The constant-volume propagating spherical flame method for laminar flame speed measurement

Laminar flame speed is one of the most important intrinsic properties of a combustible mixture. Due to its importance, different methods have been developed to measure the laminar flame speed. This paper reviews the constant-volume propagating spherical flame method for laminar flame speed measurement. This method can be used to measure laminar flame speed at high pressures and temperatures which are close to engine-relevant conditions. First, the propagating spherical flame method is introduced and the constant-volume method （CVM） and constant- pressure method （CPM） are compared. Then, main groups using the constant-volume propagating spherical flame method are introduced and large discrepancies in laminar flame speeds measured by different groups for the same mixture are identified. The sources of discrepancies in laminar flame speed measured by CVM are discussed and special attention is devoted to the error encountered in data processing. Different correlations among burned mass fraction, pressure, temperature and flame speed, which are used by different researchers to obtain laminar flame speed, are summarized. The performance of these correlations are examined, based on which recommendations are given. Finally, recommendations for future studies on the con- stant-volume propagating spherical flame method for laminar flame speed measurement are presented.

Experimental study on the laminar flame speed of hydrogen/natural gas/air mixtures

Laminar flame speeds of hydrogen/natural gas/air mixtures have been measured over a full range of fuel compositions(0-100%volumetric fraction of H_(2))and a wide range of equivalence ratio using Bunsen burner.High sensitivity scientific CCD camera is use to capture the image of laminar flame.The reaction zone area is employed to calculate the laminar flame speed.The initial temperature and pressure of fuel air mixtures are 293 K and 1 atm.The laminar flame speeds of hydrogen/air mixture and natural gas/air mixture reach their maximum values 2.933 and 0.374 m/s when equivalence ratios equal to 1.7 and 1.1,respectively.The laminar flame speeds of hydrogen/natural gas/air mixtures rise with the increase of volumetric fraction of hydrogen.Moreover,the increase in laminar flame speed as the volumetric fraction of hydrogen increases presents an exponential increasing trend versus volumetric fraction of hydrogen.Empirical formulas to calculate the laminar flame speeds of hydrogen,natural gas,and hydrogen/natural gas mixtures are also given.Using these formulas,the laminar flame speed at different hydrogen fractions and equivalence ratios can be calculated.

Experimental and kinetic study on laminar flame speeds of ammonia/syngas/air at a high temperature and elevated pressure

The laminar flame speeds of ammonia mixed with syngas at a high pressure, temperature, and different syngas ratios were measured. The data obtained were fitted at different pressures, temperatures, syngas ratios, and equivalence ratios. Four kinetic models (the Glarborg model, Shrestha model, Mei model, and Han model) were compared and validated with experimental data. Pathway, sensitivity and radical pool analysis are conducted to find out the deep kinetic insight on ammonia oxidation and NO formation. The pathway analysis shows that H abstraction reactions and NHi combination reactions play important roles in ammonia oxidation. NO formation is closely related to H, OH, the O radical produced, and formation reactions. NO is mainly formed from reaction, HNO+ H= NO+ H2. Furthermore, both ammonia oxidation and NO formation are sensitive to small radical reactions and ammonia related reactions.

Numerical study on laminar flame speed of natural gas-carbon monoxide-air mixtures

Laminar flame speeds of natural gas-carbon monoxide-air mixtures are calculated by CHEMKIN II with GRI Mech-3.0 over a large range of fuel compositions,equivalence ratios,and initial temperatures.The calculated results of natural gas are compared with previous experimental results that show a good agreement.The calculated laminar flame speeds of natural gas-carbon monoxide-air mixtures show a nonmonotonic increasing trend with volumetric fraction of carbon monoxide and an increasing trend with the increase of initial temperature of mixtures.The maximum laminar flame speed of certain fuel blend reaches its biggest value when there is 92%volumetric fraction of carbon monoxide in fuel at different initial temperatures.Five stoichiometric natural gas-carbon monoxide-air mixtures are selected to study the detailed chemical structure of natural gas-carbon monoxide-air mixtures.The results show that at stoichiometric condition,the fuel blend with 80%volumetric fraction of carbon monoxide has the biggest laminar flame speed,and the C normalized total production rate of methane with 80%volumetric fraction of carbon monoxide is the largest of the five stoichiometric mixtures.

