湍流强度对预混燃烧影响的大涡模拟研究

本文对甲烷和空气在突扩燃烧室内的预混燃烧进行了大涡模拟(LES)研究,主要关注于预混燃料的湍流强度对甲烷-空气预混燃烧效率的影响.LES数值模拟给出了不同入口湍流强度的条件下燃烧室内湍流预混燃烧反应流场的速度场,温度场和浓度场分布.分析的结果表明:随着预混燃料入口湍流强度的增大,燃烧反应更加剧烈,燃烧效率提高,火焰长度也随之变短,同时,一氧化碳的生成区域明显变小,整体生成量减少.研究的结果对有效地提高燃气轮机的燃烧效率和减少污染物的排放提供一定的参考依据.     

基于液体碳氢燃料的旋转爆轰燃烧特性研究

为了探索液体碳氢燃料参与旋转爆轰所产生的不完全燃烧现象,采用守恒元与求解元方法,开展柱坐标系下的汽油/空气两相旋转爆轰燃烧室三维数值模拟研究,针对燃料喷注压力和反应物当量比对旋转爆轰流场结构及燃烧室性能的影响进行分析。分析结果表明:保持总当量比为1.00,随着燃料喷注压力的上升,燃烧室内燃料不均匀分布增强,产生局部富燃区,燃料在燃烧室未能完全反应,导致燃烧室燃料比冲下降;保持喷注压力不变,减小当量比,在贫燃工况下依然存在局部富燃区,导致燃烧室内出现不完全燃烧现象,降低燃烧室比冲性能。由此可知,反应物喷注方案对气液两相旋转爆轰的不完全燃烧有显著影响。     

微小尺度下氢气预混燃烧的实验研究

对内径为1.66mm的不锈钢管燃烧室的氢气预混燃烧实验进行了描述，采用 红外测温仪测量了燃烧室壁面的温度场分布，获得了不同燃烧热功率下的运行界限．在突扩 段内高温回流区的作用下，在带有5mm长突扩段的燃烧室内可以实现完全预混燃 烧，最高运行界限可达1.415．由于较高的进气速度和较大的燃烧室壁面散热，在不带突扩 段的不锈钢管内无法实现完全预混燃烧．结果表明突扩段对微小尺度燃烧具有稳定火焰、拓 宽燃烧运行界限的作用．通过对火焰形状和结构的观察，结合突扩段燃烧流场的分析，合理 解释了燃烧室壁面温度场随过量空气系数的变化规律．     

富氢燃料气射流预混火焰回火特性的研究进展

燃气轮机是一种重要的动力设备,是碳中和的重要环节,燃用富氢燃料气是降低其碳排放的有效途径.由于氢气的化学反应活性高、燃烧速率快,使得燃烧室内预混射流火焰发生回火的风险大大增加,即火焰有可能从燃烧室向上游预混气管道传播.文章综述了近20年来富氢燃料气射流火焰回火的代表性实验及数值模拟的研究进展,介绍了包含燃料氢含量、来流温度及工作压力等参数、喷嘴结构与尺寸、热声振荡和微混燃烧器等对回火特性影响的研究成果,现有研究表明,边界层内火焰传播速度超过来流速度是造成回火的主要因素,控制来流速度、来流温度、改变局部燃料浓度可以克服或者减缓回火.根据目前的研究现状和发展动态,对未来的研究方向进行了展望.     

当量比对汽油燃料两相旋转爆轰发动机工作特性影响实验研究

为了研究当量比对汽油燃料两相旋转爆轰发动机工作特性的影响,开展了以高总温空气为氧化剂的气液两相旋转爆轰实验研究。旋转爆轰发动机环形燃烧室外径、内径和长度分别为202、166和155 mm。汽油和高温空气采用高压雾化喷嘴与环缝对撞喷注的方式混合,以此提高推进剂的掺混效果与活性,采用预爆轰管作为点火装置。实验通过改变汽油质量流量改变推进剂当量比,并基于燃烧室内测得的高频动态压力和平均静压,对气液两相旋转爆轰波的传播模态和传播特性以及发动机的工作特性进行了详细分析。实验结果表明:在当量比为0.79～1.25时,燃烧室内均实现了旋转爆轰波的连续自持传播,且随着当量比的增加,爆轰波传播模态从双波对撞/单波的混合模态转变为单波模态;降低当量比至0.61～0.66,爆轰波传播稳定性变差,传播模态表现为间断爆轰以及零星爆轰;进一步降低当量比至0.53,爆轰波起爆失败。此外,燃烧室平均绝对压力与爆轰波平均传播频率均随着当量比的增加呈先增大后减小的趋势,极大值出现在当量比1.19附近。在此工况下获得了最佳实验结果,旋转爆轰波的平均传播频率为1 900.9 Hz,平均传播速度为1 110.8 m/s,与高频压力信号经快速傅里叶变换得到的主频基本一致,爆轰波传播速度存在严重亏损。     

