Effect of turbulence on flame propagation in cornstarch dust-air mixtures

Following the quantitative determination of dust cloud parameters, this study investigated the flame propagation through cornstarch dust clouds in a vertical duct of 780 mm height and 160×160 mm square cross section, and gave particular attention to the effect of turbulence on flame characteristics. The turbulence induced by dust dispersion process was measured using a particle image velocimetry (PIV) system. Upward propagating dust flames were visualized with direct light and shadow photography. The results show that a critical value of the turbulence intensity can be specified below which laminar flame propagation would be established. This transition condition is about 10 cm/s. The measured propagation speed of laminar flames appears to be in the range of 0.45–0.56 m/s, consistent with the measurements reported in the literature. For the present experimental conditions, the flame speed is little sensitive to the variations in dust concentration. Some information on the flame structure was revealed from the shadow records, showing the typical heterogeneous feature of dust combustion process.      

玉米粉火焰在开口垂直管道中的传播

以对粉尘云状态参数的定量测定为基础,对玉米粉尘火焰在开口垂直管道中向上传播的过程进行了实验研究.在情形A中,火焰从管道的封闭端向开口端传播,在情形B中,从开口端向封闭端传播.实验中,观察到两种粉尘火焰,即湍流火焰和层流火焰,火焰形态转变对应的点火延迟时间约等于1.1 s,即粉尘云湍流运动强度为10cm/s.情形A中,层流火焰的传播出现周期性振荡现象,湍流火焰在传播过程中不断加速;情形B中,两种火焰都匀速传播,湍流火焰传播速度明显大于层流火焰.在所考察的实验条件下,粉尘浓度对于玉米粉尘火焰传播速度的影响不大.     

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.     

管道内甲烷火焰穿越水雾区的传播特性

利用自行设计的火焰传播实验系统研究了甲烷火焰穿越水雾区的传播现象；运用数字摄像、光电测速和温度测量等技术研究了不同水雾条件下的甲烷火焰传播速度、火焰阵面轨迹及火焰结构特性。结果发现：水雾与甲烷火焰作用后，火焰颜色明显变红。水雾量较小时，甲烷火焰会被加速；水雾量增大到一定值后，甲烷火焰会在水雾区某一位置滞留一段时间，随后火焰再加速传播或熄灭(对应更高的水雾量)。分析认为这种现象的出现与水雾在甲烷火焰区的吸热、蒸发膨胀和化学阻化等物理化学综合效应有关。     

Investigation of impinging diffusion flames with inert gas

Experimental investigations on impinging diffusion flames mixing with inert gas were conducted. The combustion results and temperature measurements show that the non-reactive gas might dilute the local fuel concentration in the diffusion process. The shape of the column flame was symmetrical due to the flame stretch force. The results showed that a conical flame was changed by the addition of inert gas to the pure methane fuel. The weakening of the stretch boundary enhanced the mixing rate between the fuel and oxidizer, which would improve the fluctuation phenomenon. The impinging flame became shorter and bluer so the combustor size can be reduced. Nitrogen gas has the advantage that we can visualize the impinging mechanism with different gases in the impinging flame. The color in the mixing plane becomes blue and transparent. The penetration length is about 8 mm near the impinging point for Re=145.     

层流预混火焰传播速度与火焰稳定传播界限的测定

采用本生灯法和直管法测定了液化石油气（LPG）、甲烷与氢燃料质子交换膜燃料电池（PEMFC）阳极尾气与空气的三种不同浓度混合气的层流火焰传播速度。此外,对三种不同浓度可燃混合气火焰的稳定传播界限也进行了测定。实验结果为多燃料燃烧器的开发提供了设计依据。     

Flame stretch analysis in diffusion flames with inert gas

1niwtonInveshgations to enhance combushon efficiencyhave been irnportan in the past decad. Most of theimProvementS were coneennd on bog thendulent fluctUations and flow intensihes. The jettodetboinging setup is widely used in rocke engines withseif ignition proPellantS. The pUrPOse of thes reseaxC istO examine the imPinging effeCt on the jet-imPingementdiffesion flame.Two asPeCs of the twinging flame, jetboingeInen heating and combushon enhaneement havebeen inveshgatal. Milson and Chig…     

