bluuebunny/crossref_metadata_2025_split huggingface dataset:
Experimental Analysis of the Pressure Field Inside a Vaneless Diffuser From Rotating Stall Inception to Surge
Authors: Bianchini Alessandro , Biliotti Davide , Rubino Dante Tommaso , Ferrari Lorenzo , Ferrara Giovanni 
Published: August 2015

An accurate estimation of rotating stall is one of the key points for high-pressure centrifugal compressors, as it is often connected with the onset of detrimental subsynchronous vibrations which can prevent the machine from operating beyond this limit.
With particular reference to vaneless diffuser rotating stall, the most common practice in industrial machines is to make use of a limited number of dynamic pressure probes to reconstruct the stall characteristics after an ensemble averaging approach.
In this study, a 1:1 model of an industrial compressor stage was tested in a dedicated test rig and equipped with 24 pressure probes properly distributed along the diffuser circumference with the scope of providing a real-time visualization of the spatial pressure distribution within the diffuser. The results allowed the assessment of some important characteristics of the stall cells that were historically supposed based on averaged data, e.g. the cells rigidity. Moreover, the present study confirmed the existence of a stall pattern with two almost axisymmetric lobes. Finally, the transient analysis of both the stall inception and the surge onset was carried out, highlighting the flow field evolution in the diffuser under these conditions.
An Experimental and Numerical Investigation on the Effects of Aerothermal Mixing in a Confined Oblique Jet Impingement Configuration
Authors: Schulz Sebastian , Schindler Alexander , von Wolfersdorf Jens 
Published: August 2015

An investigation to characterize the effect of entrainment in a confined jet impingement arrangement is presented. The investigated configuration models an impingement cooled turbine blade passage and holds two staggered rows of inclined impingement jets. In order to distinctly promote thermal entrainment phenomena the jets were heated separately. A steady-state liquid crystal technique was used to obtain near-wall fluid temperature distributions for the impingement surfaces at adiabatic conditions. Additionally, flow field measurements were undertaken using Particle Image Velocimetry (PIV). Furthermore, compressible RANS simulations were carried out with ANSYS CFX using Menter’s SST turbulence model to accompany the experiments. Distributions of effectiveness, velocity, and turbulent kinetic energy detail the complexity of the aerothermal situation. The study was conducted for a jet Reynolds number range from 10,000 to 45,000.
The experimental and numerical results are generally in good agreement. Nevertheless, the simulations predict flow features in particular regions of the geometry that are not as prominent in the experiments. These affect the effectiveness distributions, locally. The investigations revealed that the effectiveness is independent of the temperature difference between the heated and cold jet as well as the jet Reynolds number.
Ion Current Measurements as a Method for the Detection of the Reaction Rate in Combustion With Swirl Stabilized Airblast Injection Systems
Authors: Wollgarten J. Christopher , Zarzalis Nikolaos , Turrini Fabio , Peschiulli Antonio 
Published: August 2015

Due to strict emission legislation, the trend in the development of aero-engine gas turbine combustion is heading towards lean burning approaches. Lean combustion reduces the combustion temperatures and therefore also the nitrogen oxides emissions. Unfortunately, lean combustion suffers from instabilities and the operation close to the point of lean blowout increases the risk of imminent blowoff. Active stability control is therefore inevitable. The objective of this work is to evaluate the signal obtained from an ion current measurement technique to enable combustion control for aircraft propulsion applications in the near future.
In the past ion current measurements have been used in several studies as flame turbulence analyzer and to detect the reaction rate. However, investigations in lean burning and swirl stabilized airblast injection combustors for future propulsion concepts are rare. The signal obtained from an ion current detector inside a combustor depends strongly on the measurement position. In this experimental investigation field measurements at atmospheric conditions of the ion concentration in a tubular combustor with a sampling rate of 8 kHz are compared with 4 kHz time resolved temperature and OH* chemiluminescence measurements in order to determine the position of the reaction inside the combustor. Variations were performed of the air to fuel ratio (AFR), the air preheating temperature and the pressure drop across the injection system to clarify the interpretation of the ion current signal. The results indicate a strong dependence of the ion current signal on the AFR and that the technique has distinct advantages compared to OH* chemiluminescence measurements: The measurement equipment is comparable non-expensive and the results reveal that the reaction rate is measured directly and are not interpreted from a 3D image. A transition in flame shape from a compact to a tornado flame can be clearly identified with the applied probe. Furthermore, regions with high temperature fluctuations do not necessarily reveal the reaction zone in a recirculating flow field.
Dual Drive Booster for a Two-Spool Turbofan: Performance Effects and Mechanical Feasibility
Authors: Speak Trevor H. , Sellick Robert J. , Kloos Vadim , Jeschke Peter 
Published: August 2015

