bluuebunny/crossref_metadata_2025 huggingface dataset:
Oil-Air Flow Between the Cage and Inner Race of an Aeroengine Bearing
Authors: Adeniyi Akinola A. , Morvan Hervé , Simmons Kathy
Published: September 2016

In aeroengines the shafts are supported on bearings that carry the radial and axial loads. A ball bearing is made up of an inner-race, an outer-race and a cage which contains the balls, these together comprise the bearing elements. The bearings require oil for lubrication and cooling. The design of the bearing studied in this work is such that the oil is fed to the bearing through holes/slots in the inner race. At each axial feed location the oil is fed through a number of equispaced feedholes/slots but there is a different number of holes at each location. Once the oil has passed through the bearing it sheds outwards from both sides into compartments known as the bearing chambers.

A number of studies have been carried out on the dynamics of bearings. Most of the analyses consider the contributions of fluid forces as small relative to the interaction of the bearing elements. One of the most sophisticated models for a cage-raceway analysis is based on the work of Ashmore et al. [1], where the cage-raceway is considered to be a short journal bearing divided into sectors by the oil feeds. It is further assumed that the oil exits from the holes and forms a continuous block of oil that exits outwards on both sides of the cage-raceway. In the model, the Reynolds equation is used to estimate the oil dynamics.

Of interest in this current work is the behaviour of the oil and air within the space bounded by the cage and inner race. The aim is to determine whether oil feed to the bearing can be modelled as coming from a continuous slot or if the discrete entry points must be modelled. A Volume of Fluid Computational Fluid Dynamics approach is applied. A sector of a ball bearing is modelled with a fine mesh and the detailed simulations show the flow behaviour for different oil splits to the three feed locations of the bearing thus providing information useful to understanding oil shedding into the bearing chambers.

The work shows that different flow behaviour is predicted by models where the oil inlets through a continuous slot compared to discrete entry holes. The form and speed of oil shedding from the bearing is found to depend strongly on shaft speed with the shedding speed being slightly higher than the cage linear speed. The break-up pattern of oil on the cage inner surface suggests smaller droplets will be shed at higher shaft speed.
Combustion Characteristics of a Micro Gas Turbine Combustor With Gamma-Shaped Porous Media Dome
Authors: Zhang Liang , Zhang Chi , Xue Xin , Lin Peihua , Lin Yuzhen
Published: September 2016

During the past decade, increasing interest has been shown in micro gas turbines for the high-power and high-energy density. However, due to the small characteristic scale, it is still a key problem to ensure safe and reliable operation of the micro-combustor. A new micro gas turbine combustor with a Γ-shaped porous media dome was investigated in this paper. The volume of the combustor is 2.7 cm3. Dual-zone combustion (combustion zone and dilution zone) was adopted in the combustor. Combustion characteristics of the micro-combustor with different total air mass flow and total equivalence ratios were investigated by experiments at ambient temperature and atmospheric pressure. The results show that the relationship between liner pressure-loss and total air mass flow cannot be fit by a polynomial due to porous media and dilution holes combined influence. The ratio of airflow across porous media dome to total air mass flow increased with increasing total air mass flow. Stable combustion was obtained in this micro combustor as the air mass flow rate was in the range of 0.15∼1.2 g/s. With the increasing total air mass flow, the total equivalence ratios of lean ignition and blow-out limits decreased first, then increased. The exit gas temperature as high as 1460 K and power density 636 MW/m3 were achieved at the total equivalence ratio of 0.5, and total air flow rate of 1.2 g/s, the overall efficiency reached 98.5% in this condition. The results showed that safe and reliable operation can be achieved in this new micro gas turbine combustor with high overall efficiency.
Experimental Study on Frequency Band of Dynamic Vibration Absorber in Pipe Systems
Authors: Yang Xiufeng , He Lidong , Zhang Bingkang
Published: September 2016

This paper investigated the vibration control mechanism of dynamic vibration absorber (DVA) used in pipelines and other chemical equipment. For investigating the broadband characteristics, a pipe-vibration absorber test bench was built and different tests were carried out. The results showed that high mass ratio could widen the frequency bandwidth of effective vibration suppression. A new type of passive ring-shaped DVA with particle damping, employing moving steel particles in single-layer or more-layers boxes was designed, and its vibration suppression capability tested and compared with traditional DVA. The proposed damped DVA could reduce the pipeline double vibration peaks typical of the traditional DVAs. Moreover, results showed that stratifying the box was a useful method to improve the vibration suppression efficiency that increased from 59.5% (single-layer-box case) to 72.5% when two-layers-box were used and to 78.5% with three-layers-box.
Performance Prediction and Optimisation of LP Steam Turbine Radial Diffuser at Design and Off-Design Conditions Using Streamline Curvature Method
Authors: Zhang Luying , Congiu Francesco , Gan Xiaopeng , Karunakara David
Published: September 2016

