On Flame-Edge Propagation

Contemporary interest exists for understanding how reaction zones stabilize and counter-propagate against incoming reactants. Images of flame position, morphology and dynamics are presented primarily from CH planar laser-induced fluorescence (CH-PLIF) measurements. Observations of the leading-edge flame behavior with respect to upstream propagation and recession downstream are made with sequential CH-PLIF imaging, and data have been revisited in light of the recent research of McCraw et al. (Flow Turbul Combust 70(1):83–97, 2007). It is found that in cases where a distinct branch of the outer (fuel-lean) edge of the reaction zone is present, the edge of the flame is either witnessed to propagate upstream or locally disappear. In cases where no distinct branch other than the main branch is observed, the flame is witnessed to either remain stationary or drop back downstream. These observations support the notion that structures in the low speed, outer edge of the reaction zone are involved in the upstream phase of the flame propagation.     

Experimental investigation of flame/solid interactions in turbulent premixed combustion

An experimental study has been carried out to investigate the interaction between propagating turbulent premixed flames and solid obstacles. The experimental rig was configured specifically to allow detailed measurements with laser-based optical diagnostics. A wall-type solid obstacle was mounted inside a laboratory-scale combustion chamber with rectangular cross-section. The flame was initiated, by igniting a combustible mixture of methane in air at the center of the closed end of the combustion chamber. The flame front development was visualized by a high-speed (9000 frame/s) digital video camera and flame images were synchronized with ignition timing and chamber pressure data. The tests were carried out with lean, stoichiometric and rich mixtures of methane in air. The images were used to calculate highly resolved temporal and spatial data for the changes in flame shape, speed, and the length of the flame front. The results are discussed in terms of the influence of mixture equivalence ratio on the flame structure and resulting overpressure. The reported data revealed significant changes in flame structure as a result of the interaction between the propagating flame front and the transient recirculating flow formed behind the solid obstacle. Combustion images show that the flame accelerates and decelerates as it impinges on the obstacle wall boundaries. It is also found that the mixture concentrations have a significant influence on the nature of the flame/solid interactions and the resulting overpressure. The highest flame speed of 40 m/s was obtained with the unity fuel–air equivalence ratio. Burning of trapped mixture behind the solid obstruction was found to be highly correlated with the flame front length and the rate of pressure rise.     

Non-uniform velocity profile mechanism for flame stabilization in a porous radiant burner

Industrial processes where the heating of large surfaces is required lead to the possibility of using large surface porous radiant burners. This causes additional temperature uniformity problems, since it is increasingly difficult to evenly distribute the reactant mixture over a large burner surface while retaining its stability and keeping low pollutant emissions. In order to allow for larger surface area burners, a non-uniform velocity profile mechanism for flame stabilization in a porous radiant burner using a single large injection hole is proposed and analyzed for a double-layered burner operating in open and closed hot (laboratory-scale furnace, with temperature-controlled, isothermal walls) environments. In both environments, local mean temperatures within the porous medium have been measured. For lower reactant flow rate and ambient temperature the flame shape is conical and anchored at the rim of the injection hole. As the volumetric flow rate or furnace temperature is raised, the flame undergoes a transition to a plane flame stabilized near the external burner surface. However, the stability range envelope remains the same in both regimes.     

巷道支护对瓦斯爆炸火焰传播速度影响的实验研究

通过对实验室中几何尺寸为80×80mm钢制管道内部加装高度分别为4mm、8mm、12mm、16mm的四种螺旋环,模拟巷道支护结构对瓦斯爆炸火焰传播速度的影响。采用高精度动态数据采集分析系统,测量爆炸过程中的火焰传播速度,研究了不同高度的螺旋环对火焰传播速度的影响。结果表明：在螺旋环其他条件相同的情况下,管内瓦斯爆炸火焰传播速度随螺旋环高度、圈数增加而增加。并从理论上进行了分析,其主要原因为螺旋环高度越大、圈数越多,产生湍流度的程度亦愈大,对瓦斯爆炸火焰加速亦愈明显。试验结果对巷道支护的选择有指导意义。     

Modelling of a Premixed Swirl-stabilized Flame Using a Turbulent Flame Speed Closure Model in LES

This paper proposes a combustion model based on a turbulent flame speed closure (TFC) technique for large eddy simulation (LES) of premixed flames. The model was originally developed for the RANS (Reynolds Averaged Navier Stokes equations) approach and was extended here to LES. The turbulent quantities needed for calculation of the turbulent flame speed are obtained at the sub grid level. This model was at first experienced via an test case and then applied to a typical industrial combustor with a swirl stabilized flame. The paper shows that the model is easy to apply and that the results are promising. Even typical frequencies of arising combustion instabilities can be captured. But, the use of compressible LES may also lead to unphysical pressure waves which have their origin in the numerical treatment of the boundary conditions.     

