bluuebunny/crossref_metadata_2025 huggingface dataset:
Sliding Controller Design for Aero-Engines With the Rate Limitation of Actuators
Authors: Yang Shubo , Wang Xi
Published: August 2017

With the relevant theories fully developed, sliding mode control (SMC), a kind of nonlinear control strategy having particularly strong robustness and disturbance rejection properties, has been applied in a considerable number of fields, such as robotic manipulator control, power generation control in wind turbines, robust stepper motor control, etc. For aero engines, remarkable progress of adopting SMC has been made. For instance, Richter has published his research of limit management in aircraft engine controls which suggests that replacing the linear regulators with sliding controllers can overcome the obstacle of traditional min-max approach.

It is revealed from publication that researchers who design sliding controller for aero engines have made every effort to focus on the sliding surface and control law of SMC while they seldom paid attention to the constraints in actuators, such as saturation and rate limitation. In practical engineering, the performance of the ideal controller is infeasible under the situation that unavoidable constraints exist. Although the actuator saturation can be avoided by introducing a velocity form controller, rate limitation can still degenerate the control performance severely.

In this paper, therefore, the design of a sliding controller for aero engines with rate limitation is discussed. A speed tracking problem is described based on the engine model simplified from a nonlinear system to a piecewise linear system at selected equilibrium points. A sliding surface is defined as the generalized tracking error, and a SMC law is designed with Lyapunov analysis of the closed loop system. Simulation results verify the stability of the closed-loop system, and show that the proposed sliding controller is capable of regulating a turbofan engine for large thrust commands in a stable fashion with proper tracking performance, which can mitigate the negative effect of actuator rate limitation.
Investigation of the Influence of Leakages on Non Axisymmetric Endwall Contouring Applied on 3D Steam Turbine Airfoils
Authors: Zimmermann Tobias W. , Wirsum Manfred , Fowler Andrew , Patel Kush
Published: August 2017

Tangential endwall contouring is intended to improve the blading efficiency in turbomachinery. The present paper focusses on the influence of leakage flows on the performance of non-axisymmetric endwall contouring.

All tests were conducted on a 2 stage axial turbine test rig at the Institute of Power Plant Technology, Steam and Gas Turbines (IKDG) of RWTH Aachen University. The test rig is driven with air. Two sealing setups are applied to create two different leakage mass flows. Four operating points are investigated that represent the design point as well as over load and part load conditions.

The endwall contouring is applied on both hub and casing sides. Three configurations are compared. A baseline design without endwall contouring, contoured stator vanes and non-contoured rotor blades as well as contoured vanes and blades.

At first, all configurations are investigated with a negligible leakage flow rate at the casing side. The results show that the vane contoured configuration performs best in stage 1 while the fully contoured set-up loses in efficiency for the design point and in part load compared to baseline. This trend is flipped in stage 2 as the full contoured version performs best and the vane contoured configuration loses significantly. This finding is suggesting that endwall contouring has the potential to increase the efficiency.

The second focus is put on the interaction of endwall contouring and leakage flow. These investigations show that neither the vane contoured nor the fully contoured set up show an increased efficiency at any operating point. The trends within the first stage are similar to the measurements with the low amount of leakage flow. In the second stage both contouring designs perform worse than the baseline leading to the assumption that the change in efficiency is mainly caused by the re-entering leakage mass flow upstream the contouring and not by the flow that is sucked into the cavities in front of the rotor contouring.
Analysis of Aerothermal Characteristics of Surface Micro-Structures
Authors: Kapsis M. , He L.
Published: August 2017

Recent advances in manufacturing technologies, such as additive manufacturing, have raised the potential of choosing surface finish pattern as a design parameter. Hence, understanding and prediction of aerothermal effects of machined micro-structures (‘machined roughness’) would be of great interest. So far, however, roughness has been largely considered as a stochastic attribute and empirically modelled. A relevant question is: if and how would shape of the machined roughness elements matter at such fine scales?

In this paper, a systematic computational study has been carried out on the aerothermal behaviour and impact of some discrete micro-structures. Two shapes of micro-structure configurations are considered: hemispherical and rectangular elements for a Reynolds number range typical for those roughness elements (Re<5000). Several validation cases are studied as well as the turbulence modelling and grid sensitivities are examined to ensure the consistence of the numerical results. Furthermore, LES analyses are performed to contrast the behaviour in a well-established turbulent flow regime to a transitional one.

