bluuebunny/crossref_metadata_2025 huggingface dataset:
Unsteady Flow in Extraction Modules of Industrial Steam Turbines
Authors: Schramm Andreas , Müller Tim , Polklas Thomas , Brunn Oliver , Mailach Ronald
Published: September 2014

Industrial steam turbines are designed for application in power-, process- and chemical engineering. Particular modules ensure the optimum integration into power plants and other engineering processes. Extraction modules allow the controlled extraction of large steam quantities on certain and constant enthalpy levels. Valves regulate the amount of steam extracted from the turbine expansion path. Depending on the valve lift, different flow separation phenomena can occur peripherally inside the valves, causing undesired large unsteady fluid forces on the valve head and seat. Due to the compact design of the industrial steam turbines, these unsteady jets can influence the rotor dynamics as well as the blade loading of the adjacent stages. These fluctuations should be understood and avoided in order to enhance the reliability of steam turbines.

In the present study the unsteady flow phenomena due to separation occurring circumferentially inside the valve of extraction modules are investigated numerically. First, the commercial 3D RANS CFD-solver (ANSYS CFX 14) is validated in the application to experimental results. Subsequently, the various flow patterns of the examined valve design are analyzed on a standalone numerical valve model in an extensive study.

In order to assess the impact of these unsteady flow separations on other components, the complete extraction module is simulated in combination with the adjacent stages. The transient simulation results show pressure fluctuations downstream of the valves resulting in an unsteady load of the control valves, the shaft and the blading.
Phantom Cooling of Nozzle Trailing Edge Discharge on Blade Surface and Platform
Authors: Li Xueying , Ren Jing , Jiang Hongde
Published: September 2014

Phantom cooling is defined as the cooling redistribution on airfoil surfaces and endwalls due to airfoil cooling discharges and leakages. Understanding of this effect has become especially critical in recent years, because of the restricted amount of cooling air for the achievement of higher efficiency.

The phantom cooling effect of the first stage nozzle trailing edge discharge on the first stage blade surfaces and platform are studied numerically with URANS. Both time-dependent and time-averaged cooling effectiveness distributions on the rotor under the influence of vane trailing edge discharge are presented with different discharge velocity ratios. The results show that the nozzle trailing edge ejection affects the suction and pressure side cooling of the blade as well as the platform. The effects on the triangular zones of suction surface are evident, especially the bottom and top zones which are better cooled. Under the influence of passage secondary flows and rotating, different coolant discharge velocity ratios which resulted in different inlet angles have an effect on the phantom cooling distribution.

In general, the cooling air discharged from the trailing edge of the first stage nozzle influences the temperature distribution on the blade, which can substantially improve the cooling efficiency in the bottom triangular zone. This suggests that accounting for phantom cooling can improve the cooling design and if actively controlled save cooling flow.
Numerical Investigation on the Effects of Circumferential Coverage of Injection in a Transonic Compressor With Discrete Tip Injection
Authors: Wang Wei , Chu Wuli , Zhang Haoguang , Wu Yanhui
Published: September 2014

Discrete tip injection upstream of the rotor tip is an effective technique to extend stability margin for a compressor system in an aeroengine. The current study investigates the effects of injectors’ circumferential coverage on compressor performance and stability using time-accurate three-dimensional numerical simulations for multi passages in a transonic compressor. The percentage of circumferential coverage for all the six injectors ranges from 6% to 87% for the five investigated configurations. Results indicate that circumferential coverage of tip injection can greatly affect compressor stability and total pressure ratio, but has little influence on adiabatic efficiency. The improvement of compressor total pressure ratio is linearly related with the increasing circumferential coverage. The unsteady flow fields show that there exists a non-ignorable time lag of the injection effects between the passage inlet and outlet, and blade tip loading will not decline until the injected flow reaches the passage outlet. Stability improves sharply with the increasing circumferential coverage when the coverage is less than 27%, but increases flatly for the rest. It is proven that the injection efficiency which is a measurement of averaged blockage decrement in the injected region is an effective guideline to predict the stability improvement.
S-Bend Diffuser With Passive Effusion Cooling
Authors: Ng B. C. N. , Birk A. M.
Published: September 2014

This paper presents the results of an experimental study of passive effusion cooling in an S-bend channel with a diffuser. A previous paper showed the results for cold primary flow [1]. This paper deals with hot primary flow. The results show that different levels and locations of effusion injection significantly affect the primary flow field and the pressure recovery in the downstream diffuser. This leads to a significant increase in static pressure at the inlet of the S-bend section, and reduces the driving pressure for the passive effusion cooling air. This effect then leads to reduced effusion mass flow, momentum and cooling effectiveness. The paper presents data on the static pressure distributions in the device, flow distortion at the diffuser exit, flow rate of effusion injection, and effusion cooling effectiveness.
On the Reduction of the Pre-Processing Effort and the Application of a Contact Meshing Approach for Complex Jet Engine Component Assemblies
Authors: Hufenbach W. , Hornig A. , Böhm H. , Langkamp A. , Keskin A.
Published: September 2014

A significant proportion of the work effort for finite element (FE) analysis is spent for pre-processing activities, especially for complex structural components and component assemblies. An exclusive use of hexahedron (hex) elements increases the meshing effort substantially compared to tetrahedral elements. An automated method to generate high quality hexahedral meshes for an arbitrary geometry does not exist. In this work, commercially available FE software tools for meshing were investigated with the focus on an advantageous pre-processing effort. The evaluation showed that the software package NX (Siemens PLM Software) offers robust advanced semiautomatic hex meshing capabilities.

