钝体驻定湍流扩散火焰的数值研究——燃烧模型比较

分别采用标量联合的概率密度函数方法、稳态火焰面模型、Euler非稳态火焰面模型和基于有限体积/Monte Carlo混合算法的完备PDF模型对钝体驻定的Sydney湍流扩散火焰HM1进行数值模拟,以比较不同燃烧模型的性能,并比较标量联合的概率密度函数方法和Euler非稳态火焰面模型对氮氧化物排放预测的差异.计算结果和实验数据的比较表明,采用概率密度函数方法计算化学反应可以得到更好的结果但计算量较大,而用火焰面模型求解计算量较小,在接近完全燃烧的情形下,其计算结果比较合理.     

自定义标量法模拟丙烷燃烧过程

采用自定义标量法模拟丙烷扩散燃烧,该方法通过把反应组分定义为Fluent程序的自定义标量、化学反应速率作为源项求解质量、动量、能量和组分守恒方程,并用化学反应引起的能量变化修正能量方程.考虑了详细的化学反应机理,整个燃烧反应机理包括27种化学物质(不含N2)和83个基元反应.合理地模拟出了丙烷的燃烧过程,并将火焰的长度、温度、丙烷、氧气以及中间产物的分布与实验数据进行比较.     

受限湍流射流扩散火焰的PDF模拟

采用k-ε双方程模型和概率密度函数(PDF)相结合的办法,研究受限条件下的湍流射流扩散火焰,着重考虑受限条件下固体壁面、压力梯度等因素对速度场和标量场求解的影响,并在此基础上对两个不同尺寸的受限湍流燃烧场进行计算,分别研究了受限湍流射流扩散火焰的流场结构、火焰结构和火焰形状.最后给出结果,定性分析,并得出结论.     

甲醇喷雾湍流反应流的JPDF数值模拟

喷雾湍流燃烧过程中,液滴、湍流和化学反应之间强烈耦合,物理化学机理非常复杂。本文将速度-标量-频率联合概率密度函数JPDF输运方程方法应用于两相喷雾湍流反应流问题,利用火焰面模型解耦流动和化学反应动力学的耦合关系,建立起相应的数值计算模型。采用Monte-Carlo数值计算方法,针对澳大利亚悉尼大学Masri等人以甲醇为燃料所进行的湍流喷雾燃烧值班火焰这一试验进行了数值模拟,通过与Fluent下的计算结果及试验结果的对比分析,验证了本文所建模型的准确性。     

层流拉伸非预混火焰影响因素的数值研究

采用简化机理(19个基元反应,14种组分)和半详细机理(79个基元反应,32种组分)对层流拉伸非预混火焰的结构进行了数值模拟,通过与实验数据的对比发现:在由当量混合的标量耗散率表征的火焰拉伸一定时,考虑不同分子扩散系数和不同的化学反应机理均对火焰面结构的准确模拟有着重要影响.此外计算了考虑热辐射时,随着当量混合的标量耗散率的改变而变化的每单位火焰面面积放热速率和燃烧场的最高温度,并分析了层流非预混火焰的熄火现象.     

钝体后湍流预混燃烧的PDF模拟

本文采用PDF方法对矩形燃烧室内钝体后的湍流预混火焰进行了数值模拟。脉动速度-频率-标量联合的PDF输运方程用Monte Carlo方法求解,质量、动量和能量的平均值由基于无结构网格的有限体积法求解,压力通过状态方程获得。PDF方程中所需的平均密度、平均速度和压力由有限体积法提供,并将用Monte Carlo方法求出的雷诺应力、化学反应源项和比热比传递给有限体积法。本文对丙烷和空气燃烧的不同简化化学反应机理进行了研究,并与实验结果进行比较,获得满意的结果。     

不同前驱体模型对航空煤油扩散火焰碳烟颗粒生成的影响研究

采用不同的航空煤油化学反应机理和碳烟成核模型对气态航空煤油扩散火焰中碳烟颗粒的质量浓度和数量浓度进行预测.分别采用航空煤油详细化学反应机理和简化化学反应机理,结合非预混稳态扩散火焰面模型模拟燃烧反应.分别采用C2H2成核模型(基于乙炔浓度)和PAH成核模型(基于多环芳香烃浓度)预测碳烟颗粒浓度分布.研究结果表明,采用详细化学反应机理和PAH成核模型对碳烟体积分数的预测值与试验值吻合很好.相比于C2H2成核模型,采用PAH成核模型对碳烟体积分数的预测精度显著提升.     

