二甲醚-空气混合气层流燃烧速度的测定

在定容燃烧弹中利用高速纹影摄像法系统地研究了不同燃空当量比和初始压力下二甲醚-空气混合气的层流燃烧特性.利用球形扩散火焰理论分析纹影照片,获得了不同初始压力和当量比下的二甲醚-空气混合气层流燃烧速率.结果表明：随着初始压力的增大,层流燃烧速率显著减小,层流燃烧速率的峰值向浓混合气侧偏移.拉伸层流燃烧速率随拉伸率的增加而增加,拉伸层流质量燃烧速率随拉伸率的增加而减小.根据球形扩散火焰模型得到混合气的马克斯坦长度值表明：在各初始压力下,随着当量比的增加,二甲醚-空气混合气的马克斯坦长度值逐渐减小,火焰前锋面的不稳定性增加.     

二乙醚-空气预混合气层流燃烧速率测定

在定容燃烧弹内利用高速纹影摄像法系统地研究了不同初始压力、不同初始温度和不同燃空当量比下二乙醚-空气预混合气的层流燃烧特性。利用球形发展火焰分析得到了不同初始压力、不同初始温度和不同燃空当量比下二乙醚-空气预混合气的无拉伸层流火焰燃烧速率、马克斯坦长度等层流燃烧参数。研究结果表明:无拉伸层流火焰燃烧速率随初始温度的增加而增加,随初始压力的增加而降低;马克斯坦长度随着初始温度的增加而减小,随初始压力的增加而减小,随当量比的增加而减小,表明火焰前锋面不稳定性随初始温度和初始压力的增加而增加,随混合气浓度的增加而增加。基于试验数据获得了二乙醚-空气预混合气无拉伸层流燃烧速率的关系式。     

丙烷-空气-稀释气层流燃烧速率测定

利用高速纹影摄像法和球型发展火焰研究了常温常压下丙烷-空气,丙烷-空气-稀释气预混层流燃烧特性,获得了不同稀释系数(0、10%、20%、30%)和燃空当量比(0.6～2.0)下混合气的层流燃烧速率和马克斯坦长度值,分析了拉伸对火焰传播速率的影响.结果表明:丙烷-空气混合气的无拉伸火焰传播速率和无拉伸层流燃烧率在当星比1.1时达到最大值,随当量比的增加,马克斯坦长度值降低,火焰前锋面不稳定性趋势增加.当量比为1.4时,马克斯坦长度值由正值转为负值.丙烷-空气-稀释气混合气随稀释系数的增加,火焰传播速率和层流燃烧速率降低,在当量比小于1.4时,随稀释系数的增加,马克斯坦长度值增加,火焰前锋面的稳定性趋势增加.有无稀释气时无拉伸层流燃烧速率的比值仅与稀释系数有关并成线性关系而与混合气浓度无关.     

不同初始压力下掺氢天然气的燃烧特性

在定容燃烧弹内研究了不同初始压力下天然气-氢气-空气混合气的火焰传播规律,得到了不同掺氢比例和初始压力下,不同燃空当量比时混合气的层流燃烧速率,并分析了火焰的稳定性及其影响因素.研究结果表明,随着天然气中掺氢比例的增加,混合气的燃烧速率增加,且增长速率逐渐加快,而马克斯坦长度值则随着掺氢比例的增加而减小,即火焰的稳定性下降.不同初始压力下,随着燃空当量比的增加,马克斯坦长度值在不同掺氢比例下均增加,显示火焰的稳定性增加.无拉伸层流燃烧速率随着初始压力的增加略有减小,且在化学当量比附近,变化的初始压力和掺氢比对无拉伸层流燃烧速率的影响最为明显.     

初始温度/压力对天然气层流燃烧速率的影响

在定容燃烧弹内利用高速纹影摄像法研究了不同初始温度和初始压力下不同当量比的天然气-空气混合气的火焰传播过程,并结合火焰传播照片分析了初始压力和初始温度对混合气层流燃烧速率的影响.研究表明,天然气-空气混合气的无拉伸火焰传播速率和无拉伸层流燃烧速率随当量比的增加先增大后减小,且最大值出现在化学当量比附近.火焰传播速率和层流燃烧速率均随着初始压力的升高而降低,随着初始温度的升高而提高.     

