基于MATLAB的微轴承动力学特性的数值分析

微机电系统下,针对传统连续模型无法准确描述气体滑移对微轴承动力学特性影响规律的问题,引入Wu新滑移模型(任意克努森数下都与线性玻尔兹曼方程解有较高的吻合度)对传统连续模型和一阶滑移模型进行了修正。基于有限差分法,建立了用于数值分析微轴承动力学特性的动态数学模型,提出了Wu新滑移模型下动态数学模型的建立方法;在MATLAB软件平台上,对不同轴承参数下气体滑移与微小间隙下微轴承刚性转子系统动力学特性及其稳定性的变化关系进行了数值分析。研究结果表明:在微小气膜间隙下,微轴承在连续模型下预估的轴承动力学特性系数值最大,一阶滑移模型次之,Wu新滑移模型最小;即在设计MEMS下的微轴承时应采用Wu新滑移模型分析该类问题。     

微转子系统径向气体轴承特性

充分考虑滑移边界条件的影响,利用数值计算方法对微机电系统(Micro-electro-mechanical systems, MEMS)中微转子系统径向气体轴承的雷诺方程进行修正求解,得到轴承内部真实的气压分布,进而求得微气体轴承的承载能力和偏位角.通过与宏观无限短轴承模型的结果进行对比分析,发现滑移效应对气体轴承特性的影响规律.宏观无限短轴承模型和无滑移边界模型均会高估气体轴承的承载能力,特别是偏心率较高时(ε>0.6),产生的偏差更大;微转子系统在高速或超高速工况下运转,可以提高气体轴承的气压和承载能力.     

微壳体塑件充模流动三维数值模拟

针对微注塑成形中熔体充模流动行为,运用流体分析软件Fluent,对微壳体塑件熔体充模流动进行了三维稳态模拟。建立了微观黏度模型和壁面滑移模型,运用VOF多相流模型,研究了熔体充满型腔时的稳态三维流场,做出切片图分析了压力场和速度场。通过对考虑和不考虑微尺度影响(微观黏度和壁面滑移)的对比和分析,可以看到考虑微尺度因素影响时充模阻力减小约19%,而速度变化不明显。分析表明微尺度黏度对熔体流动影响显著,有利于微注塑熔体充模,而壁面滑移对充模流动影响较小。     

变高度螺旋倾斜槽气体轴承的微尺度数值模拟

基于平面螺旋槽气体止推轴承结构,结合以对数螺旋线为素线的空间螺旋曲面结构,提出一种新型气体止推轴承模型。通过坐标变换推导出适应对数螺旋线轨迹的流量方程,引入一阶滑移和新滑移边界条件修正流量方程,应用数值方法迭代求解了连续型模型和2种滑移模型下新型气体止推轴承的气膜压力分布。结果表明:微尺度下随着气膜厚度减小,边界速度滑移效应会对轴承承载力特性产生很大影响;当Knudsen数增大到过渡区时,一阶滑移和新滑移模型之间的偏差明显增大;在相对较低的转速下,新型轴承的承载力相较于传统轴承明显增大。     

微尺度下液体静压轴承性能研究

针对液体静压轴承油膜缝隙流动所出现的微尺度效应,采用计算流体力学的分析方法,应用Navier速度滑移模型,将近壁面速度滑移边界条件引入到油膜二维流动,推导出适合流体二维流动的N-S方程,并对轴承承载力和刚度进行分析。结果表明,微尺度下速度滑移对轴承的承载力和刚度造成了极大的影响,滑移长度增加时,轴承的最大承载能力将严重减小,油膜刚度将会增加;滑移的发生导致最优的油膜厚度变小。     