The present results reveal a distinctive common flow pattern change (from an ‘open separation’ to a ‘reattached separation’) associated with a drastic change of drag/loss correlation from a low to a high loss regime. The results also indicate a clear dependence of drag and heat transfer characteristics on the element pattern and orientation relative to the flow. The distinctive performance correlations with Reynolds number can be affected considerably by the element shape, for both a transitional and a turbulent flow regime. The results also consistently illustrate that conventional empirical stochastic roughness parameters would be unable to predict these trends.
Introducing a New Test Rig for Film Cooling Measurements With Realistic Hole Inflow Conditions
Authors: Fraas Marc , Glasenapp Tobias , Schulz Achmed , Bauer Hans-Jörg
Published: August 2017

Further improvements in film cooling require an in-depth understanding of the influencing parameters. Therefore, a new test rig has been designed and commissioned for the assessment of novel film cooling holes under realistic conditions.

The test rig is designed for generic film cooling studies. External hot gas flow as well as internal coolant passage flow are simulated by two individual flow channels connected to each other by the cooling holes. Based on a similarity analysis, the geometry of the test rig is scaled up by a factor of about 20. It furthermore offers the possibility to conduct experiments at high density ratios and realistic approach flow conditions at both cooling hole exit and inlet. The operational range of the new test rig is presented and compared to real engine conditions. It is shown that the important parameters are met and the transfer-ability of the results is ensured.

Special effort is put onto the uniformity of the approaching hot gas flow, which will be demonstrated by temperature and velocity profiles. A first measurement of the heat transfer coefficient without film cooling is used to demonstrate the quality of the measurement principle.
Correlation Between Pressure Recovery of Highly Loaded Annular Diffusers and Integral Stage Design Parameters
Authors: Mimic Dajan , Drechsel Bastian , Herbst Florian
Published: August 2017

Exhaust diffusers significantly enhance the available power output and efficiency of gas and steam turbines by allowing for lower turbine exit pressures. The residual dynamic pressure of the turbine outflow is converted into static pressure, which is referred to as pressure recovery. Since total pressure losses as well as construction costs increase drastically with diffuser length, it is more than favourable to design shorter diffusers with rather steep opening angles. However, those designs are more susceptible to boundary layer separation.

In this paper, the stabilising properties of tip leakage vortices generated in the last rotor row and their effect on the boundary layer characteristics are examined. Based on analytical considerations, for the first time a correlation between the pressure recovery of the diffuser and integral rotor parameters of the last stage, namely the loading coefficient, flow coefficient and reduced frequency, is established. Both, experimental data and scale resolving simulations, carried out with the SST-SAS method, show excellent agreement with the correlation.

Blade tip vortex strength predominantly depends on the amount of work performed in the rotor, which in turn is described by the non-dimensional loading coefficient. The flow coefficient influences mainly the orientation of the vortex, which affects the interaction between vortex and boundary layer. The induced velocity field accelerates the boundary layer, essentially reducing the thickness of the separated layer or even locally preventing separation.
High Aspect Ratio Blading in an Axial Compressor Stage
Authors: Schmidt Tobias , Peters Markus , Jeschke Peter , Matzgeller Roland , Hiller Sven-Jürgen
Published: August 2017

This paper majorly aims to identify and understand the driving flow phenomena when the blading aspect ratio of a 1.5-stage axial compressor is increased so that its overall axial length is reduced. The blading is representative for a state-of-the-art high-pressure compressor (HPC) front-stage design. As part of the investigation steady-state RANS simulations are performed to evaluate the impact on its performance and operability. Moreover, an optimized high aspect ratio (HAR) design is introduced to recover performance penalties.

In order to achieve the desired reduction in axial stage length at constant blade row spacing and blade height, numerous possible combinations of increased rotor and stator aspect ratios exist. The impact on compressor efficiency and surge margin will be more or less severe, depending on the chord length reduction in rotor and stator. One intermediate combination of both changes in rotor and corresponding stator aspect ratio is analyzed in detail. The results show that by reducing rotor chord length, the compressor’s stability is predominantly compromised, whereas a shorter stator chord has a bigger impact on efficiency than the rotor. For each HAR configuration, profile loss is increased through a reduced blade chord Reynolds number and a higher profile edge thickness-to-chord ratio. Secondary loss is significantly reduced. However, this effect is extenuated by an increased endwall boundary layer thickness-to-chord ratio. Ultimately, this yields a diminished overall stage efficiency. In general, current HPC blade designs exhibit a lower initial rotor aspect ratio compared to the stator vanes. Consequently, an equivalent stage length reduction has a less crucial impact on Reynolds number — hence profile loss — for rotor blades than for stator vanes. Thus, regarding efficiency, there is an optimum of balancing rotor and stator chord length reduction yielding the least efficiency drop.

On the contrary, the stability margin for the compressor stage analyzed is primarily driven by the rotor’s clearance-to-chord ratio. Hence, at constant tip clearance an increase in the rotor’s aspect ratio is proportional to the resulting lack of stability. However, specific compressor design modifications are introduced in order to recover the stability margin without adversely affecting design point efficiency, such that the optimized HAR compressor stage exhibits at least the same performance specifications of the baseline design.

