中等半径比内轴高旋圆柱间湍流场的大涡模拟

利用大涡模拟对中等半径比内轴高旋圆柱间湍流场进行了数值模拟。半径比为0.83,形状比为6,侧墙为静止侧墙、旋转侧墙及无剪切力侧墙3种边界条件。模拟结果表明,大涡模拟对该类问题有较强的预报能力。侧墙静止时,涡流始于靠近侧墙的左下方和右下方位置,然后涡心向上向中间移动,涡胞逐渐变大,外轴上形成众多小涡,小涡的涡心向下移动涡胞变大,最后涡胞混合在一起,充满整个轴间。侧墙旋转时,涡流始于靠近侧墙的左上方和右上方位置,然后涡心向上向中间移动,涡胞逐渐变大,外轴上形成众多小涡,小涡的涡心向下移动涡胞变大,最后涡胞混合在一起,充满整个轴间。在无剪切力侧墙边界条件,涡流场形成过程与侧墙旋转时形成过程相似。轴间流场最终形成固定数量的涡胞,且随着时间的推移,各个涡胞呈现此消彼长的局面,始终保持固定数涡胞的存在。在侧墙静止和无剪切力条件,流场最终形成8个涡胞;侧墙旋转时,流场最终形成6个涡胞。     

沟槽壁面和温度梯度对环隙内流场稳定性的影响

利用PIV结合折射率匹配法对光壁和沟槽模型内的Taylor-Couette流动进行测量,通过加热内圆柱构建不同的温度梯度工况以及在外圆柱壁面上安装沟槽,研究沟槽和温度梯度对泰勒涡动稳定性的影响规律。试验模型的内外圆柱的半径比为0.825,纵横比为48,雷诺数Re=80~110,研究中考察格拉晓夫数Gr=3 600,6 700两种不同的温度梯度工况对流场的影响,试验结果表明,温度梯度作用下使得不同模型内的泰勒涡具备了轴向运动的速度,同时泰勒涡的轴向尺寸也发生改变,随着温度梯度增加,环隙内向外圆柱方向的径向速度有所增加;泰勒涡的运动速度随着温度梯度的增加而增加,相同温度梯度下,随着Re的增加,泰勒涡的运动速度也随之增加;沟槽模型的存在增加了环隙内的径向速度,较光壁模型增加了130%,且受到沟槽和温度梯度的共同作用,使得泰勒涡中逆时针旋转的涡胞尺度增加,涡心位置偏离内圆柱。     

Couette-Taylor-Poiseuille流的数值模拟

对中等半径比同心旋转圆柱间Couette-Taylor-Poiseu ille流进行了数值计算,并与已有的实验数据进行比较以获得流场的更多信息。结果表明,数值计算与实验结果吻合较好,依次再现了层流涡、波动涡、非波动螺旋涡以及波动螺旋涡;轴流可以起到稳定流场的作用,轴流存在时,流场转捩的临界泰勒数Ta值会变大,涡胞变小,涡心不再位于轴间隙的中间,从左向右的轴流比较明显,交替指向内轴和外轴,并缠绕在涡胞的周围;除去平均轴流速度后,速度矢量场显示出不同的涡形,形状与相同Ta时的涡胞基本相同;在不同的泰勒数Ta和雷诺数Re下,涡心的轴向传播速度约为平均轴向流速的1.17倍,相传播速度约为内筒转速的0.42倍。     

低压差驱动下中等半径比内轴高旋圆柱间轴流量的数值计算

利用RSM湍流模型/标准壁面函数对低压差驱动下中等半径比内轴高旋圆柱间流场进行了数值模拟。圆柱的内半径为4.45 cm,外半径为5.25 cm,半径比为0.848,轴间距为0.8 cm,形状比分别为5,10,15,20,泰勒数比分别为500,625,750,875,1 000。模拟结果表明,RSM湍流模型/标准壁面函数对低压差驱动下中等半径比内轴高旋圆柱间流场有较强的预报能力。随着压差的增加,轴流量随之增加,随着压差的进一步增加,压力流的作用处于主导地位,压力流的增强最终导致涡胞消失,轴间流场特征主要表现为压力流。随着旋转速度的增加,内壁高旋产生的旋转效应增强,轴向压差对轴间流场的作用相对减弱,导致轴流量减少。随着形状比的增加,轴流量呈现逐步减小的趋势,而且随着形状比的进一步增加,轴流量减少的趋势变缓。     

Taylor-Couette 流与旋转圆柱轴间流体膜润滑与密封

对国内外Taylor-Couette流的形成与发展,旋转圆柱轴间流体膜润滑与密封的数值计算及其相关实验的研究现状进行了分析。分析结果表明,应用雷诺润滑方程求解旋转圆柱轴间流体膜润滑与密封问题有其先天的不足,雷诺润滑方程的推导过程中所采用的假设不尽合理,在雷诺润滑方程的基础上进行惯性力修正和涡粘性修正,或者是人为地对3种流动进行解耦,只能作为一种近似计算。从Taylor-Couette流的基本理论出发,计算旋转圆柱轴间流体膜润滑与密封应是计算其层流问题的发展方向。对于湍流润滑与密封,直接模拟(DNS)和大涡模拟(LES)应是首选方案。     

