theoretical predictions of the piloted ignition of black PMMA. The model for theoretical calculations included heat, momentum, mass transfer equations and reaction kinetics both in the gas phase and the solid phase, to comprehensively describe the piloted ignition. The experimental samples were thick black PMMA pieces, with the ignition time and the critical surface temperatures at ignition measured using a cone heater under different external radiation heat fluxes. The predictions from the calculations showed good agreement with the experiment at high heat flux, but the deviation was distinct at low heat fluxes, especially for the critical surface temperatures. The fail of the prediction at low heat fluxes was regarded, by analysis, as the result of the neglecting of the decomposition energy term of PMMA in the energy balance equation.     

Predicting the ignition delay of turbulent methane jets using Conditional Source-term Estimation

A predictive simulation of the autoignition process of non-premixed methane in a turbulent jet configuration was performed. Closure for the chemical source-term was obtained using Conditional Source-term Estimation with Laminar Flamelet Decomposition (CSE-LFD). The ambient oxidizer conditions – the high pressure and moderate temperatures characteristic of compression ignition engines – were chosen with the intent to validate the combustion model used under engine-relevant conditions. Validation was obtained by comparison of the predicted ignition delay to experimental results obtained from a shock-tube facility at several initial temperatures. Overall, the combination of full chemistry that has been carefully tuned to predict autoignition of premixed methane–air mixtures under similar temperature/pressure conditions with the CSE-LFD model is able to successfully predict the autoignition delay time of methane–air jets well within the scatter in the experimental data.     

激光脉冲参数对冲击点火的影响

针对激光脉冲参数对冲击点火的影响进行理论分析和数值模拟.首先从冲击波的产生和碰撞过程出发,理论分析冲击脉冲峰值功率、脉宽和上升沿时间对点火的影响;然后通过数值模拟,以点火时间窗口作为评价标准,验证理论分析结果.研究表明:冲击脉冲峰值功率是点火成败的关键因素之一,脉冲宽度则需达到百皮秒以提供足够的点火能量,上升沿时间在小于600 ps的情况下不会对点火造成明显影响. 关键词： 冲击点火 激光冲击脉冲 点火时间窗口     

A numerical simulation of transient ignition and ignition limit of a composite solid by a localised radiant source

An unsteady three-dimensional numerical model has been formulated, coded, and solved to study ignition and flame development over a composite solid fuel sample upon heating by a localised radiant beam in a buoyant atmosphere. The model consists of an unsteady gas phase and an unsteady solid phase. The gas phase formulation consists of full Navier-Stokes equations for the conservation of mass, momentum, energy, and species. A one-step, second-order overall Arrhenius reaction is adopted. Gas radiation is included by solving the radiation transfer equation. For the solid phase formulation, the energy (heat conduction) equation is employed to solve the transient solid temperature. A first-order in-depth solid pyrolysis relation between the solid fuel density and the local solid temperature is assumed. Numerical simulations provide time-and-space resolved details of the ignition transient and flame development and the existence of two types of ignition modes: one with reaction kernel initiated on the surface and the other with ignition kernel initiated in the gas phase. Other primary outputs of the computation are the minimum ignition energy (Joule) for the solid as a function of the external heating rate (Watt). Both the critical heat input for ignition and the optimal ignition energy are identified. Other parameters that were varied over the simulations include: sample thickness, ignition heat source spatial shape factor, and gravity level.     

Simulations and experiments on the ignition probability in turbulent premixed bluff-body flames

The ignition characteristics of a premixed bluff-body burner under lean conditions were investigated experimentally and numerically with a physical model focusing on ignition probability. Visualisation of the flame with a 5 kHz OH* chemiluminescence camera confirmed that successful ignitions were those associated with the movement of the kernel upstream, consistent with previous work on non-premixed systems. Performing many separate ignition trials at the same spark position and flow conditions resulted in a quantification of the ignition probability P_ign, which was found to decrease with increasing distance downstream of the bluff body and a decrease in equivalence ratio. Flows corresponding to flames close to the blow-off limit could not be ignited, although such flames were stable if reached from a richer already ignited condition. A detailed comparison with the local Karlovitz number and the mean velocity showed that regions of high P_ign are associated with low Ka and negative bulk velocity (i.e. towards the bluff body), although a direct correlation was not possible. A modelling effort that takes convection and localised flame quenching into account by tracking stochastic virtual flame particles, previously validated for non-premixed and spray ignition, was used to estimate the ignition probability. The applicability of this approach to premixed flows was first evaluated by investigating the model's flame propagation mechanism in a uniform turbulence field, which showed that the model reproduces the bending behaviour of the S_T-versus-u′ curve. Then ignition simulations of the bluff-body burner were carried out. The ignition probability map was computed and it was found that the model reproduces all main trends found in the experimental study.     

