煤粉复合旋流火焰燃烧特性研究（2）：数值模拟

本文数值模拟了煤粉旋流火焰燃烧过程，燃烧数值计算包括理论物理模型建立，数值方法两个大部分，计算模型处理了气相湍流与燃烧、气固两相流动、煤颗粒燃烧过程和辐射传热等物理化学过程，以ｋ－ε模型模拟湍流流动；ＰＤＦ法模拟气相扩散火焰燃烧；颗粒运动计算颗粒运动少颗粒湍流浓度方程模拟颗粒湍流扩散；通量法计算火焰辐射传热，煤粉颗粒复杂燃烧模型计算了颗粒尺寸、形状变化和颗粒孔隙内部燃烧、表面平度对整个颗粒的燃烧过程影响。计算获得了气相速度分布场、气相ｋ和ε分布场、气相温度场、气相组份场和颗粒浓度场及运动过程，揭示了煤粉复合旋流燃烧特性。     

PaSR湍流燃烧模型对典型湍流射流火焰的数值模拟

采用PaSR湍流燃烧模型对湍流燃烧研究中典型的甲烷湍流射流火焰进行了数值模拟.计算采用简化的化学反应机理,并将计算得到的平均温度场、速度场和各组分的分布与相应的权威实验数据进行了对比.对反应系统中流动和燃烧的不同时间尺度以及二者之间的关系作了探讨.计算结果表明,PaSR模型能够很好地模拟燃烧过程中流场和组分的变化.在火...     

三分支管3维湍流流场的模拟研究

本在网络生成，边界条件，收敛准则等方面对ＫＩＶＡ程序进行了改进，以便模拟三分支和内气体流动；利用３维粒子动态分析仪测量了矩形截面三分支管试验件的３维速度场，以验证改进后的程度软件的可靠性；最后，以某一型号发动机的ＭＰＣ排气系统为例，在只有支管进气和支管总管都进气两种情况下，模拟了ＭＰＣ模件三分支管的３维定常湍流流动。     

预混湍流燃烧的实验研究

介绍并分析了甲烷－空气混合气在定容燃烧弹内进行火花点火预混湍流燃烧的实验结果,得到了火核的初期发展,湍流参数对燃烧特性的影响以及不同电极间隙下的失火炫等具有实用参考价值的重要结论。     

受浮力作用的湍流扩散火焰的数值模拟

应用浮力修正的k-ε模型和EDC湍流燃烧模型对旋流燃烧室内具有较低燃料/空气初始动量的甲烷湍流扩散火焰进行了数值模拟，得到了两组工况下的气体时均速度场、温度场、组分浓度场和湍流脉动速度均方根值分布等．并与实验数据进行了比较，二者基本相符．同时，还将计算结果与标准k-ε模型的模拟结果进行了对比，揭示了浮力对具有较低初始动量的湍流扩散火焰的影响．     

旋流燃烧室内煤粉多相湍流流动与燃烧的数值模拟

提出了考虑湍流-颗粒反应相互作用的颗粒随机轨道模型,以此为基础建立煤粉燃烧综合理论模型并应用于旋流燃烧室内煤粉多相湍流流动与燃烧的数值模拟.模拟结果给出了气相温度场、速度场与温度脉动均方根值分布、颗粒相温度场、速度场与表观密度场以及颗粒瞬时温度与质量随时间的变化.研究表明,考虑湍流-颗粒反应相互作用对气相与颗粒相温度场的模拟结果有一定的影响,使气相温度分布与实验数据更为接近.     

燃烧系统结构对中载汽油机爆震边界内燃烧的影响

针对中载汽油机,设计开发了4种进气道和燃烧室的匹配方案,通过数值模拟的方法研究了不同燃烧系统结构对缸内宏观流场、湍流场及爆震边界内燃烧的影响。结果表明:进气道和燃烧室结构对缸内宏观流场及湍流场演化有显著影响。初期火焰传播速度主要由火花塞周围气流的平均速度决定,而主燃烧阶段的燃烧速度与该区域的湍动能大小成正比。双切向进气道匹配中置倒楔形燃烧室能够同时提高湍流强度及气流速度,从而有效缩短滞燃期及燃烧持续期,但其爆震倾向严重。采用复合进气道匹配中置倒楔形燃烧室的方案可显著抑制爆震,提前点火时刻,燃烧速度较快,可明显降低燃烧损失,提升中载汽油机的性能。     

