气固两相流场的湍流颗粒浓度理论模型

本文进行了气固两相流动颗粒湍流扩散现象的理论分析，提出了颗粒湍流扩散系数和气流弥散效应二个颗粒湍流模化新概念，在此基础上建立了气固两相流场湍流颗粒浓度模型。理论模型包括离心力和其它外加力场作用下颗粒运动和浓度分布的计算方法。运用湍流颗粒浓度模型，对直管气固两相流动、受限射流气固两相流动和９０°弯管气固两相流动等三种流动做了数值模拟，计算获得颗粒速度、颗粒浓度等主要流动参数。讨论了湍流颗粒浓度模型的适用性。     

方管内气固两相流动速度的LDV测量

介绍了气固两相流动的ＬＤＶ测试方法和有关技术，并给出了方管内网栅后气固两相流动的部分测试结果与分析。对于气相，给出了纯净气流（α＝０）、平均粒径ｄｐ＝３５０μｍ的颗粒（α＝０．１２％、０．２１％、０．３３５％）存在时的气流在管内不同断面上时均速度分布情况及不同颗粒浓度时气流速度结构的比较；对于固相，给出了三种不同固相浓度时颗粒在管内不同断面上时均速度的分布情况，并对三种固相浓度时颗粒与气相的速度分布进行了比较。     

稠密气固两相湍流流动的实验和数值模拟

基于气固两相流动模型计算循环流化床内稠密气固两相流湍流动,颗粒动理学方法模拟颗粒相湍动能,ＳＧＳ模型模拟气相湍流,采用γ－射线密度计和非等速取样管测量局部颗粒浓度和流率,利用ＦＦＴ方法计算颗粒浓度功率谱密度。模拟计算得到上升管内气相和固相速度和浓度分布等。同时数值模拟与Ｔｓｕｊｉ等和Ｋｎｏｗｌｔｏｎ等试验结果进行了比较,结果表明数值模拟计算与实验结果相吻合。     

Y型分支管内气固两相流动的数值模拟

本文采用Euler-Lagrange两相流研究方法,固相采用离散相(DPM)模型,对三种不同夹角的Y型分支管内气固两相流动进行了数值模拟.气相湍流采用Realizable k-ε模型,固相的湍流耗散采用随机轨道模型.模拟结果较好地预估了颗粒在分叉处的流动形态、颗粒在分支管内的运动轨迹,以及重新实现颗粒相流场均匀分布所需的距离.同时发现,随机轨道模型较适于评估分支与主管夹角较大情况下,固体颗粒在分叉处的运动.将分支管内固体颗粒质量分配的数值模拟结果与试验结果比较,发现两者较吻合,相对误差较小.     

环空后台阶管道内气固两相流动的数值模拟研究

环空管道后台阶突扩流动是空气正循环钻井过程中十分重要的关键部分,直接决定了钻探岩屑是否能够顺利上返地面.本文采用Euler-Lagrange两相流研究方法,气相湍流采用Realizable k-ε模型,固相采用离散相模型(DPM),固相的湍流耗散采用随机轨道模型,对环空后台阶突扩管道内气固两相流动进行了数值模拟研究.得出了气相场大涡演变规律,在此基础上研究了不同粒径时颗粒在流道中的浓度分布规律、运动轨迹,以及速度场分布规律.这为细观框架下研究气固两相相互作用规律提供了依据.     

强旋湍流气－固两相流动的颗粒随机轨道法模拟

应用颗粒随机轨道模型，并与一种新的代数Ｒｅｙｎｏｌｄｓ应力模型相结合，对新型煤粉涡旋燃烧炉内强旋湍流气。固两相流动进行了数值模拟。得到了与实验相符合的颗粒相密度分布和质量流分布。计算结果表明，在涡旋燃烧炉内的强旋湍流流场中，外壁附近颗粒浓度最高，颗粒停留时间加长，气－固两相间滑移速度增大。     

