基于RANS和LES的汽油、柴油及其掺混燃料压燃燃烧特性数值模拟研究

研究中首先采用RANS和LES两种湍流模型对汽油、柴油及汽柴油掺混燃料的喷雾进行了气液相贯穿距的标定。基于标定好的喷雾模型,采用RANS与LES对3种燃料在发动机中的燃烧过程开展了数值模拟研究。通过对比RANS与LES对部分预混燃烧数值模拟的差异,揭示了两种湍流模型对缸内流动、燃料输运及燃烧过程的影响机理。结果表明,RANS与LES都能够对柴油及掺混燃料的燃烧过程实现较好的预测,其中LES对汽油部分预混燃烧中滞燃期及放热规律的预测与试验更为接近。同时,LES对3种燃料NOx排放的计算结果都与试验更加接近,这与燃料在缸内放热的位置密切相关。     

二阶矩亚网格燃烧模型对射流火焰的大涡模拟

用二阶矩亚网格燃烧模型对美国Sandia国家实验室测量甲烷／空气射流火焰进行了大涡模拟(LES),与实验数据和用二阶矩输运方程湍流燃烧模型的雷诺平均(RANS)模拟结果进行了对比．LES得到时间平均的温度, 甲烷浓度以及温度脉动均方根值和实验值符合很好．LES时均值和RANS模拟结果接近,LES脉动均方根值优于 RANS模拟结果．LES瞬态结果显示了有燃烧时的拟序结构比无燃烧时的强,同时拟序结构强化了燃烧,湍流射流火焰呈皱折火焰面的状态．     

环境涡流对液体喷雾过程影响的大涡模拟研究

通过数值研究,以柴油机燃烧室的高温高压环境为基础分析了不同环境涡流对液体喷射过程的影响。在KIVAERC程序中加入大涡模拟(LES)程序,通过三维计算得到三个不同涡流比下液雾及蒸气的发展过程,同时对比了RANS计算结果。研究结果表明:随着流场涡流的增加,喷雾场结构变得更复杂,涡量结构得到增强。采用LES方法可以有效地反映涡流对喷雾结构的影响。     

超临界煤粉炉燃烧特性的大涡模拟研究

计算流体力学(CFD)在锅炉的研究与开发中有着广泛应用．近年来,大涡模拟(LES)对煤粉射流火焰的模拟得到不断改进和验证,是一种潜力巨大的模拟手段．本文采用LES方法模拟660 MW超超临界二次再热锅炉的炉内过程,研究了煤粉在炉内的扩散及燃烧特性．通过与雷诺时均法(RANS)对比,分析了LES方法模拟大尺度空间气固流动及反应过程中的特点．结果表明,湍流脉动对颗粒扩散行为具有重要影响,采用LES方法能够提供湍流信息在时间尺度上的发展特征,因而显著提升了对流场特征、燃料颗粒扩散以及煤粉着火等关键过程的预测精度,有助于加深对炉内燃烧过程的理解．     

旋流数对燃烧不稳定性及NO_x生成的影响

采用大涡模拟方法分析了旋流数对燃气轮机燃烧室内预混燃烧不稳定性以及NO_x生成特性的影响.结果表明:增大旋流数使得流场的扩张角增大,中心回流区范围扩大,对燃烧产物的卷吸能力增强,预混段内温度升高,高温区范围扩大,有利于燃料气流的着火与稳定燃烧,火焰长度也有所缩短;旋流数为0.7时,流场中仅存在一个进动涡核,旋流数较大时,则出现2个明显的进动涡核;增大旋流数使得涡旋周期性的脱落频率增加,破碎位置向上游移动,同时由于火焰长度缩短,热释放区域相对更为集中,从而导致燃烧室内压力脉动频率及其对应的压力峰值增大;增大旋流数也使得火焰宽度增大,峰值温度有所降低,有利于控制NO_x排放体积分数.     

