基于详细化学反应机理的DME发动机三维湍流燃烧模拟

采用基于详细化学反应机理的三维湍流燃烧数值模拟,研究直喷柴油机燃用二甲基醚(DME)的伴有化学反应的流动燃烧现象.模拟预测的缸内压力随曲轴转角的变化及NO排放浓度与实验相符.分析了计算所得的曲轴转角随缸内流场速度、温度和组分浓度的分布历程,结果表明甲醛在低中温下相对稳定,随着温度的升高,氧化反应加速进行,而由于流动及壁面传热等效应,甲醛作为不完全燃烧产物存在于排气中.     

基于FGM的旋流预混燃烧室超大涡模拟

针对航空发动机燃烧室等复杂工程中的预混燃烧问题发展高精度、高效的数值预测方法,本研究发展了火焰面生成流型(FGM)详细化学反应建表方法结合超大涡模拟方法(VLES),对工程中的GE LM6000预混旋流燃烧室燃烧开展了高精度数值研究,并与实验结果进行了比较。计算结果表明,VLES-FGM方法可以较准确地预测出旋流预混燃烧室内的流场及温度场分布。为了进一步模拟航空发动机真实的燃烧工况,对原始单头部燃烧室使用周期性边界条件来类比全环燃烧室。计算结果表明,VLES-FGM方法计算得到的周期性燃烧室流场回流区相比较固壁边界燃烧室较小,并且固壁边界燃烧室温度场具有明显的颈部结构,燃烧室下游的高温区分布更为均匀。本文计算结果表明基于FGM燃烧模型的自适应湍流模拟方法VLES对于模拟复杂航空发动机相关的旋流预混燃烧具有很大的应用潜力。     

当量比和初始混合模式对无焰燃烧的影响

对燃料一空气非预混、完全预混与部分预混三种混合模式下的无焰燃烧状态进行了实验和数值模拟研究。采用了详细化学反应机理和已被实验验证的算法进行数值模拟。研究发现三种混合模式下无焰燃烧状态的区别是由初始反应物的射流总动量的不同决定的。当炉内大尺度流场结构类似时,初始反应物射流总动量越大,炉内烟气循环越剧烈,温度分布越均匀,峰...     

超声速气流中脉冲正爆震波起爆详细化学动力学数值模拟

本文应用ce/se方法和chemkinII源代码构建了二维无粘多组分详细化学反应流计算模型,研究表明能该模型够精确地捕捉激波和燃烧波等强间断面.通过区域分裂法和MPI数据传递实现了SPMD并行计算,在一定程度上缓解了复杂化学反应对计算资源的要求.数值模拟的结果表明当来流马赫数小于CJ马赫数时,楔面可以成功地起爆向上游传播的正爆震波.     

正庚烷化学反应机理的简化与加速计算

详细化学反应机理的引入会给燃烧数值模拟带来巨大的困难:一方面,由于不同组分对应不同的特征时间,详细化学反应机理会导致燃烧模拟涉及到广泛的时间尺度;另一方面,随着燃料所含碳原子数目的增加,其详细化学反应机理中所含的组分数目与基元化学反应数目会呈指数增长,这直接导致计算量的急剧增加。为了解决这两方面的困难,本文以正庚烷氧化机理为例,通过化学反应机理简化(反应路径分析法)与加速算法(投影法)实现了在确保计算精度的条件下极大程度地提高计算效率。     

加力燃烧室热态流场的大涡模拟

本文采用k方程亚网格尺度模型对带V形槽稳定器模型加力燃烧室紊流化学反应流进行大涡模拟的研究,采用亚网格EBU燃烧模型估算化学反应速率,用热通量辐射模型估算辐射通量。数值计算表明,在稳定器后面开始在短时间内出现涡的交替脱落和逐渐消失过程,然后形成稳定的回流区,数值计算结果与实验比较吻合,表明可用大涡模拟研究实际燃烧室瞬态燃烧流场。     

合成气稀态预混燃烧器设计

稀态预混燃烧技术是实现合成气超低污染物排放利用的一条较为有前景的途径,然而预混燃烧中的燃烧稳定性问题是威胁燃气轮机安全运行的关键问题。为了研究合成气稀态预混燃烧过程的燃烧不稳定性问题,本文设计了一个模型燃烧试验器,并采用详细化学反应模型进行了数值计算,对其流场特性、预混效果及NO_x排放特性进行了详细分析。     