无约束条件下甲烷/空气预混气体燃爆特性研究

为研究无约束条件下甲烷(CH₄)/空气(air)预混气体的燃爆特性,以乳胶气球为反应容器开展了甲烷爆炸实验,结合Chemkin模拟和改进的比色测温技术,研究了不同当量比下甲烷/空气预混气体的火焰传播速度、爆炸超压及温度场分布等特征以及静置时间对预混气体燃爆特性的影响。实验结果表明：甲烷/空气预混气体的爆炸火焰传播速度呈振荡分布,当量比为0.83、1.06、1.30和1.55时的平均火焰传播速度分别为1.554m·s^-1、2.122m·s^-1、1.892m·s^-1和1.428m·s^-1;峰值超压随当量比的增加呈先增大后减小的趋势,当量比为1.06时基元反应CH₃·+O₂?O·+CH₃O·的敏感性系数最大,从而加速了生成二氧化碳(CO₂)的链反应,使得燃烧化学反应最彻底,峰值超压值最大;静置时间对火焰传播速度和爆炸峰值压力影响显著,最佳静置时间为6min;随着当量比的增大,爆炸火焰的平均温度呈现...     

超燃冲压发动机燃烧模态转换试验研究

在模拟飞行高度为25 km、来流马赫数为6的情况下,采用试验研究的方法对超燃冲压发动机燃烧模态转换进行了直连式试验。根据燃烧室壁面压力分布和一维模型分析表明,燃料喷射位置和当量比的动态改变,实现了燃烧室内燃烧模态的动态转换。不同燃料喷射位置切换顺序比较表明,燃烧室内燃烧状态的改变受燃料分布所决定,但是燃烧室自身具有一定的抗波动能力。     

点火过程和初始条件对燃烧轻气炮内弹道性能的影响

采用计算流体力学方法对燃烧轻气炮膛内燃烧过程进行数值模拟,分析不同的点火点数目和点火能量以及初始温度和压力对燃烧轻气炮内弹道特性的影响。结果表明,采用合理的点火点数目、初始温度和压力条件可以有效控制氢气的燃烧过程,减弱燃烧室内的压力波动。模拟结果对燃烧轻气炮膛内燃烧过程控制具有重要参考价值。     

微小空间内丙烷/空气火焰传播特性与加氢爆燃实验

在内径150 mm的圆盘狭缝微型燃烧室内,实验探讨了在常温常压下,不同当量比的丙烷/空气预混气以及掺氢的丙烷/空气混合气在电火花点火后向外传播的特性,通过高速摄影方法获得了在狭缝间距为2.0、2.5、3.0、5.0 mm时微燃烧室内的火焰传播形态。实验中观察到火焰传播存在光滑、皱褶和断裂三种火焰锋面形态。当量比的增加和狭缝间距的减小会使火焰更容易发生褶皱。随着火焰的传播,火焰半径逐渐增大,火焰传播速度整体呈下降趋势。火焰传播速度随着间距的减小先增大后减小,在间距3 mm时最大。因为壁面散热的影响,微尺度效应在降低火焰传播速度和增加火焰不稳定性方面具有重要作用。掺入氢气能提高预混气的火焰传播速度,在间距2.5 mm的微燃烧腔中还观察到了爆燃现象。     

