Rotordynamic Stability Research Articles

Performance Measurements of Gas Bearings With High Damping Structures of Polymer and Bump Foil Via Electric Motor Driving Tests and One Degree-of-Freedom Shaker Dynamic Loading Tests

This paper presents comprehensive test measurements for gas journal bearings with damping structures of a bump foil layer and/or a polymer layer. A one-pad top foil forms the bearing surface, under which the bearing structure and a bearing housing are located. Test bearings include gas foil bearings (GFBs), gas polymer bearings (GPBs), and gas foil-polymer bearings (GFPBs). In addition, three metal shims were employed to create wedge effects in the GFPBs. First, static load-deflection tests of test bearings estimate the radial assembly clearance. Second, shake dynamic loading tests identify frequency-dependent dynamic characteristics. An electromagnetic shaker provides flat bearing specimens with one degree-of-freedom (1DOF) vertical dynamic loading. GFPB was measured to exhibit a higher structural damping and lower stiffness than GFB. Lastly, the electric motor driving tests examine the rotordynamic stability performance. A permanent magnet (PM) synchronous motor drives a PM rotor supported on a pair of test journal bearings. As a result, the GFPBs with mechanical preloads enhanced the rotordynamic performance with no subsynchronous motions up to the maximum rotor speed of 88 krpm, and the bearing friction characteristics as well. Furthermore, they showed comparable rotordynamic performance to three-pad GFBs from a past literature, even with larger bearing clearances and small mechanical preloads.

Including Housing–Casing Fluid in a Lateral Rotordynamics Analysis on Electric Submersible Pumps

Stability and synchronous-response predictions, which were presented by Childs et al. (2014, “A Lateral Rotordynamics Primer on Electric Submersible Pumps (ESPs) for Deep Subsea Applications,” 43th International Pump Users Symposium, Texas A&M University, College Station, TX, pp. 1–18), are re-evaluated to include the effect of the fluid between the pump housing and well casing. Conclusions are made based on these new findings. The same two-line rotor–housing model is used to model the pump's rotor and its housing. The model dimensions are based on direct measurements of an ESP. The pump rotor and pump housing are only connected together at each stage by reaction forces and moments from seals and bushings. The rotor model is pinned to the housing at the rotor's ends. The housing model is pinned to ground at its ends. Synchronous response predictions are presented for: (1) relative rotor–housing motion and (2) housing velocity-response amplitudes. When handling viscosity of water, the rotor–housing model is predicted to be stable at new (centered) 1× clearances but rapidly becomes unstable with enlarged clearances (2× and 3×), primarily due to rapidly dropping rotor–housing natural frequencies. The impact of introducing effective swirl brakes for the stages' wear ring seals was investigated for a pump running at 3600 rpm. Their predicted impact on stability and synchronous response was: (1) Onset speeds of instabilities (OSIs) were elevated well above running speed and (2) synchronous response amplitudes were reduced modestly. Housing-response amplitudes varied considerably with the choice of housing-termination locations. For a pump rotor length of Lr, varying the lengths of a centered housing over 1.5 Lr, 2 Lr, and 3 Lr changes the housing's natural frequency. This natural frequency can coincide with the running speed with proper termination conditions. If the running speed and natural frequency coincide, large housing vibration amplitudes associated with resonance would exceed most vibration regulations; however, relative rotor–stator response amplitudes were a small fraction of clearances for all the cases. When handling emulsions at markedly higher viscosities, with a pump speed of 3600 rpm and new centered clearances, the predicted OSI was below 300 rpm. The OSI rapidly increased as the seals were displaced eccentrically, quickly elevating the first rotor–stator natural frequency above 1800 rpm and the OSI above 3600 rpm. With the model stabilized at 0.2 eccentricity, the synchronous relative rotor–housing amplitudes were a small fraction of seal clearances. Swirl brakes were not predicted to be effective in elevating pump OSIs for high viscosity fluids with new clearances; however, they became effective as clearances were increased. An ESP housing can contact the well casing in many possible scenarios (axial locations, contact-area length or girth, etc.). A midspan, point radial contact was examined and modeled as a stiff-spring connection from the housing to ground. For both water and oil–water emulsions, a stiff housing-to-casing contact produced major elliptical housing motion (versus circular motion without contact). However, it had a comparably minor impact on relative rotor–housing response amplitudes or rotordynamic stability.

