高压甲烷射流冲击预混火焰过程中涡量的变化

基于CONVERGE软件开展三维仿真计算，分析了定容燃烧弹内高压甲烷射流撞击当量比为1的预混气体燃烧过程中的涡量变化特征．根据射流前锋面与火焰的位置关系，将火焰发展全过程分为射流前峰面未接触火焰阶段、进入火焰阶段和离开火焰阶段3个阶段．相比于自由射流贯穿速度，在射流进入火焰阶段射流燃烧平均贯穿速度显著增加，并大于自由射流的贯穿速度和预混燃烧速度之和．产生此结果的原因是此阶段动量参数涡量的大幅增加；在轴向距离为40 mm处、火焰发展时间为1.6 ms时，射流燃烧模式平均涡量值与自由射流模式差值最大，约为260 s-1，是此时此截面预混燃烧模式平均涡量值6 s-1的43倍．     

湍流射流火焰抬举高度的实验研究

湍流射流燃烧作为工业燃烧室中普遍存在的燃烧方式,研究湍流射流火焰不仅能促进实际燃烧室的设计改造,更能增强对湍流燃烧理论的理解。在轴对称伴流射流燃烧器实验平台上,研究了湍流自由射流火焰抬举高度随射流速度的变化及氮气稀释和伴流速度对火焰抬举高度的影响。实验结果表明湍流自由射流燃烧火焰抬举高度随射流速度呈线性增长;随氮气稀释摩尔分数的增加其抬举高度的线性斜率增大,射流火焰吹出喷嘴的雷诺数降低,火焰更易发生抬举;同时,氮气稀释摩尔分数的增加也导致射流火焰发生吹熄时雷诺数减小,射流火焰在射流速度完全进入湍流之前发生吹熄;伴流速度小于0.3 m/s时对火焰抬举高度的影响不大,当伴流速度大于0.3 m/s时抬举高度随伴流速度的增加呈线性增长,当射流速度大于20 m/s时,伴流速度的影响降低;对比伴流与稀释对抬举高度的影响可知射流速度大于30 m/s时对伴流的敏感性大于稀释,而在射流速度小于30 m/s时对稀释更敏感。     

不同雾化角燃油超低NOx燃烧器的大涡模拟全文替换

采用大涡模拟方法分析不同雾化角对旋流非预混燃油低NOx燃烧器燃烧性能的影响。在充分燃尽的基础上,进行收敛性研究,对比分析两工况的温度场、速度场以及污染物排放情况,结果表明：大雾化角压力频谱低频段信息增加,燃烧室压力振荡强度增强,更易发生燃烧不稳定现象;雾化角的增大使得火焰集中分布在射流锥内侧,角回流区极少有火焰分布;大雾化角使得燃油径向雾化效果增强,轴向刺透深度减小,导致着火点提前,燃烧反应速率增大,使得炉膛最高温度升高,导致污染物排放水平升高。     

氢气/甲烷扩散火焰特性实验研究

燃烧的基本特性如抬举高度、层流燃烧速度以及射流出口速度等与燃烧装置的设计有关。对纯氢气火焰、氢气/甲烷、氢气/甲烷/CO2扩散火焰的抬举高度和射流出口速度进行了实验研究,并对层流燃烧速度进行了分析。研究认为,抬举高度随着射流出口速度的增加而线性增加。层流燃烧速度随氢气体积分数的增加呈指数增长,特别当氢气体积分数40%以后,层流燃烧速度随氢气体积分数显著增加。     

点火位置对弯管预混火焰传播特性的影响

通过自行设计的90°弯曲管道燃烧平台,结合纹影光学系统、高速摄像机和离子探针技术,研究了不同点火位置对丙烷/空气预混火焰在90°弯曲管道内传播特性的影响.实验结果表明,水平点火条件下,火焰阵面在水平管道内经历了球形、半球形、指尖形以及Tulip火焰结构4个阶段,预混火焰也从层流燃烧转变为湍流燃烧;垂直点火条件下,火焰阵面在弯曲管道内沿下壁面拉伸,进入水平管段后形成类似Tulip火焰结构,火焰基本处于层流燃烧状态.     