汽油蒸气/空气预混火焰的无拉伸层流燃烧速率

为了研究汽油蒸气/空气的爆炸特性,介绍了汽油蒸气/空气预混火焰的无拉伸层流燃烧速率。通过实验研究了无拉伸层流燃烧速率,与汽油的主要组分异辛烷和正庚烷与空气的预混火焰层流燃烧速率做了对比,发现汽油蒸气/空气的无拉伸层流燃烧速率小于异辛烷和正庚烷与空气的预混火焰无拉伸层流燃烧速率,但无拉伸层流燃烧速率随当量比的变化规律相同,随着当量比增大,无拉伸层流燃烧速率呈先增大、再减小的变化趋势,最大值在当量比为1处取得。     

LES of Chemical and Acoustic Forcing of a Premixed Dump Combustor

This paper describes the first steps in the development of a large eddy simulation (LES) code able to compute combustion instabilities in gas turbines. This code was used to compute the forcing of an experimentally investigated premixed dump combustor. It is shown that the main effect of acoustic waves entering the combustion chamber is to create large vortices and unsteady heat release when these vortices burn. Another effect of waves entering the combustor is to modulate the fuel and air flow rates produced by the feeding lines. In this case the equivalence ratio of the mixture entering the combustor may also vary. This was investigated in a “chemical effect” simulation where the inlet equivalence ratio fluctuates but the total flow rate remains constant. For perturbations from stoichiometric burning, this mechanism was shown to induce less destabilizing effects than the purely aerodynamical mechanism due to vortex formation and combustion. It is shown that the LES methodology developed is able to reproduce the experimentally observed phase shift between acoustic excitation and total reaction rate in the chamber. This revised version was published online in July 2006 with corrections to the Cover Date.     

Joint RANS/LES Approach to Premixed Flame Modelling in the Context of the TFC Combustion Model

We present an original timesaving joint RANS/LES approach to simulate turbulent premixed combustion. It is intended mainly for industrial applications where LES may not be practical. It is based on successive RANS/LES numerical modelling, where turbulent characteristics determined from RANS simulations are used in LES equations for estimation of the subgrid chemical source and viscosity. This approach has been developed using our TFC premixed combustion model, which is based on a generalization of the Kolmogorov’s ideas. We assume existence of small-scale statistically equilibrium structures not only of turbulence but also of the reaction zones. At the same time, non-equilibrium large-scale structures of reaction sheets and turbulent eddies are described statistically by model combustion and turbulence equations in RANS simulations or follow directly without modelling in LES. Assumption of small-scale equilibrium gives an opportunity to express the mean combustion rate (controlled by small-scale coupling of turbulence and chemistry) in the RANS and LES sub-problems in terms of integral or subgrid parameters of turbulence and the chemical time, i.e. the definition of the reaction rate is similar to that of the mean dissipation rate in turbulence models where it is expressed in terms of integral or subgrid turbulent parameters. Our approach therefore renders compatible the combustion and turbulent parts of the RANS and LES sub-problems and yields reasonable agreement between the RANS and averaged LES results. Combining RANS simulations of averaged fields with LES method (and especially coupled and acoustic codes) for simulation of corresponding nonstationary process (and unsteady combustion regimes) is a promising strategy for industrial applications. In this work we present results of simulations carried out employing the joint RANS/LES approach for three examples: High velocity premixed combustion in a channel, combustion in the shear flow behind an obstacle and the impinging flame (a premixed flame attached to an obstacle).     

CH4/O2/CO2预混体系爆炸动力学研究

为探究甲烷在富氧条件下的火焰动力学规律,以CH₄/O₂/CO₂预混体系为研究对象,在小尺度方形透明管道中进行了一系列爆炸实验,探讨了初始环境温度波动对爆炸参数的影响,并对预混体系的燃烧机理进行分析。结果表明:在273 K的环境温度下,化学当量比φ=0.8~1.0且氧气相对比γ<0.30和φ=1.2且γ<0.35的预混体系不能被点燃,而其他预混体系均可被点燃,最终产生郁金香与非郁金香两种火焰类型,并且根据郁金香火焰独特的演变特征,又划分为T形郁金香火焰和不对称郁金香火焰;随着γ的增大,无量纲火焰传播速度v/(S_Lσ)的变化趋势由“两升两降”转变为“一升一降”。初始环境温度的升高并未对火焰传播速度和爆炸超压的变化趋势产生影响,但是会导致最大爆炸超压p_max和最大火焰传播速度降低。值得注意的是,初始环境温度对爆炸强度的影响随化学当量比的减小而增强。另外,与最大爆炸超压相比,最大火焰传播速度与层流燃烧速度之间的关系更紧密。从敏感性分析中可知:层流燃烧速度对自由基链式反应R38(即H+O₂=O+OH)表现出最大的正敏感度,对R52(即H+CH₃(+M)=CH₄(+M))表现出最大的负敏感度,并且对自由基OH的生成速率最敏感,当初始环境温度升高至303 K时,层流燃烧速度对R38(正)和R52(负)的敏感度降低;H、O和OH自由基总摩尔分数的增大会削弱热扩散的不稳定性,增强流体力学的不稳定性。     

