利用主动轮廓算法提取火焰前锋

介绍了利用主动轮廓算法(ACM)从片光束二维激光测量图像中提取预混火焰前锋的原理,并结合V形火焰的OH平面激光诱导荧光图像分析比较了ACM和常用的阈值法．ACM从原理上消除了阈值法所包含的主观因素,自动获取最佳结果．测试结果表明当边界严重模糊时,阈值法的结果因噪声严重失去价值,而用ACM却能够获得平滑的火焰前锋．     

声波对V形预混火焰锋面皱褶的影响

对声波扰动下V形预混火焰的锋面皱褶进行了动态分析,建立了声波扰动V形预混火焰的实验台,通过精细热电偶探测火焰锋面皱褶的变化,测量了不同声波扰动频率下的V形预混火焰的锋面皱褶.实验结果与分析结果一致,表明V形预混火焰锋面皱褶的频率与声波扰动频率相同,火焰锋面皱褶的幅值随着声波扰动频率的增大而减小.     

离子信号与空燃比关系的探讨

在定容燃烧弹中通过对点火电极附近离子信号的测量,表明了该信号主要由火焰前锋和火焰后区两部分组成．理论分析证明,火焰前锋的信号主要与电极附近火核中的H3O 离子和自由电子浓度有关,火焰后区的信号主要与已燃区中高温下激态的NO 和自由电子浓度有关,特别是火焰前锋区的信号与空燃比有直接关系．实验研究发现火焰前锋和后区的信号峰值随过量空气系数的变化趋势相同,当过量空气系数为1时均达最大值．因此,利用该信号可以实现空燃比的探测．     

粒径分布对有机粉尘爆炸中火焰结构的影响

采用高速摄影及带通滤波片相结合的方法,记录并分析了火焰在开放空间两种不同粒径分布硬脂酸粉尘云中的传播特性.实验中,通过控制喷嘴压力形成两种典型粒径分布的硬脂酸粉尘云.实验发现,火焰在两种不同粒径粉尘云中传播时,具有明显不同的火焰前锋结构特征,即连续火焰前锋和离散火焰前锋.在较小粒径分布粉尘云中传播时,火焰前锋形状规则、连续,燃烧反应区均匀一致,类似于预混燃烧现象；而在较大粒径分布粉尘云中传播时,火焰前锋黄色发光区周围分布着离散的蓝色亮点.在此基础上,进一步利用CH辐射放大图像揭示了粉尘云离散火焰前锋的形成机理.理论模型的分析结果证明了粒径小于58 μm的粒子质量浓度是决定能否出现离散火焰前锋的关键参数.     

低热值气体发动机涡团与火焰面相互作用

基于大涡模拟方法对一台燃用低热值气体燃料的发动机缸内湍流涡团与火焰相互作用过程开展了模拟,分析了缸内急燃期火焰传播过程中火焰前锋面上湍流涡核的分布特征,研究湍流涡团与火焰面的相互作用机理,并研究了涡对结构对火焰面传播的影响.结果表明:急燃期燃烧过程中,缸内背景湍流和火焰面相互作用,促使高强度涡核分布在火焰前锋面上,而涡核增大火焰面褶皱程度,火焰面面积增加,有助于更多高强度火核的产生;未燃区涡对的两个涡团相互逆向旋转导致火焰面受到局部的卷吸或者拉伸等作用,是火焰面出现明显皱褶的主要原因.     

天然气直喷燃烧的可视化研究

利用高速摄影装置开展了天然气直喷燃烧的可视化研究。研究结果表明:缸内形成的分层充量分布与喷射方式有关,双点平行喷射和单点喷射将比双点对向喷射形成的充量分层强。双点平行喷射和单点喷射时火焰将向燃料喷流的下流区发展,而双点对向喷射时火焰显示出由燃烧室中心向外围方向发展的特征。天然气直喷燃烧呈现出湍流燃烧特征的皱褶火焰前锋面。通过优化喷射正时形成的可燃分层充量可实现直喷天然气超稀燃烧。     

笼式光场相机的组装及其在火焰温度场测量中的应用

为了实现燃烧火焰的三维温度分布监测,本文结合光场成像和光学分层成像技术,发展了一种基于单光场相机的火焰温度场光场层析重建方法.针对燃烧装置与火焰特性设计并研制了笼式光场相机,建立了双火焰(乙烯/蜡烛)光场层析成像实验系统.利用笼式光场相机采集火焰光场信息,利用光场成像技术获取重聚焦图像,利用光学分层成像技术重建断面辐射...     

