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.

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.

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.

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.

二甲醚燃烧机理简化与燃烧特性验证

二甲醚(DME)作为燃料具有来源广、可再生和低碳等优点,已成为最具潜力的内燃机和燃气轮机替代燃料。然而,关于DME燃烧机理的研究仍然不够深入。通过使用带有误差传播和敏感性分析的有向关系图法(DRGEPSA)对现有的高压条件下的DME燃烧详细机理进行简化获得了骨架机理(Mech_191),然后通过调整指前因子对Mech_191进行优化获得了简化机理。将应用简化机理获得的指征DME燃烧的重要参数(点火延迟时间和层流火焰速度)的预测值与文献值进行了对比。结果表明,简化机理的预测值与文献值高度吻合,验证了该机理在模拟DME燃烧特性方面的有效性。此外,还发现部分关键反应(如·CH_(3)OCH_(2)的消耗反应)的确定对于准确获取重要燃烧参数有重要作用。

预混氢气对氨气燃烧过程的加速机理研究

氨气是一种极具潜力的氢载体,但燃烧速度慢,在动力装置内单独使用存在效率低、稳定性差等问题。掺氢燃烧是加速氨气燃烧过程的有效措施。为了理解掺氢对氨气混合燃料火焰燃烧特性的影响规律,本文基于详细化学动力学计算方法对氨气掺氢的层流火焰传播及反应敏感性分析进行研究。结果表明,提高掺氢比可明显改善氨的燃烧惰性。较高的初始环境温度对燃料的反应起明显的促进作用,而较高的初始环境压力则恰恰相反;层流火焰速度最高的工况均在当量比1.12左右。

Jet-A表征燃料/乙醇掺混燃料层流燃烧特性研究

为了研究初始温度470 K、初始压力0.1 MPa下乙醇掺混对航空煤油层流火焰燃烧特性的影响,文中利用定容燃烧弹设备结合仿真进行对比分析。结果表明,乙醇的加入显著提升了航空煤油的层流火焰燃烧速度,但仿真结果与试验数据之间存在较大的偏差,在低当量比时尤为严重,需进一步优化机理。在此基础上通过敏感性分析,发现对层流火焰燃烧速度影响较大的4个基元反应,通过对4个基元反应的参数进行调整,修改得到新模型,该模型与试验值存在较好的吻合度。

Supergravity effects on flame propagation and structure in hydrogen/air mixtures

In the current work,we investigated hydrogen/air flame propagation under supergravity conditions.Results show that when gravity is in the same/opposite direction as flame propagation,it leads to acceleration/deceleration of the flame,and that such an effect could substantially modify the flame propagation and structure at high gravity levels.Furthermore,for the absolute and relative flame propagation speeds,the gravity-affected flame speed shows opposite trends as the absolute flame speed is more affected by the local induced flow field,while the relative flame speeds are controlled by the super-adiabatic or sub-adiabatic flame temperature.The gravity-affected thermal and chemical flame structures are also examined through the influence of the mixture equivalence ratio,pressure,and flame stretch.

混氢天然气燃烧特性及灭火剂抑制研究

利用Chemkin-Pro对混氢天然气层流预混火焰速度进行数值模拟研究,从化学动力学的角度讨论混氢比对混氢天然气层流预混火焰速度的影响,结果表明,FFCM1.0机理能够较准确的预测混氢天然气层流预混火焰速度;随着混氢比的增加,混氢天然气的层流预混火焰速度逐渐升高,在混氢比大于60%时,火焰速度增幅更显著。对三氟甲烷(HFC-23)、五氟乙烷(HFC-125)、七氟丙烷(HFC-227ea)三种灭火剂抑制层流火焰速度的能力进行研究分析,发现HFC-227ea对层流火焰速度具有更好的抑制效果,进一步研究了HFC-227ea对层流火焰速度的影响。结论表明:在未观察到不良反应浓度范围内,HFC-227ea浓度越高,层流火焰速度越低;燃烧当量比在1.5左右,该灭火剂对层流火焰速度有增强的效果。

RP-3航空煤油替代燃料及其化学反应动力学模型

本文提出了40%(摩尔分数,下同)正癸烷、42%正十二烷、13%乙基环己烷和5%对二甲苯的四组分RP-3航空煤油替代燃料模型,并通过实验充分验证了替代燃料模型与实际RP-3航空煤油在理化特性上的相似性.采用对冲火焰实验台架,测量了RP-3航空煤油以及四组分替代燃料的层流火焰传播速度.对比结果表明本文提出的替代燃料能够准确描述实际RP-3航空煤油的燃烧速率.进一步发展了包含168组分、1089反应的半详细反应动力学模型,验证结果表明本文机理能够准确预测RP-3航空煤油着火延迟时间和火焰传播速度.