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。     

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。     

初始压力对天然气-氢气-空气混合气火焰传播特性的影响

使用定容燃烧弹研究了不同初始压力下天然气-氢气-空气混合气的火焰传播规律,得到了初始压力、掺氢比和燃空当量比对无拉伸层流燃烧速率、质量燃烧流量的影响,结合高速纹影图片分析了影响火焰稳定性的因素(马克斯坦长度、火焰面两侧密度比和火焰厚度).结果表明,掺氢天然气无拉伸层流燃烧速率以及火焰的不稳定性受掺氢比、初始压力和燃空当量比的综合影响.结合高速纹影图片,得出火焰的稳定性会随初始压力的增加而减小;在相同的燃空当量比和掺氢比下,初始压力对密度比的影响不大,但是对火焰厚度的影响比较明显.     

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.     

高温高压下掺氢天然气的燃烧特性

在定容燃烧弹内研究了高温(450,K)、高压(0.75,MPa)条件下天然气-氢气-空气混合气的火焰传播过程,获得了不同掺氢比和不同当量比下掺氢天然气的无拉伸层流燃烧速率,并分析了火焰的稳定性。结果表明,高温高压下随着掺氢比的增加,掺氢天然气的燃烧速率增加,且增长速率逐渐加快;马克斯坦长度则随着掺氢比的增加而减小,即火焰的稳定性下降。随着当量比的增加,无拉伸层流燃烧速率呈现先增大后减小的趋势,且最大无拉伸层流燃烧速率所对应当量比的位置随着掺氢比的提高而向浓混合气移动;马克斯坦长度随当量比的增加而增大,即火焰稳定性随当量比的增加而提高。     

高温高压条件下乙醇-空气-稀释气预混合气层流燃烧特性研究

利用定容燃烧弹和高速纹影摄像手段研究了不同初始压力、初始温度、气体稀释度和燃空当量比下乙醇-空气-稀释气预混层流燃烧特性的基础特征参数,如绝热火焰温度、层流燃烧速度、层流燃烧质量流量、层流燃烧火焰厚度和已燃气体Markstein长度。研究结果表明:在给定初始压力、初始温度和气体稀释度的情况下,绝热火焰温度、质量燃烧流量和层流燃烧速度的最大值均出现在当量比1.0～1.1,层流火焰厚度在当量比1.1处取得最小值;已燃气体Markstein长度随当量比的增加呈下降趋势;在给定当量比条件下,绝热火焰温度随初始压力、初始温度的增加而增加,随氮气稀释度的增加而降低;层流燃烧速度随初始压力和氮气稀释度增加而降低,随初始温度增加而增加;层流质量燃烧流量随初始压力和初始温度的增加而增加;随氮气稀释度增加而减小;层流火焰厚度和已燃气体Markstein长度随初始压力和初始温度的增加而减小,随氮气稀释度的增加而增加。     

乙醇-氢气-空气混合物层流燃烧特性研究

氢气是一种高效的添加剂,可以改善生物质燃料的层流燃烧特性。为研究氢气对乙醇-空气预混层流火焰燃烧特性的影响,利用定容燃烧弹结合高速纹影摄像技术,系统研究了初始温度为400 K,初始压力为0.1 MPa和0.4 MPa,氢气含量为0%、10%、30%、50%、70%和90%,当量比为0.7 ~ 1.4时的氢气-乙醇-空气混合燃料的层流燃烧速度（LBV）、火焰厚度和马克斯坦长度等参数,并采用辐射校正公式使LBV更加精准。通过数值仿真构建预混火焰模型,与实验结果进行对比。结果表明,氢气比例的增加可以提高混合燃料的层流燃烧速度。当氢气比例小于50%时,LBV随氢气比例的增加线性增长。而当氢气比例大于50%,LBV随氢气的增加呈指数增长。初始压力的上升虽然降低了LBV,但提高了LBV的增长率。此外,随着氢气比例和初始压力的增加,火焰厚度减小,马克斯坦长度降低,火焰的不稳定性增强。     

