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本文对几种二阶矩湍流反应模型,包括统一二阶矩模型、只考虑温度脉动的二阶矩模型、只考虑浓度脉动关联的二阶矩模型、及同时考虑温度脉动和浓度脉动关联的二阶矩模型,进行了比较。将上述模型加入到FLUENT6.0软件平台上,模拟了不同旋流数下甲烷-空气的旋流燃烧。模拟结果和实验结果进行了比较,探讨了各关联量大小及其对时平均反应率的影响。结果表明,统一二阶矩湍流反应模型具有最好的模拟效果。其原因是,各种关联矩中,化学反应率系数与浓度间的脉动关联最重要。 相似文献
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采用亚网格动能(k方程)应力模型、二阶矩(SOM)燃烧模型和欧拉拉氏两相流模型,对乙醇-空气液雾燃烧进行了大涡模拟(LES)。瞬态结果显示:在火焰的高温区域,旋涡强度较大;在高温区边缘附近存在的拟序结构有脱落的趋势。在燃烧装置的燃料进口附近,近喷嘴中心区域,大量液滴聚集在条状湍流拟序结构的周围。LES模拟的统计结果给出的温度分布与实验结果吻合较好。说明SOM燃烧模型适用于液雾两相湍流燃烧研究,计算结果经过和实验数据对比发现,LES-SOM燃烧模型优于RANS-PDF及LES-FA计算结果。数值计算结果与实验结果的误差主要是由于采用统观一步反应机理引起的,表明燃烧模型还有待进一步改进。 相似文献
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一种改进的代数二阶矩燃烧模型被应用在Flame-D的大涡模拟中。代数二阶矩模型能够将化学反应速率的脉动项进行模拟,更准确地计算化学反应速率。在实际燃烧过程中,化学反应速率不仅与当地的温度以及组分浓度等相关,更与反应物的混合速率有密切的关系。研究考虑了亚网格尺度内混合速率对化学反应速率的影响,提出了改进的代数二阶矩模型。通过与相同模拟条件下的涡耗散概念模型的模拟结果以及实验结果进行对比,改进的代数二阶矩模型的准确性得到了验证,同时亚网格化学反应速率的模拟对总化学反应速率的影响也得到了研究。 相似文献
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旋流燃烧NO生成的USM湍流反应模型 总被引:2,自引:0,他引:2
用统一二阶矩(USM)湍流反应模型对不同旋流数下甲烷-空气旋流燃烧NO生成进行了数值模拟,并和EBU-Arrhenius(E-A)燃烧模型对燃烧的模拟结果和简化PDF湍流反应模型对NO生成的模拟结果以及相应的实验结果进行对比。结果表明,USM模型显著地优于E-A模型和简化PDF模型。E-A模型不能合理地模拟有限反应动力学,而简化PDF模型用两个单变量PDF的乘积代替联合PDF,大大地低估了NO湍流反应率。USM模型预报结果和实验结果符合最好。 相似文献
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湍流燃烧的新二阶矩模型 总被引:13,自引:0,他引:13
本文针对湍流燃烧中模拟包括污染物生成的详细有限反应动力学的要求,用二阶矩封闭与简化概率密度的概念相结合的方法,提出了湍流燃烧的新二阶矩模型。此模型可兼顾适用于工程问题中的合理性和经济性。 相似文献
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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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深入研究库仑碰撞,对两种库仑碰撞模型-Ta模型与Nanbu模型在理论上进行了对比分析,详细阐述了两种模型中散射角大小的区别.在已有的采用Ta模型的全三维粒子模拟/蒙特长罗(PIC/MCC)算法基础上,采用Nanbu模型对电子间库仑碰撞计算模块重新进行了算法设计.分别应用Ta模型、Nanbu模型和无库仑碰撞的全三维PIC/MCC算法对国外热门负氢离子源JAEA 10A中的电子能量沉积进行了模拟分析.模拟结果与实验结果的对比分析发现:1)库仑碰撞使电子能量分布更接近于麦克斯韦分布;2)相对于传统的Ta模型,Nanbu模型的散射角考虑了多体碰撞的累加效果从而具有更高的期望值,按其模拟得到的电子温度具有更高的精度.这些为国内外学者设计相关算法指明了方向. 相似文献
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Yue-Bo Liu 《中国物理 B》2021,30(12):128102-128102
The uniform distribution model of the surface donor states in AlGaN/GaN heterostructures has been widely used in the theoretical calculation. A common and a triple-channel AlGaN/GaN heterostructure Schottky barrier diodes have been fabricated to verify the models, but the calculation results show the uniform distribution model can not provide enough electrons to form three separate 2DEGs in the triple-channel AlGaN/GaN heterostructure. Our experiments indicate the uniform distribution model is not quite right, especially for the multiple-channel AlGaN/GaN heterostructures. Besides, it is found the exponential distribution model possibly matches the actual distribution of the surface donor states better, which allows the 2DEG to form in each channel structure during the calculation. The exponential distribution model would be helpful in the research field. 相似文献
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In this paper, a new mechanism for the emergence of scale-free
distribution is proposed. It is more realistic than the existing
mechanism. Based on our mechanism, a model responsible for the
scale-free distribution with an exponent in a range of 3-to-5 is
given. Moreover, this model could also reproduce the exponential
distribution that is discovered in some real networks. Finally, the
analytical result of the model is given and the simulation shows the
validity of our result. 相似文献
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对于Baraba′si提出的基于人类行为的排队模型,比较了Baraba′si的解与Va′zquez的解,指出这两个解存在差异.提出一个服务台具有服务时间的人类行为动力学模型.通过严格的解析,计算出事件的等待时间分布,得到事件在系统中的逗留时间分布(这个指标在Baraba′si模型中没有被获得).结果表明,按照优先权选择协议,这个模型的等待时间分布是指数γ=2的幂律分布,这与钱学森通信回复时间分布的指数2.1非常接近. 相似文献
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Santiago Aja-Fernández Gonzalo Vegas-Sánchez-Ferrero Antonio Tristán-Vega 《Magnetic resonance imaging》2010