Furthermore, a Contact Meshing Approach (CMA) was elaborated to reduce the effort of the challenging and time-consuming geometry decomposition significantly. Using the example of an intermediate pressure compressor it can be shown that the pre-processing effort time can be reduced up to 75%. Due to the independent meshes, element transitions in the geometry become redundant. This results in lower total element numbers and higher mesh qualities and subsequently leads to more efficient calculations. Moreover, the increased element quality has positive effects on the result quality.
Rotordynamic Investigation of Spiral Vibrations: Thermal Mode Equation Development and Implementation to Combined-Cycle Power Train
Authors: Golebiowski Mateusz , Nordmann Rainer , Knopf Eric
Published: September 2014

Rotation of vibration vector caused by thermally induced unbalance changes is a frequently observed phenomenon in large rotating machinery. The heat arising from the friction losses, which are generated at the interfaces between rotating and statoric components of the machine, is partly absorbed by the shaft. This heat input is typically not uniform around the shaft circumference and the resulting temperature difference causes the rotor to bow. The excitation resulting from the sum of mechanical unbalance and thermal bow will lead to a slowly rotating (in the synchronously rotating coordinates system) whirl vector, whose magnitude can decrease or increase in time.

A generic understanding of this effect (B.L. Newkirk in 1926, [4]) had been followed by a number of physical models representing specific heat exchange mechanisms (W. Kellenberger [3], J. Schmied [6], P. Morton [11]). A hot spot on the shaft surface can be generated at various locations of a shaft-line. Typical components responsible for thermally induced modulation of vibration vector are journal bearings, seal rings, labyrinth seals (in case of a soft rubbing). Furthermore carbon brushes sliding on the slip ring, supplying the DC current to the field winding of the generator rotor, were identified as a source of nonuniform heat input that may excite spiral vibrations (L. Eckert and J. Schmied in [7], [8]). These local heat input phenomena affect consequently the vibration behavior of the overall shaft train.

This paper provides a new approach to the quantitative description of a heat exchange mechanism which leads to the hot spot generation on the surface of a slip ring. A new thermal equation has been formulated, which determines the stability and frequency of the thermal mode. Characteristics of spiral vibration are discussed based on the analytical solution of the Jeffcott rotor model coupled with the proposed thermo-elastic equation.

The implementation of the described method to a full shaft-line model of a combined cycle, single shaft power train was done using the Finite Element Method. The results of this calculation were validated against measurement data. The paper shows how the applied computational approach can be used to extend stability margin of the spiral vibration in turbo-generator shaft trains.
Application of Unsteady CFD Methods to Trailing Edge Cutback Film Cooling
Authors: Ravelli S. , Barigozzi G.
Published: September 2014

The main purpose of this numerical investigation is to overcome the limitations of the steady modeling in predicting the cooling efficiency over the cutback surface in a high pressure turbine nozzle guide vane. Since discrepancy between Reynolds-averaged Navier–Stokes (RANS) predictions and measured thermal coverage at the trailing edge was attributable to unsteadiness, Unsteady RANS (URANS) modeling was implemented to evaluate improvements in simulating the mixing between the mainstream and the coolant exiting the cutback slot. With the aim of reducing the computation effort, only a portion of the airfoil along the span was simulated at an exit Mach number of Ma2is = 0.2. Three values of the coolant-to-mainstream mass flow ratio were considered: MFR = 0.66%, 1.05%, and 1.44%. Nevertheless the inherent vortex shedding from the cutback lip was somehow captured by the URANS method, the computed mixing was not enough to reproduce the measured drop in adiabatic effectiveness η along the streamwise direction, over the cutback surface. So modeling was taken a step further by using the Scale Adaptive Simulation (SAS) method at MFR = 1.05%. Results from the SAS approach were found to have potential to mimic the experimental measurements. Vortices shedding from the cutback lip were well predicted in shape and magnitude, but with a lower frequency, as compared to PIV data and flow visualizations. Moreover, the simulated reduction in film cooling effectiveness toward the trailing edge was similar to that observed experimentally.
NMHC and VOC Speciation of the Exhaust Gas From a Gas Turbine Engine Using Alternative, Renewable and Conventional Jet A-1 Aviation Fuels
Authors: Altaher Mohamed A. , Li Hu , Blakey Simon , Chung Winson
Published: September 2014

