共查询到20条相似文献,搜索用时 140 毫秒
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《工程热物理学报》2021,42(5):1318-1324
湍流燃烧模型在燃烧过程数值模拟中十分重要。商业软件中仍然应用的简单模型,如EBU和预设PDF模型,常常不能很好地模拟有限反应动力学。目前通行的湍流燃烧模型,如层流小火焰模型和条件矩模型,只对一定的火焰类型和火焰结构的效果较好。PDF方程模型更通用,但计算量太大,用于大涡模拟更是如此。另一类是统计矩模型,即基于湍流模型的思路,用雷诺展开和取平均,封闭未知项的二阶矩模型,但是遇到了高度非线性的温度指数函数的困难。不同研究者采取了不同的近似处理,都低估了时平均反应率。作者彻底放弃各种近似方法,构建了终版的二阶矩模型,用于不同的单相和两相燃烧的雷诺平均模拟和大涡模拟,得到了实验验证和直接数值模拟的验证。 相似文献
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本文利用直接数值模拟方法对均质压燃(HCCI, Homogeneous Charge Compression Ignition)工况下氨氢混合物的着火和燃烧特性进行了研究。结果表明,着火首先从局部孤立区域处发生,随后发展到整个计算区域;最高燃烧温度和热释率随着掺氢比的增加而增加;通过与零维计算结果对比,发现湍流和热分层使得氨氢混合物着火提前。利用直接数值模拟数据计算了反应锋面的位移速度,并据此分析了自着火和火焰传播这两种燃烧模式。发现在低掺氢比的情况下,燃烧模式以自着火为主;而在高掺氢比的情况下,燃烧模式以火焰传播为主。 相似文献
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可燃悬浮寺燃烧诱导激波可其加速过程研究 总被引:1,自引:0,他引:1
以铝粉为例,基于双流体模型,分别采用TVD格式和MacComack格式计算气相和颗粒相流场,基于Arrihenius定律及管道壁面湍流k-ε模型计算流场反应速度,对水平燃烧管内可燃悬浮粉尘在弱点火条件下激波的产生及加强过程进行了理论分析与数值模拟。研究发现,壁面湍流在火焰加速及燃烧诱导激波过程中起着关键作用,数值计算结果揭示了可燃悬浮粉尘云中压缩波到激波的转捩机制及气固两相流场参数的变化规律。计算 相似文献
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高温空气燃烧炉内湍流混合特性的数值研究 总被引:2,自引:0,他引:2
应用自行研发的三维流动、燃烧、传热和污染物NOx湍流生成的数值模拟程序,对高温空气燃烧实验模型炉进行了湍流扩散燃烧混合特性的数值模拟.数值预报了燃烧室内气体燃料和空气的混合物分数及其湍流脉动的三维分布.数值研究结果表明:在一定的几何条件和气体动力学条件下,高温空气燃烧的湍流混合在更广泛的区域内以较小梯度的进行;混合物分数的脉动主要分布在燃烧区,这表明高温空气燃烧的火焰厚度更大,具有燃烧释热更趋均匀的特性.数值模拟结果与相关的实验结果有相同的规律. 相似文献
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H. Böttler A. Scholtissek X. Chen Z. Chen C. Hasse 《Proceedings of the Combustion Institute》2021,38(2):2031-2039
Many modeling strategies for combustion rely on laminar flamelet concepts to determine structure and properties of multi-dimensional and turbulent flames. Using flamelet tabulation strategies, the user anticipates certain aspects of the combustion process prior to the simulation and selects a flamelet model which mimics local flame conditions in the more complex configuration. Flame stretch, which can be decomposed into contributions from strain and curvature, is one of the conditions influencing a flame’s properties, structure, and stability. The objective of this work is to study premixed flame structures in the strain-curvature space using a recently published composition space model (CSM) and three physical space models for canonical flame configurations (stagnation flame, spherical expanding flame and inwardly propagating flame). Flames with effective Lewis numbers both smaller and larger than unity are considered. For canonical laminar flames, the stretch components are inherently determined through boundary conditions and their specific flame configuration. Therefore, canonical flames can only represent a certain sub-set of stretch effects experienced by multi-dimensional and turbulent flames. On the contrary, the CSM allows arbitrary combinations of strain and curvature to be prescribed for premixed flames exceeding the conditions attainable with the canonical flame setups. Thereby, also influences of negative strain effects and large curvatures can be studied. A parameter variation with the CSM shows that flame structures still significantly change outside the region of the canonical flame configurations. Furthermore, limits in the strain-curvature space are discussed. The present paper highlights advantages of composition space modeling which is achieved by detaching the representation of the flame structure from a specific canonical flame configuration in physical space. 相似文献
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Lu Tian 《Combustion Theory and Modelling》2017,21(6):1114-1147
Premixed turbulent flames feature strong interactions between chemical reactions and turbulence that affect scalar and turbulence statistics. The focus of the present work is on clarifying the impact of pressure dilatation/flamelet scrambling effects with a comprehensive second-moment closure used for evaluation purposes. Model extensions that take into account flamelet orientation and molecular diffusion are derived. Isothermal pressure transport is included with an additional variable density contribution derived for the flamelet regime of combustion. Full closure is assessed by comparisons with Direct Numerical Simulations (DNSs) of statistically ‘steady’ fully developed premixed turbulent planar flames at different expansion ratios. Subsequently, the prediction of lean premixed turbulent methane–air flames featuring fractal grid generated turbulence in an opposed jet geometry is considered. The overall agreement shows that ‘dilatation’ effects contribute to counter-gradient transport and can also increase the turbulent kinetic energy significantly. Levels of anisotropy are broadly consistent with the DNS data and key aspects of opposed jet flames are well predicted. However, it is also shown that complications arise due to interactions between the imposed pressure gradient and combustion and that redistribution is affected along with the scalar flux at the leading edge. The latter is strongly affected by the reaction rate closure and, potentially, by pressure transport. Overall, the derived models offer significant improvements and can readily be applied to the modelling of premixed turbulent flames at practical rates of heat release. 相似文献