The performance of the radial diffuser of a low pressure (LP) steam turbine is important to the power output of the turbine. A reliable and robust prediction and optimization tool is desirable in industry for preliminary design and performance evaluation. This is particularly critical during the tendering phase of retrofit projects which typically cover a wide range of OEM and oOEMs designs. This work describes a fast and reliable numerical approach for the simulation of flow in the last stage and radial diffuser coupled with the exhaust hood. The numerical solver is based on a streamline curvature throughflow method and a geometry-modification treatment has been developed for off-design conditions, at which large scale flow separation may occur in the diffuser domain causing convergence difficulty. To take into account the effect of tip leakage jet flow, a boundary layer solver is coupled with the throughflow calculation to predict flow separation on the diffuser lip. The performance of the downstream exhaust hood is modeled by a hood loss model (HLM) that accounts for various loss generations along the flow paths. Furthermore, the solver is implemented in an optimization process. Both the diffuser lip and hub profiles can be quickly optimized, together or separately, to improve the design in the early tender phase. 3D CFD simulations are used to validate the solver and the optimization process. The results show that the current method predicts the diffuser/exhaust hood performance within good agreement with the CFD calculation and the optimized diffuser profile improves the diffuser recovery over the datum design. The tool provides GE the capability to rapidly optimize and customize retrofit diffusers for each customer considering different constraints.
Maximum Mistuned Vibration Amplitude With Aerodynamic Damping
Authors: Martel Carlos , Cuiping Luo
Published: September 2016

Mistuning can dramatically increase the forced response of a bladed disk. In the case of damping coming from aerodynamic effects the situation is more complicated because the value of the damping is typically different for the different travelling wave modes of the system. This damping variability changes the effect of mistuning, and it can even result in a reduction of the maximum mistuned forced response amplitude below that of the tuned case (this was not possible in the usual case of constant material damping). In this paper the Asymptotic Mistuning Model (AMM) methodology is used to analyze this situation. The AMM is a reduced model that is systematically derived from the complete mistuned bladed disk dynamical model using a perturbative procedure based on the small size of the mistuning and the damping. Several different forcing cases are analyzed, and the results from the AMM are quantitatively compared with those from a FEM model of a mistuned bladed disk. The AMM gives also precise information about the key mechanisms involved in the forced response of mistuned cyclic structures, and about the maximum amplification of the vibration amplitude that the system can experience.
Turbine Blade Life Prediction Using Fluid-Thermal-Structural Interaction Simulation
Authors: Ubulom I. A. , Fien A. , Neely A. J. , Shankar K.
Published: September 2016

In this study a fluid-thermal-structural simulation is performed to investigate cyclic stress-strain behavior and fatigue life of a gas turbine blade. The Hysteresis loop characteristic of the blade is presented under the coupled influence of various loading conditions, aerodynamic, thermal and static centrifugal loadings. Based on the predicted loading behavior, an energy-based method was used to analyze the fatigue and cumulative damage properties of the blade. The predicted hysteresis loop under aerodynamic load was purely of elastic nature and as such tends to assume a Masing behavior at the stable condition. The case for a combined thermal and aeromechanical loading showed a non-Masing behavior, but rather a temperature-dependent material softening behavior. The fatigue life was also estimated based on the energy density approach using the predicted thermal-structural predicted cyclic loops.
Experimental Investigation of the Flow in a Control Valve of an Industrial Steam Turbine
Authors: Zeß Jan-Peter , Polklas Thomas , Joos Franz
Published: September 2016

Unsteady flows in steam turbine control valves are becoming more and more a subject of interest for many reasons. In general, steam conditions have increased for both industrial and power plant turbines. Flexible operation conditions of industrial turbines are a field of research ever since, but became more and more interesting for power plants due to the significant rise of renewable energies and their natural inconstant supply.

In this paper the results of experiments in an original sized industrial steam turbine control valve are presented. The experiments with air as fluid focus on the flow in the diffuser and are recorded using PIV techniques. With the results typical flow patterns are derived and allocated to a number of operating points with variation in pressure and opening ratio and therefore also in mass flow.
Conjugate Heat Transfer Analysis for Laminated Cooling Effectiveness: Part A — Effects of Surface Curvature
Authors: Zhou Weilun , Deng Qinghua , Feng Zhenping
Published: September 2016

The laminated cooling or multi-layered impingement-effusion cooling, which originates from combustor liner cooling, combines impingement jet, rib-roughed and film cooling and results in a high overall cooling effectiveness. It’s believed to be a promising gas turbine blade cooling technique. In this paper, conjugate heat transfer analysis that has been validated by the experimental results was carried out for five laminated cooling models with different surface curvatures at a certain range of blowing ratio. The numerical results show that the curvature and blowing ratio have crucial effects on laminated cooling effectiveness. High blowing ratio results in a better overall cooling effectiveness for flat plate and concave surface, while the moderate blowing ratio performances better on convex surface. Film cooling has an interaction with the internal convective and impingement cooling, thus the optimal cooling effectiveness of laminated cooling is achieved at the condition that the improvement of internal cooling counteracts the deterioration of film cooling, instead of the condition that film cooling or internal cooling reaches the maximum respectively. Moreover, concave surfaces have the higher pressure loss in the whole range of blowing ratio, while convex surfaces have lower pressure loss than flat plate due to the turbulence intensity of external flow.
LES-Based Assessment of Rotation-Sensitized Turbulence Models for Prediction of Heat Transfer in Internal Cooling Channels of Turbine Blades
Authors: Borello Domenico , Rispoli Franco , Properzi Ermanno , Salvagni Alessandro
Published: September 2016

A comprehensive model for the prediction of flows in rotating internal cooling channel application is here presented and assessed. The flow field was modelled by using a rotation-sensitized version of the well known k-ε-ζ-f elliptic relaxation model.

