ASME Conference Presenter Attendance Policy and Archival Proceedings

2017;():V07AT00A001. doi:10.1115/GT2017-NS7A.

This online compilation of papers from the ASME Turbo Expo 2017: Turbomachinery Technical Conference and Exposition (GT2017) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference by an author of the paper, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Emerging Methods in Design and Engineering

2017;():V07AT30A001. doi:10.1115/GT2017-63446.

The contribution discusses a model update procedure and its experimental validation in the context of blisk mistuning. Object of investigation is an industrial test blisk of an axial compressor which is milled from solid using a state of the art 5-axis milling machine. First, the blisk geometry is digitized by a blue light fringe projector. Digitization is largely automated using an industrial robot cell in order to guarantee high repeatability of the measurement results. Additionally, frequency mistuning patterns are identified based on vibration measurements. Here, the system excitation is realized by a modal impact hammer. The blade response is detected using a laser scanning vibrometer. Furthermore, all blades except the currently excited one are detuned with additional masses. Applying these masses allows to identify a blade dominated natural frequency for each blade and every mode of interest. Finally, these blade dominated frequencies are summarized to mode specific mistuning patterns. The key part of the contribution presents a model update approach which is focused on small geometric deviations between real engine parts and idealized simulation models. Within this update procedure the nodal coordinates of an initially tuned finite element blisk model were modified in order to match the geometry of the real part measured by blue light fringe projection. All essential pre- and post-processing steps of the mesh morphing procedure are described and illustrated. It could be proven that locally remaining geometric deviations between updated finite element model and the optical measurement results are below 5 μm. For the purpose of validation blade dominated natural frequencies of the updated finite element blisk model are calculated for each sector up to a frequency of 17 kHz. Finally, the numerically predicted mistuning patterns are compared against the experimentally identified counterparts. At this point a very good agreement between experimentally identified and numerically predicted mistuning patterns can be proven across several mode families. Even mistuning patterns of higher modes at about 17 kHz are well predicted by the geometrically mistuned finite element model. Within the last section of the paper, possible uncertainties of the presented model update procedure are analyzed. As a part of the study the digitization of the investigated blisk has been repeated for ten times. These measurement results serve as input for the model update procedure described before. In the context of this investigation ten independent geometrical mistuned simulation models are created and the corresponding mistuning patterns are calculated.

Commentary by Dr. Valentin Fuster
2017;():V07AT30A002. doi:10.1115/GT2017-63560.

One key engine component that defines the main engine characteristics is the wheel of a high-pressure turbine. One reasonable approach for increasing the efficiency of this type of turbine is to use blade shrouding. However, this shrouding also increases the centrifugal loading on the profile part of the blades, the lock connection, and the disk. One solution to this issue is to eliminate the lock connections, i.e. to create wheels of a blisk-type design.

The bimetallic blisk was developed based on a wheel that has a lock connection for the disk and blades without shrouds. This study presents a redesign of the profile parts of the blades using computational fluid dynamics calculations, and a reintroduction of shrouds to the blade design. Connection of the blades to the disc involved a newly developed process based on powder metallurgy. The result is a bimetallic blisk consisting of single-crystal blades with shrouds and a disk consisting of a granulated heat-resistant nickel base alloy, connected by hot isostatic pressing. This bimetallic blisk satisfies the strength requirements and is detuned from the resonant frequencies. The weight is 7% lower for the developed design than for the prototype, and the turbine efficiency is increased by 2%.

Commentary by Dr. Valentin Fuster
2017;():V07AT30A003. doi:10.1115/GT2017-64064.

A new methodology is proposed for damage detection of rod fastening rotor in gas turbine. Rod fastening rotor is a vibration sensitive structure that exhibit complex and multi-modal vibration, it can show some characteristics of vibration when suffered local damage. Due to this fact, the vibration-based method could face a challenge when used for damage detection in these structures. The proposed methodology applies multi-scale space theory and data fusion approach to detect the damage from changes in modal strain energy (MSE). The multi-scale space theory is utilized to cope with the measurement noise, which is inevitable in the process of actual measurement. Based on the multi-scale space data, the proposed methodology finally uses Dempster-Shafer’s evidence theory to detect and locate single and multiple damage. The effectiveness of the methodology is analytically verified using two rod fastening rotor with various types of damage scenarios. The analytical results demonstrate that the proposed methodology has superior noise tolerant ability as well as damage detectability, with knowledge of neither the material properties nor boundary conditions. The proposed methodology can make a contribution to ensure safety and performance of gas turbines which play an important role in the power systems.

Topics: Rotors , Data fusion , Damage
Commentary by Dr. Valentin Fuster
2017;():V07AT30A004. doi:10.1115/GT2017-64126.

Quasi-stationary theory is applied to a simplified one-dimensional solid to work out warm-up thermal stress [1]. This paper verifies the quasi-stationary state exits in the three-dimensional valve body warm-up process, and the relationship of the thermal stress and the temperature difference is similar.

Dynamic analysis of warming-up at constant temperature increasing speed shows that: the ratio of the thermal stresses and the temperature difference gets the max value at the quasi-stationary state. The ratio is not influenced by the surface heat transfer coefficient, the material thermal diffusion coefficient and the temperature raise speed. It is a safety way to estimate the thermal stresses by temperature difference multiplied by this max ratio value. For quasi-stationary, once the temperature measure points be located, the ratio between stresses of any selected point and temperature difference is fixed. Where to set the measure points is unlimited, and the mean metal temperature, which is not easy to get, may not be applied anymore.

Commentary by Dr. Valentin Fuster
2017;():V07AT30A005. doi:10.1115/GT2017-64377.

Composite fan blade ply lay-up design, which includes ply drop-off/shuffle design and stacking sequence design, makes fan blade structures different from traditional composite structures. It gives designers more freedom to construct high-quality fan blades. However, contemporary fan blade profiles are quite complex and twisted, and fan blade structures are quite different from regular composite structures such as composite laminates and composite wings. The ply drop-off design of a fan blade, especially for a fully 3D fan blade, is still an arduous task. To meet this challenge, this paper develops a ply lay-up way with the help of a software called Fibersim. The fully 3D fan blade is cut into ply pieces in Fibersim. As a result, an initial ply sequence is created and ply shuffle could revise it. Because of the complexity of ply shuffling, the ply shuffle table developed in this paper mainly refers to the design experience gained from simple plate-like laminate structures and some criterion. Besides, the impact of different ply orientation patterns on the reliability of composite fan blade is studied through static and modal numerical analysis. The results show that this ply lay-up idea is feasible for aero engine composite fan blade. Under the calculated rotating speeds, the ply stacking sequence 4 (i.e.[−45°/0°/+45°/0°] with the outer seven groups are [−45°/0°/−45°/0°]) shows the greatest margin of safety compared with other stacking sequences. Modal analysis shows that plies with different angles could have relatively big different impacts on blades vibration characteristics. The composite fan blade ply design route this paper presents has gain its initial success and the results in this paper might be used as basic references for composite blade initial structural design.

Commentary by Dr. Valentin Fuster
2017;():V07AT30A006. doi:10.1115/GT2017-64412.

Turbocharger impeller wheels are traditionally manufactured using a casting process. However, with the improvement of multi-axial machining technology, machined impeller wheels are becoming popular among turbo machinery manufacturers due to their enhanced durability. Nonetheless, machining a complex impeller shape from a solid billet, results in tool marks being left on the component surface. As presented in this paper, repeatedly running a wheel to 5% beyond the design speed limit can result in fatigue failure initiating from the machining marks.

In this paper, the ‘as machined’ geometry of sample wheels has been determined using both CT scanning and optical surface measurement techniques. The data from these measurements has been used to generate solid CAD models suitable for finite element analysis to simulate the stress distribution of reverse engineered wheels. The maximum principal stress predicted is 15% higher than that obtained from the nominal CAD model. In order to model the measured geometry efficiently, a novel technique has been used to enforce cyclic symmetry on geometry that is not precisely cyclically symmetric.

The work has demonstrated that it is possible to predict the stress raising effect of the machining marks at the design stage. The analysis methodology presented in the paper will enable future integrated optimisation of both the design and manufacture of impeller wheels to ensure that wheels with a specified operating envelope are machined as efficiently as possible.

Commentary by Dr. Valentin Fuster
2017;():V07AT30A007. doi:10.1115/GT2017-64959.

There is evidence of a lack of knowledge in the design of the blade/disk attachment so that the strength of the materials is not fully exploited and the load capability of the attachment is underestimated. The aim of this work is to improve the engineers’ capability in designing the attachment so that higher loads can be carried with the same material. To this end, an optimization method has been applied to the attachment design. A dovetail blade root was chosen as case study and the objective function was the static equivalent stress in the blade and the disc. The dovetail was described by variable parameters under geometrical and physical constraints. Optimization was performed with a Genetic Algorithm (GA). The result of the optimization procedure is the optimal set of parameter values that minimizes the equivalent stress on the critical areas. Moreover, a surrogate function was utilized as a booster to the GA to save computational time. Stress analysis was performed with a commercial Finite Element (FE) software to provide the exact fitness value. An in-house code was developed to manage both the optimization process and the input/output interface with the FE software. The same code provides a decision-making core. This core checks for feasibility of the geometry of the current set of parameters. The expected result is an optimized profile in terms of Von-Misses equivalent stress.

Commentary by Dr. Valentin Fuster
2017;():V07AT30A008. doi:10.1115/GT2017-65045.

This paper deals with the estimation of forcing functions on a mistuned bladed rotor from measurements of harmonic response via Kalman Filter in time domain. An unique feature of this approach is that the number of estimated variables can be far greater than the number of measurements. The robustness of this method to measurement errors is shown. It is also shown that direct prediction of amplitude and phase of sinusoidal force vector from input/output frequency response function has a large amount of errors in the presence of unavoidable measurement noise. Numerical examples contain both frequency mistuning and geometric mistuning.

Topics: Rotors
Commentary by Dr. Valentin Fuster
2017;():V07AT30A009. doi:10.1115/GT2017-65075.

A design tool has been developed to calculate the natural frequencies of shrouded or unshrouded gas turbine blades in seconds to allow designers to perform aeromechanical frequency avoidance checks in the early concept design phase. The tool derives its inputs from a pitch-line aerodynamic calculation and a 1D structural design tool and uses a NACA-based airfoil section generator to create the airfoil sections. It then generates a shell-element based finite element model for the blade and disk sector, performs a pre-stressed modal analysis, and ranks the blades according to their frequency margins with specified aerodynamic drivers. Validation studies comparing this simplified model to high-fidelity solid element FEA models show the frequency error to be below 5% for most cases. The speed of this tool allows for frequency assessment of thousands of designs in a few hours allowing the designer to perform large spacefilling DoEs and select a flow path which minimizes the chances of fundamental mode crossings in later design stages..

Commentary by Dr. Valentin Fuster

Fatigue, Fracture and Life Prediction

2017;():V07AT31A001. doi:10.1115/GT2017-63229.

In order to operate the turbo-machineries more flexibly, a lifetime counting method was developed which enables estimating lifetime consumption of high thermal-inertia components based on the temperature history of the components. It can account for consumed fatigue life at the locations of temperature measurement during the turbo-machine operation. By considering the operation history, the structural component can be operated closer to its lifetime limits, to increase the intervals between inspections, and therefore to extend the operational lifetime of the turbo-machine.

In this method the cold start (CS) to full load cycle, with the number of cycles to crack initiation Ncs, is defined as reference load cycle. The damage weighting factor Ni/NCS for cyclic event i is then calculated based on a semi-empirical correlation between Ni/NCS and the temperature history of the part. The semi-empirical correlation can be determined for each component or each location depending on the required precision. It is determined based on the Low Cycle Fatigue (LCF) life calculated for different Gas Turbine (GT) operation scenarios. The damage weighting factors are then employed to calculate the lifetime consumption using Miner’s rule.

The predictions of this method for the turbine housings of several Gas Turbines (GTs) were evaluated against finite element (FE) results. Multiple load cases were considered for each GT. It is shown that this approach can account for the lifetime consumption using the minimum required number of GT operation parameters.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A002. doi:10.1115/GT2017-63341.

Jet engines of airplanes are designed such that in some components damage occurs and grows in service without being critical up to a certain level. Since maintenance, repair and component exchange are cost-intensive and limit the operating life of the engine, it is necessary to predict the component lifetime using an acceptable computational effort.

