Modelling of turbulent lifted jet flames using flamelets: a priori assessment and a posteriori validation

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.     

The performance of in situ adaptive tabulation in computations of turbulent flames

This paper presents a detailed characterization of the local and global errors associated with the in situ adaptive tabulation (ISAT) algorithm, which is used in conjunction with a transported PDF method. Calculations of a non-premixed turbulent methane/air piloted jet flame (Sandia flame D) using a skeletal chemical mechanism were performed using ISAT coupled with the computational fluid dynamics (CFD) code FLUENT. The three strategies implemented in ISAT for the growing of the ellipsoids of accuracy (EOAs) are discussed, and the cumulative distribution function (CDF) of the local error is presented for each of the three growing strategies. Computations are also performed to characterize the global error in the ISAT/PDF calculation. The computations used to characterize the global error were performed in parallel to achieve substantial savings in computational time.

In general the local error is well controlled, but there is a small probability of relatively large errors. Results from the investigation suggest that large retrieve errors are due to the region of accuracy (ROA) being non-convex, where the ROA is the connected region for which the error does not exceed the error tolerance, ?_tol. The global error in ISAT is found to be small compared to statistical error for ?_tol ≤ 10^?4, and is found to vary linearly with ?_tol.     

圆柱热尾流中温度的概率密度函数

本文通过实验研究圆柱热尾流中温度的概率密度函数在不同雷诺数下沿中心线的演变以及其与湍流混合程度的关系.实验中雷诺数的取值范围是1200-8600,温度是由直径为0.63 μm的冷线探针测量的.实验结果表明,温度概率密度函数在尾流中场区随空间位置变化显著.雷诺数的增加加快了这个变化过程,特别加速了尾流中心线温度的概率密度函数从非高斯向近似高斯分布的演变.     

计算机网络基础技术中的PDF格式

介绍了计算机网络基础技术中常用的PDF格式的基本结构,讨论了PDF格式的优点及对网络数据处理的影响.     

拉曼光谱在古陶片研究中的应用

利用LRS-Ⅱ型激光拉曼光谱仪对不同年代的古陶片进行了测试研究,用X射线衍射图谱分析确定了古陶片的主要成分。测试结果表明不同年代的古陶片的主要成分相同;从所获得的拉曼光谱图可以看出,随着地质年代的不同,拉曼光谱图向高波数方向移动,对其确定古陶片的历史年代提供了很重要的依据。     

A probability density function of liftoff velocities in mixed-size wind sand flux

With the discrete element method (DEM), employing the diameter distribution of natural sands sampled from the Tengger Desert, a mixed-size sand bed was produced and the particle-bed collision was simulated in the mixed-size wind sand movement. In the simulation, the shear wind velocity, particle diameter, incident velocity and incident angle of the impact sand particle were given the same values as the experimental results. After the particle-bed collision, we collected all the initial velocities of rising sand particles, including the liftoff angular velocities, liftoff linear velocities and their horizontal and vertical components. By the statistical analysis on the velocity sample for each velocity component, its probability density functions were obtained, and they are the functions of the shear wind velocity. The liftoff velocities and their horizontal and vertical components are distributed as an exponential density function, while the angular velocities are distributed as a normal density function. Supported by the Key Project of the National Natural Science Foundation of China (Grant No. 10532040)     

The effect of timescale variation in multiple mapping conditioning mixing of PDF calculations for Sandia Flame series D–F

A stochastic implementation of the multiple mapping conditioning (MMC) model has been used for the modelling of turbulence–chemistry interactions in a series of turbulent jet diffusion flames with varying degrees of local extinction (Sandia Flames D–F). The mapping function approximates the cumulative probability distribution of mixture fraction and the corresponding variance can be controlled by a standard implementation of the scalar mixing timescale. The conditional fluctuations are controlled by a minor dissipation timescale, τ_min. The results show a clear dependence of the conditional fluctuations on the choice of the minor timescale, and the appropriate value for turbulent jet flames is similar to values determined in related direct numerical simulation (DNS) studies of homogeneous turbulent reacting flows. The predictions of means and variances of temperature and species mass fractions are very good for all flames, indicating an appropriate modelling of the conditional variances. Further sensitivity studies with respect to particle number density demonstrate a relative insensitivity of the results to the particle number in the numerical solution procedure. Good results can be obtained with as few as 10 particles per cell, allowing for a computationally inexpensive implementation of a Monte Carlo/probability density function (PDF) method.     

利用Adobe Acrobat9.0 Professional软件实现方正大样的远程校队

学术期刊论文作者是否参与校对及其校对质量直接影响科技期刊编校质量的高低.为了实现作者远程校对,解决一般作者无法打开方正排版大样的问题,利用Adobe Acrobat 9.0 Professional 软件实现了方正排版大样文件与PDF文件之间的转换,并且能使编辑的意图准确表达,实现编辑和作者的直接对话.对于编排校一体化的编辑部来说,真正可以实现方正小样文件与PDF文件的无缝对接,实现作者的远程校对,从而可以有效解决传统邮寄方式所产生的种种问题,缩短校样周期,提高工作效率.     

What Changes in Topochemistry when Going from Small Molecule Dimerizations to Polymerizations in Single Crystals?

This publication promotes the increased necessity for strain management in topochemical reactions with an enlarged structural extension of the molecular products formed, i. e., when going from small molecule dimerizations, through linear polymerizations to the formation of 2D polymers. Further, it promotes to combine the trap model for photon absorption with concrete molecular scale consequences of this absorption on topochemical transformations and briefly discusses the expected consequences topological dimensionality of the forming (macro)molecular products has on trap location. The time appears ripe for going in this direction because local information concerning structural changes within single crystals is now accessible by the 3D-ΔPDF method. This method greatly facilitates the analysis of diffuse X-ray scattering providing access to concrete values of pair distribution functions and, thus, factual information on which and how distances change near a reaction site. Although only based on a first case where distance changes could be quantified in a lateral polymerization, the thoughts put forward may ignite more research towards a full understanding of all the action that occurs when a photochemically triggered topochemical reaction takes place. The 3D-ΔPDF method is so attractive for this purpose because it provides otherwise inaccessible local information in pair correlation functions rather than average structure information, which is used through the ubiquitous Bragg scattering.