This study’s findings also encourage that increasing the blading aspect ratio is a feasible measure for reducing the compressor’s overall axial length aiming a compact design. An optimized HAR compressor allows additional design flexibility, which provides potential for performance improvements.
Assessment of Unsteadiness Modelling for Transient Natural Convection
Authors: Fadl M. , He L. , Stein P. , Marinescu G.
Published: August 2017

Flexible operations of steam turbines with faster startups and shutdowns are required to accommodate emerging renewable power generations, needing more advanced prediction tools for transient thermal design and analysis. A major challenge is the time scale disparity. For a natural cooling, the physical process is typically in hours or tens of hours, but on the other hand, the time step sizes typically usable tend to be very small (in seconds or sub-seconds) due to the numerical stability requirement for natural convection as often observed. A general issue to be addressed is what time step sizes can be and should be used in terms of stability as well as accuracy.

In the present work, the impact of the temporal gradient in unsteady flow and its modelling is examined in relation to numerical stability and modelling accuracy for natural convectio n. A source term based dual timing for mulation is adopted and implemented in a commercial code, which is shown to be numerically stable for very large time steps for natural convection analysis. Furthermore, a loosely coupled partitioned procedure is developed to combine this enhanced flow solver together with a solid conduction solver for solving transient conjugate heat transfer problems for natural convection. This allows very large computational time steps to be used without any stability issues, and thus enables to assess the impact of using different time step sizes entirely in terms of the temporal accuracy requirement. Computational case studies demonstrate that the present method is more stable at a markedly shortened computational time than the baseline solver. The method is also shown to be more accurate than the commonly adopted quasi-steady methods when unsteady effects are non-negligible.
Optimizing Thermal Barrier Coating Design Using Structural Optimization Methods
Authors: Eriksson Robert , Torstenfelt Bo , Klarbring Anders , Simonsson Kjell
Published: August 2017

Thermal barrier coatings (TBCs) are used in gas turbines to reduce creep, thermo-mechanical fatigue, and oxidation, or to allow for reduced air cooling. TBCs may fail due to fatigue. Structural optimization methods were applied to optimize the TBC thickness in such a way as to increase the life of the TBC. The TBC thickness was varied for three cases: 1) minimizing TBC volume, 2) minimizing TBC maximum effective stress, and 3) minimizing compliance (minimizing the strain energy). The results from the optimization were used to estimate the relative change in TBC life via a strain energy based failure criterion and a Coffin–Manson-like expression. Minimization of volume had limited use due to limitations in the current implementation. Minimization of effective stress did not give any significant increase in life. The minimization of compliance increased the estimated TBC life at highly stressed regions.
A Study on Crack Configurations in Thermal Barrier Coatings
Authors: Eriksson Robert , Jonnalagadda Krishna Praveen
Published: August 2017

Spallation of thermal barrier coatings subjected to thermal fatigue occurs through cracking in or close to the metal–ceramic interface. To better match an experimentally observed damage progression curve with initially slow crack growth followed by rapid crack growth, a multitude of crack paths were modeled in a finite element analysis. Comparisons with experimental data enabled the most likely crack path to be identified. It was shown that the most likely failure type for the studied TBC system was crack initiation from pre-existing defects in the thermal barrier coating which propagated into the interface, leading to spallation. The results were used to fit a crack propagation model.
Turbulent Energy Budget in a Tip Leakage Flow: A Comparison Between RANS and LES
Authors: Monier Jean-François , Boudet Jérôme , Caro Joëlle , Shao Liang
Published: August 2017

An academic configuration of a single airfoil and a flat casing with clearance, put in the potential core of a jet at Rec = 9.3 × 105, is studied in order to analyse the turbulence modelling. A zonal large-eddy simulation (ZLES), validated against experimental results, is considered as a reference to evaluate two steady Reynolds-averaged Navier-Stokes (RANS) simulations. Both RANS simulations use the original Wilcox k-ω model. They differ on the constitutive relation: one uses the classical Boussinesq constitutive relation, while the other relies on the quadratic constitutive relation (QCR).

