微型开缝钝体燃烧器燃烧特性数值分析

本文将开缝钝体稳燃技术应用于微型燃烧器中,采用详细化学反应机理模拟了不同速度下微型开缝钝体燃烧器与微型常规钝体燃烧器的燃烧情况．结果表明：开缝钝体燃烧器火焰宽度一致性较好,火焰中心温度沿轴向分布更加均匀,尤其在速度较大时,开缝钝体燃烧器优势更加明显;开缝钝体燃烧器燃烧效率高于常规钝体燃烧器,速度大于25m/s时,开缝钝体燃烧器效率高出常规钝体燃烧器5%左右;由于开缝钝体中钝体缝隙过大,濒临吹熄极限时,钝体后值班火焰被吹熄,开缝钝体燃烧器吹熄极限略有降低．     

高马赫数下超声速燃烧的自点火查表方法

超燃冲压发动机燃烧室工作在高马赫数工况时, 入口来流空气的总焓非常高, 自点火在高焓条件下成为维持火焰稳定的重要物理化学过程. 本文借鉴火焰面/进度变量模型的降维思路, 发展了一种基于化学动力学的自点火建表方法. 通过定义混合分数和进度变量将复杂多维的化学反应降维, 并成功将数据库方法结合到现有的大涡模拟求解器中. 经过测试和验证, 该方法初步具备对超声速自点火燃烧进行仿真描述的能力. 针对自点火诱导的超声速燃烧问题开展数值模拟, 该方法通过查表的方式有效降低了化学反应求解过程中的计算量. 在采用详细化学反应机理时能够准确地再现自点火行为和火焰结构, 并且预测的温度和重要组分分布与实验吻合较好.      

增强型喷射器对爆轰波DDT过程的影响

在激波、火焰及射流同时存在的流场中，组织燃烧转爆轰过程是脉冲爆震发动机实现点火、起爆的关键问题。设计一类喷射器，采用C₂H₂/O₂/Ar反应，数值验证了该喷射器能增强爆震室燃料燃烧转爆轰的可行性，并讨论了流场中热点的点火机制。结果显示:该装置在流场中可激发不稳定性，产生漩涡，加速能量、质量的交换。流场产生热点，促进火焰速度加快，追赶前导激波。喷射器位置影响前导激波的运动速度。在一定范围内，前导激波速度越大，碰撞产生的热点越容易激发燃烧转爆轰过程。     

横向紊动射流的数值与实验研究进展

横向紊动射流作为流体运动的一种重要类型,广泛存在于如: 燃气轮机气膜冷却、锅炉燃烧室等的燃烧控制, V/STOL(垂直或短距离起落)飞机、废气排放的控制等工程实际应用中.由于射流的存在,增加了流场的复杂性,流场中同时存在射流剪切层涡、马蹄形涡系、反向旋涡对和尾迹涡等4种涡系结构,这对流体力学理论研究具有重要意义.长期以来,研究人员从理论分析、实验测量和数值模拟方面对横向紊动射流进行了大量的研究工作,目前已经认识了流场中的许多流动特性和流动机理.从数值模拟和实验研究两个方面,比较并分析了国内外横向紊动射流研究的现状和研究结果,评述了不同湍流模型以及不同的实验测量方法对横向紊动射流的预测能力,讨论了存在的问题并对该领域的研究方向进行了展望.      

多元混合PBX炸药冲击起爆的多元Duan-Zhang-Kim反应速率模型研究

提出了多元混合PBX炸药孔隙塌缩热点模型新的处理方法，构建了新的细观反应速率模型，系列数值模拟结果与实验结果均一致，表明该细观反应速率模型可较好地描述和预测炸药组分配比及颗粒度对多元混合PBX炸药冲击起爆过程的影响。PBX炸药冲击起爆过程主要受热点点火过程和燃烧反应过程共同作用:HMX占主导成分的PBXC03炸药，起爆压力低，冲击起爆过程受热点点火影响较明显，热点点火后的燃烧反应速度较快，表现为加速反应特性；TATB占主导成分的钝感PBXC10炸药，起爆压力高，冲击起爆过程主要受点火后的燃烧反应过程控制，且点火后燃烧反应速度较慢，表现为稳定反应特性。     