To efficiently calculate the lifetime consumption of turbine components with sufficient accuracy under operational conditions, we developed a hybrid approach, which is based on the following three steps: First, the possible operation space is analyzed and reduced to define a manageable Design-of-Experiments (DoE) space. Subsequently, precise aerodynamic and structural mechanic simulations of the component are performed at each DoE support point and the results are stored in a database. Next, the lifetime consumption of the component for the operation profile of interest is calculated based on interpolated stress and temperature fields using suitable lifetime prediction models. The implemented lifetime models are based on accepted lifetime prediction models for creep, fatigue and combined loading, which were extended to incorporate the loading situation on a high pressure turbine (HPT) blade.

Due to efficient data management, the computational time for calculating the lifetime consumption of a whole HPT blade is approximately four seconds for one take-off. Consequently, a full three dimensional lifetime consumption analysis of the lifespan of a HPT blade is possible within a few hours. Using the developed approach, it is now possible to predict the lifetime of a HPT blade for different operators with the necessary precision in an acceptable time. To demonstrate the developed approach, a HPT blade of an exemplarily chosen jet engine with known flight history and documented borescope inspections will be used. Comparing the calculated lifetime of the HPT blade with the documented findings from shop visits reveals that the simulation is in good agreement for the investigated flight mission of the chosen engine.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A003. doi:10.1115/GT2017-63366.

Siemens Energy has a large fleet of aero derivative gas turbines installed in oil & gas and power generation applications globally. Over the past several years, several incidents of disk corrosion damage have been observed during the power turbine overhauls especially for the units fielded in marine environment. Most of the corrosion attack has been observed at disk firtree (blade attachment features), gas washed disk faces and torque transmission features. In all of these cases the mechanism for corrosion has been identified as Low Temperature Hot Corrosion (LTHC) also known as Type-II sulphidation.

Siemens has explored a number of potential operating and/or design options e.g. utilization of more corrosion resistant alloys and coatings to mitigate the effects of the corrosive attack on these disks. However, many of these proposals are either intermediate or long term solutions and are addressed more towards new power turbine disks rather than those that are already in service.

In the short term, Siemens technical solution has focused on development of proprietary repair processes to effectively remove the corrosion damage and extend the operational life of in-service disks as well as salvage some of the previously scraped disks.

This Paper discusses the methodology and findings from the development and characterization of the repair of the power turbine disks when exposed to corrosion and other operational damage.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A004. doi:10.1115/GT2017-63422.

Material characterization is usually based on standard specimen testing. In contrast, small-scale specimens require less testing material and offer additional advantages like investigation of size effects with impact on material properties (specimen diameter equivalent to wall thickness of buckets and liners) and testing of samples of virgin or service-exposed turbomachinery components. The paper highlights the small-scale specimen test setup and its application for LCF- and HCF-testing. Innovative small-scale specimen testing with close-to-component structural features enables direction dependent and spatially resolved determination of material characteristics. Cast and forged gas turbine nickel-based alloys and a typical steam turbine steel are the materials of interest in this study. Substrate temperature levels ranges from room temperature up to 1000 °C. Test system, radiation heating, instrumentation and two types of small-scale specimen geometries according to German standard DIN 50100 will be presented. Benefits and limitations of application of small-scale specimens will be discussed. Finally the authors report about application of small-scale specimen testing for remaining life time determination of service exposed gas turbine buckets. Small-scale specimens have been directly extracted from a gas turbine. Bucket root material (conventionally cast nickel-base alloy IN738) and literature data serve as reference base for initial state of the material. Tensile and LCF tests have been carried out at a representative material temperature for service conditions of 850 °C. Published material data of IN738 are available at this temperature. On that basis, fatigue life consumption has been estimated. The presented procedure for remaining life time prediction can also be applied to other turbomachinery components.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A005. doi:10.1115/GT2017-63599.

For a considerable amount of time blade integrated disks (blisks) are established as a standard component of high pressure compressors (HPCs) in aero engines. Due to the steady requirement to increase the efficiency of modern HPCs, blade profiles get thinned out and aerodynamic stage loading increases. Ever since, aerofoil design has to balance structural and aerodynamic requirements. One particularity of aero engines is the possibility to ingest different kinds of debris during operation and some of those particles are hard enough to seriously damage the aerofoil. Lately, a growing number of blisk-equipped aero engines entered service and the question of foreign object damage (FOD) sensitivity relating to compressor blade high cycle fatigue (HCF) has emerged. Correct prediction of fatigue strength drop due to a FOD provides a huge chance for cost cutting in the service sector as on-wing repairs (e.g. borescope blending) are much more convenient than the replacement of whole blisks and corresponding engine strips.

The aim of this paper is to identify critical FOD-areas of a modern HPC stage and to analyze the effects of stress concentrations — caused by FOD — on the fatigue strength. A process chain has been developed, that automatically creates damaged geometries, meshes the parts and analyses the fatigue strength. Amplitude frequency strength (af-strength) has been chosen as fatigue strength indicator owing to the fact, that amplitudes and frequencies of blade vibrations are commonly measured either by blade tip timing or strain gauges. Furthermore, static and dynamic stress concentrations in damaged geometries compared to the reference design were computed. A random variation of input parameters was performed, such as the radial damage position at blade leading edge and damage diameter.

Based on results of the different samples, correlations of input parameters and the fatigue strength drop have been investigated. Evaluation shows a significant mode dependence of critical blade areas with a large scatter between drops in fatigue strength visible for mode to mode comparison. Keeping in mind the necessity of fast response times in the in-service sector, FOD sensitivity computations could be performed for all blade rows of the HPC — including the analysis of possible borescope blending geometries — in the design stage. Finally, the actual amplitude frequency levels (af-levels) of the modes excited during operation have to be appropriately taken into consideration. For example, a pronounced af-strength drop due to a FOD may not be critical for safe engine operations because the observed mode is excited by small af-levels during operation. Hence, the endurance ratio — a quotient of af-level and af-strength — is used as assessment criterion.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A006. doi:10.1115/GT2017-63775.

An energy based fatigue damage and lifing assessment method is developed for a high temperature material, Inconel 625, and Aluminum 6061-T6. A newly developed experimental method is used for interrogating accumulated fatigue damage and evolution for low and high cycle fatigue (LCF/HCF) at continuum scales. The proposed fatigue lifing assessment method is based on assessing the total strain energy dissipated to cause fatigue failure of a material, known as the fatigue toughness. From the fatigue toughness and experimentally determined fatigue lives at two different stress amplitudes, the cyclic parameters of the Ramberg-Osgood constitutive equation that describes the hysteresis stress-strain loop of a cycle are determined. Stress controlled mechanical fatigue tests are performed to construct room temperature stress-life (S-N) curves and to determine damage progression based on accumulated fatigue damage. The predicted fatigue life obtained from the present energy based approach is found in good agreement with experimental data.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A007. doi:10.1115/GT2017-63789.

Laser shock peening (LSP) is a promising surface treatment method for improving fatigue properties of turbine blades. The effect of LSP on combined low and high cycle fatigue (CCF) life of full scale turbine blade was investigated. The LSP is performed by YLSS-40 LSP equipment and the laser power density is 5.8 GW/cm2. Thirteen LSP treated turbine blades and thirteen untreated turbine blades were selected to carry out the contrast test at high temperature in a bench environment. Experimental results show that there exists a critical vibration stress of blades, below which the CCF life was significantly prolonged by LSP, and above which the LSP has no effect or an adverse effect on the CCF life. The safe life of blades can be significantly increased after treated by LSP when the total stress is below the yield stress. However, the situation is a bit different when the total stress is above the yield stress. Although the safe life of LSP blades is longer than that of untreated blades in this situation, but the median life of blades is decreased after treated by LSP. The effect of LSP on the scatter in life plays a greater role in improving the safe life that directly leads to the safe life of LSP blades longer than the safe life of untreated blades when the total stress is above the yield stress.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A008. doi:10.1115/GT2017-63857.

Slender turbine blades are susceptible to excitation. Resulting vibrations stress the blade’s fixture to the rotor or stator. In this paper, high cycle fatigue at the edge of contact between blade and rotor/stator of such fixtures is investigated both experimentally and numerically. Plasticity in the contact zone and its effects on e.g. contact tractions, fatigue determinative quantities and fatigue itself are shown to be of considerable relevance. The accuracy of the finite element analysis is demonstrated by comparing the predicted utilizations and slip region widths with data gained from tests. For the evaluation of edge of contact fatigue, tests on simple notched specimens provide the limit data. Predictions on the utilization are made for the edge of contact of a dovetail set-up. Tests with this set-up provide the experimental fatigue limit to be compared to. The comparisons carried out show a good agreement between the experimental results and the plasticity-based calculations of the demonstrated approach.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A009. doi:10.1115/GT2017-63871.

In this investigation, the variation of J-integral considering Compact Tension (CT) specimen geometry varying a/W and σ using 2D and 3D elasto-plastic Finite Element (FE) analysis have been studied. Further, the investigation has been done to examine the relationship between the J and δ for varied a/W and σ. The plane stress and plane strain elasto-plastic FE analyses have been conducted on the CT specimen with a/W = 0.45–0.65 to extract the J and Crack-tip Opening Displacement (CTOD) values for mild steel. The comparative study of the variation of dn with a/W of mild steel with earlier results of IF steel is carried out. The study clearly infers the effect of yield stress on the variation of the magnitude of dn with reference to a/W ratio. The present analysis infers that while converting the magnitude of the CTOD to J one needs to carefully evaluate the value of dn depending on the material rather than considering it to be unity. Further, the study was extended to experimental and 3D FEA wherein J-integral and CTOD were estimated using the CT specimen. Experimental results reveal that the crack length, the specimen thickness, and the loading configuration have an effect on the fracture toughness measurements. The error analysis between the results obtained by 3D FEA and experimentation were conducted and found to be within limits.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A010. doi:10.1115/GT2017-64147.

At high intermittent renewable energy share, plants are forced to operate more flexible beyond their original design intent. Many plants are older than 30 years with only limited residual lifetime. Decreasing energy prices and capacity factors will further enforce older plants to more transient operation with steeper gradients. Steam Turbine (ST) protections systems on site often are not designed for such flexible operation and therefore do not properly supervise the resulting impact on lifetime consumption.

Therefore precise lifetime management concepts are required to increase plant reliability and flexibility, mitigate risks for new implemented operation modes like faster start-ups and extended turn down.

The prerequisite for properly managing a plants lifetime is the accurate knowledge of the current state of consumed lifetime, which is evaluated in a lifetime assessment (LTA). This includes a standardized process for the calculation of creep and fatigue damage. For this long-term field operation data is used as input. The current LTA procedure typically limits the analysis to transient operation during start-up and shut-down. Other types of transient operation such as improved turn-down also need to be considered in order to take all relevant operation modes into account.

Therefore dedicated, new advanced methods are required for the precise estimation of the lifetime impact due to new operation modes with higher requirements compared to an assessment based on known current procedure. These methods will allow optimizing asset lifetime. In order to maintain power plant competitiveness, operation and maintenance cost must be reduced despite the demand for improved turn-down and more frequent start-up procedures.

This paper presents results of ST lifetime studies combining long-term operation data (including improved turn-down operation), FEM calculations and simplified and advanced constitutive material models.

In a first investigation, the rotor is modelled with FEM and thermal boundary conditions during isolated transient operation are derived from measurement data combined with generically elaborated new turn-down profiles.

The impact of the start-stop (enclosing cycle) and turndown cycle (sub-cycle) on the lifetime consumption is evaluated. For this, sub-cycles are considered in combination with the enclosing cycle and as isolated cycle.

In a second investigation, the above described calculation model is fed with 2 years of continuous operation data. Postprocessing is performed with common rain-flow methods and compared to an accumulative start-stop cycle assessment.

Based on the input of the calculation with the advanced constitutive material model further fracture mechanic investigations are performed.

The paper closes with an outlook on opportunities arising from digitalization of power generation equipment.

Topics: Steam turbines
Commentary by Dr. Valentin Fuster
2017;():V07AT31A011. doi:10.1115/GT2017-64414.

