Vibration-induced particle drift in a fluid cell under microgravity

A theoretical investigation into the effect of small vibrations on the behaviour of a small particle contained in a fluid cell under microgravity is presented. Diffusion-controlled material processing such as protein crystal growth can be adversely affected by small vibrations called g-jitter, if a relative motion is induced between the particle and surrounding fluid. When a fluid cell containing a small particle such as a protein crystal is vibrated parallel to the wall nearest to the particle, the particle oscillates with a certain amplitude and a hydrodynamic force in the direction normal to the wall is induced. Theoretical models based on an inviscid fluid assumption are used to predict the particle amplitude variation and drifting motion. Due to an external vibration such as g-jitter, the oscillating particle is predicted to drift towards the wall and the particle oscillation amplitude to decrease slightly as the distance between the particle and wall is reduced. The reduction in particle ampitude also depends on the particle-to-fluid density ratio. The particle drift towards the nearest wall acclerates due to an increasing attraction force, and the drifting speed increases with both the vibration frequency and particle diameter. Even for small protein crystals with a density close to that of the fluid, the time required to drift from the center of the fluid cell to the wall is predicted to be much shorter than the growth time.     

微重力环境低温流体无排气加注过程数值研究

针对加注系统受注贮箱,采用CFD方法就液氮贮箱无排气加注过程开展数值仿真,对比了不同重力水平下的无排气加注性能,分析了加注口结构、壁面初始温度、加注流体温度和加注流量等因素对微重力无排气加注性能的影响规律。所构建的二维轴对称模型将流体区与固壁区一起作为求解区域并划分网格,并通过植入用户自定义程序(UDF)计算加注口液体闪蒸过程及气液之间的热质交换。经过实验数据验证,该模型能够合理展示箱内温度场分布和相分布情况,并获得贮箱压力等参数变化信息。数值计算结果表明:(1)加注条件相同时,微重力工况较常重力工况体现出更好的无排气加注性能。(2)微重力条件下,无排气加注性能几乎不受加注口结构的影响,壁面初始温度和加注流体温度越高,贮箱压力越高,加注流量仅对加注时间有显著影响。     

A novel and developed approximation for motion of a spherical solid particle in plane coquette fluid flow

The present article solves the couple equations of a spherical solid particle’s motion in plane coquette fluid flow by using the HPM-Padé technique which is a combination of the Homotopy Perturbation Method (HPM) and Padé approximation. The series solutions of the couple equations are developed. Generally, the truncated series solution is adequately in a small region and to overcome this limitation, the Padé techniques which have the advantage of turning the polynomial approximation into a rational function, are applied to the series solution to improve the accuracy and enlarge the convergence domain. The current results compared with those derived from HPM and the established fourth order Runge–Kutta method in order to ascertain the accuracy of the proposed method. It is found that this method can achieve more suitable results in comparison to HPM.     

液滴在固体表面流动特性的数值模拟及验证

为了研究液滴在固体表面的流动特性,采用格子Boltzmann(LBM)分子动力学和水平集(LS)界面跟踪相结合的方法(LBM-LS)对液体在固体表面流动铺展过程进行数值模拟,用LBM求解液体运动的流场,LS捕捉运动的固液气界面。结果表明,模拟结果与实验测定结果一致,铺展接触角随时间呈指数变化规律,液体表面张力越小铺展接触角越小,证明采用LBM-LS模型模拟润湿性问题是可行的。     

Numerical and experimental study of a spherical particle flow in a cylindrical tube under vacuum conditions

The paper aims at developing a validated model that can accurately predict the flow of a particulate material. This model will serve as a virtual design tool for the design of a novel passive safety system for nuclear reactors. Therefore an experimental setup consisting of a vertical glass tube is filled with 500±30 μm spherical glass particles. The experiment is then placed in a vacuum and the particles are released by opening a valve. The velocity of the particles is recorded during their fall at three different heights using a non invasive optical tracking technique with an original implementation. The same experiment is then simulated using the Discrete Element Method and results are compared. A good agreement between the simulation and the experiment was found. The sensitivity of the simulation to a change in the contact stiffness, dynamic Coulomb coefficient of friction and tangential contact force model was investigated. The influence of the initial position of the simulated particles on the packing factor was shown to be very important. Finally the experiment proved to be extremely sensitive to a perturbation of the outflow section of the tube, something that was predicted by the simulations.     

Stability of the motion of a solid particle in a planar rotating flow

The effect of the shape of a rotating flow on the motion of a particle in it is studied. The conditions for and regions of stable motion of a solid particle are found.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 16, No. l, pp. 43–46, January, 1969.     

An experimental investigation of the behavior of solid particles during their motion in smooth and dimpled pipes

Results are given of LDA measurements of averaged and fluctuation velocities of glass particles during their deposition in smooth and dimpled narrow pipes. Experiments reveal a decrease in the axial component of averaged velocity of particles and a significant increase in fluctuation velocities of particles during their motion in a pipe with dimples.     