晶粒取向对微细加工磨削力作用机理及试验研究

微磨削加工中切削深度尺度一般小于被加工材料晶粒大小平均尺度,磨削刃作用在晶粒内部,工件材料表现为各向异性,因此,材料晶粒取向对磨削力作用较传统磨削更加显著,微磨削力的产生机理也会发生变化.为了探究晶粒取向对微细加工磨削力的作用机理,提出泰勒因子模型,量化晶粒取向对流动应力的影响,完善材料的流动应力本构模型.通过研究剪切面晶粒取向与滑移面之间的夹角以及剪切方向与滑移方向之间的夹角关系,确定激活滑移系的数量及类别,从而得到泰勒因子值.基于考虑泰勒因子模型的流动应力本构模型及平行剪切带理论,分析晶粒取向对切屑成形力作用机理并构建解析模型.通过磨棒形貌试验测量数据提取及拟合,提出新的磨棒静态磨粒密度的计算方法,进而构建动态磨粒密度模型.基于单颗磨粒磨削力模型以及磨棒动态磨粒密度模型,构建微细加工磨削力的预测模型.本模型综合考虑微磨削过程中力-热耦合效果、材料微观结构、磨棒形貌以及微磨削加工工艺,并通过微磨削试验对磨削力模型进行了验证.     

锥台型气体润滑动压轴承动力学数值模拟研究

针对高精密陀螺电机气体润滑动压轴承间隙复杂流动特点,分析轴承的静态特性和推导一阶滑移边界条件。采用基于无滑移边界和一阶滑移边界条件的计算流体力学方法模拟气体润滑动压轴承间隙复杂流动,计算获得不同偏心距轴承的静态载荷、刚度、功率和偏位角等参数,并与轴承试验结果进行对比分析。研究结果得出,在自重条件下轴承试验测量刚度结果位于无滑移和滑移边界条件数值模拟结果之间,比无滑移边界条件计算结果小约10%,比一阶滑移边界计算结果大约44%,表明轴承间隙流动只存在局部滑移;数值模拟结果的偏位角方向与试验结果一致,分别相差4°和2.6°左右。研究结果可为陀螺电机气体润滑轴承设计提供技术参考。     

微注塑成形中熔体充模流动分析及其数值模拟

借鉴宏观熔体的流变学理论和建模技术,针对微尺度流道中的聚合物熔体流动特性,采用模型修正方法,建立反映微小通道中熔体流动特性的理论模型.同时,应用数值模拟方法,研究微尺度粘度、壁面滑移和熔体与模具间的表面传热系数对微小熔体流动的影响关系,并与相关试验数据进行比较.结果表明,微流道中的熔体粘度明显小于传统理论下的粘度值,且与微流道的特征尺寸成正比.随微流道特征尺寸减小,滑移系数也明显减小,壁面滑移速度则增大.考虑局部表面传热系数时微流道中的熔体温度分布具有尺寸效应.微尺度流道中的熔体流动行为与宏观熔体有许多不同.     

超疏水表面减阻特性的研究进展

超疏水表面具有减阻效果,在提高管道传输效率、降低水下航行体和微流体器件中流动阻力等方面有着广阔的应用前景。介绍超疏水表面的制备、滑移理论以及减阻特性的研究,讨论微尺度下表面润湿性、表面微结构和流场流动状态对壁面减阻的影响,对超疏水壁面减阻的物理机制进行总结,并指出气体层不连续模型和气穴模型是分别适用于光滑疏水表面和带微结构超疏水表面的减阻模型。介绍超疏水表面减阻特性的一些应用,提出将超疏水表面应用到微流体系统中面临的问题,如微通道壁面疏水性的制备及其减阻效果的耐久性。     

稀薄效应对静压气体轴承性能影响

针对微小气膜间隙下静压气体轴承内部气体的稀薄效应,运用有限体积法离散了体现稀薄效应的滑移修正Reynolds方程,通过MATLAB软件分析稀薄效应下轴承内部压强,并对轴承承载力及最终刚度变化进行研究。通过与不考虑稀薄效应的连续模型对比发现,当气膜间隙小于1μm时,最小气膜间隙处一阶滑移及Wu新滑移模型的气压、承载力及最终刚度远低于连续模型,模型间的结果偏差随气膜间隙的减小而迅速增大。     