Numerical Study on the Morphology of a Re-Ignited Laminar Partially Premixed Flame with a Co-Axial Pilot Flame

Re-ignited partially premixed flame(PPF)is a quite extensive flame type in real applications,which is directly relevant to the local and global extinction and re-ignition phenomenon.The authors designed a model burner to establish laminar re-ignited PPFs.Numerical simulations were carried out to reveal the morphology of laminar re-ignited PPF.Based on the distributions of temperature,heat release and radicals,the morphologies of re-ignited flames were explored.W-shaped flames were formed under pilot-lean conditions.Line-shaped and y-shaped flames were formed under pilot-rich conditions.Both w-shaped and y-shaped flames had a triple-flame structure.The re-ignited flames can stand beyond the rich flammability limit.Additionally,OH distributions indicated both pilot flame and re-ignited flame well as it rapidly increased near the flame front.OH concentration did not increase visibly while CH2O concentration mildly increased during the mild re-ignition process in the pre-zone of the re-ignited PPF.According to the results of 0-D simulations using closed homogeneous reactor,both OH and CH2O reduced ignition time significantly.The results of this work are helpful for understanding re-ignited PPF more closely.     

Study on stretch extinction limits of CH4/CO2 versus high temperature O2/CO2 counterflow non-premixed flames

Extinction limits of counterflow non-premixed flames with normal and high temperature oxidizers were studied experimentally and numerically for development of new-type oxygen-enriched mild combustion furnace. Extinction stretch rates of CH₄/CO₂ (at 300 K) versus O₂/CO₂ flames at oxygen mole fractions of 0.35 and 0.40 and oxidizer temperatures of 300 K, 500 K, 700 K and 1000 K were obtained. Investigation was also conducted for CH₄/N₂ (at 300 K) versus air (O₂/N₂) flames at the same oxidizer temperatures. An effect of radiative heat loss on stretch extinction limits of oxygen-enriched flames and air flames was investigated by computations with optical thin model (OTM) and adiabatic flame model (ADI). The results show influence of radiative heat loss on stretch extinction limits was not significant in relative high fuel mole fraction regions. The extinction curve of the oxygen-enriched flames with oxygen mole fraction of 0.35 was close to that of the air flames at the oxidizer temperature of 300 K. However, the extinction curve of air flames with high temperature oxidizer was comparable with that of oxygen-enriched flames with oxygen mole fraction of 0.40. Scaling analysis based on asymptotic solution of stretch extinction was applied and it was found that stretch extinction limits can be expressed by two terms. The first term is total enthalpy flux of fuel stream based on thermo-physical parameters. The second term is a kinetic term which reflects an effect of the chemical reaction rate on stretch extinction limits. OH radicals which play important roles in chain propagating and main endothermic reactions were used to represent the kinetic term of both oxygen-enriched and air flames. The global rates of OH formation in these two cases were compared to understand the contribution of kinetic term to stretch extinction limits. Variation of extinction curves of oxygen-enriched flames and air flames was well explained by the present scaling analysis. This offers an effective approach to estimate stretch extinction limits of oxygen-enriched flames based on those of air flames at the same oxidizer temperature.     

Flame structure and global flame response to the equivalence ratios of interacting partially premixed methane and hydrogen flames

The interacting partially premixed methane and hydrogen flames established in a one-dimensional counterflow field were investigated numerically with the OPPDIF code and GRI-v3.0 was used to consider both fuels. The flame structure and response of the maximum flame temperature, heat-release rate, and flame speed to the equivalence ratios (Φ) and global strain rate (a_g) were investigated. The maximum temperature decreased with increasing a_g. The maximum temperature for cases with a stoichiometric hydrogen-side flame was higher than for other cases with the same a_g.The hydrogen-side flame played a key role in determining the maximum temperature. The maximum heat-release rates (MHRRs) for all cases show different trends. The MHRR of the methane-side flame was affected considerably by the interacting flame structure and hydrogen-side flame condition. However, the MHRRs of the hydrogen were independent of methane-side flame condition. For the cases where Φ of the methane-side flame was varied while the hydrogen-side flame was kept stoichiometric (Var-S), the MHRR and flame speed of the hydrogen-side flame were independent of the methane-side flame conditions. However, the methane-side flames had a negative flame speed except near-stoichiometric conditions. On the other hand, in the cases where Φ of the hydrogen-side flame was varied while the methane-side flame was kept stoichiometric (S-Var), the hydrogen-side flames had the MHRR and flame speed similar to those of an unstretched partially premixed hydrogen flame.     