爆炸压力积聚工况下石松子粉尘爆炸火焰传播特性

搭建了一套兼具承压和可视性能粉尘爆炸实验平台,在压力积聚工况下实验研究了石松子粉尘爆炸火焰传播特性。实验结果表明:压力积聚工况下的石松子粉尘爆炸火焰呈现空间离散的束状结构,火焰锋面呈齿状。随着粉尘浓度的提升,火焰连续性增强,锋面趋于平滑,亮度增加,并在750g/m^3达到最佳。不同浓度条件下的石松子粉尘爆炸火焰在传播过程中均呈现明显的速度脉动特征,但脉动频率随粉尘浓度的增大而减小。爆炸火焰平均传播速度随粉尘浓度的增大先增大后减小,并在750g/m^3达到最高。不同浓度条件下的石松子粉尘爆炸火焰前期传播速度均高于后期传播速度。     

Influences of acoustic instabilities on turbulent-flame propagation

Influences of acoustic instabilities on premixed turbulent-flames have been studied experimentally in a Taylor–Couette (TC) combustor for downward flame propagation in a turbulent flow-field generated in the annulus between two cylinders. Flow-field velocities were measured at a fixed location upstream of the propagating flame using laser-doppler velocimetry, while flame speeds were determined from video-recorded images. It is found that the existence of pre-ignition turbulence in the combustor (generated by rotation of the combustor-cylinder walls) does not eliminate acoustic instabilities, however as the level of pre-ignition turbulence is increased the influence of the secondary acoustic instability on the turbulent-flame speed becomes insignificant. For low intensities of pre-ignition turbulence the flame is found to accelerate during the latter stages of flame propagation, while for high levels of pre-ignition turbulence the flame propagates at a statistically constant speed, even though velocity fluctuations have been substantially amplified by the time the flame reaches the bottom end of the combustor as a result of acoustic instability.     

Soot Extraction from a Laminar Hydrocarbon Flame by Means of an Electric Field

Soot precipitation from a vertical laminar hydrocarbon flame onto a rod mounted outside the flame and having the same negative potential as the burner is investigated experimentally. The voltage-current characteristics of the burner-rod-upper grid electrode system and the currents flowing to each of these elements are determined when both negative and positive potentials are supplied to the burner. The pattern of growth of the soot deposition on the rod with increase in the negative potential is studied. The following fundamental effect is revealed: there is no soot extraction when the burner potential is positive. The experimental results obtained are qualitatively explained.__________Translated from Izvestiya Rossiiskoi Academii Nauk, Mekhanika Zhidkosti i Gaza, No. 2, 2005, pp. 6–13.Original Russian Text Copyright © 2005 by Vatazhin, Golentsov, and Likhter.     

甲烷/煤尘复合爆炸火焰的传播特性

为揭示甲烷/煤尘复合爆炸火焰的传播机理,利用气粉两相混合爆炸实验系统,在低于甲烷爆炸下限条件下,采用高速摄影机记录火焰传播图像,通过热电偶采集火焰温度,研究了煤尘种类以及甲烷体积分数对甲烷/煤尘复合火焰传播特性的影响。结果表明:挥发分是衡量煤尘燃烧特性的主导因素;随着煤尘挥发分的升高,燃烧反应增强,火焰传播速度升高,火焰温度升高;挥发分含量差异较小时,水分含量越低,燃烧反应越剧烈;在相同条件下,焦煤的燃烧反应强度最高,其次为长焰煤,最后为褐煤;随着甲烷体积分数的增加,煤尘颗粒的燃烧可由释放挥发分的扩散燃烧转变为气相预混燃烧,燃烧反应增强,火焰传播速度和火焰温度显著升高;热辐射和热对流作用促进煤尘颗粒热解,释放挥发分进行燃烧反应,维持复合火焰的持续传播;随着混合体系中甲烷体积分数的增加,混合爆炸机制由粉尘驱动型爆炸转为气体驱动型爆炸,燃烧反应增强;甲烷/煤尘复合爆炸火焰可由未燃区、预热区、气相燃烧区、多相燃烧区和焦炭燃烧区5部分组成,湍流扰动导致燃烧介质空间分布存在差异,使得燃烧区无规则交错分布。     