A model for the distribution of the sample local variance (SLV) of magnetic resonance data is proposed. It is based on a bimodal Gamma distribution, whose maxima are related to the signal and background areas of the image. The model is valid for single- and multiple-coil systems. The proposed distribution allows us to characterize some signal/background properties in MR data. As an example, the model is used to study the effect of the background size over noise estimation techniques and a method to test the validity of background-based noise estimators is presented. 相似文献
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S. Kato T. Fujimori A.P. Dowling H. Kobayashi 《Proceedings of the Combustion Institute》2005,30(2):1799-1806
This paper describes the effect of flame position and its spatial variation on prediction accuracy of combustion oscillation in a dry low emission (DLE) combustor. A one-dimensional linear model has been developed. The flame is usually treated as fixed and located at a single axial plane in conventional analysis. However, in practice, the flame position varies during operation. A new flame model considering this variation by a spatial distribution function has been developed. Variation of flame position is empirically measured by using UV images of OH radicals in the oscillating flame. A triangular distribution function is introduced into the flame model because it is similar to the experimentally obtained distribution function. A ‘top-hat’ distribution is also considered to test the influence of distribution shape on the result. Numerical results are compared with experimental data. The triangular flame model shows better prediction of the stability boundaries of combustion oscillation compared with the simple flame sheet model. The results of the top-hat flame model differ from those of the experiment. It is found that consideration of the spatial distribution yields good results for the DLE combustor. 相似文献
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A generic model is introduced for brittle fragmentation in D dimensions, and this model is shown to lead to a fragment-size distribution with two distinct components. In the small fragment-size limit a scale-invariant size distribution results from a crack branching-merging process. At larger sizes the distribution becomes exponential as a result of a Poisson process, which introduces a large-scale cutoff. Numerical simulations are used to demonstrate the validity of the distribution for D=2. Data from laboratory-scale experiments and large-scale quarry blastings of granitic gneiss confirm its validity for D=3. In the experiments the nonzero grain size of rock causes deviation from the ideal model distribution in the small-size limit. The size of the cutoff seems to diverge at the minimum energy sufficient for fragmentation to occur, but the scaling exponent is not universal. 相似文献
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The double Pareto-positive stable (dPPS) distribution is introduced as a new model for describing countries’ global current account balance data. The dPPS distribution provides a flexible model for fitting the entire range of a set of current account data (both surplus and deficit), where zero and unimodality are possible, and the double-sided Pareto distribution is included as a particular case. Expressions for the cumulative distribution, probability density and quantile functions are given. An estimation method is discussed and a simple graphical method for studying the adequacy of the data to model is given. Finally, we consider the fit of countries’ global current account balance data for several years. The new distribution is compared with four classical models: Normal, Skew Normal, Asymmetric Laplace and Hyperbolic distributions. In all the data sets considered, the dPPS distribution outperforms the fits of these four distributions. 相似文献
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