同轴离心式喷注器火焰特性实验研究

为了研究高压补燃循环液氧,煤油发动机燃烧室同轴离心式喷注器的火焰特性,分别用富氧空气和煤油蒸气以及空气和甲烷在大气环境下进行了喷注器的燃烧实验,前者采用红外热像仪测量了火焰温度场,后者采用激光诱导荧光技术测量了火焰中CO2和OH的分子浓度分布.结果表明,该型喷注器的火焰形状和燃烧产物组分随氧化剂和燃料的混合比而变化;火焰稳定在喷注器出口处,剧烈的燃烧发生在火焰中心;平面激光诱导荧光技术用于燃烧过程研究,可以提供燃烧场组分分子浓度的信息.     

Upstream flow dynamics of a laminar premixed conical flame submitted to acoustic modulations

This study is concerned with the response of conical flames to acoustic modulations. It deals with the dynamics of the velocity field in the fresh gases feeding the flame. Experiments are carried out to determine the gain and phase shift between the excitation signal and the axial velocity signal. This information, combined with PIV data, is used to identify the propagation mode in the fresh stream. Experiments indicate that three ranges can be defined based on a Strouhal number St involving the burner diameter and the upstream flow velocity. When this number is sufficiently low (St?1), the response consists in a convective wave featuring a phase velocity close to that of the mean flow. As St is augmented (1?St?Stc), where Stc depends on the flame geometry, the phase difference between the velocity oscillation and the imposed signal nearly vanishes in a finite region adjacent to the burner exhaust indicating that the perturbation propagates at the speed of sound. Further away from the burner, velocity perturbations exhibit convective features again. In the third frequency range, corresponding to higher modulation frequencies (St?Stc), velocity perturbations are dominated by acoustics in most of the experimental domain. It is shown that this behavior results from the upstream influence of the flame wrinkling. The region of influence may be deduced by considering the velocity potential associated with the flame motion. When this perturbation potential takes large values, the flow is dominated by the convective wave. This suitably reproduces experimental observations.     

组合式燃煤手烧锅炉除尘器的研究与开发

针对手烧燃煤锅炉的烟尘特性，开发了一种集多种湿法除尘脱硫技术之长于一体的新型组合式除尘脱硫装置，分析了其结构尺寸与除尘系统阻力及效率的影响，工程运行表明：该装置能有效地净化手烧锅炉燃烧时产生的烟尘，实现除尘水内循环，避免除尘废水二次污染。     

中心回燃式燃油锅炉燃烧室实验研究

中心回燃式锅炉自引进以来一直没有一种理想的燃烧室传热计算方法，本文根据中心回燃式锅炉燃烧室内射流火焰与回流烟气之间存在着强烈的掺混，认为燃烧室传热计算中不应忽略对流换热。提出了适合中心回燃式锅炉燃烧室传热的计算方法，并通过实验加以验证。     

水煤浆与其它燃料的火焰特性的比较研究

在开发和设计水煤浆锅炉的基础上 ,对水煤浆的燃烧特性和火焰特性作了深入地探讨 ,并把水煤浆跟常用的煤粉、重油、天然气 3种常用锅炉燃料的燃烧特性进行了比较 ,同时分析了同一锅炉在燃用不同燃料时出现的不同辐射特性的原因。研究结果对水煤浆锅炉的开发与设计提供了可靠的理论依据。     

Stable negative edge flame formation in a counterflow burner

In nonpremixed combustion, edge flames can form as a region of flame propagation or flame recession. Forwardly propagating edge flames, as occur in lifted flames, have a local gas velocity at the flame edge that is from unburned partially premixed fuel and air into the flame. These flames represent an ignition process, and permit the flame itself to either stabilize against an incoming gas stream or propagate into unburned fuel and air. Negative edge flames represent the opposite case of a local gas velocity from burned products through the flame edge. The negative edge flame represents a local extinction process, and occurs, for example, during vortex-induced extinction of a nonpremixed flame sheet. A technique for generating steady negative edge flames in a standard counterflow burner is presented, which permits detailed examination of their properties. A coannular counterflow burner is used to create a strain gradient that quenches a central diffusion flame. Unlike previous research on strain-induced flame edges, the axisymmetric flow field ensures gas flow from products through the edge. Measurements of the edge flame's sensitivity to global strain rates and fuel mixtures are presented, along with measurements of the edge flame structure using OH fluorescence and CH emission imaging.     