方形钝体受限绕流的三维数值模拟

采用一种具有二阶精度的分裂步有限元方法作为大涡模拟的空间离散格式,经过标准算例的验证后,对Re=1.0×104条件下的方形钝体三维受限湍流绕流流场进行了数值模拟.计算中,为消除初始效应,略去初始段的计算结果.数值分析表明在均匀来流条件下,该湍流场沿槽道轴面对称,并呈现出一定的拟周期特性.在流场特性分析的基础上,进行了湍流能耗场的分析,结果表明,方形钝体受限绕流的能耗主要集中在大涡丰富的流动区段内.计算过程反映出,采用该空间离散格式的大涡模拟方法,能够捕捉到非常丰富的涡系及涡动的时变过程,适用于方形钝体受限绕流的数值模拟.     

方形管内楔形障碍物对火焰结构与传播的影响

通过实验与数值模拟方法对CH4/空气预混火焰在有楔形障碍物的卧式燃烧方管内的传播进行了研究。采用多镜头Cranz Schardin高速摄像机和压力传感器等实验设备获得了高清晰度的障碍物诱导火焰失稳的分幅时序照片以及障碍物背风表面压力变化曲线。数值模拟则基于RANS方法与EDU-Arrhenius燃烧模型,计算结果与实验结果基本相符,反映了火焰在管内传播与变形的详细过程。通过综合分析实验与计算结果,得到了由楔形障碍物导致的火焰加速与变形的内在机理,揭示了火焰传播过程中由层流燃烧向湍流燃烧转捩的本质。     

大涡模拟在内燃机中应用的研究进展

大涡模拟 能够以比较合理的计算成本, 提供更多详细的湍流信息, 故近年来已经广泛地应用于科学和工程领域, 并也成为内燃机缸内湍流流动与燃烧过程模拟计算的最有潜力的数值方法.综合现有研究成果, 对内燃机中大涡模拟的研究进展和模拟方法进行了比较全面的评述.介绍了大涡模拟的基本概念、方法、亚网格模型,着重讨论了内燃机缸内冷态流场、燃油喷雾过程以及两相液雾湍流燃烧大涡模拟的国内外研究进展.最后论述了大涡模拟在内燃机应用中当前需要解决的问题及其发展趋势.     

开缝钝体尾迹的拟序结构

弄清开缝钝体尾迹的拟序结构是认识其火焰稳定机理的基础。在雷诺数R e为470000条件下,采用RNGk-ε模型对通道内的开缝钝体尾迹进行数值模拟来分析大涡尺度的拟序结构。模拟结果显示,偏向一侧的中缝流将近尾分成主回流区和次回流区,主回流区的漩涡脱落激发扰动,引起近尾的绝对不稳定。并提出单涡突然置于两剪切层间的漩涡脱落机理来解释拟序结构不稳定特性。为了验证上述结论,在闭式风洞中采用激光粒子测速技术(P IV)对开缝钝体的尾迹进行了实验研究,其结果与数值分析较好地吻合。     

Recent progress and challenges in fundamental combustion research（燃烧基础研究的进展和挑战）

超过80%的世界的能源转换是由燃烧方法来实现的. 发展可利用替代燃料的清洁和高效的新型发动机是解决可持续能源发展的关键之一. 在燃烧研究领域,实现这一目标的挑战是要揭示从燃料分子到发动机的多尺度燃烧过程中化学反应和火焰动力学机理,发展高效,定量的数值模拟方法和开发新的燃烧技术. 本文从7个方面综述最近几年燃烧领域的基础燃烧研究的进展和挑战. 它们包括低温清洁燃烧的发动机技术,极限条件下的燃烧机理和现象,替代燃料和混合燃料模型,多尺度化学反应模拟方法,高压燃烧反应动力学,基础燃烧的实验方法,和先进测量技术. 本文首先介绍均值充量压缩点火（HCCI）,反应控制压缩点火（RCCI）以及增压燃烧等新型发动机的概念,评述燃料特性和低温燃烧反应过程对湍流燃烧和发动机的影响,讨论发展基础燃烧研究的必要性. 第二,综述燃料浓度分层燃烧,稀薄燃烧,冷炎燃烧,以及等离子体助燃等极限燃烧条件下的新的燃烧现象和火焰机制. 第三,以航空煤油和生物柴油为例来讨论建立模拟真实燃料和替代燃料的混合燃料模型的方法. 介绍活性基指数和输运加权的反应焓的概念并用来比较燃料的高温反应特性和评价燃料的分子结构对燃烧特性的影响. 第四,评述详细化学反应机理简化的方法. 介绍多时间尺度（MTS）的化学反应的模拟和动态关联性自适应机理简化（CO-DAC）的方法来提高详细化学反应机理的计算效率. 第五,讨论高压燃烧的火焰传播速度的实验测量结果以及高压燃烧化学反应机理所存在的问题,并分析高压燃烧的关键组分和反应路径. 第六,评述测量火焰速度和组分等基础燃烧实验方法和模型中的问题和误差来源. 介绍一些改进测量方法和提高测量精度的方法. 最后,介绍测量低温燃烧中的关键组分和自由基的测量方法和最新进展.      