高压甲烷射流对层流火焰作用的试验

高压气体燃料射流与引燃的层流火焰间的相互作用决定了天然气直喷发动机的着火稳定性．在定容燃烧弹中,用点火针点燃预混甲烷形成层流火焰,并在不同火焰半径时刻进行高压甲烷射流．采用高速纹影法测试了甲烷不同喷射延时τ对预混层流火焰的影响．结果表明：甲烷喷射延时τ决定了预混层流火焰等效半径R的发展,随着τ增大,预混层流火焰等效半径R增大;射流对层流火焰发展的影响与其作用于层流火焰时火焰等效半径有关,存在一个临界火焰等效半径R₀,当R0时,射流吹熄火焰;R=R₀时,甲烷射流吹熄预混层流火焰后仍可被引燃,火焰传播速度加快;R>R₀时,甲烷射流更容易引燃成湍流燃烧火焰,同时预混火焰未受射流干扰区域仍旧保持层流火焰,此时层流火焰、湍流燃烧火焰并存,火焰传播速度加快．     

射流喷嘴旋流强度对燃烧室回流和燃烧特性的影响研究

随着燃气轮机参数的提高和稳定低排放运行工况的拓宽,对燃烧的要求也越来越高。柔和燃烧作为一种有潜力的燃烧技术,具有温度均匀、燃烧稳定和污染物排放低等优点,而如何在燃烧室内组织流动是实现柔和燃烧的关键。采用高速射流引射掺混的方式可以较好的满足柔和燃烧产生所需的条件。预混射流喷嘴结构和布置对流场和燃烧特性有重要影响,如何选择射流喷嘴结构值得进一步研究。本文通过实验和数值模拟相结合的方式,研究了柔和燃烧器中预混射流喷嘴的旋流强度对燃烧器流动结构和燃烧排放的影响。结果表明,旋流能增强燃料/空气的掺混,低旋流作用下能使喷嘴出口掺混不均匀度ISMD下降0. 15左右;但是喷嘴旋流对燃烧室的烟气回流有减弱的作用,使回流区向喷嘴和中轴线靠近;同时,旋流会造成温度场和火焰面不均匀分布,略微拓宽燃烧工况范围并略微增加火焰的稳定性。实验结果表明喷嘴旋流进气角从0°变化到45°时,NOx排放随旋流角的增大而增加。     

一次氧化剂O_2体积分数对富氧燃烧煤粉着火距离影响的数值模拟

对40kW同轴射流富氧煤粉燃烧试验系统进行数值模拟,研究了在总体O2体积分数和氧化剂过量系数一定的条件下,不同一次氧化剂O2体积分数对同轴射流富氧燃烧煤粉着火距离的影响,分析了低一次氧化剂O2体积分数下富氧燃烧煤粉着火及火焰稳定性.结果表明:在总体O2体积分数为40%和氧化剂过量系数为1.15的条件下,当一次氧化剂O2体积分数为20.9%时,着火距离较小,为附着火焰;当一次氧化剂O2体积分数减小至14.6%、10.0%和5.5%时,着火距离明显增大,着火延迟,为分离火焰;适当提高二次氧化剂预热温度有利于低一次氧化剂O2体积分数下富氧燃烧的煤粉着火及火焰稳定.     

利用高速摄影纹影法研究天然气高压喷射射流特性

在定容弹上利用高速摄影纹影法开展了不同喷射压力和背压下的天然气高压喷射射流特性试验研究。通过纹影照片得到了不同喷射压力和背压下的射流贯穿距离、射流锥角和射流体积随时间变化的规律,计算得到了射流内整体过量空气系数随时间变化的规律。结果表明:射流发展分两个阶段,射流初期阶段和射流主体阶段。在射流初期阶段,射流贯穿距离快速增加;在射流主体阶段,射流贯穿距离随时间呈线性增加趋势。射流贯穿距离、射流体积和射流内整体过量空气系数随时间而增加,射流发展过程中射流锥角基本保持不变。随着喷射压力的提高,射流贯穿距离和体积增加,射流锥角减小,射流内整体过量空气系数增大。随着容弹背压的增加,射流贯穿距离和射流体积减小,射流锥角增加,射流内整体过量空气系数增大。     