Numerical analysis of flow field and particle motion in a dynamic cyclonic selector

Computational Fluid Dynamics is employed to investigate the flow field and the fate of particles in a dynamic cyclonic classifier which is used to separate fine particles of dried sludge, produced as waste by pulp and paper-making processes. The cyclonic classifier is equipped with a rotating impeller, which improves the tangential flow, and a circular baffle, which distributes the inlet stream of gas and particles. Unsteady Reynolds-averaged Navier-Stokes equations are solved for the continuous phase, addressing the impeller motion though the Sliding Mesh approach, whereas Lagrangian tracking is employed for the particles. Surprisingly, the removal efficiency is found to be non monotonic with particle size, instead presenting a fish-hook shape. This is partly imputed to the presence of the circular baffle that promotes, in the bottom region of the cyclone, the formation of a nearly toroidal recirculation zone which entrains small particles, subsequently separated at the bottom. Moreover, too high inlet velocities were found to hamper the action of impeller rotation with a resulting detrimental effect on removal efficiency.     

微重力条件下分离结晶过程中熔体热毛细对流的数值模拟

用有限差分法对微重力条件下分离结晶生长中的熔体热毛细对流进行了数值模拟,熔体的深径比A取1和2,自由界面无因次宽度B取0.05,0.075和0.1;得到了分离结晶Bridgman生长过程中熔体热毛细对流的流函数分布和温度分布图,研究了流型的演变过程和流动的失稳机制.结果表明:当Marangoni(Ma)数比较小时,流动为稳态流动并只存在于自由界面附近,随着Ma数的增加,流动增强并逐步向熔体内部扩展,熔体内部温度分布的非线性性增加,自由界面速度增大;Ma数超过某一临界值后,流动转化为非稳态流动;流动失稳的物理机制是流速的变化和阻力的变化之间存在滞后.     

Numerical simulation of particle motion in dense phase pneumatic conveying

A gas-solids two-dimensional mathematical model was developed for plug flow of cohesionless particles in a horizontal pipeline in dense phase pneumatic conveying. The model was developed based on the discrete element method (DEM). For the gas phase, the Navier-Stokes equations were integrated by the semi-implicit method for pressure-linked equations (SIMPLE) scheme of Patankar employing the staggered grid system. For the particle motion the Newtonian equations of motion of individual particles were integrated, where repulsive and damping forces for particle collision, the gravity force, and the drag force were taken into account. For particle contact, a nonlinear spring and dash pot model for both normal and tangential components was used. In order to get more realistic results, the model uses realistic pneumatic system and material values.     

电磁颗粒阻尼器减振机理及试验研究

对直流电磁场作用下电磁颗粒阻尼器减振效果进行了理论分析和试验研究.理论和试验结果表明:在一定振动强度下,可以通过施加直流电磁场的方法,加大颗粒体与振动系统间的动量交换,提高对结构振动的抑制作用;同时增大磁颗粒之间的接触压力,由此加大摩擦力.进而提高阻尼器的摩擦耗能.该方法为扩大颗粒阻尼器的适用范围,对抑制不同强度的振动提供了基础,使颗粒阻尼能够适应不同振动环境的要求.结果说明颗粒阻尼器可以由被动振动控制方法发展为半主动振动控制手段,本文的研究为这一潜在发展提供了有益探索.     

Numerical study of the motion behaviour of three-dimensional cubic particle in a thin drum

The motion of three-dimensional cubic particles in a thin rotating drum is simulated by the SIPHPM method. The drums with frictional or smooth front and rear walls, and the particles of cubic and spherical shapes, and different particle numbers are considered to study the effect of cubic particle shape, end-wall frictions and filling levels. Different flow patterns of cubic particles are observed, which are significantly dominated by the friction from the end-walls. The probability density function of velocity components, the flatness factors are used to analyze the motion behaviour of cubic particle. The Froude number, ensemble mean and time averaged particle velocities are also analyzed. A primary and secondary mode of driving from the end-wall frictions are indicated and the mechanisms on the influences of wall friction, particle shape and filling levels are fully explained.     

Numerical and experimental investigation of a single stage centripetal pump

In the present paper a numerical and experimental investigation of a single stage centripetal pump (SSCP) is presented. The SSCP was designed using CAD and CFD tools. The performance curves of the SSCP were measured in a test facility with water involved as the working media and compared with the calculated ones. The measured performance curves are characterised by the region of hysteresis since the throttle closing performance curves do not correspond completely to the throttle opening performance curves. The delivery head and efficiency are abruptly decreased when reducing the flow rate from the point of the maximum delivery head. Due to the evident analogy of the SSCP and the axial-flow compressors performance curves and similarities in the rotor design it can be anticipated that such an operating behaviour of the SSCP is caused by the rotating stall phenomenon. CFD simulations confirm that the stalling of the rotor passages causes the steep delivery head drop when decreasing the flow rate from the point of maximum head.     

