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。     

稳态湍流非预混燃烧的小火焰模拟

以甲烷/空气的湍流射流非预混燃烧为对象,建立二维稳态湍流非预混火焰的小火焰模型.利用湍流流动模型和小火焰模型耦合求解,计算出速度、混合分数、温度以及反应标量的摩尔分数在燃烧室内的分布,模拟结果表明小火焰模型能够用来描述燃烧室内燃烧机理.     

基于深度学习的甲烷高压射流湍流燃烧火焰图像处理方法研究

基于定容弹开展了高压天然气（甲烷）射流燃烧光学测试,并分别运用深度学习方法和边缘检测算法进行了图像处理。对比结果表明,由于图像中存在射流、火焰差异大的图像识别目标,边缘检测算法无法较好识别射流和火焰,该算法适合于单一目标的火焰图像处理。深度学习方法可识别射流湍流燃烧火焰轮廓,有效地获得射流湍流燃烧火焰前锋面发展位移及火焰传播速度,该方法适用于多个目标的火焰图像处理。根据深度学习图像处理结果表明：当高压甲烷射流接触预燃球形火焰时,火焰由稳定层流速度（<3 m/s）快速上升,最大火焰传播速度高达300 m/s,形成湍流火焰,火焰沿射流方向快速向前发展,火焰面积增加。随着射流和点火时间间隔的增加,最大火焰传播速度线性下降。     

某重型燃气轮机燃烧室进气速度变化对燃烧流动特性影响规律的数值研究

燃气轮机燃烧室进气速度变化是引起燃烧不稳定的主要因素之一,本文以某重型燃气轮机单管燃烧室为研究对象,建立了不同燃烧室进气速度条件下的单管燃烧室三维数值模型,研究了预混燃烧模式下燃烧火焰面随燃烧室进气速度的变化规律。燃料和氧化剂分别为甲烷和空气,燃空当量比为0.481。基于燃气轮机实际运行流量变化试验数据,燃烧室进气速度分别取103.9 m/s、94.8 m/s、85.7 m/s、76.5 m/s、67.2 m/s。研究结果表明燃烧室进气速度越大,燃烧火焰面越长,且燃烧火焰面所包裹的体积与燃烧室进气速度基本呈正比线性关系;在进气速度为103.9 m/s时,燃烧火焰面所包裹的体积较速度为67.2 m/s增加了23.8%。     

预混湍流火焰的根部脉动特性

采用平面激光诱导荧光的测试方法捕捉本生灯预混湍流火焰的瞬态形态,重点分析火焰根部的脉动特性及其对火焰稳定的影响.采用挡环和多孔板式湍流发生器,分别形成边界层湍流和位势流湍流.实验结果表明,对于这两种湍流发生器,射流火焰根部的脉动幅度都随来流速度的增大逐渐增强;增大湍流发生器的特征尺寸或者降低混气当量比,都会加大火焰根部的脉动幅度.挡环尺度的影响则表明了临界尺度的存在.当挡环尺度小于临界尺度时,火焰根部的脉动不利于火焰的稳定,即随着挡环尺度的增加,火焰吹熄速度降低.而当挡环尺度大于临界尺度时,随着挡环尺度的增加,吹熄速度变大.因此湍流的产生区域对火焰吹熄速度有着重要影响.     