The analysis focuses on the mean velocities, the Reynolds stresses and a term-to-term decomposition of the turbulent kinetic energy budget on a plane through the clearance. RANS represents quite well the mean velocities, but under-estimates the Reynolds stresses, thus under-estimates every turbulent kinetic energy budget term. The QCR has little effect on the flow and the turbulent quantities. The method allowed a fine analysis of the physics of the turbulence.
Aero-Thermal Coupled Throughflow Method With Cooling Model Based on Flow Network Analysis
Authors: Ba Wei , Ren Xiaodong
Published: August 2017

The aero-thermal coupled phenomenon is significant in the modern cooled turbine, and it is necessary to consider the cooling effect in the throughflow design phase. A new cooling model based on flow network analysis for the aero-thermal coupled throughflow method was developed to consider the film cooling effect and predict the blade wall temperature downstream of the film cooling holes. The flow network analysis is introduced into the cooling model to determine the flow rate of each cooling hole. The mixing loss caused by film cooling is investigated as local total pressure loss, and the heat transfer influence caused by film cooling is considered by the film cooling effectiveness estimated by empirical correlation. The blade heat transfer downstream the film cooling holes is calculated from pressure and suction surfaces separately, based on main flow parameters calculated by the streamline curvature method. The experimental data of the C3X profile is selected for the cooling model verification. The film cooling flow rate calculated by the flow network analysis agrees well with the experimental data, and the calculated temperatures of both the pressure and suction surfaces downstream the film cooling holes are also in accordance with the experimental data. Therefore, aero-thermal coupled throughflow method with this cooling model can be a powerful tool for preliminary design of cooled turbine.
One Exciter per Sector Test Bench for Bladed Wheels Harmonic Response Analysis
Authors: Bertini Leonardo , Neri Paolo , Santus Ciro , Guglielmo Alberto
Published: August 2017

Bladed wheel dynamic characterization is a crucial issue to avoid resonance excitations. The test bench presented in this paper was designed to independently excite the wheel sectors with one electromagnetic shaker each blade. Since a wide frequency range (1–10 kHz) is usually considered for bladed wheels, custom electromagnetic devices were designed, and then a closed-loop control software was also implemented. The global mode shapes of the wheel were then reconstructed through subsequent accelerometer measurements on all sectors to evaluate the harmonic response. The main target of the test rig is the reproduction of any operational condition by experimentally simulating an arbitrary number of stator vanes. In this way the response levels of the differently excited modes are measured and the modal damping is optimally quantified by providing a selective excitation of any number of nodal diameters. Preliminary results showed how the test setup actually allows to excite those modes with a specific number of nodal diameters, however, also exposed some difficulties to avoid small load components with different numbers of nodal diameters.
Characterization of the Modal Characteristics of Structures Operating in Dense Liquid Turbopumps
Authors: Chiu Joseph , Brown Andrew M.
Published: August 2017

It is well-known that the natural frequencies of structures immersed in heavy liquids will decrease due to the fluid “added-mass” effect. This reduction has not been precisely determined, though, with indications that it is in the 20–40% range for water. In contrast, the mode shapes of these structures have always been assumed to be invariant in liquids. Recent modal testing at NASA/Marshall Space Flight Center of turbomachinery inducer blades in liquid oxygen, which has a density slightly greater than water, indicates that the mode shapes change appreciably, though. This paper presents a study that examines and quantifies the change in mode shapes as well as more accurately defines the natural frequency reduction. A literature survey was initially conducted and test-verified analytical solutions for the natural frequency reductions were found for simple geometries, including a rectangular plate and an annular disk. The ANSYS© fluid/structure coupling methodology was then applied to obtain numerical solutions, which compared favorably with the published results. This initial study indicated that mode shape changes only occur for non-symmetric boundary conditions. Techniques learned from this analysis were then applied to the more complex inducer model. ANSYS numerical results for both natural frequency and mode shape compared well with modal test in air and water. A number of parametric studies were also performed to examine the effect of fluid density on the structural modes, reflecting the differing propellants used in rocket engine turbomachinery. Some important findings were that the numerical order of mode shapes changes with density initially, and then with higher densities the mode shapes themselves warp as well. Valuable results from this study include observations on the causes and types of mode shape alteration and an improved prediction for natural frequency reduction in the range of 30–41% for preliminary design. Increased understanding and accurate prediction of these modal characteristics is critical for assessing resonant response, correlating finite element models to modal test, and performing forced response in turbomachinery.
Stability and Sensitivity Analysis of Hydrodynamic Instabilities in Industrial Swirled Injection Systems
Authors: Kaiser Thomas L. , Poinsot Thierry , Oberleithner Kilian
Published: August 2017

The hydrodynamic instability in an industrial, two-staged, counter-rotative, swirled injector of highly complex geometry is under investigation. Large eddy simulations show that the complicated and strongly nonparallel flow field in the injector is superimposed by a strong precessing vortex core. Mean flow fields of large eddy simulations, validated by experimental particle image velocimetry measurements are used as input for both local and global linear stability analysis. It is shown that the origin of the instability is located at the exit plane of the primary injector. Mode shapes of both global and local linear stability analysis are compared to a dynamic mode decomposition based on large eddy simulation snapshots, showing good agreement. The estimated frequencies for the instability are in good agreement with both the experiment and the simulation. Furthermore, the adjoint mode shapes retrieved by the global approach are used to find the best location for periodic forcing in order to control the precessing vortex core.
Managing Gas Turbine Combustion System Fuel Manifold Distress Through Ultrasonic Inspections and Calibrated Fracture Analyses
Authors: Keller Scott , Mohammed Afzal Pasha , Oumejjoud Khalid
Published: August 2017

