Mass,Mobility, Volatility,and Morphology of Soot Particles Generated by a McKenna and Inverted Burner

Particle mass, mobility, volatile mass fraction, effective density, mass concentration, mass–mobility exponent, and particle morphology were measured from soot generated from a premixed flame (McKenna burner) and an inverted diffusion flame over a range of equivalence ratios. It was found that the mass fraction of volatile material on the soot from the McKenna burner could be up to 0.83 at a high equivalence ratio, but there was no measurable volatile material on the soot from the inverted burner. The inverted burner can produce soot at different mass–mobility exponents, ranging from 2.23 to 2.54, over a range of global equivalence ratios of 0.53–0.67, while the mass–mobility exponent ranges from 2.19 to 2.99 for fresh soot and 2.19 to 2.81 for denuded soot for the McKenna burner at equivalence ratios of 2.0–3.75. Transmission electron microscopy analysis of inverted burner soot shows that a range of particle morphologies is present at a given global equivalence ratio, likely due to different local equivalence ratios and flame conditions in the diffusion flame. Primary particle diameter tends to increase with aggregate size, which could contribute to the mass–mobility exponent being well above 2.

Copyright 2013 American Association for Aerosol Research     

Influence of the Strain Rate,Particle Size,and Equivalence Ratio on the Combustion of the Premixed Air–Aluminum Microparticle Mixture with a Counterflow Structure

The effects of the strain rate, equivalence ratio, and particle diameter on the combustion of a mixture of aluminum microparticles with air under fuel-lean conditions are studied in the counterflow configuration with an approximate analytical perturbation method. The flame structure is assumed to consist of three zones: preheating, flame, and post-flame zones. Reasonable agreement between the current results and experimental data is obtained in terms of the flame temperature. The dimensionless ignition and ultimate flame temperatures, place of the flame starting point, and flame thickness are obtained as functions of the strain rate for different particle diameters and equivalent ratios. The results indicate that the ignition and ultimate flame temperatures and also the flame thickness decrease with increasing strain rate. With a decrease in the strain rate, the length of the preheating zone increases. With increasing particle diameter, the flame thickness increases, whereas the ignition and ultimate flame temperatures decrease. An increase in the equivalence ratio causes an increase in the ultimate flame temperature and reduction of the preheating zone and flame thickness.     

Measurement of laminar burning velocity of dimethyl ether-air premixed mixtures

Measurement of laminar burning velocity of dimethyl ether-air mixtures was taken under different initial pressures and equivalence ratios using a constant volume bomb and high-speed schlieren photography. The stretched laminar burning velocity increases with the increase of stretch rate. At equivalence ratio of 1.0, low initial pressure gives high stretched flame speed. At initial pressure less than 0.1 MPa, the stoichiometric mixture gives the higher value of stretched flame speed than those at ? = 1.2 and ? = 0.8. The Markstein numbers decrease with the increase of equivalence ratio, and this reveals that lean mixture will maintain higher stability of flame front surface than that of rich mixture in dimethyl ether-air premixed flames.     

乙醇和二甲醚微射流火焰燃烧特性研究

采用实验及数值计算研究了乙醇和二甲醚微圆管射流火焰燃烧特性。通过实验观察到不同燃料流速下乙醇和二甲醚火焰都具有四种典型的火焰形态；使用平面激光诱导荧光测试系统获得了微射流火焰的OH基元分布，实验结果表明在较高流速下稳定燃烧的乙醇火焰比二甲醚火焰直径小，且略高于二甲醚火焰；采用考虑详细化学反应机理的数值计算对乙醇和二甲醚火焰进行了数值模拟，计算结果与实验现象吻合较好；利用一维非预混对冲火焰计算进一步研究了这两种燃料的化学反应路径，分析结果表明乙醇和二甲醚火焰的中间产物有显著差异，两种燃料化学反应特性的差异导致了不同的微火焰结构。     

Measurement and chemical kinetic prediction of detonation sensitivity and cellular structure characteristics in dimethyl ether-oxygen mixtures