Flow field features in selected planes are discussed to show the changes in velocity field due to the rotation. The discussion is focused on the increase of turbulence close to the ribbed (trailing) surface when rotation is present. In this case the increase of the wall normal pressure gradient leads to an early reattachment of the large recirculation bubble downstream from the rib and to an anticipated development of the boundary layer. Furthermore, Coriolis force enhances the secondary motion. Both phenomena increase mixing and are expected to also increase heat transfer efficiency of the heated surface.

This is confirmed by the results of the temperature field. Comparisons with the available experimental results confirm the quality of the prediction.
Measured and Numerical Vibration Characteristics of Reliable 3D Designed Efficient IP Blades With a Wide Range of Operational Speed
Authors: Strauch Matthias , Beer Wolfgang , Stephan Ingo
Published: September 2016

Flexible operation at a wide range of operating conditions combined with very high efficiency is a fundamental customer requirement for industrial steam turbines today. The blading of turbines has to be designed with those objectives in mind. Especially for intermediate pressure (IP) blades operating at fixed and variable speed there is high potential for improvement with respect to efficiency. Nonetheless these blades have to satisfy rules of reliability and mechanical integrity, in many cases American Petroleum Institute (API) Standard 612 requirements. In this paper the dynamic behavior of IP blades with an improved and efficient 3D design is investigated numerically and verified by experiments. Shrouded blades are common for IP blade design to ensure a low dynamic stress level. These blades are supposed to form a closed coupling contact between adjacent blades to grant higher stiffness of the blade row and additional contact damping. Natural frequencies and mode shapes mainly remain on stiffness and mass distribution. Contact pressure at assembly state is generated by geometric interferences of the blades. Operational centrifugal forces and untwisting of blades tend to reduce the shroud contact pressure. One focus of this study is the research of different blade shroud contact conditions after the assembly process and the influence on closed shroud conditions during operation. A highly sophisticated numerical 3D model was set up to simulate and predict contact status and dynamic behavior of the blade row. Forced vibrations were imposed on the blade rows in a spin bunker to measure speed dependent frequencies and the effect of reduced shroud forces. Three-dimensional blades as well as cylindrical blades were tested on the same rotor to compare efficiency improved blades with long term industrial proven design. Results of the experiment were in good agreement with results of numerical calculations. A 10 MW steam turbine with one controlled extraction was used for validation. The turbine was operated within and beyond standard operational limits. Amongst others, the predicted contact and dynamic behavior were verified based on centrifugal forces, steam forces and real temperature distributions. Results confirmed that closed shroud contact was maintained at all operating points for properly assembled blade rows.
Experimental Analyses of a Static to Static Flex Seal and a Honeycomb Seal
Authors: Zierer Thomas , Bricaud Cyrille , Escudero-Olano Marcos , McNally Joshua , Mohammed Afzal Pasha
Published: September 2016

Improvements in turbine performance are increasingly driven by the need to control leakage both in the main gas path as well as in the secondary air flow system. Seals for static to static interfaces have a wide usage in gas turbine for component interface locations and are becoming more important as engines reach higher temperature targets and compressor pressure ratios.

Both flex and honeycomb seals have been used for many years during other OEM seal service upgrades. These seals are designed to be capable of sustaining low leakage operation whilst achieving long lifetimes. To determine the sealing capability of honeycomb and flex seals an advanced hydraulically actuated rig was designed and constructed. A series of leakage performance tests were carried out that accurately simulate engine conditions, including pressure and relative axial and radial movements.

The results of these tests are compared against previously presented data on standard membrane seals. Compared to the membrane seal, the flex seal has approximately 60% lower equivalent clearance when tested with uneven (triangular) grooves. This reduction was due to the bending of the seal and subsequent closing of the seal gap under pressure. The flex and membrane seal performed similarly well under more nominal conditions. The honeycomb seal achieved a consistently low leakage under all pressure loadings. All three sealing types have their place in the required technology mix which is essential when aiming for maximized engine performance and lifetime.
Forced Vibration of Mistuned Bladed Discs in Last Stage LP Steam Turbine
Authors: Rzadkowski Romuald , Maurin Artur , Kubitz Leszek , Szczepanik Ryszard
Published: September 2016

During the exploitation of a commercial LP steam turbine, self-excitation occurred in the last stage of slender blades, inducing high vibration amplitudes. These problems were solved by changing the geometry of certain blades (feathering) and arranging them in a specific order (alternating mistuning).