This paper investigated the emissions of individual unburned hydrocarbons and carbonyl compounds from the exhaust gas of an APU (Auxiliary Power Unit) gas turbine engine burning various fuels. The engine was a single spool, two stages of turbines and one stage of centrifugal compressor gas turbine engine, and operated at idle and full power respectively. Four alternative aviation fuel blends with Jet A-1 were tested including GTL, hydrogenated renewable jet fuel and fatty acid ester. C2-C4 alkenes, benzene, toluene, xylene, trimethylbenzene, naphthalene, formaldehyde, acetaldehyde and acrolein emissions were measured. The results show at the full power condition, the concentrations for all hydrocarbons were very low (near or below the instrument detection limits). Formaldehyde was a major aldehyde species emitted with a fraction of around 60% of total measured aldehydes emissions. Formaldehydes emissions were reduced for all fuels compared to Jet A-1 especially at the idle conditions. There were no differences in acetaldehydes and acrolein emissions for all fuels; however, there was a noticeable reduction with GTL fuel. The aromatic hydrocarbon emissions including benzene and toluene are decreased for the alternative and renewable fuels.
Variation in Laminar Burning Velocity and Markstein Length With Water Addition for Industrially Produced Syngases
Authors: Pugh Daniel , Crayford Andrew , Bowen Philip , O’Doherty Tim , Marsh Richard
Published: September 2014

An outwardly propagating spherical flame has been used to characterise the influence of water addition on the combustion of variable steelworks gas compositions. Attention was given to the ratio of hydrogen and carbon monoxide within blast furnace gas, and the catalysing influence of water addition on the preponderant reaction kinetics. A nonlinear extrapolative technique was used to obtain values of laminar burning velocity and Markstein length for atmospheric combustion with air and change in equivalence ratio. Four disparate blast furnace gas mixtures were tested with increasing volumetric proportions of hydrogen in the range of one to seven percent, displacing other constituent fractions. A non-monotonic influence was observed, with propagation accelerated for compositions comprising smaller amounts of hydrogen, and the cooling impact of water addition shown to slow faster burning flames. Water addition was also shown to increase the effects of flame stretch on observed propagation rates, and the contrasting influences resulting from vapour fraction are discussed with respect to practical combustion instability, in addition to alternative synthesised fuels. Numerically modelled results were generated using the PREMIX coded CHEMKIN-PRO, and the performance of specified chemical reaction mechanisms evaluated in relation to the obtained experimental data.
Flutter Amplitude Saturation by Nonlinear Friction Forces: Reduced Model Validation
Authors: Martel Carlos , Corral Roque , Ivaturi Rahul
Published: September 2014

The computation of the final, friction saturated Limit Cycle Oscillation amplitude of an aerodynamically unstable bladeddisk in a realistic configuration is a formidable numerical task. In spite of the large numerical cost and complexity of the simulations, the output of the system is not that complex: it typically consists of an aeroelastically unstable traveling wave (TW), which oscillates at the elastic modal frequency and exhibits a modulation in a much longer time scale. This slow time modulation over the purely elastic oscillation is due to both, the small aerodynamic effects and the small nonlinear friction forces. The correct computation of these two small effects is crucial to determine the final amplitude of the flutter vibration, which basically results from its balance. In this work we apply asymptotic techniques to consistently derive, from a bladed-disk model, a reduced order model that gives only the time evolution on the slow modulation, filtering out the fast elastic oscillation. This reduced model is numerically integrated with very low CPU cost, and we quantitatively compare its results with those from the bladed-disk model. The analysis of the friction saturation of the flutter instability also allows us to conclude: (i) that the final states are always nonlinearly saturated TW, (ii) that, depending on the initial conditions, there are several different nonlinear TWs that can end up being a final state, and (iii) that the possible final TWs are only the more flutter prone ones.
Experimental and Numerical Investigation of a Hydrogen Combustion Chamber Under Various Inlet Conditions
Authors: Malli Heidemarie , Eckerstorfer Kurt , Borm Oliver , Leitl Peter
Published: September 2014

Flameless combustion, MILD (moderate or intense low oxygen dilution) combustion and HiTAC (high temperature air combustion) all refer to a combustion regime characterized by high temperatures and a high dilution of reactants. In most cases, this is achieved by recirculating exhaust gases. This leads to comparatively low oxygen concentrations, a largely uniform temperature field and to a drastically reduced NOx formation. Up to now, the application of this combustion technology for gas turbine combustion chambers is still in an early development stage. Most investigations of flameless or MILD combustion chambers have been carried out for methane or certain fuel blends. Since this combustion technology has already successfully demonstrated low NOx emissions without the need of premixing with its potential risks of flashback and autoignition, it might be a promising technology for hydrogen burning combustion chambers. The scope of this paper is to investigate a hydrogen combustion chamber for its NOx emission characteristics and for its use in the flameless or MILD combustion regime. Thus, the influence of different inlet parameters (excess air ratio, thermal input of hydrogen, inlet velocity of the combustion air, pressure inside the combustion chamber) on the emission characteristics of the combustion chamber are examined experimentally. Additionally, for one operating point, a two–dimensional numerical simulation of the combustion chamber was carried out.
Flutter Analysis for Turbomachinery Using Volterra Series
Authors: Liou Meng-Sing , Yao Weigang
Published: September 2014

