bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Procedure for the Investigation of Combustion Dynamics in Industrial Gas Turbines: LES, RANS and Thermoacoustics
Authors: Rofi Luca , Campa Giovanni , Anisimov Vyacheslav , Daccá Federico , Bertolotto Edoardo , Gottardo Enrico , Bonzani Federico
Published: August 2015

The necessity for a combustion system to work with premixed flames and its capability to cope with rapid load variations avoiding the occurrence of thermo-acoustic instabilities, has led to investigate the complex dynamic phenomena that occur during combustion. Thanks to numerical simulations it is possible to examine these complex mechanisms getting useful information to optimize the combustion system. The aim of this work is to describe a numerical procedure developed in Ansaldo Energia for the investigation of combustion dynamics occurring in Ansaldo Energia gas turbines.

In this paper, firstly the experimental apparatus of a full scale atmospheric test rig equipped with Ansaldo Energia burner is described. Secondly, the flame behavior is studied by means of a Large Eddy Simulation (LES). Once the LES has reached a statistically stationary state, a forcing is added to the system to compute the Flame Transfer Function (FTF), in terms of amplitude n and delay time τ, related to initial phases of humming. Thirdly, the forced flame simulations are used as the input of an Helmholtz solver to analyze the acoustic behavior of the system, which is then compared to experimental data. Finally, to evaluate the feasibility of a less computationally intensive approach, a RANS simulation of the same configuration is described and the results are transferred to FEM (Finite Element Method) Helmholtz solver: a comparison between the LES approach and the RANS approach is carried out with reference to the experimental data.
A Simple Low-Fidelity Model for Embedded Blade Row Flutter Prediction
Authors: Zhao Fanzhou , Nipkau Jens , Vahdati Mehdi
Published: August 2015

Recent rig tests and CFD predictions have shown that acoustic reflections can play an important part on flutter stability of blades in multi-stage compressors. Moreover, as the use of blisks with very low mechanical damping becomes more common in modern aero-engine designs, accurate prediction of blade aerodynamic damping in a multi-row environment becomes vital. Accurate CFD predictions in such environment require unsteady computation of multi-row full annulus models which poses high demands on both computational time and resources and thus cannot be used routinely in early development stages.

The aim of this work is to develop a simple low-fidelity approach for evaluating flutter stability of embedded rotors in multi-stage compressors. The required phase and amplitude relation between the reflected waves and the outgoing waves at leading/trailing edge of the rotor is calculated based on established theories. In the approach taken here, the phase and amplitude of the reflected waves are prescribed as boundary conditions for the blade row of interest, and flutter analysis of the rotor blade is undertaken based on a single passage single-row approach using a validated 3D unsteady RANS solver. Comparison of predicted rotor aerodynamic damping shows good agreement with that obtained using a high-fidelity full annulus multi-row model and test data. An overall reduction of computational cost of roughly 360 times is achieved by the simplified model. The proposed simplified flutter prediction model provides a much faster way of evaluating the susceptibility to acoustic driven flutter, and allows one to study the effects of parameters such as axial gap, frequency and stator vane angles at early development stages on an engine. It should be emphasised that the aim of this paper is to present a low fidelity prediction model for flutter and not noise, and hence the main emphasis is modelling low nodal diameters as a compressor blade is most likely to flutter in such nodal diameters for 1F mode.
Modeling of Rain Drop Erosion in a Multi-MW Wind Turbine
Authors: Corsini Alessandro , Castorrini Alessio , Morei Enrico , Rispoli Franco , Sciulli Fabrizio , Venturini Paolo
Published: August 2015

The actual strategy in offshore wind energy development is oriented to the progressive increase of the turbine diameter as well as the per unit power. Among many pioneering technological and aerodynamic issues linked to this design trend, the wind velocity at the blade tip region reaches very high values in normal operating conditions (typically between 90 to 110 m/s). In this range of velocity, the rain erosion phenomenon can have a relevant effect on the overall turbine performance in terms of power and energy production (up to 20% loss in case of deeply eroded leading edge). Therefore, as a customary approach erosion related issues are accounted for in the scheduling of the wind turbine maintenance. When offshore, on the other hand, the criticalities inherent to the cost of maintenance and operation monitoring suggest the rain erosion concerns to be tackled at the turbine design stage. In so doing, the use of computational tools to study the erosion phenomenon of wind turbines under severe meteorological conditions could define the base-line approach in the wind turbine blades design and verification.

In this work, the authors present a report on numerical prediction of erosion on a 6 MW HAWT (horizontal axis wind turbine). Two different blade geometries of different aerodynamic loading, have been studied in a view to explore their sensitivity to rain erosion.

The fully 3D simulations are carried out using an Euler-Lagrangian approach. Flow field simulations are carried out with the open-source code OpenFOAM, based on a finite volume approach, using Multiple Reference Frame methodology. Reynolds Averaged Navier-Stokes equations for incompressible steady flow were solved with a k-ε turbulence.

An in-house code (P-Track) is used to compute the rain drops transport and dispersion, adopting the Particle Cloud Tracking approach (PCT), already validated on large industrial turbomachinery.