Effects of Unmixedness on Combustion Instabilities in a Lean-Premixed Gas Turbine Combustor

The present experimental study focuses on the effects of the degree of premixing and swirl strength on combustion instabilities occurring in a lean premixed gas turbine combustor burning natural gas and air. The combustor operated at pressurized conditions with heated air. Major measurements for the investigation of premixed combustion dynamics include pressure fluctuations, flame emissions in reacting flow, and acetone fluorescence in non-reacting flow to assess the degree of premixing between fuel and air. The acetone PLIF results revealed that the degree of premixing improves as mixing time increases. The first and second longitudinal acoustic modes were the dominant excited modes for most cases of interest. Combustion at a lean premixed condition becomes more susceptible to instabilities as the degree of premixing becomes poor, and self-excited pressure oscillations are obviously present under a fully premixed condition, even without equivalence ratio fluctuations in space. For incomplete premixing cases, local equivalence ratio fluctuations caused by poor premixing may initiate instabilities since reaction rate is sensitive to equivalence ratio fluctuations at lean conditions. Phase resolved chemiluminescence measurements show that pressure oscillations are strongly coupled with variations in flame structures.     

Large Eddy Simulation and PIV Measurements of Unsteady Premixed Flames Accelerated by Obstacles

In gas explosions, the unsteady coupling of the propagating flame and the flow field induced by the presence of blockages along the flame path produces vortices of different scales ahead of the flame front. The resulting flame–vortex interaction intensifies the rate of flame propagation and the pressure rise. In this paper, a joint numerical and experimental study of unsteady premixed flame propagation around three sequential obstacles in a small-scale vented explosion chamber is presented. The modeling work is carried out utilizing large eddy simulation (LES). In the experimental work, previous results (Patel et al., Proc Combust Inst 29:1849–1854, 2002) are extended to include simultaneous flame and particle image velocimetry (PIV) measurements of the flow field within the wake of each obstacle. Comparisons between LES predictions and experimental data show a satisfactory agreement in terms of shape of the propagating flame, flame arrival times, spatial profile of the flame speed, pressure time history, and velocity vector fields. Computations through the validated model are also performed to evaluate the effects of both large-scale and sub-grid scale (SGS) vortices on the flame propagation. The results obtained demonstrate that the large vortical structures dictate the evolution of the flame in qualitative terms (shape and structure of the flame, succession of the combustion regimes along the path, acceleration-deceleration step around each obstacle, and pressure time trend). Conversely, the SGS vortices do not affect the qualitative trends. However, it is essential to model their effects on the combustion rate to achieve quantitative predictions for the flame speed and the pressure peak.     

Partially premixed prevalorized kerosene spray combustion in turbulent flow

A detailed numerical simulation of kerosene spray combustion was carried out on a partially premixed, prevaporized, three-dimensional configuration. The focus was on the flame temperature profile dependency on the length of the pre-vaporization zone. The results were analyzed and compared to experimental data. A fundamental study was performed to observe the temperature variation and flame flashback. Changes were made to the droplet diameter, kerosene flammability limits, a combustion model parameter and the location of the combustion initialization. Investigations were performed for atmospheric pressure, inlet air temperature of 90 °C and a global equivalence ratio of 0.7. The simulations were carried out using the Eulerian Lagrangian procedure under a fully two-way coupling. The Bray–Moss–Libby model was adjusted to account for the partially premixed combustion.     