To carry out combined low and high cycle fatigue (CCF) test on turbine blades in a bench environment, it is imperative to simulate the vibration loads of turbine blades in the field. Due to the low vibration stress of turbine blades in the working state, the test time will be very long if the test vibration stress is equal to the real vibration stress in working state. Therefore, an accelerated test will be used when the test life reach the target value (typically 107). During the accelerated test, each blade is tested at two or more times than the real vibration stress. That means some specimens are tested under two vibration stress levels. In this case, a reasonable data processing method becomes very important. For this reason, a data processing method for the CCF accelerated test is proposed in this paper. These test data are iterated on the basis of S-N curve. Finally, ten real turbine blades are tested in a bench environment, one of them is tested under two vibration stress levels. The test data is processed using the method proposed above to obtain the unaccelerated life data.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A012. doi:10.1115/GT2017-64435.

The combined high and low cycle fatigue (CCF) test on full scale turbine blade in the laboratory is an important method to evaluate the life. In fact, the low cycle fatigue which is usually caused by the centrifugal force can be confirmed easily. While, the high cycle fatigue which is usually caused by the vibration and aerodynamic force is often hard to determine. So the previous scholar has proposed the contrast method to determine the high cycle load in the field. This method utilizes the new and used blades to determine the high cycle within certain limits. While it can’t be applied effectively in the whole life range with the low cycle-high cycle-ultra high cycle fatigue theory raised. So this paper put forward the modified contrast method to realize the optimization. Firstly, the CCF tests are carried out on the turbine blade systematically. Then, the CCF damage properties, including the crack propagation, the fracture morphology and the dynamic characteristic are analyzed. Lastly, the new modified contrast method is proposed with the new coordinate axes, new fitting criterions and amend method. Through comparisons we conclude that: the new method is slightly complicated, but the evaluate precision has significantly increased. So it could be used to deal with data for CCF tests on full scale turbine blade in the future.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A013. doi:10.1115/GT2017-64598.

The anisotropy is the most remarkable characteristic for single crystal nickel-based superalloys, which makes fatigue behavior and life prediction highly correlate with the crystallographic orientation. Based on critical plane approach and preferred crystallographic slip mechanism, an anisotropic LCF life model is proposed to account for orientation-dependent fatigue life in this paper. In addition, the effects of the mean stress and stress-weakening caused by asymmetric loading are also considered. The critical plane is determined by searching for 30 potential slip systems. Moreover, the slip plane with the maximum resolved shear stress amplitude in the crystallographic microstructure of the single crystal nickel-based superalloy is chosen as the critical plane.

The LCF test data are utilized to obtain the regression equation by multiple linear fitting method. The presented LCF life model is applicable for more complex stress state and has higher prediction accuracy than the CDY model.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A014. doi:10.1115/GT2017-64634.

Creep mechanisms are present in heavy duty gas turbine blades and vanes due to the simultaneous presence of high temperature and high stresses. Therefore, the microstructural phenomena (dislocation movement and atomic diffusion) that occur and accumulate during service are able to convert part of the initial elastic field of strain into permanent creep strain. This also induces a global redistribution of stresses. The progressive accumulation of creep strain can, in some extreme cases, produce changes and damage in the material (gamma prime rafting, porosity) and can eventually lead to component failure.

This work shows how the understanding of the nature of the load significantly affect the capability of creep strain to produce damage. In fact, it is shown how both primary (non-self-limiting) and secondary (self-limiting) loads are both capable to generate a significant amount of creep strain, but the microstructural damage is more easily generated by relentless primary loads, generated by external forces such as the rotor blade centrifugal force (or, in other components, external gas pressure, dead weight). In the case of turbine blades and vanes, due to the complexity of the component, it is challenging to quantitatively distinguish relentless primary from self-limiting secondary stresses or simply thermal from mechanical contributions. This work is aimed to provide the designer with tools to perform such distinction and support the interpretation of the creep calculations. The proposed methodologies are developed to improve the accuracy of the prediction of the creep damage in turbine blades and vanes, but they can also be used for other purposes (e.g. predict the hysteresis cycle shift, support the estimation of the plastic strain on the basis of an elastic FE calculation), as illustrated in the paper.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A015. doi:10.1115/GT2017-64791.

A fatigue life prediction method using strain energy density as a prediction parameter has had success predicting the lifetimes greater than 105 cycles for room and elevated temperatures under axial, bending, and shear loading for different material systems. This method uses monotonic strain energy density determined at the macroscale as a damage parameter for fatigue, despite the differences in damage behavior of static and dynamic loading. Recent studies have brought this method into question, as cyclic energy for low cycle fatigue loading has been found to be significantly greater. Amendments of the fatigue life model have addressed this discrepancy for continuum level measurements, but have yet to examine the localized effects of machined notches. This study investigates strain energy density for static and dynamic loading at cycle counts from one (monotonic) to 105 for plain and notched specimens, exposing the differences between damaging strain energy density at continuum and local length scales. Continuum level strain energy density is simply determined by using the load and strain feedback from a standard mechanical test procedure using a common extensometer and a servohydraulic load frame. Local strain energy density is determined more elaborately by using three methods. Localized energy is determined from compliance and a closed form relationship between stress intensity factor and strain energy density. The influence of the notch is considered in the stress intensity calculation, but its influence on stress concentration is disregarded. All calculations are based on the net section stress and linear elasticity is assumed. The analyses revealed two distinct groups, but one data set indicated coincidence with total accumulated strain energy density. These data also corroborate the theory that average strain energy density at the continuum level changes mechanisms governing damage evolution. Monotonic strain energy density is refuted as an appropriate damage parameter to predict fatigue lifetimes, and a statically equivalent strain energy density is proposed. An amended continuum level model is proposed, increasing prediction accuracy over fatigue lifetimes less than 106. Additionally, a localized model is proposed, expanding prediction capability to geometries with notch like features.

Topics: Density , Fatigue life
Commentary by Dr. Valentin Fuster
2017;():V07AT31A016. doi:10.1115/GT2017-64890.

Fracture mechanics analysis is essential for demonstrating structural integrity of gas turbine components. Usually, analyses based on simpler 2D stress intensity solutions provide reasonable approximations of crack growth. However, in some cases, simpler 2D solutions are too-conservative and does not provide realistic crack growth predictions; often due to its inability to account for actual 3D geometry, and complex thermal-mechanical stress fields. In such cases, 3D fracture mechanics analysis provides extra fidelity to crack growth predictions due to increased accuracy of the stress intensity factor calculations. Improved fidelity often leads to benefits for gas turbine components by reducing design margins, improving engine efficiency, and decreasing life cycle costs.

In this paper, the application of 3D fracture mechanics analysis on a gas turbine blade for predicting crack arrest is presented. A comparison of stress intensity factor values from 3D and 2D analysis is also shown. The 3D crack growth analysis was performed by using FRANC3D in conjunction with ANSYS.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A017. doi:10.1115/GT2017-65087.

The impact of non-metallic inclusions on fatigue life of various materials (steel alloys, Ni-base) has been studied extensively for more than fifty years. Specimen test procedures at various conditions (temperature, air or vacuum, LCF, HCF, VHCF) have been used to quantify the impact of inherent manufacturing induced discontinuities (ceramic inclusions, pores, carbides) on the fatigue capability of the material. Frequently, the fatigue data shows large scatter, leading to a large set of test specimens that has to be considered to quantify the lower tail of the fatigue life distribution. To understand the specimen recorded fatigue lives, assessment of the discontinuity population is usually conducted post-mortem by fractography wherein the origin of crack nucleation, size of the eventual inclusion on fracture surface, and distance from the free surface are identified.

3D characterization techniques can be leveraged to extract previously unobtainable information out of the testing specimens non-destructively. In this study, samples of different Powder Metallurgy (PM) Ni-base superalloys with different inclusion content and size were scanned to identify the Computed Tomography (CT) test setup that would provide adequate contrast to discriminate between matrix and eventual discontinuities (inclusions, pores). To further validate the capability to identify discontinuities within the matrix (Ni base alloy) using the CT technique, a set of LCF specimens were scanned prior to the test procedure. Post-failure fractography analysis showed that one of the CT indications is correlated with the failure-inducing inclusion.

Volume reconstruction and finite element meshing conclude this study to: a) further provide a size distribution of inclusions in the scanned volume as well as location of these inclusions relative to the surface of the specimen and b) connect direct measurement with engineering simulation.

Commentary by Dr. Valentin Fuster
2017;():V07AT31A018. doi:10.1115/GT2017-65189.

Considerable efforts have been conducted on the modeling of fatigue crack growth (FCG), aiming at an accurate prediction of fatigue life. However, due to the influence of microstructure, it is still challenging to describe FCG behavior, especially for small cracks.

The FCG exhibits obvious variation at small crack growth procedure. In this regard, a probabilistic model by integrating N-R model is proposed to simulate the FCG process at stage I. The concerned material is nickel based superalloy GH4169.

The proposed model involves both macroscopic and microscopic material parameters for the extension of dislocation with the impediment from grain boundary. Random grain size is represented by the fluctuation of FCG rate. Model validation is performed by comparing the simulation results and experimental data. It is revealed that the dependence tends to be less prominent on longer crack length, smaller grain size and higher applied stress.

Commentary by Dr. Valentin Fuster

Probabilistic Methods

2017;():V07AT32A001. doi:10.1115/GT2017-63289.

Low pressure turbine (LPT) rotor discs undergo high thermal and mechanical loads during normal aircraft missions. Therefore, to meet the minimum requirement for life, temperatures and stresses in the disk need to be maintained within certain limits. This is achieved by carefully designing the disk shape and the cooling system. The complexity of this multi-physics problem together with a large number of design parameters require the use of numerical optimization methods for the Secondary Air System (SAS) design. Moreover, possible variations in the boundary conditions due to ambient parameters (e.g. temperatures, pressures) and manufacturing tolerances of the SAS components should be taken into account within the system design and optimization phase. In this paper an application of robust optimization methods for the design of a LPT secondary air system is proposed. The objective is to increase the engine efficiency by minimizing the amount of cooling flow, which is needed to guarantee a minimum required number of life cycles and to keep maximal temperatures within the limits. In order to predict the disks life accurately, transient thermal-structural analysis is used, which is computationally demanding. For this reason, optimization should be performed with a very limited amount of system evaluations. The dimension of the parameter space is reduced through the application of global sensitivity analysis methods by selecting the parameters that most affect the results. Optimization methods are sped up by the use of surrogate models, created over the reduced parameter space, which approximate the objective function and the constraints.

Commentary by Dr. Valentin Fuster
2017;():V07AT32A002. doi:10.1115/GT2017-63431.

This paper illustrates a probabilistic method of studying Fan Blade Off (FBO) events which is based upon Bayesian inference. Investigating this case study is of great interest from the point of view of the engineering team responsible with the dynamic modelling of the fan. The reason is because subsequent to an FBO event, the fan loses its axisymmetry and as a result of that, severe impacting can occur between the blades and the inner casing of the engine. The mechanical modelling (which is not the scope of this paper) involves studying the oscillation modes of the fan at various release speeds (defined as the speed at which an FBO event occurs) and at various amounts of damage (defined as the percentage of blade which gets released during an FBO event). However, it is virtually infeasible to perform the vibrational analysis for all combinations of release speed and damage. Consequently, the Bayesian updating which forms the foundation of the framework presented in the paper is used to identify the most likely combinations prone to occur after an FBO event which are then going to be used further for the mechanical analysis. The Bayesian inference engine presented here makes use of expert judgements which are updated using in-service data (which for the purposes of this paper are fictitious). The resulting inputs are then passed through 1,000,000 Monte Carlo iterations (which from a physical standpoint represent the number of FBO events simulated) in order to check which are the most common combinations of release speed and blade damage so as to report back to the mechanical engineering team. Therefore, the scope of the project outlined in this paper is to create a flexible model which changes every time data becomes available in order to reflect both the original expert judgements it was based on as well as the real data itself. The features of interest of the posterior distributions which can be seen in the Results section are the peaks of the probability distributions. The reason for this has already been outlined: only the most likely FBO events (i.e.: the peaks of the distributions) are of interest for the purposes of the dynamics analysis. Even though it may be noticed that the differences between prior and posterior distributions are not pronounced, it should be recalled that this is due to the particular data set used for the update; using another data set or adding to the existing one will produce different distributions.

Topics: Blades
Commentary by Dr. Valentin Fuster
2017;():V07AT32A003. doi:10.1115/GT2017-64243.