Propagation of Flames in the Limit of Zero Ignition Temperature

This paper is devoted to the study of planar deflagration flames. The combustion takes place in a gaseous mixture in which N chemical species B ₁,…,B _N are subject to a complex chemical network of R chemical reactions: In the limit of zero ignition temperature, existence of traveling waves for any speed in a range [c ₀,+∞) is proved for exothermic networks. Non-existence is proved for c∈(0,c ^*). A new index argument is introduced which is of independent interest. In the case of Lewis number less than 1 and of non-competitive networks, the set of speeds is exactly the interval [c ₀,+∞) (c ₀?=?c ^*).     

丙烷-空气爆燃波的火焰面在直管道中的加速运动

对丙烷 空气爆燃波的火焰面 (以下简称爆燃火焰 )在直管道中加速运动的规律及其影响因素作了初步实验研究 ,包括爆燃火焰在光滑内壁管道中的传播状况 ;管道直径和点火能量的变化以及当管道内有障碍物时对火焰加速度的影响。以上研究也涉及了非稳定爆轰波的火焰面在直管道中的加速运动。根据这一研究结果 ,对目前按常规设计和使用的工业管道阻火器的安全性和可靠性提出了质疑。     

Effects of Small- and Large-Scale Structures in a Piloted Jet Diffusion Flame

Laser Doppler Anemometry (LDA) and Planar Laser-Induced Fluorescence (PLIF) measurements have been performed in a turbulent nonpremixed jet flame. One of the features of this configuration is a central co-axial fuel jet surrounded by a turbulent annular air flow. The whole is placed within a low-speed coflowing air stream. This three-flow system with turbulent primary air differs from flow systems used for nonpremixed jet flames reported in the literature and is very useful for obtaining information on the mixing process between fuel and primary air. Next to the characterization of the velocity field, special attention has been paid to the conditional seeding of the central fuel jet and of the annular air flow. Together with visualizations of the OH radical, an important combustion intermediate which is formed during combustion, and the NO radical, which is seeded to the central jet flow, the resulting statistics reveal the properties of small- and large-scale structures in the flame.     

The lateral movement of the three-dimensional cellular flame at low Lewis numbers

The lateral movement of the three-dimensional (3-D) cellular flame at low Lewis numbers is numerically investigated. The equation used is the compressible Navier–Stokes equation including a one-step irreversible chemical reaction. We superimpose the hexagonal disturbance with the peculiar wave number on the stationary plane flame and calculate the evolution of the disturbed flame. When the Lewis number is unity, i.e., only the hydrodynamic effect has an influence on the flame instability, the stationary cellular flame is formed. When the Lewis number is lower than unity, i.e., the diffusive-thermal and hydrodynamic effects have an influence, the laterally moving cellular flame is formed. With a decrease in the Lewis number, the laterally moving velocity of the cell increases. The laterally moving velocity of the three-dimensional cellular flame is much larger than that of the two-dimensional (2-D) cellular flame. Because, the increment of local temperature at the convex flame front toward the unburned gas in the three-dimensional flame is great compared with that in the two-dimensional flame.     

Studies on Lifted Jet Flames in Coflow: The Stabilization Mechanism in the Near- and Far-Fields

This paper presents the results of a parametric study concerning the phenomenon of liftoff of a nonpremixed jet flame. The dependence of liftoff height on jet exit velocity and coflow velocity is described. It is shown that lifted flames become less sensitive to jet exit velocity as the stabilization point recedes from the burner exit. The results reveal that in cases of extreme liftoff height, increases in jet exit velocity with a constant coflow cause some ethylene flames to stabilize closer to the burner. The success of current theories on lifted flame stabilization in comparison to the experimental results of this study are assessed. The existence of multiple regimes for flame stabilization, incorporating aspects of both premixed and nonpremixed combustion, is proposed.     

