弯曲扩压管道中三维不可压粘性流的数值计算及湍流模型的研究

为了便于复杂形状管道中粘性流动的数值计算,本文在任意曲线坐标系下导出了控制方程的Favre质量加权平均形式,导出了一种考虑曲率影响的湍流模型.采用抛物化方法对弯曲扩压管道内的层流和湍流流动进行了数值计算.根据计算结果对压气机静叶流道内的气动性能进行了初步的分析.     

等离子体反应器流场的数值模拟

本文采用颗粒轨道模型对等离子体反应器进行数值模拟.数值格式用SIMPLER法.湍流模型用简单的次网格模型.计算结果给出速度场、温度场及颗粒轨道,对工程设计和工艺过程参数控制有参考价值.     

基于Gauss羽流模型低阶预估旋流燃烧室中守恒标量的空间分布北大核心CSCD

混合分数是表征燃料-空气混合的守恒标量,是湍流燃烧建模的关键参考标量.其空间分布通常通过三维数值模拟获得,然而对于几何形状复杂的燃烧器,三维数值模拟耗时长、成本高,导致燃烧器迭代设计过程效率低.该研究发展了基于Gauss羽流(Gaussian plume)模型的低阶模型来计算旋流燃烧室中的混合分数场,以加速燃料-空气混合策略的评估和参数化设计过程.相比传统的构型,新推导的Gauss羽流模型包含了径向对流的影响和针对旋流来流的修正.进一步发展了镜像反射模型来模拟壁面-羽流的相互作用,并引入相关修正来确保质量守恒.将新推导的Gauss羽流模型应用于甲烷旋流燃烧室混合分数场的低阶预测.基于数值收敛的三维数值模拟生成的数据库,首先采用最小二乘法对模型参数进行优化,然后在宽范围条件下验证了模型的预测精度.该研究不仅为旋流燃烧器内混合分数的快速预测提供了一种新方法,而且为Gauss羽流模型的进一步发展和应用提供了实例.     

近壁区Reynolds应力及湍能耗散率输运特性的理论模型 *

提出在湍流边界层的近壁区采用三维不稳定波来描述湍流相干结构 ,然后根据理论模型对Reynolds应力及湍能耗散率的输运特性进行系统的计算和定量的分析 ,并计算了平均速度分布 .所得理论结果与直接数值模拟结果符合较好 ,表明本文方法正确地体现了湍流边界层近壁区的物理实质 .这不仅有益于对湍流机制的了解 ,而且可能为湍流的近壁模型化开辟一条新的途径 .     

直接数值模拟／大涡模拟中数值误差影响的研究

通过比较湍流的能谱和总动能,对数值误差(包括混淆误差、离散截断误差)、亚格子模型以及它们之间相互作用对直接数值模拟和大涡模拟的影响进行了系统研究.算例采用了三维各向均匀同性湍流.为了研究复杂几何形状,数值格式采用了谱方法和Pade紧致格式.大涡模型采用了truncated Navior-Stokes(TNS)模型结合Pade离散滤波器.结果表明直接数值模拟中离散误差对结果有很大影响,低阶格式会导致计算发散.而大涡模拟中亚格子模型不仅能表征小尺度对大尺度的影响,而且还缓解了数值误差对计算结果的影响.因而低精度格式也可取得不错的结果.     

突然扩张方管中三维湍流流动的数值模拟

本文运用SIMPLEC算法计算了突然扩张方管中的三维湍流流动,湍流模型采用k-ε模型。计算结果详细反映了突然扩张方管中三维湍流流场。从本文结果可以看出,由于突然扩张方管几何形状非轴对称,且尺寸有限,边壁对流场的作用是不可忽略的。以往文献中常见的二维突然扩张湍流的数值模拟结果与三维情况有较大差别,在靠近边壁的区域差别很大,因此对于突然扩张方管中湍流流动的数值模拟应用三维模拟。本文计算所得突然扩张截面后主回流区长度与实验结果接近。本文方法可为数值模拟突然扩张方管中湍流流场及各物理参数的分布提供有效工具。     

