Numerical simulation of rotordynamic coefficients for eccentric annular-type-plain-pump seal using CFD analysis

Annular seals are primarily used to control the leakage in turbomachinery, such as pumps. Consequently, annular seals substantially affect the stability of turbomachinery. Designing the annular seals of high performance pumps require the accurate prediction of the seal’s leakage flow rate and rotordynamic coefficients. The bulk-flow model is the traditional means for leakage flow rate analysis and predictions. Bulk-flow analysis is based on the Hirs’ lubrication equation, which simplifies the Navier-Stokes equation. However, the analysis of the bulk-flow model requires a great amount of time to develop an analysis code. Furthermore, the code possesses many constraints for analyzing seals with complicated shapes. 3D CFD simulations provide faster and less expensive estimates of the flow field for a wide variety of operating parameters and flow conditions. In this study, the flow field and the rotordynamic coefficients of a plain-eccentricannular seal were simulated with circular whirl orbits using 3D CFD code. A relative coordinate system was defined to calculate the 3D velocity profile and the dynamic pressure distribution of the seal clearance for each rotor whirling speed. The rotordynamic coefficients were determined by reaction forces of seal fluid, which were calculated by integrating the dynamic pressures to the whole area of seal. The results from our analyses were compared to existing theoretical calculations as well as compared to results acquired from experiments. The present 3D CFD results of leakage and rotordynamic coefficients of K and C showed better improvement in prediction.     

Numerical techniques for computing nonlinear dynamic characteristic of rotor-seal system

A transient CFD procedure to compute the nonlinear dynamic characteristic of the coupled rotor-seal system was presented in this study. In each time step, the displacement diffusion model was implemented to govern the mesh deformation, and the URANS (Unsteady Reynolds averaged Navier-Stokes) equations were solved to obtain the transient fluid force on the rotor surface for the free vibrations. With the obtained fluid force from the CFD solver, the nonlinear equations of motion for a simplified rotor-seal system were numerically solved on the basis of external user defined routines. During each transient time step, the computed fluid force from the CFD solver and the rotor motion from the user defined routines were transferred to each other timely. The rotor center trajectories, frequency spectrum and projection of Poincaré section were calculated to investigate the nonlinear dynamic performance of the single disk rotor-seal system. The effects of the rotational speed and pressure ratio on the vibration characteristic of the rotor-seal system were analyzed by the bifurcation theorem. The results show that the coupled rotor-seal system experiences a period-one motion, resonance, periodic-doubling motion, quasi-periodic motion, and finally possible chaotic motion as the rotor speed increases. The pressure ratio has pronounced effect on the frequency response of the first-order critical speed; however, it has little influence on the motion state as well as the frequency response of the rotating speed. Although a constant-clearance annular smooth stator seal was selected as the research object in the current paper, the presented transient CFD method is still available for other complex annular seals, such as labyrinth seal, honeycomb seal, pocket damper seal, etc.     

Prediction of leakage and rotordynamic coefficients for the circumferential-groove-pump seal using CFD analysis

The circumferential-groove seal is commonly used in various turbopumps to reduce leakage. The main goal of this paper is to develop the method of three-dimensional CFD analysis for determining leakage and rotordynamic coefficients of the circumferential-groove-pump seal. A relative coordinate system was defined for steady-state simulation to calculate the velocity and pressure distributions of the seal clearance at each rotor whirl speed. Instead of setting the inlet and outlet pressures as the boundary conditions in the three-dimensional CFD analysis, as it is more commonly done, we used the inlet velocity and outlet pressure obtained from a preliminary two-dimensional CFD analysis. For prediction leakage, the presented analysis shows improvement from the bulk-flow model analysis. For the prediction of rotordynamic coefficients of K, k and C, the presented analysis provides results in closer agreement with the experimental values than those of the bulk-flow model analysis at several rotor speeds.     

Stiffness and damping coefficients for journal bearing using the 3D transient flow calculation

The dynamic coefficients of journal bearing are necessary components in the analysis of linear stability and response of rotating dynamic systems. We propose a new method for the numerical identification of bearing support force coefficients in flexible rotor-bearing systems based on the 3D transient flow calculation. The CFD commercial software FLUENT is mainly used in this simulation, which employs a finite volume method for the discretization of the Navier-Stokes equations. To determine the dynamic coefficients, a new mesh movement approach is presented to update the volume mesh when the journal moves during the 3D transient flow calculation of a journal bearing. Existing dynamic mesh models provided by FLUENT are not suitable for the transient oil flow in journal bearings. Measurements and identification are performed on a test rotor supported on a pair of identical two-lobe fluid film bearings, and the results obtained from the CFD methods agree well with experimental results. The results indicate that the methods proposed in this paper can predict the dynamic coefficients of the journal bearing in a rotor-bearing system effectively, and provide a further tool for stability analysis.     