Theoretical analysis of the conical premixed flame response to upstream velocity disturbances considering flame speed development effects

The effect of upstream velocity pertuibations on the response of a premixed flame was investigated in terms of the flame transfer function dependency on excitation frequency.In this study,the assumption of constant flame speed was extended and the effect of flame speed development was considered;i.e.,the flame speed would grow with the time after ignition or with the distance from a flame-holder.In the present study,the kinematics of a conical flame was investigated by linearization of the front tracking equation of flame to uniform and convected fluctuations of the flow velocity and the response was compared with that of a V-shaped flame and the experimental data in the previous studies.The results show that the effect of flame speed development could influence a decreasing gain and increase the phase of the flame response to the uniform velocity oscillations in low and moderate frequencies.Comparing the variations in the gain of flame response upon normalized frequency,show that a conical flame has lower values than the V-flame.In other woods,these flames might be less susceptible to combustion instabilities than the V-flames.Furthennore,the variations in phase of the V-flames responses,which show a quasi-linear behavior with normalized frequency,have higher values than the saturated behavior in phase of the conical flame responses.Also,considering that the flame speed development induces an increase in the gain and phase of the conical flame response to the convected velocity oscillations in certain frequencies;because the developed flame front has longer length in comparison to the flame front in constant flame speed model.Therefore,the flame length may be longer than convective wavelength and the heat release would be generated in different points of the flame;consequently the flow oscillations might exert a stronger impact on the unsteady heat release fluctuations.

Review on laminar flame speed and its measurement from outwardly propagatingspherical flames

The laminar flame speed is one of the most fundamental properties of a fuel/air mixture.It determines the fuel burning rate in combustion engines and is an important target used to validate chemical models.Due to its importance,accurate measurement of the laminar flame speed receives great attention and different experimental methods have been developed.This review first introduces the laminar flame speed as well as its significance and dependence on different factors.Then,different experimental methods for the laminar flame speed measurement are described.Since the outwardly propagating spherical flame method is currently the most popular method for laminar flame speed measurement,its challenges and recent advances are reviewed.Both the constant pressure method and constant volume method using propagating spherical flames are discussed.

Study on the subsonic speed flame spraying coating of hydroxyapatite

A new method of preparation of biomaterial composite coating by the techniqueof subsonic thermal spraying was discussed in this paper. Ti_6Al_4V and pure Ti were chosen assubstrate and sublayer material respectively and the working layer was sprayed with biomaterialhydroxyapatite (HAP), forming the composite coating. The experiments of heat shock and tensilestrength showed that the bonding strength between coating and substrate is almost as same as that ofspecimen in which Ni/Al powder was adopted as sublayer. The phases of TiN, TiO_2, and Ti_2O_3 wereformed in the sublayer, which are free of toxic and have no side effects. The powder of workinglayer HAP was decomposed partly during spraying, but it can be solved by later treatment.

Modeling flame propagation speed and quenching distance of aluminum dust flame with spatially random distribution of particles

In this research combustion of aluminum dust particles in a quiescent medium with spatially discrete sources distributed in a random way was studied by a numerical approach.A new thermal model was generated to estimate flame propagation speed in a lean/rich reaction medium.Flame speed for different particle diameters and the effects of various oxidizers such as carbon dioxide and oxygen on flame speed were studied.Nitrogen was considered the inert gas.In addition,the quenching distance and the minimum ignition energy(MIE) were studied as a function of dust concentration.Different burning time models for aluminum were employed and their results were compared with each other.The model was based on conduction heat transfer mechanism using the heat point source method.The combustion of single-particle was first studied and the solution was presented.Then the dust combustion was investigated using the superposition principle to include the effects of surrounding particles.It is found that larger particles have higher values of quenching distance in comparison with smaller particles in an assumed dust concentration.With the increase of dust concentration the value of MIE would be decreased for an assumed particle diameter.Considering random discrete heat sources method,the obtained results of random distribution of fuel particles in space provide closer and realistic predictions of the combustion physics of aluminum dust flame as compared with the experimental findings.

