粉粒体慢速斜槽流的实验研究

介绍了研究粉粒体慢速斜槽流的实验装置及测量方法 ,通过测量表面速度和流层厚度 ,初步分析了斜槽倾角、流率和壁面状况对流动的影响情况     

粉粒体斜面慢速流的三维颗粒流程序仿真

为了研究粉粒体斜面慢速流的机理,在分析粉体流动性影响因素的基础上,应用颗粒离散元法三维颗粒流程序建立斜槽仿真模型并进行三维流动过程模拟,观察颗粒在斜槽内的流动情况及单个颗粒在流动过程中的受力、位移和速度,测量颗粒内部力场和颗粒速度场变化。结果表明,随着斜槽倾角的增大,粉粒流的速度呈连续、不均匀增大,堆积颗粒之间的接触力与不平衡力显著增大,并对流速产生一定的影响。     

微注塑成型流动中熔体壁面滑移的研究

基于宏观熔体流动的基本理论及其流动过程中壁面滑移机理的分析,针对微注塑成型模具中熔体充模流动时的壁面滑移行为,建立了微小通道中高聚物熔体流动的壁面滑移理论模型。并用数值模拟方法,对不同滑移系数时微小通道中熔体的壁面滑移对流动速度、熔体压力等的影响进行了研究。结果表明,微小通道中的壁面滑移可使壁面处熔体的流动速度增加,压力损失减小,有利于熔体的充模流动。     

平行平板微通道流场在双电层和边界滑移条件下的解析解

讨论了双电层和边界滑移对微通道液体流动系统的影响.利用双电层动电效应理论对控制方程进行修正,同时引入Navier滑移边界条件考虑边界滑移现象,建立了两种壁面效应同时存在时微流动控制模型,得到了流场的解析解.结果显示,双电层效应抑制流动的发展,使流体在固/液边界附近区域产生回流;边界滑移虽能促进流动发展,但当这两种效应同时存在时,边界滑移的作用变为加强抑制流动,使回流现象更加显著.     

微间距平板Poiseuille流动的稳定性

在忽略了其他微尺度效应的前提下,讨论了黏性修正和壁面滑移对微间距平板Poiseuille流动稳定性的影响．利用固壁对流体分子的作用力修正流体表观黏性,从而使流体表观黏性成为离壁面距离的函数．边界滑移修正使用纳维滑移边界模拟．利用线性小扰动法分析流动的稳定性,并用切比雪夫配置点法求解模型方程．计算结果显示,表观黏性修正在一定条件下可使平均流出现拐点,流动更易失稳;而边界滑移则增大了平均流的丰满程度,使流动更加稳定．当黏性修正模型参数ξ和n较大时,滑移长度的影响会被极大地减弱．     

微通道气体流动的格子波尔兹曼模拟

通过隐式格子波尔兹曼方程,并采用壁面平衡边界条件以及二阶关系,模拟了微通道气体流动中的非线性压力和壁面滑移速度,模拟结果与Arkilic的解析结果十分吻合,验证了格子波尔兹曼方法在滑移流区的有效性.     

考虑壁面滑移的磁流变胶泥缓冲器设计理论与落锤冲击试验

为提高汽车碰撞磁流变缓冲器力学模型的准确性,实现冲击作用下磁流变缓冲器动态特性的高精度预测,基于Herschel-Bulkley模型,同时考虑表观滑移和壁面滑移,建立了缓冲器理论力学模型。通过分析表观滑移和壁面滑移对缓冲器阻尼通道内部压力梯度的影响,结果表明,载液黏度较低时,受表观滑移影响,阻尼通道内部压力梯度有所降低,且在低速下影响更加显著;随着载液黏度的增加,在表观滑移作用下压力梯度有所增加,但对总体影响不大;壁面滑移使通道内部压力梯度明显降低,且随着滑移系数的增加,压力梯度变化更为显著;不同电流、冲击速度下的缓冲器落锤冲击试验表明,理论模型能够较好地预测、表征磁流变缓冲器的力学特性;磁流变胶泥在通道内流动主要受壁面滑移的影响,未出现明显的表观滑移。     

基于壁面滑移的薄壁注塑流动三维数值模拟

基于壁面滑移速度模型,且考虑黏度的压力依赖性,运用三维有限元数值模拟技术,对薄壁注塑流动过程进行数值模拟,研究壁面滑移对厚度方向熔体速度分布、注射时间、注射压力和锁模力的影响。结果表明,壁面滑移对薄壁注塑流动过程具有重要的影响。通过模拟结果与实验结果进行对比,以验证数值模拟结果的可靠性。结果还表明,只有同时考虑壁面滑移效应和黏度的压力依赖性,才能更准确地描述薄壁注塑流动过程。     

