Robert Liebich

The Influences of Thermal Effects Induced by the Light-Rub Against the Brush Seal to the Rotordynamics of Turbo Machines

Brush seals find increasing use in turbomachinery substituting conventional labyrinth seals thanks to their excellent leakage characteristics and convenient integration. Brush seals have very small clearances during operation. In case of contacts between rotor and brush seals, contact forces will be low due to the compliant behaviour of the bristles. While short term contacts between seal and rotor have no significant influence on the rotordynamics, longer-lasting rub can lead to thermally induced rotor-vibrations, also known as the Newkirk-effect. Light partial rub and the subsequently dissipated heat that enters into the shaft may yield a thermal bow performing spiral-vibrations regarding rotating coordinates. Depending on thermal coefficients and rotating speed, this thermal bow may effect instable behaviour with high amplitudes and a possible damage of the machine. At the Chair of Engineering Design and Product Reliability at Berlin Institute of Technology investigations of light partial rub of a rotor against a brush seal are conducted. A test rig is under construction in order to validate the numerically calculated parameters. Investigations are setting up on a thermoelastic model, developed by Kellenberger for a real rotor model. The goals of the investigations are to verify and to extend the model for brush seals and finally to formulate guidelines for the safe use of brush seals in turbomachinery concerning rotordynamics. The difficulty of defining stability statements is to quantify the required thermal parameters. Hence, the three dimensional temperature distribution inside the rotor, which depends on the rotating speed as well, must be known. In order to calculate this temperature distribution the three dimensional Laplace-Equation in cylindrical coordinates is solved for the different convection coefficients by means of Finite-Volume-discretization. Subsequently the required parameters are calculated by numerical integration of the 3-D-structure. The stiffness of the brush seal with respect to a partial rub is calculated using beam theory and continuous elastic support. This paper shows the numerical results of the 3-D temperature distribution, the numerically identified parameters that drive the thermal bow and stability charts regarding spiral vibrations for a chosen brush seal configuration.Copyright

Non-linear Stability Analysis of a Modified Gas Foil Bearing Structure

Gas foil bearings (GFBs) have been successfully introduced in the field of high speed turbo machineries. A combination of low power loss, high speed operation and the omission of an oil system heighten the importance for small and medium sized turbo machineries, e.g. turbochargers or range extenders. However, experimental and numerical investigations have shown subsynchronous vibrations, which affect the rotor dynamic behaviour. Structural damping generated by friction contacts inside the compliant structure may reduce vibrations up to a certain level. In addition, several proved methods and devices, e.g. side feed pressurerisation, pre-loading due to shims and viscoelastic foil bearings are common techniques to decrease non synchronous vibrations. However, far too little attention has been paid to the causes of these non-linear effects. Understanding the causes may results in a higher knowledge of the overall GFB dynamic behaviour. Thus, the aim of this paper is to analyse the causes of these non-linear vibrations. A hypothesis is stated, that the non-linear vibrations are influenced by a self excitation and a forced non-linearity. The non-linear compressible transient Reynolds equation is discretised by a hybrid finite difference scheme with an implicit time discretisation while the pressure field is coupled with a 2D plate model. This plate model is linked to a spring-damper configuration. The time domain analysis shows, that the subsynchronous frequencies may excite the system eigenfrequency. In addition, good correlations between the onset speed of sub synchronous vibrations of the time domain simulations and the linearised frequency domain analysis are shown. In the second part of this paper, the effects of different bump foil configurations (bump-type GFB, shimmed GFB and a lobed GFB) on the dynamic performance are considered. It is shown, that an effective reduction of sub synchronous vibrations due to a non-uniform circumferential stiffness distribution and the use of shims is possible. Especially, the low loaded case (5 N) has an increase of onset speed of subsynchronous vibration of ≈173 %, compared to the same bearing setup without shims.

