IMECH-IR  > 非线性力学国家重点实验室
非预混湍流燃烧亚格子混合模型研究
高福杰
Thesis Advisor何国威 ; 张健
2014
Degree Grantor中国科学院研究生院
Place of Conferral北京
Subtype博士
Degree Discipline流体力学
Abstract非预混湍流燃烧是航空发动机等工程问题中的重要物理过程。非预混湍流燃烧的核心问题是湍流和化学反应的相互作用,而湍流混合是该问题的一个关键方面。通过湍流对流作用,大尺度的燃料和氧化剂被撕裂(smear)成小尺度,并通过小尺度的对流和分子扩散作用达到分子水平混合,最终发生化学反应;所以,小尺度的湍流混合是大尺度湍流混合的结果,也是达到分子水平混合并发生化学反应的必要过程,该过程决定了非预混燃烧的化学反应速率以及热释放率。以往关于湍流混合的研究集中于湍流混合的整体(global)特性,随着大涡模拟在非预混湍流燃烧中的发展和应用,我们需要系统研究局部(local,这里指亚格子内)的湍流混合特性。 湍流混合的主要困难在于该物理过程的多尺度性质以及这些尺度之间的相互作用。直接数值模拟方法可以解析湍流混合和燃烧的全部尺度,因此,针对上述问题,本文通过直接数值模拟稳态均匀各项同性湍流场中的被动标量混合,得到了相应的DNS数据库,并据此研究了亚格子湍流混合特性以及整体二元混合特性,考察了亚格子被动标量的假设滤波密度函数(FDF)方法,根据亚格子湍流混合特性,提出了亚格子被动标量的假设双高斯FDF模型,并对该模型进行了先验检验。 本文的主要创新性工作包括: 1. 首次使用直接数值模拟方法在充分发展的均匀各向同性湍流混合过程中观察到了亚格子内非平衡态的二元混合特性,验证了射流实验中类似结论。本文考察了被动标量的滤波密度函数、耗散率的条件滤波、扩散率的条件滤波等物理量――这些物理量刻画了被动标量在亚格子内的混合特性――观察到在亚格子标量方差很大的情况下,这些物理量随亚格子标量的变化曲线依次具有双峰、U型和S型形状;这些现象表明亚格子内的混合具有非平衡态混合特性。前人认为这种局部的非平衡态混合特性,与被动标量的扩散层结构(薄层内的强耗散结构)有关,本文用三维标量场数据显示并考察了扩散层结构,验证了之前关于扩散层结构的推断。本文发现上述亚格子湍流混合特性,在典型的湍流流动――均匀各向同性湍流和射流中一致,证明了上述亚格子混合特性对湍流流动类型不敏感,是一般的亚格子混合特性。本文关于亚格子混合特性的结果,为理解亚格子内的湍流混合过程,及构造新的亚格子湍流混合模型奠定了物理基础。 2.根据上述亚格子湍流混合特性,本文提出了亚格子被动标量的假设双高斯FDF模型,并进行了先验检验。本文指出了扩散层结构将亚格子被动标量混合,分为两个均匀并相对独立的部分,标量在每一个均匀部分接近高斯分布;不存在扩散层的亚格子,混合较为充分和均匀,这时亚格子内的标量场可以视为独立同分布的两部分的合并;本文据此提出了亚格子被动标量的双高斯分布模型。通过二元混合的DNS数据库,本文考察了新的亚格子混合模型预测结果,并与beta模型及DNS的结果进行了比较,验证结果表明假设双高斯FDF模型具有良好的预测亚格子湍流混合的能力。 3. 本文在非预混湍流燃烧中,先验验证了被动标量的假设双高斯FDF模型。首先,使用无限快化学反应速率的化学平衡模型和二元混合DNS数据库验证了上述混合模型,其次,使用有限快化学反应速率条件下的小火焰模型以及过程变量模型结合非预混湍流燃烧DNS数据库,先验验证了上述混合模型;验证结果表明,亚格子被动标量的双高斯FDF模型在非预混燃烧中具有良好的预测能力,能够作为一种候选的混合模型应用于非预混燃烧的大涡模拟。
Other AbstractNonpremixed turbulent combustion is an important physical process in engineering.The interaction of turbulence and chemical reaction is the main problem in nonpremixedcombustion, in which turbulent mixing plays an important role. Turbulentconvection smears the fuel and oxidizer from large scales to small scales; through theconvection at these small scales and the di usion under the smallest turbulent scales,the mixing of fuel and oxidizer reaches molecular level; at last the chemical reactionoccurs. Therefore, mixing at small scales determines the chemistry reaction rate aswell as the heat release rate. In previous studies, the global statistical characteristics of turbulent mixing are adequately investigated. With the development of the LES in turbulent combustion, the characteristics of sub-grid scale (SGS) turbulent mixing need to be studied systematically. The multi scales and interactions among these scales of turbulent mixing process lead great challenges in the study of this problem. Direct numerical simulation (DNS) resolves all scales of a physical process. In the present work, DNSs of nonpremixed combustion in homogeneous and isotropic turbulence under various conditions are implemented. These data are first employed to study the SGS mixing characteristics systematically, then to test the proposed assumed double Gaussian FDF model of SGS scalar mixing. In present work, the new assumed FDF model is also tested in nonpremixed turbulent combustion in a priori way. These studies provide a fundamental understanding of the SGS mixing; the assumed double Gaussian FDF