bluuebunny/crossref_metadata_2025 huggingface dataset:
Research on the Applicability of Using a Curved Blade to Optimize the Flow Loss Weight Distribution of a Linear Compressor Cascade With Different Flow Separation Types
Authors: Kan Xiaoxu , Wang Songtao , Luo Lei , Su Jiexian
Published: August 2018

Due to the influence of an adverse pressure gradient, three types of flow separation occur on the suction surface of a linear compressor cascade: full-span open separation, corner closed separation and a corner stall condition. A numerical simulation was performed using a topological analysis method to determine the applicability of using a curved blade to improve the total pressure loss coefficient of a cascade with different types of flow separation situations on the suction surface. First, the accuracy of the ANSYS CFX program was verified against existing experimental data to balance the relationship between the calculation accuracy and the time savings. Second, the incidence characteristics of the cascade were analyzed to determine the incident conditions for three types of flow separation. Three factors of the cascade were considered to analyze a cascade with and without a curved blade: the transferring process of the topological structure on the blade suction surface, the evolution process of the vortex structure in the cascade passage and the weight distribution of the total pressure loss coefficient. The results indicate that the strength and scale of the concentrated shedding vortex (CSV) is distinctly reduced in the corner stall condition due to the influence of radial migration by the curved blade; thus, the flow loss is observably reduced. In the corner closed separation condition, the scale of the corner separation was too small to be notably reduced. In the full-span open separation condition, the curved blade not only reduced the total pressure loss coefficient but also increased the strength and scale of the passage vortex (PV). Finally, the curved blade method improved the applicability of reducing the flow loss in the corner stall condition and increased the stability and margin of a highly loaded compressor.
Capturing Radial Mixing in Axial Compressors With CFD
Authors: Cozzi Lorenzo , Rubechini Filippo , Giovannini Matteo , Marconcini Michele , Arnone Andrea , Schneider Andrea , Astrua Pio
Published: August 2018

Due to the generally high stage and blade count, the current standard industrially adopted to perform numerical simulations on multistage axial compressors is the steady-state analysis based on the Reynolds-averaged Navier-Stokes approach (RANS), where the coupling between adjacent rows is handled by means of mixing planes. In addition to the well-known limitations of a steady-state picture of the flow, namely its inherent inability to capture the potential interaction and the wakes from the upstream blades, there is another flow feature which is lost through a mixing-plane, and which is believed to be a major accountable for the radial mixing: the transport of stream-wise vorticity. Streamwise vorticity arises throughout a compressor for various reasons, mainly associated with secondary and tip-clearance flows. A strong link does exist between the strain field associated with the transported vortices and the mixing augmentation: the strain field increases both the area available for mixing and the local gradients in fluid properties, which provide the driving potential for mixing itself.

Especially for the rear stages of a multistage axial compressor, due to high clearances and low aspect ratios, only accounting for the development along the meridional path of secondary and clearance flow structures it is possible to properly predict the spanwise mixing.

In this work, the results of steady and unsteady RANS simulations on the high-pressure section of an industrial heavy-duty axial compressor are presented and compared with experimental data acquired during a test campaign. Adopting an unsteady full-annulus URANS approach, the enhanced radial mixing in the rear stages of the compressor is properly captured, obtaining a really good agreement with experimental data both in terms of total temperature and pressure outlet radial distributions. On the contrary, with a steady-state modelling, the radial transport is strongly underestimated, leading to results with marked departures from experiments. Examining what occurs across the inter-row interfaces for RANS and URANS solutions, a possible explanation for this underestimation is provided. In particular, as the stream-wise vorticity associated with clearance flows is one of the main drivers of radial mixing, restraining it by pitch-averaging the flow at mixing planes of a steady-state analysis is the reason why this simplified approach is not able to properly predict the radial transport of fluid properties in the rear part of the axial compressor.
Performance Testing of an Axial Flow Fan Designed for Air-Cooled Heat Exchanger Applications
Authors: Wilkinson Michael B. , van der Spuy Johan , von Backström Theodor W.
Published: August 2018

An axial flow fan developed in previous study is tested in order to characterise its performance. The M-fan, a 7.3152 m diameter rotor only axial flow fan was designed to perform well under the challenging operating conditions encountered in air-cooled heat exchangers. Preliminary CFD results obtained using an actuator disk model as well as a periodic three dimensional model indicate that the fan meets the specified performance targets, with an expected total-to-static efficiency of 59.4 % and a total-to-static pressure rise of 114.7 Pa at the operating point.

Experimental tests are performed on the M-fan in order to determine its performance across a full range of flow rates. A range of fan configurations are tested in order to ascertain the effect of tip clearance, blade angle and hub configuration on fan performance. Due to the lack of a suitable facility for testing a large diameter fan, a scaled 1.542 m diameter model is tested on the BS 848 (ISO 5801) type A fan test facility at Stellenbosch University. A RANS CFD model representing the M-fan in the test facility is also developed in order to provide additional insight into the flow field in the vicinity of the fan blades. The results of the CFD model will be validated using the experimental data obtained. Both the CFD results and the experimental data obtained are compared to the initial CFD results for the full scale fan, as obtained in the previous study, by means of fan scaling laws.