稠密气固两相流各向异性颗粒相矩方法

基于气体分子动力学和颗粒动理学方法,考虑颗粒速度脉动各向异性,建立颗粒相二阶矩模型.应用初等输运理论,对三阶关联项进行模化和封闭.考虑颗粒与壁面之间的能量传递和交换,建立颗粒相边界条件模型.采用Koch等计算方法模拟气固脉动速度关联矩.考虑气体-颗粒间相互作用,建立稠密气体-颗粒流动模型.数值模拟提升管内气固两相流动特性,模拟结果表明提升管内颗粒相湍流脉动具有明显的各向异性.预测颗粒速度、浓度和颗粒脉动速度二阶矩与Tartan等实测结果相吻合.模拟结果表明轴向颗粒速度脉动强度约为平均颗粒相脉动强度的1.5倍,轴向颗粒脉动能大约是径向颗粒脉动能3.0倍.     

水平流动边界层内气固相间作用的试验研究

应用三维粒子动态分析仪（threedimensionalparticledynamicsanalyzer）,测量了含有230μm颗粒的气固两相水平流的特性,特别是壁面边界层内的两相流动特性．结果表明颗粒载荷比（质量流率）对相间作用有较大影响,随颗粒流率的增加颗粒对气流平均速度和湍流的影响增大,颗粒使气流速度边界展变薄．颗粒和气流相互作用在不同方向上呈各向异性,颗粒对气流垂直方向的脉动影响较大．颗粒与湍流边界层气流的作用行为大致可以分成三个区域：贴壁区、中间区和外流区．     

大速差射流预燃室内三维回流两相湍流的数值模拟

本文由多流体两相流模型、气相湍流κ-ε模型和颗粒湍流代数模型出发,成功地模拟了真实形状大速差射流预燃室中三维湍流回流两相流动,得到了这类复杂的气固两相流中不同纵横截面上气相速度场、颗粒速度场及浓度场和两相湍流度场的分布,并且获得了与实验定性一致的合理结果,揭示了预燃室中气固两相流动与混合的主要物理特征,探讨了大速差射流技术稳焰和强化燃烧的两相流动机理。     

竖直管道中固—液两相流动流态化实验探讨

主要探讨竖直管道中不同颗粒级配、流体流速条件下的固-液两相流动的流态化规律.首先通过量纲分析获得关键控制参数,然后以玻璃珠(粒径:0.25 mm～2.0 mm)、粉细砂(d10=0.044 mm)两种固体和水为实验介质,开展了两相流动流态化实验,考虑流体流速(相对于管道雷诺数介于640～3 300之间)和颗粒级配的影响.通过分析发现:具有均匀粒径分布的玻璃珠床,床层膨胀高度随流速的增加而增加,流速与浓度的对数呈线性关系,与Richardson--Zaki公式一致;具有较宽粒径分布的粉细砂,细颗粒随水流逐渐流出管道,剩余颗粒质量与雷诺数呈指数递减趋势.     

Particle/Turbulence Interactions, Mass Transfer and Gas/Solid Chemistry in a CFBC Riser

Gas/solid chemistry in the upper, dilute region of a circulating fluidised bed combustor (CFBC) riser is addressed. The limitations of turbulent mixing are illustrated by the example of the heterogeneous NO/CO/char reaction, relevant in CFB combustion of nitrogen-containing solid fuels. The mass transfer of the gaseous reactants to the char surface is determined, and how the conversion is influenced by the degree of mixing of the multiphase system by turbulent dispersion. Particle/turbulence interactions are taken into account by a (Lagrangian) frequency spectrum of velocity fluctuations, which determine the dispersion of the char particles described here with the Tchen–Hinze model. Chars from solid fuels characterised by fuel ratio (FR) ranging from 0.1 (wood) via 0.5 (peat) and 1 (coal A) to 2 (coal B) were considered. The effective rate of the NO/CO/char reaction is determined as a function of the size and type of the char particle, temperature, particle concentration, reactor dimensions and fluidization velocity, at atmospheric pressure. It was found that for this case the effective gas/solid conversion rate in the upper, dilute region of the riser is much lower than the gas/solid chemistry, mainly due to mass transfer limitations for char particles with sizes of typically 300 μm. The concentration of NO at the char particle surface is only a few % of that in the bulk gas phase. Strong influences were found for particle size and temperature, whilst the fluidisation velocity and the reactor size have only a small influence. It is concluded that for a typical CFB riser, for particles larger than approx. 20 μm, mass transfer has a stronger influence on the heterogeneous NO/CO/char reduction mechanism than the ``unmixedness' due to particle eddy dispersion limitations. It is recommended that this or a similar approach to turbulent dispersive mixing is implemented in CFD codes when these are used for boiler and furnace calculations. This revised version was published online in July 2006 with corrections to the Cover Date.     