One of the common issues within the industrial gas turbine fleet is the susceptibility of a can-annular combustors’ fuel manifold cover (support housings) to develop embedded cracks. These cracks, located in the assembly joint of cover plates that create internal passages for fuel delivery to the combustion system, have enough of a driving force to propagate to the surface of the component. Once a crack propagates to the surface, gas has the potential to leak into the enclosure, posing a potential fire and safety risk. Furthermore, cracked fuel manifold covers are prone to increased NOx levels and excessive dynamics. Consequently, operators have the potential for a forced outage due to the failure of the fuel manifold. Currently, the existing solution is to replace the support housings with new or refurbished housings, with prior analyses requiring near perfect fusion.

An ultrasonic inspection procedure has been developed to inspect a combustor’s fuel manifold cover for embedded cracks, which are not currently detectable with FPI or X-ray methods. Through this method, the amount of fusion in the assembly joint is readily obtained, including the ability to understand if the crack is partial-thickness or through-thickness. Parametric fracture analyses, utilizing experimental material test data calibrated to service-exposed components, are conducted to predict the residual life. Coupled with the engine operating data, including the use of cold- or heated-fuels, a recommendation for the remaining useful operation of the support housings can be provided. Ultimately, by completing the ultrasonic inspection and analysis on the support housing/fuel manifold, both the risk of an unplanned outage in the future and the lifecycle management cost to the operator is reduced.
The Role of Tip Leakage Flow in Spike-Type Rotating Stall Inception
Authors: Hewkin-Smith M. , Pullan G. , Grimshaw S. D. , Greitzer E. M. , Spakovszky Z. S.
Published: August 2017

This paper describes the role of tip leakage flow in creating the leading edge separation necessary for the onset of spike-type compressor rotating stall. A series of unsteady multi-passage simulations, supported by experimental data, are used to define and illustrate the two competing mechanisms that cause the high incidence responsible for this separation: blockage from a casing-suction-surface corner separation and forward spillage of the tip leakage jet. The axial momentum flux in the tip leakage flow determines which mechanism dominates. At zero tip clearance, corner separation blockage dominates. As the clearance is increased, the leakage flow reduces the blockage, moving the stall flow coefficient to lower flow, i.e., giving a larger unstalled flow range. Increased clearance, however, means an increase in leakage jet momentum and the contribution to leakage jet spillage. There is thus a clearance above which jet spillage dominates in creating incidence, so the stall flow coefficient increases and the flow range decreases with clearance. As a consequence there is a clearance for maximum flow range; for the two rotors in this study, the value was approximately 0.5% chord. The chord-wise distribution of the leakage axial momentum is also important in determining stall onset. Shifting the distribution towards the trailing edge increases the flow range of a leakage jet dominated geometry and reduces the flow range of a corner separation dominated geometry. Guidelines are developed for flow range enhancement through control of tip leakage flow axial momentum magnitude and distribution. An example is given of how this might be achieved.
Transient Thrust Forces on a Twin Scroll Turbocharger
Authors: Thiyagarajan Janakiraman , Halldorf Erik , Fridh Jens
Published: August 2017

The bearing system of turbochargers used in trucks needs to be optimized in order to reduce the frictional losses. This helps in transmitting the exhaust energy more efficiently to the compressor wheel to increase boost pressure. Understanding the thrust loading on the axial bearing helps in optimal design of the bearing and the associated lubrication system. With the advent of twin scroll turbochargers, it is necessary to understand the thrust load behaviour at different operating conditions. This paper pioneers in studying the unsteady axial loads measured on a twin scroll turbocharger mounted on a 6 cylinder, 13 litre diesel engine used in the truck industry along with the corresponding analytical predictions for varied engine speeds and loading conditions. Transient thrust forces were measured using a weakened bearing in the experimental approach along with transient pressure measurments on the turbocharger. The axial bearing weakening required a design trade-off between flexibility and rigidity of the bearing. The results from the experimental and analytical methods provide better understanding of the characteristics of transient thrust forces that act on a turbocharger mounted on an engine of a heavy duty truck along with its design implications. The maximum normalized axial load measured and predicted were −90 N and −100 N, respectively.
The Use of Air-Measured Profile Data for Application in a High Pressure Steam Turbine
Authors: Britz Marcus , Jeschke Peter , Brunn Oliver , Polklas Thomas
Published: August 2017

This paper investigates the validity of the current industrial procedure of measuring optimized blade profiles in a wind tunnel under air condition although they are applied in a steam turbine. Therefore, it is important to analyze the possibility of using air-measured profile data for optimizing steam turbine blades. To this end, experimental data is collected using the cylindrical datum blade of a steam turbine in a three-stage high pressure steam turbine and in an annular air cascade wind tunnel. Three-dimensional CFD simulations are separately performed for both setups and show a good agreement with the experimental data. The numerical simulations can therefore be assumed to represent the real flow conditions.