凹腔稳燃超声速燃烧火焰闪回不稳定性的数值研究北大核心CSCD

针对等直截面超燃冲压发动机燃烧室中火焰闪回低频燃烧振荡现象,采用延迟分离涡模拟(DDES)的混合RANS/LES方法结合PaSR湍流燃烧模型进行了三维模拟研究.计算得到了完整的燃烧振荡周期,与实验中的低频燃烧振荡现象较为一致.低频燃烧振荡周期可分为凹腔火焰稳定、火焰回传、火焰吹熄3个阶段.通过分析低频燃烧振荡周期中不同阶段的燃烧流动状态,给出了可能的低频燃烧振荡的形成机制.研究结果表明,在整个低频燃烧振荡周期中燃烧室内没有发生热壅塞,燃烧室提供的背压和燃烧释热是燃烧室内形成低频燃烧振荡的关键.     

颗粒湍能输运方程的两相湍流模型和平面闭式两相射流的数值模拟

在现有的两相湍流数值模拟中,对颗粒湍流普遍采用以局部追随概念为基础的代数模型,其预报结果在很多情况下与实验不符。本文提出了以颗粒湍能输运方程为基础的κ-ε-kκ两相湍流模型,并以平面闭式两相射流为例进行了数值模拟,预报结果与实验符合良好,表明此模型明显优于k-ε-A.P.的颗粒湍流代数模型。     

可压缩均匀各向同性湍流的直接数值模拟

采用8阶精度的中心差分格式及7阶精度的迎风偏斜格式对Re_l = 72~153, M_t= 0.2~0.7的均匀各向同性湍流进行了直接数值模拟, 建立了湍流数据库. 与他人的计算结果吻合十分理想, 说明方法的有效性. 数值结果表明, 采用适当的迎风型差分格式可以克服起动问题(start-up problem)对湍流Mach数的限制, 提高可计算的湍流Mach数, 是可压湍流直接数值模拟的有效方法. 分析了压缩性效应对湍流统计量的影响, 发现压缩性使得湍动能的衰减加快. 探讨了可压湍流中微激波产生的机理, 对流场进行了标度律分析. 发现在本文的Reynolds数和湍流Mach数条件下, 流场中扩展自相似性仍然成立, 同时发现压缩性对标度指数影响不大.     

HF化学激光放大器的交叉弛豫动力学研究

在低气压超声速cwHF化学激光放大器中 ,假设平动和转动非平衡交叉弛豫同时起作用 .对于气流介质的层流和湍流扩散混合两类流动模型 ,分别导出了饱和增益谱的表示式 ,并进行了数值计算 .计算结果表明 ,湍流扩散混合模型与实验结果符合得更好 .     

Evaluation of the approximated diffusion flamelet concept using fuels with different chemical complexity

The ability of flamelet models to reproduce turbulent combustion in devices such as diesel engines or gas turbines has enhanced the usage of these approaches in Computational Fluid Dynamics (CFD) simulations. The models based on turbulent look-up tables generated from counterflow laminar diffusion flames (DF model) permit drastic reduction of the computational cost of the CFD calculation. Nevertheless, for complex molecular fuels, such as n-heptane, the oxidation process involves hundreds of species and the calculation of the transport equations together with the ODE system that models the chemical kinetics for the DF solution becomes unaffordable for industrial devices where hundreds of flamelets are required. In this context, new hypotheses have to be introduced in order to reduce the computational cost maintaining the coherence of the combustion process. Recently, a new model known as Approximated Diffusion Flamelet (ADF) has been proposed with the aim of solving the turbulent combustion for complex fuels in a reduced time. However, the validity of this model is still an open question and has to be verified in order to justify subsequent CFD calculations. This work assesses the ADF model and its ability to reproduce accurately the combustion process and its main parameters for three fuels with different chemical complexity and boundary conditions by its comparison with the DF model. Results show that although some discrepancies arise, the ADF model has the ability to correctly describe the ignition delay and the combustion structure in the auto-ignition zone that is the most relevant one for industrial processes.     