高温空气燃烧燃气热态试验炉非稳态数值模拟

采用双方程湍流模型、概率密度函数(PDF)燃烧模型和离散坐标(DO)辐射模型，对采用高温空气燃烧技术的燃气热态试验炉炉内工况进行了非稳态数值模拟，预测热态试验炉运行时的炉内温度场、组分场和速度场，由数值模拟结果分析比较了不同喷口间距五喷口燃烧器对炉内工况的影响，为采用高温空气燃烧技术的燃气燃烧器设计提供参考。     

低浓度瓦斯脉动燃烧过程的数值模拟

选取k-ε湍流双方程模型、概率密度方程(PDF)湍流燃烧模型及部分预混燃烧模型,利用流体分析软件Fluent对煤矿低浓度瓦斯在Helmholtz型脉动燃烧器中的燃烧过程进行了数值模拟。通过对燃烧室内低浓度瓦斯脉动燃烧压力场和速度场的分布进行模拟研究,并与理论燃烧过程相比较,其结果表明:低浓度瓦斯脉动燃烧数值模拟结果符合实际脉动燃烧规律,说明脉动燃烧这一燃烧方式适合低浓度瓦斯的研究利用,并为以后的研究提供参考。     

旋流煤粉燃烧NO生成的AUSM湍流反应模型

针对煤粉燃烧NO生成提出一种湍流反应统一二阶矩代数(AUSM)模型．用纯双流体模型，包括κ—ε—κp两相湍流模型、EBU—Arrhenius燃烧模型、六热流辐射模型、NO生成湍流反应的AUSM模型和原有二阶矩代数模型，对旋流煤粉燃烧器内相流动、煤粉燃烧和NO生成进行了数值模拟．两相流动的模拟结果和PDPA实验结果符合较好。热态模拟结果和文献中的实验结果的对照指出，AUSM模型的模拟结果比ASM模型的模拟结果更合理．ASM模型由于采用温度指数函数的级数展开近似，舍去了并非小量级的高阶项，低估了NO生成率．这和文献中用ASM模型模拟甲烷—空气燃烧低估了NO生成率的趋势是一致的。     

在定容燃烧弹中湍流特征参数对预混燃烧的影响

:给出在定容燃烧弹中火花点燃 CH4-空气充量进行湍流预混合燃烧的试验结果并进行了分析 ,得到一些有价值的结论 :如在火核起始发展期中存在一个最小火焰传播速度 ,此时的火核半径与湍流积分长度标尺大致相等 ,增加湍流强度 (u <1 .8m/s) ,瞬时燃烧率增加 ,燃烧持续期缩短 ,相对缓燃期增加 ,相对主燃期缩短 ,这是组织湍流可以提高火花点火发动机热效率的主要原因。此外本文还给出不同间隙的失火率并指出减少火核向电极传热是减少失火率的主要措施     

Experimental study on induced accelerated combustion of premixed hydrogen-air in a confined environment

The study on induced accelerated combustion of premixed hydrogen-air in a confined environment is of great significance for the efficient utilization of hydrogen energy in internal combustion engines. The accelerated flame induced by the orifice plate is more stable and easy to control, which is beneficial to achieve controlled and rapid turbulent combustion. In this work, the accelerated combustion process induced by the orifice plate, and the influence of the orifice structure and initial conditions on the flame propagation and combustion characteristics were investigated by constant volume combustion bomb and schlieren method. The results show that the combustion process induced by the orifice plate consists of three stages: the initial stage of propagation, the accelerated stage of the orifice plate, and the end combustion stage. The reduction in aperture induces greater turbulence intensity and increases the perturbation of the orifice plate to the flame, resulting in a substantial increase in flame propagation speed through the orifice plate. As the initial pressure and the equivalence ratio increase, the velocity of turbulent flame induced by the orifice plate and the change rate of the velocity before and after the orifice plate increase. As the initial temperature increases, the turbulent flame propagation velocity does not change much, and the velocity change rate before and after the orifice plate decreases. The effect of the initial conditions on flame acceleration induced by the orifice plate is essentially the influence of flame propagation speed and instability. The greater the flame propagation speed and the stronger the flame instability, the stronger the induced turbulence and the greater the influence of the turbulent flow disturbance, and the greater the velocity of the turbulent flame induced by the orifice plate. There exists an optimum aperture for the shortest combustion duration at any initial conditions, but the optimal diameter is not sensitive to changes in initial conditions. The effect of orifice-induced combustion acceleration is remarkable, and the combustion durations induced by each orifice plate are shortened by more than 50%.     