Firstly, for analyzing aerodynamic transferability, two optimized profiles are measured in the annular air cascade wind tunnel at Reynolds number of 6 × 105. These profile sections are designed for high and intermediate pressure applications by employing an optimizer. The optimization is performed with the focus on reducing the profile loss for steam conditions. The experimental data verifies that the losses of the optimized profiles are reduced significantly compared to the datum blade profile measured in the same air rig. Secondly, the air-measured optimized blade profiles are used to design a 3D-optimized blade. In a numerical investigation, this optimized blade is analyzed in the steam turbine by applying steam conditions. The outlet Reynolds number of the 2nd stage is 8 × 105. This configuration is compared with the numerical results of the datum blade profile simulations.

The relative isentropic total-to-total efficiency is increased by 0.6% due to the use of the optimized rotor blades. The benefit persists also for a maximum outlet Reynolds number of 9 × 106.
Analysis of a Tilted Turbine Housing Volute Design Under Pulsating Inlet Conditions
Authors: Lee Samuel P. , Jupp Martyn L. , Nickson Ambrose K. , Allport John M.
Published: August 2017

Radial inflow turbines are widely used in the automotive turbocharger industry due to the greater amount of work that can be extracted per stage and their ease of manufacture compared with equivalent axial designs [1]. The current industry trend towards downsized engines for lower emissions has driven research to focus on improving turbine technologies for greater aero-thermal efficiency. Consequently, mixed flow turbines have recently received significant interest due to a number of potential performance benefits over their radial counterparts, including reduced inertia and improved performance at low velocity ratios.

This paper investigates the performance of a tilted volute design compared with that of a radial design, under steady state and pulsating flow conditions. The tilted volute design was introduced in an attempt to improve inlet flow conditions of a mixed flow turbine wheel and hence improve performance. The investigation is entirely computational and the approach used was carefully validated against gas stand test results.

The results of the study show that under steady state conditions the tilted volute design resulted in stage efficiency improvements of up to 1.64%. Under pulsating flow conditions, the tilted housing design resulted in a reduction in incidence angle and a maximum cycle averaged rotor efficiency improvement of 1.49% while the stage efficiencies resulted in a 1.23% increase. To assess the loss mechanisms within the rotor, the entropy flux generation through the blade passage was calculated. The tilted housing design resulted in reductions in leading edge suction and shroud surface separation resulting in the improved efficiency as observed.
Complete Characterization of a Highly Loaded Low Pressure Compressor at Different Reynolds Numbers for CFD Simulations
Authors: Hadavandi Ruzbeh , Fontaneto Fabrizio , Desset Julien
Published: August 2017

CFD is nowadays extensively used for turbomachinery design and performance prediction. Nevertheless, compressors numerical simulations still fail in correctly predicting the stall inception and the post-stall behavior. Several authors address such a lack of accuracy to the incomplete definition of the boundary conditions and of the turbulence parameters at the inlet of the numerical domain. The aim of the present paper is to contribute to the development of compressors CFD by providing a complete set of input data for numerical simulations.

A complete characterization has been carried out for a state-of-art 1.5 stage highly loaded low pressure compressor for which previous CFD analyses have failed to predict its behavior. The experimental campaign has been carried out in the R4 facility at the Von Karman Institute for Fluid Dynamics. The test item has been tested in different operative conditions for two different speed lines (90% and 96% of the design speed) and for two different Reynolds numbers. Stable and unstable operative conditions have been investigated along with the stalling behavior, its inception and the stall-cell flow field. Discrete hot-wire traverses have been performed in order to characterize the span-wise velocity field and the turbulence characteristics.
Ported Shroud Flow Processes and Their Effect on Turbocharger Compressor Operation
Authors: Sharma Sidharath , Jupp Martyn L. , Nickson Ambrose K. , Allport John M.
Published: August 2017

The ported shroud (PS) self-recirculating casing treatment is widely used to delay the onset of the surge by enhancing the aerodynamic stability of the turbocharger compressor. The increase in the stable operation region of the turbocharger compressor is achieved by recirculating the low momentum fluid that blocks the blade passage to the compressor inlet through a ported shroud cavity. While the ported shroud design delays surge, it comes with a small penalty in efficiency.

