生物质燃气预混层流燃烧特性的实验研究

在密闭燃烧容器中对常温、常压环境下的生物质燃气预混层流燃烧特性进行了实验研究,研究了不同燃气组分、不同当量比对生物质燃气预混层流火焰传播速度、火焰表面拉伸率和层流燃烧速度的影响规律。研究结果表明:发酵法制取的生物质燃气中甲烷含量越高,其层流火焰传播速度就越快;相同尺寸的火焰锋面上拉伸率越大,层流燃烧速度则越快;随着当量比的增大,层流火焰传播速度、层流燃烧速度呈现出先增大后减小的趋势。     

异丁醇/辛酸甲酯混合燃料预混层流火焰速度的研究

针对生物柴油与醇类混合燃料燃烧机理研究的需求,采用高速纹影光学诊断方法和定容燃烧弹系统试验研究了异丁醇/辛酸甲酯混合燃料的预混层流燃烧特性。测量了不同当量比和初始压力条件下的不同配比混合燃料—空气预混合气的层流燃烧火焰速度,火焰拉伸率以及马克斯坦长度。分析了燃烧初始条件及异丁醇掺混比例对混合燃料的无拉伸层流燃烧速度及火焰不稳定性的影响规律。结果表明:异丁醇/辛酸甲酯混合燃料的拉伸层流火焰传播速度和层流火焰燃烧速度随着当量比的增加先增加后减少,随着初始压力的增加而减小;马克斯坦长度随着当量比和初始压力的增加而减小;异丁醇掺混比例的增加加快了层流火焰燃烧速度,但使得火焰的不稳定性倾向增加。     

氮气稀释气对天然气燃烧特性的影响

为获得氮气稀释气对天然气燃烧特性的影响规律,在定容燃烧反应器中对不同当量比与初始压力下天然气的火焰传播特性、燃烧稳定性及燃烧特性进行了试验测试,并分析了氮气稀释度对天然气火焰传播特性、燃烧稳定性及燃烧特性的影响规律。研究结果表明:随着初始压力与氮气稀释度的升高,火焰前锋面将出现细小裂纹,火核逐渐向定容燃烧反应器上部漂移,火焰稳定性变差;随着初始压力的提高,马克斯坦长度明显变短,火焰稳定性变差,无拉伸火焰传播速度与层流燃烧速度明显降低,但最大燃烧压力显著升高。随着当量比的提高,层流燃烧速度与最大燃烧压力出现先增加后降低的趋势,两者的最大值出现在当量比为1.0时。马克斯坦长度随氮气稀释度的增加逐渐变短,表明火焰逐渐趋于不稳定;同时,无拉伸火焰传播速度、层流燃烧速度与最大燃烧压力随氮气稀释度的增加显著降低。     

乙醇-空气预混层流火焰特性的试验研究

利用纹影高速摄像技术,在定容燃烧弹内试验研究了温度为358~500 K,当量比从0.7到1.4的乙醇—空气预混层流火焰的传播特性。通过研究乙醇—空气火焰传播速度与层流火焰拉伸的关系,获得了乙醇—空气火焰无拉伸层流燃烧速度。结合先前研究结论,总结得出了乙醇—空气层流火焰无拉伸层流燃烧速度的经验公式。通过计算乙醇—空气层流火焰质量燃烧速率,确定了乙醇—空气层流火焰的全局活化温度以及Zeldovich数随混合气当量比的变化关系,并由此提出了乙醇—空气层流火焰燃烧速度的的替代拟合公式。通过比较,发现本研究结论与以前结果很吻合。     

仲丁醇-空气预混层流燃烧速度的研究

基于定容燃烧弹,利用纹影法和球型火焰扩散法研究了不同燃空当量比、环境温度和环境压力下仲丁醇-空气预混层流燃烧速度.通过对仲丁醇-空气拉伸层流火焰传播速度与拉伸率之间关系的分析,获得了无拉伸火焰层流燃烧速度和马克斯坦长度.研究结果表明:随着环境压力的上升,仲丁醇-空气层流燃烧速度降低,马克斯坦长度降低,火焰不稳定性增加;随着环境温度的增加,无拉伸层流燃烧速度增加,马克斯坦长度减小,表明燃烧火焰不稳定性增加;随着燃空当量比的增加,马克斯坦长度减小,火焰不稳定性增加;燃空当量比Φ=1.1左右时,火焰传播速度和无拉伸层流燃烧速度达到最大值.     