Effects of the external turbulence on centrally-ignited spherical unstable Ch4/H2/air flames in the constant-volume combustion bomb

In the present study, we conducted experiments to investigate the effects of external turbulence on the development of spherical H₂/CH₄/air unstable flames developments at two different equivalence ratios associated with different turbulent intensities using a spherical constant-volume turbulent combustion bomb and high speed schlieren photography technology. Flame front morphology and acceleration process were recorded and different effects of weak external turbulent flow field and intrinsic flame instability on the unstable flame propagation were compared. Results showed the external turbulence has a great influence on the unstable flame propagation under rich fuel conditions. For fuel-lean premixed flames, however, the effects of external turbulence on the morphology of the cellular structure on the flame front was not that obvious. Critical radius decreased firstly and then kept almost unchanged with the augment of the turbulence intensity. This indicated the dominating inhibiting effect of flame stretch on the turbulent premixed flame at the initial stage of the flame front development. Beyond the critical radius, the acceleration exponent was found increasing with the enhancement of initial turbulence intensity for fuel-lean premixed flames. For fuel-rich conditions, however, the initial turbulence intensity had little effect on acceleration exponent. In order to evaluate the important impact of the intrinsic flame instability and external turbulent flow field for spherical propagating premixed flames, intrinsic flame instability scale and average diameter of vortex tube were calculated. Intrinsic flame instability scale decreased greatly and then stayed unchanged with the propagation of the flame front. The comparison between intrinsic flame instability scale and average diameter of vortex tube demonstrated that the external turbulent flow filed will be more important for the evolution of wrinkle structure in the final stage of the flame propagation, when the turbulence intensity was more than 0.404 m/s.     

Experimental study on the turbulent premixed combustion characteristics of 70%H2/30%CO/air mixtures

To investigate the effect of equivalence ratio and turbulence intensity on the combustion characteristics of syngas/air mixtures, experiments involving premixed combustion of 70% H₂/30% CO/air mixtures at various equivalence ratios and turbulence intensities were conducted in a turbulent combustion bomb at atmospheric temperature and pressure. The turbulent burning velocity and flame curvature were used to study turbulent combustion characteristics. The results show that the turbulent burning velocity grew nonlinearly as the equivalence ratio increased, while the normalized turbulent burning velocity tended to decrease. When the equivalence ratio was relatively low, the turbulence intensity was a greater determinant of the burning velocity. The normalized turbulent burning velocity increased as the turbulence intensity increased. Re and Da were found to be directly and inversely proportional to u’/u_L, respectively. A linear relationship was observed between u_T/u_L and ln Re. As the turbulence intensity increased or equivalence ratio decreased, the wrinkle degree of the flame front increased, and the maximum and minimum values of flame front curvature increased and decreased, respectively. Meanwhile, the range of the flame front curvature increased gradually. The proportion of components with smaller absolute value of flame front curvature gradually decreases.     

Experimental and numerical investigation of gaseous fuel combustion in swirl chamber

In this paper the results of experimental and numerical investigations of swirl burner were presented. Mathematical model for prediction of velocity, temperature and concentration fields of axisymmetrical confined swirl turbulent flame was developed. Model consists of few mutually coupled segments related to basic processes in turbulent flows with combustion. The original combustion rate model based on the ideal reacting hypothesis within fine structure of turbulence was applied. The comparison of the experimental results with computation showed satisfactorily agreement between the model and the experiment. This analysis also showed the importance of the proposed combustion rate model with simultaneous influence of both chemical kinetics and turbulent effects.     

缸内直接喷射式汽油机点火稳定性的数值分析

缸内直接喷射式汽油机的一个显著特点是依靠火花塞点燃喷入缸内的汽油油束。由于缸内混合气浓度极不均匀，所以其点火及火焰传播过程与普通均质燃烧式发动机有很大的不同。火焰核心的稳定形成及初始火焰发展对缸内的整个燃烧过程有极其重要的影响。本文利用二维两相混合模型模拟喷雾过程，利用一个详细的准维模型模拟火花塞的点火过程，并采用特殊处理方法使两个子模型相匹配，计算了缸内直接喷射式汽油机从喷雾到形成稳定火核的全过程，分析了多种因素对点火稳定性的影响，尤其是对涡流比、点火时刻和喷油定时之间的适当配合进行了模拟分析。计算结果对优化实验有明显的指导作用。     

The influence of turbulence on the structure and propagation of enclosed flames

This investigation was undertaken to examine the influence of turbulence on burning velocity and on the physical structure of the flame surface under flow conditions similar to those experienced in turbojet afterburner systems. Uniform propane-air mixtures were supplied to a combustion chamber 12 in. long and of 4 in. × 4 in. cross section. Control over the turbulence level was achieved by means of grids located at entry to the chamber. Schlieren photographs were taken through transparent side walls at turbulence levels ranging from 2 to 14 per cent and at velocities up to 250 ft/sec. These photographs provided the basic data for the investigation. Turbulent flume velocities were derived as the product of the inlet velocity and the sine of the angle between the flow direction and the mean surface of the flame.