喷管中非平衡化学反应气体高速流动的数值模拟

本文给出了非平衡化学反应气体流动的控制方程,并对高速流动情况下控制方程的求解方法进行了详细讨论。本文采用定常方法模拟定常化学反应流动,避免了时间相关法求解中的方程“病态”问题,采用压力主变量原参数求解的方法数值模拟化学反应高速流动,国内外尚未见先例。本文对H_2-F_2化学反应系统在喷管中的流动进行了数值模拟得到了一些有用的结果。     

甲烷预混燃烧火焰的详细数值模拟

将六步简化甲烷燃烧反应模型与层流Ｎ－Ｓ方程相结合，数值模拟了定常与非定常的甲烷预混燃烧火焰．得到了速度、温度和包括ＣＯ和ＮＯ在内的十种组份的浓度分布；预测了进气流量脉动时的燃烧流场特性．该模型及其计算软件可较推确地模拟化学反应动力学起主导作用的甲烷预混燃烧现象，为研究燃烧过程特性，发展高效低污染燃烧技术与设备提供了有力的数值研究工具．     

MILD煤粉燃烧湍流与化学反应相互作用的数值分析

采用计算流体动力学(CFD)数值模拟方法分析了湍流与化学反应相互作用下MILD煤粉燃烧的微观特征。比较了涡耗散模型(EDM模型)和涡耗散概念模型(EDC模型)对MILD煤粉燃烧的影响,两者得到的速度场、温度场、烟气成分等结果与实验结果符合较好.在此基础上分析了MILD煤粉燃烧湍流和化学反应的微观特征尺度,发现采用EDC模型能更准确地反映MILD煤粉燃烧的微观特征,即山于强烈的烟气内部再循环导致的高湍流混合和烟气稀释后低氧浓度下的缓慢化学反应.     

An adaptive high-dimensional stochastic model representation technique for the solution of stochastic partial differential equations

A computational methodology is developed to address the solution of high-dimensional stochastic problems. It utilizes high-dimensional model representation (HDMR) technique in the stochastic space to represent the model output as a finite hierarchical correlated function expansion in terms of the stochastic inputs starting from lower-order to higher-order component functions. HDMR is efficient at capturing the high-dimensional input–output relationship such that the behavior for many physical systems can be modeled to good accuracy only by the first few lower-order terms. An adaptive version of HDMR is also developed to automatically detect the important dimensions and construct higher-order terms using only the important dimensions. The newly developed adaptive sparse grid collocation (ASGC) method is incorporated into HDMR to solve the resulting sub-problems. By integrating HDMR and ASGC, it is computationally possible to construct a low-dimensional stochastic reduced-order model of the high-dimensional stochastic problem and easily perform various statistic analysis on the output. Several numerical examples involving elementary mathematical functions and fluid mechanics problems are considered to illustrate the proposed method. The cases examined show that the method provides accurate results for stochastic dimensionality as high as 500 even with large-input variability. The efficiency of the proposed method is examined by comparing with Monte Carlo (MC) simulation.     

High dimensional model representation method for fuzzy structural dynamics

Uncertainty propagation in multi-parameter complex structures possess significant computational challenges. This paper investigates the possibility of using the High Dimensional Model Representation (HDMR) approach when uncertain system parameters are modeled using fuzzy variables. In particular, the application of HDMR is proposed for fuzzy finite element analysis of linear dynamical systems. The HDMR expansion is an efficient formulation for high-dimensional mapping in complex systems if the higher order variable correlations are weak, thereby permitting the input-output relationship behavior to be captured by the terms of low-order. The computational effort to determine the expansion functions using the α-cut method scales polynomically with the number of variables rather than exponentially. This logic is based on the fundamental assumption underlying the HDMR representation that only low-order correlations among the input variables are likely to have significant impacts upon the outputs for most high-dimensional complex systems. The proposed method is first illustrated for multi-parameter nonlinear mathematical test functions with fuzzy variables. The method is then integrated with a commercial finite element software (ADINA). Modal analysis of a simplified aircraft wing with fuzzy parameters has been used to illustrate the generality of the proposed approach. In the numerical examples, triangular membership functions have been used and the results have been validated against direct Monte Carlo simulations. It is shown that using the proposed HDMR approach, the number of finite element function calls can be reduced without significantly compromising the accuracy.     

