基于格子Boltzmann方法的分叉微通道内非牛顿流体流动特性研究

为探究分叉微通道内非牛顿流体的流动特性,将非牛顿流体幂律模型引入牛顿流体格子Boltzmann模型,在不同分叉角度矩形截面微通道内数值模拟不同质量分数剪切稀化流体的流动行为;通过分析流动过程中密度随时间的变化趋势以及稳态流动下的密度,得到微通道内压力的分布以及流动区间的压力降;分析溶液质量分数、入口速度与分叉角度对非牛...     

Comparison of flow in numerical and physical models of a ventricular assist device using low- and high-viscosity fluids

The flows in a model of a ventricular assist device (VAD) were investigated numerically and experimentally for two different Newtonian test fluids. These were a blood analogue fluid and a much higher viscosity fluid. A finite volume method was employed to solve the governing equations for a three-dimensional unsteady laminar flow on a transient grid. The numerical solutions were compared with experimental results from an identical physical model. The experimental flows were investigated by flow visualization and by laser Doppler velocity measurements at selected points in the flow field. The validation was based on comparisons of flow patterns and of non-component velocity-time histories. The maximum Reynolds numbers in the inflow tube of the model VAD were approximately 460 and 3300 using the high- and low-viscosity fluids respectively. The investigation showed that the flow patterns were better predicted for the high-viscosity fluid. However, the agreement between the velocity-time histories was found to be slightly better for the low-viscosity fluid. The discrepancies in the flow patterns may be due to intermittent turbulence with a further contribution from numerical diffusion.     

Fluid flow signals processing based on fractional Fourier transform in a stirred tank reactor

The analysis of fluid flow signals and the characterization of fluid flow behavior are of great importance for two-phase flow studies. In this work, the fractional Fourier transform (FRFT), which was based on the optimum order calculated by stepping search method, was proposed to extract the characteristics of fluid flow signals. Meanwhile, the largest Lyapunov exponent (LLE), which is an indication of the chaotic degree of mixing process, was adopted to quantify fluid flow behavior. The maximum amplitude (MA) and LLE value were taken together to inquire into the relationship between the characteristics of fluid flow signals and the characterization of fluid flow behavior. In addition, differences between the two adjacent values (AD) and the maximum differences (MD) are employed to further analyze the differences in behavioral characterization with MA and LLE. The results show that the MA value performs the same increasing trend as the LLE value when the gas flow rate and agitation speed increase. AD and MD values of the MA are one to two orders of magnitude greater than those of the LLE. The eigenvalues (MA) solved by the FRFT method is facilitates capturing small changes owing to changes in external conditions. These findings can provide new ideas for the extraction and characterization of fluid flow behavior.     

Modeling and simulation of useful fluid flow rate in grinding

This research established a mathematical model of the useful grinding fluid flow rate of a rough grinding wheel. The abrasive distribution matrix of the grinding wheel surface topography was programmed on the MATLAB software platform to obtain the grinding wheel porosity φ at different particle sizes. The grinding fluid flow field was simulated and studied by using the volume of fluid multiphase flow model of FLUENT. Results showed that given a certain circular velocity of the grinding wheel, a larger grinding fluid jet velocity resulted in greater useful grinding fluid flow. When the grinding fluid jet velocity was set, the useful grinding fluid flow increased with increasing circular velocity of the grinding wheel. With the increasing velocity of the grinding wheel, as affected by the airbond layer, the increasing rate of the useful grinding fluid flow decreased, and the flow likewise showed a tendency to decrease. With a certain grinding fluid jet velocity, the useful flow rate of the grinding fluid was positively proportional to the useful flow. When the grinding fluid jet velocity changed and grinding wheel velocity was set, the grinding fluid jet velocity increased as the useful flow rate decreased. When the grinding fluid jet velocity was equivalent to the grinding wheel velocity, the useful flow rate of the grinding fluid was positively proportional to the useful flow. When the minimum clearance of grinding zone h increased, the useful grinding fluid flow and useful flow rate likewise increased. When the grinding fluid jet velocity was equivalent to the grinding wheel velocity, a larger nozzle gap width increased the flow supply for the grinding fluid and the useful grinding fluid flow. However, the increase in the useful flow rate of the grinding fluid was significantly smaller than that of the nozzle flow. This condition decreased the useful flow rate of the grinding fluid.     