U-RANS／PDF方法计算钝体后漩涡脱落

本研究发展了U-RANS/PDF混合算法研究湍流和化学反应相互作用对燃烧稳定性的影响,采用有限体积和Monte Carlo相结合的方法在非结构网格中求解相容的U-RANS方程和脉动速度-湍流频率-标量的联合PDF方程.本文对钝体火焰驻定器后冷态流场进行了计算,结果表明此混合算法能够捕捉流场中非稳态的漩涡脱落现象.着重研究了湍流频率模型系数的改变对漩涡脱落频率以及拟序结构在动量输运中的作用的影响.     

湍流扩散燃烧的数值研究-PDF方法和火焰面模型的性能比较

采用标量概率密度函数(PDF)方法、稳态和非稳态火焰面模型三种方法对一个值班湍流CH_4/O_2/N_2射流扩散火焰(Sandia Flame D)进行数值计算,以比较不同燃烧模型的性能。PDF方法通过计算反应标量的PDF输运方程来得到标量分布,而火焰面模型只求解单标量混合物分数的PDF方程,组分和温度分布通过火焰面方程的求解或者火焰面数据库的插值得到。计算结果和实验数据对比表明PDF方法计算结果最好但计算量相当大,稳态火焰面模型则反之。综合而言,非稳态火焰面模型的预测结果相对稳态模型有了非常大的改进,而计算量仍然容易接受,非常适合工程应用。     

抬举湍流H2/N2射流火焰的PDF模拟

采用数值目的研究了一个高温燃烧产物环境中的抬举湍流H₂/N₂射流火焰,对火焰的自然和抬举特性进行了研究.采用标量联合概率密度函数(PDF)目的处理详细的化学动力学过程,而湍流流场采用一个多时间尺度(MTS)k-ε湍流模型计算.计算中结合了一套描述氢气氧化的详细化学反应动力学机理.计算结果和实验数据进行了对比,表明所采用的模型可以精确的模拟火焰抬举高度和自然的过程.     

Transported PDF simulation of turbulent CH4/H2 flames under MILD conditions with particle-level sensitivity analysis

Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH₄/H₂ flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.     

An abstraction layer for efficient memory management of tabulated chemistry and flamelet solutions

A large number of methods for simulating reactive flows exist, some of them, for example, directly use detailed chemical kinetics or use precomputed and tabulated flame solutions. Both approaches couple the research fields computational fluid dynamics and chemistry tightly together using either an online or offline approach to solve the chemistry domain. The offline approach usually involves a method of generating databases or so-called Lookup-Tables (LUTs). As these LUTs are extended to not only contain material properties but interactions between chemistry and turbulent flow, the number of parameters and thus dimensions increases. Given a reasonable discretisation, file sizes can increase drastically. The main goal of this work is to provide methods that handle large database files efficiently. A Memory Abstraction Layer (MAL) has been developed that handles requested LUT entries efficiently by splitting the database file into several smaller blocks. It keeps the total memory usage at a minimum using thin allocation methods and compression to minimise filesystem operations. The MAL has been evaluated using three different test cases. The first rather generic one is a sequential reading operation on an LUT to evaluate the runtime behaviour as well as the memory consumption of the MAL. The second test case is a simulation of a non-premixed turbulent flame, the so-called HM1 flame, which is a well-known test case in the turbulent combustion community. The third test case is a simulation of a non-premixed laminar flame as described by McEnally in 1996 and Bennett in 2000. Using the previously developed solver ‘flameletFoam’ in conjunction with the MAL, memory consumption and the performance penalty introduced were studied. The total memory used while running a parallel simulation was reduced significantly while the CPU time overhead associated with the MAL remained low.     