LES/FGM investigation of ignition and flame structure in a gasoline partially premixed combustion engine

This paper presents a joint numerical and experimental study of the ignition process and flame structures in a gasoline partially premixed combustion (PPC) engine. The numerical simulation is based on a five-dimension Flamelet-Generated Manifold (5D-FGM) tabulation approach and large eddy simulation (LES). The spray and combustion process in an optical PPC engine fueled with a primary reference fuel (70% iso-octane, 30% n-heptane by volume) are investigated using the combustion model along with laser diagnostic experiments. Different combustion modes, as well as the dominant chemical species and elementary reactions involved in the PPC engines, are identified and visualized using Chemical Explosive Mode Analysis (CEMA). The results from the LES-FGM model agree well with the experiments regarding the onset of ignition, peak heat release rate and in-cylinder pressure. The LES-FGM model performs even better than a finite-rate chemistry model that integrates the full-set of chemical kinetic mechanism in the simulation, given that the FGM model is computationally more efficient. The results show that the ignition mode plays a dominant role in the entire combustion process. The diffusion flame mode is identified in a thin layer between the ultra fuel-lean unburned mixture and the hot burned gas region that contains combustion intermediates such as CO. The diffusion flame mode contributes to a maximum of 27% of the total heat release in the later stage of combustion, and it becomes vital for the oxidation of relatively fuel-lean mixtures.     

多孔焦炭燃烧特性的模型构建及数值模拟

焦炭燃烧在固体含碳燃料燃烧进程中占有主导地位,常规燃烧温度范围内(1273～1700 K)的焦炭燃烧过程研究及其模型化对于燃烧设备的设计和优化具有重要的意义。本文根据实际炉内的炭粒燃烧情况,将焦炭燃烧的模拟过程分解成几个环节分别进行研究,即热解后焦炭的初始化学反应活性、焦炭燃烧中化学活性变化、外部氧的扩散对于内孔燃烧的影响,并给出了相关过程的模型计算式。通过与已有管式炉实验结果的比较,新模型的预测结果能较好地反映焦炭的真实燃烧状况。与目前常用的焦炭燃烧模型相比,本模型具备一定的燃料通用性,计算负荷低且能保持相当的预测精度,可耦合到大型燃烧计算程序之中,更为有效地指导实际燃烧设备的优化设计。     

利用火焰发射光谱来研究汽油机的燃烧过程

本文用一套精密的光电转换系统，采集了一台汽油机燃烧过程中火焰辐射在可见光到近紫外波段内的光谱，探测到了燃烧中间产物ＣＨ、ＣＮ、Ｃ２、Ｈ２Ｏ等的特征光谱，并分析了这些产物在燃烧过程中的变化规律，以及随过量空气系数，缸内压力的变化．实验结果表明，汽油机三个不同的燃烧阶段具有不同的燃烧光谱特征：着火过程中，存在着大量的处于激发态的分子、原子、离子、自由基等活化中心的束缚态光谱，随着燃烧发展，ＣＨ、Ｃ２自由基的光谱强度明显加强；当减小过量空气系数时，光谱强度变弱并且着火延迟期增长；自由基特征光谱的光强变化曲线可以反映它们在燃烧过程中的浓度变化．所以火焰发射光谱是实时检测燃烧中间产物，特别是ＣＨ、Ｃ２等有害排放物变化规律的有效手段，可以为分析、模拟燃烧过程，控制排放提供有用的实验数据．     

An analysis of the in-cylinder soots generated from the main- and post-injection combustion in diesel engines