基于旋流火焰根部熄火现象的FGM模型研究

针对原尺寸的复杂几何预混旋流燃烧器开展了同步OH/CH_2O-PLIF测量,并利用发展的带拉伸查表法(SFGM)耦合大涡模拟(LES)的方法模拟了相同雷诺数下(Re=10 000)稳定火焰及临近吹熄下的旋流燃烧特性.发展的SFGM模型在化学反应源项中引入与拉伸率直接相关的着火因子,如此拉伸率则无需与进展变量耦合建表,极大地缩小了建表工作量.随后将模拟结果与实验结果对比,验证了LES耦合SFGM模型可以较为准确地捕捉复杂几何旋流预混燃烧的火焰形态(OH及CH_2O×OH分布):如预测到预混管出口处的局部熄火位置,临近吹熄的火焰相比于稳燃火焰有更高的抬升高度.在强旋流下,湍流火焰的传播速度和湍流速度的脉动值成正相关,火焰前锋面在上游侧的边缘位置很大程度上由流场向下游扩张的轴向速度和中央回流区(CRZ)中的主体火焰向上游传播的速度共同决定.随着当量比降低到熄火极限,CRZ内的混合物被不断稀释,火焰逐渐破碎成孤立的火核;与此同时,燃烧温度下降导致火焰传播速度降低,而流场速度不变,火焰前锋面被吹向下游远离CRZ并最终被吹熄.     

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。     

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。     

基于FGM模型的同轴射流非预混火焰大涡模拟

采用直接数值模拟方法对二甲醚(Dimethyl Ether,DME)射流推举燃烧进行了研究（DNS）,分析了DME射流推举火焰结构、燃烧模式和推举稳定机理。数值模拟工况条件为：燃料由狭缝射出,初始温度500 K,射流速度138 m/s;伴流空气的初始温度1 000 K,流速3 m/s,压力为0506 6 MPa。研究表明：DME射流推举火焰与传统的边火焰有很大不同,在射流核心区内存在1条低温放热分支以及紧随其后的中温着火分支,并且推举稳定点位于贫燃侧;DME湍流射流推举火焰包含冷焰反应区(Cool Flame Zone,CFZ)、中温反应区(Intermediate Temperature Zone,ITZ)、富燃高温区(High Temperature Rich Burn Zone,HTR)以及贫燃高温区(High Temperature Lean Burn Zone,HTL)4种模式;在CFZ与ITZ区内湍流混合占主导,并且湍流混合会抑制低温放热;在HTR与HTL区内放热速率占主导地位,但是湍流会显著增强超贫燃区间内的高温放热速率;大部分热量在HTL和HTR区产生,而CFZ和ITZ区对总体产热的贡献微乎其微,但是所产生的中低温组分加快了高温着火过程;射流推举稳定性由贫燃侧的高温自着火反应机制所控制。     

焦炉气同轴射流自由扩散火焰的特性

以焦炉气为燃料,纯氧气为氧化剂,采用双通道烧嘴在不同的射流速度下于敞开空间中进行垂直自由扩散燃烧,考察了Fr数、Re数对火焰长度的影响,以及焦炉气射流速度与火焰脱火高度的关系.实验表明,火焰由浮力控制区向动量控制区转变时的Fr数约为197,由层流区向湍流区转变时的Re数约为16000;火焰发生脱火的临界速度约为20 m/s.     

Flame stabilization in a lifted non-premixed turbulent hydrogen jet with coaxial air

To understand hydrogen jet liftoff height, the stabilization mechanism of turbulent lifted jet flames under non-premixed conditions was studied. The objectives were to determine flame stability mechanisms, to analyze flame structure, and to characterize the lifted jet at the flame stabilization point. Hydrogen flow velocity varied from 100 to 300 m/s. Coaxial air velocity was regulated from 12 to 20 m/s. Simultaneous velocity field and reaction zone measurements used, PIV/OH PLIF techniques with Nd:YAG lasers and CCD/ICCD cameras. Liftoff height decreased with increased fuel velocity. The flame stabilized in a lower velocity region next to the faster fuel jet due to the mixing effects of the coaxial air flow. The non-premixed turbulent lifted hydrogen jet flames had two types of flame structure for both thin and thick flame base. Lifted flame stabilization was related to local principal strain rate and turbulent intensity, assuming that combustion occurs where local flow velocity and turbulent flame propagation velocity are balanced.     

