Delay time for fine particle ignition within gas with fluctuating temperature

The Pontryagin equation was applied to calculating the average time for the random process escaping the assign interval: this gives the average delay time for waiting of particle ignition moment in a turbulent flow of gas. A direct numerical simulation method was developed for gas temperature fluctuations with assigned autocorrelation function and particle temperature fluctuations due to exothermal chemical reaction. The method was based on numerical solution of a system of stochastic differential equations. Results of direct simulation were validated through comparing with the analytical solution available for particles without exothermal reaction. Analytical calculations and results of direct numerical simulation for the delay time of particle ignition are in agreement.     

Phase equilibria and transport in polymer colloids

Colloidal dispersions of uniform spherical particles, achievable through special emulsion polymerization techniques, show order-disorder transitions in which the ordered phase consists of particles spaced on a close-packed lattice. This colloidal phenomenon is an example of condensation of a hard-sphere gas, the so-called Kirkwood-Alder transition. The colloid transition can be simulated by using the DLVO pair potential either in numerical integration of Newton's equations o r in a Monte Carlo method. Although suitable for equilibrium properties, these methods can not yield transport properties of colloids, due to neglect of interactions between particle and medium, Substitution of Langevin's equation for Newton's introduces both frictional and stochastic forces on the particles. Computer simulation of particle motions by integration of Langevin's equation yields correct equilibrium properties, and permits the calculation of transport properties as well. Some results of computer simulation of diffusion are presented.     

A Theoretical and Experimental Study to Measure the Concentration and Particle Size Distribution in Two‐Phase Flows

Based on light scattering theory, an optical method is presented for measuring the concentration and particle size distribution of the dispersed phase in two‐phase flows. A prototype was also constructed. Comprehensive computer simulation and numerical calculations were carried out to calibrate the correctness of this method. An experimental study was also performed in gas–solid and gas–liquid two‐phase flows. The results of the measurements are given and discussed in detail.     

A hybrid particle approach for continuum and rarefied flow simulation

A hybrid particle scheme is presented for the simulation of compressible gas flows involving both continuum regions and rarefied regions with strong translational nonequilibrium. The direct simulation Monte Carlo (DSMC) method is applied in rarefied regions, while remaining portions of the flowfield are simulated using a DSMC-based low diffusion particle method for inviscid flow simulation. The hybrid scheme is suitable for either steady state or unsteady flow problems, and can simulate gas mixtures comprising an arbitrary number of species. Numerical procedures are described for strongly coupled two-way information transfer between continuum and rarefied regions, and additional procedures are outlined for the determination of continuum breakdown. The hybrid scheme is evaluated through a comparison with DSMC simulation results for a Mach 6 flow of N₂ over a cylinder, and good overall agreement is observed. Large potential efficiency gains (over three orders of magnitude) are estimated for the hybrid algorithm relative to DSMC in a simple example involving a rarefied expansion flow through a small nozzle into a vacuum chamber.     

Optimal design of a convergent-barrel cold spray nozzle by numerical method

A convergent-barrel (CB) cold spray nozzle was designed through numerical simulation. It was found that the main factors influencing significantly particle velocity and temperature include the length and diameter of the barrel section, the nature of the accelerating gas and its pressure and temperature, and the particle size. Particles can achieve a relatively low velocity but a high temperature under the same gas pressure using a CB nozzle compared to a convergent-divergent (CD) nozzle. The experiment results with Cu powder using the designed CB nozzle confirmed that particle deposition can be realized under a lower gas pressure with a CB nozzle.     