At the impact on blade, erosion is modelled accounting for the main quantities affecting the phenomenon, which are impact velocity and material properties of the target surface.

Results provide the regions of the two blades more sensitive to erosion, and the effect of the blade geometry on erosion attitude.
Experimental and Numerical Benchmarking of an Improved Meanline Modelling Method for Automotive Turbocharger Centrifugal Compressors
Authors: Harley Peter , Spence Stephen , Filsinger Dietmar , Dietrich Michael , Early Juliana
Published: August 2015

After the development of a new single-zone meanline modelling technique, benchmarking of the technique and the modelling methods used during its development are presented. The new meanline model had been developed using the results of three automotive turbocharger centrifugal compressors, and single passage CFD models based on their geometry.

The target of the current study was to test the new meanline modelling method on two new centrifugal compressor stages, again from the automotive turbocharger variety. Furthermore the single passage CFD modelling method used in the previous study would be again employed here and also benchmarked.

The benchmarking was twofold; firstly test the overall performance prediction accuracy of the single-zone meanline model. Secondly, test the detailed performance estimation of the CFD model using detailed interstage static pressure tappings.

The final component of this study exposed the weaknesses in the current modelling methods used (explicitly during this study). The non-axisymmetric flow field at the leading and trailing edges for the two compressors was measured and is presented here for the complete compressor map, highlighting the distortion relative to the tongue.
Numerical Investigation on the Vibration of Steam Turbine Inlet Valves and the Feedback to the Dynamic Flow Field
Authors: Domnick Clemens Bernhard , Benra Friedrich-Karl , Brillert Dieter , Dohmen Hans Josef , Musch Christian
Published: August 2015

The power output of steam turbines is controlled by steam turbine inlet valves. These valves have a large flow capacity and dissipate in throttled operation a huge amount of energy. Due to that, high dynamic forces occur in the valve which can cause undesired valve vibrations.

In this paper, the structural dynamics of a valve are analysed. The dynamic steam forces obtained by previous computational fluid dynamic (CFD) calculations at different operating points are impressed on the structural dynamic finite element model (FEM) of the valve. Due to frictional forces at the piston rings and contact effects at the bushings of the valve plug and the valve stem the structural dynamic FEM is highly nonlinear and has to be solved in the time domain.

Prior to the actual investigation grid and time step studies are carried out. Also the effect of the temperature distribution within the valve stem is discussed and the influence of the valve actuator on the vibrations is analysed.

In the first step, the vibrations generated by the fluid forces are investigated. The effects of the piston rings on the structural dynamics are discussed. It is found, that the piston rings are able to reduce the vibration significantly by frictional damping. In the second step, the effect of the moving valve plug on the dynamic flow in the valve is analysed. The time dependent displacement of the valve is transferred to CFD calculations using deformable meshes. With this one way coupling method the response of the flow to the vibrations is analysed.
Film Cooling Effectiveness of Transonic Turbine Vane Suction Side With Compound-Angle Shaped Hole Configuration
Authors: Yang Shang-Feng , Han Je-Chin , MirzaMoghadam Alexander , Riahi Ardeshir
Published: August 2015

This paper studies the effect of transonic flow velocity on local film cooling effectiveness distribution of turbine vane suction side, experimentally. A conduction-free Pressure Sensitive Paint (PSP) method is used to determine the local film cooling effectiveness. Tests were performed in a five-vane annular cascade at Texas A&M Turbomachinery laboratory blow-down flow loop facility. The exit Mach numbers are controlled to be 0.7, 0.9, and 1.1, from subsonic to transonic flow conditions. Three foreign gases N2, CO2 and Argon/SF6 mixture are selected to study the effects of three coolant-to-mainstream density ratios, 1.0, 1.5, and 2.0 on film cooling. Four averaged coolant blowing ratios in the range, 0.7, 1.0, 1.3 and 1.6 are investigated. The test vane features 3 rows of radial-angle cylindrical holes around the leading edge, and 2 rows of compound-angle shaped holes on the suction side. Results suggest that the PSP technique is capable of producing clear and detailed film cooling effectiveness contours at transonic condition. The effects of coolant to mainstream blowing ratio, density ratio, and exit Mach number on the vane suction-surface film cooling distribution are obtained, and the consequence results are presented and explained in this investigation.
A Neural Network Identification Technique for a Foil-Air Bearing and its Application to Unbalance Response Analysis
Authors: Hassan Mohd Firdaus Bin , Bonello Philip
Published: August 2015

