湍流燃烧的统一二阶矩模型

提出了一种湍流燃烧统一二阶矩模型。其特点是对包括反应率系数k的脉动和浓度脉动关联在内的所有关联量都统一形式的二阶矩输运方程加以封闭和求解。考虑了化学反应对关联量耗散的影响，代替了已经的两种湍流燃烧二阶矩模型中，或用温度指数函数的级数展开近似，或用单变量概率密度函数乘积模拟联合概率密度函数的近似。用本模型对甲烷－空气射流湍流扩散燃烧进行了数值模拟，将其结果和EBU模型以及以前提出的两种二阶矩模型的模拟结果进行了比较，并用美国国家实验室的标准实验数据加以检验，证实本模型更好，本模型可用于模拟详细反应动力学，其计算量远小无PDF方程模型的计算量。     

分级进风旋流燃烧室内湍流燃烧的数值模拟

应用考虑湍流-化学反应相互作用的代数二阶矩-概率密度函数(PDF)湍流燃烧模型,对分级进风旋流燃烧室内两组工况下的甲烷湍流燃烧进行了数值模拟,得到的二氧化碳浓度和气体轴向脉动速度均方根值分布与实验数据相符合,得到的气体轴向和切向速度、轴向一切向脉动速度关联量、温度和氧气浓度分布与实验数据基本相符合.研究结果表明,选取适当的二次风率可以起到优化燃烧过程的作用.     

旋流燃烧室内甲烷湍流燃烧的数值模拟

为合理考虑湍流-复杂化学反应的相互作用,建立了甲烷湍流四步反应的温度脉动简化概率密度函数(PDF)模型.应用该模型对TECFLAM燃烧室内的甲烷湍流旋流燃烧进行了数值模拟,得到了与实验相符合的气体轴向、径向与切向速度、温度、温度脉动均方根值及甲烷、氧气、二氧化碳与水蒸气质量分数分布.得到的一氧化碳和氢气质量分数分布与实验基本符合.     

高温空气燃烧的模型比较数值研究

为验证适用于高温空气燃烧过程的燃烧模型,应用EBU模型、E-A模型、PDF模型三种燃烧模型模拟了一个2m×2m×6.25m的高温空气燃烧室的燃烧过程,并根据国际火焰协会的实验数据对模拟结果进行了验证与比较。湍流输运模型和辐射传热模型分别采用了Reynolds应力模型(RSM)湍流模型和离散坐标(DO)辐射传热模型。结果表明,在预测燃烧室温度、燃料组分体积分数和出口NO体积分数上,EBU模型预测值比E-A模型和PDF模型更符合实验测量值。EBU模型是三种模型中最适合模拟高温空气燃烧的燃烧模型。     

TRI对湍流火焰模拟中辐射源项的影响

以美国Sandia实验室Flame D为研究对象,探讨由温度脉动引起的湍流辐射交互作用(TRI)对湍流射流火焰模拟中辐射源项的影响.采用κ-ε湍流模型和标量概率密度函数(PDF)结合详细的化学反应机理模拟燃烧过程,球谐函数(P1近似)法、窄带模型的灰带近似模拟辐射换热.吸收系数脉动与辐射强度脉动关联项采用湍流涡团光学薄...     

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。     

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．     

层流小火焰模型在柴油机湍流燃烧中的应用

将湍流燃烧的层流小火焰模型应用于典型的柴油机扩散燃烧过程.以混合分数为自变量,以标量耗散率为参数,建立相空间中的层流小火焰数据库.应用KIVA-3程序模拟内燃机缸内多维湍流流场,并补充求解混合分数的时均值和脉动均方值的湍流输运方程.将两部分结果通过Beta概率密度函数进行耦合积分,便可得到组分质量分数和温度等参数在柴油机工作过程中的时间、空间分布.对一台直喷式柴油机的湍流燃烧过程进行了模拟计算,所得结果符合实际.     