The Impact of Modified Corrugated Bump Structures on the Rotor Dynamic Performance of Gas Foil Bearings

Gas foil bearings (GFBs) have been successfully introduced in the field of high speed turbo machineries. Low drag friction, high speed operation and the omission of an oil system are some advantages of bump type foil bearings. However, experimental and numerical investigations have shown sub synchronous vibrations, which affect the rotor dynamic behavior. Several methods and devices have been introduced to decrease these vibrations (e.g. viscoelastic foil bearings, shims and side feed pressurization). This current paper examines the effect of different bump foil configurations on the rotor dynamic performance. Different bump stiffness distributions in axial and circumferential direction are considered for a set of loadings (5, 15, 30 and 60 N) and rotor speeds (6,000–70,000 rpm). To evaluate the onset speed of sub synchronous vibration a linear stability analysis is applied. It uses the linearized bearing parameters stiffness and damping. The results show, that a variation of stiffness distributions may enlarge the stability range. Simulations indicate that a non-uniform circumferential stiffness distribution is very effective to avoid sub synchronous vibrations, due to smaller cross coupling effects.Copyright

Customer loads of two-wheeled vehicles

ABSTRACT Customer usage profiles are the most unknown influences in vehicle design targets and they play an important role in durability analysis. This publication presents a customer load acquisition system for two-wheeled vehicles that utilises the vehicles onboard signals. A road slope estimator was developed to reveal the unknown slope resistance force with the help of a linear Kalman filter. Furthermore, an automated mass estimator was developed to consider the correct vehicle loading. The mass estimation is performed by an extended Kalman filter. Finally, a model-based wheel force calculation was derived, which is based on the superposition of forces calculated from measured onboard signals. The calculated wheel forces were validated by measurements with wheel–load transducers through the comparison of rainflow matrices. The calculated wheel forces correspond with the measured wheel forces in terms of both quality and quantity. The proposed methods can be used to gather field data for improved vehicle design loads.

Road classification for two-wheeled vehicles

ABSTRACT This publication presents a three-part road classification system that utilises the vehicles onboard signals of two-wheeled vehicles. First, a curve estimator was developed to identify and classify road curves. In addition, the curve estimator continuously classifies the road curviness. Second, the road slope was evaluated to determine the hilliness of a given road. Third, a modular road profile estimator has been developed to classify the road profile according to ISO 8608, which utilises the vehicles transfer functions. The road profile estimator continuously classifies the driven road. The proposed methods for the classification of curviness, hilliness, and road roughness have been validated with measurements. The road classification system enables the collection of vehicle-independent field data of two-wheeled vehicles. The road properties are part of the customer usage profiles which are essential to define vehicle design targets.

Investigations on Non-steady Behaviour of Rotors Due to Light Rubbing to Brush Seals

One option to increase efficiency of turbomachines is the reduction of leakage losses by using and improving sealing concepts with minimized clearance, accepting the risk of rub of rotors to the seals under operational conditions. Beside wear and subsequently increased leakage losses, rub affects rotordynamics of turbomachines with reaction to fluid dynamics again. Brush seals consider these issues and combine excellent leakage characteristics with compliant behaviour under rub conditions. While brush seal related topics like leakage characteristics, fluid dynamics, design issues and wear were investigated in detail during the last decades; publications concerning impact of brush seals on rotordynamics are more or less rare. The following paper deals with the influence of light rub of rotors to brush seals on rotordynamics. Light rub implies frictional heat, partly entering the shaft and leading to thermal expansion. A not uniform heat distribution of the shaft through eccentric rub can lead to thermal bending of the shaft causing further growth of shaft deflection. This phenomenon is known as Newkirk-Effect also referred to as spiral vibrations because of the spirally shaped shaft orbit in rotating coordinates. Depending on thermal parameters and damping in particular, stable and unstable operating areas can be identified. In extension to existing steady state descriptions this paper pursues a transient consideration of spiral vibrations due to rub to brush seals. The objective is a better understanding of the magnitude and implications of spiral vibration and its possible prediction during run-up and rundown. For that purpose the power loss due to rub is taken into account and a rotordynamic analysis is conducted for a simple rotor model consisting of a bulky shaft rubbing to a brush seal supported by rigid bearings. In a first step a run-up with a constant low drive torque is considered with focus on behaviour at stability limit near first eigenfrequency.