The objective of this paper is to describe an accurate and efficient reduced order modeling method for aeroelastic (AE) analysis and for determining the flutter boundary. Without losing accuracy, we develop a reduced order model based on the Volterra series to achieve significant savings in computational cost. The aerodynamic force is provided by a high-fidelity solution from the Reynolds-averaged Navier-Stokes (RANS) equations; the structural mode shapes are determined from the finite element analysis. The fluid-structure coupling is then modeled by the state-space formulation with the structural displacement as input and the aerodynamic force as output, which in turn acts as an external force to the aeroelastic displacement equation for providing the structural deformation. NASA’s rotor 67 blade is used to study its aeroelastic characteristics under the designated operating condition. First, the CFD results are validated against measured data available for the steady state condition. Then, the accuracy of the developed reduced order model is compared with the full-order solutions. Finally the aeroelastic solutions of the blade are computed and a flutter boundary is identified, suggesting that the rotor, with the material property chosen for the study, is structurally stable at the operating condition, free of encountering flutter.
Effects of Coolant Injection Configuration on the Cooling Performance of Gas Turbine Stage Using CFD and Conjugate Heat Transfer Method
Authors: Gao Qing , Li Jun , Feng Zhenping
Published: September 2014

The effects of coolant injection configuration on the cooling performance in the wheel space cavity and rotating components of gas turbine stage and the aerodynamic performance influenced by the interaction between rim seal purge flow and main flow of the stage were numerically investigated using CFD and conjugate heat transfer method. Three-dimensional Reynolds-Averaged Navier-Stokes (RANS) and SST turbulent model were utilized to analyze the flow and heat transfer characteristics of gas turbine stage model with vane, blade and inter wheel space cavity. Fluid domain including stator and rotor blade passage, wheel space cavity as well as rim seal and solid domain including solid blade and rotating disk were considered in the present computational domains. Center coolant injection, center coolant injection combined with high position pre-swirl coolant injection, and high position pre-swirl coolant injection configuration for the axial rim seal were used to investigate their influence on the cooling effectiveness and aerodynamic performance of the turbine stage. In addition, the influence of rim seal configuration with center coolant injection on the cooling effectiveness and aerodynamic performance of the computational domains is also investigated. Numerical results show that the coolant injection configuration influences both the cooling effectiveness and flow field in the wheel space cavity and the aerodynamic performance of the turbine stage. The rim seal configuration mainly influences the cooling effectiveness in the wheel space cavity but its effect on aerodynamic performance is ignorable in this work. The internal flow and heat transfer behavior of stage passage and wheel space cavity, as well as the cooling effectiveness of the rotor blade and rotating disk were also illustrated and discussed.
Numerical Investigation of Aerothermal Characteristics of the Blade Tip and Near-Tip Regions of a Transonic Turbine Blade
Authors: Arisi A. , Xue S. , Ng W. F. , Moon H. K. , Zhang L.
Published: September 2014

In modern gas turbine engines, the blade tips and near-tip regions are exposed to high thermal loads caused by the tip leakage flow. The rotor blades are therefore carefully designed to achieve optimum work extraction at engine design conditions without failure. However, very often gas turbine engines operate outside these design conditions which might result in sudden rotor blade failure. Therefore, it is critical that the effect of such off-design turbine blade operation be understood to minimize the risk of failure and optimize rotor blade tip performance. In this study, the effect of varying the exit Mach number on the tip and near-tip heat transfer characteristics was numerically studied by solving the steady Reynolds Averaged Navier Stokes (RANS) equation. The study was carried out on a highly loaded flat tip rotor blade with 1% tip gap and at exit Mach numbers of Mexit = 0.85 (Reexit = 9.75 × 105) and Mexit = 1.0 (Reexit = 1.15 × 106) with high freestream turbulence (Tu = 12%). The exit Reynolds number was based on the rotor axial chord. The numerical results provided detailed insight into the flow structure and heat transfer distribution on the tip and near-tip surfaces. On the tip surface, the heat transfer was found to generally increase with exit Mach number due to high turbulence generation in the tip gap and flow reattachment. While increase in exit Mach number generally raises he heat transfer over the whole blade surface, the increase is significantly higher on the near-tip surfaces affected by leakage vortex. Increase in exit Mach number was found to also induce strong flow relaminarisation on the pressure side near-tip. On the other hand, the size of the suction surface near-tip region affected by leakage vortex was insensitive to changes in exit Mach number but significant increase in local heat transfer was noted in this region.
Dynamic Performance of Power Generation Systems for Off-Shore Oil and Gas Platforms
Authors: Pierobon Leonardo , Iyengar Krishna , Breuhaus Peter , Kandepu Rambabu , Haglind Fredrik , Hana Morten
Published: September 2014

On off-shore oil and gas platforms two or more gas turbines typically support the electrical demand on site by operating as a stand-alone (island) power system. As reliability and availability are major concerns during operation, the dynamic performance of the power generation system becomes a crucial aspect for stable operation and prevention of unwanted shut down in case of disturbances in the local grid.