This paper proposes and studies the non-parametric system identification of a foil-air bearing (FAB) and its application to the frequency-domain nonlinear analysis of a foil-air bearing rotor system. This research is motivated by two advantages: (i) it removes computational limitations by replacing the air film and foil structure state equations by a displacement/force relationship; (ii) if the identification is based on empirical data, it can capture complications that cannot be easily modelled. A numerical model of the FAB is identified using a recurrent neural network (RNN). The training data sets are taken from the simultaneous time domain solution of the air film, foil and rotor equations. The RNN FAB model identified at a single speed is then validated over a range of speeds in two ways: (i) by subjecting it to several sets of input-output data that are different from those used in training; (ii) by using it in the harmonic balance (HB) solution process for the unbalance response of a rotor-bearing system. In either case, the test results using the identified model show good agreement with the exact results obtained using the air film and foil equations, demonstrating the great potential of this method, in the absence of self-excitation effects.
Ultra Lean Combustion Characterization in a Pilot-Scale Gas Turbine Burner Using Image Processing Techniques
Authors: De Giorgi Maria Grazia , Sciolti Aldebara , Campilongo Stefano , Ficarella Antonio
Published: August 2015

The aim of the present investigation is the characterization of the behavior of a lean partially-premixed liquid fuel gas turbine near lean blowout limit.

At this combustion regime the onset of instability will occur with negative impacts on combustion efficiency.

The identification of the instability occurrence permits an efficient flame control adjusting the combustion parameters (as fuel or air mass flow, temperature, pressure, etc.) to stabilize the flame or designing opportunely flame control system.

High-speed images of the flame under stable and near blowout condition were captured in conjunction with simultaneous optical data in order to better understand the phenomenology of the flame blowout process and the onset of instability.

In particular the experimental characterization was performed through a High Speed Digital Camera, an Infrared camera and a Photomultiplier Tube (PMT) in association with the use of optical filter (OH*). The data collected with these instrumentations produce useful features for the development of an efficient tool for the flame control in industrial and aeronautical burners. The images acquired by the different cameras were processed considering the luminosity signal of each pixel and evaluating the frequency behavior, the variations of amplitude of the signals and some other descriptive parameters able to define the regime of the flame. Spectral analysis and Wavelet transform of pixel intensities of flame images were used and entropy and energy contents were evaluated. The spatial maps of the different spectral and statistical parameters were shown at different fuel/air equivalence ratio. The OH* emissions data measured by the PMT were processed and compared with the data obtained from the images processing.
Investigation on the Impact of Circumferential Grooves on Aerodynamic Centrifugal Compressor Performance
Authors: Bareiß Simon , Vogt Damian M. , Chebli Elias
Published: August 2015

Casing treatments are widely used in compressors for increasing range, stability and aerodynamic performance. However, applications in centrifugal compressors, as commonly used in turbochargers, are rare and mostly applied at the inlet region in terms of bleed slots. This paper presents the application of circumferential grooves, which are known to increase stability and performance in axial compressors, to the rear part of the impeller shroud casing in a centrifugal compressor. Experimental and numerical investigations of three different configurations have been performed and compared with the initial geometry. Experiments were conducted on a hot gas test facility where static pressure and temperature measurements up- and downstream of the compressor were acquired. The results indicate only small changes in operating range except for one speedline, where a considerable improvement is observed. Efficiency remains nearly unaffected for all configurations whereas the pressure ratio is increased at some operating points. For detailed examination of the compressor flow field, CFD analyses were conducted using steady-state RANS calculations. Structured meshes with node to node connections were used to suspend any possible influences stemming from interfaces in regions of interest. Validation with test data yields good agreement concerning choke margin and gradient trends. CFD results confirm that the investigated configurations of circumferential grooves have only small impact on efficiency and pressure ratio. Investigations on the mechanism which balances the additional losses due to increased groove surface friction and increases pressure rise are presented.
Influence of Pre-Flame and Post-Flame Mixing on NOx-Formation in a Reacting Premixed Jet in Hot Cross Flow
Authors: Ahrens Denise , Kolb Michael , Hirsch Christoph , Sattelmayer Thomas
Published: August 2015

Axial staging in premixed gas turbine combustors is a promising option for the increase of firing temperature without NOx-penalty and for the improvement of turndown ratio, which is limited by the onset of CO-emissions. The configuration of greatest interest is the combination of state of the art premixed combustion in the primary stage with secondary injectors near the turbine inlet, which feed additional jets of premixed combustible mixture into the hot cross flow. Regarding NOx this configuration is particularly beneficial (1) if the overall mixing quality in the first stage is limited, (2) if the difference between primary zone flame temperature and turbine inlet temperature due to air addition along the combustor is large and (3) if a high degree of mixing in the second stage is achieved. The potential of this promising combustion concept was investigated in a large scale atmospheric test rig. For the study presented below scaling of the second stage according to Karlovitz number similarity was chosen. This leads to smaller jet diameters and higher injection velocities compared to scaling based on Damköhler number applied in an earlier study. The impact of the higher velocities at the injector outlet on the flow field, on the lift-off height of the flame and on NOx-formation is analyzed. A chemical network model is presented, which illustrates the effects of pre-flame and post-flame mixing on NOx-formation under atmospheric and high pressure conditions. In addition this model is used to study the interactions of chemistry with mixing between the reacting jet and cross flow. On the basis of atmospheric testing and reactor modeling, predictions for engine pressure are made assuming similar lift-off as well as pre- and post-flame mixing. These results are further analyzed regarding the NOx-reduction potential at different equivalence ratios and residence times. Finally, it is discussed under which conditions the investigated configuration can be beneficially applied to reduce NOx-emissions of real engines.
Impact of Passive Tip-Injection on Tip-Leakage Flow in Axial Low Pressure Turbine Stage
Authors: Ghaffari Pouya , Willinger Reinhard , Bauinger Sabine , Marn Andreas
Published: August 2015