Two-sided ignition of a thin PMMA sheet in microgravity

Numerical computations and a series of experiments were conducted in microgravity to study the ignition characteristics of a thin polymethylmethacrylate (PMMA) sheet (thicknesses of 0.2 and 0.4 mm) using a CO₂ laser as an external radiant source. Two separate ignition events were observed, including ignition over the irradiated surface (frontside ignition), and ignition, after some delay, over the backside surface (backside ignition). The backside ignition was achieved in two different modes. In the first mode, after the laser was turned off, the flame shrank and stabilized closer to the fuel surface. This allowed the flame to travel from the frontside to the backside through the small, open hole generated by the laser’s vaporization of PMMA. In the second mode, backside ignition was achieved during the laser irradiation. The numerical calculation simulating this second process predicts fresh oxygen supply flows from the backside gas phase to the frontside gas phase through the open hole, which mixes with accumulated hot MMA fuel vapor which is ignited as a second flame in the frontside gas phase above the hole. Then, the flame initiated from the second ignition travels through the hole to ignite the accumulated flammable mixture in the backside gas phase near the hole, attaining backside ignition. The first backside ignition mode was observed in 21% oxygen and the second backside ignition mode in 35%. The duration of the laser irradiation appears to have important effects on the onset of backside ignition. For example, in 21% oxygen, the backside ignition was attained after a 3 s laser duration but was not observed after a 6 s laser duration (within the available test time of 10 s). Longer laser duration might prevent two-sided ignition in low oxygen concentrations.     

DNS and LES of spark ignition with an automotive coil

The cycle to cycle combustion variability which is observed in spark-ignition engines is often caused by fluctuations of the early flame development. LES can be exploited for a better understanding and mastering of their origins. For that purpose appropriate models taking into account energy deposition, mixture ignition and transition to propagation are necessary requirements. This paper presents first DNS and LES of spark ignition with a real automotive coil and simplified pin-pin electrodes. The electrical circuit characteristics are provided by ISSIM while the energy deposition is modelled by Lagrangian particles. The ignition model is first evaluated in terms of initial spark radius on a pin-pin ignition experiment in pure air performed at CORIA and EM2C laboratories, showing that it pilots the radius of the torus formed by the initial shock wave. DNS of a quiescent lean propane/air mixture are then performed with this ignition system and a two-step mechanism. The impact of the modelled transferred energy during glow phase as well as the initial arc radius on the minimum ignition energy (MIE) are examined and compared to experimental values. Replacing the two-step chemistry by an analytically reduced mechanism leads to similar MIE but shows a different ignition kernel shape. Finally, LES of turbulent ignition using a Lagrangian arc model show a realistic prediction of the arc shape and its important role on the energy transfer location and thus on the flame kernel shape.     

Effects of blending 2,5-dimethylfuran on the laminar burning velocity and ignition delay time of isooctane/air mixture

The prospects of 2,5-dimethylfuran (DMF) as a bio-derived fuel that can be blended with gasoline are believed to be impressive. However, the effects of blending DMF on the key combustion parameters like the laminar burning velocity and ignition delay time of gasoline/air mixture need to be studied extensively for the successful implementation of the fuel mixture in spark ignition engines. Therefore, a skeletal chemical kinetic mechanism, comprising of 999 reactions among 218 species, has been developed in the present work for this purpose. The proposed chemical kinetic model has been validated against a wide range of experimental data for the laminar burning velocity and ignition delay time of isooctane (representing gasoline), DMF and their blends. It has been found from the present study that the thermal diffusivity of the unburnt gas mixture changes by a very small amount from the corresponding value for the pure isooctane/air mixture when DMF is added. Unlike isooctane, the DMF molecule does not consume H radicals during its primary breakup. Therefore, the maximum laminar burning velocity increases marginally when 50% DMF is blended with isooctane due to the increased presence of H radicals in the flame. The negative temperature coefficient behaviour in the ignition delay time of the isooctane fuel vanishes when 30% DMF (v/v) is blended to it.     

