新型涡流槽密封泄漏特性与动力特性研究

提出一种密封入口周向均匀设置有涡流槽的新型密封结构,建立了传统迷宫密封与新型涡流槽密封泄漏特性及动力特性求解模型,在实验验证数值计算方法准确性的基础上,通过比较分析了传统迷宫密封与新型涡流槽密封在不同进出口压比、预旋比工况下的泄漏特性与动力特性,研究了新型涡流槽结构对密封泄漏特性及动力特性的影响机理。研究结果表明：随着涡流槽数量的增加,涡流槽密封的泄漏量逐渐降低;在同一压比下,不同涡流槽数新型密封的泄漏量之间差值随着压比的增大而增大。当压比为6时,64涡流槽的新型密封较传统迷宫密封,泄漏量下降了3.37%;在高预旋比的工况下,不同涡流槽数量密封的切向气流力均与转子涡动方向相反,起到抑制转子涡动的作用,且随着涡流槽数量的增加,切向气流力也随着增大;随着转子涡动频率的增大,三种不同涡流槽数量密封的交叉刚度先减小到负值然后增大到正值。涡流槽密封的有效阻尼均高于传统迷宫密封,新型涡流槽密封可以提高转子系统的稳定性。     

可倾/固定密封内流体激振力计算与试验比较

提出可倾式密封减小密封内气流激振力的理论,其本质在于通过密封块的自适应摆动从而改变流场.利用ANSYS CFX建立可倾密封流固耦合分析模型,计算时应用动网格的方法来实现密封块的自适应摆动.计算并比较了两种形式密封内气流激振力随入口压力、转子偏心以及转速的变化情况.结果表明,随着三个参数的增大,密封内气流激振力均普遍增大.在所有工况下,可倾密封气流激振力均小于固定密封气流激振力,两种密封气流激振力的差值随三个参数的增大而增大.设计密封汽流激振力的试验装置,应用传递函数法识别了密封气流激振力,通过共振的方法提高了识别精度.试验证实了计算得到的现象.对于现代超临界、超超临界机组,两种密封内气流激振力的差别预计会更大.     

高低齿迷宫式汽封泄漏流动特性研究

针对汽轮机高低齿迷宫式隔板汽封的复杂结构和轴的旋转效应,采用数值求解三维Navier-Stokes方程技术研究其泄漏流动特性。采用有限体积方法离散Navier-Stokes方程和标准k-ε两方程紊流模型封闭方程组。数值求解方法采用SIMPLE算法,迎风二阶格式求解对流项,迎风一阶格式求解扩散项。研究分析了在相同汽封轴向距离和汽封间隙条件下,三种不同齿数的高低齿迷宫式隔板汽封在不同压比下的泄漏流动特性,并计算了相应的量纲一流量系数。数值模拟结果证明了在相同压比和汽封轴向距离条件下齿数减小会造成泄漏量增加,在相同的齿数条件下,量纲一流量系数随着压比增加而增大。研究工作为工程设计高低齿迷宫式汽封提供了理论依据和技术支撑。     

密封动力特性系数试验识别方法及影响因素分析

引入转子动力学双平面平衡理论,将多级密封内的分布气流力等效到气缸两个端面上,建立多级密封动力特性系数识别模型.该模型考虑气缸平动和偏摆混合运动情况,并通过共振的方法放大气流力影响,从而提高识别精度.构建密封动力特性试验台,研究进出口压比、转速、偏心、间隙等因素对密封动力特性系数的影响.试验结果表明,8个刚度阻尼系数随着进出口压比增大有明显增大,且垂直和水平方向存在耦合效应.随转速增大,交叉耦合刚度系数和直接阻尼系数随转速变大趋势明显,而直接刚度系数和交叉阻尼系数变化不很明显.偏心状态下的交叉耦合刚度系数比同心状态下变大,而直接阻尼系数则减小,说明转子在偏心状态下比同心状态下更容易失稳.随密封平均间隙变小,8个动力特性系数绝对值增大.     