In addition to geometrical modifications of the blade tip for reducing tip-leakage mass flow rate the method of passive tip-injection serves as an aerodynamic resistance towards the tip-leakage flow. The impact of this method has been investigated thoroughly at unshrouded blades in linear cascades. Furthermore combinations of shrouded blades with passive tip-injection have been investigated analytically as well as via numerical simulations for incompressible flow in linear cascades. The objective of this paper is to consider a real uncooled low pressure turbine stage with shrouded blades and to investigate the effect of passive tip-injection on various operational characteristics. CFD calculations have been carried out in a rotational frame taking into consideration compressible flow and serve for evaluating the method of passive tip-injection in the given turbine stage. Experimental data obtained from the machine without tip-injection serve as boundary conditions for the CFD calculations.
Full Coverage Shaped Hole Film Cooling in an Accelerating Boundary Layer With High Free-Stream Turbulence
Authors: Kingery J. E. , Ames F. E.
Published: August 2015

Full coverage shaped-hole film cooling and downstream heat transfer measurements have been acquired in the accelerating flows over a large cylindrical leading edge test surface. The shaped holes had an 8° lateral expansion angled at 30° to the surface with spanwise and streamwise spacings of 3 diameters. Measurements were conducted at four blowing ratios, two Reynolds numbers and six well documented turbulence conditions. Film cooling measurements were acquired over a four to one range in blowing ratio at the lower Reynolds number and at the two lower blowing ratios for the higher Reynolds number. The film cooling measurements were acquired at a coolant to free-stream density ratio of approximately 1.04. The flows were subjected to a low turbulence condition (Tu = 0.7%), two levels of turbulence for a smaller sized grid (Tu = 3.5%, and 7.9%), one turbulence level for a larger grid (8.1%), and two levels of turbulence generated using a mock aero-combustor (Tu = 9.3% and 13.7%). Turbulence level is shown to have a significant influence in mixing away film cooling coverage progressively as the flow develops in the streamwise direction. Effectiveness levels for the aero-combustor turbulence condition are reduced to as low as 20% of low turbulence values by the furthest downstream region. The film cooling discharge is located close to the leading edge with very thin and accelerating upstream boundary layers. Film cooling data at the lower Reynolds number, show that transitional flows have significantly improved effectiveness levels compared with turbulent flows. Downstream effectiveness levels are very similar to slot film cooling data taken at the same coolant flow rates over the same cylindrical test surface. However, slots perform significantly better in the near discharge region. These data are expected to be very useful in grounding computational predictions of full coverage shaped hole film cooling with elevated turbulence levels and acceleration. IR measurements were performed for the two lowest turbulence levels to document the spanwise variation in film cooling effectiveness and heat transfer.
CAD-Based Adjoint Shape Optimisation of a One-Stage Turbine With Geometric Constraints
Authors: Xu Shenren , Radford David , Meyer Marcus , Müller Jens-Dominik
Published: August 2015

An extension of the CAD-based parametrisation termed ‘NURBS-based parametrisation with complex constraints’, or NsPCC, is developed and applied to the aerodynamic shape optimisition of a one-stage high pressure turbine. NsPCC uses a test-point approach to impose various geometric constraints such as continuity, thickness and trailing edge radius constraint.

To perform the shape optimisation using NsPCC, The surface sensitivity is first computed efficiently using a discrete adjoint solver. The displacements of the control points of the NURBS patches are used as the design variables and linked to the surface sensitivity through consistent application of Automatic Differentiation. A robust mesh deformation based on linear elasticity and further enhanced with sliding mesh capability is used to deform the mesh at each design step. Finally, the optimised rotor shape is exported as a STEP file. The method is demonstrated on a turbine stage where isentropic efficiency is improved by over 0.4% with both the inlet capacity and rotor reaction ratio deviation below the prescribed thresholds. Satisfaction of the G1 continuity, thickness and trailing radius constraints is verified.
Characterization of Turbine Vane Roughness Variations Using Self-Organizing Maps
Authors: McClain Stephen T.
Published: August 2015