转/静子齿对迷宫密封泄漏特性与动力特性影响机制研究

迷宫密封齿设计于转子或静子上对迷宫密封的性能影响较大.为揭示转/静子齿对迷宫密封泄漏特性与动力特性的影响机制,应用非定常动网格技术建立了迷宫密封泄漏特性与动力特性多频椭圆涡动求解模型,在验证求解模型准确性基础上,分析转/静子齿迷宫密封的泄漏特性与动力特性,并讨论2种结构对迷宫密封转子系统稳定性的影响.结果表明:转/静子...     

Numerical prediction of rotordynamic coefficients for an annular-type plain-gas seal using 3D CFD analysis

Annular-type gas seals in many types of compressors and turbines are designed to reduce leakage and enhance the vibrational stability of the turbo machinery. Many researchers have attempted precise theoretical evaluation of the leakage and the rotordynamic coefficients from reaction forces of small seal gaps. The Bulk-flow model, which is based on Hirs’ lubrication equation, is a general method with advantages of simplicity and short computing time. However, due to the disadvantage of the long time required to develop analysis code, and constraints from complicated seal shapes, CFD analysis is currently preferred. In the present, the method for determining the rotordynamic coefficients of an annular plain-gas seal, which is the simplest shape of gas seals, is suggested by extending the analysis of an annular plain-pump seal. A relative coordinate system for steady-state simulation is defined to calculate the compressible flow field and dynamic pressure distribution of the seal gap. The present analysis is verified by comparison with results acquired from Bulk-flow analysis code and published experimental results. The 3D CFD rotordynamic coefficients results of direct stiffness(K) and cross coupled stiffness(k) show better improvements in prediction.     

Experimental Study of the Flow Field Inside a Whirling Annular Seal

The flow field inside a whirling annular seal has been measured using a 3-D Laser Doppler Anemometer (LDA) system. The seal investigated has a clearance of 1.27 mm, a length of 37.3 mm and is mounted on a drive shaft with a 50 percent eccentricity ratio. This results in the rotor whirling at the same speed as the shaft rotation (whirl ratio = 1.0). The seal is operated at a Reynolds number of 12000 and a Taylor number of 6300 (3600 rpm). The 3-D LDA system is equipped with a rotary encoding system which is used to produce phase averaged measurements of the entire mean velocity vector field and Reynolds stress tensor field from 0.13 mm upstream to 0.13 mm downstream of the seal. The mean velocity field reveals a highly three-dimensional flow field with large radial velocities near the inlet of the seal as well as a recirculation zone on the rotor surface. The location of maximum mean axial velocity migrates from the pressure side of the rotor at the inlet to the stiction side at the exit. Turbulence production is a maximum near the seal inlet as indicated by the rapid increase of the turbulence kinetic energy (κ). However, turbulence production and dissipation attain equilibrium fairly quickly with K remaining relatively constant over the last half of the seal.     

汽轮机叶顶间隙汽封内泄漏流动的数值研究

采用计算流体动力学软件分析某汽轮机高压级叶顶汽封内的泄漏流动特性及泄漏流在级后与主流的掺混过程,并比较有无汽封情况下间隙流场的气动性能。结果表明:汽封间隙内充满复杂的涡运动,使泄漏流的动能得到充分的耗散,且齿数增加可使耗散更加充分;增加叶顶汽封装置可有效降低级后间隙泄漏流与主流的掺混损失,增加汽封齿数能够在一定程度上削弱级后掺混带来的流动影响。     

基于相似原理的环瓣式石墨密封泄漏流动特性求解模型

环瓣式石墨密封因其泄漏通道尺寸微小，导致其建模、网格划分以及计算困难。基于相似原理方法建立环瓣式石墨密封泄漏通道求解模型，采用方程分析法推导环瓣式石墨密封泄漏通道内流体流动相似准则，获得遵循几何相似和力学相似的映射模型，并采用建立的泄漏通道映射模型分析环瓣式石墨密封的泄漏流动特性，并与实际模型计算结果进行比较。研究结果表明：泄漏通道内气体流动相似性可综合采用弗劳德、欧拉、雷诺相似准则表征；映射前后模型相同结构位置处流体压力、速度分布具有较好的一致性。通过映射模型求解的泄漏量与实际模型求解的泄漏量相对误差在误差允许范围内，验证了推导的泄漏通道流体流动相似准则和映射方法的可靠性，为研究环瓣式石墨密封微小泄漏通道泄漏流动特性提供新方法。     