The results fully supported the wrinkled laminar flame concept of turbulent flame propagation. Turbulent flame speed was found to increase with increases in laminar flame speed, turbulent velocity and flow velocity. Under turbulent flow conditions the flame surface was characterized by a cellular structure, the average cell size diminishing with increase in approach stream velocity and turbulence. However, the main effect of turbulence was in lacerating and disrupting the flame and thereby increasing its surface area.

The results of previous investigations were confirmed in regard to the relatively slight dependence of flame spreading rate on inlet velocity, especially at high velocities. However, flame spreading rate was found to vary appreciably with turbulence and also with fuel-air ratio, a result which was consistent with the wrinkled laminar flame model, but which contradicted previous findings on enclosed flames.     

Understanding ignition processes in spray-guided gasoline engines using high-speed imaging and the extended spark-ignition model SparkCIMM: Part B: Importance of molecular fuel properties in early flame front propagation

Recent high-speed imaging of ignition processes in spray-guided gasoline engines has motivated the development of the physically-based spark channel ignition monitoring model SparkCIMM, which bridges the gap between a detailed spray/vaporization model and a model for fully developed turbulent flame front propagation. Previously, both SparkCIMM and high-speed optical imaging data have shown that, in spray-guided engines, the spark plasma channel is stretched and wrinkled by the local turbulence, excessive stretching results in spark re-strikes, large variations occur in turbulence intensity and local equivalence ratio along the spark channel, and ignition occurs in localized regions along the spark channel (based upon a Karlovitz-number criteria).In this paper, SparkCIMM is enhanced by: (1) an extended flamelet model to predict localized ignition spots along the spark plasma channel, (2) a detailed chemical mechanism for gasoline surrogate oxidation, and (3) a formulation of early flame kernel propagation based on the G-equation theory that includes detailed chemistry and a local enthalpy flamelet model to consider turbulent enthalpy fluctuations. In agreement with new experimental data from broadband spark and hot soot luminosity imaging, the model establishes that ignition prefers to occur in fuel-rich regions along the spark channel. In this highly-turbulent highly-stratified environment, these ignition spots burn as quasi-laminar flame kernels. In this paper, the laminar burning velocities and flame thicknesses of these kernels are calculated along the mean turbulent flame front, using tabulated detailed chemistry flamelets over a wide range of stoichiometry and exhaust gas dilution. The criteria for flame propagation include chemical (cross-over temperature based) and turbulence (Karlovitz-number based) effects. Numerical simulations using ignition models of different physical complexity demonstrate the significance of turbulent mixture fraction and enthalpy fluctuations in the prediction of early flame front propagation. A third paper on SparkCIMM (companion paper to this one) focuses on the importance of molecular fuel properties and flame curvature on early flame propagation and compares computed flame propagation with high speed combustion imaging and computed heat release rates with cylinder pressure analysis.The goals of SparkCIMM development are to (a) enhance our fundamental understanding of ignition and combustion processes in highly-turbulent highly-stratified engine conditions, (b) incorporate that understanding into a physically-based submodel for RANS engine calculations that can be reliably used without modification for a wide range of conditions (i.e., homogeneous or stratified, low or high turbulence, low or high dilution), and (c) provide a submodel that can be incorporated into a future LES model for physically-based modeling of cycle-to-cycle variability in engines.     

Effect of initial pressure on flame–shock interaction of hydrogen–air premixed flames

By utilizing a newly designed constant volume combustion bomb (CVCB), turbulent flame combustion phenomena are investigated using hydrogen–air mixture under the initial pressures of 1 bar, 2 bar and 3 bar, including flame acceleration, turbulent flame propagation and flame–shock interaction with pressure oscillations. The results show that the process of flame acceleration through perforated plate can be characterized by three stages: laminar flame, jet flame and turbulent flame. Fast turbulent flame can generate a visible shock wave ahead of the flame front, which is reflected from the end wall of combustion chamber. Subsequently, the velocity of reflected shock wave declines gradually since it is affected by the compression wave formed by flame acceleration. In return, the propagation velocity of turbulent flame front is also influenced. The intense interaction between flame front and reflected shock can be captured by high-speed schlieren photography clearly under different initial pressures. The results show that the propagation velocity of turbulent flame rises with the increase of initial pressure, while the forward shock velocities show no apparent difference. On the other hand, the reflected shock wave decays faster under higher initial pressure conditions due to the faster flame propagation. Moreover, the influence of initial pressure on pressure oscillations is also analyzed comprehensively according to the experimental results.