气温和颗粒密度对声场中颗粒动力学影响的数值模拟

综合考虑黏性夹带力、Basset力、虚拟质量力和压力梯度力,建立颗粒在声场中的动力学模型,利用变步长四阶RungeKutta算法和二阶隐式Adams插值算法对颗粒的受力和运动进行数值模拟。将模拟和实验得到的颗粒运动特性进行对比,验证数值模拟的正确性。在此基础上,研究气温和颗粒密度对颗粒动力学的影响规律。结果表明,黏性夹带力对颗粒运动起主导作用;气温升高,压力梯度力与黏性夹带力之间的相位差减小,Basset力、虚拟质量力与黏性夹带力之间的相位差增大。研究还发现,气温较低时,颗粒密度对颗粒运动有重要影响,夹带系数随着密度的增加而迅速下降;气温较高时,颗粒密度对颗粒运动的影响较小,颗粒位移振幅和夹带系数相对低温时明显增加。      

三维颗粒气体相分离现象

颗粒体系是一类复杂的耗散体系.在颗粒气体中,耗散性质会使其内部形成局部的凝聚,类似于真实气体中亚稳分解形成的液滴,因此被认为是颗粒气液两相分离的过程. 零重力环境下二维颗粒气体相分离现象已有成熟的流体静力学理论解释,将该理论模型推广到三维情形,发现相分离现象依然存在且具有同样的不稳定性根源,通过理论计算给出了三维相分离发生的具体条件. 同时,用分子动力学方法模拟检验了理论结果,并给出了三维颗粒气体相分离的新形貌. 关键词： 颗粒气体 耗散 相分离 分子动力学模拟     

3-D numerical analysis of EHD turbulent flow and mono-disperse charged particle transport and collection in a wire-plate ESP

The present study attempts to develop a detailed numerical approach and a simulation procedure to predict the motion of gas, ions and particles inside a simple parallel plate channel containing a single corona wire. A hybrid Finite Element (FEM)-Flux Corrected Transport (FCT)-Finite Volume (FVM) method is used: the FEM–FCT numerical algorithm is applied for modeling the steady-state corona discharge, while the turbulent gas flow and the particle motion under electrostatic forces are modeled using the commercial CFD code FLUENT. Calculations for the gas flow are carried out by solving the Reynolds-averaged Navier–Stokes equations and turbulence is modeled using the k–? turbulence model. An additional source term is added to the gas flow equation to include the effect of the electric field, obtained by solving a coupled system of the electric field and charge transport equations using User-Defined Functions (UDFs). The particle phase is simulated based on the Lagrangian approach, where a large number of particles is traced with their motion affected by the gas flow and electrostatic forces using the Discrete Phase Model (DPM) in FLUENT. The developed model is useful to gain insight into the particle collection phenomena that take place inside an ESP.     

COIL压力恢复系统气流主动冷却技术数值模拟

 引射式压力恢复系统是化学激光系统应用中的一个重要组成部分,为了提高引射器引射效率,减小整个系统尺寸,可以采用主动降低光腔出口气流温度的方法。通过数值模拟,开展了用热交换器降低光腔出口气流温度的研究。给出了混合气体的热物理性质、热交换器建模方法及数值模拟条件,比较了不同条件下热交换器性能的差异,发现由于出口气流密度很低,热交换器的总传热系数、压力损失比常规条件下有较大幅度的减小。此时适当增大椭圆管尺寸,采用高翅片换热管,可以有效地提高热交换器的换热能力。     

二维拉格朗日坐标系下气粒混合双向耦合对激波流场影响的计算

喷射颗粒与气体混合是内爆压缩领域的热点和难点. 针对喷射混合中的气粒双向耦合问题, 开展了理论建模、离散算法以及颗粒反馈对激波流场的影响研究. 建立了拉格朗日计算框架下的数学模型; 给出了耦合源项的离散算法; 开展了平面及汇聚构型条件下, 气粒双向耦合的数值模拟研究; 发现了颗粒反馈导致气体激波提速现象以及气区流场物理量分布形态的改变, 初步获得了量化分析结果. 本文建立的数学模型、计算方法和获得的新的物理认识, 为深入理解喷射混合现象、解决相关工程应用问题提供了重要理论支撑.     