甲烷预混多喷嘴阵列燃烧器热声振荡模态实验研究

为研究燃气轮机模型燃烧室的非预混燃烧流场,采用大涡模拟方法分别结合火焰面生成流形模型（FGM）和部分预混稳态火焰面模型（PSFM）对甲烷/空气同轴射流非预混燃烧室开展了数值模拟研究,并与试验结果进行对比。结果表明：FGM所预测的速度分布、混合分数分布、燃烧产物及CO分布与试验结果更符合;两种模型均能捕捉到燃烧室中的火焰抬举现象;燃烧过程中的火焰结构较为复杂,同时存在预混燃烧区域和扩散燃烧区域,扩散燃烧主要分布在化学恰当比等值线附近,预混燃烧区域主要分布在贫油区。     

基于分离涡方法的定日镜风效应研究

选用RANS湍流模型、大涡模拟与分离涡模型等计算流体动力学(CFD)方法,应用由新的湍流脉动流场产生方法DSRFG(discretizing and synthesizing random flow generation)模拟风场实际的湍流边界条件,对定日镜结构的风压分布及流场进行分析。将数值模拟结果与风洞试验数据进行对比,给出风向角为0°工况下定日镜结构的风压系数分布规律及特点,分析5种模型下定日镜周围的流场分布和涡量分布。结果表明,RANS模型虽然能够模拟出结构周围流场的基本规律,但却无法模拟出风场中的紊流和涡旋的分离扩散情况。大涡模拟及分离涡模型对风场中的紊流和涡旋的分离扩散有较好的模拟效果,在风压系数的分布上也与风洞试验数据拟合较好。在定日镜周围流场中脉动风压系数的分布上,分离涡模型的模拟结果较大涡模拟更为接近风洞试验值,且耗时更少。     

稀疏化动态模态分解算法在发动机缸内流场研究中的应用

采用大涡数值模拟方法模拟了发动机缸内冷态流场,连续计算100个周期,获得了缸内多循环流场数据库,模拟结果通过粒子图像速度场测量技术(PIV)测量试验进行了验证.然后,采用动态模式分解(DMD)算法分析动态非线性系统流场数据库,以识别其流动特性.结果表明:DMD算法能够有效识别缸内涡团脉动频率,提取对应的流场结构,有利于发现在发动机整个工作过程中具有大衰变率的不稳定流场结构.此外,改进的"稀疏化"DMD算法可有效地对最重要的流场结构进行低维近似,这将有利于寻找影响和控制发动机缸内流场动态演化的方法.     

旋转流线涡技术对驻涡燃烧室性能的影响

基于凹腔驻涡燃烧室的基本结构,提出了一种新的燃烧流场组织技术———旋转流线涡技术,并通过数值模拟,对比分析了不同进气条件下该技术与传统驻涡燃烧技术对燃烧室涡及燃烧性能的影响。结果表明:与传统驻涡燃烧室相比,旋转流线涡技术燃烧室有更好的流场形式,其在凹腔内形成的涡更靠近凹腔顶端、范围更大,更不易脱落,空气与燃料的掺混更加充分。旋转流线涡燃烧室的燃烧效率高于传统驻涡燃烧室,且火焰长度更短,但总压损失比传统驻涡燃烧室大2%左右,损失了一定的经济效益。     

顺压梯度环境下成型孔气膜冷却射流的大涡模拟

为了研究燃气轮机透平内部流向压力梯度(Streamwise Pressure Gradient, SPG)对气膜冷却效果的影响。采用大涡模拟(Large Eddy Simulation, LES)方法研究了在密度比1.5,吹风比1.6,孔出口加速系数K=1×10~(-6)条件下成型孔的冷却特性。通过实验对LES结果进行验证,比较零压梯度(Zero Pressure Gradient, ZPG)和顺压梯度(Favorable Pressure Gradient, FPG)的雷诺时均模拟(Reynolds Average Navier-Stokes, RANS)结果,并分析LES得到的时均流场和瞬时流场。结果表明:FPG促进发卡涡生成,促进主流和射流的掺混,同时抑制发卡涡抬升,对冷却有利。     

Comparison of Reynolds averaged Navier-Stokes based simulation and large-eddy simulation for one isothermal swirling flow

The flow structure of one isothermal swirling case in the Sydney swirl flame database was studied using two numerical methods. Results from the Reynolds-averaged Navier-Stokes (RANS) approach and large eddy simulation (LES) were compared with experimental measurements. The simulations were applied in two different Cartesian grids which were investigated by a grid independence study for RANS and a post-estimator for LES. The RNG k-ε turbulence model was used in RANS and dynamic Smagorinsky-Lilly model was used as the sub-grid scale model in LES. A validation study and cross comparison of ensemble average and root mean square (RMS) results showed LES outperforms RANS statistic results. Flow field results indicated that both approaches could capture dominant flow structures, like vortex breakdown (VB), and precessing vortex core (PVC). Streamlines indicate that the formation mechanisms of VB deducted from the two methods were different. The vorticity field was also studied using a velocity gradient based method. This research gained in-depth understanding of isothermal swirling flow.     