旋转管式膜分离器流型的研究

采用三相异步电动机的输入电磁转速与三相异步电动机的实际转速之差的变化来表征两旋转圆筒间流场特性的参数。主要研究了清水介质时无轴向流和有轴向流条件下流型的划分,并将无轴向流时的流型分成了6种：Couette层流,层流泰勒涡,层流波涡,层流调制波涡、湍流泰勒涡和湍流;有轴向流时的流型分成了4种：Couene层流、层流泰勒涡、湍流泰勒涡和完全湍流。并且验证了流型转变的滞后特性。     

湍流边界层内钝体扰流的流动与传热特性

应用大涡模拟方法对小尺度开缝圆柱涡流发生器强化传热和流动减阻的机理进行研究。水平开缝圆柱置于充分发展湍流边界层内,分析不同间隙比对开缝圆柱尾流、湍流边界层拟序结构以及槽道底面流动与换热特性的影响。为验证所采用数值方法的准确性与可靠性,将矩形空槽道的计算结果与前人直接数值模拟结果及与采用相关准则关系式所得结果进行对比。计算结果表明：湍流边界层内钝体扰流的尾迹流与壁面边界层的相互作用能够显著提高槽道的换热性能。与未开缝的基准圆柱相比,间隙比小于2.0时,开缝圆柱通道的整体热性能较好;间隙比为2.0时,其综合性能系数最大;间隙比大于2.0时,整体热性能较差。与矩形空槽道相比,最大努塞尔数可提高17.45%,最小摩擦因数可减小4.94%。     

后台阶流场大涡模拟的入口边界条件研究

对不同入口边界条件下后台阶下游三维流场进行了大涡模拟。讨论了层流及湍流入口流速剖面下边界层厚度及湍流度对台阶下游平均流速剖面、再附长度的影响。模拟结果表明,对于层流入口流速剖面,边界层厚度对再附长度的影响很大,随边界层厚度的增大,再附长度将增大。对于湍流入口流速剖面,边界层厚度对再附长度的影响不大,而入口湍流度则对再附长度有较大影响。随着入口湍流度的增加,再附长度将会变短矗在雷诺数为5000情况下,当采用湍流入口流速剖面并选择湍流度为0．5％时,大涡模拟能得到与直接数值模拟基本一致的结果。     

广义边界下含双相流水平井钻柱频率特性分析

钻柱在内流作用和旋转因素的影响下容易产生耦合振动,发生疲劳失效。本文基于微分求积法（DQM）对含双相流水平井钻柱耦合动力学特性进行了研究。利用扩展的Hamilton变分原理建立了计入内流、轴向压力及旋转等因素影响的水平井钻柱动力学方程。在振动问题中考虑了广义边界条件,通过改变边界等效弹簧刚度将模型简化为简支、悬臂等简单边界条件模型进行研究。通过分析旋转角速度、轴向压力、液相流速、气体体积分数等因素对模型频率特性的影响,得到了无量纲固有频率随不同参数变化的特征曲线。分析结果表明：不同边界条件下模型的频率特性曲线有很大的差别;气体体积分数对临界流速的影响在悬臂管系统中表现的更为明显;在简支管模型中,随着轴力的增大会产生模态耦合颤振。此外,通过液相流速和旋转角速度的频率云图展示了两种因素对钻柱频率特性的影响。     

小尺度圆柱涡流发生器的传热与流动特性

应用大涡模拟对设置小尺度圆柱涡流发生器矩形槽道底面的流动与传热特性进行研究,小尺度圆柱涡流发生器置于湍流边界层内。分析不同间隙比对槽道流动结构、槽道底面Nusselt数、摩擦因数以及综合性能系数的影响。此外,采用大涡模拟所得槽道计算结果与前人直接数值模拟结果一致性很好,验证所采用数值方法的准确性与可靠性。研究结果表明,与未设置小尺度圆柱涡流发生器的矩形槽道相比,设置小尺度圆柱涡流发生器槽道底面的换热性能可以得到显著提高,同时其流动阻力的增加亦得到有效抑制。当间隙比为2.0时,槽道底面换热性能最佳,其Nusselt数可提高18.76%;而当间隙比为0.5时,槽道底面减阻效果最佳,摩擦因数可减小3.77%。     

Experimental study on the vortex flow in a concentric annulus with a rotating inner cylinder