Rotation of a 1-GeV particle beam by a fan system of thin crystals

The deflection of a 1-GeV charged particle beam by a system formed by fan-oriented thin silicon wafers has been studied theoretically and experimentally. Software has been developed for numerical simulation of a particle beam transmission through a fan crystal system. In the U-70 experiment on a proton beam, the particles were deflected by such a system through an angle exceeding 1 mrad. Thus, a new method has been demonstrated for rotating a particle beam, which can be used for creating accelerator beams for medical purposes.     

循环流化床气固曳力模型

气固曳力是稠密气固两相流动,尤其是垂直流动中的主要作用力,相应的模型也是数值模拟中准确描述气固两相运动的关键.为了解决现有经验或半经验模型的普适性问题,合理描述流动中经常发生的颗粒团聚现象及其对气固曳力的影响,从理论分析入手,运用最小能量的概念,将传统的CFD方法与宏观的系统分析方法相结合,建立了一个新的计及颗粒团聚效应的气固曳力理论模型.与现有模型相比,新模型不仅具有相同的函数变化关系,可合理地描述气固两相相互作用的物理过程,而且避免了以往经验系数不准确导致的各种误差,为稠密气固两相流动的数值描述提供了重要依据.     

Improvement of the pressure algorithm of the stand-alone PDF method to treat unsteady three-dimensional turbulent reacting flows

This paper deals with the particle-mesh probability density function (PDF) method. It shows how an existing but less precise pressure algorithm for the stand-alone method can be improved. The present algorithm is able to handle the general case of an unsteady three-dimensional turbulent reacting flow. The transport equation of the joint PDF of velocity and composition is solved with a particle method. Open boundary conditions are realized and for statistical reasons a simple but effective particle splitting procedure is applied.

Based on a simple configuration, the properties of the presented improved pressure algorithm are analysed. It is shown which numerical condition must be taken care of so that the algorithm is able to correct the particle positions such that the normalization condition is fulfilled as accurately as specified.

To verify the algorithm the combustion of a methane–air mixture enclosed in an open simulation volume is calculated. It is shown that the simple particle splitting algorithm works very effectively in the studied case. The behaviour of the improved pressure algorithm is examined by different calculations. To analyse the convergence of the algorithm, the particle number per cell and the grid spacing are varied. To demonstrate the accuracy, a statistically stationary inflow/outflow configuration is used and the numerical solution is compared to an analytical one. For a less symmetric test case, the previous unsteady combustion problem is simulated, including an additional mean velocity in one direction.

The presented improved pressure algorithm provides the opportunity to calculate unsteady three-dimensional turbulent reacting flows with a stand-alone method, and offers an alternative to the complex hybrid finite-volume/particle PDF method.     

Simulation of front motion in a reacting condensed two phase mixture

The computational technique is developed in order to provide the scale capturing for numerical simulation of the thermal processes. The thermal front motion and gas flow dynamics as well as the rate of particle growth during the Carbon Combustion Synthesis of Oxides (CCSO) were predicted using the numerical simulation. In CCSO the exothermic oxidation of carbon nanoparticles generates a self-sustained thermal reaction front that propagates through the solid reactant mixture converting it to the desired complex oxides. The combusted carbon is emitted from the sample as carbon dioxide and its high rate of release increases the product porosity and friability. It was shown that the complicated finger front instability can be developed during the carbon combustion synthesis. This phenomenon results from a vortex gas flow in the reaction zone fed by the carbon dioxide co-flow and oxygen counter-flow filtration.     

Lattice Boltzmann方法模拟多孔介质Klinkenberg效应

格子Boltzmann数值模拟方法是研究复杂的多孔介质结构特别是Klinkenberg效应的有效方法之一，对处理复杂边值问题尤其有效，用格子Boltzmann方法研究了气流穿越多孔介质问题，并将数值计算结果与实验结果进行了比较，结果表明格子Boltzmann方法是数值模拟气流穿越多孔介质问题的有效方法之一。     