In this work, experiments have been performed to measure the detonation velocities and characteristic cell sizes in the dimethyl ether (DME) fuel-oxygen mixtures. Equilibrium calculation and detailed chemical kinetics modeling of the ZND structure of detonations are also carried out to investigate the detonation characteristics of DME. Detonation cell sizes estimated using a correlation model by Ng et al. [Ng HD, Ju Y, Lee JHS. Assessment of detonation hazards in high-pressure hydrogen storage from chemical sensitivity analysis. Int J Hydrogen Energy 2007;32:93-99] are in good agreement with experimental data. It is found that the cell size values for DME-oxygen mixtures are comparable to those of propane or ethane fuels. At low initial pressure, double cell like detonation structures have been observed in all equivalence ratios considered in this study. Chemical kinetic results reveal that DME oxidation under detonation environment exhibits similarly a two-stage heat release process inside the reaction zone. This effect may play a significant role in the existence and scaling of the multi-cell detonation pattern in stoichiometric and fuel-rich DME mixtures. On the lean side, multiple cells appear to be caused primarily by the strong intrinsic instability of the unsteady detonation front. The present experimental results and chemical kinetic sensitivity analyses provide some basic information to assess detonation hazards in DME-based mixtures.     

Ammonia combustion at elevated pressure and temperature conditions

This numerical study examines the combustion characteristics of premixed ammonia-air mixtures, with equivalence ratios around unity, at elevated pressure and temperature conditions which are encountered in SI engine operations. The laminar burning velocity, final flame temperature and species concentrations were determined using Konnov’s mechanism [18]. A flat, freely propagating flame was considered. Both equivalence and compression ratios have an important impact on both the laminar burning velocity and the adiabatic flame temperature. Furthermore, only the variation of the equivalence ratio has a major impact on the formation of nitrogen monoxide. It was found that the compression ratio and the final temperature do not have a significant impact on NO yields for equivalence ratios above unity.     

二甲醚火焰传播速度的实验测定

二甲醚被誉为21世纪的绿色燃料，是目前理想的内燃机与民用替代燃料．为了获得二甲醚应用的重要基础参数——火焰传播速度，在定容球弹法、本生火焰法和平面火焰法作出比较分析后，采用本生火焰法实验测定了二甲醚火焰的传播速度．得出了二甲醚火焰传播速度与一次空气系数的关系曲线，并且在一次空气系数约为0．85处，二甲醚火焰传播速度的峰值为0．48m／s．     

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 H₂) 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.     

自由堆积多孔介质内预混燃烧火焰传播

为了解多孔介质内预混燃烧火焰前沿的传播特性,对不同化学当量比（=0.7～1.0）的甲烷/空气预混气体在不同孔隙率（ε为0.37和0.42）的多孔介质内的火焰前沿传播特性进行了研究,多孔介质采用3 mm和6 mm直径的Al₂O₃小球在陶瓷管中堆积而成。结果表明,预混气体在多孔介质中能够形成低速燃烧的稳定燃烧波;其火焰传播速度随化学当量比增大而加快,最大的火焰传播速度为3.52×10^-3 cm·s^-1;多孔介质的结构对火焰前沿传播速度影响很大,即使在孔隙率差别不大的情况下,大球堆积而成的多孔介质比小球具有更高的火焰前沿传播速度。     

Laminar flame speeds and extinction limits of conventional and alternative jet fuels

Experimental results of laminar flame speeds and extinction stretch rates for the conventional (Jet-A) and alternative (S-8) jet fuels are acquired and compared to the results from our earlier studies for neat hydrocarbon surrogate components, including n-decane and n-dodecane. Specifically, atmospheric pressure laminar flame speeds are measured using a counterflow twin-flame configuration for Jet-A/O₂/N₂ and S-8/O₂/N₂ mixtures at preheat temperatures of 400, 450, and 470 K and equivalence ratios ranging from 0.7 to 1.4. The flow field is recorded using digital particle image velocimetry. Linear extrapolation is then applied to determine the unstretched laminar flame speed. Experimental data for the extinction stretch rates of the nitrogen diluted jet fuel/oxidizer mixtures as a function of equivalence ratio are also obtained. In addition, the experimental data of Jet-A are compared to the computed values using a chemical kinetic mechanism for a kerosene surrogate reported in literature. A sensitivity analysis is further performed to identify the key reactions affecting the laminar flame speed and extinction stretch rate for this kerosene surrogate.     

