R. Gordon Kirk

Application of Computational Fluid Dynamics Analysis for Rotating Machinery—Part II: Labyrinth Seal Analysis

Labyrinth seals are used in various kinds of turbo machines to reduce internal leakage flow. The working fluid, or the gas passing through the rotor shaft labyrinth seals, often generates driving force components that may increase the unstable vibration of the rotor. It is important to know the accurate rotordynamic force components for predicting the instability of the rotor-bearing-seal system. The major goals of this research were to calculate the rotordynamic force of a labyrinth seals utilizing a commercial CFD program and to further compare those results to an existing bulk flow computer program currently used by major US machinery manufacturers. The labyrinth seals of a steam turbine and a compressor eye seal are taken as objects of analysis. For each case, a 3D model with eccentric rotor was solved to obtain the rotordynamic force components. The leakage flow and rotor dynamics force predicted by CFX TASCFlow are compared with the results of the existing bulk flow analysis program DYNLAB. The results show that the bulk flow program gives a pessimistic prediction of the destabilizing forces for the conditions under investigation. Further research work will be required to fully understand the complex leakage flows in turbo machinery.

Application of CFD Analysis for Rotating Machinery—Part I: Hydrodynamic, Hydrostatic Bearings and Squeeze Film Damper

The traditional method for bearing and damper analysis usually involves a development of rather complicated numerical calculation programs that may just focus on a simplified and specific physical model. The application of the general CFD codes may make this analysis available and effective where complex flow geometries are involved or when more detailed solutions are needed. In this study, CFX-TASCflow is employed to simulate various fixed geometry fluid-film bearing and damper designs. Some of the capabilities in CFX-TASCflow are applied to simulate the pressure field and calculate the static and dynamic characteristics of hydrodynamic, hydrostatic, and hybrid bearings as well as squeeze film dampers. The comparison between the CFD analysis and current computer programs used in industry has been made. The results show reasonable agreement in general. Some of the possible reasons for the differences are discussed. It leaves room for further investigation and improvement on the methods of computation.

Turbocharger On-engine Experimental Vibration Testing:

Many automotive turbochargers operate in the self-excited unstable region. In the past these instabilities have been accepted as unavoidable, but recent developments in analysis and instrumentation may make it possible to reduce or eliminate them. A test stand being developed at Virginia Tech has been used to measure the vibrations of a 3.9 liter diesel engine stock turbocharger with floating bushing journal bearings. Vibration spectrum content clearly identifies the shaft instabilities and provides the basis for additional evaluation of future bearing design modifications. This paper provides additional experimental vibration data reduction that will be useful for future research direction to fully understand the turbocharger dynamic instability.

Expert System Source Identification of Excessive Vibration

The importance of vibration data in determining the condition of rotating machinery is well established in both the aircraft and the heavy equipment industries. Installation of noncontact displacement probes for shaft motion and either velocity or acceleration sensors for bearing cap or foundation motion is standard practice for equipment manufacturers. Automation of the diagnostic evaluation of certain simple faults can be easily implemented. The advances in computer languages in recent years have made it difficult to keep pace with the graphical capabilities available to the programmers. One major concern is the content of the knowledge base and the method of modifying the knowledge base or the procedure of evaluating the confidence of a given identified possible cause of a problem. This article addresses the application of one such expert system to a recent vibration problem on a 7-megawatt steam turbine‐driven generator located at the power plant of Virginia Polytechnic Institute.

Analysis of thermal gradient effects in oil ring seals

The analysis of ring type seals is of great concern for the designer of high speed and high pressure centrifugal gas compressors. Computers now play a major role in allowing for a quick evaluation of the floating ring seal. Since the accuracy of the rotor shaft stability evaluation depends on the validity of the dynamic seal characteristics, every step in the evaluation must be carefully scrutinized and updated to include, the latest in theory and practical operating experience. This paper presents the theory and results of one such extension to the current published analysis of oil ring seals. The new analysis includes the influence of the sealing fluid temperature gradients on the leakage flow rate and seal dynamic characteristics for multi-land ring configurations. Using classical short bearing theory but allowing for temperature and pressure dependent viscosity variations across the seal, the resulting pressure profiles have been determined numerically utilizing a finite difference solution technique....