This paper presents free and forced vibrations of various mistuned steam turbine bladed discs. The natural frequencies and mode shapes of the steam turbine bladed discs were calculated using FEM models. Two different approaches to mistuning were applied: either the blade geometry or the Young’s Modulus were changed. Next, the results were compared. This showed that blade geometry mistuning gave the best results for long blades in the case of higher mistuning. The forced vibration analysis showed that the maximal blade stress location differed, depending on the kind of mistuning. The application feathering and alternating mistuning showed lower stress levels than the tip-timing measured standard mistuning pattern.
Numerical Investigation of the Flow Behavior Inside a Supercritical CO2 Centrifugal Compressor
Authors: Ameli Alireza , Turunen-Saaresti Teemu , Backman Jari
Published: September 2016

Centrifugal compressors are one of the best choices among compressors in supercritical Brayton cycles. A supercritical CO2 centrifugal compressor increases the pressure of the fluid which state is initially very close to the critical point. When the supercritical fluid is compressed near the critical point, wide variations of fluid properties occur. The density of carbon dioxide at its critical point is close to the liquid density which leads to reduction in compressor work.

The investigated compressor is a centrifugal compressor tested in the Sandia supercritical CO2 compression loop. In order to get results with the high accuracy and take into account the non-linear variation of the properties near the critical point, the CFD flow solver is coupled with a lookup table of properties of fluid. Behavior of real gas close to its critical point and the effect of the accuracy of the real gas model on the compressor performance are studied in this paper and the results are compared with the experimental data from the Sandia compression facility.
Effect of Bleed Position and Bleed Angle on Impingement Cooling of a Spherical-Dimpled Surface
Authors: Liu Zhao , Yang Xing , Li Jun , Feng Zhenping , Simon Terrence
Published: September 2016

Numerical simulations were performed to document cooling of the lower surface of a channel. Through the channel top wall and normal to the channel flow are cooling jets that impinge on the channel lower surface. This surface has a regular array of dimples with centers in line with the impingement jet axes. Also on the channel lower surface is a series of purge holes through which coolant flow is extracted. The effects of location of the purge holes relative to the dimples and angle of the purge holes on the channel lower surface cooling performance are documented. A 3D RANS analysis with SST k-ω turbulence closure modeling was conducted with a Reynolds number of the impingement jet of 35,000. The position of the purge hole relative to the dimple was varied from −0.34 to 0.50 dimple spacing, the purge hole angle varied as 30° and 150° and three different ratios of bleed flow rate to impingement jet flow rate, 2.5%, 5.0% and 12.0%, were documented. The results show how the combined effects of impingement jet flow, channel crossflow and purge flow affect heat transfer characteristics on the channel lower surface for this assembly. Also shown is how the dimpled surface heat transfer benefits from putting the purge flow holes slightly upstream in the channel flow from the dimples. The geometry under study may be applied for cooling a gas turbine endwall, where the bottom side of the channel lower wall of the assembly under study would be the passage endwall and the purge flow would be discrete hole film cooling flow.
Comparison of RANS and DES Modeling Against Measurements of Leading Edge Film Cooling on a First-Stage Vane
Authors: Ravelli S. , Barigozzi G.
Published: September 2016

The performance of a showerhead arrangement of film cooling in the leading edge region of a first stage nozzle guide vane was experimentally and numerically evaluated. A six-vane linear cascade was tested at an isentropic exit Mach number of Ma2s = 0.42, with a high inlet turbulence intensity level of 9%. The showerhead cooling scheme consists of four staggered rows of cylindrical holes evenly distributed around the stagnation line, angled at 45° towards the tip. The blowing ratios tested are BR = 2.0, 3.0 and 4.0. Adiabatic film cooling effectiveness distributions on the vane surface around the leading edge region were measured by means of Thermochromic Liquid Crystals technique. Since the experimental contours of adiabatic effectiveness showed that there is no periodicity across the span, the CFD calculations were conducted by simulating the whole vane. Within the RANS framework, the very widely used Realizable k-ε (Rke) and the Shear Stress Transport k-ω (SST) turbulence models were chosen for simulating the effect of the BR on the surface distribution of adiabatic effectiveness. The turbulence model which provided the most accurate steady prediction, i.e. Rke, was selected for running Detached Eddy Simulation at the intermediate value of BR = 3. Fluctuations of the local temperature were computed by DES, due to the vortex structures within the shear layers between the main flow and the coolant jets. Moreover, mixing was enhanced both in the wall-normal and spanwise direction, compared to RANS modeling. DES roughly halved the prediction error of laterally averaged film cooling effectiveness on the suction side of the leading edge. However, neither DES nor RANS provided the expected decay of effectiveness progressing downstream along the pressure side, with 15% overestimation of ηav at s/C =0.2.
Cooling Injection Effect on a Transonic Squealer Tip: Part 1 — Experimental Heat Transfer Results and CFD Validation
Authors: Ma H. , Zhang Q. , He L. , Wang Z. , Wang L.
Published: September 2016