This paper aims at developing and validating a dynamic model of the gas turbine-based power generation system installed on the Draugen off-shore oil and gas platform (located in the North Sea, Norway). The dynamic model of the SGT-500 gas turbine includes dynamic equations for the combustion chamber and for the high pressure, low pressure and turbine shafts. The low and high pressure compressors are modeled by using quasi steady-state conditions by scaling the maps of axial compressors employing a similar design point. For the turbines, the Stodola equation as well as a correlation relating the isentropic efficiency and the non-dimensional flow coefficient is utilized. The model is implemented in the Modelica language.

The dynamic model of a single SGT-500 gas turbine is first verified by comparing the transient response for a given load variation with the results of a non-physical Matlab model developed by the gas turbine manufacturer and adapted to the power set-point of the original engine installed on Draugen. Subsequently, the complete power generation system consisting of three gas turbines is simulated during transient operation and the results are compared with operational data provided by the platform operator. The model is also applied to evaluate the transient response of the system during peak loads. The results suggest that the highest accuracy (average relative error ∼1%) arises on the prediction of the rotational speed of the high pressure shaft, while the largest deviation (average relative error ∼20%) occurs in the evaluation of the pressure at the outlet of the low pressure turbine.

As waste heat recovery units (e.g. organic Rankine cycles) are likely to be implemented in future off-shore platforms, the proposed model may serve in the design phase for a preliminary assessment of the dynamic response of the power generation system and to evaluate if requirements such as minimum and maximum frequency during transient operation and the recovery time are satisfied. Furthermore, as the model is based on physics it can be coupled with the measuring instruments to monitor the thermodynamic variables at the inlet and at the outlet of each engine component.
Investigation of Large-Scale Cooling System Fan Blade Vibration
Authors: Muiyser Jacques , Els Danie N. J. , van der Spuy Sybrand J. , Zapke Albert
Published: September 2014

Fans operating at the edges of large-scale air-cooled steam condensers often do so under distorted inlet air flow conditions. These conditions create variations in the aerodynamic loads exerted on a fan blade during rotation which causes it to vibrate. In order to isolate the sources of the unsteady aerodynamic loads as well as their effects on blade vibration, a potential flow fluid dynamics code was written to determine the aerodynamic loads exerted on a fan blade as a function of its rotation. The lift and drag forces were exported to a finite element code approximating the fan blade as a cantilever beam. With these two sets of code the response of the blade when subjected to varying aerodynamic loads could be determined. Furthermore, the effect of changing certain parameters such as blade stiffness or damping can be investigated. It was found that the blade’s response closely resembles that which was measured at the full-scale facility and that slight changes to the blade’s stiffness can potentially reduce the vibrational amplitude but may also lead to resonance.
Combustion Monitoring in Gas Turbines and Enhanced Stability at Very Lean Conditions
Authors: Giacomazzi E. , Giulietti E. , Stringola C. , Cassani S. , Pagliari L. , Chiocchini S.
Published: September 2014

In the near future, combustion dynamics is expected to become a more critical issue in operation of lean premixed gas turbines. Firstly, due to the lack of a quality harmonization code for the gas injected in the distribution network: this increases fuel composition variability at the inlet of gas turbine combustors. Secondly, due to the increasing share of renewable sources of energy in the electricity grid: this causes more likely power fluctuations. Gas turbines will be required to perform quicker loading and unloading phases than current ones (while maintaining their stability and low pollutant emissions) to guarantee the grid stability. Such a scenario highlights the importance of developing reliable and robust combustion instability sensing techniques to be integrated into control systems. Besides, it also shows the need for new and more efficient gas turbine “parking” strategies with lower minimum environmental loads. With these needs in mind, this work compares an optical device (ODC, Optical Diagnostics of Combustion) with standard high-temperature pressure-transducers while monitoring the stable and unstable conditions of an experimental combustor equipped with an ANSALDO-SIEMENS V64.3A burner fed with lean CH4/Air mixtures. Pressure and flame radiant energy signals are compared in terms of frequencies and associated amplitudes. It is also suggested how they can be used for the early detection of thermo-acoustic instabilities, i.e., for the real-time identification of instability precursors. Then, tests are repeated at the same conditions using a V64.3A burner modified to operate in the volumetric combustion regime. Tests prove its enhanced stability, thus suggesting volumetric combustion as a strategy to operate gas turbines at lower loads.
Simplified Method to Evaluate the “Under Platform” Damper Effects on Turbine Blade Eigenfrequencies Supported by Experimental Test
Authors: Bessone Andrea , Traversone Laura
Published: September 2014

Many turbine blades are assembled on rotors with “under platform” dampers. These devices have two main effects: increase structural damping and modify the blade eigenfrequencies. During the design development of new turbine blades it can be useful to evaluate the damper effect in terms of eigenfrequencies with a simplified method incorporating OEM experiences.