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James C. Sutherland Philip J. Smith Jacqueline H. Chen 《Combustion Theory and Modelling》2013,17(2):287-303
In recent years, direct numerical simulations have been used increasingly to evaluate the validity and performance of combustion reaction models. This study presents a new, quantitative method to determine the ideal model performance attainable by a given parameterization of the state variables. Data from direct numerical simulation (DNS) of unsteady CO/H2–air jet flames is analysed to determine how well various parameterizations represent the data, and how well specific models based on those parameterizations perform. Results show that the equilibrium model performs poorly relative to an ideal model parameterized by the mixture fraction. The steady laminar flamelet model performs quite well relative to an ideal model parameterized by mixture fraction and dissipation rate in some cases. However, at low dissipation rates or at dissipation rates exceeding the steady extinction limit, the steady flamelet model performs poorly. Interestingly, even in many cases where the steady flamelet model fails (particularly at low dissipation rate), the DNS data suggests that the state may be parameterized well by the mixture fraction and dissipation rate. A progress variable based on the CO2 mass fraction is proposed, together with a new model based on the CO2 progress variable. This model performs nearly ideally, and demonstrates the ability to capture extinction with remarkable accuracy for the CO/H2 flames considered. 相似文献
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Giampaolo Maio Mélody Cailler Nasser Darabiha Benoît Fiorina 《Proceedings of the Combustion Institute》2021,38(2):2559-2569
Multiple flame regimes are encountered in industrial combustion chambers, where premixed, stratified and non-premixed flame regions may coexist. To obtain a predictive tool for pollutant formation predictions, chemical flame modeling must take into account the influence of such complex flame structure. The objective of this article is to apply and compare two reduced chemistry models on both laminar and turbulent multi-regime flame configurations in order to analyze their capabilities in predicting flame structure and CO formation. The challenged approaches are (i) a premixed flamelet-based tabulated chemistry method, whose thermochemical variables are parameterized by a mixture fraction and a progress variable, and (ii) a virtual chemical scheme which has been optimized to retrieve the properties of canonical premixed and non-premixed 1-D laminar flames. The methods are first applied to compute a series of laminar partially-premixed methane-air counterflow flames. Results are compared to detailed chemistry simulations. Both approaches reproduced the thermal flame structure but only the virtual chemistry captures the CO formation in all ranges of equivalence ratio from stoichiometry premixed flame to pure non-premixed flame. Finally, the two chemical models combined with the Thickened Flame model for LES are challenged on a piloted turbulent jet flame with inhomogeneous inlet, the Sydney inhomogeneous burner. Mean and RMS of temperature and CO mass fraction radial profiles are compared to available experimental data. Scatter data in mixture fraction space and Wasserstein metric of numerical and experimental data are also studied. The analyses confirm again that the virtual chemistry approach is able to account for the impact of multi-regime turbulent combustion on the CO formation. 相似文献
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Shaohong Ruan Nedunchezhian Swaminathan Oliver Darbyshire 《Combustion Theory and Modelling》2013,17(2):295-329