On the stabilization of flames on multijet industrial burners

Measurements of velocity and temperature characteristics, together with the analysis of the process of flame extinction, are reported for a range of high-intensity flames stabilized on a model of an industrial oxyfuel burner installed in a divergent quarl. The burner consists of a central axisymmetric jet surrounded by 16 circular jets, simulating the injection of oxygen in practical burners. A laser-Doppler velocimeter was used to measure density-weighted velocity characteristics, and bare-wire thermocouples were used to measure near unweighted temperature characteristics. Experiments were carried out to improve knowledge of the flow in the near field of multijet burner heads, which is essential to design further modifications in their geometry and to predict their effects. Isothermal and combusting flows are studied; for the latter, the experiments quantify the effect of quarl geometry, fuel-to-air ratio, swirl number, and central-to-peripheral jet velocity ration on the flame characteristics.

The results show that flame stabilization occurs in the vicinity of the quarl and is affected by its geometry owing to changes in the rate of entrainment of cold air. Increasing the swirl level and decreasing the peripheral airflow improves flame stability by promoting the mixing of fuel and air along the annular stabilization region. Turbulence measurements show common features with and without combustion and suggest the absence of large-scale mixing in the present flames. Although the laminar flamelet concept may represent most of the features of the flames investigated, the local quenching of burning flamelets is shown to preclude the internal ignition of flame gases in a way that influences the process of flame stabilization.     

Application of laser ignition on laminar flame front investigation

The first stages of laser-induced spark ignition were investigated as a function of time. Experiments were conducted using a premixed laminar CH₄/air burner. Laser-induced breakdown was achieved by focusing a 532-nm nanosecond pulse from a Q-switched Nd:YAG laser. An anti-reflection coated lens with a focal length of 100 mm was used. The results obtained from an intensified high-speed and PIV CCD camera and a Cassegrain optics system coupled to an ICCD spectrometer provided information about the formation of laser-induced plasma and its transition to a flame kernel and a self-sustaining flame. The localization of the kernel and its time development were reproducible. Two types of flame fronts develop: one that expands against the flow direction, and one that moves with the flow. The initial flame expansion along the laser axis is asymmetric because of the shape of the plasma, different ionization levels inside the plasma, and the shock-wave expansion. Development of the fast flame occurs behind the shock wave induced by the plasma. This is important when laser ignition is used as a flame holder. An ICCD spectrometer coupled to an optical fiber permitted chemiluminescence visualization. The spectrum obtained during the plasma and flame kernel formation defined different stages in flame formation. The results obtained with these two optical techniques were synchronized to obtain the temporal resolution of the flame kernel evolution. Laser-induced ignition of a very lean mixture can be controlled to provide local heat release and extinction in a flame.     

Non-uniform velocity profile mechanism for flame stabilization in a porous radiant burner

Industrial processes where the heating of large surfaces is required lead to the possibility of using large surface porous radiant burners. This causes additional temperature uniformity problems, since it is increasingly difficult to evenly distribute the reactant mixture over a large burner surface while retaining its stability and keeping low pollutant emissions. In order to allow for larger surface area burners, a non-uniform velocity profile mechanism for flame stabilization in a porous radiant burner using a single large injection hole is proposed and analyzed for a double-layered burner operating in open and closed hot (laboratory-scale furnace, with temperature-controlled, isothermal walls) environments. In both environments, local mean temperatures within the porous medium have been measured. For lower reactant flow rate and ambient temperature the flame shape is conical and anchored at the rim of the injection hole. As the volumetric flow rate or furnace temperature is raised, the flame undergoes a transition to a plane flame stabilized near the external burner surface. However, the stability range envelope remains the same in both regimes.     