切削液油雾形成机制及影响因素的实验研究

对切削液油雾产生的机制进行理论分析,并对切削液油雾形成的影响因素进行实验研究。实验在密闭的车床内进行,利用车床的供液系统采用浇注式供给磨削液,在油雾产生密集区域内安装有磨削液雾滴粒径的测量装置,该装置中的共聚焦显微镜可观察切削液雾滴在空气中的分布,精确扫描单颗雾滴的形态轮廓,通过计算机可计算出雾滴的分布和直径尺寸。分别控制主轴转速和切削液流量,探究对所形成的油雾浓度和平均直径的影响。实验结果表明:金属加工过程中主轴转速和切削液流量对切削液油雾特性的影响显著,随着主轴转速的增加,油雾的平均直径减小,而油雾浓度却呈上升趋势;随着切削液流量的增加,油雾的平均直径变大,并且油雾浓度同样呈上升趋势。     

混合流动式磁流变阀压降性能试验分析

针对目前磁流变阀结构形式单一且体积大的不足,设计了一种结构紧凑的混合流动式磁流变阀,该磁流变阀阻尼间隙液流通道由轴向圆环流动和径向圆盘流动共同组成。介绍了混合流动式磁流变阀结构及工作原理;同时建立了其压降数学模型。在搭建的磁流变阀测试试验台上对其压降性能进行测试,具体分析了加载电流及模拟外加负载对所设计的磁流变阀进出口压差的影响。     

Using µPIV to Investigate Fluid Flow in a Pocketed Thrust Bearing

This article presents the results of an experimental and analytical investigation of the fluid flow in a pocketed thrust bearing. An experimental test rig was designed, developed, and used to visualize fluid flow in pocketed thrust bearings. Microparticle image velocimetry (μPIV) was used to measure fluid flow inside the pocket of a thrust bearing. The thrust bearings were constructed by gluing precision shim stocks to a flat BK7 glass disk in contact with a polished steel disk. The precision shim stock provides the desired pocket depth for the bearing. A polished steel disk in contact with the thrust bearing was driven by a motor in order to induce fluid flow within the pockets. μPIV was then employed to measure the shear-driven cavity flow and generate the quiver plots of the flow field. Three different lubricants were used at various speeds and a constant load to measure the effects of speed and viscosity on the flow out of the pocketed thrust bearing. In order to achieve the analytical aspect of this research, a model was developed to predict the film thickness, cavitation area, and pressure distribution generated within the bearing. The cavitation areas obtained from the model were compared with the experimental results. The results corroborate well. The calculated pressure and film thickness were then used to determine the 3D velocity profiles within the pocketed thrust bearing. The measured velocities obtained from the experimental images were compared to the analytical velocity fields. Comparing the measured velocities with the analytical model, the depth of the microparticles in the bearing pocket was determined. Using this approach, the μPIV-measured 2D velocity field was converted into a 3D velocity field, which illustrates the fluid motion inside a pocketed thrust bearing at various speeds and viscosities.     

毛细管基微流体放大系统流体流量分配的研究

微流体在平行通道中的均匀分配是实现微流体控制放量制备粒径单分散性功能微球的重要技术。本研究设计了毛细管基微流体控制放大装置,详细研究了毛细管尺寸、毛细管长度、流体流量以及流体粘度对微流体流量均分的影响。研究结果表明,随着毛细管内径的减小、毛细管长度的增加、流体流量的增大、流体粘度的增加,流体流量分配均匀性更好,即流体流量变异系数(CV)变小。研究获得了描述CV与其影响参数之间关系的经验公式。     

基于CFD的压裂施工中定压节流器结构优化

为了减轻压裂过程中流体对定压节流器和外面套管的损坏,利用流体动力学分析软件FLUENT中的标准k-ε模型,对节流器附近流场和压力分布进行数值模拟分析,进而对套管内封隔器分段压裂工艺中定压节流器的入口形状和出口形状进行优化。计算结果表明,该模型中的节流器空腔内壁存在冲蚀破坏,这主要是由于节流器入口附近有旋流场存在;节流器入口采用流线形过渡形状,可以减弱入口处的旋流场;近似流线形的节流器出口形状能保证在较高的节流器底部压力的条件下,降低工具内壁的动压力,在射流处起到整流作用的同时,降低湍流强度,有利于减弱工具内壁所受冲蚀;组合使用入口过渡形状和流线形出口结构能有效减弱压裂过程中流体对压裂工具的冲蚀破坏。     