火焰拉伸对甲烷/空气和丙烷/空气层流燃烧速度的影响

通过拓展层流火焰消耗速度的概念,将其定义与反应进程变量（progress variable）的定义相结合,给出了一个积分层流燃烧速度的广义定义。在准一维稳态系统中,分析了积分层流燃烧速度,以及其与未燃气体的位移速度和已燃气体的位移速度之间的关系。对甲烷-空气和丙烷-空气拉伸层流预混火焰在常温常压下进行了数值计算,研究了在不同当量比下,火焰拉伸对层流燃烧速度的影响,并得出了马克斯坦长度。对基于通过火焰前锋放热率的积分层流燃烧速度和基于燃料消耗率的积分层流燃烧速度进行了比较。结论表明低拉伸火焰的马克斯坦数与渐进分析一致,也与球形火焰获得的实验数据吻合。     

层流预混滞止火焰结构及传播速度的数值模拟

在考虑了甲烷与空气燃烧过程中１７种分子、原子和基团的４６个基元反应的基础上,采用数值模拟方法求解了甲烷层流滞止火焰结构,给出了各种组分与温度的空间分布,计算了不同拉伸率下的甲烷滞止火焰的传播速度,导出了层流预混火焰的传播速度。     

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。     

湍流条件下高当量比合成气着火实验研究

为了研究高当量比合成气着火极限条件下初始湍流环境对火焰初始发展阶段的影响规律,在湍流定容燃烧实验装置中开展了常温、常压条件下50%CO/50%H2合成气相关湍流燃烧实验,并研究了火焰等效半径和火焰传播速度变化的规律及影响因素.研究结果表明:在本文实验条件下层流环境中,火焰传播速度呈现先减小后增加的趋势,并且随着当量比的增加而逐渐减小,合成气着火极限当量比为5.8;在高当量比混合气条件下,初始湍流强度的增加可以拓宽可燃混合气的着火极限;在高当量比着火极限条件下,火焰等效半径随着湍流强度增大而增加,火焰传播速度随着湍流强度的增加而增大,同一湍流强度环境中,火焰传播速度总体呈现出先减小后增大的趋势.     

甲醇裂解气对甲烷-空气预混层流燃烧特性的影响

采用定容燃烧弹-纹影系统,将H_2与CO按体积比为2∶1的混合气来模拟真实甲醇裂解气,进行了初始温度为343,K、初始压力为0.3,MPa下的甲烷-甲醇裂解气-空气预混燃烧试验,研究了不同当量比(0.6~1.8)和不同添加比例(20%,~80%,)的甲醇裂解气(其中V(H_2)∶V(CO)=2∶1)对甲烷-空气层流火焰燃烧速度、马克斯坦长度、火焰胞状结构及其影响参数等层流燃烧特性的影响,并在相同条件下单独添加CO,探究CO在甲醇裂解气中的作用.结果表明:甲醇裂解气能提高混合气层流火焰燃烧速度,在整个当量比范围尤其是稀燃时加强火焰不稳定性,促进胞状结构的产生.CO也能提高燃烧速度,但提升幅度比甲醇裂解气小,而且只有在大比例添加且当量比为1.2附近时才对火焰胞状不稳定性产生明显促进作用,即甲醇裂解气中对甲烷层流燃烧速度和火焰稳定性起主要影响的成分为H_2.     