迷宫气封三维非定常流场及转子动特性数值仿真

通过计算流体力学方法,采用近似的解析变换网格,以及混合有限分析格式,对迷宫气封流场进行三维非定常数值求解,进而求得密封气流对转子的气流激振力,获得转子的动特性系数。将计算结果与试验结果以及双控制体计算结果相比较,结果令人满意,说明了该计算程序是正确可靠的。该方法精度较高,适用范围广,可以应用于各种复杂的涡动情况及各种齿型,对气封转子的深入研究有重要意义。计算结果还显示了主刚度、主阻尼对气流进口周向速度不敏感,而交叉刚度对气流进口周向速度非常敏感。由于交叉刚度对转子的稳定性影响相当大,这一点是值得注意和利用的。     

入口长度对旋转直通式迷宫气封流场及流场力的影响

迷宫密封的转子和定子部件之间的流场和流场力直接影响转子的振动和稳定性。为了深入了解迷宫密封的流场和流场力特性,选取旋转直通式迷宫气封为研究对象,利用稳态CFD分析方法,研究密封入口长度对旋转直通式迷宫气封泄漏、流场和流场力的影响。考虑和不考虑入口预旋的影响,计算不同入口长度的旋转直通式迷宫气封的泄漏量、流场速度流线、总压强分布、流体压力和流体黏滞力。结果表明:随着入口长度的增大,密封流场泄漏量先增加后减小,转子表面流体压力和流体黏滞力先增加后快速减小;入口长度影响着直通式迷宫气封流场力,存在一个特定的入口长度,使转子表面的流体压力和流体黏滞力最大;当考虑入口预旋时,泄漏量、流体压力和流体黏滞力均有所减小。     

反旋流对密封静力与动力特性影响的理论与试验研究

设计加工无/有反旋流共4种密封结构,从理论与实验两个方面研究反旋流对密封静力与动力特性的影响规律。建立反旋流密封静力特性CFD模型,理论分析反旋流对密封间隙流体切向速度、周向压力分布以及泄漏特性的影响;设计搭建反旋流密封动力特性试验台,试验测试无/有反旋流密封的泄漏特性,应用不平衡同频激励法试验研究反旋流对密封动力特性的影响。研究结果表明:反旋流可减小密封间隙流体的切向速度,进而降低密封间隙流体的周向压力差,且密封间隙流体周向压差随切向速度的减小而降低,这是反旋流抑制密封气流激振力的主要原因;密封的泄漏量随进出口压比的增加而增大,两者近似呈线性关系;与无反旋流密封相比,反旋流密封增加了密封的泄漏量,且随着进出口压比的增加,两者泄漏量差异增大;密封的动力特性系数的随密封进出口压比与转速的增加而增大。在相同工况下,主刚度大于交叉刚度约一个数量级,主阻尼与交叉阻尼数量级相同,且主阻尼大于交叉阻尼;反旋流可有效降低密封的等效刚度,增加密封的等效阻尼,提高密封的稳定性。     

燃气轮机透平喷嘴迷宫式汽封泄漏流动特性研究

采用CFD软件对某燃气轮机喷嘴内围带迷宫式汽封进行了三维流动特性数值模拟,详细分析了汽封内部流场结构和汽封泄漏量特性的变化。结果表明,在汽封室内流动是具有复杂涡系结构的湍流流动,在相同的汽封尺寸参数下,汽封泄漏量随着压比的增加而增大,在相同的压比下,汽封泄漏量随着汽封齿长度的增加而减小、随着汽封齿宽度的增加而增大。     

篦齿封严气弹稳定性影响因素研究

为探究篦齿封严气弹失稳机制,理论分析了篦齿封严结构动力学与气弹稳定性判定公式,建立了篦齿封严结构动力特性数值求解模型与气弹稳定性求解模型,并验证该了方法的准确性;研究了篦齿环齿间距、壁厚以及转速对篦齿封严固有频率与气弹稳定性的影响规律,揭示了篦齿封严气弹失稳影响机制。研究表明：齿间距、壁厚对固有频率的影响程度随着节径数的增加而增大,对前三阶固有频率影响不大;转速对于第一阶固有频率影响最大;篦齿封严齿间距的增加会导致固有频率和气弹稳定性的下降;篦齿封严壁厚的增加可引起固有频率的升高,并且可以有效改善气弹稳定性;升高转速会引起篦齿环固有频率较明显的增加,对气弹稳定性影响较小。     