In this paper a FE modal analysis of a turbine bladed disc assembly is described, incorporating the 3D CAD simplifications and FE model boundary conditions. The ANSYS program has been used for the simulation and the different configurations for this type of analysis have been investigated. Furthermore the “under platform” damper effects have been included in the post-processing of the analysis as a percentage contribution derived from technical literature. The calculated eigenfrequencies of the blade have been compared with experimental testing in an over-speed cell to validate the methodology. These experimental tests have been performed with strain gauges applied to blades. The complete stage row has been assembled in a particular balanced configuration with and without “under platform” dampers. The data capture allows the identification of the damper effect on the blade eigenfrequencies.

Finally the Campbell diagram with the different boundary conditions during the gas turbine start-up has been prepared. Comparing calculated eigenfrequencies with gas turbine excitation sources it is possible to evaluate the blade dynamic behavior and to study different design solutions if the component has insufficient separation with the engine order excitation.
Mistuning and Damping Analysis of a Radial Turbine Blisk in Varying Ambient Conditions
Authors: Beirow Bernd , Maywald Thomas , Kühhorn Arnold
Published: September 2014

A mistuned radial turbine impeller is analyzed with respect to the impact of varying ambient pressures and temperatures as well on frequency response functions and modal damping ratios. Beginning at room conditions, a finite element model of an impeller wheel at rest is updated based on experimentally determined mistuning in terms of blade dominated frequencies. The following numerical forced response analyses yield a maximum blade displacement amplification of 67% compared to the tuned reference. In addition, modal damping ratios are determined in dependence on the ambient pressure ranging from technical vacuum at 1 mbar up to 6000 mbar in a pressure chamber. Shaker excitation and laser Doppler vibrometry response measurement is employed in this context. A linear dependence of modal damping ratios on ambient pressure and a dominating damping contribution of the surrounding air even for higher modes could be proved. Moreover, the experimental determination of frequency response functions (FRF) at technical vacuum yields a better separation of resonance peaks compared to room conditions at 1013 mbar and hence, this data allows for more accurate model-updates in principle. It is proved that numerical models updated regarding mistuning at room conditions are well suited to predict the forced response at arbitrary pressures if measured modal damping ratios at these pressures are considered. Finally, within analyzing the effect of increasing structural temperatures with the surrounding air at 1013 mbar included slightly decreasing resonance frequencies but strongly increasing FRF-amplitudes are determined.
The Impact of Turbulence Model and Numerical Scheme on the Performance of a Linear Compressor Cascade
Authors: Sans J. , Resmini M. , Brouckaert J. F. , Hiernaux S.
Published: September 2014

In the use of RANS models, it is well known that the selection of the turbulence model and the numerical scheme may have a critical impact not only in terms of convergence, but also on the reliability to simulate separated or secondary flows in general. The aim of the investigation, performed using the commercial software FINE/Turbo, is the understanding and the quantification of the effects of these two numerical parameters on the performance and the stability of a state-of-the-art controlled diffusion airfoil compressor cascade. A mesh sensitivity analysis has been carried out at both design and off-design conditions. The behaviour of the main flow parameters have been investigated over the whole incidence working range, considering a variation of the inlet Mach number between 0.35 and 0.65. Five different turbulence models have been tested: Baldwin-Lomax, Spalart-Allmaras, k–ε Yang-Shih, k–ε Launder-Sharma and k–ω SST. In a specific combination of incidences and Mach numbers, the impact of turbulence model settings has been assessed imposing boundary conditions according to different criteria. Two different numerical schemes have been tested: a Jameson central scheme and a second order upwind scheme. The results between the different simulations are discussed in terms of loss coefficient distribution and incidence range; considering the turbulence model comparison, the differences are significant in the whole incidence range, specially approaching the stall limit. Baldwin-Lomax and Spalart-Allmaras simulations present the same value of last stable incidence, while Yang-Shih and SST are characterized by a reduced stall margin. In many operating conditions, simulations computed with centered scheme present negative losses in a wide area of the outlet sections. This problem is reduced if an upwind scheme is used, but causes a substantial reduction of the incidence range.
The Influence of Roughness on a High-Pressure Steam Turbine Stage: An Experimental and Numerical Study
Authors: Bellucci Juri , Rubechini Filippo , Marconcini Michele , Arnone Andrea , Arcangeli Lorenzo , Maceli Nicola , Dossena Vincenzo
Published: September 2014