This paper will present a way to capture the geometric blade by blade variations of a milled from solid blisk as well as the manufacturing scatter. Within this idea it is an essential task to digitize the relevant airfoil surface as good as possible to create a valid surface mesh as the base of the upcoming evaluation tasks. Since those huge surface meshes are not easy to handle and are even worse in getting quantified and easy interpretable results, it should be aimed for an easily accessible way of presenting the geometric variation.

The presented idea uses a section based airfoil parametrization that is based on an extended NACA-airfoil structure to ensure the capturing of all occurring characteristic geometry variations. This Paper will show how this adapted parametrization method is suitable to outline all the geometric blade by blade variation and even more, refer those airfoil design parameters to modal analysis results such as the natural frequencies of the main mode shapes. This way, the dependencies between the modal and airfoil parameters will be proven.

Commentary by Dr. Valentin Fuster
2017;():V07AT32A004. doi:10.1115/GT2017-64408.

A probabilistic risk assessment for low cycle fatigue (LCF) based on the so-called size effect has been applied on gas-turbine design in recent years. In contrast, notch support modeling for LCF which intends to consider the change in stress below the surface of critical LCF regions is known and applied for decades. Turbomachinery components often show sharp stress gradients and very localized critical regions for LCF crack initiations so that a life prediction should also consider notch and size effects. The basic concept of a combined probabilistic model that includes both, size effect and notch support, is presented. In many cases it can improve LCF life predictions significantly, in particular compared to E-N curve predictions of standard specimens where no notch support and size effect is considered. Here, an application of such a combined model is shown for a turbine vane.

Commentary by Dr. Valentin Fuster
2017;():V07AT32A005. doi:10.1115/GT2017-64811.

We developed and successfully applied a direct simulation Monte-Carlo scheme to quantify the risk of fracture for heavy duty rotors commonly used in the energy sector. The developed Probabilistic Fracture Mechanics high-performance computing methodology and code ProbFM routinely assesses relevant modes of operation for a component by performing billions of individual fracture mechanics simulations. The methodology can be used for new design and life-optimization of components, as well as for the risk of failure quantification of in service rotors and their re-qualifications in conjunction with non-destructive examination techniques, such as ultrasonic testing. The developed probabilistic scheme integrates material data, ultra-sonic testing information, duty-cycle data, and finite element analysis in order to determine the risk of failure. The methodology provides an integrative and robust measure of the fitness for service and allows for a save and reliable operation management of heavy duty rotating equipment.

Commentary by Dr. Valentin Fuster

Bearing and Seal Dynamics

2017;():V07AT34A001. doi:10.1115/GT2017-63012.

The influence of sealing components on the stability of turbomachinery has become a key topic because oil and gas market is increasingly requiring high rotational speed and high efficiency, which implies the clearance reduction in the seals. The accurate prediction of the effective damping of the seals is critical to avoid instability issues. In recent years, “negative-swirl” swirl brakes have been employed to reverse the circumferential direction of inlet flow, changing the sign of the cross-coupled stiffness coefficients and generating stabilizing forces. Industries started to investigate, by experiments, the dynamical behavior of labyrinth seals. The experimental results of a 14 teeth-on-stator labyrinth seal with nitrogen, performed in the high-pressure seal test rig owned by GE Oil&Gas, are presented in the paper. Both experimental tests with positive and negative pre-swirl values were performed in order to investigate the pre-swirl effect on the cross-coupled stiffness coefficients.

Concerning with the dynamic characterization of the seal, the fluid-structure interaction into the seal can be modelled by the bulk-flow numeric approach that is still more time efficient than computational fluid dynamics (CFD). Dealing with the one-control volume bulk-flow model, the thermodynamic process in the seal is considered isenthalpic, despite an expected enthalpy variation along the seal cavities, both for gas and steam applications.

In this paper, the authors improve the state-of-the-art one-control volume bulk-flow model [1], by introducing the effect of the energy equation in the zero-order solution. In this way, the real gas properties are evaluated in a more accurate way because the enthalpy variation, expected through the seal cavities, is taken into account in the model. The authors, considering the energy equation only in the zero-order solution, assume that the enthalpy is not a function of the clearance perturbation (i.e. of the rotor perturbed motion).

The energy equation links the continuity and the circumferential momentum equations. The density, in the leakage correlation, depends on the enthalpy, which is calculated (in the energy equation) on the basis of the circumferential velocity and of the fluid/rotor shear stress. Therefore, the leakage mass-flow rate and the fluid thermodynamic properties depend, indirectly, on the shear stresses. This fact is proved in the literature by several CFD simulations that investigate the leakage in the straight-through labyrinth seals, hence, the energy equation allows to better characterize the physics of the problem.

Overall, by taking into account the energy equation, a better estimation of the coefficients in the case of negative pre-swirl ratio has been obtained (as it results from the comparison with the experimental benchmark tests). The numerical results are also compared to the state-of-the-art bulk-flow model developed by Thorat and Childs (2010), highlighting the improvement obtained.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A002. doi:10.1115/GT2017-63014.

Since the 80s, academic research in the rotordynamics field has developed mathematical treatment for the prediction of the dynamic coefficients of sealing components. Dealing with the straight-through labyrinth seal, Iwatsubo [1], at a first stage, and Childs [2], later on, have developed the one-control volume bulk flow model. The model allows evaluating the surrounding fluid forces acting on the rotor, analyzing the fluid dynamics within the seal: the continuity, circumferential momentum and energy equations are solved for each cavity. To consider axial fluid dynamics, correlations, aiming to estimate the leakage and the pressure distribution, are required. Several correlations have been proposed in the literature for the estimation of the leakage, of the kinetic energy carry-over coefficient (KE), of the discharge coefficient and of the friction factor.

After decades of research in the field of seal dynamics, the bulk-flow model has been confirmed as the most popular code in the industries, however, it is not clear which is the best set of correlations for the prediction of seal dynamic coefficients.

This paper allows identifying the most accurate combination of correlations to be implemented in the bulk-flow model. The correlations are related to: the leakage formula, the flow coefficient, the KE and the friction factor. Investigating the results of several models (32 models), which consider different sets of correlations, in comparison to the experimental data (performed by General Electric Oil & Gas), it is possible to observe the dependence, of the model correlations, on the operating conditions.

The experimental results, performed using a 14 teeth-on-stator labyrinth seal, investigate several operating conditions of pressure drop.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A003. doi:10.1115/GT2017-63152.

Gas turbine aircraft engine manufacturers push for simple squeeze film damper (SFD) designs, short in length, yet able to provide enough damping to ameliorate rotor vibrations. SFDs employ orifices to feed lubricant directly into the film land or into a deep groove. The holes, acting as pressure sources (or sinks), both disrupt the film land continuity and reduce the generation of squeeze film dynamic pressure. Overly simple predictive formulations disregard the feedholes and deliver damping (C) and inertia (M) force coefficients not in agreement with experimental findings. Presently, to bridge the gap between simple theory and practice, the paper presents measurements of the dynamic forced response of an idealized SFD that disposes of the feedholes altogether. The short-length SFD, whose diameter D = 125 mm, has one end submerged (flooded) within a lubricant bath and the other end exposed to ambient. ISO VG 2 lubricant flows by gravity through the film land of length L = 25.4 mm and clearance c = 0.122 mm. From dynamic load tests over excitation frequency range 10–250 Hz, experimental damping coefficients (CXX, CYY) from the flooded damper agree well with predictions from the classical open ends model with a full film for small amplitude whirl motions (r/c << 1), centered and off-centered. Air ingestion inevitably occurs for large amplitude motions (r/c > 0.4) thus exacerbating the difference between predictions and tests results. For reference, identical tests were conducted with a practical SFD supplied with lubricant (Pin = 0.4 bar) via three orifice feedholes, 120° apart at the film land mid plane. A comparison of test results shows as expected, that for small amplitude (r/c ∼ 0.05) orbits, the flooded damper generates on average 30% more damping than the practical configuration as the latter’s feedholes distort the generation of pressure. For large amplitude motions (r/c > 0.4), however, the flooded damper provides slightly lesser damping and inertia coefficients than the SFD with feedholes whose pressurized lubricant delivery alleviates air ingestion in the film land. The often invoked open ends SFD classical model is not accurate for the practical engineered design of an apparently simple mechanical element.

Topics: Dampers
Commentary by Dr. Valentin Fuster
2017;():V07AT34A004. doi:10.1115/GT2017-63254.

Wet gas compression systems and multiphase pumps are enabling technologies for the deep sea oil and gas industry. This extreme environment determines both machine types have to handle mixtures with a gas in liquid volume fraction (GVF) varying over a wide range (0 to 1). The gas (or liquid) content affects the system pumping (or compression) efficiency and reliability, and places a penalty in leakage and rotordynamic performance in secondary flow components, namely seals. In 2015, tests were conducted with a short length smooth surface annular seal (L/D = 0.36, radial clearance = 0.127 mm) operating with an oil in air mixture whose liquid volume fraction (LVF) varied to 4%. The test results with a stationary journal show the dramatic effect of a few droplets of liquid on the production of large damping coefficients. This paper presents further measurements and predictions of leakage, drag power, and rotordynamic force coefficients conducted with the same test seal and a rotating journal. The seal is supplied with a mixture (air in ISO VG 10 oil), varying from a pure liquid to an inlet GVF = 0.9 (mostly gas), a typical range in multiphase pumps. For operation with a supply pressure (Ps) up to 3.5 bar (a), discharge pressure (Pa) = 1 bar (a), and various shaft speed (Ω) to 3.5 krpm (ΩR = 23.3 m/s), the flow is laminar with either a pure oil or a mixture. As the inlet GVF increases to 0.9 the mass flow rate and drag power decrease monotonically by 25% and 85% when compared to the pure liquid case, respectively. For operation with Ps = 2.5 bar (a) and Ω to 3.5 krpm, dynamic load tests with frequency 0 < ω < 110 Hz are conducted to procure rotordynamic force coefficients. A direct stiffness (K), an added mass (M) and a viscous damping coefficient (C) represent well the seal lubricated with a pure oil. For tests with a mixture (GVFmax = 0.9), the seal dynamic complex stiffness Re(H) increases with whirl frequency (ω); that is, Re(H) differs from (K2M). Both the seal cross coupled stiffnesses (KXY and −KYX) and direct damping coefficients (CXX and CYY) decrease by approximately 75% as the inlet GVF increases to 0.9. The finding reveals that the frequency at which the effective damping coefficient (CXXeff = CXX-KXY/ω) changes from negative to positive (i.e., a crossover frequency) drops from 50% of the rotor speed (ω = 1/2 Ω) for a seal with pure oil to a lesser magnitude for operation with a mixture. Predictions for leakage and drag power based on a homogeneous bulk flow model match well the test data for operation with inlet GVF up to 0.9. Predicted force coefficients correlate well with the test data for mixtures with GVF up to 0.6. For a mixture with a larger GVF, the model under predicts the direct damping coefficients by as much as 40%. The tests also reveal the appearance of a self-excited seal motion with a low frequency; its amplitude and broad band frequency (centered at around ∼12 Hz) persist and increase as the gas content in the mixture increase. The test results show that an accurate quantification of wet seals dynamic force response is necessary for the design of robust subsea flow assurance systems.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A005. doi:10.1115/GT2017-63276.

SFD (short for squeeze film damper) is a kind of passive vibration isolator widely used in rotor supporting structures of aero-engines for stabilization and vibration control. However, the conventional SFDs are highly nonlinear in terms of damping coefficient, which lead to complex response such as bitable state. In this paper, numerical simulations are carried out to investigate a new kind of SFD, elastic ring squeeze film damper (ERSFD). The elastic ring is modeled by FEM and the film is analyzed by CFD, the orifices on the ring is also included. An FSI approach is introduced to account for the influence of elastic ring’s deformation on oil film thickness. The Zwart-Gerber-Belamri model is included to account for air ingestion and cavitation in the damper land. The characteristics such as pressure distribution, oil film force and the deformation of the ring are obtained and compared with the results without FSI to reveal the self-adaptive mechanism of film thickness. The force coefficients for ERSFD are derived and gained by the FFT method. The dynamic coefficients for ERSFD versus whirl frequency are obtained and compared with corresponding air volume fraction.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A006. doi:10.1115/GT2017-63284.