Turbine vane roughness and blade roughness produce adverse effects including increased aerodynamic losses and increased heat transfer into the turbine components. Consequently, measuring levels of roughness and characterizing the roughness development as a function of operating time and conditions are critical to developing proper engine maintenance and replacement schedules. Because of real blade and vane three-dimensional geometries, historical roughness studies have employed two-dimensional traverses of surface elevation measurements performed at predetermined blade locations using stylus-based profilometry. While laser-based and white-light surface metrology has advanced considerably in the past decades, capturing the mean component geometry at the resolution required to characterize the roughness has proven to be difficult given the high surface curvature and typical surface reflectivities. Even on two-dimensional cascade vanes, local regions of high surface curvature result in situations where characterizing the nature of surface roughness is challenging. In this study, a single-vane cascade flow tunnel based on the VKI transonic vane shape was constructed to create deposition-like roughness. The roughness was generated using textured paint sprayed into the inlet plenum of the tunnel. The vane was then scanned using a NextEngine 3D Scanner HD laser scanning system. A combined self-organizing map and multi-dimensional statistics approach, which was developed by McClain and Kreeger (2013) for characterizing airframe ice roughness, is employed to represent the mean blade profile, to unwrap the surface topography, and to evaluate the roughness characteristics as a function of surface distance from the leading edge. The approach enables new insights into the variations of roughness parameters along the surface of engine components and test models. Finally, difficulties in extending the approach to real engine components with twist and other three-dimensional features are discussed.
Experimental and Numerical Investigation of the Nonlinear Vibrational Behavior of Steam Turbine Last Stage Blades With Friction Bolt Damping Elements
Authors: Drozdowski R. , Völker L. , Häfele M. , Vogt D. M.
Published: August 2015

Low-pressure last stage blades of industrial steam turbines are subjected to high dynamic loading. Especially in variable speed applications resonant blade vibration cannot be avoided. Thus, the aim of the blade layout is to reach a robust design that can cover high vibrational amplitudes while still keeping good efficiency. An effective way to keep vibration amplitudes low is the introduction of friction damping elements to the blades. In this paper the structural behavior of a low-pressure last stage blade coupled by friction bolt damping elements is described by means of linear and nonlinear Finite Element Method. Special focus is put on the nonlinear effects of the contact between blade and damping element to investigate the frictional damping performance of the system. The obtained numerical results are validated by strain gauge and tip timing measurements in a full scale test turbine under real steam conditions at the Institute of Thermal Turbomachinery and Machinery Laboratory of the University of Stuttgart.
Numerical Investigation of the Non-Axisymmetric End Wall Application to the White Cascade
Authors: Li Deying , Chen Huanlong , Song Yanping , Cui Ke
Published: August 2015

A profiling method, in terms of the trigonometric function and considering about the different axial location of the non-axisymmetric end wall warping, is developed firstly. The axial and the circumferential location of the end wall warping are defined by the cosine function and the sine function respectively.

To investigate the effects of the non-axisymmetric end wall on the flow property and the steam condensation, the profiling method is applied to the aft-loaded White cascade with the revised nucleation model of the two-phase wet steam flow. The results show that it has very little influence on the performance while the non-axisymmetric end wall warping is closing to the leading edge. If the non-axisymmetric end wall warping locates nearby the trailing edge, the aerodynamic loss increases significantly with a sharp flow separation on the corner of the suction side. While the crest is in the middle of the axial chord, the aerodynamic loss nearby the end wall decreases about 2.0%, implying a well improvement in the aerodynamic performance. Besides, the steam condensation nearby the end wall is restrained significantly while the non-axisymmetric end wall warping is in the middle axial chord or closing the trailing edge.

The proper designed non-axisymmetric end wall, which is able to increase the pressure of the zone with the wet steam nucleation, is beneficial to improve the aerodynamic performance and control the steam condensation in the passage of the low pressure steam turbine.
Experimental Investigation on the Vibration Tuning of a Beam With Shape Memory Alloy
Authors: Ma Yanhong , Wang Mingzhang , Yang Xin , Zhang Dayi , Hong Jie
Published: August 2015

The paper describes the experimental investigation on a smart beam with variable stiffness made of shape memory alloy (SMA). Quasi static experiments were performed on Ti50Ni41Cu9 SMA samples in order to obtain the mechanical characteristics including the Young’s modulus and the loss factor. The test results showed, Ti50Ni41Cu9 SMA exhibited excellent damping performance in Martensite, and the storage modulus varied significantly while SMA transferred from Martensite to Austenitic. The dynamic parameters of SMA were affected by temperature and strain amplitude.

Based on the mechanical characteristics of SMA tested, SMA was applied in the vibration tuning of a beam through experiments. The steady and transient response was suppressed using an effective control strategy according to the variable stiffness control theory. The steady experiments revealed that the first three order natural frequencies could be adjusted significantly by heating/cooling the SMA parts, for example the first order natural frequency varied from 60Hz to 100Hz. The temperature changing rate affected the response amplitude a lot and showed some interesting phenomenon in frequency domain. The transient experiments exhibited that the resonant response was controlled efficiently by changing the temperature of SMA, and the influence laws of the stiffness varying rate on the nonlinear dynamic characteristics was obtained.
Active Flow Separation Control Using Endwall Vortex Generator Jets in Highly Loaded Compressor Cascades
Authors: Feng Yanyan , Song Yanping , Chen Fu , Liu Huaping
Published: August 2015