PDMS在大振幅剪切流动中的壁面滑移

本文采用旋转流变仪研究了聚二甲基硅氧烷(PDMS)在两平行板间的大振幅剪切流动,其中的关键参数是应变幅值和角频率.随应变幅值增加,依次观察到没有波形畸变→明显的波形畸变→波形畸变减弱直至消失,对应着熔体与壁面之间的粘附→弱滑移→粘附/滑移→强滑移.同时角频率增加,样品的非线性增加,滑移也增强.从应力的间距依赖性和波形畸变等力学响应角度讨论了大振幅剪切流中的壁面滑移现象,同时使用应力分解的方法对大振幅剪切流中样品的非线性强弱进行了度量,提出了PDMS在大振幅剪切流中的动力学图谱.     

壁面特征对磁流变液传力性能的影响

为了保证磁流变传动装置的动力传递效果,采用实验方法,分析了不同壁面材料、表面粗糙度大小、壁面形貌、滑差转速及工作间隙对磁流变传力性能的影响规律。研究表明壁面材料对磁流变液动力传递效果影响较为明显,材料磁导率越低,传递转矩越小,容易发生壁面滑移现象;壁面形貌亦为影响传力效果的主要因素,凹凸同心圆表面减弱了磁流变液的传力性能;高滑差转速时,磁流变液颗粒成链较为复杂,其传递转矩随滑差转速的增加而逐渐降低,而工作间距对其传力性能影响不大。     

Position dependent velocity profiles in granular avalanches

We present velocity profile measurements in granular avalanches flowing down a flat chute with wide rectangular cross section. The flow is recorded through a transparent side-wall by a high-speed camera, which is able to capture 1,825 pictures in a second. Due to the high frame rate of the camera, several flow features can be observed. Quantitative statements can be made by analysing the images with a pattern matching algorithm. This provides us with flow-normal velocity profiles with a very high temporal and spatial resolution. We find that even on flat surfaces, velocity profiles are strongly changing through the flow and for the range of investigated chute angles (from 26° to 36°) clear trends can be recognised. In the head of the avalanche the velocity is highest, decreasing continuously over the length of the avalanche. Thus, the investigated granular avalanches stretch through the flow. The experimental method allows us to study the evolution of characteristic flow properties such as depth averaged velocity, slip velocity, surface velocity, shear rates or flow depth. Side-wall friction effects are estimated.     

POWDER FLOW MONITORING USING MICROWAVE DOPPLER RADAR

A computerised, microwave Doppler effect flowmeter has been developed and applied to the flow of model powders down a chute. The instrument enabled the velocity and distribution of velocities of moving powders to be determined simultaneously.

The velocity profiles on a 1.8m stainless steel chute of rectangular cross-section have been obtained for sand in the size range 355-425µm at chute inclinations ranging from 30 to 65 degrees. Freely accelerating flows, which occurred at high chute inclinations, led to beds of open structure while flow at lower angles led to much denser beds being formed. A greater range of velocities existed within the denser beds at low angles than was present in the more open beds at higher angles.     

EFFECT OF MASS FLOWRATE ON POWDER FLOW VELOCITIES

The effect of mass flowrate on powder velocities during flow down a chute and in free fall from a hopper was studied using a microwave Doppler technique. The velocities on an inclined chute, which was either smooth or had its surface roughened by coating with a layer of the sample, increased as the mass flowrate increased; this effect was most significant at large angles of inclination. Free fall from a hopper also led to higher velocities being monitored as the mass flowrate increased.

The distribution of particle velocities was also estimated; during flow on a chute at high angles of inclination the distribution of velocities was greater for low flowrates while at low angles, where the powder only just managed to flow, the trend was reversed. There was no difference in the distribution of velocities measured for the free fall of particles as the mass flowrate varied.     

Wall slip phenomena in talc-filled polypropylene compounds

Slip velocities of unfilled and talc-filled polypropylene (PP) compounds, detectable at the die wall during pressure driven shear flow, have been determined using capillary rheometry. The presence of low molar mass, polar additives is responsible for the detection of wall slip in unmodified PP. Slip velocity increases with shear stress, beyond the critical onset condition. Increasing talc concentration in the PP compounds reduces slip velocity systematically, according to the talc volume fraction, whilst talc particle morphology appears to modify the wall slip behaviour to a greater extent than particle size. In comparison to PP-talc composites based on untreated filler, the presence of surface coatings tends to increase wall slip velocity, at any given shear stress, when the coating concentration exceeds monolayer level. These observations are explained in terms of a mechanism for wall slip in a low cohesive strength interphase, rich in low molar mass amide species, close to the flow boundary. This behaviour has also been modelled using a power law, to define wall slip parameters as a function of shear stress and talc concentration that can be used to enhance process simulation. It is demonstrated that the onset and magnitude of wall slip may be controllable by compound formulation and process conditions, creating exploitation potential to enhance process control and product properties of particle-modified PP composites.     