A detailed elasto-gasdynamic model of a preloaded three-pad air foil journal bearing is presented. Bump and top foil deflections are herein calculated with a nonlinear beamshell theory according to Reissner. The 2D pressure distribution in each bearing pad is described by the Reynolds equation for compressible fluids. With this model, the influence of the assembly preload on the static bearing hysteresis as well as on the aerodynamic bearing performance is investigated. For the purpose of model validation, the predicted hysteresis curves are compared with measured curves. The numerically predicted and the measured hysteresis curves show a good agreement. The numerical predictions exhibit that the assembly preload increases the bearing stiffness (in particular for moderate shaft displacements) and the bearing damping.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A007. doi:10.1115/GT2017-63380.

Circumferentially grooved, annular liquid seals typically exhibit good whirl frequency ratios and leakage reduction, yet their low effective damping can lead to instability. The current study investigates the rotordynamic behavior of a 15 stage groove-on-rotor annular liquid seal by means of CFD, in contrast to previous studies which focused on a groove-on-stator geometry. The seal dimensions and working conditions have been selected based on experiments of Moreland and Childs. The precessional frequency ratios as high as 4 have been studied. The CFD model replicates the whirling motion imposed by the 2D shaker apparatus in Moreland and Childs experimental setup. Implementation of pressure-pressure inlet and outlet conditions obviates the need for loss coefficients at the entrance and exit of the seal. A computationally efficient quasi-steady approach is used to obtain impedance curves as functions of excitation frequency Ω. The effectiveness of steady-state CFD approach is validated by comparison with the experimental results of Moreland and Childs. Results show good agreement in terms of leakage, pre-swirl ratio and rotordynamic coefficients. Leakage is shown to decrease with spin rotational speed ω, whirl speed Ω and surface roughness . The variation of pre-swirl ratio (PSR) and outlet-swirl ratio (OSR) with these parameters is presented. It was found that PSR will be about 0.3–0.4 at the entrance of seal in the case of radial injection and OSR always converges to values near 0.5 for current seal and operational conditions. The rotordynamic coefficients show negligible dependence on Ω in agreement with experiments. The small negative value of direct stiffness coefficients, large cross-coupled stiffness coefficients and small direct damping coefficients explain the destabilizing nature of these seals. Finally, influence of surface roughness on leakage, PSR, OSR and stiffness coefficients is discussed.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A008. doi:10.1115/GT2017-63444.

In civil aircraft aeroengine bearing chambers it is sometimes difficult to feed oil to bearings using the traditional under-race or targeted jet approaches. In such situations one proposed solution is that of a scoop delivery system. Published experimental investigations into scoop performance show that scoop collection efficiency (the percentage of oil delivered by the scoop system to its destination compared to that supplied by the feed jet) is a function of many operational and geometric parameters. However even with high speed imaging it is impossible to experimentally determine in detail the factors that most contribute to reduction in collection efficiency and it is here particularly that a computational fluid dynamics (CFD) investigation has value.

In the work reported here a commercial CFD code (ANSYS Fluent) is used to investigate vortex formation at the scoop tips and the effect these structures have on scoop collection efficiency. The computational domain, a 2D slice through the chosen scoop system, is discretized utilizing ANSYS Meshing. A Volume of fluid (VOF) method is used to model the multiphase flow of oil and air in the system and the RNG k-ε turbulence model is employed.

The results obtained show that the formation of vortices from the tip of the rotating scoops leads to a reduction in pressure in the region near the tip of the oil jet, subsequently causing part of the jet to divert upwards away from the scoop creating a plumed tip. The pluming effect reduces capture efficiency because the oil plume moves outwards under centrifugal effects and this oil is not captured. The frequency of vortex shedding from the scooped rotor was investigated and the Strouhal numbers obtained were around 0.132. This compares well to 0.15 for an inclined flat plate. Two potential methods to reduce the jet pluming effect are investigated one in which the sharp tip of the scoop is blunted and the other in which the jet direction is reversed. The blunt tip increased capture efficiency by almost 2%. Reversing the jet orientation reduces jet pluming but also significantly reduces capture efficiency; it was found to be 10% lower for the case investigated.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A009. doi:10.1115/GT2017-63448.

The present work advances experimental results and analytical predictions on the dynamic performance of an integral squeeze film damper (ISFD) for application in a high-speed super-critical CO2 (sCO2) expander. The test campaign focused on conducting controlled orbital motion mechanical impedance testing aimed at extracting stiffness and damping coefficients for varying end seal clearances, excitation frequencies, and vibration amplitudes. In addition to the measurement of stiffness and damping; the testing revealed the onset of cavitation for the ISFD. Results show damping behavior that is constant with vibratory velocity for each end seal clearance case until the onset of cavitation/air ingestion, while the direct stiffness measurement was shown to be linear. Measurable added inertia coefficients were also identified. The predictive model uses an isothermal finite element method to solve for dynamic pressures for an incompressible fluid using a modified Reynolds equation accounting for fluid inertia effects. The predictions revealed good correlation for experimentally measured direct damping, but resulted in grossly overpredicted inertia coefficients when compared to experiments.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A010. doi:10.1115/GT2017-63451.

High loads and bearing life requirements make journal bearings the preferred choice for use in high power, epicyclic gearboxes in jet engines. In contrast to conventional, non-orbiting journal bearings in epicyclic star gearboxes, the kinematic conditions in epicyclic planetary arrangements are much more complex. With the planet gears rotating about their own axis and orbiting around the sun gear, centrifugal forces generated by both motions interact with each other and affect the external flow behavior of the oil exiting the journal bearing.

This paper presents a literature and state-of-the-art knowledge review to identify existing work performed on cases similar to external journal bearing oil flow. In order to numerically investigate external journal bearing oil flow, an approach to decompose an actual journal bearing into simplified models is proposed. Later, these can be extended in a step-wise manner to allow key underlying physical phenomena to be identified. Preliminary modeling considerations will also be presented. This includes assessing different geometrical inlet conditions with the aim of minimizing computational requirements and different numerical models for near-wall treatment. The correct choice of near-wall treatment models is particularly crucial as it determines the bearing’s internal and external thermal behavior and properties. The findings and conclusions are used to create a three dimensional (3D), two-component computational fluid dynamic (CFD) sector model with rotationally periodic boundaries of the most simplistic approximation of an actual journal bearing: a non-orbiting representation, rotating about its own axis, with a circumferentially constant, i.e. concentric, lubricating gap. The inlet boundary conditions for simulating the external oil flow are generated by partly simulating the internal oil flow within the lubricating gap. In order to track the phase interface between the oil and the air surrounding the bearing, the Volume of Fluid (VoF) method is used. The quality of the CFD simulations of the domain of interest is not only dependent on the accuracy of the inlet conditions, but is also dependent on the computational mesh type, cell count, cell shape and numerical methods used. External journal bearing oil flow was simulated with a number of different mesh densities and the effect on the flow field behavior will be discussed. Two different operating temperatures, representing low and high viscosity oil, were used and their effect on the flow field behavior will also be assessed.

In order to achieve the future objective of creating a design tool for routine use, key areas will be identified in which further progress is required. This includes the need to progressively increase the model fidelity to eventually simulate an orbiting journal bearing in planetary configuration with an eccentric, i.e. convergent-divergent, lubricating gap.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A011. doi:10.1115/GT2017-63492.

In straight-through centrifugal pumps, a grooved seal acts as a balance piston to equilibrate the full pressure rise across the pump. As the groove pattern breaks the development of fluid swirl, this seal type offers lesser leakage and lower cross-coupled stiffnesses than a similar size and clearance annular seal. Bulk-flow models predict expediently the static and dynamic force characteristics of annular seals; however they lack accuracy for grooved seals. Computational fluid dynamics (CFD) methods give more accurate results, but are not computationally efficient. This paper presents a modified bulk-flow model to predict the rotordynamic force coefficients of shallow depth circumferentially grooved liquid seals with an accuracy comparable to a CFD solution but with a simulation time of bulk-flow analyses. The procedure utilizes the results of CFD to evaluate the bulk flow velocity field and the friction factors for a 73 grooves annular seal (depth/clearance dg/ Cr = 0.98 and length/diameter L/D = 0.9) operating under various sets of axial pressure drop and rotor speed. In a groove, the flow divides into a jet through the film land and a strong recirculation zone. The penetration angle (α), specifying the streamline separation in the groove cavity, is a function of the operating conditions; an increase in rotor speed or a lower pressure difference increases α. This angle plays a prominent role to evaluate the stator friction factor and has a marked influence on the seal direct stiffness. In the bulk-flow code the friction factor model (f = nRem) is modified with the CFD extracted penetration angle (α) to account for the flow separation in the groove cavity. The flow rate predicted by the modified bulk-flow code shows good agreement with a measured result (6% difference). A perturbation of the flow field is performed on the bulk-flow equations to evaluate the reaction forces on the rotor surface. Compared to the rotordynamic force coefficients derived from the CFD results, the modified bulk-flow code predicts rotordynamic force coefficients within 10%, except that the cross-coupled damping coefficient is over-predicted up to 14%. An example test seal with a few grooves (L/D = 0.5, dg/Cr = 2.5) serves to further validate the predictions of the modified bulk-flow model. Compared to the original bulk-flow analysis, the current method shows a significant improvement in the predicted rotordynamic force coefficients, the direct stiffness and damping coefficients in particular.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A012. doi:10.1115/GT2017-63495.

Air foil bearing (AFB) technology has made substantial advancement during the past decades and found its applications in various small turbomachinery. However, rotordynamic instability, friction and drag during the start/stop, and thermal management are still challenges for further application of the technology. Hybrid air foil bearing (HAFB), utilizing hydrostatic injection of externally pressurized air into the bearing clearance, is one of the technology advancements to the conventional AFB. Previous studies on HAFBs demonstrate the enhancement in the load capacity at low speeds, reduction or elimination of the friction and wear during starts/stops, and enhanced heat dissipation capability. In this paper, the benefit of the HAFB is further explored to enhance the rotordynamic stability by employing a controlled hydrostatic injection. This paper presents the analytical and experimental evaluation of the rotordynamic performance of a rotor supported by two three-pads HAFBs with the controlled hydrostatic injection, which utilizes the injections at particular locations to control eccentricity and attitude angle. The simulations in both time domain orbit simulations and frequency-domain modal analyses indicate a substantial improvement of the rotor-bearing performance. The simulation results were verified in a highspeed test rig (maximum speed of 70,000 rpm). Experimental results agree with simulations in suppressing the subsynchronous vibrations but with a large discrepancy in the magnitude of the subsynchronous vibrations, which is a result of the limitation of the current modelling approach. However, both simulations and experiments clearly demonstrate the effectiveness of the controlled hydrostatic injection on improving the rotordynamic performance of AFB.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A013. doi:10.1115/GT2017-63558.

Counter-rotation angled injection employed for aerostatic hybrid bearings reduces the cross coupling stiffness that may lead to whirl-whip instabilities at high rotation speeds. The benefits of counter-rotation injection have been known for years. Theoretical investigations were performed for water or air fed hybrid bearings but experiments were conducted only for water fed bearings. The present work is the first effort dedicated to angled injection in air fed hybrid bearings. The tests were performed for a simple rotor supported by two identical hybrid bearings. The hybrid bearings are provided with small size, shallow pockets and are fed with air via counter-rotation oriented orifice type restrictors. An impulse turbine fed with air entrains the rotor. An impact gun applies dynamic excitations and the rotordynamic coefficients are identified from the equations of motion of the rotor. Different air feeding pressures are tested as well as high rotational speeds.

Compared to the dynamic characteristics of radial injection hybrid bearings, the direct stiffness of counter-rotation injection bearings has slightly lower values and the direct damping is higher but the main impact is the drastic reduction of the cross-coupling stiffness that may have even negative values.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A014. doi:10.1115/GT2017-63615.

Potential geometrical deviations in bump foil bearings due to manufacturing uncertainty can have significant effects on both the local stiffness and clearance, and hence, affecting the overall bearing performance. The manufacturing uncertainty of bump type foil bearings was investigated, showing large geometrical deviations, using a developed measurement tool for the formed bump foils. A reduced order foil bearing model was used in a Monte Carlo simulation studying the effect of manufacturing noise on the onset of instability, highlighting the sensitivity of the rotor-bearing system to such manufacturing deviations. It was found that 30% of the simulated cases resulted improvements in stability, the remaining cases underperformed. Attempting to increase the robustness of the bearing, two other compliant structures replacing the classical gen-II bump foils were investigated from a manufacturing perspective. The first is a modified bump type Sinusoidal foil, and the second is the Cantilever beam foil. Consequently, quasi-static load-displacement tests were executed showing deviations in local clearance and stiffness for the classical bump type compliant structure compared to the other designs. It was found that the Cantilever beam foils yield more robustness compared to the bump type foils. Finally, an analytical model for the sequential engagement of the compliant structure is presented and validated with experimental measurements for both bump type and Cantilever structures.

