水雾作用下甲烷/空气预混火焰的光谱特性

利用中阶梯光栅光谱仪,对甲烷,空气层流预混火焰以及水雾作用过程中的火焰发射光谱特性进行了实验研究,对比分析了水雾对预混火焰燃烧过程中自由基离子发射光谱的影响,探讨了水雾抑制甲烷燃烧过程中的化学作用机理.结果表明,火焰阵面OH*、CH*自由基离子发射光谱强度随着火焰阵面水雾载荷比的增大而减小;足量的水雾作用可抑制预混火焰中OH*、CH*和HCO*等链引发自由基的生成,增强甲烷燃烧链式反应中的三体反应过程,促进甲烷预混火焰的链销基反应的发生,在抑制甲烷预混火焰燃烧过程中起重要作用.     

催化重整反应对层流火焰传播速度的影响

通过对冲火焰实验台测量了不同当量比下的甲烷与空气的预混气和经预催化后的预混气的层流火焰传播速度。结果表明，当甲烷和水蒸气经过催化重整反应后，预混气的火焰传播速度明显提高。同时，反应产生的氢气还扩展了甲烷的火焰传播界限（贫限）。此研究从实验角度证明催化重整反应产生的氢气对烃类燃料的火焰传播和稳定均有积极的影响。     

氢气和水蒸气对甲烷/空气层流火焰传播速度的影响

为了较为系统地认识氢气和水蒸气对火焰传播的影响，应用CHEMKIN-Ⅱ程序计算了甲烷/空气层流预混火焰传播速度，并就水蒸气和氢气对火焰传播速度的影响做了定量计算与分析．结果表明：氢气能使火焰传播速度大幅提高，且当初始温度升高时，氢气对火焰传播速度的提高作用增大；水蒸气的加入会使甲烷/空气层流预混火焰传播速度降低，并且在空气过量时燃料越少其影响越弱；当氢气和水蒸气同时加入预混气时，水蒸气的加入会使氢气对火焰传播速度的提高作用减弱．     

障碍物结构对管道中预混火焰加速的影响

在一端封闭、一端开口的火焰传播管中均匀布置障碍物，研究了障碍物结构对管道中预混火焰传播的影响。结果表明，由于障碍物的扰动，火焰不断加速，在阻塞比相同的条件下，最终的火焰稳态速度与障碍物的形状和间距基本无关，其中障碍物间距仅仅影响火焰的加速速率，在障碍物间距约等于火焰传播管内径（W/D≈1.0）时，平均火焰速度达到最大值，火焰到达稳态传播的距离最短。同时，本文用一维简化模型模拟了火焰在障碍物管道中的加速过程，计算结果与实验测试结果在定性上比较吻合，说明在管内火焰速度较低的情况下，用一维可压缩流动近似处理能初步揭示管内火焰的加速机制。     

微尺度甲烷扩散火焰特性的数值解析

以均匀空气流中圆管形成的甲烷扩散火焰为对象，用数值解析的方法研究了微尺度扩散火焰的火焰结构和燃烧特性．燃烧反应采用甲烷／空气一步总括反应，喷管壁面绝热．在Re一定的情况下，改变喷口尺寸和喷口流速，考察了微扩散火焰的结构和火焰熄灭的尺度效应．计算结果表明，Re=12条件下，喷口直径为0．07mm时达到熄灭极限；稳定燃烧区的最小总放热率约为0．5W；微尺度条件下，Da对火焰结构和火焰熄灭有显著影响，熄火附近的Da的数量级在0．01．     

微细通道中甲烷与氧气的预混燃烧

对微细通道中甲烷/氧气预混火焰传播性质进行了实验研究．确定了微细通道中不同甲烷浓度下的火焰传播速度,以及混合气体流量与火焰传播速度的关系．结果表明,混合气体流量对火焰传播速度有显著的影响,在微细通道中火焰传播速度的分布趋势与宏观尺度下火焰传播速度的分布趋势基本相同,但在数值上随着流量的不同相差较大．实验证明,在室温条件下,甲烷和氧气预混火焰可以在细管中稳定停留在一点燃烧,并且可以很好地控制其移动;当量比为1．0时火焰传播速度受流量影响最大．     