射流剪切层及相关预混火焰的被动控制

研究了网格湍流的被动控制方式对射流剪切层以及相关预混火焰的影响.利用热线风速仪测量冷态流场结构,结果显示,网格湍流显著减小了剪切层的湍流强度和尺度,使湍流更趋于各向同性,并增强了流场的稳定性,减弱了剪切层失稳程度,表明了网格湍流能对剪切层中的大尺度拟序结构起到抑制和破坏作用.此外,用高频响细丝热电偶研究了网格湍流以及所引起的剪切层改变对预混火焰结构的影响,结果显示,网格湍流一方面通过改变当地射流剪切层的流动特性抑制了火焰皱褶低频运动;另一方面通过在火焰根部施加强制扰动产生皱褶从而强化燃烧,这说明网格湍流能够同时产生使剪切层中预混火焰强化和稳定两方面的效应.     

CNG/CO_2气体发动机湍流火焰结构的大涡模拟

采用大涡模拟的方法对一台燃用压缩天然气(CNG)/CO2混合气体燃料的火花点火发动机缸内湍流涡团与火焰面的相互作用过程开展了模拟计算研究.基于OpenFOAM开源CFD程序包建立了AKTIMEuler点火模型和PaSR-LES燃烧模型,并采用耦合多面体顶点运动与梅斯基特算法的求解方法构建了发动机动网格.经过对计算结果的分析可知:随着可燃气体中惰性气体组分的增加,湍流火焰传播速率下降,涡对结构对火焰面皱褶的作用减弱,火焰面在达到相同皱褶度的时间向后推迟,其皱褶度的升高率也随之下降;随着发动机转速的增大,火焰面受到涡团卷吸与拉伸的作用增大,局部容易形成环形火焰面的现象.     

低排放火花点火稀燃的研究

首先制定了单缸燃烧程序以便全面分析并说明几种不同燃烧室的燃烧性能.试验台取得的性能数据和排放数据由其他分析技术取得的数据来补充;电离传感器装在燃烧室的关键部位来探测火焰前锋的传播;用热线风速计和流动显形技术对燃烧室内的动态流场进行定量分析,从而补充了传统的稳态涡流和流动测量;燃烧性能和发动机摩擦状况由缸内实测的压力来确定.     

Observation of premixed flame fronts by laser tomography

The principle of combustion field detection by using laser tomography, as well as exploitation of the laser tomography apparatus and the tool for image processing is described. An experiment detecting flame fronts by laser tomography was made by employing a V-shaped premixed flame. The results show that the instantaneous geometric shape of flame wrinkles within the light sheet can be clearly resolved. The contours of the flame fronts are precisely tracked through active contour models (ACM) from the digital images of laser tomography, laying the basis for the quantitative analysis of flame wrinkling and propagation.     

Observation of premixed flame fronts by laser tomography

The principle of combustion field detection by using laser tomography, as well as exploitation of the laser tomography apparatus and the tool for image processing is described. An experiment detecting flame fronts by laser tomography was made by employing a V-shaped premixed flame. The results show that the instantaneous geometric shape of flame wrinkles within the light sheet can be clearly resolved. The contours of the flame fronts are precisely tracked through active contour models (ACM) from the digital images of laser tomography, laying the basis for the quantitative analysis of flame wrinkling and propagation. __________ Translated from Journal of Combustion Science and Technology, 2007, 13(3): 275–279 [译自: 燃烧科学与技术]     

Numerical investigation on the effects of reaction orders on the flame propagation dynamic behaviors for premixed gas in a closed tube

In this paper, the premixed flame propagation in a closed tube is surveyed using Computational Fluid Dynamics. The propagation characteristics of premixed flame are obtained coupling a single-step reaction mechanism with a laminar flame model. Three single-step reaction mechanisms are established with different reaction orders for hydrocarbon fuels. This study is to establish a wider range of reaction mechanisms and represent actual experimental conditions better. The numerical simulation results demonstrate that reaction orders can affect the tulip flame development. As the flame spreads, the tulip flame fronts become wrinkled. When the reaction order is 2, there are more wrinkles in the flame front and the degree of wrinkles is more obvious. Reaction orders also affect the flame tip velocity and the flame skirt velocity. The main reason is that laminar flame speeds are significantly different. When the reaction orders are 1.5 and 2, laminar flame speeds are mainly affected by temperature, which respectively increase by about 25% and 75%. When the reaction order is 1, the pressure is crucial for the variation of laminar flame speed. The laminar flame speed decreases by about 33%.     