Induced Unbalance as a Method for Improving the Dynamic Stability of High-Speed Turbochargers

The high-speed diesel engine turbocharger is known to have subsynchronous vibrations for a wide speed range. The bearing fluid-film instability is the main source of the vibration. The nonlinear forces inside the bearings are causing the rotor to whirl in a limit cycle. This study presents a new method for improving the dynamic stability by inducing the turbocharger rotor unbalance in order to suppress the subsynchronous vibration. The finite-element model of the turbocharger with floating-ring bearings is numerically solved for the nonlinear time-transient response. Both compressor and turbine added unbalance are induced and the dynamic stability is computed. The turbocharger model with linearized floating-ring bearings is also solved for eigenvalues to predict the modes of instability. The linear analysis demonstrates that the forward whirling mode of the floating-ring at the compressor end also becomes unstable at the higher turbocharger speeds, in addition to the unstable forward conical and cylindrical modes. The numerical predictions are also compared to the former experimental results of a similar turbocharger. The results of the study show that the subsynchronous frequency amplitude of the dominant first mode is reduced when inducing either the compressor or the turbine unbalance at a certain level.

Turbocharger vibration shows nonlinear jump

Automotive turbochargers are known to operate into the self-excited unstable region. In the past these instabilities have been accepted as unavoidable, but recent developments in analysis and instrumentation may make it possible to reduce or eliminate them. A test stand has been developed at Virginia Tech to measure the vibrations of a 3.9 liter diesel engine stock turbocharger with both stock floating ring journal bearings and also custom design fixed geometry bearings. Vibration spectrum content clearly identifies the shaft instabilities and provides the basis for additional evaluation of future improved bearing design modifications. The current results, for custom fixed journal bearings, have clearly revealed a distinct jump with associated shift in the spectrum frequency content. This paper will document the recent tests of custom design fluid film bearings that have experienced this nonlinear jump condition.

Application of CFD Analysis for Rotating Machinery: Part 2 — Labyrinth Seal Analysis

Labyrinth seals are used in various kinds of turbo machines to reduce internal leakage flow. The working fluid or, the gas passing through the rotor shaft labyrinth seals, often generates driving force components that may increase the unstable vibration of the rotor. It is important to know the accurate rotordynamic force components for predicting the instability of the rotor-bearing-seal system. The major goals of this research was to calculate the rotordynamic force of a labyrinth seals utilizing a commercial CFD program and to further compare those results to an existing bulk flow computer program currently used by major US machinery manufacturers. The labyrinth seals of a steam turbine and a compressor eye seal are taken as objects of analysis. For each case, a 3D model with eccentric rotor was solved to obtain the rotordynamic force components. The leakage flow and rotor dynamics force predicted by CFX TASCFlow are compared with the results the existing bulk flow analysis program DYNLAB. The results show that the bulk flow program gives a pessimistic prediction of the destabilizing forces for the conditions under investigation. Further research work will be required to fully understand the complex leakage flows in turbo machinery.Copyright

Diesel Engine Turbocharger Rebuild and Experimental Testing

The objective of the 2006–07 Virginia Tech Turbocharger Project was to obtain and analyze vibrations data from different bearing designs. The supporting tasks were divided amongst various sub-teams; a turbocharger team, an engine test operating team, a dynamometer team, and an ADRE software team. The turbocharger team has successfully developed a procedure for disassembly and reassembly of the turbocharger. A consistent and well-documented assembly procedure has ensured turbocharger operation free from human error. Consistency in the rebuild process is vital to properly compare vibration data with the different shaft bearings. The most important aspect of this reassembly is the torque applied to the tie-bolt nut. This torque causes the tie-bolt to stretch, and this stretch must be accounted for because it is a measure of preload force on the compressor wheel. The team designed and made a stand to carefully apply this torque on the tie-bolt nut without creating a bending moment. The original stock bearings were used for the first turbocharger rebuild to see if the rebuild procedure would lead to similar data. Two other bearings have been tested and analyzed. This paper will give a brief overview of the results to date.© 2007 ASME

Design Tool for Prediction of Thermal Synchronous Instability

The investigation of thermal synchronous instability has been active since the mid 1990’s and the number of machines experiencing this type of instability continues to rise. The initial design threshold speed criteria was developed and compared to available case studies during 1998–2001. Further advances were made from 2002–2007 in the prediction of the bearing journal thermal hot spot in addition to improved pre and post processing capability. These improvements made the design tool more useful for prediction of thermal synchronous instability for shaft overhangs. More recent work reported in 2011, extended the theory to treat rotor center-span instability, but the interest in shaft overhang instability is still dominant from a practical view. This paper will document a new threshold speed criteria being used in a modified design tool that is considered to be more realistic for a wider range of operating speed conditions. The paper will discuss the improved threshold criteria and illustrate why it is a more reliable design tool.Copyright