障碍物管道中湍流火焰发展的数值模拟

应用湍流马赫数修正的非稳态可压缩性K－ε-f-gr四方程湍流模型，模拟了半开口狭长管道中重复布置的障碍物引起的湍流火焰加速现象。结果表明，障碍物产生的扰动对加强燃烧和湍流输运的影响很大。随着火焰向前传播，火焰穿过障碍物时发生变形，反应区越来越长，且火焰速度逐渐上升。同时，火焰速度和管内压力的计算结果与实验测量值吻合良好，修正后的湍流模型能较真实地模拟障碍物管内预混火焰的发展过程。     

爆炸波除灰器中火焰传播及压力波形研究

燃烧气脉冲发生器应用于电站锅炉除灰。其工作原理是预混可燃气体在右端部分开口,内部有障碍物的容器中快速燃烧。形成一定的压力脉冲。并产生作用于积灰表面的射流和冲击波。火焰在湍流扰动装置的作用下不断加速。容器中的压力不断上升。火焰传播愈快,压力波形愈陡。压力锋值愈高。针对这些现象。主要研究了乙块、水煤气、液化石油气和甲烷四种燃料,在不同燃料浓度、不同阻塞比时对火焰传播的影响,分析了不同燃料浓度下对压力波形的影响。     

催化重整反应加氢对预混气火焰传播速度的影响

根据“驻定火焰法”基本原理，利用对冲火焰实验装置系统测量了甲烷与少量氢气预混气的火焰传播速度。结果表明，与纯甲烷时相经，含氢条件下的火焰传播速度有了明显的提高。实验氢气是甲烷与水蒸汽在催化重整反应中产生的。这说明，催化剂的存在使得掺水燃料或乳化油在燃烧的同时产生氢气。氢气的加入提高了混和气的火焰传播速度，改善了燃烧过程。这项技术在实践中有着重要意义。还对实验条件进行了数值模拟和计算，计算结果与实验结果符合较好。     

微尺度条件下反复熄燃火焰特性的实验研究

在可视化微尺度燃烧实验台上进行甲烷和氧气的燃烧试验,利用高速数码照相机捕捉到了火焰面在微通道内的传播过程,测试分析了不同进气流量下反复熄燃火焰的可燃极限、火焰传播速度和火焰间隔时间,获得了反复熄燃火焰(Flames with repetitive extinction and ignition, FREI)的燃烧特性。结果表明,随着甲烷进气流速的增加,可以形成FREI火焰的氧气进气流速范围也在扩大;在甲烷进气流速一定的情况下,随着氧气进气流速的增加,火焰的传播速度也逐渐增加,并且火焰重复点燃的间隔时间呈现先变大后逐渐变小的规律,即火焰重复点燃的频率先变慢后又逐渐变快直至火焰熄灭。     

Large eddy simulation of premixed hydrogen/methane/air flame propagation in a closed duct

In this paper, large eddy simulation (LES) is performed to investigate the propagation characteristics of premixed hydrogen/methane/air flames in a closed duct. In LES, three stoichiometric hydrogen/methane/air mixtures with hydrogen fractions (volume fractions) of 0, 50% and 100% are used. The numerical results have been verified by comparison with experimental data. All stages of flame propagation that occurred in the experiment are reproduced qualitatively in LES. For fuel/air mixtures with hydrogen fractions of 0 and 50%, only four stages of “tulip” flame formation are observed, but when the hydrogen fraction is 100%, the distorted “tulip” flame appears after flame front inversion. In the acceleration stage, the LES and experimental flame speed and pressure dynamic coincide with each other, except for a hydrogen fraction of 0. After “tulip” flame formation, all LES and experimental flame propagation speeds and pressure dynamics exhibit the same trends for hydrogen fractions of 0 and 100%. However, when the hydrogen fraction is 50%, a slight periodic oscillation appears only in the experiment. In general, the different structures displayed in the flame front during flame propagation can be attributed to the interaction between the flame front, the vortex and the reverse flow formed in the unburned and burned zones.     