Experimental study of industrial gas turbine flames including quantification of pressure influence on flow field,fuel/air premixing and flame shape

A commercial swirl burner for industrial gas turbine combustors was equipped with an optically accessible combustion chamber and installed in a high-pressure test-rig. Several premixed natural gas/air flames at pressures between 3 and 6 bar and thermal powers of up to 1 MW were studied by using a variety of measurement techniques. These include particle image velocimetry (PIV) for the investigation of the flow field, one-dimensional laser Raman scattering for the determination of the joint probability density functions of major species concentrations, mixture fraction and temperature, planar laser induced fluorescence (PLIF) of OH for the visualization of the flame front, chemiluminescence measurements of OH^* for determining the lift-off height and size of the flame and acoustic recordings. The results give insights into important flame properties like the flow field structure, the premixing quality and the turbulence–flame interaction as well as their dependency on operating parameters like pressure, inflow velocity and equivalence ratio. The 1D Raman measurements yielded information about the gradients and variation of the mixture fraction and the quality of the fuel/air mixing, as well as the reaction progress. The OH PLIF images showed that the flame was located between the inflow of fresh gas and the recirculated combustion products. The flame front structures varied significantly with Reynolds number from wrinkled flame fronts to fragmented and strongly corrugated flame fronts. All results are combined in one database that can be used for the validation of numerical simulations.     

Swirl effects on harmonically excited,premixed flame kinematics

This paper describes the response of a swirling premixed flame with constant burning velocity to non-axisymmetric harmonic excitation. This work extends prior studies of axisymmetric forcing, which have shown that wrinkles are excited on the flame that propagate downstream along the mean flame surface at a speed given by U_o cos ψ, where U_o is the mean flow velocity and ψ is the flame angle. The swirl component in the flow field introduces an azimuthal transport mechanism for disturbances on the flame. As such, the flame response at any given position is a superposition of flame wrinkles excited at earlier times, upstream axial locations, and different azimuthal positions. These swirl transport effects do not arise in problems where axisymmetric flames are subjected to axisymmetric excitation, but enter quite prominently in the presence of non-axisymmetries, such as when the flame is subjected to transverse excitation. The solution characteristics are strongly dependent upon the ratio of angular rotation rate to excitation frequency, denoted by σ = Ω/ω, which describes the fraction of azimuthal rotation a disturbance makes in one acoustic period. When σ ? 1 and σ ? 1, the axial wavelength of flame wrinkles scales with the convective wavelength, λ_c, but becomes much longer for σ ～ O(1). The spatial variation in phase of flame wrinkling is also strongly dependent upon σ. Regardless of swirl number, flame wrinkles propagate in helical spirals along the solution characteristics at a phase speed equal to the local tangential velocity. The axial phase characteristics of flame wrinkling at a fixed azimuthal location, as would be measured by laser sheet imaging, are much more complex. For σ < 1, the wrinkles exhibit the familiar negative roll-off character for the phase with axial downstream distance, indicative of an axially convecting disturbance. The slope of this phase roll-off decreases with increasing σ, however, and becomes zero at σ = 1 for a compact flame. For σ > 1, the wrinkles actually have a positive roll-off character for the phase with axial downstream distance, indicating a flame wrinkle with a negative trace velocity, but whose actual propagation velocity is positive. Finally, these results show that while the flame response to transverse acoustic excitation is quite strong locally, its spatially integrated effect is much smaller for acoustically compact flames. This suggests that the dominant mechanism through which the flame responds globally to transverse excitation is the induced vortical and longitudinal acoustic fluctuations.     