Recent studies have demonstrated that the aerothermal characteristics of turbine rotor blade tip under a transonic condition are qualitatively different from those under a low speed subsonic condition. The cooling injection adds further complexity to the Over-Tip-Leakage (OTL) transonic flow behavior and aerothermal performance, particularly for commonly studied shroudless tip configurations such as a squealer tip. However there has been no published experimental study of a cooled transonic squealer. The present study investigates the effect of cooling injection on a transonic squealer through a closely combined experimental and CFD effort. Part 1 of this two-part paper presents the first of the kind tip cooling experimental data obtained in a transonic linear cascade environment (exit Mach number 0.95). Transient thermal measurements are carried out for an uncooled squealer tip, and six cooling configurations with different locations and numbers of discrete holes. High resolution distributions of heat transfer coefficient and cooling effectiveness are obtained. ANSYS Fluent is employed to perform numerical simulations for all the experimental cases. The mesh and turbulence modeling dependence is first evaluated before further computational studies are carried out. Both the experimental and computational results consistently illustrate strong interactions between the OTL flow and cooling injection. When the cooling injection (even with a relatively small amount) is introduced, distinctive series of stripes in surface heat transfer coefficient are observed with an opposite trend in the chordwise variations on the squealer cavity floor and on the suction surafce rim. Both experimental and CFD results have also consistently shown interesting signatures of the strong OTL flow–cooling interactions in terms of the net heat flux reduction distribution in areas seemingly unreachable by the coolant. Further examinations and analyses of the related flow physics and underlining vortical flow structures will be presented in Part 2.
Unsteady Forces in Last Stage LP Steam Turbine Rotor Blades With Exhaust Hood
Authors: Rzadkowski Romuald , Surwiło Jan , Kubitz Leszek , Lampart Piotr , Szymaniak Mariusz
Published: September 2016

Several high vibration amplitude problems have been reported regarding the slender last stage blades of commercial LP steam turbines. This paper presents a numerical study of unsteady forces acting on rotor blades using ANSYS CFX. A 3D transonic viscous flow through the stator and rotor blades with an exhaust hood was modelled. The last stage was modelled as a full blade annulus, so that the axial, radial and circumferential distribution of flow patterns and blade forces could be examined. An unsteady flow analysis was conducted on a typically designed last stage and exhaust diffuser, with measured and calculated downstream static pressure distribution as the outlet boundary condition. The results showed that under off-design conditions, vortices occurred in the last stage and diffuser.

Unsteady aerodynamic forces were found at high frequencies (stator passing frequencies) and low frequencies (generated from asymmetric pressure distributions behind the rotor), with the relative dominance of these forces/frequencies shifting as a function of radial span. An FFT analysis was carried out. Three sections were investigated: the hub, midspan and peripheral (tip) section. The steady pressure behind the rotor blade was compared with experimental results in the LP last stage behind the rotor blades and in a specified cross-section of the exhaust hood. The lower frequency unsteady forces had a higher relative contribution towards the tip of the blade.
A Reliable Transition Duct With Separation Planes for the Annular Combustor of GT13E2 Gas Turbine
Authors: Tschuor Remigi , Trbojevic Bogdan , Markovic Alen
Published: September 2016

Designing a state-of-the-art combustor requires an iterative process where mechanical design solutions in the early concept phase are continuously assessed using analytical cooling and lifetime assessments, which are later backed up with experimental investigations and validation measures.

This paper describes the integrated design process in terms of design, cooling, manufacturing, aerodynamic and mechanical integrity for the transition duct of the GT13E2 combustor, which houses the flame development region of the combustor.

The objective was to develop a retrofit design with reduced lifecycle costs, with no impact on the combustor-turbine interface and no impact on the overall engine performance. The major challenges were the on-site welding of the two half-shells as well as the increasing demand for cyclic operation of the engine.

During the development process, the focus was on selecting reliable and robust cooling schemes on the inner and outer shell to reduce mechanical deflections and thus to reduce the overall stress distribution.

Key features of the GT13E2 combustor zone 2 are the following:

• Drastically reduced lifecycle cost with no performance penalty (retrofit)

• Separation planes at 3 and 9 o’clock position with mechanical locking connections (bridges) and a recess weld with film cooling

• Bellmouth shaped inlet of cooling channel with improved impingement and convective cooling

• Unique membrane seals with great mechanical flexibility around the turbine inlet outer diameter including mitigation against hot gas ingress (bow wave effect)

• Improved cooling during transient conditions to reduce mechanical deflections

• Reduced reconditioning effort due to innovative manufacturing methods

The cooling schemes were successfully validated against laboratory experiments and integrated into a stable manufacturing process. Increased dimensional stability was achieved through higher rigidity and led to an improved on-site welding process for the separation planes. All the aforementioned features result in a marked improvement of the cyclic lifetime with no performance penalty. This is proven by the fact that the GT13E2 with annular combustor transition duct has now accumulated more than 30,000 operating hours with 1,200 starts.
Three-Dimensional Turbulent Thermo-Elastohydrodynamic Analyses of Hybrid Thrust Foil Bearings Using Real Gas Model
Authors: Xu Fangcheng , Kim Daejong
Published: September 2016