Effect of initial pressure on laminar combustion characteristics of hydrogen enriched natural gas

Flame propagation of premixed natural gas–hydrogen–air mixtures was studied in a constant volume combustion bomb. Laminar burning velocities and mass burning fluxes were obtained under various hydrogen fractions and equivalence ratios with various initial pressures, while flame stability and their influencing factors (Markstein length, density ratio and flame thickness) were obtained by analyzing the flame images at various hydrogen fractions, initial pressures and equivalence ratios. The results show that hydrogen fraction, initial pressure as well as equivalence ratio have combined influence on both unstretched laminar burning velocity and flame instability. Meanwhile, according to flame propagation pictures taken by the high speed camera, flame stability decreases with the increase of initial pressures; for given equivalence ratio and hydrogen fraction, flame thickness is more sensitive to the variation of the initial pressure than to that of the density ratio.     

钝体燃烧器湍流预混燃烧的数值模拟

采用湍流火焰封闭燃烧模型(TFC)模拟了钝体燃烧器的湍流预混燃烧,比较了基于火焰褶皱率和湍流燃烧速度2种源项解法对钝体预混燃烧的预测,对3个不同湍流燃烧速度表达式模拟的性能进行了比较,采用粒子成像测速技术(PIV)测量了燃烧器中心射流出口的速度分布,并将其作为边界条件代入计算.结果表明:不同湍流燃烧速度公式的计算结果在火焰刷厚度、位置及火焰前锋位置方面存在较大差别;Gulder公式的计算结果最接近试验数据,火焰刷厚度与试验结果吻合较好,但火焰刷位置与试验结果差别较大;Dinkelacker的火焰褶皱率模型主要模拟燃烧器在高压条件下的燃烧,在运行压力接近标准大气压的情况下,计算结果与试验值存在较大误差.     

Characteristics of Lean Premixed Flame in Various Oxygen Concentrations Measured by Laser Imaging Techniques

For heavy-duty gas-turbine engines, one of the promising approaches to reducing NO_X emissions is the adoption of lean premixed combustion. This technique could be combined with the conventional technique of exhaust gas recirculation (EGR). However, the reduction in the oxygen concentration will influence the burning velocity and reaction zone characteristics of the lean premixed flame. To elucidate this effect, in this study, we measured the lean premixed flame temperature and OH concentration distributions for various oxygen concentrations instantaneously and simultaneously using laser imaging techniques. Based on the results, we investigated the characteristics of a lean premixed flame under various oxygen concentrations and found that the OH laser-induced fluorescence (LIF) intensity in the reaction zone decreased with the oxygen concentration, as did the flame temperature at a given axial distance from the exit nozzle. The characteristics of the premixed flame changed from a small-scale convexo-concave surface to a smoother one, leading to a decrease in the ratio of turbulent to laminar burning velocity. In addition, local extinction of the premixed flame was observed under conditions with a high air ratio and low oxygen concentration.     

Enhancement of biogas/air combustion by hydrogen addition at elevated temperatures

In this study, combustion characteristics of various biogas/air mixtures with hydrogen addition at elevated temperatures were experimentally investigated using bunsen burner method. Methane, CH₄, was diluted with different concentrations of carbon dioxide, CO₂, 30 to 40% by volume, to prepare the biogas for testing. It is followed by the hydrogen, H₂, enrichment within the range of 0 to 40% by volume and the temperature elevation of unburned gas till 440 K. Blowoff velocities were measured by lowering the jet velocity until a premixed flame could be stabilized at the nozzle exit, while laminar burning velocities were calculated by analyzing the shape of the directly captured premixed bunsen flames. The results showed that hydrogen had a positive effect on the blowoff velocity for all three fuel samples. Nonlinear growth of the blowoff velocity with hydrogen addition was associated to the dominance of methane-inhibited hydrogen combustion process. It was also observed that the increase in the initial temperature of the unburned mixture led to a linear increase of the blowoff velocity. Moreover, specific changes in flame structure such as flame height, standoff distance, and the existence of tip opening were attributed to the change in the blowoff velocity. The effect of CO₂ content in the mixture was examined with regards to laminar burning velocity for all compositions. The outcome of the experiment showed that the biogas mixture with higher content of CO₂ possessed lower values of laminar burning velocity over the wide range of equivalence ratios. A reduced GRI-Mech 3.0 was used to simulate the combustion of biogas/air mixtures with different compositions using ANSYS Fluent. The numerically simulated stable conical flames were compared with the experimental flames, in terms of flame structure, showing that the reduced GRI-Mech 3.0 was suitable for modeling the combustion of biogas/air mixtures.     