干气密封启动过程研究

为了深入研究干气密封启动过程，提高密封启动稳定性，基于N—S方程、Laminar模型和SIMPLEC算法，在动环转速和端面压差变化的多种情况下，对于气密封端面流场进行了数值模拟，重点考察了端面开启力的构成及变化情况。数值结果表明，在密封启动过程中，转速上升到一定数值后，端面才能脱开，而流体动压力是影响端面脱开的主要因素，在脱开前两端面间存在严重摩擦和磨损，会对端面造成破坏。针对此提出研究一种能减小或消除启动过程中端面接触时间的新型密封。     

车用液力变矩器混合流道CFD仿真方法

针对车用液力变矩器复杂动态过程中工作相位随时发生转换,不能及时判断相应流场结构的改变,难以对瞬时流场特性进行准确仿真的问题,基于传统变矩器CFD流道模型和导轮空转无叶片模型,建立了液力变矩器混合流道CFD仿真模型。该仿真模型可以自动识别变矩器变矩、偶合和功率反传等工作相位及其相位转换过程,并根据导轮是否空转自动选择相应流道模型。对某变矩器进行了一系列稳态通用特性和动态特性的仿真与试验研究,对比结果表明,液力变矩器混合流道CFD仿真方法对变矩器稳态和动态特性仿真精度较高,有效解决了变矩器复杂动态过程难以快速实时仿真的问题,具有一定的工程实际意义。     

主动电磁轴承失效后转子坠落在备用轴承过程中的非线性动力学

用有限元法建立了任意个由滚动轴承组成的固定间隙备用轴承-主动电磁轴承-多盘多质量转子系统在主动电磁轴承失效前后的动力学方程,以一个多盘转子系统为例分析了主动电磁轴承失效后转子坠落在备用轴承过程中的瞬态非线性动力学特性,讨论了柔性备用轴承的支撑刚度、支撑阻尼和间隙对转子在坠落过程中动力特性的影响。结果表明采用具有较大阻尼柔性结构的备用轴承能够显著地改善转子坠落在备用轴承上的瞬态动力特性,抑制转子在坠落过程中可能出现具有较大振动和冲击力的全间隙区的涡动运动,减小转子坠落对备用轴承寿命和可靠性的影响。柔性结构备用轴承的支撑阻尼越大,支撑刚度越小,间隙越大,转子坠落后出现全间隙区涡动运动的可能性越小。     

CFD Study on Stepped and Drum Balance Labyrinth Seal

The stability of rotors in turbomachines is affected by the labyrinth seal where driving forces are generated. Recent research results have shown that the preswirl has a significant influence on the rotordynamic characteristics of the straight labyrinth seal including both tooth-on-stator and tooth-on-rotor configurations. However, modern turbomachines require higher and higher pressure drop for the seal so that the eye seal is usually designed as stepped labyrinth, whereas the balance drum usually employs a tooth-interlocking labyrinth and a larger number of teeth. Both designs could generate greater forces on the rotor than the straight labyrinth seal. Therefore, it is necessary to further study the influence of preswirl on stepped and tooth-interlocking labyrinth seals. This article employed 3D calculational fluid dynamics (CFD) to solve the flow in those seals. Then the rotordynamic coefficients of the two seals at different preswirl rates were calculated based on the CFD results. The results were compared with test data and bulk flow results. A rotor stability analysis was conducted considering the balance drum seal with the coefficients predicted by the numerical method.     

Influence of rub groove on rotordynamics associated with leakage air flow through a labyrinth seal

An extensive investigation of influence of rub grooves on dynamics and stability of the rotor, which is subjected to aerodynamic forcing associated with the leakage flow through 44 straight-through seals, was performed by using numerical calculations based on the single control-volume method and the perturbation analysis. Three cases of different groove configurations were chosen for the comparative study, e.g., the seal without rub groove (case1), the seal with upstream shifting of the seal tooth in respect to the rub groove (case2) and the seal with location of the seal tooth in the middle station of the rub groove (case3). The orifice contract coefficient adopted in reduction of rotordynamic coefficients was provided by using Computational Fluid Dynamics (CFD). Influence of rub grooves on the leakage flow was obtained in terms of the close-up view of the flow pattern near the seal tooth, leakage flow rate, distributions of the mean pressure and circumferential velocity in cavities. In comparison to case 1, the leakage flow in case 3 is considerably intensified, while which in case 2 is slightly increased. Dynamics and instability of the rotor in all cases was discussed in terms of the rotordynamic coefficients and the logarithmic decrement, respectively. The results disclosed that the aerodynamic forcing in case 2 intensified the destabilization of the rotor system.     