On the slow motion of a solid submerged in a fluid with a surfactant surface film

Summary This paper continues the work of Shail and Gooden [1–4] on the motion generated in a semi-infinite fluid by a singularity or submerged solid moving particle when the surface of the fluid is contaminated with a surfactant film. The fluid motion is assumed to be slow and quasi-steady, but the restriction to axially symmetric flows of earlier investigations is removed. The various linearised models of Shail and Gooden [3,4] governing the variation of film concentration are discussed, the constitutive properties of the film being expressed in terms of Boussinesq coefficients of surface shear and dilatational viscosities. The resulting film boundary conditions are applied to solve the non-axially symmetric problem of a Stokeslet placed in the bulk fluid with its axis parallel to the surface (assumed planar throughout the motion), and the results used to calculate approximate expressions for the resistive force on a particle which translates far from and parallel to the surface. A similar analysis is given for the case of a rotelet whose axis is parallel to the surface.     

Numerical and experimental investigation of polycarbonate vacuum-forming process

This research investigates the influence of process variables on the quality of vacuum-formed thermoplastic products and provides guidelines for polycarbonate (PC) thermoforming optimization. The process variables investigated include: operating temperature, non-uniform temperature of the PC sheet, and applied pressure. A series of thermo-coupled numerical simulations are conducted to examine the quality of the vacuum-formed parts. Three general hyperelastic constitutive models are used to describe the mechanical behavior of PC during the thermoforming process and to examine its reliability. The forming limit and the thickness distribution predicted by the numerical simulations for the axisymmetric vacuum-formed parts are compared with the experimental results of vacuum-forming. Good agreement is found between the numerical simulations and the experimental results. In addition, the effect of temperature on the surface smoothness and the deformation results of the thermoformed parts are discussed. Based on the results of the experiments and numerical analyses, guidelines for the PC thermoforming process are proposed.     

Numerical investigation of electrostatic effect on particle behavior in a 90 degrees bend

Particle behavior in a turbulent circular-sectioned 90° bend under electrostatic field at three air flow rates (1600 L/min, 1100 L/min and 950 L/min, the corresponding bulk Reynolds numbers are 58,000, 40,000, 34,000) is simulated by a Large Eddy Simulation-Lagrangian particle tracking technique (LES-LPT) method coupled with electrostatic field model by Coulomb’s law. This numerical simulation is dedicated to study the electrostatic effect on particle behavior and erosion occurred in the dilute particle-laden bend flow. Forces considered acting on particles includes drag, lift, gravity and electrostatic force. Results obtained for the fluid phase are in good agreement with experimental and numerical data. Predictions show that electrostatic field does affect the particle motion in the pipe bend. At higher air flow rate with higher electrostatics at the inner arc the increasement of impact angle is lower than that at lower flow rate with lower electrostatics. The same conclusion can be found at the outer arc. In addition, electrostatic effect does increase particle-wall impact velocity while such trend decreases with flow rate. Erosion rate increases with increasing air flow rate, which is independent of electrostatics. However, given the same flow rate, the electrostatics reduces the occurrence of erosion at the bend. The erosion rate under electrostatic effect is found to approach that without electrostatics as the flow rate increases. Therefore, the effect of electrostatics on erosion decreases with the air flow rate.     

The motion of bubbles in a sinusoidally oscillating liquid in microgravity

The sinusoidal motion of single, spherical bubbles in microgravity was studied experimentally aboard the U.S. Space Shuttle. Tests were performed to determine the effect of frequency, acceleration amplitude, bubble size, and fluid viscosity on bubble motion. Five test cells each containing a single bubble were subjected to rectilinear, sinusoidal oscillations. Three nominal bubble sizes and three liquids were used to cover a range of Stokes numbers from 1.3 to 21 and Reynolds numbers from 0.6 to 75. Bubble motion was recorded by video. The ratio of bubble motion amplitude to container motion amplitude was found to be essentially independent of the actual container motion amplitude. Therefore, this ratio could be plotted against frequency to obtain a frequency response for each case. This ratio was found to rise sharply from zero at zero frequency and then approach an asymptote at high frequencies. The strong effect of the walls in these experiments caused the amplitude of bubble motion to be reduced somewhat from that expected for an infinite fluid.     

Analytical investigation on acceleration motion of a vertically falling spherical particle in incompressible Newtonian media

The settling behavior of solid particles is of fundamental importance in natural and artificial applications. In current study, the unsteady motion of a spherical particle falling in a Newtonian fluid was analyzed using a drag of the form given by Oseen/Ferreira, for a range of Reynolds numbers. Particle equation of motion involved added-mass term and neglected the Basset term. Using the homotopy perturbation method (HPM) analytical expressions for the instantaneous velocity, acceleration and position of the particle were derived. The practical applications of the results were discussed. The presented investigation showed the effectiveness of HPM.