Laminar burning velocity and Markstein length of nitrogen diluted natural gas/hydrogen/air mixtures at normal,reduced and elevated pressures

Flame propagation of premixed nitrogen diluted natural gas/hydrogen/air mixtures was studied in a constant volume combustion bomb under various initial pressures. Laminar burning velocities and Markstein lengths were obtained for the diluted stoichiometric fuel/air mixtures with different hydrogen fractions and diluent ratios under various initial pressures. The results showed that both unstretched flame speed and unstretched burning velocity are reduced with the increase in initial pressure (except when the hydrogen fraction is 80%) as well as diluent ratio. The velocity reduction rate due to diluent addition is determined mainly by hydrogen fraction and diluent ratio, and the effect of initial pressure is negligible. Flame stability was studied by analyzing Markstein length. It was found that the increase of initial pressure and hydrogen fraction decreases flame stability and the flame tends to be more stable with the addition of diluent gas. Generally speaking, Markstein length of a fuel with low hydrogen fraction is more sensitive to the change of initial pressure than that of a one with high hydrogen fraction.     

Measurements of laminar burning velocities and onset of cellular instabilities of methane–hydrogen–air flames at elevated pressures and temperatures

An experimental study on laminar burning velocities and onset of cellular instabilities of the premixed methane–hydrogen–air flames was conducted in a constant volume combustion vessel at elevated pressures and temperatures. The unstretched laminar burning velocity and Markstein length were obtained over a wide range of hydrogen fractions. Besides, the effects of hydrogen addition, initial pressure and initial temperature on flame instabilities were analyzed. The results show that the unstretched flame propagation speed and the unstretched laminar burning velocity are increased with the increase of initial temperature and hydrogen fraction, and they are decreased with the increase of initial pressure. Early onset of cellular instability is presented and the critical radius and Markstein length are decreased with the increase of initial pressure, indicating the increase of hydrodynamic instability with the increase of initial pressure. Flame instability is insensitive to initial temperature compared to initial pressure. With the increase of hydrogen fraction, significant decrease in critical radius and Markstein length is presented, indicating the increase in both diffusional-thermal and hydrodynamic instabilities as hydrogen fraction is increased.     

A computational study and correlation of premixed isooctane–air laminar reaction front properties under spark ignited and spark assisted compression ignition engine conditions

To address the need for reliable premixed laminar burning velocity and thickness information within the spark assisted compression ignition (SACI) combustion regime, a large dataset of simulated reaction fronts has been generated in this work. A transient one dimensional premixed laminar flame simulation was applied to isooctane–air mixtures using a 215 species chemical kinetic mechanism. The simulation was exercised over fuel–air equivalence ratios, unburned gas temperatures and pressures ranging from 0.1 to 1.0, 298 to 1000 K and 1 to 250 bar, respectively, a range that extends beyond that of previous researchers. Steady reaction fronts with burning velocities in excess of 5 cm/s could not be established under all of these conditions, especially when burned gas temperatures were below 1500 K and/or when characteristic reaction front times were on the order of the unburned gas ignition delay. Steady premixed laminar burning velocities were correlated using a modified two-equation form based upon the asymptotic structure of a laminar flame, which produced an average error of 2.5% between the simulated and correlated laminar burning velocities, with a standard deviation of 3.0%. Additional correlations were constructed for reaction front thickness and adiabatic flame temperature. The resulting premixed laminar burning velocity correlation showed good agreement with experiments and existing correlations within the spark-ignited (SI) regime. Analysis of the simulated characteristic reaction front times and ignition delays suggests that homogeneous SACI combustion is most useful under medium and high load operating conditions.