陶瓷空心球形复合超硬磨料低温磨削特性实验研究

针对空心球形复合超硬磨料的多微孔结构对磨削温度影响问题,研究了空心球形复合超硬磨料的结构,建立了空心球形磨料砂轮的磨削模型.从理论上分析了多微孔结构对砂轮的切削锋利度和携带冷却液能力的影响机制;运用温度测量系统,进行了陶瓷空心球形复合超硬磨料砂轮在不同磨削切深和不同冷却液供给量条件下的平面磨削温度实验研究,并分析了磨削温度随不同切深和冷却液供给量的变化趋势.研究结果表明,空心球形磨料砂轮的磨削温度明显低于传统超硬磨料砂轮,验证了空心球形磨料砂轮具有较锋利的切削刃和更好的携带冷却液能力,可有效降低磨削温度,是实现低温磨削的一种新途径.     

滑片式压缩机卸荷过程的静态特性建模与仿真

为研究滑片压缩机卸荷过程的静态特性,建立了流体进入进气口及卸荷阀的流量模型。通过压力一流量特性分析发现,量纲一流量与压力成抛物线关系,并随着压力的增大而减小;通过压力增益特性分析发现,增益曲线的斜率随出口与进口流量系数的比值增大而增大;通过位移一流量特性分析发现,流量随滑柱位移的增大而增大。22kW滑片压缩机的静态特性测试验证了仿真结果。     

Influence of slip velocity in Herschel-Bulkley fluid flow between parallel plates - A mathematical study

This theoretical study investigates three types of basic flows of viscous incompressible Herschel-Bulkley fluid such as (i) plane Couette flow, (ii) Poiseuille flow and (iii) generalized Couette flow with slip velocity at the boundary. The analytic solutions to the nonlinear boundary value problems have been obtained. The effects of various physical parameters on the velocity, flow rate, wall shear stress and frictional resistance to flow are analyzed through appropriate graphs. It is observed that in plane Poiseuille flow and generalized Couette flow, the velocity and flow rate of the fluid increase considerably with the increase of the slip parameter, power law index, pressure gradient. The fluid velocity is significantly higher in plane Poiseuille flow than in plane Couette flow. The wall shear stress and frictional resistance to flow decrease considerably with the increase of the power law index and increase significantly with the increase of the yield stress of the fluid. The wall shear stress and frictional resistance to flow are considerably higher in plane Poiseuille flow than in generalized Couette flow.

     

Flow visualization of oscillation characteristics of liquid and vapor flow in the oscillating capillary tube heat pipe

The two-phase How patterns for both non-loop and loop type oscillating capillary tube heat pipes (OCHPs) were presented in this study. The detailed flow patterns were recorded by a high-speed digital camera for each experimental condition to understand exactly the operation mechanism of the OCHP. The design and operation conditions of the OCHP such as turn number, working fluid, and heat flux were varied. The experimental results showed that the representative flow pattern in the evaporating section of the OCHP was the oscillation of liquid slugs and vapor plugs based on the generation and growth of bubbles by nucleate boiling. As the oscillation of liquid slugs and vapor plugs was very speedy, the How pattern changed from the capillary slug flow to a pseudo slug flow near the annular flow. The flow of short vapor-liquid slug-train units was the flow pattern in the adiabatic section. In the condensing section, it was the oscillation of liquid slugs and vapor plugs and the circulation of working fluid. The oscillation flow in the loop type OCHP was more active than that in the non-loop type OCHP due to the circulation of working fluid in the OCHP. When the turn number of the OCHP was increased, the oscillation and circulation of working fluid was more active as well as forming the oscillation wave of long liquid slugs and vapor plugs in the OCHP. The oscillation flow of R-142b as the working fluid was more active than that of ethanol and the high efficiency of the heat transfer performance of R-I42b was achieved.     

间隙宽度对类迷宫密封性能的影响

类迷宫密封由于其结构的特殊性,内部流体流动也有别于迷宫密封。利用FLUENT软件分析密封间隙对类迷宫密封性能的影响,并与传统迷宫密封内部流场进行对比。结果表明：在同等尺寸条件下,类迷宫密封湍流流体流动在槽向上比迷宫密封明显,在研究类迷宫密封时槽向流动不可忽略;间隙宽度对泄漏特性影响较大,随着间隙宽度的增大,流体速度降低,节流效应减弱,泄漏量增大。     