An active flow control technique of endwall vortex generator jets (VGJs) was used in two kinds of highly loaded compressor cascades. Numerical investigations were carried out on a NACA 65 profile with a large camber angle at low subsonic and high subsonic speeds, and a CDA profile at high subsonic speed respectively. The results indicate that the endwall VGJs can restrain flow separation effectively by reenergizing the boundary layer fluid and resisting the transverse movement of endwall secondary flow. At Mach number 0.23, the results of the jet blowing ratio study illustrate that the increasing jet velocity shows noteworthy potential to improve the cascade aerodynamic performance. The double jets structures were investigated yet gains weaker beneficial effects than single jet. It is probably attributed to the complex flow structure, leading to strong disturbance and large-scale mixing loss. Under −5°, 0° and +5° angles of attack, the loss coefficients are maximally reduced by 4.1%, 9.5% and 17.3% respectively. Under high subsonic conditions, the endwall VGJs still has significantly positive effects on NACA 65 profile. Considering the small separation region of CDA, the loss coefficients increase slightly although the flow separation is weakened further by VGJ.
Low-Pressure Turbine Cascade Performance Calculations With Incidence Variation and Periodic Unsteady Inflow Conditions
Authors: Marconcini Michele , Pacciani Roberto , Arnone Andrea , Bertini Francesco
Published: August 2015

The performance of a Low-Pressure Turbine (LPT) cascade were investigated under both steady and periodic unsteady inflow boundary conditions with different Reynolds numbers and a reduced frequency representative of real LPT working conditions. A sensitivity analysis to the variation of the inlet flow angle was performed to assess the performance during off-design operation.

The numerical framework is based on a steady/unsteady Reynolds Averaged Navier-Stokes (RANS/URANS) flow solver which includes some state-of-the-art transition-sensitive turbulence closures. Boundary conditions for the time-accurate computations upstream of the cascade were derived from the experimental characterization of a moving bar system used to generate the wake periodic perturbations.

The computed performance of the cascade, as a function of Reynolds number and incidence angle variation, is discussed in comparison with experimental data. Steady and unsteady boundary layer quantities are also compared with measurements in order to gain more insights into the loss generation process and to better support the numerical results. Such an assessment proves that the proposed procedure is adequate for the analysis of the off-design behavior of cascades operating in low pressure turbine conditions.
Experimental and Numerical Calculation of Turbulent Timescales at the Exit of an Engine Representative Combustor Simulator
Authors: Koupper Charlie , Bacci Tommaso , Facchini Bruno , Picchi Alessio , Tarchi Lorenzo , Gicquel Laurent , Duchaine Florent , Bonneau Guillaume
Published: August 2015

To deepen the knowledge of the interaction between modern lean burn combustors and high pressure turbines, a non-reactive real scale annular trisector Combustor Simulator (CS) has been assembled at University of Florence, with the goal of investigating and characterizing the combustor aerothermal field as well as the hot streak transport towards the high pressure vanes. To generate hot streaks and simulate lean burn combustor behaviors, the rig is equipped with axial swirlers fed by a main air flow stream that is heated up to 531 K, while liners with effusion cooling holes are fed by air at ambient temperature. Detailed experimental investigations are then performed with the aim of characterizing the turbulence quantities at the exit of the combustion module, and specifically evaluating an integral scale of turbulence. To do so, an automatic traverse system is mounted at the exit of the CS and equipped to perform Hot Wire Anemometry (HWA) measurements. In this paper, two-point correlations are computed from the time signal of the axial velocity giving access to an evaluation of the turbulence timescales at each measurement point. For assessment of the advanced numerical method that is Large Eddy Simulation (LES), the same methodology is applied to a LES prediction of the CS. Although comparisons seem relevant and easily accessible, both approaches and contexts have fundamental differences: mostly in terms of duration of the signals acquired experimentally and numerically but also with potentially different acquisition frequencies. In the exercise that aims at comparing high-order statistics and diagnostics, the specificity of comparing experimental and numerical results is comprehensively discussed. Attention is given to the importance of the acquisition frequency, intrinsic bias of having a short duration signal and influence of the investigating windows. For an adequate evaluation of the turbulent time scales, it is found that comparing experiments and numerics for high Reynolds number flows inferring small-scale phenomena requires to obey a set of rules, otherwise important errors can be made. If adequately processed, LES and HWA are found to agree well indicating the potential of LES for such problems.
An Experimental and Numerical Assessment of Airfoil Polars for Use in Darrieus Wind Turbines: Part 2 — Post-Stall Data Extrapolation Methods
Authors: Bianchini Alessandro , Balduzzi Francesco , Rainbird John M. , Peiro Joaquim , Graham J. Michael R. , Ferrara Giovanni , Ferrari Lorenzo
Published: August 2015

Accurate post-stall airfoil data extending to a full range of incidences between −180° to +180° is important to the analysis of Darrieus vertical-axis wind turbines (VAWTs) since the blades experience a wide range of angles of attack, particularly at the low tip-speed ratios encountered during startup.

Due to the scarcity of existing data extending much past stall, and the difficulties associated with obtaining post-stall data by experimental or numerical means, wide use is made of simple models of post-stall lift and drag coefficients in wind turbine modeling (through, for example, BEM codes). Most of these models assume post-stall performance to be virtually independent of profile shape.