Numerical study of the ignition behavior of a post-discharge kernel in a turbulent stratified crossflow

Ensuring robust ignition is critical for the operability of aeronautical gas-turbine combustors. For ignition to be successful, an important aspect is the ability of the hot gas generated by the spark discharge to initiate combustion reactions, leading to the formation of a self-sustained ignition kernel. This study focuses on this phenomena by performing simulations of kernel ignition in a crossflow configuration that was characterized experimentally. First, inert simulations are performed to identify numerical parameters correctly reproducing the kernel ejection from the ignition cavity, which is here modeled as a pulsed jet. In particular, the kernel diameter and the transit time of the kernel to the reacting mixture are matched with measurements. Considering stochastic perturbations of the ejection velocity of the ignition kernel, the variability of the kernel transit time is also reproduced by the simulations. Subsequently, simulations of a series of ignition sequences are performed with varying equivalence ratio of the fuel-air mixture in the crossflow. The numerical results are shown to reproduce the ignition failure that occurs for the leanest equivalence ratio ($?=0.6$). For higher equivalence ratios, the simulations are shown to capture the sensitivity of the ignition to the equivalence ratio, and the kernel successfully transitions into a propagating flame. Significant stochastic dispersion of the ignition strength is observed, which relates to the variability of the transit time of the kernel to the reactive mixture. An analysis of the structure of the ignition kernel also highlights the transition towards a self-propagating flame for successful ignition conditions.     

Glowing ignition of wood: the onset of surface combustion

A theoretical model for wood pyrolysis including char surface oxidation is presented. The main objective is to expose the physical mechanisms governing glowing ignition. By “glowing ignition,” we mean the onset of surface combustion. The char surface oxidation, which can lead to glowing ignition, is considered at the surface boundary condition. Two regimes of char surface oxidation, namely, kinetic and diffusion-controlled, are distinguished. Depending on the char surface oxidation resistances, the char surface oxidation as either kinetic- or diffusion-controlled can be identified. A criterion for glowing ignition is developed based on a surface energy balance. A numerical result shows that according to the present glowing ignition criteria, an inflection point of the surface temperature history can indicate glowing ignition. Generally, a good agreement between theoretical and experimental results at glowing ignition is obtained.     

激光聚变冲击点火物理特性研究

冲击点火是一种新型的惯性约束聚变点火方案,该点火方案具有低点火能量阈值、高增益和较好的流体力学稳定性等优点.本文针对冲击点火的物理特性进行理论分析和数值模拟.首先以一维平板模型对冲击波碰撞的物理过程进行理论研究和分析;然后通过辐射流体力学数值模拟对冲击点火的物理特性进行研究,验证理论分析结果,并考察冲击加载时间对点火的影响. 关键词： 冲击点火 冲击波碰撞 辐射流体力学模拟     

快点火中相对论电子束能量沉积的动理学研究

针对相对论快电子束在高密度压缩芯区等离子体中的能量沉积过程开展物理建模、程序研制和数值模拟研究。从等离子体粒子碰撞的基本物理出发,综合考虑了高能电子与背景等离子体之间的短程两体碰撞过程和长程集体效应,建立了相对论Fokker-Planck动理学模型,通过采用球谐展开的方法,推导得到了适于数值求解的方程形式并根据方程特点开展相应的数值算法研究及程序研制并完成了物理考核,对快点火能量沉积的典型物理算例进行了模拟研究,并针对即将在神光Ⅱ升级装置上开展的快点火物理实验进行了初步的物理分析。     

惯性约束聚变中氘氚燃料整体点火与燃烧条件研究

在局域热动平衡近似下, 利用能量平衡关系, 建立热核系统整体点火能量平衡方程, 对该方程求解得到热核反应系统点火阈值. 在计算和分析的基础上给出参数空间的点火关系, 以及该条件受装量、核子数比以及返照率等因素的影响情况. 点火时刻面密度越大, 则对应的点火温度越低, 并且电子-辐射温度脱离越小, 越接近三温平衡的点火状态; 反之则在点火时刻对应较大温度脱离. 更重要的是, 该分析方法还可以根据点火时刻系统的物理状态, 通过线性稳定性分析方法, 描述出系统的后续行为, 也就是说, 可以判断出这样的热核系统能否继续升温并实现深度燃烧.     