LES of a Confined Configuration using Oscillating Inflow Conditions

In actual turbulent flow problems, in particular in gas turbine combustion chambers, flows are three dimensional, recirculating and turbulent in confined and complex geometries. Instationarities due to turbulence, combustion instabilities and oscillating inflow conditions require a highly time and spatial resolving procedure. 3D Large Eddy Simulations (LES) are therefore used in this work to study two confined coaxial air jets with oscillating velocity inflow conditions. The influences of the inflow, in particular frequency (f = 0 ÷ 1600 [Hz]) and amplitude, on the velocity are analysed.     

Numerical simulation of two-stage combustion in SI engine with prechamber

Burning lean mixture in spark ignition (SI) engine leads to decrease in temperature of combustion process and is one of the methods of limiting nitric oxide emission and increasing the engine efficiency. The two-stage combustion system of stratified mixture (engine with prechamber) can be an effective method of lean mixture combustion. The paper presents the results of three-dimensional modeling of fuel mixture preparation and combustion in SI engine with sectional combustion chamber powered by liquefied fuel. Three dimensional modeling was performed in KIVA-3V code. The modeling results were compared with results obtained from the analysis of experimental measurements of two-stage combustion test engine operating at the Institute of Internal Combustion Engines and Control Engineering (Czestochowa University of Technology). The performed simulations of the combustion process provided data concerning the spatial and temporal distributions of turbulent kinetic energy, pressure, temperature and nitric oxides concentration in the combustion chambers of the engine. The engine model with two-stage combustion system properly represents the real processes which occur in the combustion chambers of the test engine. Pressure and temperature courses in function of CA obtained from the experiment and modeling were in good qualitative and quantitative consistence. Comparison of modeled and measured nitric oxide emissions revealed relatively significant discrepancies. In case of λ = 1.4, the measured values of NO_x concentration were 1.75 times higher than the modeled values. In case of λ = 2.0, the modeled and measured values were close to each other and were within the range of measurement error.     

Embedding quasi laminar 1D flame profiles to model turbulent premixed combustion with a joint PDF method

A new model for turbulent premixed combustion is presented which is based on a joint velocity composition probability density function (JPDF) method. The key idea is a scale separation approach. The method combines the model by Bray, Moss and Libby [1] (BML) for premixed combustion with the flamelet approach for nonpremixed combustion. Here, a Lagrangian formulation of the BML model is considered. The progress variable used by the BML model becomes a computational particle property and its value is triggered by the arrival of the flame front at the particle's position. Similar as in the flamelet approach we assume that the smallest eddies are not small enough to disturb the reactive diffusive flame structure. To resolve the (embedded) quasi laminar flame structure, a flame residence time is introduced. With that residence time, the evolution of the particle composition, including enthalpy, can be determined from precomputed laminar 1D flames. The main challenge with this approach is to model the probability that an embedded flamefront arrives at the particle location, which is necessary to close the chemical source term. Numerical experiments of a turbulent premixed flame show good agreement with experimental data. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Computation of turbulent reactive flows in industrial burners

This paper presents models that are suitable for computing steady and unsteady gaseous combustion with finite rate chemistry. Reynold averaging and large eddy simulation (LES) techniques are used to model turbulence for the steady and unsteady cases, respectively. In LES, the Reynold stress terms are modelled by a linear combination of the scale-similarity and eddy dissipation models while the cross terms are of the scale-similarity type. In Reynold averaging, the conventional k–ε two-equation model is used. For the chemical reactions, a 3-step mechanism is used for methane oxidation and the extended Zeldovich and N₂O mechanism are used for NO formation. The combustion model is a hybrid model of the Arrhenius type and a modified eddy dissipation model to take into account the effects of reaction rate, flame stretch and turbulent intensity and scale. Numerical simulations of a flat pulse burner and a swirling burner are discussed.     

Simulation of methane turbulent swirling flame in the TECFLAM combustor

A presumed probability density function (PDF) model for temperature fluctuation is proposed and formulated in this paper. It incorporates the four-step reaction mechanism of methane combustion. A set of analytical expressions is derived for the time-averaged four-step reaction rates. The model is employed to numerically simulate methane turbulent swirling flame in the TECFLAM combustor. The calculated gas axial, radial and tangential velocities, species mass fractions, temperature, and temperature fluctuation are compared with the measured test data. Agreement is achieved between the calculation and the measurement.     