喷动流化床的数值模拟研究

本文借鉴 Ｃｕｎｄａｌｌ＆ Ｓｔｒａｃｋ提出的碰撞模型,将欧拉方法和拉格朗日法结合起来,建立了喷动流化床气固两相流动的数学模型,在计算机上对试验台进行了模拟,探讨了床内颗粒的运动特征。按１：１的比例建立了喷动流化床的模型试验台并进行了试验,对模拟结果和试验结果进行了比较与分析。颗粒在喷动床内的运动特征,模拟结果都能够再现出来,模拟结果与试验结果取得了很好的一致性。     

Numerical simulation of modes of combustion and detonation of hydrogen-air mixtures in porous medium in the framework of the mechanics of two-phase reaction mediums

Numerical simulation is carried out for combustion and detonation waves propagating through a motionless gas mixture in a porous inert charge. Computations are performed in a one-dimensional approximation by means of an EFAE computer program that was developed in the framework of the mechanics of multiphase reaction mediums. The chemical conversion of gas is modeled by a one-stage reaction of the Arrhenius type with constants selected based on existing experimental data on the ignition lags behind the reflected shock waves. Computations are performed for hydrogen-air mixtures with 35 and 15% hydrogen and compared with literature experimental data in which the initial pressure and the diameter of charged particles are varied. All three combustion modes (slow, fast, and supersonic) observed in the experiment and combustion failure under conditions lower than threshold are followed by numerical simulation. In addition, the computations qualitatively reproduced experimental data on the change of the combustion mode in the case of transfer from stoichiometric to a lean mixture and data on the combustion wave velocity and limiting conditions of combustion mode transition and failure of flame as a function of the initial pressure and the charged particle size. It is shown that supersonic waves propagating with a velocity of lower than 1100 m/s do not have a Chapman-Jouguet surface in the end of the reaction zone and it is evident that they can be related to detonation, as in the cited literature.     

A unified gas-kinetic scheme for continuum and rarefied flows

With discretized particle velocity space, a multiscale unified gas-kinetic scheme for entire Knudsen number flows is constructed based on the BGK model. The current scheme couples closely the update of macroscopic conservative variables with the update of microscopic gas distribution function within a time step. In comparison with many existing kinetic schemes for the Boltzmann equation, the current method has no difficulty to get accurate Navier–Stokes (NS) solutions in the continuum flow regime with a time step being much larger than the particle collision time. At the same time, the rarefied flow solution, even in the free molecule limit, can be captured accurately. The unified scheme is an extension of the gas-kinetic BGK-NS scheme from the continuum flow to the rarefied regime with the discretization of particle velocity space. The success of the method is due to the un-splitting treatment of the particle transport and collision in the evaluation of local solution of the gas distribution function. For these methods which use operator splitting technique to solve the transport and collision separately, it is usually required that the time step is less than the particle collision time. This constraint basically makes these methods useless in the continuum flow regime, especially in the high Reynolds number flow simulations. Theoretically, once the physical process of particle transport and collision is modeled statistically by the kinetic Boltzmann equation, the transport and collision become continuous operators in space and time, and their numerical discretization should be done consistently. Due to its multiscale nature of the unified scheme, in the update of macroscopic flow variables, the corresponding heat flux can be modified according to any realistic Prandtl number. Subsequently, this modification effects the equilibrium state in the next time level and the update of microscopic distribution function. Therefore, instead of modifying the collision term of the BGK model, such as ES-BGK and BGK–Shakhov, the unified scheme can achieve the same goal on the numerical level directly. Many numerical tests will be used to validate the unified method.     

基于简正波理论的浅海低频混响时间序列仿真

混响场时域信号序列的数值实现对主动声纳目标模拟器研制具有重要意义。本文所述适用于水平变化浅海环境混响时间序列的一种实现方法,基于现有的浅海简正波相干混响理论。所述方法的思路是通过相干散射场随机序列与发射信号的离散卷积来构成混响时域序列,文中以Pekeris均匀浅海环境为例,给出了相应的混响时间序列仿真结果,并对仿真结果的频谱特性、统计特性、强度衰减以及垂直相关特性等进行了系统检验。结果表明,由仿真混响时间序列所获得的混响特性与理论结果相符,可用于浅海混响序列的仿真。