DNS Analysis on the Indirect Relationship between the Local Burning Velocity and the Flame Displacement Speed of Turbulent Premixed Flames

The local burning velocity and the flame displacement speed are the dominant properties in the mechanism of turbulent premixed combustion. The flame displacement speed and the local burning velocity have been investigated separately, because the flame displacement speed can be used for the discussion of flame-turbulence interactions and the local burning velocity can be used for the discussion of the inner structure of turbulent premixed flames. In this study, to establish the basis for the discussion on the effects of turbulence on the inner structure of turbulent premixed flames, the indirect relationship between the flame displacement speed and the local burning velocity was investigated by the flame stretch, the flame curvature, and the tangential strain rate using DNS database with different density ratios. It was found that for the local tangential strain rate and the local flame curvature, the local burning velocity and the flame displacement speed had the opposite correlations in each density ratio case. Therefore, it is considered that the local burning velocity and the flame displacement speed have a negative correlation.

Consideration on the Flow Velocity in the Experimental Analysis of the Flame Displacement Speed Using DNS Data of Turbulent Premixed Flames with Different Lewis Numbers

The flame displacement speed is one of the major characteristics in turbulent premixed flames. The flame displacement speed is experimentally obtained from the displacement normal to the flame surface, while it is numerically evaluated by the transport equation of the flame surface. The flame displacement speeds obtained both experimentally and numerically cannot be compared directly because their definitions are different. In this study, two kinds of experimental flame displacement speeds—involving the mean inflow velocity and the local flow velocity—were simulated using the DNS data with the different Lewis numbers, and were compared with the numerical flame displacement speed. The simulated experimental flame displacement speed involving the mean inflow velocity had no correlation with the numerical flame displacement speed, while the simulated displacement speed involving the local flow velocity had a clear correlation with the numerical displacement speed in the cases of higher Lewis number than unity. The correlation coefficient of the simulated displacement speed involving the local flow velocity with the numerical displacement speed had a maximum value on the isosurface of the reaction progress variable with the maximum temperature gradient where the dilation effect of the flame is strongest.

Investigation on 1-Heptene/Air Laminar Flame Propagation under Elevated Pressures

The laminar flame propagation of 1-heptene/air mixtures covering equivalence ratios from 0.7 to 1.5 is investigated in a constant-volume cylindrical combustion vessel at 373K and elevated pressures (1, 2, 5, and 10 atm). Laminar flame speed and Markstein length are derived from the recorded schlieren images. A kinetic model of 1-heptene combustion is developed based on our previous kinetic model of 1-hexene. The model is validated against the laminar flame speed data measured in this work and the ignition delay time data in literature. Modeling analyses, such as sensitivity analysis and rate of production analysis, are performed to help understand the high temperature chemistry of 1-heptene under various pressures and its influence on the laminar flame propagation. Furthermore, the laminar flame propagation of 1-heptene/air mixtures is compared with that of n-heptane/air mixtures reported in our previous work. The laminar flame speed values of 1-heptene/air mixtures are observed to be faster than those of n-heptane/air mixtures under most conditions due to the enhanced exothermicity and reactivity.

Laminar Diffusion Flames of Methane in a Co-annular Jet of Oxygen-Enriched Air

Oxygen rich combustion is a mean to increase the energy efficiency and to contribute to CO2 capture. Influence of oxygen enriched air on the stability of methane flames from non premixed laminar jets has been investigated experimentally. The burner consists of two coaxial jets： methane flowing out of the inner, oxidizer from the outer. The flame behavior is studied according to the proportion of oxygen in the oxidizer jet, the oxidizer and the methane jets velocities. The flame is either anchored to the burner, lifted, stationary or not or blown-out. The addition of oxygen produces a decrease of the lift height, a reduction of the length of the reaction zone and an increase in the soot emission. These results have been reported into diagrams of stability where the flame configurations are connected to the competition between the dynamic effect of the injection velocity and the chemical effect of oxygen addition.