This work deals with the influence of roughness on high-pressure steam turbine stages. It is divided in three parts. In the first one, an experimental campaign on a linear cascade is described, in which blade losses are measured for different values of surface roughness and in a range of Reynolds numbers of practical interest. The second part is devoted to the basic aspects of the numerical approach, and consists of a detailed discussion of the roughness models used for computations. The fidelity of such models is then tested against measurements, thus allowing their fine-tuning and proving their reliability. Finally, comprehensive CFD analysis is carried out on a high-pressure stage, in order to investigate the influence of roughness on the losses over the entire stage operating envelope. Unsteady effects that may affect the influence of the roughness, such as the upcoming wakes on the rotor blade, are taken into account, and the impact of transition-related aspects on the losses is discussed.
Experimental Analysis of the Dynamic Characteristics of a Foil Thrust Bearing
Authors: Balducchi Franck , Arghir Mihai , Gauthier Romain
Published: September 2014

The paper deals with the experimental analysis of the dynamic characteristics of a foil thrust bearing (FTB) designed following the specifications given by NASA in 2009. The start-up characteristics of the same foil bearing were investigated in a recently published paper. The test rig used for start-up measurements was adapted for dynamic measurements. The paper presents the test rig in detail as well as its identified dynamic models. Measurements of the dynamic characteristics of the bump foil structure were performed for static loads comprised between 30 N and 150 N while measurements for the FTB were performed at 35 krpm for 30 N, 60 N and 90 N. Excitation frequencies were comprised between 150 Hz and 750 Hz. Results showed that the dynamic stiffness of the FTB increase with excitation frequency while the equivalent damping decreases. Both stiffness and damping increase with the static load but are smaller at 35 krpm compared to 0 rpm.
Turbulent Unsteady Flow Analysis of Horizontal Axis Wind Turbine Airfoil Aerodynamics Based on the Harmonic Balance Reynolds-Averaged Navier-Stokes Equations
Authors: Campobasso M. Sergio , Gigante Fabio , Drofelnik Jernej
Published: September 2014

Several horizontal axis wind turbine unsteady flows, such as that associated with the yawed wind regime, are predominantly periodic. Harmonic balance Reynolds-averaged Navier-Stokes solvers can be used to accurately analyze such flows substantially faster than what their time-domain counterparts can do. The paper presents the mathematical and numerical features of a new turbulent harmonic balance Navier-Stokes solver using Menter’s shear stress transport model for the turbulence closure. The effectiveness of the developed technology is demonstrated by using two-dimensional harmonic balance flow simulations to determine the periodic aerodynamic loads acting on a blade section of a 164 m-diameter wind turbine rotor in yawed wind. Presented results highlight that the turbulent harmonic balance solver can compute the sectional hysteresis force cycles more than 10 times faster than its time-domain counterpart, and with an accuracy comparable to that of the time-domain solver.
Compressible Reynolds-Averaged Navier-Stokes Analysis of Wind Turbine Turbulent Flows Using a Fully-Coupled Low-Speed Preconditioned Multigrid Solver
Authors: Campobasso M. S. , Yan M. , Drofelnik J. , Piskopakis A. , Caboni M.
Published: September 2014

The high-fidelity aeromechanical analysis and design of multi-megawatt horizontal axis wind turbines can be performed by means of Reynolds-averaged Navier-Stokes codes. The compressible or incompressible formulation of the fluid equations can be used. One of the objectives of the paper is to quantify the effects of flow compressibility on the aerodynamics of large turbine rotors with particular attention to the tip region of a 82 m rotor blade featuring a relative Mach number of about 0.3 near rated conditions. Noticeable local static pressure variations due to compressibility are observed. Such variations point to the better suitability of compressible solvers for turbine aerodynamics, not only when the solver is used for direct aeroacoustic simulation of the near field noise propagation, but also when it is used to provide the surface static pressure to be used as input for acoustic analogy noise propagation codes. On the numerical side, a novel numerical approach to low-speed preconditioning of the mean flow and turbulence model equations for the fully coupled integration of the flow equations coupled to a two-equation turbulence model is presented and implemented in a compressible Navier-Stokes research code for the steady and yawed wind-induced time-dependent flows analyzed herein.
Tilting Pad Journal Bearings: On Bridging the Hot Gap Between Experimental Results and Model Predictions
Authors: San Andrés Luis , Tao Yujiao , Li Yingkun
Published: September 2014