A novel two-spool turbofan engine configuration is described which uses a booster powered by both the LP and HP spool. Design and off-design performance analysis shows the operating characteristics of the configuration and a mechanical feasibility study of the gearbox is presented.
The trends towards ever higher engine overall pressure ratio and bypass ratio have resulted in a combination of higher pressure ratio and lower blade speed in the booster compressor of conventional two spool turbofans. This combination gives rise to many stages in the booster and/or lower booster efficiency and also a higher degree of off-design miss-match between the core compressors. The current paper describes an engine architecture which aims to alleviate both these issues by powering the booster compressor from both LP and HP spools through an epicyclic gear system. We have called this engine architecture the Dual Drive Booster. The concept gives the engine designer greater flexibility to optimize component performance and work split, resulting in the potential for lower cruise SFC and higher hot day take-off thrust capability than current engine configurations.
The gear system is described along with the mathematical derivation of the booster rotational speed in terms of LP and HP spool speeds. Both design point and off-design performance modelling has been conducted and comparison is made between a conventional turbofan and a turbofan fitted with the Dual Drive Booster. The results show a significant enhancement in take-off thrust due to the better speed match of the booster. The paper also describes the results of a preliminary study into the design and mechanical feasibility of the engine architecture and gear system.
The presented concept is an alternative to the conventional turbofan and should be considered during the conceptual design of future aircraft engines.
Robust Design Optimization Applied to a High Pressure Turbine Blade Based on Surrogate Modelling Techniques
Authors: Wagner Frank , Kühhorn Arnold , Parchem Roland 
Published: August 2015

To achieve reverse objectives in engine design, advanced modelling and analysis methods are among the key research technologies. In the presented work, a robust design optimization of a first stage high pressure turbine blade has been carried out. This blade derives from a current production of a Rolls-Royce aero engine.
The motivation of this work is to show that the methodology of robust design optimization can be applied to high pressure turbine blades. A fully automated workflow, which encapsulated the integral blade design and analysis process, has been used. The main workflow objective is a representative life value of the external surface of the blade. In addition, the workflow enables the engineering uses to consider sub objectives like mass, efficiency and life at critical locations of the blade. These can also be taken into account in the multi-objective robust design optimization. This research also focuses on the use of surrogate models, with attention to the delivery of a physically correct result. For this purpose, the validation of the applied methods has a huge significance and a toolbox was created to generate and evaluate the quality of the surrogate models.
In the present case sixteen geometry parameters were considered. In order to show that this methodology is not limited to geometry variation, parameters for material specification and for boundary conditions were varied in addition. The surrogate model was trained by the workflow generated DoE-data and could be used for different kinds of optimization.
As a conclusion, it has been demonstrated that the methodology can be used for the engineering design process of turbine blades, while delivering physically correct results. The different techniques for surrogate modelling were examined and compared. With the help of these surrogate models, an optimization of life, mass and efficiency with 22.5 million evaluations was possible. Finally, an overview of the methodology for the case of a real world turbine blade could be given, and an improved blade in the sense of multi-objective robust design was found.
Unsteady Operative Conditions of Bleeding Lines in Heavy Duty Axial Gas Turbine
Authors: Cioffi Marco 
Published: August 2015

The proper design and operation of air bleeding pipes (blow-off lines) from axial compressors in heavy duty gas turbines is relevant to protect the compressor during start-ups and shut-downs by avoiding dangerous flow instabilities in the first stages. The blow-off lines are usually equipped by valves, which are closed during normal gas turbine operation and opened at low rotor speed. During gas turbine shut-downs the blow-off valves open instantaneously.
In this paper the unsteady flow behavior in blow-off lines following the valve opening is presented together with numerical results based on available field data. The paper main scope is to address and to help the design of experimental activities on production gas turbines and to make available some simple numerical tools to be adopted during the industrial design of an axial compressor and its auxiliary systems. The performed analysis results have been used to define the structural requirements and the correct positioning of the measuring probes installed in blow-off lines. In addition the presented models are part of the compressor design loop, used to compute a fast evaluation of the limiting mass flow rate, which characterizes the blow-off pipes as gas turbine safety devices.
Aerodynamic Optimization of the Nozzle for the Last Stage of Steam Turbine
Authors: Macálka Aleš , Synáč Jaroslav , Váchová Jana , Hajšman Miroslav 
Published: August 2015

In order to meet the requirements of rising energy demand, one goal in the design process of modern steam turbines is to achieve high efficiencies. A major gain in efficiency is expected from the optimization of the last stage of a low pressure turbine (LP).
This paper focuses on aerodynamic study by compound lean, axial sweep and hub end-wall of nozzle. The objective function is to maximize efficiency and minimize the leaving losses under the assumption of constant mass flow rate.
The aerodynamic design process involves commercial 3D CFD tools. The maximization of the objective functions is achieved by means of a Design of Experiment (DoE) method “Optimal Space Filling” based on “Latin Hypercube Sampling”.
Finally, the optimized design is analyzed by a transient (Time Transformation) CFD simulation. Furthermore, the detailed flow pattern of the optimized and the initial design is analyzed and compared.
Hybrid RANS-LES Modeling of a Hot Streak Generator Oriented to the Study of Combustor-Turbine Interaction
Authors: Andreini A. , Facchini B. , Insinna M. , Mazzei L. , Salvadori S. 
Published: August 2015