Numerical solution of gas–solid flow in fluidised bed at sub-atmospheric pressures

Fluidised beds are characterised by excellent thermal and chemical uniformity and have a wide application range including heat and surface treatment, ore roasting and catalyst production. However, compared to other gas-based systems, to fluidise a particulate mass, a significant quantity of gas is required. To conserve gas there is potential to operate the fluid bed under low-pressure conditions. It is also observed that heat transfer remains constant with reduction in pressure. The present work has numerically studied the nature of hydrodynamics in fluidised bed at sub-atmospheric conditions and a new drag law is proposed to account for the increased mean free path of the fluid. A wide range of sub-atmospheric pressures were considered such that slip flow regime, which is characterised with Kn ～ 1, is applicable. An open source code (MFIX) is used to numerically solve the multiphase problem of a jet in the fluidised bed column with an immersed surface at vacuum pressure conditions. Bubbling fluidisation in shallow and deep beds are also solved. The new drag model takes into consideration the effect of slip flow to model drag force on the particles and the results of velocity distributions in the column and around the submerged surface is presented. The results of velocity distributions from the slip flow model are compared with the existing Gidaspow’s model. Significant differences were observed in the simulation results of velocity distributions and flow structure in the fluidised bed under vacuum conditions.     

Effect of surface roughness on gas flow in microchannels by molecular dynamics simulation

Understanding the effect of surface roughness on gas flow in microchannels is highly desirable in microfluidic devices. Non-equilibrium molecular dynamics simulation is applied to investigate the effect of the surface roughness on slip flow of gaseous argon in submicron platinum channels. The geometries of the surface roughness are modeled by triangular, rectangular, sinusoidal and randomly triangular waves respectively. The results show that the boundary conditions of velocity slip, including slip, no-slip and negative slip, depend not only on the Knudsen number but also on the surface roughness. Induced by the roughness, the slip length of gas microflow over a rough surface is less than that predicted by the Maxwell model and shows a non-linear relationship with the Knudsen number. The friction coefficient increases not only with decreasing the Knudsen number but also with increasing the surface roughness. The impacts of the surface roughness and the gas rarefaction on the friction coefficient of gas microflow are strongly coupled. The roughness geometry also shows significant effects on the boundary conditions and the friction characteristics. The distortion of the streamlines and the enhancement of the penetrability near the rough surface are demonstrated to be responsible for the roughness effect.     

Dilatant double shearing theory applied to granular chute flow

Summary Although the steady flow of a granular material down a plane inclined slope has been exhaustively examined from both theoretical and experimental points of view, there is still no general agreement concerning the basic flow properties such as density and velocity profiles. The majority of studies assume that the velocity component of the material perpendicular to the inclined plane is sufficiently small to assume that it is everywhere zero. However, recent dynamical modelling of granular chute flow indicates that this component of velocity, although small, is actually non-zero. In this paper, we examine a dilatant double shearing theory for chute flow assuming that the perpendicular component of velocity is non-zero. An explicit analytical form for the perpendicular velocity profile is deduced which gives rise to an integral expression for the chute stream velocity. Assuming a linear decreasing density profile, numerical integration for the chute stream velocity predicts a non-linear profile which is concave in shape and which is in agreement with recent results from computer simulation and existing experimental data in the literature.     

Falling process of a rectangular granular step

This study experimentally investigates the falling process of a dry granular step in a transparent plexiglass chute by particle image analysis. Three types of uniform spherical beads and one type of quartz sand were piled up with various bed slopes and widths to elucidate their flow characteristics. The surface angles during the early slipping phase are close to the failure angles that are associated with the active earth pressure, based on the Mohr-Coulomb friction law. For a given size of particles (d) and slope (θ), the retreating upper granular surface follows a theoretical curve, and dimensionless mobile length decreases as the dimensionless time parameter t* increases. Velocity profiles measured at the side wall exhibit an exponential-like tail close to the static region at the bottom of the chute. As determined by the conservation of mass and momentum, the relationship between the characteristic velocity and the characteristic depth is linear in the transient flow.