Nonpremixed bluff-body burner flow and flame imaging study

A nonpremixed bluff-body burner flow and flame have been studied using planar flow visualization and species concentration imaging techniques. The burner consists of a central jet of CH ₄ in a cylindrical bluff-body and an outer coflowing-air stream. Planar flow visualization, using Mie scattering from seed particles added to the fuel jet, Raman scattering from CH ₄ and laser-induced fluorescence of CH combined with Raman scattering of CH ₄ provided information on turbulent flow, mixing and combustion. The CH ₄ imaging system utilized two cameras, which enhanced the dynamic range of the diagnostic system by a factor of 10 over a single-camera system. It was observed that the fuel jet stagnated on the axis due to interaction with the high velocity air flow. The flow and mixing were found to have significant coherent and noncoherent, large-scale, time-varying structures. The detailed CH ₄ Raman and CH fluorescence measurements of an air-dominated bluff-body flame revealed that the stagnation zone governs mixing and flame stability. Through large-scale mixing, the stagnated jet feeds the recirculation zone and also creates a favorable condition to stabilize the flame detached from the bluff-body. The instantaneous flame zone, as defined by CH, was found to be narrow and concentrated in an envelope around the stagnation zone. This narrow flame characteristic is consistent with that observed for jet flames. Although the internal structure of the flame envelops have not yet been defined, these results suggest that this bluff-body flame can be modeled by a flame sheet type approach, where the reaction front is captured by the large-scale structures. This should simplify the development of modeling approaches for these flows since molecular mixing and chemical reaction, which occur within the flame sheet, can be separated from the large-scale mixing process.     

Assessing the Predictive Capabilities of Combustion LES as Applied to the Sydney Flame Series

Large Eddy Simulation (LES) and flamelet-based combustion models were applied to four bluff-body stabilized nonpremixed and partially premixed flames selected from the Sydney flame series, based on Masri’s bluff-body test rig (University of Sydney). Three related non-reacting flow cases were also investigated to assess the performance of the LES solver. Both un-swirled and swirled cases were studied exhibiting different flow features, such as recirculation, jet precessing and vortex breakdown. Due to various fuel compositions, flow rates and swirl numbers, the combustion characteristics of the flames varied greatly. On six meshes with different blocking structure and mesh sizes, good prediction of flow and scalar fields using LES/flamelet approaches and known fuel and oxidizer mass fluxes was achieved. The accuracy of predictions was strongly influenced by the combustion model used. All flames were calculated using at least two modeling strategies. Starting with calculations of isothermal flow cases, simple single flamelet based calculations were carried out for the corresponding reacting cases. The combustion models were then adjusted to fit the requirements of each flame. For all flame calculations good agreement of the main flow features with the measured data was achieved. For purely nonpremixed flames burning attached to the bluff-body’s outer edge, flamelet modeling including strain rate effects provided good results for the flow field and for most scalars. The prediction of a partially premixed swirl flame could only be achieved by applying a flamelet-based progress variable approach.     

Flow Physics of a Bluff-Body Swirl Stabilized Flame and their Prediction by Means of a Joint Eulerian Stochastic Field and Tabulated Chemistry Approach

In the frame of this work a transported joint scalar probability density function (PDF) method is combined with the flamelet generated manifolds (FGM) tabulated chemistry approach for large eddy simulation (LES) modeling of a three-dimensional turbulent premixed swirl burner. This strategy accounts for the turbulence-chemistry interaction at reasonable computational costs. At the same time, it allows the usage of detailed chemistry mechanisms for the creation of the chemical database. The simulation results obtained are comparatively assessed along with complementary measurements. Furthermore, transient and time-averaged data are used to provide insight into the flow physics of the bluff-body swirl stabilized flame considered. The sensitivity of the results to different modeling approaches regarding the predicted flame shape and its dynamics is also investigated, where the implemented approach is compared with the well-established artificially thickened flame (ATF) combustion model. Consequently, the investigation conducted in this work aims to provide a complete picture on the ability of the proposed combustion model to reproduce the flow conditions within complex bluff-body swirl stabilized flames.     