Topics: Foil bearings
Commentary by Dr. Valentin Fuster
2017;():V07AT34A015. doi:10.1115/GT2017-63662.

The catcher bearing is a crucial part of the magnetic bearing system. It can support the rotor when the magnetic bearing is shut down or malfunctioning and limit the rotor’s position when large vibration occurs. The sleeve bearing has the advantages of a relatively large contact surface area, simple structure and an easily replaced surface. There are already many applications of the sleeve type catcher bearings in the industrial machinery supported by the magnetic bearings. Few papers though provide thorough investigations into the dynamic and thermal responses of the sleeve bearing in the role of a catcher bearing. This paper develops a coupled elastic deformation — heat transfer finite element (FEM) model of the sleeve bearing acting as a catcher bearing. The FEM model investigates the dynamic and thermal behavior when a flexible rotor drops onto the sleeve catcher bearing. The thermal load caused by the thermal expansion is also considered. The flexible rotor is composed of Timoshenko beam elements. A coulomb friction model is used to model the friction force between the rotor and the sleeve bearing surface. The contact force and 2-D temperature distribution of the sleeve bearing are obtained by numerical integration. To validate the FEM code developed by the author, firstly, both the mechanical and thermal static analysis results of the sleeve bearing model are compared with the results calculated by the commercial software, “SolidWorks Simulation”. Secondly, the transient analysis numerical results are compared with the rotor drop test results in reference 13. Additionally, this paper explores the influences of different surface lubrication conditions, different materials, such as stainless steel, bronze, and aluminum, on rotor-sleeve bearing’s dynamic and thermal behavior. This paper lays the foundation of the fatigue life calculation of the sleeve bearing and provides the guideline for the sleeve type catcher bearing design.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A016. doi:10.1115/GT2017-63687.

In this paper, the extended Reynolds equation proposed by Dousti et al. [1] is applied to predict the dynamic behavior of different fixed geometry bearings used in vertical water pumps. The influence of convective and temporal inertia effects is studied in regular and preloaded multi-lobe bearings. It is shown that the convective inertia is more influential at the presence of preload and higher rotational speeds and alters the stiffness and damping properties of the bearing. The temporal inertia leads to the prediction of considerable lubricant added mass coefficients in the order of journal mass. The stability analysis shows depending upon the geometry of the bearing, the new extended Reynolds equation may predict higher or lower logarithmic decrement.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A017. doi:10.1115/GT2017-63813.

In a civil aero-engine transmission system a number of bearings are used for shaft location and load support. A bespoke experimental test facility in the University of Nottingham’s Gas Turbine and Transmissions Research Centre (G2TRC) was created to investigate oil shedding from a location bearing. An engine representative ball bearing was installed in the rig and under-race lubrication was supplied via under-race feed to three locations under the inner race and cage. The oil was supplied in an engine representative manner but the delivery system was modified to provide circumferentially even flow. An electromagnetic load system was designed and implemented to allow engine representative axial loads between 5 and 35 kN to be applied to the bearing. In this phase of testing the rig was operated at shaft speeds between 1,000 rpm and 7,000 rpm for a range of oil flow rates and low and high load conditions. The rig was designed with good visual access and high speed imaging was used to investigate film formation and movement on surfaces close to the bearing.

This paper presents images and qualitative observations of thin film formed on the static surfaces forming the outer-periphery of the bearing compartment as well as the gap between orbiting cage and static outer race. Quantitative film thickness was obtained at two circumferential locations (90° and 270° from top dead centre) and three axial locations, through sophisticated analysis of the high speed images. The effect on film thickness of the varied parameters rotational speed, axial load and oil feed input flow rate are presented in this paper.

It was observed that for all axial planes of measurement in both co-current and counter-current regions film thickness decreases with increase in shaft rotational speed. At 5,000 and 7,000 rpm film thicknesses are around 0.75 mm – 1 mm and are similar at 90° and 270°; at 3,000 rpm films tend to be somewhat thicker at around 1.5 mm – 2 mm and are thicker in the counter current region, particularly closer to the bearing. It is suggested that at higher shaft speeds interfacial shear dominates whereas at lower speed the effect of gravity in slowing the film in the counter-current region causes a measureable difference.

It was further observed that increasing the input oil flow rate from 5.2 litres per minute to 7.3 litres per minute did not produce significant effect on film thickness. However, the increase of axial bearing load from 10 kN to 30 kN yielded thicker films at the location above the cage.

In all cases there was waviness on the film surface at the bearing outer periphery; imaging was not sufficient to see if the film surface close to the bearing is wavy.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A018. doi:10.1115/GT2017-63815.

In civil aero-engine transmission system bearings are used for shaft location and load support. An experimental test facility in the University of Nottingham’s Gas Turbine Transmissions Research Centre (G2TRC) was designed and commissioned to investigate oil behaviour as it exits an engine-representative ball bearing. In the rig, oil is delivered to the bearing inner race and cage via under-race feed at three delivery locations i.e. front, mid and rear of the bearing assembly. An electromagnetic load system is designed and implemented to allow engine representative axial loads up to 35 kN to be applied to the bearing. This paper details the rig design including the load and under-race lubrication systems and gives information about bearing oil shedding mechanisms observed.

In this phase of testing high speed images are acquired at shaft speeds between 1000 and 7000 rpm at an oil flowrate of 5.2 litres per minute and bearing axial load of 10 kN. The work presented here focusses on oil shedding from the bearing cage. Oil shedding behaviour from aeroengine ball bearing is identified to share many similarities to that observed in the past for shedding from rotating disks and cups. However, it is shown that it not possible to predict the conditions at which transition in flow regimes will occur for the aeroengine bearing on the basis of correlations for simpler geometries (spinning disks and cups). The work presented here is the first observation of flow regimes in an aeroengine ball bearing involving high-resolution highspeed imaging.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A019. doi:10.1115/GT2017-63822.

The dynamic characteristics of foil bearings operating at high rotation speeds depend very much on the mechanical characteristics of the foil structure. For this reason, the stiffness and damping of the structure of foil bearings is a problem that is the focus of many analyses. The mechanical characteristics of the foil structure (top and bump foil) are analyzed either by using a simple approach obtained for an isolated bump modeled as a beam or with more elaborate ones taking into account the three-dimensional nature of the bumps and their mutual interactions. These two kinds of models give different foil structure stiffness, with lower values for the simplified model. However, the published experimental results of the foil bearing structure tend to validate the simplified model. The present paper explains the differences between the simplified and the elaborate models by taking into account the manufacturing errors of the foil structure.

A three-dimensional model based on the non-linear theory of elasticity is developed. The model offers a unique insight into the way the bearing structure deforms when the rotor is incrementally pushed into the foil structure. Three realistic manufacturing errors, bump height, bump length and radius of the bump foil are analyzed. Bump height and length vary following a normal distribution with a given standard deviation while the radius of the bump foil is given a waviness form with an imposed peak-to-peak amplitude. Three to five cases were calculated for each kind of error. Results show that only the manufacturing errors of the bump height affect the stiffness of the foil structure by diminishing its values. Height errors of 20 μm standard deviation (4% of the average bump height and 60% of the radial clearance) may induce a 40–50% reduction of the stiffness of the foil structure, i.e. in the range of the predictions of the simplified model.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A020. doi:10.1115/GT2017-63847.

Owing to their low cost and reduced power losses, floating bush bearings are extensively used in high-speed rotors. The advantages are mainly the result of the rotation of the bush. When shaft speed is within a low speed range, bush rotation speed increases linearly with shaft speed. However, the bush-to-shaft speed ratio decreases sharply when the shaft speed reaches a certain range. The mechanism of this phenomenon is not completely clear yet, and a precise prediction method has not been established. The traditional theoretical model predicts that the speed ratio remains constant even when the shaft speed reaches the certain range. Some researchers have attempted to improve the prediction model by considering thermal effect on the assumption that a temperature increase decreases the viscosity of the inner oil film and consequently reduces the speed ratio. However, temperature rise alone is insufficient to induce that much drop of speed ratio. This paper focuses on the effect of air invasion flow in the inner oil film from the axial ends and evaluates the importance of air invasion and thermal effects. Computational fluid dynamics (CFD) modeling is adopted in this study because of its capacity to handle complicated calculation domain and calculate air-oil two-phase flow. Three series of CFD simulations with different models are conducted. These models consider the thermal effect (thermal model), the air invasion effect (air model), and the combination of the thermal and air invasion effects (hybrid model). CFD results of the different models are compared to weigh the importance of each effect. The CFD calculation indicates that a substantial amount of air invades the inner oil film when the shaft speed reaches a certain range. Speed ratio drop is not caused by a single factor, but it is the result of the combination of the air invasion and thermal effects. Air invasion, which researchers previously ignored, plays a greater role than the thermal effect.

Topics: Bearings
Commentary by Dr. Valentin Fuster
2017;():V07AT34A021. doi:10.1115/GT2017-63891.

Under high pressure, the elastic deformation of floating ring seals has a significant influence on the seal leakage flowrate and rotordynamic coefficients. The present study establishes a coupled model of the clearance flow and elastic structure of floating rings with bulk flow model and finite element method. The results show that the structural deformation reduces the clearance thickness and decreases the leakage flowrate. On the other hand, the elastic deformation reduces the direct stiffness, and increases the cross-coupled damping and inertia coefficients. In addition, the deformation induces the cross-coupled stiffness and direct damping to increase rapidly with increasing eccentricity ratio. Generally, the structural deformation deteriorates the rotordynamic characteristics of floating ring seals. In order to alleviate the adverse influences, a deformation compensation method is presented and proved to be effective.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A022. doi:10.1115/GT2017-63909.

With the development of high speed rotating machinery, the flow regime in bearings changes from laminar to superlaminar, that is, the flow is between laminar and fully developed turbulent. The superlaminar oil flow in an oil–lubricated tilting–pad journal bearing is analyzed in this study. A three–dimensional model for the oil domain is established and the CFD results obtained using laminar and seventeen turbulence models are compared with the experimental results obtained by S.Taniguchi. The seventeen turbulence models are divided into three groups, namely, classical fully developed turbulence models, transition turbulence models, and turbulence models with low–Re correction. The laminar and classical turbulence models cannot simulate the superlaminar flow correctly; accordingly, corrections should be applied to classical fully developed turbulence models for superlaminar flows to consider the turbulent effect properly. However, not all corrections are suitable. Among all the compared turbulence models, the SST model with low–Re correction performs the best. Furthermore, this model can capture the turbulent effect in superlaminar oil flow, as indicated in the analysis of turbulent viscosity ratio. A comparison of the velocity profiles shows that the mechanism of the superlaminar flow in journal bearings is near–wall turbulence. The buffer layer plays an important role in superlaminar flows. The SST model with low–Re correction can likewise capture the characteristics of the buffer layer and simulate the near–wall turbulence properly in superlaminar flows. Thus in superlaminar journal bearings, the low–Re correction is the most suitable correction for the SST turbulence model for simulating oil flows.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A023. doi:10.1115/GT2017-63988.

A 2-phase annular seal stand (2PASS) has been developed at the Turbomachinery Laboratory of Texas A&M University to measure the leakage and rotordynamic coefficients of division wall or balance-piston annular seals in centrifugal compressors. 2PASS was modified from an existing pure-air annular seal test rig. A special mixer has been designed to inject the oil into the compressed air, aiming to make a homogenous air-rich mixture.

Test results are presented for a smooth seal with an inner diameter D of 89.306 mm, a radial clearance Cr of 0.188 mm, and a length-to-diameter ratio L/D of 0.65. The test fluid is a mixture of air and Silicone oil (PSF-5cSt). Tests are conducted with inlet LVF = 0%, 2%, 5%, and 8%, shaft speed ω = 10, 15, and 20 krpm, and pressure ratio PR = 0.43, 0.5, and 0.57. The test seal is concentric with the shaft (centered), and the inlet pressure is 62.1 bars.