Heat transfer characteristics of gaseous slip flow in a micro-channel

Two-dimensional compressible momentum and energy equations with slip boundary conditions are solved to obtain the heat transfer characteristics of gaseous slip flow in a micro-channel with CWT (constant wall temperature) whose temperature is lower or higher than the inlet temperature (cooled case or heated case). The numerical methodology is based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The stagnation temperature is fixed at 300 K and the computations were done for the wall temperature which ranges from 250 K to 350 K. The channel height ranges from 2 to 10 μm and the channel aspect ratio is 200. The stagnation pressure is chosen in such a way that the exit Mach number ranges from 0.1 to 0.7. The outlet pressure is fixed at atmospheric condition. The bulk temperature and the total temperature of the heated case are compared with those of the cooled case and also compared with temperatures of the incompressible flow in a conventional sized channel. Heat transfer characteristics of the gaseous flow are different from those of the liquid flow. And they are also different from each cooled and heated case. A correlation for the prediction of the heat transfer rate of the gaseous slip flow in a micro-channel is proposed.     

随机粗糙表面对微气体轴承性能的影响

采用尺度独立的分形参数表征粗糙表面,修正气体的雷诺方程,研究滑块轴承中气体薄膜在稀薄效应和粗糙度耦合作用影响下的压强分布、承载能力及承载力的主流向位置。结果表明:由于粗糙度的影响,气体轴承内的压强分布具有非线性特征,承载能力也不会简单地增大或减小,但承载力的主流向位置会变大。     

Three-dimensional flow analysis and improvement of slip factor model for forward-curved blades centrifugal fan

This work developed improved slip factor model and correction method to predict flow through impeller in forward-curved centrifugal fan. Both steady and unsteady three-dimensional CFD analyses were performed to validate the slip factor model and the correction method. The results show that the improved slip factor model presented in this paper could provide more accurate predictions for forward-curved centrifugal impeller than the other slip factor models since the present model takes into account the effect of blade curvature. The correction method is provided to predict mass-averaged absolute circumferential velocity at the exit of impeller by taking account of blockage effects induced by the large-scale backflow near the front plate and flow separation within blade passage. The comparison with CFD results also shows that the improved slip factor model coupled with the present correction method provides accurate predictions for mass-averaged absolute circumferential velocity at the exit of impeller near and above the flow rate of peak total pressure coefficient.     

滑动轴承二维流场的滑移现象研究

目前对于二维流场及复杂流场的界面滑移分析很少,根据螺旋油楔滑动轴承能使润滑剂产生周向和轴向二维流动的独特的结构特点,考虑周向和轴向两方向的滑移建立基于极限切应力的数学模型,并通过试验和理论对比验证模型的正确性。试验方面运用"目标速度跟踪法"证实了周向和轴向都存在滑移,获知随着供油压力的提高滑移速度有所提高,并且提出轴瓦和轴表面的极限切应力;理论方面运用有限差分法和试验测得的轴瓦和轴表面极限切应力,求解四种状态的广义雷诺方程,发现滑移发生在极限切应力大、间隙小和油膜的封油面区域;考虑界面滑移时,螺旋油楔滑动轴承的承载力和摩擦阻力有所降低;偏心率、螺旋角和转速的变化,影响着承载力和摩擦阻力降低的幅度。     

Numerical and experimental analysis of aerostatic thrust bearings with porous restrictors

The finite element method is used to predict the performance of aerostatic thrust bearings with a complete porous surface. Results obtained by a 1D and 3D source flow model derived from D’Arcy’s law are compared for rectangular porous bearings having an infinite width. It turns out that the 1D source flow model is adequate for practical design parameters. For a circular aerostatic porous thrust bearing results calculated with several mathematical models for the source flow and slip flow are compared with experimental results. A relatively simple model incorporating unflatness and deformation of the bearing surface correlates well with the experimentally determined bearing performance.     