In this study, wind tunnel tests were carried out on a standard NACA0018 airfoil and a NACA 0018 conformally transformed to mimic the “virtual camber” effect imparted on a blade in a VAWT with a chord-to-radius ratio c/R of 0.25.

Unsteady CFD results were taken for the same airfoils both at stationary angles of attack and at angles of attack resulting from a slow VAWT-like motion in an oncoming flow, the latter to better replicate the transient conditions experienced by VAWT blades.

Excellent agreement was obtained between the wind tunnel tests and the CFD computations for both the symmetrical and cambered airfoils. Results for both airfoils also compare favorably to earlier studies of similar profiles. Finally, the suitability of different models for post-stall airfoil performance extrapolation, including those of Viterna-Corrigan, Montgomerie and Kirke, was analyzed and discussed.
Combined Acoustic Damping-Cooling System for Operational Flexibility of GT26/GT24 Reheat Combustors
Authors: Schuermans Bruno , Bothien Mirko , Maurer Michael , Bunkute Birute
Published: August 2015

In the development process of gas turbine combustion chambers, finding countermeasures for thermoacoustically induced pressure pulsations is a major focus. This paper presents a novel system consisting of a multi-layered and multi-functional high frequency damping and cooling structure that is implemented on the sequential burner front panel of the GT26/GT24 gas turbines. The device features multiple single Helmholtz dampers and an advanced convective near wall cooling system to improve the cooling capability and to reduce the cooling mass flow and thereby reducing NOx emissions. The acoustic properties of the dampers and their placement have been defined as function of the identified acoustic mode shapes. The latter is very important since the dampers are designed to counteract screech tones that have acoustic wave lengths of the order of one burner front face width. In order to identify the acoustic mode shapes, multiple dynamics pressure measurements are applied in the full scale engine. The near-wall cooled damping front panel design represents a new technology which has been developed and successfully validated at engine level in fuel gas and oil operation. The restrictions of the stable operating range due to pulsations are completely eliminated resulting in an increase of operational flexibility and lifetime. In addition to a thorough treatment of the damper’s acoustic performance, information on the improved near wall cooling scheme is given in the paper, too.
The Impact of Pad Flexibility on the Rotordynamic Coefficients of Tilting-Pad Journal Bearings
Authors: Gaines Jennifer E. , Childs Dara W.
Published: August 2015

Static and dynamic load tests were performed on a three-pad, rocker-pivot, tilting-pad journal bearing (TPJB) with three interchangeable pad configurations, each with measurably different pad flexibilities. Measured dynamic-stiffness data for the bearing were readily fitted by a frequency-independent, constant-coefficient [K][C][M] model.

The test bearing had a 101 .74 mm diameter with L/D = 0.6 Tests were conducted over the speed range of 6 to 12 krpm, with unit loads varying from .172 to 1.724 MPa. An ISO VG 46 lubricant was used as the test fluid.

Pad flexibility was characterized as the change in the pad’s bending stiffness or the change in pad thickness. A finite-element model was created to predict the structural bending stiffness of each pad configuration, showing a significant pad-flexibility increase as pad thickness decreased.

To examine the effect of pad flexibility on the rotordynamic coefficients, the measured results were compared across pad configurations and showed that the pad-flexibility increase reduced the direct damping coefficients by 12–20%. As pad flexibility increased, the direct stiffness coefficients could increase or decrease, depending on the unit load. They varied from an increase of 12% at low unit loads to a decrease of 3% at high unit loads.

Results show that the pad’s structural bending stiffness or flexibility is important when predicting the bearing’s dynamic performance. Damping is consistently over-predicted when neglecting pad flexibility.

A non-dimensional pad flexibility parameter αflex was developed. It related the average deflection across the pad surface to the pad’s arc length and was to relate the pad flexibility of multiple bearings of different sizes.

A bearing code was used to predict the percent change in direct damping coefficients for rigid-pad/flexible-pivot and flexible-pad/flexible-pivot models for a surface speed of 54 m/s and a unit load of 783 kPa for the three pad configurations tested here plus five additional tested bearings from the literature. For the minimum-pad-thickness configuration tested here, the code predicted a 20% drop in predicted Cxx (off-load-axis direct damping) when comparing a model that included pad flexibility with a model that neglected pad flexibility. In terms of αflex, the two thinnest pad configurations tested here are quite flexible compared to both TPJB’s pads used in industry and previously-tested TPJB pads.
Effects of Vortex Generator Jet on Flow Separations in Bowed Compressor Cascades
Authors: Li Longting , Song Yanping , Chen Fu , Liu Huaping
Published: August 2015