甲烷/空气预混气体火焰的传播特征

利用高速纹影摄像等技术探讨了密闭管道内不同当量比的甲烷/空气预混气体火焰的传播特征。结果表明,当甲烷含量接近当量值时,预混气体火焰传播中会发生火焰阵面由向未燃区弯曲到向已燃区弯曲的转折过程,逐渐由层流燃烧转变成湍流燃烧,并形成Tulip火焰结构;当甲烷含量偏离当量值一定程度时,预混火焰呈现出典型的层流燃烧特征,不会发生火焰阵面由向未燃区弯曲到向已燃区弯曲的转折过程。Tulip火焰结构形成于火焰传播速度迅速降低的区间里,且只有当减速阶段的最大加速度的绝对值大于某一数值时才能形成;Tulip火焰结构是预混火焰由层流燃烧向湍流燃烧转变的一个中间过程。     

Effect of oxygen fraction on local entropy generation in a hydrogen–air burner

This study considers numerical simulation of the combustion of hydrogen with air, including oxygen and nitrogen, in a burner and the numerical solution of local entropy generation rate due to the high temperature and velocity gradients in the combustion chamber. The effects of equivalence ratio (ϕ) and oxygen percentage (γ) on the combustion and entropy generation rate are investigated for different ϕs (from 0.5 to 1.0) and γs (from 10 to 30%). The combustion is simulated for the fuel mass flow rate providing the same heat transfer rate to the combustion chamber in the each case. The numerical calculation of combustion is performed individually for all cases with the help of the Fluent CFD code. Furthermore, a computer program has been developed to calculate numerically the volumetric entropy generation rate distributions and the other thermodynamic parameters by using the results of the calculations performed with the FLUENT code. The calculations bring out that the increase of ϕ (or the decrease of λ) reduces significantly the reaction rate levels. The average temperatures in the combustion chamber increase about 70 and 23% with the increases of γ (from 10 to 30%) and ϕ (from 0.5 to 1.0), respectively. With the increase of γ from 10 to 30%, the volumetric local entropy generation rates decrease about 9 and 4% in the cases of ϕ=0.5 and 1.0, respectively, and while the total entropy generation rates decrease exponentially, the merit numbers increase. The useful energy transfer rate to irreversibility rate therefore improves as the oxygen percentage increases.     

飞行Ma12条件超燃发动机流场及燃烧特征分析

为提升针对高马赫数发动机的模拟能力,对计算方法进行了可压缩性修正,并针对飞行Ma12条件下超燃冲压发动机进行了多状态三维数值模拟,分析了发动机内波系、参数以及燃烧性能特征.研究结果表明:(1)修正后的方法计算所得激波位置及强度与试验值吻合,在激波串模拟、高马赫数发动机模拟上均展现了更优的能力.(2)发动机内形成激波与反...     

Experimental investigation of flame/solid interactions in turbulent premixed combustion

An experimental study has been carried out to investigate the interaction between propagating turbulent premixed flames and solid obstacles. The experimental rig was configured specifically to allow detailed measurements with laser-based optical diagnostics. A wall-type solid obstacle was mounted inside a laboratory-scale combustion chamber with rectangular cross-section. The flame was initiated, by igniting a combustible mixture of methane in air at the center of the closed end of the combustion chamber. The flame front development was visualized by a high-speed (9000 frame/s) digital video camera and flame images were synchronized with ignition timing and chamber pressure data. The tests were carried out with lean, stoichiometric and rich mixtures of methane in air. The images were used to calculate highly resolved temporal and spatial data for the changes in flame shape, speed, and the length of the flame front. The results are discussed in terms of the influence of mixture equivalence ratio on the flame structure and resulting overpressure. The reported data revealed significant changes in flame structure as a result of the interaction between the propagating flame front and the transient recirculating flow formed behind the solid obstacle. Combustion images show that the flame accelerates and decelerates as it impinges on the obstacle wall boundaries. It is also found that the mixture concentrations have a significant influence on the nature of the flame/solid interactions and the resulting overpressure. The highest flame speed of 40 m/s was obtained with the unity fuel–air equivalence ratio. Burning of trapped mixture behind the solid obstruction was found to be highly correlated with the flame front length and the rate of pressure rise.