Autoignited laminar lifted flames of propane in coflow jets with tribrachial edge and mild combustion

Characteristics of laminar lifted flames have been investigated experimentally by varying the initial temperature of coflow air over 800 K in the non-premixed jets of propane diluted with nitrogen. The result showed that the lifted flame with the initial temperature below 860 K maintained the typical tribrachial structure at the leading edge, which was stabilized by the balance mechanism between the propagation speed of tribrachial flame and the local flow velocity. For the temperature above 860 K, the flame was autoignited without having any external ignition source. The autoignited lifted flames were categorized in two regimes. In the case with tribrachial edge structure, the liftoff height increased nonlinearly with jet velocity. Especially, for the critical condition near blowout, the lifted flame showed a repetitive behavior of extinction and reignition. In such a case, the autoignition was controlled by the non-adiabatic ignition delay time considering heat loss such that the autoignition height was correlated with the square of the adiabatic ignition delay time. In the case with mild combustion regime at excessively diluted conditions, the liftoff height increased linearly with jet velocity and was correlated well with the square of the adiabatic ignition delay time.     

二甲醚湍流射流推举火焰的燃烧机理研究

对长、宽、高为650 mm×400 mm×12 mm的半闭口狭窄矩形通道（海伦-肖装置）内的甲烷/空气层流预混火焰传播过程进行了实验研究,探究当量比φ在0.6~1.2范围内、火焰传播角度ω在垂直向下-90°至垂直向上90°区间对火焰前锋轮廓发展及非标准层流火焰速度的影响。结果表明：火焰在通道内的传播分为热膨胀、准稳态传播和端壁效应3个阶段,每个阶段具有各自不同的前锋轮廓特征。由于瑞利-泰勒不稳定性机制的作用,所有当量比工况下向上传播的火焰均在准稳态传播阶段中呈现出明显的锋面褶皱与胞状结构;对向下传播的火焰而言,其在贫燃工况（φ为0.6,0.8）下的胞状不稳定性得到了有效抑制,而在当量比φ=1.0及富燃工况（φ=1.2）下,该稳定性效应并不显著。火焰瞬时速度与标准层流速度的比值Ui/UL,在φ=0.6的极贫燃工况与其他当量比工况下展现出明显不同的发展特性,极贫燃工况火焰向上传播时（ω=90°）,Ui/UL随着传播过程的进行一直增大,直到火焰触碰壁面末端熄灭,整个过程Ui/UL与火焰传播方向呈正相关关系;而对于其他当量比工况,Ui/UL在传播过程中均先升高后下降,火焰触碰壁面末端熄灭前其值趋于稳定,其平均速度与标准层流速度的比值Ua/UL在水平传播（ω=0°）时达到最大值。     

Stabilization of air coflowed ammonia jet flame at elevated ambient temperatures

Ammonia is a promising carbon-free fuel, while, understanding of ammonia jet flame is still in lack. In this work, a novel facility was applied and air coflowed ammonia jet flames were achieved in an elevated ambient temperature range, 723–923K. Stabilization regimes and limits were investigated. Stable lifted flame with a classical triple structure was observed, and critical aerodynamic parameters were measured at three specific regimes, liftoff, reattachment and blowoff. Attached flame can only be retained under laminar conditions with flow Reynolds number <150. A linear correlation between velocities of fuel jet and coflow under critical conditions was uncovered, which is different from the literature research on methane flames. Effects of partially premixing and N₂ dilution were considered. Partially premixing was found harmful to stabilization at 823K, while this influence becomes unclear at 923K. Differently, a linearly adverse effect was observed under both N₂-diluted jet and coflow conditions at different temperatures.     

Electric fields effect on liftoff and blowoff of nonpremixed laminar jet flames in a coflow

The stabilization characteristics of liftoff and blowoff in nonpremixed laminar jet flames in a coflow have been investigated experimentally for propane fuel by applying AC and DC electric fields to the fuel nozzle with a single-electrode configuration. The liftoff and blowoff velocities have been measured by varying the applied voltage and frequency of AC and the voltage and the polarity of DC. The result showed that the AC electric fields extended the stabilization regime of nozzle-attached flame in terms of jet velocity. As the applied AC voltage increased, the nozzle-attached flame was maintained even over the blowout velocity without having electric fields. In such a case, a blowoff occurred directly without experiencing a lifted flame. While for the DC cases, the influence on liftoff was minimal. There existed three different regimes depending on the applied AC voltage. In the low voltage regime, the nozzle-detachment velocity of either liftoff or blowoff increased linearly with the applied voltage, while nonlinearly with the AC frequency. In the intermediate voltage regime, the detachment velocity decreased with the applied voltage and reasonably independent of the AC frequency. At the high voltage regime, the detachment was significantly influenced by the generation of discharges.     