Effect of initial pressure on flame–shock interaction of hydrogen–air premixed flames

By utilizing a newly designed constant volume combustion bomb (CVCB), turbulent flame combustion phenomena are investigated using hydrogen–air mixture under the initial pressures of 1 bar, 2 bar and 3 bar, including flame acceleration, turbulent flame propagation and flame–shock interaction with pressure oscillations. The results show that the process of flame acceleration through perforated plate can be characterized by three stages: laminar flame, jet flame and turbulent flame. Fast turbulent flame can generate a visible shock wave ahead of the flame front, which is reflected from the end wall of combustion chamber. Subsequently, the velocity of reflected shock wave declines gradually since it is affected by the compression wave formed by flame acceleration. In return, the propagation velocity of turbulent flame front is also influenced. The intense interaction between flame front and reflected shock can be captured by high-speed schlieren photography clearly under different initial pressures. The results show that the propagation velocity of turbulent flame rises with the increase of initial pressure, while the forward shock velocities show no apparent difference. On the other hand, the reflected shock wave decays faster under higher initial pressure conditions due to the faster flame propagation. Moreover, the influence of initial pressure on pressure oscillations is also analyzed comprehensively according to the experimental results.     

Comprehensive study of the evolution of an annular edge flame during extinction and reignition of a counterflow diffusion flame perturbed by vortices

The structure of a time-dependent methane/enriched-air flame established in an axisymmetric, laminar counterflow configuration is investigated, as the flame interacts with two counterpropagating toroidal vortices. Computationally, the time-dependent equations are written using a modified vorticity–velocity formulation, with detailed chemistry and transport, and are solved implicitly on a nonstaggered, nonuniform grid. Boundary conditions are chosen to create local extinction and reignition in the vicinity of the axis of symmetry. Experimentally, CO planar laser-induced fluorescence (PLIF), OH PLIF, and an observable proportional to the forward reaction rate (RR) of the reaction CO+OH→CO₂+H are measured. Particle image velocimetry (PIV) is used to characterize the velocity field of the vortical structures and to provide detailed boundary conditions for the simulations. Excellent agreement is found between model and experiments to the minutest morphological details throughout the interaction. The validated model is then used to probe the dynamics of the two-dimensional extinction process with high temporal resolution. During the initial phase of the interaction, the flame is locally extinguished by the two vortices. The resulting edge flame propagates outward as an extinction front, with a structure that does not depart significantly from that of a diffusion flame. The front recedes from the axis of symmetry with a negative propagation speed that reaches a value as large as six times that of the freely propagating laminar flame with the same reactant concentrations found at the stoichiometric surface. As the front propagates outward, it transitions to an ignition front, and it reaches a positive propagation speed comparable to that of the freely propagating laminar flame. During this transition, it develops a characteristic premixed “hook,” with a lean premixed branch, a stoichiometric segment that evolves into the remnant of the original primary diffusion flame, and a much weaker secondary diffusion flame resulting from a secondary peak in heat release in the original unperturbed diffusion flame. No evidence of a distinct rich premixed flame is found. The edge flame stabilizes at a radial location where the local gaseous speed equals the propagation speed of the front. When the local perturbation has decayed below the flame propagation speed, the flame edge starts reigniting the mixing layer as an ignition wave that propagates with an essentially frozen structure along the stoichiometric surface until the original diffusion flame structure is fully recovered. Implications for flamelet modeling of turbulent flames with local extinction are discussed.     

Interaction of pressure wave and propagating flame during knock

To determine the mechanism of interaction between a pressure wave and a propagating flame during knock, normal combustion and knock are numerically modeled in a simplified one-dimensional hydrogen-fueled spark ignition engine. The heat release rate of the flame front during knock abruptly increases when the pressure wave propagates through the reaction zone. The pressure wave in the diffusion zone perturbs temperature and thus causes thermal runaway at positions with low temperature and high reactant concentrations. Analysis of the Damköhler number (the ratio of gas dynamic time to chemical reaction time) and the estimated overpressure revealed that abruptly raised heat release rate during knock facilitates the amplification of the pressure wave and reinforces the interaction between pressure wave and chemical heat release.     

The effect of a perforated plate on the propagation of laminar hydrogen flames in a channel – A numerical study

Laminar hydrogen flame propagation in a channel with a perforated plate is investigated using 2D reactive Navies-Stokes simulations. The effect of the perforated plate on flame propagation is treated with a porous media model. A one step chemistry model is used for the combustion of the stoichiometric H₂–air mixture. Numerical simulations show that the perforated plate has considerable effect on the flame propagation in the region downstream from the perforated plate and marginal effect on the upstream region. It is found to squeeze the flame front and result in a ring of unburned gas pocket around the flame neck. The resulting abrupt change in flow directions leads to the formation of some vortices. Downstream of the perforated plate, a wrinkled “M”-shape flame is observed with “W” shape flame speed evolution, which lastly turns back to a convex curved flame front. Parametric studies have also been carried out on the inertial resistance factor, porosity, perforated plate length and its location to investigate their effects on flame evolution. Overall, for parameter range studied, the perforated plate has an effect of reducing the flame speed downstream of it.     