Turbine entry conditions are characterized by unsteady and strongly non-uniform velocity and temperature and pressure fields. The uncertainty and the lack of confidence associated to these conditions require the application of wide safety margins during the design of the turbine cooling systems, which are detrimental for the efficiency of the engine. These issues have been further complicated by the adoption of lean-burn technology in modern aeroengines, identified by many manufacturers as the most promising solution for a significant reduction of NOx emission. Such devices are in fact characterized by a very compact design, whereas the strong swirl component generated by the injector is maintained up to the end of the flametube due to the absence of dilution holes, which in conventional combustors provides the required pattern factor.
Bearing in mind complexity and costs associated to the experimental investigation of combustor-turbine interaction, CFD has become a key and complementary tool to understand the physical phenomena involved. Due to the well-known limitations of the RANS approach and the increase in computational resources, hybrid RANS-LES models, such as Scale Adaptive Simulation (SAS), are proving to be a viable approach to resolve the main structures of the flow field.
This paper reports the main findings of the numerical investigation of a hot streak generator for the study of combustor-turbine interaction. The results were compared to experimental data obtained from a test rig representative of a lean-burn, effusion cooled, annular combustor, developed in the context of the EU project FACTOR. Steady RANS and unsteady SAS runs were carried out in order to assess the improvements related to hybrid models. Additional simulations were performed to investigate the effect of the periodicity assumption and the impact of liner cooling modelling on the exit conditions.
Impact of Swirl Flow on Combustor Liner Heat Transfer and Cooling: A Numerical Investigation With Hybrid RANS-LES Models
Authors: Mazzei L. , Andreini A. , Facchini B. , Turrini F. 
Published: August 2015

This paper reports the main findings of a numerical investigation aimed at characterizing the flow field and the wall heat transfer resulting from the interaction of a swirling flow provided by lean burn injectors and a slot cooling system, which generates film cooling in the first part of the combustor liner. In order to overcome some well-known limitations of RANS approach, e.g. the underestimation of mixing, the simulations were performed with hybrid RANS-LES models, namely SAS-SST and DES-SST, which are proving to be a viable approach to resolve the main structures of the flow field. The numerical results were compared to experimental data obtained on a non-reactive three sector planar rig developed in the context of the EU project LEMCOTEC.
The analysis of the flow field has highlighted a generally good agreement against PIV measurements, especially for the SAS-SST model, whereas DES-SST returns some discrepancies in the opening angle of the swirling flow, altering the location of the corner vortex. Also the assessment in terms of Nu/Nu0 distribution confirms the overall accuracy of SAS-SST, where a constant over-prediction in the magnitude of the heat transfer is shown by DES-SST, even though potential improvements with mesh refinement are pointed out.
Best Strategies for the Development of a Holistic Porosity Model of a Heat Exchanger for Aero Engine Applications
Authors: Yakinthos K. , Misirlis D. , Vlahostergios Z. , Flouros M. , Donnerhack S. , Goulas A. 
Published: August 2015

In an attempt to manage CFD computations in aero engine heat exchanger design, this work presents the best strategies and the methodology used to develop a holistic porosity model, describing the heat transfer and pressure drop behavior of a complex profiled tubular heat exchanger for aero engine applications. Due to the complexity of the profile tube heat exchanger geometry and the very large number of tubes, detailed CFD computations require very high CPU and memory resources. For this reason the complex heat exchanger geometry is replaced in the CFD computations by a simpler porous medium geometry with predefined pressure loss and heat transfer.
The present work presents a strategy for developing a holistic porosity model adapted for heat exchangers, which is capable to describe their macroscopic heat transfer and pressure loss average performance. For the derivation of the appropriate pressure loss and heat transfer correlations, CFD computations and experimental measurements are combined. The developed porosity model is taking into consideration both streams of the heat exchanger (hot and cold side) in order to accurately calculate the inner and outer pressure losses, in relation to the achieved heat transfer and in conjunction with the selected heat exchanger geometry, weight and operational parameters. For the same heat exchanger, RAM and CPU requirement reductions were demonstrated for a characteristic flow passage of the heat exchanger, as the porosity model required more than 80 times less computational points than the detailed CFD model. The proposed porosity model can be adapted for recuperation systems with varying heat exchanger designs having different core arrangements and tubes sizes and configurations, providing an efficient tool for the optimization of the heat exchangers design and leading to an increase of the overall aero engine performance.
Failure Analysis and Life Assessment of Thermal Fatigue Crack Growth in a Nickel-Base Superalloy Based on EBSD Method
Authors: Kobayashi Daisuke , Miyabe Masamichi , Achiwa Masahiro 
Published: August 2015