Study of laminar combustion characteristics of gasoline surrogate fuel-hydrogen-air premixed flames

This paper investigated the effects of hydrogen addition to gasoline surrogates fuel-air mixture on the premixed spherical flame laminar combustion characteristics. The experiments were carried out by high speed Schlieren photography on a constant-volume combustion vessel. Combining with nonlinear fitting technique, the variation of flame propagation speed, laminar burning velocity, Markstein length, flame thickness, thermal expansion coefficient and mass burning flux were studied at various equivalence ratios (0.8–1.4) and hydrogen mixing ratios (0%–50%). The results suggested that the nonlinear fitting method had a better agreement with the experimental data in this paper and the flame propagation was strongly effected by stretch at low equivalence ratios. The stretched propagation speed increased with the increase of hydrogen fraction at the same equivalence ratio. For a given hydrogen fraction, Markstein length decreased with the increase of equivalence ratio; flame propagation speed and laminar burning velocity first increased and then decreased with the increase of equivalence ratio while the peaks of the burning velocity shifted toward the richer side with the increase of hydrogen fraction.     

Measurement of laminar burning velocity and Markstein length relative to fresh gases using a new postprocessing procedure: Application to laminar spherical flames for methane,ethanol and isooctane/air mixtures

The purpose of this study is to present a new tool for extracting the laminar burning velocity in the case of spherically outward expanding flames. This new procedure makes it possible to determine the laminar burning velocity directly based on the flame displacement speed and the global fresh gas velocity near the preheat zone of the flame front. It therefore presents a very interesting alternative to the standard method (commonly used in the literature), which is based on the flame front displacement and the ratio of unburned and burned gas densities. The influence of external flame stretching on the burning velocity can be characterized and the Markstein length relative to the unburned gases (i.e., fresh gases) can be deduced by using this new tool. Contrary to the standard procedure, the unstretched laminar burning velocity is determined directly without using the fuel mixture properties. The temporal evolution of the flame front is visualized by high-speed laser tomography and the algorithm, based on a tomographic image correlation method, makes it possible to accurately measure the fresh gas velocity near the preheat zone of the flame front. The measurements of laminar flame speeds are carried out in a high-pressure and high-temperature constant-volume vessel over a wide range of equivalence ratios for methane, ethanol, and isooctane/air mixtures. To validate the experimental facility and the postprocessing of the flame images, fresh gas velocities and unstretched laminar burning velocities, as well as Markstein lengths relative to burned and unburned gases, are presented and compared with experimental and numerical results of the literature for methane/air flames. New results concerning ethanol/air and isooctane/air flames are presented for various experimental conditions (373 K, equivalence ratios range 0.7–1.5, pressure range 0.1–5 MPa).     

Experimental and modeling study of laminar burning velocity of biomass derived gases/air mixtures

Laminar burning velocities of four biomass derived gases have been measured at atmospheric pressure over a range of equivalence ratios and hydrogen contents, using the heat flux method on a perforated flat flame burner. The studied gas mixtures include an air-blown gasification gas from an industrial gasification plant, a model gasification gas studied in the literature, and an upgraded landfill gas (bio-methane). In addition, co-firing of the industrial gasification gas (80% on volume basis) with methane (20% on volume basis) is studied. Model simulations using GRI mechanisms and detailed transport properties are carried out to compare with the measured laminar burning velocities. The results of the bio-methane flame are generally in good agreement with data in the literature and the prediction using GRI-Mech 3.0. The measured laminar burning velocity of the industrial gasification gas is generally higher than the predictions from GRI-Mech 3.0 mechanism but agree rather well with the predictions from GRI-Mech 2.11 for lean and moderate rich mixtures. For rich mixtures, the GRI mechanisms under-predict the laminar burning velocities. For the model gasification gas, the measured laminar burning velocity is higher than the data reported in the literature. The peak burning velocities of the gasification gases/air and the co-firing gases/air mixtures are in richer mixtures than the bio-methane/air mixtures due to the presence of hydrogen and CO in the gasification gases. The GRI mechanisms could well predict the rich shift of the peak burning velocity for the gasification gases but yield large discrepancy for the very rich gasification gas mixtures. The laminar burning velocities for the bio-methane/air mixtures at elevated initial temperatures are measured and compared with the literature data.     