This work presents an investigation of the flow processes associated with a ported shroud compressor and quantifies the effect of these flow mechanisms on the compressor operation. The full compressor stage is numerically modelled using a Reynolds Averaged Navier-Stokes (RANS) approach employing the shear stress transport (SST) turbulence model for steady state simulations at the design and near surge conditions. The wheel rotation is modelled using a multiple reference frame (MRF) approach. The results show that the flow exits the PS cavity at the near surge condition in the form of three jet-like structures of varying velocity amplitudes. Net entropy generation in the compressor model is used to assess the influence of the ported shroud design on the compressor losses, and the results indicate a small Inlet-PS mixing region is the primary source of entropy generation in the near surge conditions. The analysis also explores the trends of entropy generation at the design and the near surge condition across the different speed lines. The results show that the primary source of entropy generation is the impeller region for the design condition and the inlet-PS cavity region for the near surge condition.
URANS Simulations and Experimental Investigations on Unsteady Aerodynamic Effects in the Blade Tip Region of a Shrouded Fan Configuration
Authors: Na Gi-Don , Kameier Frank , Springer Nils , Mauß Michael , Paschereit C. O.
Published: August 2017

The acoustical characteristics of cooling fans are an essential criterion of product quality in the automotive industry. Fan modules have to suffice growing customer expectations which are reflected in the comfort requirements set by car manufacturers around the world. In order to locate dominant acoustic sources and to reduce the noise emission generated by a shrouded fan configuration, numerical simulations and experimental investigations are performed. The working approach considers variously modified fan geometries and their evaluation regarding arising vortex flow phenomena and their effect on a decreased sound pressure level (SPL) in consideration of an improvement or the constancy of aerodynamic fan performance. Particular emphasis lies on the analysis of secondary flows in the blade tip region by post-processing CFD-results. Due to the large number of geometrical modifications investigated and the importance of highly resolved eddy structures, a hybrid approach is chosen by applying the SAS-SST turbulence model in URANS simulations. The SAS (Scale Adaptive Simulation) delivers LES (Large Eddy Simulation) content in unsteady regions of a RANS-simulation and exhibits not nearly the high computational effort needed to perform a full scale LES. An assessment of the actual propagation of noise emission into the far-field is made by performing experimental investigations on the most promising modifications. The acoustic measurements are carried out in a fan test stand in the anechoic chamber of Duesseldorf University of Applied Sciences. The aerodynamic performance is measured in a fan test rig with an inlet chamber setup in accordance to ISO 5801. The measured acoustical and aerodynamic performances are validated by the industrial partner. The results of the acoustic measurements are in turn utilized to determine indicators of noise radiation in the numerical simulation. Within this work an innovative geometry modification is presented which can be implemented into shrouded fan configurations with backward-skewed blades. The new design exhibits a reduced SPL (A-weighted) of approx. 4 dB over the entire operating range while showing no significant deterioration on the aerodynamic performance. While the design was registered for patent approval cooperatively by the industrial partner and Duesseldorf University of Applied Sciences, further investigations regarding variations of design parameters are performed and presented in this paper. All numerical simulations are performed with ANSYS CFX, a commercial solver widely spread in the industry. Methods similar to those shown in this work can be implemented in the design phase of axial fans in order to develop acoustically optimized fan geometries.
The Effect of Fuel Staging on the Structure and Instability Characteristics of Swirl-Stabilized Flames in a Lean Premixed Multi-Nozzle Can Combustor
Authors: Samarasinghe Janith , Culler Wyatt , Quay Bryan D. , Santavicca Domenic A. , O’Connor Jacqueline
Published: August 2017

Fuel staging, or fuel splitting, is a commonly used strategy for the suppression of combustion instabilities in gas turbine engines. In multi-nozzle combustor configurations, this is achieved by varying the fuel flow rate to the different nozzles. The effect of fuel staging on flame stabilization and heat release rate distribution (referred to as flame structure), and self-excited instability characteristics is investigated in a research can combustor employing five small-scale lean-premixed industrial nozzles. The nozzles are arranged in a “four-around-one” configuration and fuel staging is achieved by injecting additional fuel to the middle nozzle. An operating condition was identified where all five nozzles were fueled equally and the combustor was subject to a self-excited instability. At the operating condition considered, the self-excited instabilities are suppressed with fuel staging: this is true for cases where overall equivalence ratio is increased by staging (by only increasing the fuel flow rate to the middle nozzle) as well as cases where overall equivalence ratio is kept constant while staging (by simultaneously decreasing the fuel flow rate of the outer nozzles while increasing the fuel flow rate to the middle nozzle).