Experimental analysis of a double-spark ignition system

The spark that ignites the combustible mixtures is a discharge produced between the electrodes of a spark plug, connected to the secondary of a coil at the high voltage. Optimum combustion requires a steady spark, in a volume as large as is possible, and with maximum energy. We propose a solution to increase the plasma volume and present electrical discharge parameters as a function of inter-electrode distances, pressures in the test-reactor and the width of the electrical pulses of the power supply.     

Detailed investigation of ignition by hot gas jets

Experimental and numerical investigations of the ignition of hydrogen/air mixtures by jets of hot exhaust gases are reported. An experimental realisation of such an ignition process, where a jet of hot exhaust gas impinges through a narrow nozzle into a quiescent hydrogen/air mixture, possibly initiating ignition and combustion, is studied. High-speed laser-induced fluorescence (LIF) image sequences of the hydroxyl radical (OH) and laser Schlieren methods are used to gain information about the spatial and temporal evolution of the ignition process. Recording temporally resolved pressure traces yields information about ambient conditions for the process. Numerical experiments are performed that allow linking these observables to certain characteristic states of the gas mixture. The outcome of numerical modelling and experiments indicates the important influence of the hot jet temperature and speed of mixing between the hot and cold gases on the ignition process. The results show the quenching of the flame inside the nozzle and the subsequent ignition of the mixture by the hot exhaust jet. These detailed examinations of the ignition process improve the knowledge concerning flame transmission out of electrical equipment of the type of protection flameproof enclosure.     

激光二极管点火系统研究进展

综述了激光点火方式、激光二极管点火系统的组成及最新发展。提出了一种激光二极管点火系统结构,描述了各部分的关键技术。介绍了国外在光回路检测、保险与解保险及多路点火方面的研究成果,以及国内在温度控制、光纤芯径对点火阈值功率的影响和激光点火器设计的研究成果。指出了当前激光点火技术在武器装备中的应用和现阶段激光点火遇到的技术难点。     

金锥-聚合物球壳靶制备初步研究

 报道了研制快点火物理实验用带金锥固体球壳靶时,采用多次乳化技术制备聚合物空心微球,精密车床加工和电镀技术制备金锥,精密加工实现聚合物微球表面打孔的工艺。窄分布聚苯乙烯制备的空心微球直径与壁厚分布较窄。电镀液的配方、pH值、镀前处理、尖端效应等对镀层的表面质量和镀层与芯轴之间的结合力有重要的影响。采用精密车床加工可以在聚合物微球表面获得直径约120 μm的孔,初步实现金锥与聚合物微球的连接。     

Numerical simulations of the ignition of n-heptane droplets in the transition diameter range from heterogeneous to homogeneous ignition

Spontaneous ignition of single n-heptane droplets in a constant volume filled with air is numerically simulated with the spherical symmetry. The volume is closed against mass, species, and energy transfer. The numerical model is fully transient. It continues calculation even after the droplet has completely vaporized, and therefore can predict pre-vaporized ignition. Initial pressure and initial air temperature are fixed at 3 MPa and 773 K, respectively. The droplet is initially at room temperature, and its diameter is between 1 and 100 μm. When the overall equivalence ratio is fixed to be sufficiently large, there exists no ignition limit in terms of initial droplet diameter d₀, and the ignition delay takes a minimum value at certain d₀. In such a case, transition from the heterogeneous ignition to the homogeneous ignition with decreasing d₀ is observed. When d₀ is fixed to be so small that the ignition would not occur in an infinite volume of air, the ignition delay takes a minimum value at certain , which is less than unity. Two-stage ignition behavior is investigated with this model. Ignition delay of a cool flame has the dependence on d₀ that is similar to that of ignition delay of a hot flame when is unity. When is almost zero, the ignition limit for cool flame in terms of d₀ is not identified unlike that for hot flame.     

超音速等离子体点火过程的三维数值模拟

为了研究等离子体点燃超音速混合气流的过程,设计并验证了超音速燃烧室的三维计算模型,计算出了燃烧室等离子体点火时的流场参数和化学反应规律,分析了等离子体点火对燃烧室内燃烧的影响。计算结果表明:高温等离子体射流的滞止作用通过增加混合气在燃烧室内的停留时间提高了点火效率; 等离子体点火时燃烧区域的压力扩散比较充分,内部为压力相对平衡的低速流动; 高温等离子体射流高速射向混合气流时产生的速度矢量偏移扩大了点火面积,从而使点火效率得到提高; 氢气、空气燃烧的燃烧产物主要是水,燃烧区域局部温度主要受局部放热反应的影响。