节流短管内制冷剂流动的两流体模型

制冷剂在短管内的两相节流过程存在显著的动力学和热力学非平衡特性。建立了描述节流短管内制冷剂两相流的六方程两流体模型,模型中考虑了相间速度滑移和温度滑移。计算表明,制冷剂在短管内比在毛细管(长管)内的闪蒸过程具有更大的相间速度滑移和温度滑移。对R134a在短管内的流量特性进行了预测,与试验数据偏差小于±20％。     

微通道内流体流动及换热特性的数值分析

采用Navier—Stokes方程与滑移边界条件联立的理论分析模型，对等壁温、等热流及无温度梯度工况下，气体在微通道中的流速分布、阻力系数变化趋势（Cf·Re）和传热特性（努塞尔数）进行了数值研究。结果表明：气体稀薄效应可显著减小管内的摩擦阻力和努塞尔数，增大气体流速；壁面的速度滑移和温度跳跃对微圆管内换热特性的影响相反，温度跳跃的影响更大；等热流加热与等壁温加热两种情况下，努塞尔数随克努森数的变化趋势明显不同。     

稀薄效应对动压气体轴承静动特性的影响

为研究稀薄效应对微小间隙下动压气体轴承静动态特性的影响,分别采用连续模型、一阶滑移模型以及在任意克努森数下都与线性玻尔兹曼方程解有较高吻合的WU新滑移模型,基于有限体积法建立考虑稀薄效应的静动态Reynolds方程,给出3种模型下轴承静态承载力与偏心率、轴颈扰动频率与轴承动特性系数的变化关系。数值分析结果表明:随偏心率的变化,连续模型预测的静态承载力最大,一阶滑移次之,WU新滑移模型预测的值最小;随着扰动频率的增加,考虑滑移模型计算的主刚度系数和主阻尼系数均有减小的趋势,且WU新滑移模型计算的主刚度系数和主阻尼系数明显低于连续模型和传统的一阶滑移模型。传统连续模型和一阶滑移模型过高地估计了轴承的静动特性系数,WU新滑移模型计算的结果更为准确。     

Numerical prediction of slip flow effect on gas-lubricated journal bearings for MEMS/MST-based micro-rotating machinery

The fluid dynamics of gas-lubricated journal bearings in micro-rotating machinery is different from those of larger size. One point to be considered is a slip flow effect. In this paper, the slip flow effect is considered in order to estimate load-carrying capacity and dynamic coefficients of micro gas-lubricated journal bearings. Based on a modified compressible Reynolds equation including slip flow effect, the first slip approximation was applied. To extract dynamic coefficients, the linearized dynamic equations were formulated by the perturbation method. Numerical predictions compared the static and dynamic characteristics considering slip flow at room-to-high temperature in a range of bearing numbers. The results including load-carrying capacity and dynamic coefficients demonstrate that the slip flow effect becomes significant with temperature increase as well as in the lower range of bearing numbers.     

Performance model evaluation of turbine flow meter in vertical gas-liquid two-phase flows

Aiming at the need for flow measurement of gas-liquid flows in domestic gas well production, this paper proposes a measurement method based on the combination of the turbine flow meter (TFM) and a rotating electric field conductance sensor (REFCS). In experiments, the REFCS is used for the measurement of the gas holdup. To verify the applicability of the TFM models investigated in the previous study, for the modeling part, the mass, momentum and torque models are evaluated in vertical upward gas-liquid two-phase flows. In our model test, the meter factor model of TFM considers the effects of the slip ratio between the gas and liquid phases and flow patterns. In particular, the gas holdup involved in calculating the slip ratio in the model evaluation is obtained from the REFCS measurements. Model test results show the torque model has better volumetric flow rate prediction accuracy than the mass and momentum models. In the present study, the ranges of the liquid and gas phases are Q_w = 2–30 m³/d and Q_g = 1–16 m³/d, it was found that the average absolute deviation (AAD) in the predicted volume flow rate is equal to 1.23 m³/d and the average absolute percentage deviation (AAPD) is equal to 7.69%. The evaluated results presented in this paper will allow better estimates of the volumetric flow rates of gas-liquid flows based on the combined TMF and REFCS measurements during the monitoring of gas well production.