This study focuses on the modelling of turbulent lifted jet flames using flamelets and a presumed Probability Density Function (PDF) approach with interest in both flame lift-off height and flame brush structure. First, flamelet models used to capture contributions from premixed and non-premixed modes of the partially premixed combustion in the lifted jet flame are assessed using a Direct Numerical Simulation (DNS) data for a turbulent lifted hydrogen jet flame. The joint PDFs of mixture fraction Z and progress variable c, including their statistical correlation, are obtained using a copula method, which is also validated using the DNS data. The statistically independent PDFs are found to be generally inadequate to represent the joint PDFs from the DNS data. The effects of Z–c correlation and the contribution from the non-premixed combustion mode on the flame lift-off height are studied systematically by including one effect at a time in the simulations used for a posteriori validation. A simple model including the effects of chemical kinetics and scalar dissipation rate is suggested and used for non-premixed combustion contributions. The results clearly show that both Z–c correlation and non-premixed combustion effects are required in the premixed flamelets approach to get good agreement with the measured flame lift-off heights as a function of jet velocity. The flame brush structure reported in earlier experimental studies is also captured reasonably well for various axial positions. It seems that flame stabilisation is influenced by both premixed and non-premixed combustion modes, and their mutual influences. 相似文献
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W. J.S. Ramaekers J. A. van Oijen L. P.H. de Goey 《Combustion Theory and Modelling》2013,17(6):943-975
In this paper it is investigated whether the Flame Surface Density (FSD) model, developed for turbulent premixed combustion, is also applicable to stratified flames. Direct Numerical Simulations (DNS) of turbulent stratified Bunsen flames have been carried out, using the Flamelet Generated Manifold (FGM) reduction method for reaction kinetics. Before examining the suitability of the FSD model, flame surfaces are characterized in terms of thickness, curvature and stratification. All flames are in the Thin Reaction Zones regime, and the maximum equivalence ratio range covers 0.1?φ?1.3. For all flames, local flame thicknesses correspond very well to those observed in stretchless, steady premixed flamelets. Extracted curvature radii and mixing length scales are significantly larger than the flame thickness, implying that the stratified flames all burn in a premixed mode. The remaining challenge is accounting for the large variation in (subfilter) mass burning rate. In this contribution, the FSD model is proven to be applicable for Large Eddy Simulations (LES) of stratified flames for the equivalence ratio range 0.1?φ?1.3. Subfilter mass burning rate variations are taken into account by a subfilter Probability Density Function (PDF) for the mixture fraction, on which the mass burning rate directly depends. A priori analysis point out that for small stratifications (0.4?φ?1.0), the replacement of the subfilter PDF (obtained from DNS data) by the corresponding Dirac function is appropriate. Integration of the Dirac function with the mass burning rate m=m(φ), can then adequately model the filtered mass burning rate obtained from filtered DNS data. For a larger stratification (0.1?φ?1.3), and filter widths up to ten flame thicknesses, a β-function for the subfilter PDF yields substantially better predictions than a Dirac function. Finally, inclusion of a simple algebraic model for the FSD resulted only in small additional deviations from DNS data, thereby rendering this approach promising for application in LES. 相似文献
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Kaidi Wan Luc Vervisch Jun Xia Pascale Domingo Zhihua Wang Yingzu Liu Kefa Cen 《Proceedings of the Combustion Institute》2019,37(3):2791-2799