Complex dynamic stiffness coefficients are measured for the seal. The real parts are generally too dependent on excitation frequency Ω to be modeled by constant stiffness and virtual mass coefficients. The direct real dynamic stiffness coefficients are denoted as KΩ; the cross-coupled real dynamic stiffness coefficients are denoted as kΩ. The imaginary parts of the dynamic stiffness coefficients are modeled by frequency-independent direct C and cross-coupled c damping coefficients.

Test results show that the leakage and rotordynamic coefficients are remarkable impacted by changes in inlet LVF. Leakage mass flow rate drops slightly as inlet LVF increases from zero to 2%, and then increases with further increasing inlet LVF to 8%. As inlet LVF increases from zero to 8%, KΩ generally decreases except it increases as inlet LVF increases from zero to 2% when PR = 0.43. kΩ increases virtually with increasing inlet LVF from zero to 2%. As inlet LVF further increases to 8%, kΩ decreases or remains unchanged. C increases as inlet LVF increases; however, its rate of increase drops significantly at inlet LVF = 2%. Effective damping Ceff combines the stabilizing impact of C and the destabilizing impact of kΩ. Ceff is negative (destabilizing) for lower Ω values and becomes more destabilizing as inlet LVF increases from zero to 2%. It then becomes less destabilizing as inlet LVF is further increased to 8%.

Measured and rotordynamic coefficients are compared with predictions from XLHseal_mix, a program developed by San Andrés [1] based on a bulk-flow model, using the Moody wall-friction model while assuming constant temperature and a homogenous mixture.

Predicted values are close to measurements when inlet LVF = 0 and 2%, and are larger than measured values when inlet LVF = 5% and 8%. As with measurements, predicted drops slightly as inlet LVF increases from zero to 2%, and then increases with increasing inlet LVF further to 8%. However, in the inlet LVF range of 2∼8%, the predicted effects of inlet LVF on are weaker than measurements.

XLHseal_mix poorly predicts KΩ in most test cases. For all test cases, predicted KΩ decreases as inlet LVF increases from zero to 8%. The increase of KΩ induced by increasing inlet LVF from zero to 2% at PR = 0.43 is not predicted. C is reasonably predicted, and predicted C values are consistently smaller than measured results by 14∼34%. Both predicted and measured C increase as inlet LVF increases. kΩ and Ceff are predicted adequately at pure-air conditions, but not at most mainly-air conditions. The significant increase of kΩ induced by changing inlet LVF from zero to 2% is predicted. As inlet LVF increases 2% to 8%, predicted kΩ continue increasing versus that measured kΩ typically decreases. As with measurements, increasing inlet LVF from zero to 2% decreases the predicted negative values of Ceff, making the test seal more destabilizing. However, as inlet LVF increases further to 8%, the predicted negative values of Ceff drops versus measured values increase. For high inlet LVF values (5% and 8%), the predicted negative values of Ceff are smaller than measurements. So, the seal is actually more stable than predicted for high inlet LVF cases.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A024. doi:10.1115/GT2017-64050.

Active magnetic bearings (AMBs) have the well-documented advantage of reduced operational power losses when compared to conventional fluid-film bearings; however, they have yet to be widely adopted in industry due to the high initial costs of manufacturing and supporting power electronics. As AMBs look to become more cost competitive in more widely based applications, permanent magnet biased designs seek to reduce both the operating electrical power losses and the power electronic hardware costs while maintaining normal load and maximum load capacities. In these new designs, permanent magnet components are used to provide the necessary bias magnetic flux in the bearing usually provided by an electrical bias current in traditional all electromagnetic AMB designs. By eliminating electrical bias currents, operating electrical power losses can be significantly reduced while allowing for smaller, cheaper electronic components. This paper provides a comparison of the performance of permanent magnet biased thrust and radial bearing designs with conventional, all electromagnetic bearing designs. The thrust bearings are designed with nominal and maximum load capacities of 1,333 N and 4,000 N, while the radial bearings are designed with nominal and maximum load capacities of 1,000 N and 3,000 N. The shaft diameter is considered to be 70 mm for all bearings. Finite element modeling is used to calculate load capacities and operating electrical power requirements. Power requirements for a number of loads ranging from nominal to maximum capacity are presented for the permanent magnet biased and all electromagnetic bearing designs. A significant reduction in electrical power requirements under maximum load conditions is shown in the permanent magnet biased designs. This reduction is further magnified under nominal load conditions. Additionally, the number of pole wire turns and maximum wire currents are adjusted to realize even greater electrical power losses. The required bias magnetic flux can be generated with reduced wire currents by increasing the number of wire turns. While reducing wire currents also reduces electrical power requirements, the increase in wire turns increases the circuit induction. This increase in induction decreases the bearing slew rate and, in turn, the bandwidth. This study looks at a number of wire turns and current combinations. Tradeoffs between reduced electrical power losses and bearing bandwidth are presented and discussed. The permanent magnet biased AMB designs are shown to significantly reduce electrical power losses having the potential to improve overall machine efficiency. Implications of adopting this technology to both operating and manufacturing costs are discussed. The use of permanent magnets in AMBs is shown to make the costs of these systems more competitive with oil lubricated bearings when compared to conventional AMB designs.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A025. doi:10.1115/GT2017-64151.

This study presents the dynamic motion of a ball bearing cage submerged in a cryogenic fluid under high-speed conditions. The dynamic motion of the cage has been studied as a function of the race land–cage and ball–cage pocket clearances for different inner race rotation speeds under light load conditions. In addition, this study conducted computational fluid dynamics (CFD) analysis using commercial software to analyze the fluid dynamic forces on the cage. The hydraulic force obtained from the CFD analysis was coded in commercial ball bearing analysis software as a function of the eccentricity ratio and rotation speed of the cage. Finally, the dynamic motion of the ball bearing cage considering the effects of fluid dynamic forces has been studied. The results include the cage whirling amplitude, fluctuation of cage whirling speed, and cage wear for various cage clearances and rotation speeds. The cage outer guidance clearances studied were 1.14, 1.04, 0.94, 0.84, and 0.74 mm and the ball–pocket clearances were 0.62, 0.92, 1.22, 1.52, and 1.82 mm. The rotation speeds of the inner race were 5,000, 8,000, and 11,000 rpm. The cage whirling amplitude decreases as the outer guidance clearance decreases, and it decreases as the rotation speed increases up to 11,000 rpm because of the increasing hydrodynamic force of the liquid nitrogen (LN2). However, the probability density function (PDF) curves indicate that an increase in the rotor speed increases the standard deviation in the cage whirling frequency. The wear loss of the cage was greatest for the largest race land–cage and the smallest ball–cage pocket clearances, owing to the increased number of intermittent collisions between the cage and the ball bearings (ball–race). Consequently, the analysis results for various operating conditions (inner race rotation speeds, cage clearances, traction coefficients, etc.) are in good agreement with the reference results.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A026. doi:10.1115/GT2017-64251.

The article presents the calculation results of dynamic coefficients of hydrodynamic slide bearings obtained using three different methods. The numerical analysis based on a linear and nonlinear algorithm was carried out. The software developed at the Institute of Fluid-Flow Machinery PAN was applied for this purpose. In the experimental research, we used the impulse response method for the determination of dynamic coefficients of hydrodynamic bearings. This method is based on a linear algorithm and allows the calculation of stiffness, damping and mass coefficients for the rotor – bearings system. It allows calculating the complete set of coefficients in only one calculation step. During experimental research, vibrations of the rotor supported on two slide bearings are excited using an impact hammer at the middle part of the shaft. Stiffness, damping and mass coefficients are determined after the analysis of displacements of the journals and the registered excitation forces. The shaft mass is known, therefore mass coefficients can be used for quick verification of the obtained results — by comparing their values with the shaft mass.

The experimental tests were carried out on the test rig produced by SpectraQuest. The basic dynamic characteristics of the test rig were determined in the framework of this research, including vibration trajectories of the journals for both bearings — at rotational speeds between 2250 and 6000 rpm. In this speed range, a resonant speed of the rotor was noticed. The vibration trajectories of the journals were used to verify calculated coefficients of the hydrodynamic slide bearings.

Since the tested system exhibits nonlinear properties, the three different calculation methods produced large differences in results. The numerical calculations conducted with a linear algorithm provide only one set of stiffness and damping coefficients for each rotational speed (two main and two cross-coupling coefficients for each bearing). In the case of calculations utilizing nonlinear algorithm, the values of coefficients vary over time, notwithstanding the fact that the rotational speed is a fixed value. In each time step, we have different values of stiffness and damping coefficients of the hydrodynamic bearings. In the case of calculations based on the results obtained from experimental research, we receive one set of coefficients for each rotational speed, just as it is for the linear algorithm. The mean and standard deviation of stiffness, damping and mass coefficients are obtained by repeating many times experimental tests followed by statistical calculations. To get values of the coefficients for more than one rotational speed, calculations must be made for each one separately.

As a matter of fact, most mechanical systems exhibit certain nonlinear properties. In rotating machinery, we sometimes face distortions of their operation caused by, for example, couplings or supporting structures. In the ideal case, if the system had linear properties and operated correctly without any distortion, the results obtained from all three methods would be the same. The reality is that the experimentally and numerically determined coefficients of the hydrodynamic journal bearings differed, and their differences were discussed in this article. These differences arise from the adopted assumptions and abilities of the three different calculation models. In numerical calculations based on a linear algorithm, it is assumed that the bearing journal is in static equilibrium during its operation. For nonlinear calculations, the solution is found by iterations. The stiffness and damping coefficients are determined for sufficiently small time steps such that we can replace their values in every position of the bearing journal. In experimental research, it is essential that the bearing operation should be considered as the operation in the linear range.

Topics: Bearings
Commentary by Dr. Valentin Fuster
2017;():V07AT34A027. doi:10.1115/GT2017-64263.

Tilting-pad journal bearings (TPJBs) are dominant as shaft support in high-parameter turbomachinery, particularly in highspeed applications. Bearing test technology and high-precision model simulation are the key technologies for the development of high-end bearings. A bearing test platform which is based on the active magnetic bearing (AMB) loading and excitation is first designed and developed. Two large AMBs are used in the test rig as loader and exciter, and the system has a whole loading capacity up to 20000 N. As the bearing load is not always along the vertical direction, the performance of five-pads TPJBs with inner diameter of 140 mm is tested under the different load directions and load magnitudes in the speed range of 2000 to 12000 r/min. The results indicate that the direction and magnitude of the bearing load have a considerable effect on the experimental characteristics of TPJBs, especially under high-speed and heavy-load operation conditions. The bearing load has less influence on power loss than the rotation speed. By changing the direction of load, the bearing pad in the load direction has much higher temperature than the opposite pad. The change of pad temperature caused by the change of bearing load direction becomes greater under a larger bearing load. By analyzing the influence of the magnitude and direction of the bearing load on the rotor vibration, the mutual relationship among the dynamic stiffness, bearing load and rotor vibration are revealed. The performance test of TPJBs under different load directions with the AMB loading is beneficial to design the tilting pad bearing with high performance, to reduce the power loss of bearing, and to verify the correctness of the numerical model analysis. The results can be used for the performance prediction of the integrally geared centrifugal compressor because of their ever varying bearing load direction and load magnitude.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A028. doi:10.1115/GT2017-64665.

Designing high-performance industrial machinery with fluid-film bearings for safe and stable operation relies on accurate stability prediction. One element of accurate stability prediction is fluid-film bearing properties. Bearing codes are utilized to predict fluid-film bearing properties and these prediction models are validated using various test rigs. Due to systematic errors present in test rigs, estimated bearing coefficients have associated uncertainty. Uncertainty in fluid-film bearing coefficient estimation due to displacement measurement error, force measurement error, and rotor flexibility is analyzed. A single-degree-of-freedom system model is analyzed to understand trends in uncertainties. Computer modeling software is used to develop a modular, high-fidelity model to analyze a test rig for worst-case uncertainty in fluid-film bearing coefficient estimation for a tilting-pad journal bearing. The results show that for high excitation frequencies the uncertainty is large enough that estimated coefficients are practically useless.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A029. doi:10.1115/GT2017-64745.

Full three-dimensional CFD simulations are carried out using ANSYS CFX to obtain the detailed flow field and to estimate the rotordynamic coefficients of a labyrinth seal for various inlet swirl ratios.