用二阶矩亚网格尺度燃烧模型对丙烷-空气旋流扩散燃烧的大涡模拟

用二阶矩(SOM)亚网格尺度燃烧模型对环缝进燃料的丙烷-空气旋流湍流扩散燃烧进行了大涡模拟(LES).模拟得到统计平均的热态3个方向的速度、湍流度、温度、丙烷、氧和CO2浓度分布,其值与实验数据符合很好.结果表明,二阶矩(SOM)亚网格尺度燃烧模型适用于大涡模拟.环缝进气使湍流脉动强度、各向异性程度和温度分布趋于均匀.     

湍流-化学双时间尺度PDF湍流燃烧模型及其检验

高精度航空发动机燃烧室数值模拟需要准确描述湍流和详细化学反应机理之间的强烈非线性相互作用,采用概率密度函数湍流燃烧模型(PDF)的大涡模拟方法(LES)可望达到此目的,现有PDF小尺度混合模型需要考虑化学反应的影响.本文在前人研究基础上,对小尺度混合模型中的混合时间尺度进行了修正,提出3种含湍流-化学反应双时间尺度的PDF模型,在基于LES-PDF方法的Aero Engine Combustor Simulation Code(AECSC)程序基础上,对Sandia实验室的甲烷射流火焰Flame D和Flame E进行了数值模拟,其瞬态结果显示,几种模型都能预测射流火焰的瞬时流动状态.将计算得到的标量平均值统计结果与实验数据进行对比,结果表明,采用算术平均修正的PDF模型计算结果与实验数据最接近.湍流-化学反应双时间尺度PDF模型需要进一步检验.     

Comparison between a composition PDF transport equation model and an ASOM model for simulating a turbulent jet flame

A composition PDF transport equation (PDF) model and an algebraic second-order moment (ASOM) model of turbulent combustion are used to simulate a methane–air turbulent jet flame, measured by the Sandia National Laboratory. In most regions, both PDF and ASOM predictions are in agreement with the experimental data with not too much difference. The PDF modeling results give the second-order moments with distributions having a similar trend as those given by the ASOM closure model. Although in general the PDF modeling results are somewhat better than the ASOM results, but considering that the computation time of the ASOM model is almost in two orders of magnitude smaller than that of the PDF model, the ASOM model is suggested for simulating large-size engineering facilities.     

Simulation of Swirling Turbulent Flow and Combustion in a Combustor

An algebraic concentration moment (ACM)-PDF turbulent combustion model is proposed. It is formulated by jointly utilizing the explicit algebraic expressions for the second-order-moment of concentration fluctuation and the presumed probability density function (PDF) of gas instantaneous temperature. A set of analytical expressions for the time-averaged temperature relevant quantity is obtained for the closure of the time-averaged reaction rate. The model is applied to the simulation of turbulent flow and combustion in a swirl combustor. The calculated gas velocity, temperature, species concentrations, and turbulent fluctuating velocity are in agreement with the measured data. They are much improved over those obtained by the EBU-Arrhenius model.     

DNS-LES Validation of an Algebraic Second-Order-Moment Combustion Model

Direct numerical simulation (DNS) of three-dimensional turbulent reacting channel flows with buoyancy is carried out using a spectral method. Statistical results from the DNS database are used to validate an algebraic second-order-moment sub-grid-scale (ASOM-SGS) combustion model and show that the ASOM-SGS model is reasonable. Furthermore, a methane–air jet flame is simulated by large–eddy simulation (LES) using the ASOM-SGS model and indicates that the Reynolds–averaged Navier-Stokes ASOM combustion model is a reasonable model.     