有限长大间隙环流中偏心转子动特性系数的简化分析方法

给出一种分析有限长大间隙环流中偏心转子动特性系数的简化数值分析方法。该方法基于有限长大间隙环流中同心转子的动特性系数计算了三维有限长大间隙环流中偏心转子的动特性系数。数值计算结果表明该方法行之有效。与已有的环压密封的其它数值计算结果和实验结果比较,该方法简单且具有较好的精度。     

反旋向供油环形密封的流体动力特性

对双出口反旋向供油环形密封（有进动偏心）的流体动力特性提出了一种计算方法，建立了计算模型。导出了一阶流场的常微分方程组和边界条件的解析提法，实现了边界条件的转化。计算表明，反旋向喷流导致进动频率比大大降低，从而提高了转子／密封稳定性。     

间隙环流中同心涡动转子动特性的研究

考虑轴向流动因素的影响，采用紊流整体流动模型和Moody壁面摩擦系数方程，建立间隙环流3D非线性微分方程，运用摄动法和线性振动理论分析求解间隙环流中同心涡动转子动特性系数。实例计算结果表明，间隙环流中同心涡动转子动特性系数与间隙的大于有关，同时分析轴向压差、壁面粗糙度、长径比及入口预旋等参数对大、小间隙环流中同心涡动转子的动特性系数的影响。     

环境激励下的旋转机械工作模态参数辨识

伴随着高速透平机械向大跨度、柔性结构方向发展,转子-轴承系统的稳定性将面临着严峻考验。在出厂测试阶段,确保机组转子系统具有足够稳定性裕度是降低生产现场机组发生失稳故障风险的重要手段。采用适用于随机平稳环境激励下的随机子空间法,对机组的模态参数进行辨识,可规避在转子非驱动端增设电磁激振器的传统测试方法。通过分析转子振型进动方向,区分一阶正反进动的模态参数。结合3-σ统计聚类算法,剔除非稳定的噪声或物理极点,形成了区分转子系统的正反进动的稳态图。数值仿真表明,随机子空间法可以有效地辨识系统的模态参数,利用旋转机械的振型进动方向分析方法可以区分正反进动。此外,通过传统扫频激励模态参数辨识试验,验证了随机子空间方法的辨识精度和工程测试可行性。研究结果可为透平机组的稳定性测试提供技术和理论支撑。     

Application of computational fluid dynamics and fluid structure interaction techniques for calculating the 3D transient flow of journal bearings coupled with rotor systems

Journal bearings are important parts to keep the high dynamic performance of rotor machinery. Some methods have already been proposed to analysis the flow field of journal bearings, and in most of these methods simplified physical model and classic Reynolds equation are always applied. While the application of the general computational fluid dynamics (CFD)-fluid structure interaction (FSI) techniques is more beneficial for analysis of the fluid field in a journal bearing when more detailed solutions are needed. This paper deals with the quasi-coupling calculation of transient fluid dynamics of oil film in journal bearings and rotor dynamics with CFD-FSI techniques. The fluid dynamics of oil film is calculated by applying the so-called ’’dynamic mesh’’ technique. A new mesh movement approach is presented while the dynamic mesh models provided by FLUENT are not suitable for the transient oil flow in journal bearings. The proposed mesh movement approach is based on the structured mesh. When the journal moves, the movement distance of every grid in the flow field of bearing can be calculated, and then the update of the volume mesh can be handled automatically by user defined function (UDF). The journal displacement at each time step is obtained by solving the moving equations of the rotor-bearing system under the known oil film force condition. A case study is carried out to calculate the locus of the journal center and pressure distribution of the journal in order to prove the feasibility of this method. The calculating results indicate that the proposed method can predict the transient flow field of a journal bearing in a rotor-bearing system where more realistic models are involved. The presented calculation method provides a basis for studying the nonlinear dynamic behavior of a general rotor-bearing system.     

应用动网格技术模拟分析滚动转子压缩机的瞬态流动

滚动转子式压缩机是通过偏心转子的转动与滑动档板的平动,完成吸气、压缩、排气等功能。本文采用局部弹性变形与网格重组的CFD动网格技术,对制冷型滚动转子压缩机瞬时动态流场进行了数值模拟。可以计算压缩理想气体为工作介质,满足流体控制方程及气体状态方程,湍流模型采用标准k-e模型。计算得到了压缩机主要性能参数随时间变化的谐波规律、流场分布及压力分布,观测到滚动转子式压缩机内旋涡生成、运动、增大或缩小等现象。