通道形状对活塞冷却油腔内工作流体阻力及流动特性的影响

为揭示通道形状对活塞冷却油腔内工作流体的阻力及流动特性的影响,将冷却油腔的直壁通道改为波壁通道,并采用压力差和可视化的方法对通道内工作流体的阻力及流动特性展开了实验研究。压力差实验结果表明,与水在直壁通道内的流动相比,水在波壁通道内层湍转捩点大大提前,将非牛顿流体替换水作为工作流体后,层湍转捩点再次提前;可视化实验结果表明,波壁通道内工作流体在雷诺数较低时就可呈现不稳定流动形态。为在较低雷诺数下继续强化活塞冷却油腔内的热质传递,进一步开展了脉动流场下非牛顿流体在波壁通道内的流动特性实验和数值模拟研究,结果表明：脉动流场的操作参数对波壁通道内非牛顿流体的流动不稳定率有较大影响,且影响了通道内非牛顿流体的质量传递。     

连续注排式气体离心密封的性能分析

针对普通离心密封用于气体场合下存在的问题,提出一种带有连续注排密封液结构的新型离心密封,介绍该密封的工作原理及特点。基于VOF方法,利用Fluent软件对离心密封内的气液两相流进行数值模拟,求得密封液流量、气液交界面位置以及压力折减系数等密封特性参数,研究操作参数和结构参数对密封性能的影响。结果表明:该密封适用于高转速下的气体密封,在一定的密封间隙范围内,增加转轴转速会减少密封液的流量,而在密封液进口处设置环形积水槽会增加密封液的流量;增加密封液的注入压力,减小密封间隙有助于降低环形漩涡的影响。     

垂直管中定常螺旋流涡量特性的PIV试验研究

为了研究螺旋流场中的涡量特征分布,提高流体传质效率,利用激光粒子测速系统(PIV)对垂直管中不同介质的螺旋流场进行了测量,并利用Tecplot对螺旋流的涡量进行了显示和局部涡量场提取,研究了不同切向速度下,不同介质的螺旋流涡量场分布特点和其局部涡量特征。结果显示,螺旋流的涡量具有贴壁特征,随着切向速度的增加,螺旋流的涡量强度增大,正涡和负涡交换的频率增大,有利于提高流体传质效率。     

高压子母叶片泵流量均匀性

针对液压用油的可压缩性,研究高压子母叶片泵流量均匀性及其改善的办法。对液压传动领域中传统的忽略油液可压缩性的理论瞬时流量公式进行修正,建立考虑油液可压缩性时的理论瞬时流量数学模型,绘出理论瞬时流量波形图,计算出流量不均匀系数。通过研究泵的工况参数变化与流量不均匀系数的关系,确定影响高压子母叶片泵流量均匀性的主要因素是油液的体积弹性模量、泵的工作压力、转速和阻尼的结构型式及参数,而不是排油腔几何容积变化率的不均匀性。研究结果表明,选择合适的阻尼结构型式及参数,使工作腔在预升压过程中,由机械闭死压缩、阻尼倒灌及吸油区叶片伸出耗油而产生的瞬时损失流量在时域上叠加,并将叠加后的幅值近似为常值,可有效地改善泵的瞬时流量均匀性。     

实验分析两端支承管道的参数共振

用实验方法研究了一端固定、一端铰支输流管道在脉动内流作用下的参数共振问题。对实验管道在几种不同的平均流速下做了多次重复实验,获得了第一振型1／2次谐波参数共振相关实验数据。实验结果表明：当平均流速达到一定值时,一端固定、一端铰支管道在一定的脉动流振幅和脉动频率下会产生第一振型1／2次谐波参数共振;脉动流振幅越大,发生参数共振的频率范围也越大,当脉动流振幅小于一定值时,不再发生参数共振;平均流速越大,发生参数共振所需要的脉动流频率就越小,即低频脉动时平均流速越大管道越容易失稳。实验结果与理论分析结果是一致的。对可能引起定量误差的原因进行了较详细的分析。     

高粘度流体在三维内肋管中层流强化传热性能研究

以润滑油为工质,采用正交原理试验设计方法,对高粘度流体在叉排列三维内肋管中的流动和传热性能进行了研究。结果表明:离散的三维内肋结构能够促进高粘度流体在较低的雷诺数下完成从层流向湍流的转变。说明在高粘度流体的换热问题中,采用三维内肋管可以有效促进流态转变,并因此获得明显的传热强化效果;对试验数据采用最小二乘法进行多元线性回归,获得了三维内肋管中高粘度流体在层流区的流阻和换热准则方程式;根据Webb定义的热力性能系数,作为强化传热性能的判断指标,得到了性能最优的三维肋结构组合,为结构优化指出了方向。