Environment-friendly power generation systems are active area of research. Among many systems, closed loop Brayton cycles using super critical CO2 (S-CO2) is attractive alternative to conventional power cycles due to very high efficiency and power density. When converting low temperature thermal energy such as waste heat to electrical power, closed loop organic Rankine cycles (ORC) using refrigerants are very popular. Large utility scale systems adopting S-CO2 or ORC cycles require traditional bearing systems with dry gas seals, but small systems with shaft power less than 1MW are best suited with gas bearings lubricated with the cycle fluids. Foil gas bearings, which have been successfully applied to the air blowers/compressors and small power generation gas turbines, are the best candidate for the small S-CO2 or ORC cycle systems. However, design/analysis tool of the foil bearings with these non-ideal gases is rare. In addition, thrust foil bearings are technically more challenging compared to radial foil bearings due to low load capacity and large power loss due to high flow turbulence. This paper presents high level analysis tool involving three-dimensional thermo-hydrodynamic analyses of hybrid thrust foil bearings employing real gas effect and flow turbulence inside the film. The pressure distribution, temperature distribution, load capacity, film thickness, and power loss of 154mm hybrid thrust foil bearings are presented.
A Simple Recuperated Ceramic Microturbine: Design Concept, Cycle Analysis, and Recuperator Component Prototype Tests
Authors: Vick Michael , Young Trent , Kelly Matthew , Tuttle Steven , Hinnant Katherine
Published: September 2016

Ceramic recuperators could enable microturbines to achieve higher fuel efficiency and specific power. Challenges include finding a suitable ceramic fabrication process, minimizing stray heat transfer and gas leakage, mitigating thermal stress, and joining the ceramic parts to neighboring metal components. This paper describes engine and recuperator design concepts intended to address these obstacles. The engine is sized to produce twelve kilowatts of shaft power, and it has a reverse-flow compressor and turbine. Motivations for this layout are to balance axial thrust forces on the rotor assembly; to minimize gas leakage along the rotating shaft; to reduce heat transfer to the compressor diffuser; to enable the use of a simple, single-can combustor; and to provide room for lightweight ceramic insulation surrounding all hot section components. The recuperator is an annular, radial counterflow heat exchanger with the can combustor at the center. It is assembled from segmented wafers made by ceramic injection molding (CIM). These are housed in a pressure vessel to load the walls mainly in compression, and are joined together by flexible adhesives in the cool areas to accommodate thermal expansion. A representative wafer stack was built by laser-cutting, laminating, and sintering tapecast ceramic material. The prototype was tested at temperatures up to 675°C, and the results were used to validate analytical and computational fluid dynamics (CFD) models, which were then used to estimate the effectiveness of the actual design. Turbomachinery efficiencies were also calculated using CFD, and allowances were made for additional losses like bearing friction and gas leakage. Based on these component performance estimates, a cycle model indicates the engine could achieve a net fuel-to-electrical efficiency of 21%, at a core weight including the recuperator of 11 kg, or about 1 kg/kW electric output.
Oil-Free Automotive Turbochargers: Drag Friction and On-Engine Performance Comparisons to Oil-Lubricated Commercial Turbochargers
Authors: Ryu Keun , Ashton Zachary
Published: September 2016

Oil-free bearings for automotive turbochargers (TCs) offer unique advantages eliminating oil-related catastrophic TC failures (oil coking, severe bearing wear/seizure, and significant oil leakage, for example) while increasing overall system reliability and reducing maintenance costs. The main objective of the current investigation is to advance the technology of the gas foil bearings (GFBs) for automotive TCs by demonstrating their reliability, durability, and static/dynamic force characteristics desirable in extreme speed and temperature conditions. The paper compares drag friction and on-engine performances of an oil-free TC supported on GFBs against an oil-lubricated commercial production TC with identical compressor and turbine wheels. Extensive coast-down and fast acceleration TC rotor speed tests are conducted in a cold-air driven high-speed test cell. Rotor speed coast-down tests demonstrate that the differences in the identified rotational viscous drag coefficients and drag torques between the oil-free and production TCs are quite similar. In addition, rotor acceleration tests show that the acceleration torque of the oil-free TC rotor, when airborne, is larger than the production TC rotor due to the large mass and moment of inertia of the oil-free TC rotor even though air has lower viscosity than the TC lubricant oil. Separate experiments of the oil-free TC installed on a diesel engine demonstrate the reliable dynamic forced performance and superior rotor dynamic stability of the oil-free TC over the oil-lubricated TC. The post on-engine test inspection of the oil-free TC test hardware reveals no evidence of significant surface wear between the rotor and bearings, as well as no dimensional changes in the rotor outer surfaces and bearing top foil inner surfaces. The present experimental characterization and verified robustness of the oil-free TC system continue to extend the GFB knowledge database.
Design and Calibration of a Full Scale Active Magnetic Bearing Based Test Facility for Investigating Rotordynamic Properties of Turbomachinery Seals in Multiphase Flow
Authors: Voigt Andreas Jauernik , Mandrup-Poulsen Christian , Nielsen Kenny Krogh , Santos Ilmar F.
Published: September 2016