Effect of differential diffusion on turbulent lean premixed hydrogen enriched flames through structure analysis

This study presents the flame structure influenced by the differential diffusion effects and evaluates the structural modifications induced by the turbulence, thus to understand the coupling effects of the diffusively unstable flame fronts and the turbulence distortion. Lean premixed CH₄/H₂/air flames were conducted using a piloted Bunsen burner. Three hydrogen fractions of 0, 30% and 60% were adopted and the laminar flame speed was kept constant. The turbulence was generated with a single-layer perforated plate, which was combined with different bulk velocities to obtain varied turbulence intensities. Quasi-laminar flames without the plate were also performed. Explicit flame morphology was obtained using the OH-PLIF. The curvature, flame surface density and turbulent burning velocity were measured. Results show that the preferential transport of hydrogen produces negatively curved cusps flanked with positively curved bulges, which are featured by skewed curvature pdfs and consistent with the typical structure caused by the Darrieus-Landau instability. Prevalent bulge-cusp like wrinkles remain with relatively weak turbulence. However, stronger turbulence can break the bulges to be finer, and induce random positively curved cusps, therefore to destroy the bulge-cusp structures. Evident positive curvatures are generated in this process modifying the skewed curvature pdfs to be more symmetric, while the negative curvatures are not affected seriously. From low to high turbulence intensities, the hydrogen addition always strengthens the flame wrinkling. The augmentation of flame surface density and turbulent burning velocity with hydrogen is even more obvious at higher turbulence intensity. It is suggested that the differential diffusion can persist and even be strengthened with strong turbulence.     

Modification of premixed combustion in shear layers by grid turbulence

The influence of grid turbulence on the shear layer of a jet and the premixed flames embedded in it was investigated in the present study. The velocity field of the jet was measured by using hot-wire anemometry. It was found that grid turbulence reduced turbulence intensities in the shear layer and suppressed low frequency fluctuation. Moreover, the energy contained in small-scale fluctuation was increased and turbulence became homogeneous. The results indicate that grid turbulence inhibits the formation of a large-scale coherent structure in the shear layer. Flame temperature was measured by using a compensated fine-wire thermocouple. It was found that grid turbulence reduced low frequency fluctuation of the flame fronts, increased the small-scale wrinkles and elevated the mean temperature of the flame zone. The results show that grid turbulence can enhance and stabilize premixed flames in shear flow. Translated from Journal of Engineering Thermophysics, 2006, 27(1): 159–162 [译自: 工程热物理学报]     

Experimental analysis of an oblique turbulent flame front propagating in a stratified flow

This paper details the experimental study of a turbulent V-shaped flame expanding in a nonhomogeneous premixed flow. Its aim is to characterize the effects of stratification on turbulent flame characteristics. The setup consists of a stationary V-shaped flame stabilized on a rod and expanding freely in a lean premixed methane-air flow. One of the two oblique fronts interacts with a stratified slice, which has an equivalence ratio close to one and a thickness greater than that of the flame front. Several techniques such as PIV and CH^* chemiluminescence are used to investigate the instantaneous fields, while laser Doppler anemometry and thermocouples are combined with a concentration probe to provide information on the mean fields. First, in order to provide a reference, the homogeneous turbulent case is studied. Next, the stratified turbulent premixed flame is investigated. Results show significant modifications of the whole flame and of the velocity field upstream of the flame front. The analysis of the geometric properties of the stratified flame indicates an increase in flame brush thickness, closely related to the local equivalence ratio.     

Flame wrinkle destruction processes in harmonically forced,laminar premixed flames

This paper describes numerical and theoretical analyses of the nonlinear dynamics of harmonically forced, stretch-sensitive premixed flames. A key objective of this work is to analyze the relative contributions of kinematic restoration and flame stretch upon the rate at which flame wrinkles, excited by harmonic forcing, are smoothed out. Kinematic restoration is an intrinsically nonlinear process with a two spatial-zone structure, whose amplitude dependence is fundamentally different near and far from the wrinkle excitation source. Flame stretch processes appear even in the small perturbation limit, and smooth out flame wrinkles in thermodiffusively stable mixtures. Which process dominates is a function of the perturbation amplitude, frequency, stretch sensitivity of the mixture, and spatial location. This paper presents computed results illustrating the solution characteristics, as well as key dimensionless parameters controlling the solution based upon a third order perturbation analysis.