Modeling and Designing of a Radial Anode Off-Gas Recirculation Fan for Solid Oxide Fuel Cell Systems

To improve the industry benchmark of solid oxide fuel cell (SOFC) systems, we consider anode off-gas recirculation (AOR) using a small-scale fan. Evolutionary algorithms compare different system design alternatives with hot or cold recirculation. The system performance is evaluated through multi-objective optimization (MOO) criteria, i.e., maximization of electrical efficiency and cogeneration efficiency. The aerodynamic efficiency and rotordynamic stability of the high-speed recirculation fan is investigated in detail. The results obtained suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable, including for small power scale (10 kWe).

Rotordynamic Computational and Experimental Characterization of a Convergent Honeycomb Seal Tested With Negative Preswirl, High Pressure With Static Eccentricity and Angular Misalignment

Hole-pattern or honeycomb seals have been commonly used for many years in the Oil & Gas industry as damper seals for turbomachinery. The main motivation has been to introduce additional damping to improve the shaft rotordynamic stability operating under high-pressure conditions. Experience has shown that the dynamic and even static characteristics of those seals are very sensitive to the operating clearance profile as well as the installation tolerances. Rotordynamic stability is related not only to the seal effective damping but to the effective stiffness as well. In fact, for this kind of seal, the effective stiffness can be high enough to alter the rotor system's natural frequency. The seal stiffness is strictly related to the tapering contour: if the clearance profile changes from divergent to convergent, the effective stiffness may change from a strong negative to a strong positive magnitude, thus avoiding the rotor natural frequency drop as it is detrimental for the stability. Unfortunately, the effective damping is reduced at the same time but this effect can be mitigated using proper devices to keep the preswirl low or even negative (e.g., swirl brakes and shunt holes). This paper presents the results from an extended test campaign performed in a high-speed rotor test rig equipped with active magnetic bearings (AMBs) working under high pressure (14 krpm, 200 bar gas inlet pressure), with the aim to validate the rotordynamic characteristics of a negative preswirl, convergent honeycomb seal and demonstrate its ability to effectively act as a gas bearing as well as a seal. The test plan included variations of inlet pressure, differential pressure (given the same inlet pressure), as well as rotational speed in order to fully validate the seal behavior. This kind of test was performed in a “dynamic mode” that is to say exciting the spinning test rotor through a pair of AMBs along linear orbits. Additionally, the impact of the seal to rotor static eccentricity and the seal to rotor angular misalignment were both experimentally investigated and compared to relevant computational fluid dynamics (CFD) simulations. This kind of test was performed in a “static mode,” that is to say imposing through the AMBs the required eccentricity/angular misalignment and then measuring the forces needed to keep the rotor in the original position. Dynamic mode test was also performed in order to check the impact of the seal static eccentricity on its dynamic behavior. Finally, the test results were compared with predictions from a state of the art bulk-flow code in order to check the predictability level for future design applications.

On the rotordynamic stability of two‐pole synchronous electric machinery considering different load cases

AbstractThis paper discusses stationary rotordynamics of synchronous electric machinery, considering different load cases. The model comprises the electrical operation in a rigid network and in an isolated condition. The mechanical part is modelled as a Laval Rotor (Jeffcott Rotor) with a noncircular shaft, accounting for both static‐ and dynamic rotor eccentricities. The results show, that the machine's electrical operation may influence the occurence of mechanical vibrations significantly and therefore demonstrate the importance of analysing the electromechanical interaction. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Oil-Free Automotive Turbochargers: Drag Friction and On-Engine Performance Comparisons to Oil-Lubricated Commercial Turbochargers