Laminar burning characteristics of premixed methane-dissociated methanol-diluent mixtures

The influence of dissociated methanol (DM) and diluent (CO₂ and N₂) addition on methane was investigated in a constant volume chamber under initial conditions of 3 bar and 343 K. CO was also added in separate proportions instead of DM under the same conditions to assess its effect. The laminar burning velocity, Markstein length and flame instability were analyzed systematically under various equivalence ratios (0.8–1.4), dissociated methanol gas ratios (40 and 80%), CO ratios (40 and 80%) and dilution ratios (0–15%). Furthermore, the flame speed of the fuel mixture and the production rate of key reactants were analyzed based on the calculation results of the Aramco Mech 2.0 mechanism to determine the influence principles of dilution. The results show that dissociated methanol gas increases the flame speed of the mixtures and promotes instability of the flame, and H₂ is the dominant component in enhancing the combustion process. Within the dilution ratio range of this study, the diluents decrease the laminar burning velocity of the mixtures since the addition of diluent gas decreases the concentration of key reactants, such as H and OH. The addition of diluent gas can inhibit the flame instability, but the effect is not clear. Compared with N₂, the effect of CO₂ is more significant.     

Measurements of laminar burning velocities and flame stability analysis for dissociated methanol–air–diluent mixtures at elevated temperatures and pressures

The laminar burning velocities and Markstein lengths for the dissociated methanol–air–diluent mixtures were measured at different equivalence ratios, initial temperatures and pressures, diluents (N₂ and CO₂) and dilution ratios by using the spherically outward expanding flame. The influences of these parameters on the laminar burning velocity and Markstein length were analyzed. The results show that the laminar burning velocity of dissociated methanol–air mixture increases with an increase in initial temperature and decreases with an increase in initial pressure. The peak laminar burning velocity occurs at equivalence ratio of 1.8. The Markstein length decreases with an increase in initial temperature and initial pressure. Cellular flame structures are presented at early flame propagation stage with the decrease of equivalence ratio or dilution ratio. The transition positions can be observed in the curve of flame propagation speed to stretch rate, indicating the occurrence of cellular structure at flame fronts. Mixture diluents (N₂ and CO₂) will decrease the laminar burning velocities of mixtures and increase the sensitivity of flame front to flame stretch rate. Markstein length increases with an increase in dilution ratio except for very lean mixture (equivalence ratio less than 0.8). CO₂ dilution has a greater impact on laminar flame speed and flame front stability compared to N₂. It is also demonstrated that the normalized unstretched laminar burning velocity is only related to dilution ratio and is not influenced by equivalence ratio.     

Propagation speed of tribrachial (triple) flame of propane in laminar jets under normal and micro gravity conditions

The propagation speed of tribrachial (triple) flames in laminar propane jets has been investigated experimentally under normal and micro gravity conditions. We found in the present experiment that the displacement speed varied nonlinearly with axial distance because the flow velocity along the stoichiometric contour was comparable to the propagation speed of tribrachial flame. Approximate solutions for the velocity and concentration accounting density difference and virtual origins have been used in determining the propagation speed of tribrachial flame and the concentration field was validated from the measurement of Raman scattering. Under the microgravity condition, the results showed that the propagation speed of tribrachial flame decreased with the mixture fraction gradient, in agreement with previous studies. The limiting maximum propagation speed under the microgravity condition is in good agreement with the theoretical prediction, ie, the ratio of maximum propagation speed to the stoichiometric laminar burning velocity is proportional to the square root of the density ratio of unburned to burnt mixture.     