航空发动机篦齿封严特性数值分析

为了解航空发动机空气系统中封严篦齿的流动及封严情况,采用数值模拟方法对二维蓖齿封严特性进行研究.使用商业软件FLUENT6.3对直通型蓖齿齿数N为3、5、7、9,间隙比t/c为0.5、1、2时不用工况下进行计算,得出各种情况下流量系数随压比变化关系.主要结论为:流量系数随压比的增加而增大;齿数N越多,封严效果越好;缝隙越小,间隙比t/c越大,封严效果越好.同时比较了齿数N为3时的直通型与非直通型封严效果,得出非直通型封严效果更好.     

A novel negative dislocated seal and influential parameter analyses of static/rotordynamic characteristics

This paper proposes a new type of negative dislocated seal (NDS) based on the dislocated bearing theory to investigate the influential parameters of static rotor eccentricity and dislocated ratio on the static and rotordynamic characteristics of an example seal solved with computational fluid dynamics (CFD) the flow field. The rotordynamic characteristics of the NDS were investigated in respect of the effects of rotor whirling frequency on response force, stiffness coefficients, damping coefficients and rotor system stability, by multifrequency elliptical orbit rotor whirling model. Based on the studies, we reached the following conclusions. The circumferential pressure distribution of NDS and conventional labyrinth seal (LS) presents sine periodic variation approximately, while relative to the LS, the NDS has two divergent wedge gaps and reduces the hydrodynamic pressure effects, then the circumferential pressure difference and tangential force on rotor surface decreases by about 40 %~190 %. The leakage of the NDS linearly increases with the rising eccentricity ratio and dislocation ratio approximately, and is larger (about 0.9 %~1.5 %) than the LS. The direct stiffness coefficients of the two seals increase with the rise of rotor whirling frequency, while the absolute values of both the cross-coupled stiffness coefficients and the damping coefficients decrease with raising rotor whirling frequency. Compared with the LS, the NDS has smaller (about 28.8 %~206.2 %) cross-coupled stiffness coefficient, larger (about 26.15 %~60.39 %) effective damping coefficient, and good stability of rotor system. This study has developed a novel seal structure to improve the seal performance of turbomachinery such as aeroengine.     

An experimental identification model of rotordynamic coefficients of seals using unbalanced synchronous excitation method

This study presents an improved impedance method based on unbalanced synchronous excitation to identify the rotordynamic coefficients of labyrinth seals. The rotordynamic coefficient test is implemented near the cylinder resonance frequency to enlarge the influence of seal force. The force generated by the rotor unbalance is used to provide synchronous frequency excitation for the rotordynamic coefficient test. Four unique equations are set up under two sets of different rotor unbalance conditions to obtain four unknown complex rotordynamic coefficients. The factors that influence the rotordynamic coefficients of seals, namely, unbalance mass, inlet/outlet pressure ratio, and rotating speed, are considered. The dynamic coefficients are minimally affected by different rotor unbalances. The direct items are nearly equal with same signs, whereas the cross-coupled items are nearly equal with opposite signs. All coefficients increase with increasing inlet/outlet pressure ratio and rotating speed. The direct stiffness coefficients increase more quickly than the cross-coupled items. In addition, the effect stiffness and effect damping coefficients are analyzed; results indicate that both coefficients increase with increasing rotating speed.     

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.     

迷宫密封对转子系统稳定性影响研究

透平机械中汽流激振力主要产生于各种形式的密封处,其对转子系统的稳定性有不良影响。因迷宫密封的存在而引起汽流激振,其各有关结构和工况参数对转子动力特性均存在影响。本文主要分析转子转速、入口预旋比、密封间隙、压差、齿数变化等一系列因素对转子系统稳定性的影响,为迷宫密封汽流激振的防治和迷宫密封设计中各参数的最优确定提供参考依据。     

Measurements of Flow Rate and Force Coefficients in a Short-Length Annular Seal Supplied with a Liquid/Gas Mixture (Stationary Journal)