二甲醚湍流射流推举火焰的燃烧机理研究

对长、宽、高为650 mm×400 mm×12 mm的半闭口狭窄矩形通道（海伦-肖装置）内的甲烷/空气层流预混火焰传播过程进行了实验研究,探究当量比φ在0.6~1.2范围内、火焰传播角度ω在垂直向下-90°至垂直向上90°区间对火焰前锋轮廓发展及非标准层流火焰速度的影响。结果表明：火焰在通道内的传播分为热膨胀、准稳态传播和端壁效应3个阶段,每个阶段具有各自不同的前锋轮廓特征。由于瑞利-泰勒不稳定性机制的作用,所有当量比工况下向上传播的火焰均在准稳态传播阶段中呈现出明显的锋面褶皱与胞状结构;对向下传播的火焰而言,其在贫燃工况（φ为0.6,0.8）下的胞状不稳定性得到了有效抑制,而在当量比φ=1.0及富燃工况（φ=1.2）下,该稳定性效应并不显著。火焰瞬时速度与标准层流速度的比值Ui/UL,在φ=0.6的极贫燃工况与其他当量比工况下展现出明显不同的发展特性,极贫燃工况火焰向上传播时（ω=90°）,Ui/UL随着传播过程的进行一直增大,直到火焰触碰壁面末端熄灭,整个过程Ui/UL与火焰传播方向呈正相关关系;而对于其他当量比工况,Ui/UL在传播过程中均先升高后下降,火焰触碰壁面末端熄灭前其值趋于稳定,其平均速度与标准层流速度的比值Ua/UL在水平传播（ω=0°）时达到最大值。     

Experimental study and three-dimensional simulation of premixed hydrogen/air flame propagation in a closed duct

An experimental and numerical study of premixed hydrogen/air flame propagation in a closed duct is presented. High-speed schlieren photography is used in the experiment to record the changes in flame shape and location. The pressure transient during the combustion is measured using a pressure transducer. A dynamic thickened flame model is applied to model the premixed combustion in the numerical simulation. The four stages of the flame dynamics observed in the experiment are well reproduced in the numerical simulation. The oscillations of the flame speed and pressure growth, induced by the pressure wave, indicate that the pressure wave plays an important role in the combustion dynamics. The predicted pressure dynamics in the numerical simulation is also in good agreement with that in the experiment. The close correspondence between the numerical simulation and experiment demonstrate that the TF approach is quite reliable for the study of premixed hydrogen/air flame propagation in the closed duct. It is shown that the flame wrinkling is important for the flame dynamics at the later stages.     

The response of a propane-air counter-flow diffusion flame subjected to a transient flow field

OH planar laser-induced fluorescence (PLIF) and particle image velocimetry have been used to study the frequency response of laminar C₃H₈-air counterflow diffusion flames to assess the adequacy of the steady-flamelet models. Particle image velocimetry was used to determine the flame strain rate, while OH PLIF was used both to measure temperature at the flame front, using the two-line PLIF technique, and the reaction-zone width. Both measurements demonstrate that the frequency response of flames subjected to a time-varying flow field is diffusion-limited. At the 30-Hz and 50-Hz forcing frequencies, the maximum reaction-zone temperature and width were found to respond quasi-steadily. However, at higher forcing frequencies-i.e., 100 and 200 Hz-transient behavior is evident from the phase relationship between the imposed sinusoidal strain rate and the resulting peak temperature and reaction-zone width. The measured values of the OH-field widths were well fit by an offset sine function. In all cases when the oscillation amplitude is normalized by the cycle mean strain rate and plotted against the non-dimensional flow field frequency, the results collapse onto a single line having a steep negative slope.