In the case of failure incidents involving important components, it is necessary to clarify the fracture mechanism by failure analysis. In the case of conventional steel materials, according to the individual fracture mode the fracture surfaces have unique fracture morphology corresponding to tensile, impact, creep and fatigue conditions. We can identify the mechanism of a fracture by observing its fracture surface, and this is known as the fractography. However regarding nickel-base superalloys, any differences in fracture morphology are unfortunately barely distinguishable, which makes it difficult to conduct fractography.
In this paper, in order to characterize the damage behavior of IN738LC, the misorientation analysis within grains by using the electron backscattered diffraction (EBSD) method across almost all the whole range of specimens has been carried out. As a result, it was found that the cross section of fracture samples have unique distinguishable morphology corresponding to the individual fracture mode. Furthermore, the striations corresponding to the fatigue crack growth rate was found in the crack cross-sectional sample. It was considered that the EBSD striation observed on the cross section reveals the fatigue crack growth rate, as with striations found in the fatigue fracture surface such as conventional steel materials.
On the case study of the actual (service and damaged) gas turbine blade, the EBSD analysis as the fractography revealed the mechanism of cracking and the fatigue crack growth rate. Thus, it is concluded that the misorientation analysis of damage materials allows the qualitative estimation of the fracture mode and the quantitative life assessment of the fatigue crack growth.
Volume of Fluid (VOF) Analysis of Oil-Jet Lubrication for High-Speed Spur Gears Using an Adaptive Meshing Approach
Authors: Fondelli Tommaso , Andreini Antonio , Da Soghe Riccardo , Facchini Bruno , Cipolla Lorenzo 
Published: August 2015

In high speed gearbox systems, the lubrication is generally provided using nozzles to create small oil jets that feed oil into the meshing zone. It is essential that the gear teeth are properly lubricated and that enough oil gets into the tooth spaces to permit sufficient cooling and prevent gearbox failure. A good understanding of the oil behaviour inside the gearbox is therefore desirable, to minimize lubrication losses and reduce the oil volume involved, and ensure gearbox reliability.
In order to reach these objectives, a comprehensive numerical study of a single oil jet impinging radially on a single spur gear teeth has been carried out using the Volume of Fluid (VOF) method. The aims of this study are to evaluate the resistant torque produced by the oil jet lubrication, and to develop a physical understanding of the losses deriving from the oil-gear interaction, studying the droplets and ligaments formation produced by the breaking up of the jet as well as the formation of an oil film on the surface of the teeth.
URANS calculations have been performed with the commercial code ANSYS FLUENT and an adaptive mesh approach has been developed as a way of significantly reducing the simulation costs. This method allows an automatic mesh refinement and/or coarsening at the air-oil interface based on the volume of fluid gradient, increasing the accuracy of the predictions of oil break-up as well as minimizing numerical diffusion of the interface. A global sensitivity analysis of adopted models has been carried out and a numerical set-up has been defined. Finally several simulations varying the oil injection angle have been performed, in order to evaluate how this parameter affects the resistant torque and the lubrication performances.
Influence of Turbulent Flow Characteristics and Coherent Vortices on the Pressure Recovery of Annular Diffusers: Part A — Experimental Results
Authors: Kuschel Marcus , Drechsel Bastian , Kluß David , Seume Joerg R. 
Published: August 2015

Exhaust diffusers downstream of turbines are used to transform the kinetic energy of the flow into static pressure. The static pressure at the turbine outlet is thus decreased by the diffuser, which in turn increases the technical work as well as the efficiency of the turbine significantly. Consequently, diffuser designs aim to achieve high pressure recovery at a wide range of operating points.
Current diffuser design is based on conservative design charts, developed for laminar, uniform, axial flow. However, several previous investigations have shown that the aerodynamic loading and the pressure recovery of diffusers can be increased significantly if the turbine outflow is taken into consideration. Although it is known that the turbine outflow can reduce boundary layer separations in the diffuser, less information is available regarding the physical mechanisms that are responsible for the stabilization of the diffuser flow.
An analysis using the Lumley invariance charts shows that high pressure recovery is only achieved for those operating points in which the near-shroud turbulence structure is axi-symmetric with a major radial turbulent transport component. This turbulent transport originates mainly from the wake and the tip vortices of the upstream rotor. These structures energize the boundary layer and thus suppress separation. A logarithmic function is shown that correlates empirically the pressure recovery vs. the relevant Reynolds stresses. The present results suggest that an improved prediction of diffuser performance requires modeling approaches that account for the anisotropy of turbulence.