Oil-free bearings for automotive turbochargers (TCs) offer unique advantages eliminating oil-related catastrophic TC failures (oil coking, severe bearing wear/seizure, and significant oil leakage, for example), while increasing overall system reliability and reducing maintenance costs. The main objective of the current investigation is to advance the technology of the gas foil bearings (GFBs) for automotive TCs by demonstrating their reliability, durability, and static/dynamic force characteristics desirable in extreme speed and temperature conditions. The paper compares drag friction and on-engine performances of an oil-free TC supported on GFBs against an oil-lubricated commercial production TC with identical compressor and turbine wheels. Extensive coastdown and fast acceleration TC rotor speed tests are conducted in a cold air-driven high-speed test cell. Rotor speed coastdown tests demonstrate that the differences in the identified rotational viscous drag coefficients and drag torques between the oil-free and production TCs are quite similar. In addition, rotor acceleration tests show that the acceleration torque of the oil-free TC rotor, when airborne, is larger than the production TC rotor due to the large mass and moment of inertia of the oil-free TC rotor even though air has lower viscosity than the TC lubricant oil. Separate experiments of the oil-free TC installed on a diesel engine demonstrate the reliable dynamic-forced performance and superior rotor dynamic stability of the oil-free TC over the oil-lubricated TC. The post on-engine test inspection of the oil-free TC test hardware reveals no evidence of significant surface wear between the rotor and bearings, as well as no dimensional changes in the rotor outer surfaces and bearing top foil inner surfaces. The present experimental characterization and verified robustness of the oil-free TC system continue to extend the GFB knowledge database.

Rotordynamic Force Prediction of a Shrouded Centrifugal Pump Impeller—Part II: Stability Analysis

In Paper I, some test cases of centrifugal pump impellers which showed unconventional impedances curves were reviewed and possible sources of the bump and dip in the impedance curves were investigated by simulating a wear-ring seal pump impeller. In this paper, the unconventional impedances determined in Paper I are converted into a form for inclusion in rotordynamic stability and forced response analyses. First of all, a finite element (FE) rotor model is considered to investigate the influence of the bump and dip in the impedance curves on the rotordynamic stability. With the same FE model, speed-dependent impedances are calculated to obtain unbalance frequency response. Finally, a new linear curve-fit approach is developed to model the fluctuating impedances since the unconventional impedance cannot be expressed by the second-order polynomials with the rotordynamic coefficients (stiffness, damping, and mass). In order to validate the newly developed method, a Jeffcott rotor model with the impeller forces is considered and rotordynamic stability analysis is implemented. The results of the analysis show that the existence of the bump and dip in the impedance curves may further destabilize the rotor system.

Numerical Analysis of the Power Balance of an Electrical Machine With Rotor Eccentricity

The power balance in the numerical simulation of a cage induction machine with eccentric rotor has been studied. The asymmetrical air-gap flux density distribution caused by the non-uniform air gap due to eccentricity produced forces that play an important role in the rotor dynamic stability. These forces act both in the radial and the tangential directions. The tangential force together with the whirling motion produces additional power in the shaft. If the power balance of the simulation satisfies, the power due to the whirling can be calculated from the power balance. This could also give a new approach to compute the forces due to eccentricity or verify the existing force computation methods. The error in the power balance of an eccentric machine has been calculated and the sources of the errors have been illustrated and discussed.

Numerical and Experimental Investigation on the Effect of Swirl Brakes on the Labyrinth Seals

Swirl brake influences the static and rotordynamic characteristics of labyrinth seal which are important in the prediction of turbomachine stability. To study the influence of the swirl brakes on improving seal stability, the effects of swirl brakes on the static and rotordynamic characteristics of labyrinth seals were investigated by the combination of numerical simulation and experiment. First, it was performed to the effects of swirl brake on the static flow characteristics of labyrinth seal with swirl ratio and pressure distribution based on computational fluid dynamics (CFD). And then a comparison between leakage predicted by the CFD model and measurement was presented to verify the accuracy of the simulation. Moreover, an experiment was implemented to analyze the rotordynamic characteristics of labyrinth seal using an improved impedance method based on an unbalanced synchronous excitation method on a rotor test rig. The influences of swirl brake density, length, inlet/outlet pressure ratio, and rotating speed were measured and discussed, respectively. The CFD numerical results show that the swirl brake effectively reduces the seal swirl ratio (∼60–75% less), circumferential pressure difference (∼25–85% less) so that the seal destabilizing forces decrease. With the increasing of the swirl vanes density and length, the seal leakage drops (∼8–20% less). The experimental rotordynamic characteristics results show that it is more obvious to reduce the cross-couple stiffness (∼50–300% less) and increase the direct damping (∼50–60% larger) with the increasing in the number and length of the swirl vanes, and thus the swirl brake improves the seal rotordynamic stability. The efforts of this paper provide a useful insight to clearly understand the effects of swirl brakes on the labyrinth seal static and rotordynamic characteristics, which is beneficial to improve the design of annular seals.