Deep sea compression systems must work under strenuous conditions with either gas in liquid or liquid in gas mixtures, mostly inhomogeneous. Off-design operation affects the mechanical system's overall efficiency and reliability, with penalties in leakage and rotordynamic performance of secondary flow components, namely, seals. This article introduces a test rig to characterize the leakage and dynamic force coefficients of a short-length annular seal (L/D = 0.36, clearance = 0.127 mm) operating under various flow regimes ranging from pure gas, to bubbly (liquid in gas), to foamy (gas in liquid), to pure liquid. The test rig includes of rotating journal and a softy supported cartridge that make a clearance annular seal that is supplied with a liquid/gas mixture. Flowmeters record the fluid's passage, and with manual control of the streams, the mixture has a known liquid (or gas) volume fraction at the seal inlet plane. Two orthogonally mounted electromagnetic shakers excite the cartridge with periodic (single-frequency) forces spanning a wide frequency range. Eddy current sensors and accelerometers record the seal cartridge motions and a frequency domain parameter identification method delivers the seal dynamic force coefficients.

For tests with a pressure supply/pressure discharge ratio = 3.0 and 3.5 and a nonrotating journal, the article reports the flow rate for an ISO VG10 oil in air mixture with liquid volume fraction (LVF) at the inlet plane increasing from pure gas to pure liquid. Wet seal stiffness and mass and damping force coefficients follow for a seal operating with a pressure supply/pressure discharge ratio = 2.0 and operating with air (only) and also with an oil-in-air mixture with inlet LVF = 2% and 4%. The experimental results, the first reported, reveal that a small amount of liquid increases the damping coefficients of the wet seal 10-fold (or more). Predictions from a computational bulk flow model also demonstrate that the seal damping coefficient varies greatly with small contents of liquid in the oil/gas mixture, although agreement with the experimental force coefficients is not compelling due to the likely inhomogeneity of the mixture flowing though the seal.     

Experimental identification of fluid-induced force in labyrinth seals

The seal force is an important factor in turbomachineries. Therefore, the current paper puts forward an expanded seal force identification model. A seal test rig consisting of several sets of seals was prepared. Using the double-plane unbalance force identification theory in rotordynamics, the distributed seal force in the cylinder became equivalent to two selected planes. Considering the complex cylinder vibration with increasing rotating speed and inlet pressure, the cylinder was regarded as a vibration system with 4 degrees of freedom. The 4×4 impedance matrix was tested at the two selected planes using a shaker in two orthogonal directions. The equivalent seal force can be obtained by multiplying the impedance matrix with the measured change in the cylinder vibration. In the seal rig, tests were performed on the influence of inlet pressure, rotating speed, eccentricity ratio, rotor vibration, and clearance. The seal force increases almost linearly with the increasing inlet pressure, eccentricity ratio, and vibration amplitude. Furthermore, the seal force is strongly sensitive to the change in clearance between the cylinder and the rotating rotor. The phase difference between the seal force and the vibration influences the work done. If the phase difference is nearly 90°, then the work is at maximum. Moreover, the seal force applies positive force on the cylinder and negative force on the rotor.     

Annular seal rotordynamic stability enhancement with circumferential spiral flow control using helical deflectors

Rotordynamic instability problem caused by seal force widely exists in turbo-machines. The fluid-induced force in the seal is determined by the pressure distribution associated with the flow field structure, particularly with the circumferential flow characteristics. In the previous study, the authors found that the helix-comb seal can effectively change the circumferential flow field. The specially designed helical deflectors can reverse the circumferential flow direction of the fluid inside the seal. A considerable static stability enhancement is obtained. However, the transient rotordynamic characteristics of the helix-comb seal are not reported. In the present study, the transient rotordynamic characteristics of the helix-comb seal are investigated further. The rotordynamic coefficients, effective damping, and aerodynamic work of the helix-comb seal are calculated and compared with those of the labyrinth seal. Results reveal that compared with the labyrinth seal, the helix-comb seal considerably improves rotordynamic stability, particularly at a high preswirl ratio. The desirable rotordynamic stability can be obtained with flow field control.

     

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.