In modern diesel engines, the exhaust soot primarily comes from the main-injection combustion and post-injection combustion. Therefore, to reduce the diesel soot emissions, it is essential to better understand the soots generated from the main-injection combustion (main-soot) and from the post-injection combustion (post-soot). This work focused on the properties of the main-soot and post-soot during the combustion process, including the primary particle size, nanostructure and soot mass. The in-cylinder soot samples were obtained using a self-developed total cylinder sampling system, and the primary particle size and nanostructure were determined using high-resolution transmission electron microscopy. The isolation of the post-soot was achieved by adding dimethyl ether to the intake gas instead of the real main-injection to create a simulated main-injection combustion environment for post-soot formation. Combustion analysis and numerical simulation results showed that the simulated combustion environment for post-soot formation generated by the DME combustion was very similar to that generated by the real main-injection combustion. During the combustion process, although the main-soot and post-soot exhibit similar variations in the primary particle size, the maximum primary particle size of the post-soot is smaller than that of the main-soot (23.38 nm for the main-soot and 20.51 nm for the post-soot). The main-soot and post-soot show almost the same trends in the nanostructure, as characterized by the fringe length, separation and tortuosity, throughout the combustion process. The introduction of the post-injection accelerates the reduction of the primary particle size of the main-soot and the increase in the structural order of the main-soot. Because a large number of the main-soot particles are oxidized during the post-injection combustion, the post-soot accounts for a considerable proportion in the engine-out soot (i.e., 42%).     

Combustion kinetic model development using surrogate model similarity method

An ideal combustion kinetic model needs to be validated by different experimental targets over a wide range of temperatures and pressures that represent operating conditions in real engines. However, conditions of laboratory experiments for model validation are often limited by the constraint of experimental techniques. In order to improve model predictions under certain conditions (for example, at a relatively higher pressure), it is often needed to use the experimental data obtained under other conditions. In this work, the surrogate model similarity (SMS) method is proposed to find the experimental conditions or targets for model optimisation under certain conditions where the experiments are hard to be conducted. The similarity coefficient is calculated by the cosine similarity between the characteristic coefficients (vectors) of the High Dimensional Model Representation (HDMR) models for different model predictions. A larger similarity coefficient represents a closer relationship between two model predictions. The experimental data with larger similarity coefficients could be more effective to model uncertainty reduction under the concerned conditions. To demonstrate this method, simulations were conducted for two selected combustion systems with hydrogen or methanol as the fuel. In addition to its strength in available experimental data selection for model optimization, this method can be used to screen out experimental targets with strong constraint effect beforehand, thus providing an effective way to maximise utilisation of experimental resources.     

新型燃煤锅炉燃烧过程稳定性评价指数CSI

本文简要分析了现有燃烧稳定性评价方法不具有通用性的原因，然后提出了基于BIBO稳定性理论的新型燃煤钢炉燃烧过程稳定性评价指数CSI，该指数用稳定燃烧过程能够克服的最大燃料扰动率来定量。燃烧过程动态模拟分析和试验测试结果初步验证了CSI指数的有效性。可望在此基础上建立一种全面的评价电站燃煤锅炉燃烧过程相对稳定性的理论体系。     

燃烧室部件传热空间非均匀性对缸内燃烧的影响

将所有燃烧室部件(气缸盖-气缸套-活塞组-润滑油膜)作为一个耦合体,在对耦合体进行三维传热数值模拟的基础上,利用分区求解、边界耦合法建立缸内工作过程与燃烧室部件的三维耦合计算模型,从而实现缸内工作过程与燃烧室部件的耦合三维仿真模拟,以此考察燃烧室部件传热空间非均匀性对缸内燃烧的影响。研究结果表明,燃烧室部件传热空间非均匀性在喷雾过程几乎对燃烧没有任何影响,可以忽略不计;燃烧过程后期,这种传热空间非均匀性才越来越明显,由此可以推断,燃烧室部件传热空间非均匀性的影响在排气过程将进一步加剧。     

Combustion at the focus: laser diagnostics and control

Fifty years after the foundation of the Combustion Institute and almost 150 years after Michael Faraday’s famous lectures on the combustion of a candle, combustion diagnostics have come a long way from visual inspection of a flame to detailed analysis of a combustion process with a multitude of sophisticated techniques, often using lasers. The extended knowledge on combustion phenomena gained by application of these diagnostic techniques, combined with equally advanced numerical simulation of the process, has been instrumental in designing modern combustion devices with efficient performance and reduced pollutant emission. Also, similar diagnostic techniques are now employed to develop sensors for process control in combustion. This article intends to give a perspective on the potential of combustion diagnostics by highlighting selected application examples and by guiding the reader to recent literature. In particular, techniques are emphasized, which permit measurement of important features of the chemical composition, sometimes in conjunction with flow field parameters. Although a complete image of present research and applications in combustion diagnostics and control is beyond the scope of this article, this overview may be a starting place where ideas may be found to solve specific combustion problems with the aid of diagnostics.