The intricate coupling between coal pyrolysis, gas phase combustion and the emissions of alkali metal, such as sodium, is studied in the early stage of a temporally evolving three-dimensional planar turbulent jet carrying pulverized-coal particles. Complex chemistry is used to account for both the combustion of volatile hydrocarbons and the sodium containing species. The response of the sodium chemistry is analyzed in the mixture fraction space, along with the topology of the reactions zones. Combustion is found to start preferentially in partially premixed flames, which then evolve toward diffusion-like reactive layers and reach chemical equilibrium. From the direct numerical simulation (DNS) database, the possibility of modeling the dynamics of sodium species using one-dimensional premixed flamelet generated manifolds (FGM) is investigated. A chemical lookup table is constructed for the combustion of the partially premixed volatiles and an additional three-dimensional simulation is performed to compare the tabulated sodium species against their reference counterparts with complex chemistry. Quantitative analysis of the performance of the developed chemistry tabulation confirms the validity of the approach. Perspectives for the modeling of sodium emissions in pulverized-coal furnaces and boilers are finally drawn. 相似文献
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Andrei Nikolaevich Lipatnikov Shinnosuke Nishiki Tatsuya Hasegawa 《Combustion Theory and Modelling》2013,17(3):309-328
The linear relation between the mean rate of product creation and the mean scalar dissipation rate, derived in the seminal paper by K.N.C. Bray [‘The interaction between turbulence and combustion’, Proceedings of the Combustion Institute, Vol. 17 (1979), pp. 223–233], is the cornerstone for models of premixed turbulent combustion that deal with the dissipation rate in order to close the reaction rate. In the present work, this linear relation is straightforwardly validated by analysing data computed earlier in the 3D Direct Numerical Simulation (DNS) of three statistically stationary, 1D, planar turbulent flames associated with the flamelet regime of premixed combustion. Although the linear relation does not hold at the leading and trailing edges of the mean flame brush, such a result is expected within the framework of Bray's theory. However, the present DNS yields substantially larger (smaller) values of an input parameter cm (or K2 = 1/(2cm ? 1)), involved by the studied linear relation, when compared to the commonly used value of cm = 0.7 (or K2 = 2.5). To gain further insight into the issue and into the eventual dependence of cm on mixture composition, the DNS data are combined with the results of numerical simulations of stationary, 1D, planar laminar methane–air flames with complex chemistry, with the results being reported in terms of differently defined combustion progress variables c, i.e. the normalised temperature, density, or mole fraction of CH4, O2, CO2 or H2O. Such a study indicates the dependence of cm both on the definition of c and on the equivalence ratio. Nevertheless, K2 and cm can be estimated by processing the results of simulations of counterpart laminar premixed flames. Similar conclusions were also drawn by skipping the DNS data, but invoking a presumed beta probability density function in order to evaluate cm for the differently defined c's and various equivalence ratios. 相似文献
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S. K. Sadasivuni W. Malalasekera S. S. Ibrahim 《Russian Journal of Physical Chemistry B, Focus on Physics》2010,4(3):465-474
The unsteady flamelet/progress variable approach has been developed for the prediction of a lifted flame to capture the extinction
and re-ignition physics. In this work inclusion of the time variant behavior in the flamelet generation embedded in the large
eddy simulation technique, allows better understanding of partially premixed flame dynamics. In the process sufficient simulations
to generate unsteady laminar flamelets are performed, which are a function of time. These flamelets are used for the generation
of the look-up table and the flamelet library is produced. This library is used for the calculation of temperature and other
species in the computational domain as the solution progresses. The library constitutes filtered quantities of all the scalars
as a function of mean mixture fraction, mixture fraction variance and mean progress variable. Mixture fraction and progress
variable distributions are assumed to be β-PDF and δ-PDF respectively. The technique used here is known as the unsteady flamelet
progress variable (UFPV) approach. One of the well known lifted flames is considered for the present modeling which shows
flame lift-off. The results are compared with the experimental data for the mixture fraction and temperature. Lift off height
is predicted from the numerical calculations and compared with the experimentally given value. Comparisons show a reasonably
good agreement and the UFPV combustion model appears to be a promising technique for the prediction of lifted and partially
premixed flames. 相似文献