The recent move towards subsea oil and gas production brings about a requirement to locate process equipment in deepwater installations. Furthermore, there is a drive towards omitting well stream separation functionality, as this adds complexity and cost to the subsea installation. This in turn leads to technical challenges for the subsea installed pumps and compressors that are now required to handle multiphase flow of varying gas to liquid ratios. This highlights the necessity for a strong research focus on multiphase flow impact on rotordynamic properties and thereby operational stability of the subsea installed rotating machinery. It is well known that careful design of turbomachinery seals, such as interstage and balance piston seals, is pivotal for the performance of pumps and compressors. Consequently, the ability to predict the complex interaction between fluid dynamics and rotordynamics within these seals is key. Numerical tools offering predictive capabilities for turbomachinery seals in multiphase flow are currently being developed and refined, however the lack of experimental data for multiphase seals renders benchmarking and validation impossible. To this end, the Technical University of Denmark and Lloyd’s Register Consulting are currently establishing a purpose built state of the art multiphase seal test facility, which is divided into three modules. Module I consists of a full scale Active Magnetic Bearing (AMB) based rotordynamic test bench. The internally designed custom AMBs are equipped with an embedded Hall sensor system enabling high-precision non-contact seal force quantification. Module II is a fully automatised calibration facility for the Hall sensor based force quantification system. Module III consists of the test seal housing assembly. This paper provides details on the design of the novel test facility and the calibration of the Hall sensor system employed to measure AMB forces. Calibration and validation results are presented, along with an uncertainty analysis on the force quantification capabilities.
Investigations of Flutter and Aero Damping of a Turbine Blade: Part 2 — Numerical Simulations
Authors: Ren Wei-Min , Seeley Charles E. , Zhang Xuefeng , Mitchell Brian E. , Ju Hongbin
Published: September 2016

There is a demand for Modern Heavy Duty Gas Turbines (HDGT) to provide greater MW power output with higher efficiency. This trend leads to longer and slimmer turbine last stage blades with exposure to higher aerodynamic loadings. As a result, they are more prone to flutter risks. Turbine flutter risks must be addressed during the design phase and this requires the accurate prediction of the flutter boundary and the aero damping ratios with quantified uncertainties.

Numerical simulations, based on Computational Fluid Dynamics (CFD), of a two-passage linear turbine cascade were carried out. The results are presented in this paper aimed to study aspects of flutter. A time-linearized Navier–Stokes approach was applied to predict the aeroelastic response. Simulations were carried out for Mach numbers ranging from 0.4 to 1.2. The objective of this study is to quantify the aero damping prediction uncertainty.

CFD results showed successful prediction of the transonic flutter boundary. The predicted aero work was compared with the experimental data and good agreements were demonstrated for both the subsonic and supersonic flows. The steady and unsteady surface pressures on the test rig sidewalls, predicted by CFD, were then compared with the test data in great detail. Limitations of the linear approach are also discussed.
Detailed Large Eddy Simulations (LES) of Multi-Hole Effusion Cooling Flow for Gas Turbines
Authors: Sung Yonduck , Dord Anne L. , Laskowski Gregory M. , Shunn Lee , Natsui Greg , Kapat Jay
Published: September 2016

We present highly resolved large eddy simulation (LES) of a realistic effusion cooling geometry. The test case features mixing between a high-density working fluid (CO2) and a low-density crossflow (air) to mimic the density stratification in typical gas turbine cooling flows. The coolant CO2 is fed from a plenum into the air channel through a series of 52 inclined orifices in a staggered arrangement. Highly detailed LES computations presented in this study resolved entire 52 cooling holes to accurately capture realistic jet-to-jet interactions which are critical in cooling film formation. Two different blowing ratios (M) are analyzed, comparing predictions obtained from wall-modeled and wall-resolved grids. M is defined as the density times the velocity of the coolant divided by that of the air channel. These values are chosen so that the coolant jets are attached to the wall (M=0.457) in one case, and detached (M=1.22) in the other. Results show that for the two blowing ratios analyzed, the wall-resolved adiabatic effectiveness based on CO2 concentration compares favorably with that obtained from pressure sensitive paint (PSP) measurements. The mixing and turbulence characteristics upstream and downstream of the jets are characterized using the probability density function of CO2 concentration and its impact on cooling effectiveness. It is found that the lower blowing ratio case provides more initial cooling than the higher blowing ratio case because the cooling jets attach to the surface and induce interactions among adjacent columns of jet streams. This provides more uniform film coverage and therefore higher initial cooling effectiveness. In the higher blowing ratio case, however, this interaction is significantly delayed due to lift-off of the coolant jets. PDF analysis shows interactions between adjacent cooling stream columns near the wall do not occur until the last jets. Although both the wall-resolved (WR) and wall-modeled (WM) cases show consistent trends with the PSP measurement, the wall-resolved cases show better quantitative agreement overall.
A Thermo-Mechanical Fatigue Crack Growth Accumulative Model for Gas Turbine Blades and Vanes
Authors: Riva Andrea , Costa Alessio , Dimaggio Dalila , Villari Paolo , Kraemer Karl Michael , Mueller Falk , Oechsner Matthias
Published: September 2016