Numerical Analysis of Explosion Characteristics of Vent Gas From 18650 LiFePO_(4) Batteries With Different States of Charge

The combustion and explosion characteristics of lithium-ion battery vent gas is a key factor in determining the fire hazard of lithium-ion batteries.Investigating the combustion and explosion hazards of lithium-ion batteries vent gas can provide guidance for rescue and protection in explosion accidents in energy storage stations and new energy vehicles,thereby promoting the application and development of lithium-ion batteries.Based on this understanding and combined with previous research on gas production from lithium-ion batteries,this article conducted a study on the combustion and explosion risks of vent gas from thermal runaway of 18650 LFP batteries with different states of charge(SOCs).The explosion limit of mixed gases affected by carbon dioxide inert gas is calculated through the“elimination”method,and the Chemkin-Pro software is used to numerically simulate the laminar flame speed and adiabatic flame temperature of the battery vent gas.And the concentration of free radicals and sensitivity coefficients of major elementary reactions in the system are analyzed to comprehensively evaluate the combustion explosion hazard of battery vent gas.The study found that the 100%SOC battery has the lowest explosion limit of the vent gas.The inhibitory elementary reaction sensitivity coefficient in the reaction system is lower and the concentration of free radicals is higher.Therefore,it has the maximum laminar flame speed and adiabatic flame temperature.The combustion and explosion hazard of battery vent gas increases with the increase of SOC,and the risk of explosion is the greatest and most harmful when SOC reaches 100%.However,the related hazards decrease to varying degrees with overcharging of the battery.This article provides a feasible method for analyzing the combustion mechanism of vent gas from lithium-ion batteries,revealing the impact of SOC on the hazardousness of battery vent gas.It provides references for the safety of storage and transportation of lithium-ion batteries,safety protection of energy storage stations,and the selection of related fire extinguishing agents.

基于5 kHz平面激光诱导荧光的射流火焰局部熄火时空特性分析

基于高频光学测量手段研究射流火焰局部熄火的分布特性,对于分析燃烧场不稳定性机理有重要意义。建立5 kHz的平面激光诱导荧光测量系统,对高速射流火焰开展实验研究,并获得火焰结构的动态发展过程;将双边滤波、竖直滑窗局部自适应阈值等图像处理方法引入平面激光诱导荧光图像的预处理,在局部断裂结构数量特征的基础上,提取局部断裂结构长度和空间位置等特征;基于上述特征深入解析局部熄火的时空分布特性,进一步完善高频平面激光诱导荧光诊断与分析方法。研究表明:双边滤波算法处理后,图像峰值信噪比为34.3 dB,结构相似度为0.93,噪声水平显著降低,获得了较好的去噪效果;使用竖直滑窗局部自适应方法进行图像分割的F1分数达到93.30%;断裂结构累计数量随时间线性增加,当射流马赫数由0.5增至1.6时,单侧图像断裂结构的每秒累计数量从790.7增至9 217.2,并且局部熄火频率与射流马赫数呈指数关系;断裂结构主要分布区域为火焰臂及上侧中央燃料区。本研究进一步拓展了高频平面激光诱导荧光技术的应用能力,证明了高频平面激光诱导荧光技术用于局部熄火时空分布特性研究的可行性。

泡沫铜对密闭管道内合成气爆炸特性影响的实验研究

为研究泡沫铜孔隙密度和H2体积分数对合成气爆炸特性的影响,在封闭的管道中安装了孔隙密度为15、25和40 ppi的泡沫铜,实验分析了当量比为1的合成气-空气在不同H_(2)体积分数时的火焰结构、尖端速度和超压等参数变化规律。实验结果表明:火焰在泡沫铜上游的行为是受“郁金香”火焰形成过程的影响,泡沫铜对其没有影响。但是孔隙密度和H2体积分数的改变不仅会影响“郁金香”火焰的形成时间,还会影响变形“郁金香”火焰的形成。泡沫铜将火焰分割促使其从层流向湍流转化,对爆炸火焰传播起到加速作用。泡沫铜会引起管道内超压和火焰尖端速度的极大提升,且孔隙密度越小,H_(2)体积分数越大,火焰穿过泡沫铜后的最大火焰尖端速度越大,压力上升幅度越大,超压峰值越高。