A combined flow control was performed by using a vortex generator jet (VGJ) in bowed compressor cascades. Loss calculations were done over a wide range of parameters including jet mass flow, jet direction and jet location, to determine their effectiveness in controlling flow separation. The topology theory is introduced in this paper to analyze the changes of separation structures in bowed cascades caused by VGJ. The results in this paper indicate that not all kinds of VGJs made up of jet parameters in any arrangement can improve the flow conditions of bowed cascades, but that there are optimal combinations of jet parameters attaining the largest total pressure loss reduction of up to 2.7% and 9.1% for positively and negatively bowed blades, respectively. The VGJ comprised of optimal jet parameters make the flow separation on the suction side of the positively bowed blade basically disappear. The VGJ in the optimal case renders flow separation on the suction side of the negatively bowed blade be relieved so that a separation line on it disappears accordingly. In addition, the mechanisms of VGJs in cases which increase the loss in bowed cascades were chosen to be analyzed in detail. The consequences show that VGJ, in that case, causes that two passage vortices are produced in the positively bowed blade, so the flow field is deteriorated. For the negatively bowed blade, the VGJ leads to the result that the flow separation on the suction side is severer and the separation nodal point corresponding to the corner vortex is transformed from a degenerately nodal point to a spiral point which means that the loss caused by the corner vortex increases.
Part Load Operation of a Multiple-Injection Dry Low NOx Combustor on Hydrogen-Rich Syngas Fuel in an IGCC Pilot Plant
Authors: Asai Tomohiro , Dodo Satoschi , Karishuku Mitsuhiro , Yagi Nobuo , Akiyama Yasuhiro , Hayashi Akinori
Published: August 2015

The successful development of coal-based integrated gasification combined cycle (IGCC) technology requires gas turbines capable of achieving the dry low-nitrogen oxides (NOx) combustion of hydrogen-rich syngas for low emissions and high plant efficiency. Mitsubishi Hitachi Power Systems, Ltd. (MHPS) has been developing a “multiple-injection combustor” to achieve the dry low-NOx combustion of hydrogen-rich syngas. This study suggests an advanced fuel staging comprising a hybrid partial combustion mode to improve the combustor’s part load performance. The purposes of this paper are to present the test results of the combustor with the advanced staging on a syngas fuel in an IGCC pilot plant, and to evaluate its performance. The syngas fuel produced in the plant contained approximately 50% carbon monoxide, 20% hydrogen, and 20% nitrogen by volume. In the test, the advanced staging reduced the maximum NOx at part load to 44 ppm (at 15% oxygen) compared with the initial staging with a maximum NOx of 75 ppm, and attained higher combustion efficiency above 98.7% over the part load range than the initial staging with combustion efficiency above 97.1%. In conclusion, the advanced staging improved the part load performance by achieving lower NOx emissions and higher combustion efficiency.
Experimental Investigation of the Combustion Characteristics of a Double-Staged FLOX®-Based Combustor on an Atmospheric and a Micro Gas Turbine Test Rig
Authors: Zanger Jan , Monz Thomas , Aigner Manfred
Published: August 2015

To establish micro gas turbine (MGT) systems in a wide field of CHP applications, innovative combustion concepts are needed to meet the demands for low exhaust gas emissions, high efficiency and reliability as well as high fuel flexibility. A promising technology for future MGT combustion is the FLOX® concept. The goal of the presented work is to prove the feasibility of a double–staged, FLOX®–based MGT combustion system on a MGT test rig. The paper reports a reliable operating behavior of a Turbec T100 MGT in combination with the new FLOX®–based combustion chamber utilizing natural gas. The measured exhaust gas emissions are compared for different configurations of the combustion chamber and the standard Turbec system. It is shown that the carbon monoxide emissions are reduced whereas the nitrogen oxide emissions exceed the emission levels of the standard MGT burner. However, they still fall far below the German legal limits. For helping to interpret the results of the MGT combustion system, the double–staged combustor is compared to a single–staged FLOX®burner on basis of atmospheric measurements. Here, it is shown that the margin to lean blow–off is substantially increased by the fuel staging. Moreover, it is demonstrated that the exhaust gas emissions of the double–staged combustor could be kept at a similar very low level by applying the staging. Additionally, the overall reaction regions are reported by OH* chemiluminescence imaging as a function of burner air number. Based on this atmospheric study the transfer to MGT conditions is made and appropriate measures are derived to optimize the exhaust gas emissions of the MGT FLOX® combustion system.
Model and Method for a Time-Efficient Analysis of Lateral Drillstring Dynamics
Authors: Gorelik Ilja , Neubauer Marcus , Wallaschek Jörg , Höhn Oliver
Published: August 2015

The lumped parameter model and numerical method proposed in this paper aim at gaining a better understanding of the mechanisms leading to harmful lateral drillstring vibrations in inclined boreholes. The shooting method is applied to the equations of motion in order to skip transients and to arrive quickly at a steady state solution. In combination with a sequential continuation technique, parameter maps are generated that show regions where harmful vibrations can be avoided. Comparisons to a finite element model show that the steady state is predicted accurately, while consuming only a fraction of the computational time. The proposed model is experimentally validated on a test rig. Special emphasis is put upon the evaluation of contact forces and the frequency content of the signals. The presented investigations create the basis for real-time analysis of drillstring dynamics and can be used to give recommendations to adjust operational parameters.