Numerical Comparison of Rotordynamic Characteristics for a Fully Partitioned Pocket Damper Seal and a Labyrinth Seal With High Positive and Negative Inlet Preswirl

Pocket damper seals (PDSs) are used as replacements for labyrinth seals in high-pressure centrifugal compressors at the balance-piston location or center seal location to enhance rotordynamic stability. A concern exists that this enhanced stability will be lost at high positive inlet preswirl. Numerical results of frequency-dependent rotordynamic force coefficients and leakage flow rates were presented and compared for a fully partitioned PDS (FPDS) and a labyrinth seal at high positive and negative inlet preswirl, using a proposed transient computational fluid dynamics (CFD) method based on the multifrequency elliptical orbit whirling model. The negative preswirl indicates a fluid swirl in a direction opposite to rotor rotation at seal inlet. Both seals have identical diameter and sealing clearance. The full 3D concentric CFD model and mesh were built for the labyrinth seal and FPDS, respectively. The accuracy and availability of the present transient CFD numerical method were demonstrated with the experiment data of frequency-dependent rotordynamic coefficients of the labyrinth seal and FPDS at zero and high positive preswirl conditions. The numerical boundary conditions include two high positive preswirl, two high negative preswirl, and a zero preswirl. Numerical results show that the effect of inlet preswirl on the direct force coefficients is weak, but the effect on the cross-coupling stiffness and effective damping is dramatic. Both seals possess negative effective damping at lower excitation frequencies due to positive preswirl, and the crossover frequency of effective damping term increases with increasing positive preswirl. Negative preswirl produces negative cross-coupling stiffness and positive effective damping over the whole excitation frequency range. Increasing negative preswirl is a stabilizing factor for annular gas seals, which results in a significant increase in the effective damping and a decrease in the crossover frequency. It is desirable to reduce the inlet preswirl to zero or even negative through applications of negative-swirl brakes and negative injection devices.

Bump-Type Foil Bearings and Flexure Pivot Tilting Pad Bearings for Oil-Free Automotive Turbochargers: Highlights in Rotordynamic Performance

Oil-free turbochargers (TCs) require gas bearings in compact units of enhanced rotordynamic stability, mechanical efficiency, and improved reliability with reduced maintenance costs compared with oil-lubricated bearings. Implementation of gas bearings into automotive TCs requires careful thermal management with accurate measurements verifying model predictions. Gas foil bearings (GFBs) are customarily used in oil-free microturbomachinery because of their distinct advantages including tolerance to shaft misalignment and centrifugal/thermal growth, and large damping and load capacity compared with rigid surface gas bearings. Flexure pivot tilting pad bearings (FPTPBs) are widely used in high-performance turbomachinery since they offer little or no cross-coupled stiffnesses with enhanced rotordynamic stability. The paper details the rotordynamic performance and temperature characteristics of two prototype oil-free TCs; one supported on foil journal and thrust bearings and the other one is supported on FPTP journal bearings and foil thrust bearings of identical sizes (outer diameter (OD) and inner diameter (ID)) with the same aerodynamic components. The tests of the oil-free TCs, each consisting of a hollow rotor (∼0.4 kg and ∼23 mm in OD at the bearing locations), are performed for various imbalances in noise, vibration, and harshness (NVH; i.e., cold air driven rotordynamics rig) and gas stand test facilities up to 130 krpm. No forced cooling air flow streams are supplied to the test bearings and rotor. The measurements demonstrate the stable performance of the rotor–gas bearing systems in an ambient NVH test cell with cold forced air into the turbine inlet. Post-test inspection of the test FPTPGBs after the hot gas stand tests evidences seizure of the hottest bearing, thereby revealing a notable reduction in bearing clearance as the rotor temperature increases. The compliant FPTPGBs offer a sound solution for stable rotor support only at an ambient temperature condition while demonstrating less tolerance for shaft growth, centrifugal, and thermal, beyond its clearance. The current measurements give confidence in the present GFB technology for ready application into automotive TCs for passenger car and commercial vehicle applications with increased reliability.