氨气/甲烷贫预混旋转湍流火焰稳定性及NO生成

为了研究氨气/甲烷掺混燃气在贫预混旋转湍流状态下的火焰稳定性及NO的排放特性,设计建造了一个可视化的旋转湍流燃烧装置,开展了一系列的实验测量研究。研究表明：随着当量比增大,氨气火焰稳定燃烧的范围有所扩大,但当氨气掺混比大于0.60时火焰出现上下振荡现象,继续增加将导致火焰吹熄;NO的排放水平随当量比增加而提高;但在相同的当量比下,NO的排放随氨气掺混比的增加先升高再下降。此外,分别采用化学反应器网络（CRN）方法和一维层流预混火焰计算方法,对相应的火焰状态进行了数值计算分析,虽然计算结果与实验结果误差较大,但其预测的NO排放特性随氨气掺混比、当量比的变化趋势是一致的,对三者之间误差的来源进行了分析。     

氨气/甲烷贫预混旋转湍流火焰稳定性及NO生成

为了研究氨气/甲烷掺混燃气在贫预混旋转湍流状态下的火焰稳定性及NO的排放特性,设计建造了一个可视化的旋转湍流燃烧装置,开展了一系列的实验测量研究。研究表明：随着当量比增大,氨气火焰稳定燃烧的范围有所扩大,但当氨气掺混比大于0.60时火焰出现上下振荡现象,继续增加将导致火焰吹熄;NO的排放水平随当量比增加而提高;但在相同的当量比下,NO的排放随氨气掺混比的增加先升高再下降。此外,分别采用化学反应器网络（CRN）方法和一维层流预混火焰计算方法,对相应的火焰状态进行了数值计算分析,虽然计算结果与实验结果误差较大,但其预测的NO排放特性随氨气掺混比、当量比的变化趋势是一致的,对三者之间误差的来源进行了分析。     

The Generation of a Reduced Mechanism for Flame Inhibition by Phosphorus Containing Compounds Based on Path Flux Analysis Method

In order to analyze the complex chemical kinetic mechanism systematically and find out the redundant species and reactions, a numerical platform for mechanism analysis and simplification is established basing on Path Flux Analysis （PFA）. It is used to reduce a detailed mechanism for flame inhibited by phosphorus containing compounds, a reduced mechanism with 65 species and 335 reactions is obtained. The detailed and reduced mechanism are both used to calculate the freely-propagating premix C3H8/air flame with different dimethyl methylphosphonate doped over a wide range of equivalence ratios. The concentration distributions of free radicals and major species are compared, and the results under two different mechanisms agree well. The laminar flame speed obtained by the two mechanisms also matches well, with the maximum relative error introduces as a small value of 1.7%. On the basis of the reduced mechanism validation, the correlativity analysis is conducted between flame speed and flee radical concentrations, which can provide information for target species selection in the further mechanism reduction. By analyzing the species and reactions fluxes, the species and reaction paths which contribute the flame inhibition significantly are determined.     

Combustion of Boron Particles in Premixed Methane/Air Flames

Because of its high energy density, boron particles have been a subject of interest for the use as propellant in propulsion systems for many years. A cheap and fast opportunity to investigate multiphase reacting flows in such systems is offered by numerical simulations. Therefore, a detailed knowledge of the chemistry and kinetics of boron combustion in different gaseous surroundings is required. The main topic of this contribution is the experimental investigation of the influence of the equivalence ratio of reacting methane‐air mixtures on the combustion time of boron particles. Additionally, numerical calculations were performed using a combustion model proposed by Yeh et al. [1] and modified by Hussmann et al. [2, 3]. The experimental results show that for small particles there exists an optimal stoichiometry, at which the combustion time of boron particles is minimized. High equivalence ratios yield larger burning times because of a low concentration of oxygen. Low equivalence ratios are accompanied by low flame temperatures also leading to large burning times, because of a slow reactive evaporation of the boron‐oxide layer. With increasing particle size the burning process is dominated by the evaporation process. Numerical results are in a good agreement with the experiments for small particles. For larger particles, the predicted burning time is too high due to the fact that boron particles cannot be treated as a sphere as assumed in the original model. By implementing a sphericity factor good agreement with the experiments can be achieved.     