Liftoff heights of turbulent non-premixed flames in co-flows diluted by CO2/N2

The objective of this study was to propose a new model for the prediction of the liftoff heights of turbulent flames diluted by the entrainment of burned gases. In combustion furnaces with the internal recirculation of burned gases, mixtures of fuel and oxidizer are diluted with recirculated burned gases through entrainment into the gas jets. We focused on the effects which dilution resulting from entrainment has on the stabilization mechanism of lifted flames. In order to investigate the effects of dilution on liftoff height, we employed a concentric burner incorporating fuel, oxidizer and co-flow gas nozzles. The recirculated burned gas was simulated by co-flow air diluted with either N₂ or CO₂ gas. Liftoff heights were observed to increase with decreasing O₂ concentrations in the co-flow gas when maintaining a constant O₂ concentration in the oxidizer, due to dilution resulting from entrainment of the diluted co-flow gas. The liftoff heights obtained with co-flow gases diluted by CO₂ were greater than those obtained when diluting with N₂ due to both thermal and chemical dilution effects. The conventional premixed model was not able to predict the liftoff trends observed in this study and we therefore propose a modified premixed model which takes into account the dilution effect resulting from entrainment. In this model, the amount of entrained co-flow gas is evaluated according to the self-similarity law of a round jet. Non-dimensional liftoff heights based on this modified model exhibit excellent linear correlation with non-dimensional fuel gas velocities, even when various co-flow gases are used for dilution. The conventional large eddy model was also modified in the same manner and the results obtained from the modified model exhibit satisfactory correlation.     

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.     

可控活化热氛围下柴油喷雾自燃特性的研究

利用可控活化热氛围燃烧试验系统，结合连续喷射系统对柴油燃料在高温热氛围下的自燃特性进行了研究，并利用高速摄影技术，得出柴油燃料喷雾在不同协流温度下的滞燃期、自燃点位置以及稳定火焰起升高度。结果表明，随协流温度的升高，柴油喷雾滞燃期减小，自燃点位置至喷嘴口距离减小。协流温度低于1022K时，不能形成稳定的起升火焰；协流温度处于1022～1074K范围内，火焰起升高度随温度升高急剧降低，而起升高度的均方差也较大。温度继续增加时，起升高度变化趋缓，火焰稳定。     

Acoustic enhancement of combustion in lifted nonpremixed jet flames

Lifted nonpremixed jet flames are often used in industrial processes and present inherent difficulties such as their reattachment to the burner, blowout, and poor combustion. One solution is to control the jet by acoustic forcing. For flames lifted in the hysteresis zone where anchoring may occur, forcing at high amplitudes and middle frequencies (around 200 Hz) changes the combustion regime and prevents reattachement. The common long yellow plume, due to soot radiation, vanishes. The flame becomes shorter, totally blue and stabilizes at a higher position above the burner. The phenomenon is explained using the results obtained by analyzing the flow dynamics with high-speed laser tomography, laser Doppler anemometry, particle image velocimetry, and Mie scattering techniques. Measurements show that the excitation periodically generates axial velocities higher than the maximum velocity of the hysteresis zone, leading to flame liftoff. Some primary and streamwise eddy vortices similar to natural instabilities develop during the jet deceleration. Contrary to the unexcited case, these structures, disorganized by the superimposition of the forcing wave, lead to quasi-homogeneous turbulence which provides efficient mixing and improves the combustion regime. Finally, the frequency is sufficiently high to avoid excessive fluctuations of the lift-off height and the reattachment to the burner.