Rotors supported by elastomer-ring-dampers – experimental and numerical investigations

Rubber or elastomer rings are simple devices for rotor damping. Rotordynamics are hard to predict when using elastomer damping devices due to nonlinearities of the material parameters. Complex material tests have to be performed which results in material master curves that can be transformed into classical stiffness and damping parameters generally used in rotordynamic numerical simulation and analyses. A small test rig has been built which is used for the experimental validation of the numerical simulation. The experimental results show a good agreement with the simulation.

Experimental Structural Analysis of Gas Foil Bearings

Gas foil Bearing (GFB) are oil-free, high-speed and light bearings, which work according to the principle of fluid film lubrication. Thanks to their elastic structure, GFBs are able to compensate for minor pressure changes in the lubrication films. This paper presents the experimental structural analysis of first-generation gas foil bearings. The aim of the experimental investigation is to determine the behaviour of GFBs at static and dynamic loads. The tests are carried out with rotor speeds close to 0 rpm. In the course of the static investigation, the GFB was mounted on shafts with different diameters and loaded with a force of −150 N to 150 N. Results from the static measurement show that not only the shaft diameter plays a role in determining the bearing clearance but also the number of activated bumps. It also shows that with a small bearing clearance (\( {\le }10\,{\upmu }\mathrm{m} \)), the GFB has an almost linear static stiffness. In the dynamic study, the GFB was mounted on a non rotating shaft and was excited by the shaker with a mono-frequency load. The goal of the dynamic investigation was to determine the dynamic stiffness behaviour and damping behaviour of GFBs at different amplitudes (2 \(\upmu \)m, 6 \(\upmu \)m and 10 \(\upmu \)m) and over the frequency range of 30 Hz to 1000 Hz. In addition, this study aimed to find out whether the formation of subharmonic vibrations observed in the rotordynamic investigation can be attributed to the GFB structure. These subharmonic vibrations, as previous studies show, occur at speeds starting at about 20 000 rpm (333 Hz). For this reason, the dynamic measurement was performed up to 1000 Hz. The results show that the damping decreases with increasing frequency up to 490 Hz before rising again. This behaviour is amplitude independent. The stiffness of the bearings increases with increasing frequency. To verify the formation of subharmonic vibrations through the structure of GFBs, a Fourier transformation of the measurement signal was performed. However, no subharmonic vibration can be detected.

Numerical Study on the Influence of Gas Foil Thrust Bearings on the Vibrational Behavior

Gas Foil Bearings (GFBs) have a promising future in high-speed turbomachinery such as air cycle machines and turbochargers. To achieve complete oil-free operation of the rotor support structure Gas Foil Thrust Bearings (GFTBs) can be used in combination with Gas Foil Journal Bearings (GFJBs). The present study numerically investigates the influence of GFTBs on the rotordynamic behaviour. In a first step the perturbation method is used to calculate linearized stiffness and damping coefficients. The perturbation approach used in this study is widely used in GFJBs resulting in uncoupled first-order equations to calculate the stiffness and damping parameters. Previously published approaches for GFTBs were relying either on coupled first order equations or were independent of excitation frequency. The calculated bearing parameters are validated against numerically calculated data published in the available literature. Linear stability analysis for a rigid rotor supported by GFTBs is performed and later extended for a rotor supported by both GFTBs and GFJBs.

Explicit Finite Element Analyses of Drop Tests With Thin-Walled Steel Sheet Containers for the Konrad Repository

Alternatively to experimental drop tests, the mechanical safety analyses of containers for final disposal of radioactive waste with negligible heat generation in the German Konrad repository may be carried out by numerical simulations within the safety assessment procedure. In the past, safety assessments for thin-walled steel sheet containers have been done exclusively by prototype tests and unfavorable drop scenarios were determined by engineering judgment. So far, reliable numerical simulations do not exist. Therefore, a research project was started to develop numerical simulation approaches for drop test analyses and to determine existing safety margins. Comparisons of experimental and numerical results confirm that the Finite Element (FE) model represents the general mechanical behavior of the steel sheet container sufficiently. Simulations have been used to determine an unfavorable drop scenario resulting in large deformation and damage. This paper presents the investigations carried out as well as the further development of the FE model in terms of damage mechanics.Copyright