Enhancement of evaporation and combustion of liquid fuels through porous media

Existing designs of most conventional liquid fuel burners have relied solely on spray atomizers, with a large amount of very fine droplets forming in a relatively large combustion chamber, resulting in a relatively low combustion intensity. Against this background, a novel down-flow compact porous burner system was developed for burning kerosene without the need of using a spray atomizer. Successive development on this burner research is important in view of the need to create energy by an efficient device based on simple technology. The focus has been on the introduction of the packed bed emitter installed downstream of the porous burner. The evaporation process and combustion phenomena that occurred are described through the coupled interaction of the solid phase (porous burner), the liquid phase (kerosene) and the gas phase. Enhancement of evaporation and combustion are evaluated through the measured thermal structures in terms of temperature distribution along the burner length and emission characteristics at the burner exit. Stable combustion with low emission of pollutants was realized even though the combustion flame was confined in-between the porous burner and the packed bed emitter with an increase in the back-pressure. The effects of various parameters including heat input and equivalence ratio on the combustion characteristics were clarified to confirm improvement in mixing of the fuel vapor/air mixture and turn-down ratio of the burner. The effect of the introduced packed bed emitter with suitable bed length and its installation location is investigated as an efficient method for enhancement of evaporation and combustion of the liquid fuels without a spray atomizer. Future applications of this type of burner system are suggested.     

Numerical Simulation of Non-premixed Turbulent Combustion Using the Eddy Dissipation Concept and Comparing with the Steady Laminar Flamelet Model

Numerical simulations of the Sandia flame CHNa and the Sydney bluff-body stabilized flame HM1E are reported and the results are compared to available experimental data. The numerical method is based on compressible URANS formulations which were implemented recently in the OpenFOAM toolbox. In this study, the calculations are carried out using the conventional compressible URANS approach and a standard k- ?? turbulence model. The Eddy Dissipation Concept with a detailed chemistry approach is used for the turbulence-chemistry interaction. The syngas (CO/H₂) chemistry diluted by 30 % nitrogen in the Sandia flame CHNa and CH₄/H₂ combustion in the Sydney flame HM1E are described by the full GRI-3.0 mechanism. A robust implicit Runge-Kutta method (RADAU5) is used for integrating stiff ordinary differential equations to calculate the reaction rates. The radiation is treated by the P1-approximation model. Both target flames are predicted with the Steady Laminar Flamelet model using the commercial code ANSYS FLUENT as well. In general, there is good agreement between present simulations and measurements for both flames, which indicates that the proposed numerical method is suitable for this type of combustion, provides acceptable accuracy and is ready for further combustion application development.     

Flame-Flow Interaction in Premixed Turbulent Flames During Transient Head-On Quenching

This paper reports on experimental investigations of turbulent flame-wall interaction (FWI) during transient head-on quenching (HOQ) of premixed flames. The entire process, including flame-wall approach and flame quenching, was analyzed using high repetition rate particle image velocimetry (PIV) and simultaneous flame front tracking based on laser-induced fluorescence (LIF) of the OH molecule. The influence of convection upon flame structures and flow fields was analyzed qualitatively and quantitatively for the fuels methane (CH₄) and ethylene (C₂H₄) at ? = 1. For this transient FWI, flames were initialized by laser spark ignition 5 mm above the burner nozzle. Subsequently, flames propagated against a steel wall, located 32 mm above the burner nozzle, where they were eventually quenched in the HOQ regime due to enthalpy losses. Twenty ignition events were recorded and analyzed for each fuel. Quenching distances were 179 μm for CH₄ and 159 μm for C₂H₄, which lead by nondimensionalization with flame thickness to Peclet numbers of 3.1 and 5.5, respectively. Flame wrinkling and fresh gas velocity fluctuations proved flame and flow laminarization during wall approach. Velocity fluctuations cause flame wrinkling, which is higher for CH₄ than C₂H₄ despite lower velocity fluctuations. Lewis number effects explained this phenomenon. Results from flame propagation showed that convection dominates propagation far from the wall and differences in flame propagation are related to the different laminar flame speeds of the fuels. Close to the wall flames of both fuels propagate similarly, but experimental results clearly indicate a decrease in intrinsic flame speed. In general, the experimental results are in good agreement with other experimental studies and several numerical studies, which are mainly based on direct numerical simulations.