Numerical investigation of high pressure and high Reynolds diffusion flame using Large Eddy Simulation

Today,with nonstop improvement in computational power,Large-Eddy Simulation（LES） is a high demanding research tool for predicting engineering flows.Such flows on high pressure condition like diesel engines is extensively employed in ground and marine transportation,oblige the designer to control and predict toxic pollutants,while maintaining or improving their high thermal efficiency.This becomes one of the main challenging issues in decades.In the present work,numerical investigation of diffusion flame dynamics is performed in the near-field of high-Reynolds jet flow on high pressure condition encountered in diesel engine applications.This work discusses the implementation of Partially Stirred Reactor（PaSR） combustion model by the approaches of large eddy simulation（LES）.The simulation results show that LES,in comparison with Reynolds-Averaged Navier-Stokes（RANS） simulation predicts and captures transient phenomena very well.These phenomena such as unsteadiness and curvature are inherent in the near-field of high Reynolds diffusion flame.The outcomes of this research are compared and validated by other researchers＇ results.Detailed comparisons of the statistics show good agreement with the corresponding experiments.     

Numerical simulation of turbulent flow in a Ranque–Hilsch vortex tube

The present work is about numerical simulations of the internal flow in a commercial model of a Ranque–Hilsch vortex tube (RHVT) operating in jet impingement. Simulation of the turbulent, compressible, high swirling flow was performed by both RANS and LES techniques. The effect of different turbulence closure models have been tested in RANS simulations using a first order closure RNG k–ε and, for the first time in this kind of flow, a second order RSM (Reynolds Stress differential Model) closure. RANS computations have been executed on an axis-symmetric two-dimensional mesh and results have been compared with LES ones, obtained over a three-dimensional computational grid. Smagorinsky sub-grid scale model was used in LES. All the calculations were performed using FLUENT^? 6.3.26. The use of a common code for the different simulations allowed the comparison of the performances of the different techniques and turbulence models, avoiding the introduction of other variables.In all the simulations performed, consistency with the real commercial vortex tube model in jet impingement operation has been followed by substituting an axial hot computational exit to the usual radial one. Comparison of the results between RANS simulations performed on both a traditional radial hot outlet computational domain and one with an axial hot outlet, demonstrates the suitability of the computational model adopted in this work, closer to the real geometry of the device, particularly in RANS RSM simulations. Results in different sections of the tube show significant differences in the velocity profiles, temperature profiles and secondary vortex structures, varying turbulence model.The accurate numerical simulation of the flow in a RHVT, resulting in an improved prediction capability of the kinematic and thermal properties of outgoing jets, could allow a correct estimation of the cooling performance of this device in jet impingement operation.     

A fully-coupled simulation of vortical structures in a large-scale buoyant pool fire

A numerical study simulating the temporal vortical structures of a large-scale buoyant pool fire has been carried out using a fully-coupled Large Eddy Simulation (LES) model which incorporates all essential subgrid scale (SGS) turbulence, combustion, radiation and soot chemistry considerations. Based on the strained laminar flamelet approach, a scalar dissipation conditioned SGS combustion model is introduced to distinguish the highly non-equilibrating burn and extinguishment of flamelets commonly found in pool fires. Numerical results from the present model are validated and compared against a one-meter diameter methane pool fire experimental data and predictions from other LES field models. The predicted time-averaged velocity and temperature profiles have been found to be in good agreement with the experimental data and those numerical results. Qualitative comparisons of instantaneous velocity field against experimental data have revealed that the dynamic phenomena of large-scale vortical structures and its associated puffing behaviour of pool fire are well captured. Quantitative comparisons of velocity time history and pulsation frequency also show close agreement against experimentally evaluated quantities.     