稀释气对高甲烷含量天然气燃烧特性的影响

针对高甲烷含量天然气在实际发动机中燃烧温度过高、NOx排放过高的问题,利用定容燃烧弹实验和Chemkin软件模拟计算相结合的方法,对其预混层流燃烧特性进行研究,分析了不同稀释比和稀释气种类(N2和CO2)对混合气的层流火焰速度、NOx摩尔分数、燃烧压力和燃烧期等燃烧特性参数的影响。研究表明,层流火焰速度、质量燃烧率和热释放率均随稀释比的增加而减小,稀释气添加导致火焰温度下降,从而降低了NOx摩尔分数。Markstein长度和火焰厚度都随稀释比的增加而增加,火焰流动不稳定性得到抑制。添加稀释气导致燃烧压力峰值和压力升高率降低、燃烧期延长,与N2相比,CO2对混合气燃烧特性的稀释效果更加显著,从而为通过废气再循环技术路径降低高甲烷含量天然气发动机燃烧温度,控制NOx排放提供了理论指导。

CH_4/O_2/CO_2层流预混火焰传播速度实验研究

采用本生灯法，测量了CH4／O2／CO2混合气在不同工况下的层流预混火焰传播速度．采用基于火焰图像的全面积法计算本生灯火焰的传播速度．主要研究了化学当量比（0．8～1．2）、氧气体积分数（25％～35％）和稀释剂种类（N2，CO2）对火焰传播速度的影响规律．结果表明：当化学当量比为1时火焰传播的速度达到最大值，在其两侧火焰传播速度逐渐下降；火焰传播速度随着氧气体积分数的增加而增加，且火焰传播速度与氧气体积分数呈二次函数关系；与N2相比，高浓度CO2降低了火焰传播速度，CH4在O2／CO2气氛下的火焰传播速度约为O2／N4气氛下的五分之一。

三种典型低热值气体燃料的层流火焰速度测量

实验采用热流量炉方法,在常温常压下对3种典型低热值气体(生物质气化合成气、煤气化合成气、高炉煤气)以及甲烷和合成氨驰放气的层流火焰速度进行了测量。所测气体的当量比从0.62变化到2.09。另外,通过Chemkin对实验气体的层流火焰速度进行了模拟计算。对比实验值和计算值,发现两者非常接近。比较不同种类气体的实验结果和模拟计算结果显示:由于所测低热值气体具有较低的低位发热量,其层流火焰速度普遍较低,在当量比1处,大约为20 cm/s;H2对层流火焰速度具有很大的影响,几种低热值气体中H2含量越高,其层流火焰速度越大;另外,低热值气体中的CO直接影响层流火焰速度的峰值当量比,使其偏离1,位于比较富燃的区域。

甲烷/乙烷-空气预混层流燃烧特性试验和数值模拟研究

利用高速纹影摄像法在定容燃烧弹内研究了不同初始压力、初始温度、当量比和甲烷含量条件下甲烷/乙烷-空气预混层流燃烧特性,得到了马克斯坦常数和层流火焰燃烧速率等数据,并进行了化学特性分析。研究结果表明:层流火焰燃烧速率随初始压力的增加而减小,随着初始温度的增加而增加,最大值在当量比约为1.1取得,甲烷含量增加层流火焰速率略微减小;马克斯坦常数随初始压力的增加而减小,随着当量比的增加而增加;数值模拟得到的一维自由传播火焰的层流火焰速率与试验结果吻合良好。

甲醇空气预混层流燃烧的简化化学反应动力学机理

基于甲醇氧化的详细反应历程，利用敏感性分析的方法，提出了一个用于描述甲醇空气预混层流燃烧速度的包含18种组分、28步基元反应的简化化学反应动力学机理．研究发现，在甲醇的氧化过程中，甲醇的分解反应及H、OH等自由基的链锁反应具有十分高的敏感性，其中HCO＋M和H＋O2分别是产生H、OH自由基的主要反应．计算结果与实验结果对比表明，该简化机理可以较合理地模拟当量比为0．6～1．2以及不同初始温度下的层流燃烧速度和火焰结构．与详细机理相比，该机理更适合与CFD三维数值模拟软件耦合．

正十四烷低温点火及燃烧机理的构建和简化

为了开展燃烧流场数值模拟,构建了包含71个组分391个反应方程的正十四烷低温点火及燃烧骨架反应动力学机理(C14_SK71);采用计算奇异摄动法(CSP)和准稳态假设法(QSSA)对骨架机理进行简化,得到包含44个组分40个反应方程的总包简化机理(C14_Red44);通过实验测得的点火延迟时间、火焰传播速度以及射流搅拌反应器(JSR)组分浓度数据对机理进行了计算和验证。结果表明:该十四烷燃烧机理能够比较准确地预测温度700K^1350K内点火延迟数据,再现中低温条件下的负温度效应(NTC);较好地模拟了当量比0.7~1.4内的正十四烷/空气预混气的层流火焰传播速度,以及温度650K^1050K内正十四烷氧化过程中的组分分布。与现有的正十四烷氧化反应机理相比,该骨架机理和总包简化机理规模较小,为进一步开展燃烧流场数值模拟提供了可用的反应机理模型。

流量非对称性对层流火焰传播速度测量的影响

对冲火焰被广泛地应用于测量层流火焰传播速度,但实验中常常出现两个喷嘴的流量非对称,造成流场的非对称性。通过数值模拟发现,流量不对称性使得火焰锋面不断移向流量较小的一侧,两侧火焰速度均发生变化,随着流量差增大,流量较大侧得到的层流火焰速度也稍有增加,而流量较小侧的与对称条件下的一致。建议尽量使用流量较小侧作为测量区域。在常规实验的流量偏差内,使用局部拉伸率拟合所得到的层流火焰传播速度是可靠的。