Structural and Rotordynamic Force Coefficients of a Shimmed Bump Foil Bearing: An Assessment of a Simple Engineering Practice

High speed rotors supported on bump-type foil bearings (BFBs) often suffer from large subsynchronous whirl motions. Mechanically preloading BFBs through shimming is a common, low cost practice that shows improvements in rotordynamic stability. However, there is an absence of empirical information related to the force coefficients (structural and rotordynamic) of shimmed BFBs. This paper details a concerted study toward assessing the effect of shimming on a first generation BFB (L = 38.1 mm and D = 36.5 mm). Three metal shims, 120 deg apart, are glued to the inner surface of the bearing cartridge and facing the underside of the bump foil strip. The shim sets are of identical thickness, either 30 μm or 50 μm. In static load tests, a bearing with shims shows a (nonlinear) structural stiffness larger than for the bearing without shims. Torque measurements during shaft acceleration also demonstrate a shimmed BFB has a larger friction coefficient. For a static load of 14.3 kPa, dynamic loads with a frequency sweep from 250 Hz to 450 Hz are exerted on the BFB, without and with shims, to estimate its rotordynamic force coefficients while operating at ∼50 krpm (833 Hz). Similar measurements are conducted without shaft rotation. Results are presented for the original BFB (without shims) and the two shimmed BFB configurations. The direct stiffnesses of the BFB, shimmed or not, increase with excitation frequency, thus evidencing a mild hardening effect. The BFB stiffness and damping coefficients decrease slightly for operation with rotor speed as opposed to the coefficients when the shaft is stationary. For frequencies above 300 Hz, the direct damping coefficients of the BFB with 50 μm thick shims are ∼30% larger than the coefficients of the original bearing. The bearing structural loss factor, a measure of its ability to dissipate mechanical energy, is derived from the direct stiffness and damping coefficients. The BFB with 50 μm thick shims has a 25% larger loss factor—average from test data collected at 300 Hz to 400 Hz—than the original BFB. Further measurements of rotor motions while the shaft accelerates to ∼50 krpm demonstrate the shimmed BFB (thickest shim set) effectively removes subsynchronous whirl motions amplitudes that were conspicuous when operating with the original bearing.

An identification method for damping ratio in rotor systems

Centrifugal compressor testing with magnetic bearing excitations is the last step to assure the compressor rotordynamic stability in the designed operating conditions. To meet the challenges of stability evaluation, a new method combining the rational polynomials method (RPM) with the weighted instrumental variables (WIV) estimator to fit the directional frequency response function (dFRF) is presented. Numerical simulation results show that the method suggested in this paper can identify the damping ratio of the first forward and backward modes with high accuracy, even in a severe noise environment. Experimental tests were conducted to study the effect of different bearing configurations on the stability of rotor. Furthermore, two example centrifugal compressors (a nine-stage straight-through and a six-stage back-to-back) were employed to verify the feasibility of identification method in industrial configurations as well.

Semi-Analytical Dynamic Analysis of Noncontacting Finger Seals

Noncontacting finger seals represent a new noncontacting and compliant seal in gas turbine sealing technology. The compliance and noncontacting nature make this kind of seals fully adaptive to rotor excursions in the radial direction without damaging the seal performance. A new semi-analytical method is developed for characterizing the linearized dynamic performance of noncontacting finger seals. The linearized dynamic characteristics of the gas film are numerically computed using the step jump method and then approximated analytically in the equations of motion using a Prony series. By combining the gas film analytical dynamic characterization with the constitutive model for the dynamic properties of noncontacting finger seals, the dynamic equation of motion for the system is derived in analytical form. The results are presented of the gas film properties, natural transient response to initial conditions, steady-state response to rotor excursion, transmissibility ratios and dynamic stability analysis using the analytical model. It is demonstrated that the steady-state responses from the closed-form solutions agree well with those by the numerical simulation, and the analytical solutions can be used as the benchmark for calibrating the results from numerical analysis.