Effects of the external turbulence on centrally-ignited spherical unstable Ch4/H2/air flames in the constant-volume combustion bomb

In the present study, we conducted experiments to investigate the effects of external turbulence on the development of spherical H₂/CH₄/air unstable flames developments at two different equivalence ratios associated with different turbulent intensities using a spherical constant-volume turbulent combustion bomb and high speed schlieren photography technology. Flame front morphology and acceleration process were recorded and different effects of weak external turbulent flow field and intrinsic flame instability on the unstable flame propagation were compared. Results showed the external turbulence has a great influence on the unstable flame propagation under rich fuel conditions. For fuel-lean premixed flames, however, the effects of external turbulence on the morphology of the cellular structure on the flame front was not that obvious. Critical radius decreased firstly and then kept almost unchanged with the augment of the turbulence intensity. This indicated the dominating inhibiting effect of flame stretch on the turbulent premixed flame at the initial stage of the flame front development. Beyond the critical radius, the acceleration exponent was found increasing with the enhancement of initial turbulence intensity for fuel-lean premixed flames. For fuel-rich conditions, however, the initial turbulence intensity had little effect on acceleration exponent. In order to evaluate the important impact of the intrinsic flame instability and external turbulent flow field for spherical propagating premixed flames, intrinsic flame instability scale and average diameter of vortex tube were calculated. Intrinsic flame instability scale decreased greatly and then stayed unchanged with the propagation of the flame front. The comparison between intrinsic flame instability scale and average diameter of vortex tube demonstrated that the external turbulent flow filed will be more important for the evolution of wrinkle structure in the final stage of the flame propagation, when the turbulence intensity was more than 0.404 m/s.     

Effects of combined obstacles on deflagration characteristics of hydrogen-air premixed gas

To study the mechanism by which an increase in the number of obstacles affects the propagation of hydrogen-air premixed gas explosions under a constant overall volume of obstacles, a large eddy simulation method was used to carry out numerically simulate configurations with different distribution modes of combined obstacles. The study focused on the flame structure, evolution process of overpressure dynamics, and flame-flow coupling relationship. The results showed that the flame propagation velocity and flame front area are increased during the conversion of the combined obstacles from 1-30 mm to 4–7.5 mm, while the flame front area logarithmically depends on the number of obstacles. The flames gradually develop from “corrugated flamelets” to “thin reaction zones” in different distribution modes. In addition, the results showed that although increasing dispersion increases the explosion overpressure, a critical number of obstacles likely exist. Beyond the critical point, explosion overpressure peak no longer strongly varies with the number of obstacles. Furthermore, for working configurations with different numbers of obstacles, an increase in the overall number of obstacles before reaching the same number of obstacles weakly affects the flame shape and flow rate of the flame front. This study provides theoretical guidelines for safety designs to prevent hydrogen-air premixed gas explosion in obstructed spaces.     

Experimental study of flame propagation across a perforated plate

Flame propagation across a single perforated plate was experimentally studied in a square cross-section channel. Experiments were performed in premixed hydrogen-air mixture with different equivalence ratios and initial pressures, aiming at identifying the parametric influence. High-speed schlieren photography and pressure records were used to capture the flame front and obtain the pressure build-up. Four stages for the flame front crossing the perforated plate were obtained, namely, laminar flame, jet flame, turbulent flame and secondary flame front. Following ignition, a laminar flame was obtained, which was nearly not affected by the confinement. This laminar flame was squeezed to pass through the perforated plate, producing the jet flame with a step change on velocity. Turbulent flame was generated by merging the jets, which facilitated the acceleration of the flame front. Secondary flame front induced by Rayleigh-Taylor instability was clearly observed in the process of the turbulent front moving forward. Both velocity and pressure are enhanced in this stage. Parametric studies suggested that the secondary flame front is more obvious in the stoichiometric mixture with higher initial pressure, and characterized by a faster propagation velocity and a bigger pressure rise.