Measurements of laminar burning velocities and Markstein lengths for methanol-air-nitrogen mixtures at elevated pressures and temperatures

The laminar burning velocities and Markstein lengths for the methanol-air mixtures were measured at different equivalence ratios, elevated initial pressures and temperatures, and dilution ratios by using a constant volume combustion chamber and high-speed schlieren photography system. The influences of these parameters on the laminar burning velocity and Markstein length were analyzed. The results show that the laminar burning velocity of the methanol-air mixture decreases with an increase in initial pressure and increases with an increase in initial temperature. The Markstein length decreases with an increase in initial pressure and initial temperature, and increases with an increase in the dilution ratio. A cellular flame structure is observed at an early stage of flame propagation. The transition point is identified on the curve of flame propagation speed against stretch rate. The reasons for the cellular structure development are also analyzed.     

Structures and burning velocity of biomass derived gas flames

Biomass derived gases produced via gasification, pyrolysis, and fermentation are carbon neutral alternative fuels that can be used in gas turbines, furnaces, and piston engines. To make use of these environmentally friendly but energy density low fuels the combustion characteristics of these fuels have to be fully understood. In this study the structure and laminar burning velocity of biomass derived gas flames are investigated using detailed chemical kinetic simulations. The studied gaseous fuels are the air-blown gasification gas, co-firing of gasification gas with methane, pyrolysis gases, landfill gases, and syngas, a mixture of carbon monoxide and hydrogen. The simulated burning velocities of reference fuel mixtures using two widely used chemical kinetic mechanisms, GRI Mech 3.0 and the San Diego mechanism, are compared with the experimental data to explore the uncertainties and scattering of the simulation data. The different chemical kinetic mechanisms are shown to give a reasonable agreement with each other and with experimental data, with a discrepancy within 7% over most of the conditions. The results show that the structures of typical landfill gas flames and co-firing of methane/gasification gas flames share essential similarity with methane flames. The reaction zones of these flames consist of a thin inner layer and a relatively thick CO/H₂ oxidation layer. In the inner layer hydrocarbon fuel (methane) is converted through chain reactions to intermediates such as CH₃, CH₂O, CO, H₂, etc. The structures of gasification gas flames, pyrolysis gas flames, syngas flames share similarity with the oxidization layer of the methane/air flames. Overall, the chemical reactions of all biomass derived gas flames occur in thin zones of the order of less than 1 mm. The thickness of all BDG gas flames is inversely proportional to their respective laminar burning velocity. The laminar burning velocities of landfill gases are found to increase linearly with the mole fraction of methane in the mixtures, whereas for gasification gas, syngas and pyrolysis gas where hydrogen is present, the laminar burning velocities scale linearly with the mole fraction of hydrogen.     

Effect of nitrogen on deflagration characteristics of hydrogen / methane mixture

In order to study the influence of nitrogen on the deflagration characteristics of premixed hydrogen/methane, the explosion parameters of premixed hydrogen/methane within various volume ratios and different dilution ratios were studied by using a spherical flame method at room temperature and pressure. The results are as follows: The addition of nitrogen makes the upper limit of explosion of hydrogen/methane premixed gas drop, and the lower limit rises. For explosion hazard (F-number), hydrogen/methane premixed fuel with a hydrogen addition ratio of 10% has the lowest risk, and nitrogen has a greater impact on the dangerous degree of hydrogen and methane premixed gas whose hydrogen addition ratio does not exceed 30%. In terms of flame structure, the spherical flame was affected by buoyancy instability as the percentage of nitrogen dilution increased, but the buoyancy instability gradually decreased as the percentage of hydrogen addition increased. The addition of diluent gas reduces the spreading speed of the stretching flame and reduces the stretching rate in the initial stage of flame development. The laminar flame propagation velocity calculated by the experiment in this paper is consistent with the laminar flow velocity of the hydrogen/methane premixed gas calculated by GRI Mech 3.0. Considering the explosion parameters such as flammability limit, laminar combustion rate and deflagration index, when hydrogen is added to 70%, it is the turning point of hydrogen/methane premixed fuel.