Experimental studies on the thickness of upward flame over poly(methyl methacrylate) slabs

Experiments were performed to investigate the flame thickness of the upward flame on the poly(methyl methacrylate) slabs with width of 100‐400 mm. The results indicated that the flame thickness exhibited an increase first and then decrease trend in the upright orientation, and the maximum thickness location was approximately equal to the pyrolysis front location. The thickness of the flame along the lateral side of slab was less than that of the interior flame. The flame maximum thickness as a function of pyrolysis height and width was obtained, which showed the maximum thickness provided a power law increase with the pyrolysis height and width. Furthermore, the maximum thickness exhibited a power law increase with the total heat release rate as well. Based on the obtained flame thickness, the radiation heat flux at the pyrolysis height was estimated by using a simplified model. Comparison with the calculated convective heat flux revealed approximated pyrolysis heights for upward flame transition from convective heat flux controlled to radiation heat flux controlled.     

Flame structure and laminar burning speeds of JP-8/air premixed mixtures at high temperatures and pressures

Jet propellant 8 (JP-8)/air laminar burning speed was experimentally measured and its flame structure was studied at high temperatures and pressures using a high-speed camera. The experimental facilities included a spherical vessel, used for the measurement of burning speed, and a cylindrical vessel, used in a shadowgraph system to study flame shape and structure and to measure burning speed. A thermodynamic model was developed to calculate burning speeds using the dynamic pressure rise in the vessel due to the combustion process. The model consists of a central burned gas core of variable temperature surrounded first by a reaction sheet, then by an unburned gas shell with uniform temperature and lastly by thermal boundary layers at the wall and electrodes. Radiation from burned gases to the walls was also included in the model. Burning speeds of laminar flames of JP-8/air were calculated for a wide range of conditions. A Power law correlation was developed to calculate laminar burning speed at temperatures ranging from 500-700 K, pressures of 1-6 atm and equivalence ratios of 0.8-1. Flame structure and cell formations were observed using an optical system. Experimental results showed that pressure and the fuel-air equivalence ratio have a strong influence on flame structure.     

Experimental investigation of flow through multi-layered preforms

This research investigates resin flow phenomena through preforms composed of multiple layers of reinforcement material. Two complete studies were performed: the first investigates the effect of varying the order of lay-up of a fixed number of plies, and the second studies the impact of varying the thickness of the individual layers of a thick preform. In each case, a set of compression experiments were first performed to characterize the preforms so that in-situ layer thicknesses can be calculated. Next, flow experiments were performed upon the preforms to determine the permeability and flow front behavior. In the first study, it was found that the weighted average scheme provides a reasonable estimate for the effective permeabilities of the preforms, with errors ranging from 14.2% to 23.8%. In the second study, however, the weighted average scheme was found to provide only a rudimentary estimate of the permeability, with an error that was seen to increase in magnitude during the course of each experiment. In addition, computer simulations were performed for the second study, which show the possibility of deriving transverse permeability values without direct measurement, although significant errors in inlet pressures were once again evident.     

Experimental study of multispecies gas combustion in a several-section porous media burner

Experiments with combustion of diluted liquefied petroleum gases used as a fuel premixed with air are performed. From the experiment results, one can see that low-heat-value gases are capable of stable burning in a porous media burner. Distributions of species concentrations and flame temperature are measured. Based on these data, the flame is found to be most stable if the equivalence ratio is equal to 0.8. To improve the burner performance, experiments with different characteristics of porous media are performed. Optimal parameters of porous media are confirmed by subsequent numerical simulations. It is demonstrated that the properties of combustion in the porous media burner are superior as compared to those in the free flame burner.     

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.     

高浓度丙酮VOCs高温预热近极限贫燃预混火焰特性的数值分析

为进一步对一种丙酮挥发性有机化合物（VOCs）焚烧炉进行设计优化和运行参数调节,本文对其在不同的燃料当量比、预热温度下的火焰特性进行了数值模拟,分析了其绝热火焰温度、着火延迟时间、火焰传播速度和一维火焰产物分布特性。研究结果表明：典型当量比（约0.113）下的绝热火焰温度为850~900℃,属于中低温燃烧,绝热火焰温度随预热温度和当量比（0.06~0.4）的升高均线性升高。预热温度和化学当量比对着火延迟时间的影响十分敏感。在其典型贫燃条件下,层流火焰传播速度随预热温度升高呈指数函数关系增大,随化学当量比增大而缓慢升高,且其层流火焰传播速度不超过150cm/s。反应过程首先发生丙酮的分解和部分氧化,并持续时间较长,仅当混合物的温度升高一定程度后才发生较剧烈的CO氧化。