Fuel staging causes variations in the distribution of time-averaged heat release rate in the regions where adjacent flames interact. The locations of highest heat release rate fluctuation are not altered with increased fuel staging but the fluctuation amplitude is reduced. A breakup in the monotonic phase behavior that is characteristic of convective disturbances is observed with increased fuel staging, resulting in a lower pressure fluctuation amplitude. In particular, the monotonic variation in phase in the middle flame and the region where adjacent flames interact is out-of-phase with that of the outer flames, resulting in a cancellation of the global heat release rate oscillations. The distribution of local Rayleigh integral within the combustor shows that during a self-excited instability, the regions of highest heat release rate fluctuation are in phase-with the pressure fluctuation. When staging fuel is introduced, these regions fluctuate out-of-phase with the pressure fluctuation, further illustrating that fuel staging suppresses instabilities by altering the phase relationship of convective disturbances that travel along the flame front.
Leading-Edge Slots for Improving the Aerodynamic Performance of Cambered Airfoils in Horizontal Axis Wind Turbine Blades
Authors: Amano Ryoichi S. , Beyhaghi Saman
Published: August 2017

In the present work, a narrow span-wide rectangular channel (referred to as a slot) is introduced and drilled near the leading edge of a finite-span cambered airfoil to study its impact on the overall aerodynamic performance. These slots are proposed to have two legs, where the first leg starts from the vicinity of the leading edge, and the second leg exits from the pressure-side of the airfoil. NACA 4412 is used as the baseline airfoil profile, and the influence of several geometrical parameters of the slots at different angles of attack (AoAs) ranging from 0 to 16 degrees are investigated on the lift and drag coefficients. The influence of slot’s length, width, and exit angles are studied, and it is demonstrated that longer and narrower slots that exit more aligned with the pressure-side streams are generally more suitable, and can result in better performance over the entire range of AoA. For the best case considered, a lift coefficient improvement as large as 15% is observed, while the drag penalty is insignificant.

Furthermore, the inlet angle and the vertical position of slots are independently varied within reasonable ranges to characterize the slots further. Computational fluid dynamics (CFD) is used for modeling and analysis. Simulations are performed at the chord-based Reynolds number of 1.6E6. Results are validated against published data and the results from a set of wind-tunnel experiment.
Rejuvenation Heat Treatment of Single Crystal Gas Turbine Blades
Authors: Kuipers J. , Wiens K. , Ruggiero B.
Published: August 2017

Thermal degradation of precipitation-hardened nickel based superalloys has been demonstrated to be reversible through full solution rejuvenation heat treatment processing. The specific concern with full solution rejuvenation heat treatment of single crystal alloys is the formation of recrystallized grains on surfaces with residual stress. The threshold temperature for recrystallization and the effect of heat treatment temperature and time on recrystallization depth were evaluated on service run industrial gas turbine blades comprised of nickel based single crystal alloys René N5 and RR2000. Recrystallization of rejuvenated blades was observed on the root surfaces of blades which had been shot peened at original manufacture and/or during a prior repair. Blades which did not receive peening at manufacture were free of recrystallization in critical areas following full solution rejuvenation heat treatment. Given that gas turbine blade roots operate at relatively low temperatures compared to the airfoil, creep is not considered a life limiting damage mechanism for this region of the blade. Rather, high cycle fatigue is considered the primary damage mechanism of concern. As such, fatigue testing of shot peened and heat treated (recrystallized) René N5 specimens was carried out at 650°C at various stress levels in comparison with baseline (non-recrystallized) specimens to determine the extent to which recrystallization would limit fatigue endurance at blade root operating conditions. It was found that recrystallization did not reduce the fatigue endurance relative to baseline samples at the tested conditions. The findings indicate that repair including full solution rejuvenation heat treatment of previously peened blades comprised of René N5 alloy is feasible provided that recrystallization be limited to root surfaces.
Quantification of X-Ray Measurement Uncertainty Based on Optical Measurement Data of Turbine Blades
Authors: Högner Lars , Knebel Sebastian , Voigt Matthias , Mailach Ronald , Meyer Marcus
Published: August 2017

Probabilistic design approaches taking geometric variation due to manufacturing scatter or wear into account reflect a relevant part of today’s design process in turbomachinery. This process is characterized by a high degree of multidisciplinarity involving complex geometric features, which demands high standards to incorporate geometric variability. The significance of the employed probabilistic approaches considering real geometry effects is based on the information about the existing variability including distributions, correlations as well as the respective uncertainty of employed measurement techniques.

Within the scope of mechanical probabilistic analysis of cooled turbine blades, the variability of the internal geometry is of particular interest since this in combination with the external geometry variation leads to a change in wall thickness, which may influence stress and life significantly. Computed tomography (CT) represents a non-destructive technology that enables the detection of internal geometric features.

The present paper address the uncertainty quantification (UQ) of CT for a high-pressure turbine (HPT) blade. Measurement data derived by optical techniques are used by way of comparison. A parametric turbine blade model is employed to quantify the deviations between optical and tomographic measurement data. Furthermore, five blades were cut open by wire erosion, which enables a comparative investigation in terms of wall thickness. The observed deviations are related to inherent sources of uncertainties in CT.