An All-Metal Compliant Seal Versus a Labyrinth Seal: A Comparison of Gas Leakage at High Temperatures

Parasitic secondary flows (seals' leakage) in centrifugal compressors and gas and steam turbines represent a substantial loss in efficiency and power delivery with an increase in specific fuel consumption. Labyrinth seals (LS) are the most common and inexpensive means of reducing secondary leakage, albeit wearing out with operation and thereby penalizing performance and even affecting rotordynamic stability. The novel hydrostatic advanced low leakage (HALO) seal is an all-metal seal with flexibly supported shoes that enable clearance self-control to effectively reduce leakage, in particular for operation with high pressure ratios and at high surface rotor speeds. This paper presents leakage tests with hot air (max. 300 °C) conducted in a test rig holding a LS and a HALO seal, both of similar diameter, axial length, and clearance. The novel seal leaks much less than the LS as the supply/discharge pressure ratio (Ps/Pa) increases. The leakage reduction is ∼50% for (Ps/Pa) < 2 and continuously dropping to 70% for (Ps/Pa) > 3.0. Thus, the savings in leakage are maximized for operation with a high pressure differential. Leakage measurements with a rotor spinning to a maximum speed of 2700 rpm (surface speed ∼24 m/s) produce a slight decrease in leakage for both seals. Characterization of seal leakage in terms of a flow factor removes the effect of temperature and supply pressure; the LS showing a constant flow factor for (Ps/Pa) > 2. Application of the novel seal technology will aid to increase system efficiency by reducing leakage and will extend maintenance intervals since it eliminates wear of components.

Rotordynamic Analysis of an Oil-Free Turbocharger Supported on Lobed Gas Foil Bearings—Predictions versus Test Data

This article presents the predictions of the rotordynamic performance of oil-free turbochargers (TCs) for various lobed gas foil bearings (GFBs) with the first-generation bump-type configuration and compares these predictions to the measured test data reported earlier. The three-lobed GFBs used for the simulations have increased bearing clearances and mechanical preloads with a fixed minimum bearing clearance, thus yielding larger average bearing clearances and higher wedge effects at the same time. The engine-induced TC housing excitation is found to have no influence on the critical speeds and damping ratios of the rotor rigid modes. The synchronous and asynchronous GFB analyses imply decreases in destabilizing forces for the highly lobed GFBs. The predicted critical speed and damping ratio of the TC rotor supported on the lobed GFBs demonstrate increasing system natural frequency and enhanced rotordynamic stability with increasing mechanical preload. The rotordynamic and bearing analyses of the lobed GFBs validate the beneficial effects of the mechanical preload on the rotordynamic stability of GFBs even with an increased average bearing clearance, which leads to a decreased bearing friction torque as well.

가스 포일 베어링으로 지지된 고속 회전체의 경사각과 베어링의 기계적 예압이 고유 진동수와 불안정성 발생 속도에 미치는 영향

School of Mechanical Systems Engineering, Kookmin University(Received 2014 March 20, 2014 ; Revised 2014 May 1, 2014 ; Accepted 2014 May 4, 2014) Abstract − This study investigates the effects of the slopes of the rotational axis and bearing preloads on the nat-ural frequencies and onset speeds of the instability of a rotor supported on gas foil bearings (GFBs). The pre-dictive model for the rotating system consists of a rigid rotor supported on two gas foil journal bearings (GFJBs)and a pair of gas foil thrust bearings (GFTBs). Each GFJB supports approximately half the rotor weight. As theslope of the rotational axis increases from 0° (horizontal rotor operation) to 90° (vertical rotor operation), theapplied load on the GFJB owing to the rotor weight decreases. The predictions show that the natural frequencyand onset speed of instability decrease significantly with an increase in the slope of the rotational axis. In a para-metric study, the nominal radial clearance and preload for the GFJB were changed. In general, a decrease in thenominal radial clearance lead to an increase in the natural frequency and onset speed of instability. For constantassembly clearance, the decrease in the preload changed the natural frequency and onset speed of instability withinsignificant improvements in the rotordynamic stability. The present predictions can be used as design guide-lines for GFBs for oil-free high-speed rotating machinery with improved rotordynamic performance.Keywords −gas foil journal bearing (가스 포일 저널 베어링), preload (예압 ), natural frequency (고유 진동수), instability(불안정성), slope of rotational axis (회전축 경사각)