Flow fields in the labyrinth seal with the eccentricity of the rotor are observed in detail and the detailed mechanisms that increase the destabilizing forces at high inlet swirl ratios are discussed based on the fluid governing equations associated with the flow fields. By evaluating the contributions from each term of the governing equation to cross coupled force, it is found that circumferential velocity and circumferential distribution of axial mass flow rate play key roles in generating cross coupled forces. In the case that circumferential velocity is high and decreases along the axial direction, all contributions from each term are positive cross coupled force. On the other hand, in the case that circumferential velocity is low and increases along the axial direction, one contribution is positive but the other is negative. Therefore, cross coupled force can be negative in the local chamber depending on the balance even if circumferential velocity is positive.

CFD predictions of cross coupled stiffness coefficients and direct damping coefficients show better agreement with experimental results than a bulk flow model does by considering the force on the rotor in the inlet region. Cross coupled stiffness coefficients derived from the force on the rotor in the seal section agree well with those of the bulk flow model.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A030. doi:10.1115/GT2017-64822.

Direct lubrication tilting pad journal bearings (TPJBs) require of less oil flow, reduce power consumption and offer cooler pad temperatures for operation at high surface speeds. Although apparently free of a hydrodynamic instability, the literature shows that direct lubrication TPJBs exhibit unexpected shaft vibrations with a broadband low frequency range, albeit small in amplitude. Published industrial practice demonstrates the inlet lubrication type, a reduced supply flow rate causing film starvation, and the bearing discharge conditions (evacuated or end sealed) affect the onset, gravity and persistency of the sub synchronous (SSV) hash motions. The paper presents a physical model to predict the performance of TPJBs with flow conditions ranging from over flooded to extreme starvation. Lubricant starvation occurs first on an unloaded pad, thus producing a (beneficial) reduction in drag power. As the supplied flowrate decreases further, fluid starvation moves towards the loaded pads and affects the film temperature and power loss, increases the journal eccentricity, and modifies the dynamic force coefficients of each tilting pad and thus the whole bearing. For a point mass rotor supported on a TPJB, the analysis produces eigenvalues and frequency response functions (FRFs) from three physical models for lateral rotor displacements: one with frequency reduced (4×4) bearing stiffness (K) and damping (C) coefficients and another with constant K-C-M (inertia) coefficients over a frequency range. The third model keeps the degrees of freedom (tilting) of each pad and incorporates the full matrices of force coefficients including fluid inertia. Predictions of rotordynamic performance follow for two TPJBs: one bearing with load between pads (LBP) configuration, and the other with a load on a pad (LOP) configuration. For both examples, under increasingly poor lubricant flow conditions, the damping ratio for the rotor-bearing low frequency (SSV) modes decreases, thus producing an increase in the amplitude of the FRFs. For the LOP bearing, a large static load produces a significant asymmetry in the force coefficients; the rotor-bearing has a small stiffness and damping for shaft displacements in the direction orthogonal to the load. A reduction in lubricant flow only exacerbates the phenomenon; starvation reaches the loaded pad to eventually cause the onset of low frequency (SSV) instability. The bearing analyzed showed similar behavior in a test bench. The predictions thus show a direct correlation between lubricant flow starvation and the onset of SSV.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A031. doi:10.1115/GT2017-64875.

The performance of annular seals depends on the geometry of the leakage path as it facilitates the dissipation of the fluid kinetic energy. The goal of this study is to investigate potential correlations between the characteristics of the alternately arranged surface pattern and the corresponding rotor dynamic properties of the seal in addition to mapping its performance.

Various patterning arrangements lining the stator surface are considered and the relative seal performance change is investigated using a hybrid method that calculates the seal dynamic response for each point in the design space. The design parameters selected in this DOE study are: the diameters of alternately arranged holes that are replicated in both axial and circumferential direction to construct the pattern, the hole depths for both types of holes, and the number of holes in both axial and circumferential directions. A sensitivity study is conducted to analyze the influence of each geometrical parameter on the seal response. Regression models are then generated for each response, including the leakage rate and the rotor dynamic coefficients. Quadratic regression models are used in this study to represent the relationship between the objective functions and the design parameters. The goal is to achieve a minimum leakage rate as well as an improved dynamic response. The results of the baseline model and the best performing design are compared. The results show that the patterning arrangements have crucial effects on the leakage rate as well as the dynamic coefficients of the seal. The results of this study are found to be helpful in designing a hole-pattern seal that can concurrently satisfy constraints on both the leakage rate and the rotor dynamic response while maintaining same design envelope.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A032. doi:10.1115/GT2017-64949.

Fluid film bearings for turbomachinery are designed to support the loads applied by the rotor system, often at high speeds when power loss in the bearing becomes significant and bearing temperatures can reach levels that can be detrimental to the long-term reliability of the support system. These requirements of supportive bearings require an intimate understanding of how bearing design variables affect their overall performance. Ideal bearings minimize power loss to increase machine efficiency and maintain low operating temperatures to ensure long-term reliability while meeting other design criteria such as minimum film thickness to provide proper support and avoiding high fluid pressures that can be harmful to the bearing structure. However, real world designs are often forced to sacrifice some of these design goals in order to preserve others. Therefore, further understanding of the relative opportunity costs associated with optimizing the bearing design with differently weighted performance metrics and their relationships to bearing design variables is invaluable to design engineers.

This study explores the impact of eight bearing design variables on the performance of two tilting pad journal bearings supporting an eight-stage centrifugal compressor using design of experiments techniques applied to an established thermoelastohydrodynamic (TEHD) bearing model of tilting pad bearing performance. The bearing design variables analyzed include the radial clearance, pad arc spacing, pad axial length, pivot offset, preload, working fluid viscosity and viscosity index, and the number of pads. Each of the design variables — excluding the number of pads which was realistically constrained — were first varied over five levels each in a central composite design. These central composite designs were repeated for each of three values for number of pads. The responses obtained from the TEHD numerical simulations for each bearing design point were power loss, maximum pad temperature, minimum film thickness, and maximum fluid film pressure. The results from the central composite studies were fit with a multivariate least-squares regression model and a secondary series of experimental design studies were simulated around potential optimum design points to obtain a learning set to initialize direct optimization methods.

Two direct multi-objective optimization methods, a sequential quadratic programming method and a multi-island genetic algorithm, were performed using Isight, a commercial software. A range of weighting parameters were selected for the optimization functions to find bearing designs that minimized power loss and pad temperature while maintaining pressure and film thickness criteria within acceptable design ranges for fluid film bearings. The resulting optimum design points allowed for a comparison between the design optimization approaches. The various strengths and weaknesses of the different methods are discussed. This study demonstrates how designers can use these approaches to view the relationships between design variables and important performance metrics to design better bearings for a wide range of applications.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A033. doi:10.1115/GT2017-65055.

Higher energy level and more compact structure are the trend of centrifugal compressor, which may lead to the rotordynamics instability problems and high vibration. These problems restrict the improvement of the the rotating machinery operation efficiency and bring about potential safety problems, especially, such as the subsynchronous vibration and unbalance vibration. Aiming at the potential instability caused by the excitation of the seal, a seal combined with electromagnetic damping function was designed in “Investigation on the seal structure design and rotor vibration controller for back-to-back centrifugal compressor (GT2016-56900)”. Here, the capability of the Active Magnetic Bearing (AMB)-Hole Pattern Seal (HPS) is investigated in the aspect of function and load characteristic. Furthermore, the electromagnetic damping seal actuator also has the potential to implement the unbalance control. In the aspect of control theory, we combined the damping control algorithm, which is used to control the rotor instability vibration, with the self-optimizing control algorithm for fundamental vibration control of rotor bearing system. The independently optimized control strategy was chosen according to the different frequency vibration waveform. Moreover, we built a comprehensive vibration control experiment platform that is mainly combined with the rotor, tilting pad bearings and electromagnetic damping seal actuator. We did experiment on simulating the instability condition, the control of the instable low frequency vibration, and the control of fundamental frequency vibration. The experimental results show that with the control hardware of high speed FPGA module and the self-optimizing control algorithm, the electromagnetic damping sealing can not only solve the instability caused by the original seal of centrifugal compressor to improve the system stability, but also be used as the active actuator to effectively control the vibration of the rotor system. At the same time, it is also found in the experiment that how to effectively restrain the influence of irrelevant noise and the reasonable selection of initial control parameters have a very important influence on the overall control effect, which is the urgent problem to be solved in the following work. This paper also provides a new design idea and a set of feasible active vibration control strategies for the high speed centrifugal compressor and has guiding significance for solving the problem of multi-frequency vibration and rotor stability of high speed centrifugal compressor.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A034. doi:10.1115/GT2017-65105.

In recent years, gas foil bearings have gained increased attention due to potential applications in aerospace systems. Research and development efforts have been focused towards simplifying design and analysis methods or experimentally demonstrating stable bearing performance under various operating conditions. Many researchers have proposed design guidelines for parameters such as load capacity, stiffness, and damping etc., for extending the state of the art based on experimental data available in existing literature. The authors previously presented scaling laws for radial clearance and support structure stiffness of radial foil bearings. In that study, the criteria for selecting radial clearance and support structure stiffness for scale up or scale down of an existing bearing design was presented. In addition, the results from that paper showed that a hydrodynamic film could be sustained for large bearings (up to 300 mm diameter) demonstrating that the bearings would have adequate load capacity. However, the rotordynamic effects for the various bearing sizes were not considered in that study. This paper serves as an extension of the paper on scaling laws by the same authors. The subject of this paper is a four degree of freedom (4-DOF) rotordynamic analysis performed for turbomachinery systems that employ bearings designed using the scaling laws for radial clearance and support structure stiffness. Further, case studies to show feasibility of foil bearings for applications in Mega Watt range turbo blowers and turbo compressors is presented.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A035. doi:10.1115/GT2017-65233.

Foil bearings are one type of hydrodynamic air/gas bearings but with a compliant bearing surface supported by structural material that provides stiffness and damping to the bearing. The hybrid foil bearing (HFB) in this paper is a combination of a traditional hydrodynamic foil bearing with externally-pressurized air/gas supply system to enhance load capacity during the start and to improve thermal stability of the bearing. The HFB is more suitable for relatively large and heavy rotors where rotor weight is comparable to the load capacity of the bearing at full speed and extra air/gas supply system is not a major added cost. With 4,448N∼22,240N thrust class turbine aircraft engines in mind, the test rotor is supported by HFB in one end and duplex rolling element bearings in the other end. This paper presents experimental work on HFB with diameter of 102mm performed at the US Air force Research Laboratory. Experimental works include: measurement of impulse response of the bearing to the external load corresponding to rotor’s lateral acceleration of 5.55g, forced response to external subsynchronous excitation, and high speed imbalance response. A non-linear rotordynamic simulation model was also applied to predict the impulse response and forced subsynchronous response. The simulation results agree well with experimental results. Based on the experimental results and subsequent simulations, an improved HFB design is also suggested for higher impulse load capability up to 10g and rotordynamics stability up to 30,000rpm under subsynchronous excitation.

Commentary by Dr. Valentin Fuster
2017;():V07AT34A036. doi:10.1115/GT2017-65240.

Tilting pad journal bearing has its wide application in turbomachinery. For years, frictionless contact has been assumed for all the contact types in bearing coefficients prediction of tilting pad bearings. Existing experimental data demonstrates contact friction does present in pivots and can prevent pads from effective tilting, which is especially true for the surface contact pivot, e.g. ball-socket pivot. However, no open literature is available to discuss individual pad motion with consideration of pivot contact friction and document the effects on the dynamic behaviors of tilting pad bearings. This paper tries to improve the understanding in this area by discussing the pad motion of a tilting pad bearing with ball-socket pivots subject to pivot friction. A model, which couples the journal motion, hydrodynamic pressure from oil film and contact pressure within the pivots, is established. Instead of Hertz contact, a conformal contact model is applied for the ball-socket contact. Through the coupled transient analyses, the effects of contact friction coefficients, bearing dynamic and static loading, and pivot size on pads motion of a 5-pads load-on-pad bearing are considered and compared.

The results show that the presence of friction within ball-socket pivots does not guarantee “locked up” pads motion, but only limits pads from effective tilting for a given range of friction coefficients. Larger dynamic load increases pad tilting amplitude; this does not mean that enlarged tilting magnitudes reduce pivot friction moments for the pads. With varying bearing loads, the pivot friction could result in non-synchronous motion of pads tilting and pivots deformation and bring these non-synchronous components into the journal vibration. Size-reduced pivots are found to diminish the effects of pivot friction and offer preferred pads dynamics and journal vibration.

Commentary by Dr. Valentin Fuster

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