The First Turbulence and First Fossil Turbulence

A model is proposed connecting turbulence, fossil turbulence and the big-bang origin of the universe. While details are incomplete, the model is consistent with our knowledge of these processes and is supported by observations. Turbulence arises in a hot big-bang quantum gravitational dynamics scenario at Planck scales. Chaotic, eddy-like motions produce an exothermic Planck particle cascade from 10^?35 m at 10³² K to 10⁸ larger, 10⁴ cooler, quark-gluon scales. A Planck-Kerr instability gives high Reynolds number (Re ～ 10⁶) turbulent combustion, space-time-energy-entropy and turbulent mixing. Batchelor-Obukhov-Corrsin turbulent-temperature fluctuations are preserved as the first fossil turbulence by inflation stretching the patterns beyond the horizon ct of causal connection faster than light speed c in time t～ 10^?33 sec. Fossil big-bang temperature turbulence reenters the horizon and imprints nucleosynthesis of H-He densities that seed fragmentation by gravity at 10¹² s in the low Reynolds number plasma before its transition to gas at t～ 10¹³ s and T～ 3000 K. Multiscaling coefficients of the cosmic microwave background (CMB) temperature anisotropies closely match those for high Reynolds number turbulence, Bershadskii, A. and Sreenivasan, K.R., Phys. Lett. A 299 (2002) 149-152; Bershadskii, A. and Sreenivasan, K.R., Phys. Lett. A 319 (2003) 21-23. CMB spectra support the interpretation that big-bang turbulence fossils triggered fragmentation of the viscous plasma at supercluster to galaxy mass scales from 10⁴⁶ to 10⁴² kg, Gibson, C.H., Appl. Mech. Rev. 49 (5) (1996) 299-315; Gibson, C.H., J. Fluids Eng. 122 (2000) 830-835; Gibson, C.H., Combust. Sci. Technol. (2004, to be published).     

Ideal Turbulence

Ideal turbulence is a mathematical phenomenon which occurs in certain infinite-dimensional deterministic dynamical systems and implies that the attractor of a system lies off the phase space and among the attractor points there are fractal or even random functions. A mathematically rigorous definition of ideal turbulence is based on standard notions of dynamical systems theory and chaos theory. Ideal turbulence is observed in various idealized models of real distributed systems of electrodynamics, acoustics, radiophysics, etc. In systems without internal resistance, cascade processes are capable to birth structures of arbitrarily small scale and even to cause stochastization of the systems. Just these phenomena are inherent in ideal turbulence and they help to understand the mathematical scenarios for many features of real turbulence.     

Turbulence Plus

This paper attempts to give a concise overview of the turbulence research performed at the Laboratory for Aero and Hydrodynamics of the Delft University of Technology under the guidance of Frans Nieuwstadt. Frans Nieuwstadt was appointed in 1986 as director of the laboratory, and he held this position until his sudden death in 2005. Frans’ principal interest was to investigate turbulence at a fundamental level, but also to consider turbulence and its role in other processes. He coined a name for this research: turbulence plus.     