Large-eddy simulations (LES) of temperature fluctuations in a mixing tee with/without a porous medium

Temperature fluctuations in a mixing tee were simulated with and without a porous media in FLUENT using the LES turbulent flow model with the sub-grid scale (SGS) Smagorinsky–Lilly (SL) model with buoyancy. The normalized mean and fluctuating temperatures are used to describe the time-averaged temperatures and the time-averaged temperature fluctuation intensities. For the tee junction without the porous media, the predicted normalized mean temperature and temperature fluctuations compare well with previous experimental data. Comparison of the numerical results with the porous media with both experimental and numerical data without porous media shows that the porous media significantly reduces the temperature fluctuations. Moreover, analysis of the temperature fluctuations and the power spectrum densities (PSD) at the locations having the strongest temperature fluctuations in the tee junction shows that the porous media significantly reduces the thermal fatigue effects and can be useful in various structures such as tee junctions, elbows, piping systems.     

Lifted methane-air jet flames in a vitiated coflow

The present vitiated coflow flame consists of a lifted jet flame formed by a fuel jet issuing from a central nozzle into a large coaxial flow of hot combustion products from a lean premixed H₂/air flame. The fuel stream consists of CH₄ mixed with air. Detailed multiscalar point measurements from combined Raman-Rayleigh-LIF experiments are obtained for a single base-case condition. The experimental data are presented and then compared to numerical results from probability density function (PDF) calculations incorporating various mixing models. The experimental results reveal broadened bimodal distributions of reactive scalars when the probe volume is in the flame stabilization region. The bimodal distribution is attributed to fluctuation of the instantaneous lifted flame position relative to the probe volume. The PDF calculation using the modified Curl mixing model predicts well several but not all features of the instantaneous temperature and composition distributions, time-averaged scalar profiles, and conditional statistics from the multiscalar experiments. A complementary series of parametric experiments is used to determine the sensitivity of flame liftoff height to jet velocity, coflow velocity, and coflow temperature. The liftoff height is found to be approximately linearly related to each parameter within the ranges tested, and it is most sensitive to coflow temperature. The PDF model predictions for the corresponding conditions show that the sensitivity of flame liftoff height to jet velocity and coflow temperature is reasonably captured, while the sensitivity to coflow velocity is underpredicted.     

Simulations of transient n-heptane and n-dodecane spray flames under engine-relevant conditions using a transported PDF method

Time-dependent Reynolds-averaged CFD is performed for transient turbulent spray flames in a high-pressure, constant-volume chamber for two single-component fuels using skeletal chemical mechanisms. The simulations span a range of initial pressures, temperatures and compositions that correspond to conventional and advanced (e.g., low-temperature) compression-ignition engine combustion. The objectives are to establish the extent to which turbulent fluctuations in composition and temperature influence ignition delays and lift-off lengths and turbulent flame structure under engine-relevant conditions, and to provide insight into turbulence-chemistry interactions. This is done by comparing results from a model that accounts for turbulent fluctuations using a transported composition probability density function (PDF) method with those from a model that ignores the influence of turbulent fluctuations on local mean reaction rates (a locally well-stirred reactor – WSR – model). For robust diesel combustion conditions, the WSR and PDF computed ignition delays and lift-off lengths are close to each other, and both are in good agreement with experiment. For lower initial temperatures, ignition delays and lift-off lengths from the two models are significantly different, and the results from the PDF model are in better agreement with experiment. The differences are especially striking for n-dodecane. There the PDF-model computed ignition delays and lift-off lengths are within 10% of measured values for initial temperatures of 900 K and higher (for 22.8 kg/m³ density, 15% oxygen), while the WSR model predicts an ignition delay that is three times the measured value at 900 K. At an initial temperature of 800 K, the WSR model fails to ignite, whereas the PDF model computed ignition delay and lift-off length are within 30% of the measured values. In all cases, the WSR and PDF models produce significantly different turbulent flame structures, and the differences increase with decreasing initial temperature and oxygen level. The WSR model produces a thin laminar-like flame, while the PDF model gives a broadened turbulent flame brush that is qualitatively more consistent with what is expected for these highly turbulent flames and what is observed experimentally. Thus, while it may be possible to reproduce some global ignition characteristics using a WSR model (depending on the choice of chemical mechanism), turbulent fluctuations play an increasingly important role at lower initial temperatures and oxygen levels.