M80S20喷涂层,喷熔层及激光涂敷层的显微组织和耐磨性之研究

本文采用(Fe,Cr,Ni,W,Mo)_(80)(B,Si,C)_(20)铁基非晶自熔合金粉末对20钢材进行了热喷涂、喷熔和激光涂敷等表面处理,研究了这3种涂层的显微组织、成分及硬度分布和耐磨性。作者指出,喷涂层由于层内有一定数量高硬度的合金非晶质点存在,其耐磨性是钢基体的4.9倍;喷熔层内合金晶化产生的晶粒细小,合金元素能起固溶强化作用,也能生成化合物产生弥散硬化作用,故此耐磨性是钢基体的6.3倍;激光涂敷层除所含合金元素产生的固溶强化和弥散硬化作用外,表层组织为高硬度的枝晶+共晶和马氏体,故其耐磨性是钢基体的8.8倍。     

Flame holding downstream from a co-flow injectorAccrochage d'une flamme en aval d'un injecteur co-courant

We present numerical results on the flame attachment in the downstream vicinity of the co-flow injector lip that separates the reactive fluids at injection. Two stability diagrams show the domains where the flame is anchored, blown off, or extinguished, in terms of separating plate thickness and injection velocities of both fluids. Different anchoring modes—stagnation point counter-flow holding or edge flame anchorage—are described, depending particularly on the plate rim thickness. To cite this article: C. Nicoli et al., C. R. Mecanique 334 (2006).     

LES of the Sandia Flame D Using Laminar Flamelet Decomposition for Conditional Source-Term Estimation

A variation of the Laminar Flamelet Decomposition (LFD) method for the Conditional Source Term (CSE) model developed by Bushe and Steiner (Phys Fluids 15:1564–1575, 2003) is implemented into an existing LES code. In this approach, the set of basis functions, on which the decomposition is based, is reduced using the mixture fraction dissipation rate as external parameter for the selection. It was found that reducing the basis improves and stabilises the inversion, resulting in reasonably accurate approximation for the average conditional quantities. Some modifications have been introduced to improve the inversion process by reducing the number of flamelets. This modification is found to help stabilize the inversion and keep the dimension of the linear system small. The model is used to simulate the turbulent non-premixed piloted SANDIA Flame D. Reasonably good predictions for conditional and unconditional average variables were found for different planes and at centreline of the flow field. However, an over prediction of the consumption rate in the near field of the flame is found, which may be partially attributed to the use of the Steady Laminar Flamelets (SLF) as functions for the decomposition and the use of a constant boundary condition for the species mass fractions in solving the flamelets. The present simulation of a turbulent reacting jet is the first test of the LFD approach in a realistic scenario using only the temperature field to calculate the inversion. The model is found to be computationally inexpensive.     

方形管内楔形障碍物对火焰结构与传播的影响

通过实验与数值模拟方法对CH4/空气预混火焰在有楔形障碍物的卧式燃烧方管内的传播进行了研究。采用多镜头Cranz Schardin高速摄像机和压力传感器等实验设备获得了高清晰度的障碍物诱导火焰失稳的分幅时序照片以及障碍物背风表面压力变化曲线。数值模拟则基于RANS方法与EDU-Arrhenius燃烧模型,计算结果与实验结果基本相符,反映了火焰在管内传播与变形的详细过程。通过综合分析实验与计算结果,得到了由楔形障碍物导致的火焰加速与变形的内在机理,揭示了火焰传播过程中由层流燃烧向湍流燃烧转捩的本质。     

Modelling and simulation of sprays in laminar flames

In the present paper we model and numerically simulate steady, laminar, premixed spray flames. The gasphase is described in Eulerian form by the equations governing the conservation of overall mass, momentum, energy and species mass. The liquid phase is described in Lagrangian form by the overall continuity equation, which reduces to an equation for the droplet size, the equations of motion, the energy equation and a droplet density function transport equation. The latter is the so-called spray equation, which, at any position in the chemically reacting flowfield, describes the joint distribution of droplet size, droplet velocity and droplet temperature. Herein the spray equation is solved using a Monte Carlo method. Detailed models of the exchange of mass, momentum and energy between the gaseous and the liquid phase are taken into account. The results presented in this paper are for an octane-air flame, where small amounts of liquid octane in form of a liquid spray are added to a fresh, unburnt gaseous octane-air mixture.Presented at Euromech Colloquium 324: The Combustion of Drops, Sprays and Aerosols, 25th–27th July 1994, Marseilles, France.