氨/氢/柴油三燃料燃烧机理构建及研究

使用Chemkin软件中的Reaction Workbench模块,构建了由70种物种和392步基元反应组成的氨/氢/柴油(ammonia/hydrogen/diesel, AHD)三燃料燃烧机理。利用Chemkin软件对AHD机理进行化学动力学验证,同时通过将AHD机理与计算流体力学(computational fluid dynamics, CFD)模型耦合的方法,对发动机不同燃料燃烧模式下的燃烧过程和排放进行对比验证。结果表明,AHD机理对氨、氢、柴油的点火延迟时间、层流火焰传播速度、射流搅拌反应器(jet stirred reactor, JSR)氧化关键组分浓度变化的预测值与试验结果吻合较好,平均误差均在一个数量级内;通过CFD模型与AHD机理耦合,在纯柴油、氨/柴油、氨/氢/柴油三种燃烧模式下,发动机缸内压力和放热率的模拟值与试验结果的变化趋势一致,最大误差均在10%以内。在标定工况(1 800 r/min)下,随着氢能比增大,点火延迟期缩短,燃烧速率加快,CO和碳氢化合物排放逐渐下降,NOx排放呈上升趋势。

合成气/CO_(2)火焰传播过程中辐射再吸收的影响

针对生物质制取合成气燃料制备和应用过程中CO_(2)掺混的影响,对合成气/空气/CO_(2)预混火焰中开展数值模拟,通过不同的辐射模型研究辐射再吸收对火焰传播速度的影响。结果表明,辐射再吸收提高了火焰传播速度,最大提升幅度可达11.15%。随当量比增加,辐射再吸收对火焰传播速度的提升幅度从11.15%减小至3.12%,受预热导致的化学效应控制。H自由基在贫燃工况占据主导地位,辐射再吸收通过反应H+O_(2)==O+OH影响火焰传播速度;而富燃工况下,与OH自由基相关的反应H+OH+M=H_(2)O+M是辐射再吸收影响火焰传播速度的主导反应。

光流法测速在点火过程研究中的应用

光流法作为一种运动图像分析方法,正在越来越广泛地被应用于燃烧诊断领域,它能够定量获得火焰面位移速度.在强迫点火过程研究中,准确测量火核的演变速度能够更清楚地揭示其物理化学过程本质,基于此,本文验证了利用光流法研究点火过程的可行性,并分析了其准确度,研究了静止混气和扩散射流中的强迫点火过程,定量展示了强迫点火过程中火核的发展速度,可为点火模型的建立提供数据支撑.结果显示,射流中火核的发展速度呈现先慢后快的特点,并且在发展过程中,其速度矢量场中会出现多个发散源.

不同管径内瓦斯爆炸压力与火焰波传播实验研究

为减少低浓度瓦斯输送过程中输送管道爆炸事故,利用公称直径分别为700、500、200 mm的钢制爆炸实验管道,实验研究了管道中瓦斯爆炸压力及火焰波的传播规律。结果表明:爆炸传播过程中最大爆炸压力随长径比变化曲线呈波动上升趋势,管径变化会改变压力峰值出现的时间空间位置,700、500、200 mm管道最大爆炸压力分别出现在长径比115、103、103处,分别为0.59、0.28、0.09 MPa。管道直径越大,最大爆炸压力越大。火焰传播速度随长径比变化曲线起伏波动,整体呈上升趋势,通过分析可将火焰传播过程分为自由传播、反弹加速、稳定加速3个阶段,700、500、200 mm管道最大火焰传播速度分别出现在长径比83、108、96处,分别为846、943、250 m/s。实验中700、500 mm管道出现爆燃转爆轰现象,火焰速度可达800 m/s以上,管径越大爆燃转爆轰过程越早。研究结果可为工程实践中瓦斯管道输送和利用的安全防护提供有力支撑。

封闭体系内丁烷-空气预混气体爆炸的试验研究

为研究不同浓度丁烷-空气预混气体在封闭管道内的燃爆特性,利用方形密闭爆炸试验管道对不同体积分数的丁烷-空气预混气体进行爆炸试验。结果表明:气体爆炸先后经历了压力上升第一阶段、压力上升第二阶段和压力下降阶段;随着丁烷气体浓度的上升,爆炸压力上升速率、最大火焰速度、火焰加速度都呈先升高、后降低的趋势;其中,当丁烷体积分数为5%时,上述参数均达到峰值;含水管道中,气相与液相的爆炸压力趋势基本一致,但相较于无水管道中的压力变化更为平缓,并且最大爆炸压力及升压速率都较低。为可燃气体燃爆问题研究提供理论参考。