The accurate prediction of the forced performance of tilting pad journal bearings (TPJB) relies on coupling a fluid film model that includes thermal energy transport, and on occasion fluid inertia, to the structural stiffness of the pads’ pivots and the thermomechanical deformation of the pads’ surfaces. Often enough, the flexibility of both pads and pivots is ignored prior to the bearing actual operation; practice dictating that force coefficients, damping in particular, decrease dramatically due to pivot flexibility. Even in carefully conducted experiments, components’ flexibilities are invoked to explain dramatic differences between measurements and predictions. A multiple-year test program at TAMU has demonstrated the dynamic forced response of TPJBs can be modeled accurately with matrices of constant stiffness K, damping C, and added mass M coefficients. The K-C-M model, representing frequency independent force coefficients, is satisfactory for excitation frequencies less or equal to the shaft synchronous speed. However, as shown by the authors in Ref. [1], pivot flexibility reduces the applicability of the simple constant parameter model to much lower excitation frequencies. Presently, a fluid film flow model predicts the journal eccentricity and force coefficients of a five-pad rocker-back TPJB tested at TAMU under a load-between-pad (LBP) configuration. The predictions agree well with the test results provided the model uses actual hot bearing clearances and an empirical characterization of the pivot stiffness. A study follows to determine the effects of pad preload, r¯p = 0.0, 0.27 (test article) and 0.50, as well as the load orientation, LBP and load-on-pad (LOP), on bearing performance with an emphasis on ascertaining the configuration with most damping and stiffness, largest film thickness, and the least drag friction. In the study, a rigid pivot and two flexible pivots are considered throughout. Further examples present the effective contribution of the pads’ mass and mass moment of inertia and film fluid inertia on the bearing force coefficients. To advance results of general character, predictions are shown versus Sommerfeld number (S), a design parameter proportional to shaft speed and decreasing with applied load. Both LBP and LOP configurations show similar performance characteristics; the journal eccentricity increasing with pivot flexibility. For LBP and LOP bearings with 0.27 preload, pivot flexibility decreases dramatically the bearing damping coefficients, in particular at the low end of S, i.e., large loads. The model and predictions aid to better design TPJBs supporting large specific loads.
Aerodynamics and Acoustics of Turning Mid Turbine Frames in a Two-Shaft Test Turbine
Authors: Faustmann C. , Göttlich E.
Published: September 2014

The paper deals with the investigation on the aerodynamics and the acoustics of two different turning mid turbine frames (TMTF) in the two-stage two-spool test turbine located at the Institute for Thermal Turbomachinery and Machine Dynamics (ITTM) of Graz University of Technology. The facility is a continuously operating cold-flow open-circuit plant which is driven by pressurized air. The flow path consists of a transonic turbine stage (HP) followed by a low pressure turbine stage made of a turning mid turbine frame (TMTF) and a counter-rotating low pressure rotor.

The two TMTF setups have been investigated at engine like flow conditions. The first configuration consists of 16 highly 3D-shaped turning struts. The goal of the second design was to reduce the length of the TMTF by 10% without increasing the losses and providing comparable inflow to the LP turbine rotor. This was achieved by applying 3D-contoured endwalls at the hub.

To estimate the pressure loss over the duct aerodynamic measurements are performed at the inlet and the outlet of both turning mid turbine frames by using 5-hole probes (FHP) and total pressure rakes. The FHP-measurements at the inlet of the TMTF were performed in three different ways to obtain the influence of probe positioning and traversing on the results. While the 5-hole probe was traversed only in a sector the rakes were traversed over the full circumference. The comparison between the two turning mid turbine frame setups shows from an aerodynamic point of view that it is possible to reduce the engine weight by designing a 10% shorter TMTF with endwall contouring providing the same pressure loss and comparable inflow conditions for the LP turbine rotor.

Due to the fact that noise becomes more and more an issue additional acoustic measurements were carried out downstream of the low pressure turbine. By comparing the two setups in terms of noise generation the propagating modes due to the HP turbine were found to be the same, while an increase of 10dB in amplitude of the modes related to the LP turbine was found in the 10% shorter setup. This is in good accordance with previous studies, where reducing the distance between stator and rotor increases the emitted sound.
The Design of a Steady Aero Thermal Research Turbine (START) for Studying Secondary Flow Leakages and Airfoil Heat Transfer
Authors: Barringer Michael , Coward Andrew , Clark Ken , Thole Karen A. , Schmitz John , Wagner Joel , Alvin Mary Anne , Burke Patcharin , Dennis Rich
Published: September 2014

This paper describes a new gas turbine research facility that was designed and is currently being constructed with a number of unique features that allow experiments which aim at developing and improving sealing and cooling technologies at a reasonable cost. Experiments will include engine-representative rotating turbine hardware in a continuous, steady-state, high-pressure flow environment. The facility includes a 1.5 stage turbine that will simulate the aerodynamic flow and thermal field interactions in the engine between the hot mainstream gas path and the secondary air flows at relevant corrected operating conditions and scaling parameters. Testing in the new facility is planned to begin in 2014 and will take place in two phases. The first phase is focused on understanding the behavior of inner-stage gap flow leakages in the presence of the main gas path and the rotating blade platform. The second phase is focused on developing and testing novel cooling methods for turbine airfoils, platforms, and disks, ultimately leading to an integrated approach for more effective use of the secondary cooling flow. The uniqueness of this facility includes a continuous duration facility with engine-relevant rotational and axial Reynolds and Mach numbers at the blade inlet.