An Approach to Model Partially Premixed Turbulent Combustion withProbability Density Function (PDF) Methods

In turbulent combustion one distinguishes between premixed, non-premixed and partially premixed combustion. While laminar flamelet models proved to be extremely valuable for a wide range of non-premixed flame simulations, similar approaches are more problematic in the partially premixed regime. Here the laminar flamelet concept for non-premixed turbulent combustion simulations is generalized for the partially premixed regime. Similar as in the unsteady flamelet approach, the joint statistics of a progress variable, mixture fraction and scalar dissipation rate is used to obtain the joint statistics of the compositions from pre-computed flame tables. The required distribution is computed with a joint PDF method and the main differences between the new approach and previous ones, are the pre-computed tables and the way the evolution of the progress variable is calculated. Instead of evolving 1D flamelets, steady 2D solutions of burning flamelets propagating into unburned mixtures with varying mixture fraction are considered. The location of a fluid particle in this 2D laminar flame is defined by its mixture fraction and a burning time, which are modeled for each computational particle used in the PDF method. Numerical experiments of a turbulent lifted diffusion flame and a premixed Bunsen flame demonstrate that this approach can be employed for a wide range of applications. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A self-similar premixed turbulent flame model

This paper is devoted to premixed combustion modelling in turbulent flow. First, we derive a model for the turbulent flame velocity based on the observed self-similarity of the turbulent flame. The model uses the local flame brush width as a fundamental parameter and, therefore, we show how it can be retrieved for numerical implementation. The diffusive property of the brush width is treated in such a way as to theoretically let the brush have a clearly defined boundary propagating at finite velocity. The model, implemented in Star-CD CFD software through user programming, is then numerically tested on three configurations for which another model, the Turbulent Flame Closure model, is known to give very good agreement. Some effects of numerics are commented and results for both models are compared. While based on very different approaches the two models lead to substantially similar results. In this way, we have shown that the local brush width can effectively be used, giving an additional degree of freedom for premixed turbulent combustion modelling.     

Modelling Intermediate Species in Partially Premixed Turbulent Combustion Based on the LES-FGM Method北大核心CSCD

The LES of partially premixed turbulent flame MRB in TU Darmstadt was conducted based on the flamelet-tabulated combustion model FGM, and effects of premixed and partially premixed tabulations on the modelling results were studied. The results show that, different methods of tabulation exhibit limited influences on the predictions of the flame structure, velocity, and major species, but using a partially premixed tabulation largely improves the reliability of modelling intermediate minor species CO and H2. The underlying reason lies in a better inclusion of the fuel-air mixing effects through the partially premixed tabulation, which is built based on laminar counter-flow flames. Adding extra transport equations for the intermediate species improves the predictions of intermediate species, especially given a premixed tabulation adopted; meanwhile, the stretch effects in this turbulent flame are ignorable. The results are significant to guide the high-fidelity simulation of partially premixed turbulent flames based on the flamelet-tabulated combustion model. © 2023 Editorial Office of Applied Mathematics and Mechanics. All rights reserved.     

泄爆诱导的湍流、旋涡和外部爆炸

基于k-ε湍流模型和Eddy-dissipation燃烧模型,采用同位网格SIMPLE算法,对充满甲烷-氧气预混气的带导管柱形泄爆容器向空气中泄爆的情形进行了数值模拟．根据计算结果,分析了泄爆后外流场中可燃云团、火焰和压力的变化过程．结果表明,外部爆炸是因射流火焰点燃高压区中的可燃云团,从而引起的剧烈湍流燃烧所致．同时还讨论了外流场湍流和涡量的分布特征．射流火焰进入外部可燃云团后,湍流主要分布在平均动能梯度较大的区域,而不在火焰阵面上．涡量分布主要受斜压效应的影响,在压力和密度梯度斜交区域,其值较大．