Vertical Motions of Heavy Inertial Particles Smaller than the Smallest Scale of the Turbulence in Strongly Stratified Turbulence

We study the statistics of the vertical motion of inertial particles in strongly stratified turbulence. We use Kinematic Simulation (KS) and Rapid Distortion Theory (RDT) to study the mean position and the root mean square (rms) of the position fluctuation in the vertical direction. We vary the strength of the stratification and the particle inertial characteristic time. The stratification is modelled using the Boussinesq equation and solved in the limit of RDT. The validity of the approximations used here requires that $ \sqrt{{L}/{g}} < {2\pi}/{\mathcal{N}} < \tau_{\eta} $ , where τ _η is the Kolmogorov time scale, g the gravitational acceleration, L the turbulence integral length scale and $\mathcal{N}$ the Brunt–Väisälä frequency. We introduce a drift Froude number $Fr_{d} = \tau_p g / \mathcal{N} L$ . When Fr _d ?<?1, the rms of the inertial particle displacement fluctuation is the same as for fluid elements, i.e. $\langle(\zeta_3 - \langle \zeta_3 \rangle)^2\rangle^{1/2} = 1.22\, u'/\mathcal{N} + \mbox{oscillations}$ . However, when Fr _d ?>?1, $\langle(\zeta_3 - \langle \zeta_3 \rangle)^2\rangle^{1/2} = 267 \, u' \tau_p$ . That is the level of the fluctuation is controlled by the particle inertia τ _p and not by the buoyancy frequency $\mathcal{N}$ . In other words it seems possible for inertial particles to retain the vertical capping while loosing the memory of the Brunt–Väisälä frequency.     

A Century of Turbulence

A brief, superficial survey of some very personal nominations for highpoints of the last hundred years in turbulence. Some conclusions can be dimly seen. This field does not appear to have a pyramidal structure, like the best of physics. We have very few great hypotheses. Most of our experiments are exploratory experiments. What does this mean? We believe it means that, even after 100 years, turbulence studies are still in their infancy. We are naturalists, observing butterflies in the wild. We are still discovering how turbulence behaves, in many respects. We do have a crude, practical, working understanding of many turbulence phenomena but certainly nothing approaching a comprehensive theory, and nothing that will provide predictions of an accuracy demanded by designers. This revised version was published online in July 2006 with corrections to the Cover Date.     

DNS of Fractal-Generated Turbulence

An innovative approach which combines high order compact schemes, Immersed Boundary Method and an efficient domain decomposition method is used to perform high fidelity Direct Numerical Simulations (DNS) of four spatially evolving turbulent flows, one generated by a regular grid and three generated by fractal square grids. The main results which we have been able to obtain from these simulations are the following: the vorticity field appears more clustered when generated by fractal square grids compared to a regular grid; fractal square grids generate higher vorticities and turbulence intensities than a regular grid; the flow holds clear geometrical imprints of the fractal grids far downstream, a property which could be used in the future for flow design, management and passive control; the DNS obtained with fractal grids confirmed the existence of two turbulent regions, one where the turbulence progressively amplifies closer to the grid (the production region) followed by one where the turbulence decays; the energy spectra of fluctuating turbulent velocities at various locations in the production region of the flow provide some information on how the turbulence is generated at the smallest scales first near the grid where the smallest wakes are dominant, followed by progressively smaller turbulent frequencies further downstream where progressively larger wakes interact.     

Stratified Turbulence in the Atmospheric Mesoscales

The first author proposed earlier that the atmospheric energy spectrum in the mesoscale range is controlled by upscale energy transport in stratified and geostrophic turbulence, with the source of the energy probably convective clouds and storms. This hypothesis is reviewed in the light of a variety of theoretical and mechanistic tests, mostly involving numerical simulation. Some conflicting results are noted, with fluid dynamics simulations mostly negative and meteorological simulations positive, including one new set presented here. The rapid increase in larger-scale energy shown in our simulations should, however, be ascribed in part to a different mechanism, involving the rapid growth of the unconstrained outflow and its further spreading by mean shear. Received 23 May 1997 and accepted 21 February 1998