Large eddy simulation and laser diagnostic studies on a low swirl stratified premixed flame

This paper presents numerical simulations and laser diagnostic experiments of a swirling lean premixed methane/air flame with an aim to compare different Large Eddy Simulations (LES) models for reactive flows. An atmospheric-pressure laboratory swirl burner has been developed wherein lean premixed methane/air is injected in an unconfined low-speed flow of air. The flame is stabilized above the burner rim in a moderate swirl flow, triggering weak vortex breakdown in the downstream direction. Both stereoscopic (3-component) PIV and 2-component PIV are used to investigate the flow. Filtered Rayleigh scattering is used to examine the temperature field in the leading flame front. Acetone-Planar Laser Induced Fluorescence (PLIF) is applied to examine the fuel distribution. The experimental data are used to assess two different LES models; one based on level-set G-equation and flamelet chemistry, and the other based on finite rate chemistry with reduced kinetics. The two LES models treat the chemistry differently, which results in different predictions of the flame dynamic behavior and statistics. Yet, great similarity of flame structures was predicted by both models. The LES and experimental data reveal several intrinsic features of the low swirl flame such as the W-shape at the leading front, the highly wrinkled fronts in the shear layers, and the existence of extinction holes in the trailing edge of the flame. The effect of combustion models, the numerical solvers and boundary conditions on the flame and flow predictions was systematically examined.     

基于大涡模拟的发动机缸内湍流流动及拟序结构

应用大涡模拟方法对发动机缸内湍流流场进行了三维瞬态数值分析.主要从湍流脉动、湍动能和缸内拟序结构演变等方面考察了发动机缸内流场特性.计算结果表明:相比雷诺平均模型,大涡模拟方法可以更真实地反映发动机循环过程中缸内气体流动的细节和规律.利用大涡模拟结合Q准则判别法可以较好地识别缸内大尺度湍流拟序结构;拟序结构对于缸内大尺度动能的产生及湍流的维持具有关键的作用.RANS类模型则不具备充分捕获大尺度拟序结构的能力.湍流脉动与活塞平均运行速度接近于成正比.     

Turbulent opposed-jet flames: A critical benchmark experiment for combustion LES

Turbulent opposed-jet configurations have gained attention as a challenging test case to validate the mixing and combustion models used in the simulation of turbulent combustion. In general, validation requires comprehensive experimental information on flow and scalar fields, and the emergence of combustion large-eddy simulation (CLES) necessitated more advanced diagnostics. These laser-optical techniques allow measurements not only of single-point statistics but of structural information of the flame, such as correlations, gradients, and structure functions. This paper presents thorough experimental and numerical investigations of one isothermal and two reacting turbulent opposed jets with fuel jets consisting of partially premixed methane. Its focus is on one configuration at and one configuration close to the highest possible Reynolds numbers where flames could be stabilized. The experimental data presented comprise information on axial velocity, main species concentrations, temperature, mixture fraction, scalar dissipation rate, joint probability density functions, and structure functions. These quantities are compared to results of highly resolved CLES to show the configuration's suitability as a critical benchmark for state-of-the art combustion LES.     

Large eddy simulation of a 660 MW utility boiler under variable load conditions

Large eddy simulation (LES) has become a promising tool for pulverized coal combustion with the development of computational fluid dynamics (CFD) technologies in recent years. LES can better capture the unsteady features and turbulent structures of coal jet flame than Reynolds averaged Navier Stokes (RANS). The coal-fired power plants in China are now required to be operated in a wide load range and quickly respond to the electric grid. The boiler performance of variable loads should be evaluated in terms of flow, heat transfer, and combustion processes. In this paper, LES was applied to simulate a 660 MW ultra-supercritical boiler under BMCR (boiler maximum continue rate), 75%THA-100, and 50%THA-100 conditions. The predicted gas velocities agree well with the thermal calculation and the temperature error is less than 130 K. The simulation results show that the operation load has significant effects on the boiler performance. It is also proved that LES can provide guidance for the design and operation of advanced coal-fired boilers.     

格子Boltzmann方法模拟层流对冲预混火焰

运用格子Boltzmann方法对气体燃烧进行了模拟,其中包括了对流、扩散和反应等过程．在模拟中假设化学反应对流场没有影响,因而流场、温度场和组分场没有相互耦合,可以分别用LB方程进行求解．选择层流对冲火焰作为对燃烧的基础计算模拟．该模型的几何特征是有两个相对的相同燃烧喷口喷出燃料与空气的混合气体,而形成稳定的流场．计算结果与传统的Navier—Stokes方法计算得到的结果进行了对比,结果能够较好地吻合,说明格子Boltzmann方法可以对燃烧进行模拟．