Application of the conditional source-term estimation model for turbulence–chemistry interactions in a premixed flame

Conditional Source-term Estimation (CSE) is a closure model for turbulence–chemistry interactions. This model uses the first-order CMC hypothesis to close the chemical reaction source terms. The conditional scalar field is estimated by solving an integral equation using inverse methods. It was originally developed and has been used extensively in non-premixed combustion. This work is the first application of this combustion model for a premixed flame. CSE is coupled with a Trajectory Generated Low-Dimensional Manifold (TGLDM) model for chemistry. The CSE-TGLDM combustion model is used in a RANS code to simulate a turbulent premixed Bunsen burner. Along with this combustion model, a similar model which relies on the flamelet assumption is also used for comparison. The results of these two approaches in the prediction of the velocity field, temperature and species mass fractions are compared together. Although the flamelet model is less computationally expensive, the CSE combustion model is more general and does not have the limiting assumption underlying the flamelet model.     

Turbulent diamagnetism in flowing liquid sodium

The nature of Ohm's law is examined in a turbulent flow of liquid sodium. A magnetic field is applied to the flowing sodium, and the resulting magnetic field is measured. The mean velocity field of the sodium is also measured in an identical-scale water model of the experiment. These two fields are used to determine the terms in Ohm's law, indicating the presence of currents driven by a turbulent electromotive force. These currents result in a diamagnetic effect, generating magnetic field in opposition to the dominant fields of the experiment. The magnitude of the fluctuation-driven magnetic field is comparable to that of the field induced by the sodium's mean flow.     

Evaluation of a flamelet/progress variable approach for pulverized coal combustion in a turbulent mixing layer

A steady flamelet/progress variable (FPV) approach for pulverized coal flames is employed to simulate coal particle burning in a turbulent shear and mixing layer. The configuration consists of a carrier-gas stream of air laden with coal particles that mixes with an oxidizer stream of hot products from lean combustion. Carrier-phase DNS (CP-DNS) are performed, where the turbulent flow field is fully resolved, whereas the coal is represented by Lagrangian point particles. CP-DNS with direct chemistry integration is performed first and provides state-of-the-art validation data for FPV modeling. In a second step the control variables for FPV are extracted from the CP-DNS and used to test if the tabulated manifold can correctly describe the reacting flow (a priorianalysis). Finally a fully coupled a posteriori FPV simulation is performed, where only the FPV control variables are transported, and the chemical state is retrieved from the table and fed back to the flow solver. The a priori results show that the FPV approach is suitable for modeling the complex reacting multiphase flow considered here. The a posteriori data is similarly in good agreement with the reference CP-DNS, although stronger deviations than a priori can be observed. These discrepancies mainly appear in the upper flame (of the present DNS), where premixing and highly unsteady extinction and re-ignition effects play a role, which are difficult to capture by steady non-premixed FPV modeling. However, the present FPV model accurately captures the lower, more stable flame that burns in non-premixed mode.     

Identifying turbulent jet sound sources

Theoretical models of the formation of the acoustic field of a nonisothermal turbulent flow are analyzed. The paper presents possible methods of constructing solutions to the convective wave equation for a shear parallel nonisothermal turbulent flow. A simplified mathematical model of noise sources in the field of a nonisothermal shear turbulent flow is considered, based on real physical notions on the processes occurring in the jet mixing range, making it possible to approximately estimate the spectral noise characteristics.     

A priori investigation of the constructed PDF model

The constructed probability density function (PDF) model approximates the species and temperature at a point in a general turbulent reacting flow by the species and temperature that evolved in an independent homogeneous turbulent flow. The thermo-chemical PDF is parameterized by a suitable set of lower moments, and tabulated for retrieval in 3D CFD codes. The Linear Eddy Model is used to resolve, affordably, detailed kinetic calculations in the homogeneous turbulence geometry. In this work, the constructed PDF is parameterized by the first two moments of the mixture fraction, and tested against the equilibrium, assumed-shape PDF model, which is parameterized by the same two moments. The models are evaluated by comparing mean species and temperature predictions with experimental measurements at three points in a turbulent, piloted, jet diffusion flame. The constructed PDF model exhibits consistently improved predictions, and is able to capture super-equilibrium intermediate species as well as species governed by slow kinetics, such as the pollutant NO. The advantage of the constructed PDF model is the capability to decouple the finite-rate chemistry from the multi-dimensional CFD simulation, allowing rapid CFD simulations on large meshes.