一次氧化剂O2体积分数对富氧燃烧煤粉着火距离影响的数值模拟

对40 kW同轴射流富氧煤粉燃烧试验系统进行数值模拟,研究了在总体O2体积分数和氧化剂过量系数一定的条件下,不同一次氧化剂O2体积分数对同轴射流富氧燃烧煤粉着火距离的影响,分析了低一次氧化剂O2体积分数下富氧燃烧煤粉着火及火焰稳定性.结果表明:在总体O2体积分数为40％和氧化剂过量系数为1.15的条件下,当一次氧化剂O2体积分数为20.9％时,着火距离较小,为附着火焰;当一次氧化剂O2体积分数减小至14.6％、10.0％和5.5％时,着火距离明显增大,着火延迟,为分离火焰;适当提高二次氧化剂预热温度有利于低一次氧化剂O2体积分数下富氧燃烧的煤粉着火及火焰稳定.     

High-fidelity resolution of the characteristic structures of a supersonic hydrogen jet flame with heated co-flow air

In the present paper, direct numerical simulation (DNS) is performed to analyze the characteristic structures of a supersonic jet lifted hydrogen-air flame with Reynolds number of 22, 000, and Mach number of 1.2. The fuel consisting of 85% H₂ and 15% N₂ by volume is injected into hot co-flow air from a round orifice. Overall 975 million grids are used to compute the complex multi-scales phenomena. A Damköhler number and a flame index are defined to analyze combustion modes and the mixedness of the flame. Complicated characteristic elements of the supersonic jet lifted flame are observed, i.e. a stable laminar flame base with auto-ignition as the stabilization mechanism, a violent mixing region in which vigorous turbulent combustion occurs with both fuel-lean and fuel-rich mixtures, and a flame region consisting of outer diffusion combustion and inner weaker premixed combustion in the far field. At the leading edge of the fame base, auto-ignition takes place primarily in the fuel-lean mixture where the mixedness mode is opposed. Downstream of the laminar flame base, the combustion becomes turbulent due to the intensified mixing of fuel and air, which results in the subequilibrium values of temperature and OH concentration. Detonation occurs near the sonic layer, and then sustains the combustion in higher dissipative mixture. The flame near the stochiometric condition keeps non-premixed, and the other non-premixed flame elements could be observed in the very fuel-rich region. Through the reacting field the premixed flame appears near the shear layer. The combustion intensity decreases in the far field where the inner non-premixed flame disappears gradually.     

Investigation on combustion mode and heat release in a model scramjet engine affected by shocks

A model scramjet engine in which the 1.0 Ma hydrogen jet mixes and reacts with the 2.0 Ma surrounding airstream is investigated using large eddy simulation. The flame structure is analyzed with a focus on the relationship between premixed/diffusion combustion mode and heat release in the supersonic reacting flow. The flame filter is used to evaluate the contributions to heat release rate by different combustion modes qualitatively and quantitatively. Results show that the heat is released from a combination of premixed combustion mode and diffusion combustion mode even when the fuel and airstream are injected into the combustor separately. Local mode-transition occurs as the supersonic jet flame propagates and interacts with shocks. The diffusion combustion mode dominates during the ignition stage and the premixed combustion becomes dominant during the intensive combustion region. When the shock wave impinges on the flame, the combustion area decreases a little due to the compression effects of the shock. However, the heat release rate is significantly improved in the interaction region since the shock could increase the air entrainment rate by directing the airflow toward the fuel jet and enhance the mixing rate by inducing vorticity due to baroclinic effects, which is good for flame stabilization in the supersonic flow. For the present case, 33.3% of the heat is released by diffusion combustion and 66.7% of the heat is released by premixed combustion. Thus the premixed combustion mode is dominant in terms of its contributions to heat release in the model scramjet engine.     