model proposed in the present work is proved to be an excellent alternative in large eddy simulation (LES) of turbulent combustion. The main innovative work of this thesis are as follows: 1. The nonequilibrium SGS mixing characteristics are investigated by first using DNS of developed homogeneous and isotropic turbulent mixing. This work confirms similar conclusions of SGS mixing in experimental studies of a turbulent jet. Using the conditional statistics of the filtered density function (FDF), conditionally filtered dissipation (CFD), conditionally filtered di usion (CFDIF),and so on, the SGS mixing characteristics are investigated. The results show that for large SGS scalar variance these quantities are bimodal, U shaped, transversally S shaped, respectively, which means the SGS mixing is non equilibrium. In the present work, the di usion layer like structure is investigated and visualized using DNS databases of steady turbulent scalar field in a cube, which confirms the inference that it leads locally non-equilibrium mixing characteristic. It is found from the present studies that, these non-equilibrium SGS mixing characteristics are consistent in both homogeneous and isotropic turbulence and a turbulent jet, which means they are general local mixing characteristics.These results on the SGS mixing characteristics provide a fundamental understanding for the SGS mixing and its modeling. 2. An assumed double Gaussian FDF model for the SGS mixing is proposed based on the SGS mixing characteristics. This work confirms that the passive scalar in a subgrid is separated into two relatively well mixed parts by a diffusion layer like structure. The fact is further stressed that the two well mixed parts are relatively independent and the scalar in each of them has an approximately Gaussian distribution. According to the SGS mixing characteristics an assumed double Gaussian FDF model for SGS mixing is proposed and its parameters are determined. The results predicted by the new model is supported by the DNS data of binary mixing in homogeneous isotropic turbulence and compared with the assumed Beta FDF model. The assumed double Gaussian is proved that it grasps the nonequilibrium SGS mixing characteristicsand has an excellent ability to prescribe the mixing in a subgrid. 3. In this work, the assumed double Gaussian FDF model is verified in a priori way in non-premixed turbulent combustion of one step, second order, isothermal reaction. Firstly, DNS databases of binary mixing in homogeneous and isotropic turbulence are used with equilibrium chemistry reaction to test the model. Secondly, flamelet model and flamelet progress method are used with the DNS databases of homogeneous and isotropic turbulent combustion to test the model. The results show that this model has a good predictive ability on SGS mixing. The model is an alternative for scalar mixing at subgrid scales in LES of turbulent combustion.
Language中文
Document Type学位论文
Identifierhttp://dspace.imech.ac.cn/handle/311007/49418
Collection非线性力学国家重点实验室
Recommended Citation
GB/T 7714
高福杰. 非预混湍流燃烧亚格子混合模型研究[D]. 北京. 中国科学院研究生院,2014.
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