프로세스 고속 경량 원심 압축기의 로터다이나믹 안정성 강화를 위한 설계해석 - Part II: 로터다이나믹 안정성 개선

In this Part II study, rotordynamic stability analyses were carried out to confirm improvements to the stability of a process high-speed lightweight centrifugal compressor, depending on the effects of tilting pad journal bearing designs. The bearing design variables considered were the clearances, LBPs, LOPs, and preloads. The results showed that the rotordynamic stability of the subject compressor rotor-bearing system improves exactly in accordance with the effects of the bearing design variables, which were determined in the preceding Part I study, owing to reduced bearing stiffnesses. Specifically, it was confirmed that the stability of the rotor system can be greatly improved by increasing both the machined and assembled bearing clearances, but there were no stability improvements by simply changing from an LBP to an LOP design. In addition, it was confirmed that for given fixed machined bearing clearances, the stability can be additionally improved by decreasing the preloads, i.e., by increasing the assembled clearances. In conclusion, it may be necessary to improve the designs of the original tilting pad bearings to obtain a sufficient margin of rotordynamic stability against a possible aerodynamic cross-coupled stiffness in a process high-speed centrifugal compressor. Thus, increasing the machined and assembled bearing clearances and decreasing the preload could be effective solutions.

Numerical and Experimental Stability Investigation of a Flexible Rotor on Two Different Tilting Pad Bearing Configurations

Rotordynamic stability is crucial for high performance centrifugal compressors. In this paper, the weighted instrumental variable (WIV) based system identification method for rotating machinery stability is investigated based on a sine sweep forward excitation with an electromagnetic actuator. The traditional multiple input multiple output (MIMO) frequency response function (FRF) is transformed into a directional frequency response function (dFRF). The rational polynomial method (RPM) combined with WIV is developed to identify the rotor’s first forward mode parameters. This new approach is called the COMDYN method. Experimental work using the COMDYN method is carried out under different rotating speeds, oil inlet temperatures, and pressure conditions. Two sets of bearings with preloads 0.1 and 0.3 are investigated. A numerical rotor-bearing model is also built. The numerical results correlate reasonably well with the experimental results. The investigation results indicate that the new method satisfies the desired features of rotating machine stability identification. Furthermore, the system log decrement was improved somewhat with the increase of oil inlet temperature. The increase of oil supply pressure affects the rotor-bearing system stability very slightly. The results of this paper provide new and useful insights for potentially avoiding instability faults in centrifugal compressors.

프로세스 고속 경량 원심 압축기의 로터다이나믹 안정성 강화를 위한 설계해석 - Part I: 베어링 설계의 영향

Part I of this study analyzed the effects of tilting pad bearing designs to reduce the stiffness of the bearings used in a process high-speed lightweight centrifugal compressor intended for a domestic refinery use. This was done in an attempt to enhance the robustness of its rotordynamic stability against possible aerodynamic cross-coupled stiffness. The bearing design variables reviewed were the clearances, LBPs, LOPs , and preloads. The results showed that there was practically no difference between the LBP and LOP designs in terms of the bearing stiffness, because the compressor rotor was lightweight and the bearings had relatively high preloads. Increasing both the machined and assembled clearances in bearing designs has resulted in the bearing stiffness being greatly reduced. In addition, it has been confirmed that an additional reduction in the bearing stiffness can be obtained for given fixed machined clearances by decreasing the preloads, i.e., by increasing the assembled clearances. Keywordsprocess compressor (프로세스 압축기), rotordynamic stability (로터다이나믹 안정성), effects of bearing design (베어링 설계의 영향), tilting pad bearing (틸팅 패드 베어링), reducing bearing stiffness (베어 링 강성 감소), bearing clearance (베어링 틈새)