甲烷预混多喷嘴阵列燃烧器热声振荡模态实验研究

为研究燃气轮机模型燃烧室的非预混燃烧流场,采用大涡模拟方法分别结合火焰面生成流形模型（FGM）和部分预混稳态火焰面模型（PSFM）对甲烷/空气同轴射流非预混燃烧室开展了数值模拟研究,并与试验结果进行对比。结果表明：FGM所预测的速度分布、混合分数分布、燃烧产物及CO分布与试验结果更符合;两种模型均能捕捉到燃烧室中的火焰抬举现象;燃烧过程中的火焰结构较为复杂,同时存在预混燃烧区域和扩散燃烧区域,扩散燃烧主要分布在化学恰当比等值线附近,预混燃烧区域主要分布在贫油区。     

Effect of the Structure Parameters of a Low Swirler on Premixed Characteristics

Combustion with lean premixed and low swirl is an effective way of flame organization.It can improve the flame stability and reduce NOX emission.In this kind of combustion,one of the most important issues is fuel/air premixed characteristics.How the structure parameters influence that issue is figured out through numerical simulation.The structure parameters concerned in the study are as follows.They are shape of blades,number of blades,location and shape of gas jet.The influences of them are analysed with comprehensive consideration of many aspects.With the same light shading rate and stagger angle,the axial swirler with curved blades has worse premixed uniformity and lower pressure loss than the one with straight blades.With the same structure of each blade,the decrease of the quantity of blades does influence the pressure loss,while the quantity of gas jets changes correspondingly.But it has little effect on premixed uniformity in a certain range.However,more blades make contribution to better premixed performance.When the total flow area is the same,the axial and circumferential positions of the fuel jets also greatly influence the premixing process.When the fuel jets are upstream the blades and locate at middle of the vanes,the premixing performance is the best.Meanwhile,the jet direction of the fuel jets is a very important influencing factor of the premixing process.When the fuel jet direction is oblique downward at an angle of 30°to the horizontal,the premixing effect is better than the horizontal outflow,which is better than the oblique upward structure.     

Pre-chamber turbulent jet ignition of methane/air mixtures with multiple orifices in a large bore constant volume chamber: effect of air-fuel equivalence ratio and pre-mixed pressure

Liquefied natural gas (LNG), mainly composed of methane, is in progress to substitute diesel fuel in heavy-duty marine engine for practical, economic, and environmental considerations. However, natural gas is relatively difficult to be ignited in a large bore combustion chamber. A combustion enhancement technique called pre-chamber turbulent jet ignition (TJI) can permit combustion and flame propagation in a large-bore volume. To investigate the effect of air-fuel equivalence ratio and pre-mixed pressure on pre-chamber TJI of methane/air mixtures with multiple orifices in a large bore volume, experimental tests and computational simulations were implemented to study the discharge of hot turbulent jets from six orifices of the pre-chamber. Different initial pressures and air-fuel equivalence ratios were considered to analyze the characteristics of TJI. The asymmetry of the turbulent jet actuated from six different orifices were found due to the asymmetric orientation of the spark plug, resulting in the inhomogeneous distribution of combustion in the constant volume chamber, which should be considered seriously in the marine engine design. Besides, as the premixed pressure increases, it has more effect on the flame propagation and plays a more important role, as it further increases.     

Parametric and statistical investigation of the behavior of a lifted flame over a turbulent free-jet structure

Partially premixed combustion is involved in many practical applications, due to partial premixing of combustible and oxidant gases before ignition, or due to local extinctions, which lead to mixing of reactants and burned gases. To investigate some features of flames in stratified flows, the stabilization processes of lifted turbulent jet flames are studied. This work offers a large database of liftoff locations of flames stabilized on turbulence-free jets for different fuels and nozzle diameters studied over their flame stability domains. Methane, propane, and ethylene flames are investigated for nozzle diameters of 2, 3, 4, and 5 mm. Blowout velocities are measured and compared with an approach based on large-scale structures of the jet. The axial and radial locations of the flame base are measured by planar laser-induced fluorescence (PLIF) of the OH radical through high sampling (at least 5000 points). From this large database the average locations of the flame base are analyzed for the fuels investigated. The pdfs exhibit an evolution of their shapes according to the region of the turbulent jet where the flame stabilizes (potential core, transition to turbulence, or fully developed turbulence regions). This dependence is probably due to the interaction of the flame with the jet structures. This is confirmed by the comparison between the amplitude of the height fluctuations and the local size of the large-scale structures deduced from particle image velocimetry measurements and self-similarity laws for velocity. The results show the flame can be carried over a distance equal to the local diameter of the jet within the region of fully developed turbulence for propane and ethylene, and over a slightly larger distance for methane.     