This experimental study concerns the characteristics of vortex flow in a concentric annulus with a diameter ratio of 0.52, whose outer cylinder is stationary and inner one is rotating. Pressure losses and skin friction coefficients have been measured for fully developed flows of water and of 0.4% aqueous solution of sodium carboxymethyl cellulose (CMC), respectively, when the inner cylinder rotates at the speed of 0-600 rpm. Also, the visualization of vortex flows has been performed to observe the unstable waves. The results of present study reveal the relation of the bulk flow Reynolds number Re and Rossby number Ro with respect to the skin friction coefficients. In somehow, they show the existence of flow instability mechanism. The effect of rotation on the skin friction coefficient is significantly dependent on the flow regime. The change of skin friction coefficient corresponding to the variation of rotating speed is large for the laminar flow regime, whereas it becomes smaller as Re increases for the transitional flow regime and, then, it gradually approach to zero for the turbulent flow regime. Consequently, the critical (bulk flow) Reynolds number Re_c decreases as the rotational speed increases. Thus, the rotation of the inner cylinder promotes the onset of transition due to the excitation of Taylor vortices.     

Axial slit wall effect on the flow instability and heat transfer in rotating concentric cylinders

The slit wall effect on the flow instability and heat transfer characteristics in Taylor-Couette flow was numerically studied by changing the rotating Reynolds number and applying the negative temperature gradient. The concentric cylinders with slit wall are seen in many rotating machineries. Six different models with the slit number 0, 6, 9, 12, 15 and 18 were investigated in this study. The results show the axial slit wall enhances the Taylor vortex flow and suppresses the azimuthal variation of wavy Taylor vortex flow. When negative temperature gradient exists, the results show that the heat transfer augmentation appears from laminar Taylor vortex to turbulent Taylor flow regime. The heat transfer enhancement become stronger as increasing the Reynolds number and slit number. The larger slit number model also accelerates the flow transition regardless of the negative temperature gradient or isothermal condition.     

Viscous Flow between Rotating Concentric Cylinders with a Circumferential Pressure Gradient at Speeds above Critical

For sufficiently high speeds of the inner cylinder, the flow between concentric rotating cylinders with a circumferential pressure gradient is unstable. By considering the full nonlinear equations, the amplitude of the resulting vortex motion is computed. The effect of the vortex motion on the shear stress at the inner cylinder and the relation between the mean flow and the pressure gradient are discussed.     

Stroboscopic two-dimensional ultrasonic velocity profiling for measuring flow transition in Taylor Couette systems

Flow characterization in a Taylor Couette system was made by investigating the radial velocity component with Ultrasonic Doppler Velocimetry based flow mapping. With the technique presented in this work, it is possible to measure the radial velocity components for variable axial position in a Couette cell within Taylor vortex flow (TVF), wavy vortex flow (WVF), modulated vortex flow (MVF) as well as spiral vortex domains in a conical shaped gap. The resulting maps for the different flow states show the location of vortices in the annular gap between the inner and outer cylinder. Cylindrical and conical concentrically rotating inner bodies were applied and respective flow patterns were analyzed. The method uses a stroboscopic triggering to synchronize flow measurements and rotational motion. The oscillation frequency f of unsteady motion in WVF, MVF, and spirals can be obtained from the power spectrum of velocity. The UVP transducer was preferably positioned in radial direction, perpendicular to the surface of the inner rotating body for measuring the radial velocity component. At the same time, the transducer was moved with constant velocity vertically along the outer cylinder height.     

掠形对高负荷风扇转子失稳机理的影响

以超高负荷两级风扇第一级前掠转子为研究对象,通过定常和非定常数值模拟手段对近失速点叶尖流场进行了深入分析。为探索失稳机理,与同等气动性能下的后掠转子进行了对比。结果表明:前掠转子在近失速工况下泄漏涡涡心并未发生破碎,其流动失稳是由于端壁机匣处的低能流体集聚所致。而后掠转子的泄漏涡与激波干涉导致其向下游发展中发生破碎,表现出与常规转子相似的失速特征。     

边界滑移对柱塞偶件间油膜流动规律的影响

斜盘式轴向柱塞泵内柱塞偶件间油膜为相对运动的偶件提供润滑及密封作用。油膜流动将直接影响柱塞偶件的工作性能。深入分析偶件间油膜的流动规律对设计与优化柱塞偶件有重要意义。基于Navier-Stokes（N-S）方程,引入Navier边界滑移推导偶件间油膜流动方程,根据柱塞运动的周期性规律,分析单个周期内滑移长度和柱塞泵转速对油膜流动剪应力及流量的影响。研究发现：吸油阶段时近柱塞壁面处油膜剪应力随滑移长度增大而减小,流量随着滑移长度增大而增大,柱塞运动速度最大且滑移长度由1 μm增大到3 μm后,剪应力减小18%,流量增大13.59%;排油阶段柱塞运动速度越大,近柱塞壁面处剪应力和油膜流量与无滑移条件下的差距越小。在滑移长度为1 μm的条件下柱塞泵转速由1 500 r/min增大到4 000 r/min时,近柱塞壁面处的油膜剪应力与无滑移条件下相比降低明显,一个周期内油膜总流量与无滑移条件下相比差距减小。