Heavy duty gas turbine blades and vanes are operated at high temperatures and high stresses, condition where several damage mechanisms can simultaneously be present. For example creep, fatigue and oxidation play an important role in the propagation of existing cracks. Crack growth models are employed for assessment criteria, interpretation of the field feedback and non-conformities management and they are required to be as accurate as possible when predicting crack propagation under the combined effect of all the three phenomena.

In this work, a Linear Elastic Fracture Mechanics (LEFM) model based on isothermal experimental tests and validated by Thermo-Mechanical-Fatigue Crack Growth tests (TMFCG), is employed to predict crack propagation of a cast Ni-base superalloy used in gas turbine blades and vanes. When calculating the individual propagation fractions of creep and fatigue crack growth, the model accounts for the instantaneous stress state and temperature in transient regime (i.e. a complete cycle of start-up, base-load and shut-down). The loss of γ’- precipitates at the crack tip due to surface oxidation is interpreted as environmental damage fraction.

A complete workflow for the systematic use of the approach, comprising an in-house software, has been defined and developed. Stress intensity factors used for LEFM calculations are determined either using tabulated weight functions or with the aid of Finite Element Analysis (FEA). This flexible approach is consistent with the industrial need of a given fracture mechanics calculation, which might require different levels of accuracy and resources/time consumption case by case. The software identifies the fraction of propagation caused by oxidation, creep crack growth or fatigue crack growth. This allows checking the physical realism of the results by comparing to metallographic analysis of fracture surfaces from broken TMFCG test specimen and/or real component damage information from field. Besides, this feature can be helpful to support the engineer in residual life evaluation under damage tolerant approach because it allows the identification of the type of operational regime that minimizes crack propagation. The software also allows the execution of sensitivity analyses via Monte-Carlo calculations, identifying for a given component and operational condition the more relevant calculation inputs. This feature also quantitatively supports the engineers in the identification of the most appropriate safety margins.
Forced Response Analysis of Supercritical CO2 Radial Inflow Turbine Designed for Concentrating Solar Power
Authors: Modir Shanechi Mohsen , Veidt Martin , Hooman Kamel
Published: September 2016

The forced response of high-pressure sCO2 radial-inflow turbine blisk is studied with regards to internal mistuning and inherent characteristics of supercritical Brayton cycle. A novel preliminary meanline analysis led to the generation of turbine designs for the sCO2 Brayton cycle with respect to concentrating solar power (CSP) applications. Details of mentioned study are published in a separate paper. The sCO2 turbine with a pressure ratio of 2.2 and the mild inlet temperature of 560 C is studied for rotational speed ranging between 75000 and 125000 RPM.

Aiming to achieve an enhanced understanding of the fluid-structure-interaction in sCO2 radial-inflow turbine, a numerical method capable of predicting the forced responses of tuned and intentionally mistuned blisks due to aerodynamic excitation is presented. The numerical work involves the simulation of the transient flow field, and then the unsteady aerodynamic excitation forces on the blades are determined by modelling various resonance condition, including the influence of the operating condition and stator number. Performing the forced response of the structure, the transient and spatially resolved pressure distribution is used as a boundary condition in an FE model. As a result, the response amplifications of sCO2 turbines are eventually compared.

The similar geometrical turbine was designed and manufactured to be operated in subcritical state for the sake of validation. The results of the subcritical turbine are derived by means of experimental and numerical analyses. To update the effect of mistuning in the FE model, blade by blade measurements using the example of a subcritical turbine blisk is performed and results of well correlated FRFs are used as inputs to adjust the blade individual Young’s modulus. The tendency of results is approved by previous works done in subcritical state. The structural damping information to be considered in the update process is taken from results of an experimental modal analysis and the aerodynamic damping induced by blade vibration is computed using an energy balance technique.

It has been found that increase of the maximum forced response beyond that of the sCO2 turbine with higher rotational speed is not significant due to the existence of high pressure-density sCO2. This implies an occurrence of high aerodynamic damping which would cause a low vibrational amplitude in case of a mistuned blisk. Considering aeroelastic coupling, in supercritical turbine with small mistuning, no change of maximum response magnitudes is achieved for the fundamental bending mode; however, with large mistuning pattern, aerodynamic damping can cause significantly better response level. This result indicates considerable contrast with responses obtained from subcritical model which would be expressed by either characteristic or state of working fluid.