Lifted methane-air jet flames in a vitiated coflow

The present vitiated coflow flame consists of a lifted jet flame formed by a fuel jet issuing from a central nozzle into a large coaxial flow of hot combustion products from a lean premixed H₂/air flame. The fuel stream consists of CH₄ mixed with air. Detailed multiscalar point measurements from combined Raman-Rayleigh-LIF experiments are obtained for a single base-case condition. The experimental data are presented and then compared to numerical results from probability density function (PDF) calculations incorporating various mixing models. The experimental results reveal broadened bimodal distributions of reactive scalars when the probe volume is in the flame stabilization region. The bimodal distribution is attributed to fluctuation of the instantaneous lifted flame position relative to the probe volume. The PDF calculation using the modified Curl mixing model predicts well several but not all features of the instantaneous temperature and composition distributions, time-averaged scalar profiles, and conditional statistics from the multiscalar experiments. A complementary series of parametric experiments is used to determine the sensitivity of flame liftoff height to jet velocity, coflow velocity, and coflow temperature. The liftoff height is found to be approximately linearly related to each parameter within the ranges tested, and it is most sensitive to coflow temperature. The PDF model predictions for the corresponding conditions show that the sensitivity of flame liftoff height to jet velocity and coflow temperature is reasonably captured, while the sensitivity to coflow velocity is underpredicted.     

Ignition of hydrogen jet fires from high pressure storage

Self-ignition behaviour of highly transient jets from hydrogen high pressure tanks were investigated up to 26 MPa. The jet development and related ignition/combustion phenomena were characterized by high speed video techniques and time resolved spectroscopy. Video cross correlation method BOS, brightness subtraction and 1-dimensional image contraction were used for data evaluation. Results gained provided information on ignition region, flame head jet velocity, flame contours, pressure wave propagation, reacting species and temperatures. On burst of the rupture disc, the combustion of the jet starts close to the nozzle at the boundary layer to the surrounding air. Combustion velocity decelerated in correlation to an approximated drag force of constant value which was obtained by analysing the head velocity. The burning at the outer jet layer develops to an explosion converting to a nearly spherical volume at the jet head; the movement of the centroid is nearly unchanged and follows the jet front in parallel. The progress of the nearly spherical explosion could be evaluated by assuming an averaged flame ball radius. An apparent flame velocity could be derived to be about 20 m/s. It seems to increase slightly on the pressure in the tank or the related initial jet momentum. Self-initiation is nearly always achieved especially induced the interaction of shock waves and their reflections from the orifice. The combustion process is composed of shell combustion of the jet cone at the bases with a superimposed explosion of the decelerating jet head volume.     

零碳燃料对燃气锅炉燃烧过程调控作用

利用小型化模拟炉膛开展了零碳燃料氢气对燃气锅炉燃烧过程调控作用实验研究,研究了掺氢比对炉膛内部预混火焰宏观形态、炉膛温度均匀性、炉膛污染物排放规律的影响,并总结了CO及NO_x的排放规律。实验结果表明：随着预混当量比增加,纯甲烷火焰长度逐渐缩短;对于20%掺氢火焰,随着预混程度的提高,火焰长度降低明显;不同火焰条件下,炉膛温度只由燃烧功率控制;改变燃烧条件时,处于壁面附近位置的温度变化较为平稳,而靠近火焰处温度变化较大;天然气中掺入氢气,燃烧时可以有效降低未燃CO排放;在相同预混程度下,全局当量比减小导致未燃空气增加,热量被稀释,火焰温度降低,热力型NO_x的生成降低;随着掺氢比的增加,燃烧时火焰温度升高,导致热力型NO_x排放增加。     

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.