Experimental data indicates that the M-fan does not meet the pressure requirement set out in the initial study, at the design blade setting angle of 34 degrees. Under these conditions the M-fan attains a total-to-static pressure rise of 102.5 Pa and a total-to-static efficiency of 56.4%, running with a tip gap of 2 mm. Increasing the blade angle is shown to be a potential remedy, improving the total-to-static pressure rise and efficiency obtained at the operating point. The M-fan is also shown to be highly sensitive to increasing tip gap, with larger tip gaps substantially reducing fan performance. The losses due to tip gap are also shown to be overestimated by the CFD simulations. Both experimental and numerically obtained results indicate lower fan total-to-static efficiencies than obtained in the initial CFD study. Results indicate that the M-fan is suited to its intended application, however it should be operated with a smaller tip gap than initially recommended and a larger blade setting angle. Hub configuration is also shown to have an influence on fan performance, potentially improving performance at low flow rates.
Improved Low Cycle Fatigue Analysis for Ni-Based Turbine Nozzles
Authors: Maggiani Gianluca , Roy Matthew J. , Colantoni Simone , Withers Philip J.
Published: August 2018

The requirements for cleaner energy have driven industrial gas turbines manufacturers to increase firing temperatures and improve cooling of nozzles. The application of high temperature alloys having adequate thermo-mechanical requirements is critical, as assessed by low cycle fatigue performance. The effect of higher firing temperatures combined with higher cooling efficiencies has lead to operating cycles where the level of plastic strain imparted define component life. The capability of material models to account for non-linear effects such as ratchetting or shakedown, cyclic hardening or softening as well as Bauschinger or relaxation effects have been highlighted in this context. Neglecting these effects can lead to over and under-conservative life assessment analysis, while accounting for them using standard multilinear material models lead to convergence issues in finite element analysis. In this paper, Chaboche viscoplastic model has been applied to a transient structural of a first stage gas turbine nozzle. Fitting of the model based on experimental mechanical test data on MAR-M-247 alloy will be described, followed by an overview of how the model may be implemented to a benchmark nozzle thermo-mechanical transient analysis. Finally the details how the Chaboche-type model has provided up to 50% decrease in computation time when compared to using a standard multi-linear material modelling approach.
On the Use of the Automatic Differentiation for Developing a Linear Harmonic Solver
Authors: Zhang Wei , Wang Dingxi , Huang Xiuquan , Yang Tianxiao , Yan Hong , Li Jianling
Published: August 2018

The linear and nonlinear harmonic methods are efficient frequency domain methods for analyzing time periodic unsteady flow fields. They have been widely used in both academia and industry. But the cost and complexity of developing a linear harmonic solver has been limiting its wider applications. On the other hand, the automatic differentiation (AD) has long been used in the CFD community with a focus on generating adjoint codes in a reverse mode. All those AD tools can do a much better job in generating linearized codes in a tangent mode, but so far very little, if any, attention is paid to using AD for developing linear harmonic solvers.

The linear harmonic method, in comparison with the harmonic balance method, has its own advantages. For example, it can capture small disturbances very effectively, and avoids aliasing errors which can lead to solution instability since each wave component is solved for separately.

This paper presents the effort of using an AD tool to generate major source codes for the development of a linear harmonic solver for analyzing time periodic unsteady flows. It includes the procedures and advice of using AD for such a purpose. A case study is also presented to validate the developed linear harmonic solver.
Aerodynamic Characteristics and Noise Analysis of a Low-Speed Axial Fan
Authors: Luo Bo , Chu Wuli , Dong Wei , Chen Xiangyi
Published: August 2018

Axial fans are widely used in modern industry and new regulations and stringent environmental concerns are prompting manufacturer to design efficient low-noise axial fans. This paper is focused on improving the aerodynamic performances and reducing the tonal noise at BPF and its harmonics by the optimum choice of lean-swept blade and the stacking line for the low-speed axial fan. The aerodynamic characteristics of the axial fan with a shroud are explored by CFD with ANASYS CFX. A hybrid method, SST turbulence model for flow and FW-H equation for acoustics, is chosen to predict the radiated noise. The accuracy and reliability of predicted aerodynamic and aeroacoustics results are verified by comparing both computation and experimental data. A number of modified blades with different leaned angle, swept angle and the stacking lines are modeled and analyzed, and the investigation into the optimum choice of lean-swept blade and the stacking line is conducted according to aerodynamic performances and tonal noise. Q-criterion which can visualize the major flow disturbances is applied for the purpose of identification of acoustic sources. The turbulent flow structures on the leading edge, tip and suction side of the blade are main noise sources. An optimal modification is determined through the analysis of the aerodynamic performances and noise, which is to achieve the desired performances by blade sweep and lean and adjusting the stacking line. The results show that aerodynamic and acoustic performances of the optimized fan are better than that of the original fan and the improvement is more obvious to change the stacking line with centre of gravity compare to blade sweep and lean for the low-speed axial fan.
1D and 3D-Design Strategies for Pressure Slope Optimization of High-Flow Transonic Centrifugal Compressor Impellers
Authors: Hildebrandt A. , Ceyrowsky T.
Published: August 2018

The present paper deals with the numerical and theoretical investigations of the effect of geometrical dimensions and 1D-design parameters on the impeller pressure slope of a transonic centrifugal compressor stage for industrial process application. A database being generated during the multi-objective and multi-point design process of a high flow coefficient impeller, comprising 545 CFD (Computational Fluid Dynamics) designs is investigated in off-design and design conditions by means of RANS (Reynolds Averaged Navier Stokes) simulation of an impeller with vaneless diffuser. For high flow coefficients of 0.16 < phi < 0.18, the CFD-setup has been validated against measurement data regarding stage and impeller performance taken from MAN test rig experimental data for a centrifugal compressor stage of similar flow coefficient. The paper aims at answering the question how classical design parameter, such as the impeller blade angle distribution, impeller suction diameter and camber line length affect the local and total relative diffusion and pressure slope towards impeller stall operation. A second order analysis of the CFD database is performed by cross-correlating the CFD data with results from impeller two-zone 1D modelling and a rapid loading calculation process by Stanitz and Prian. The statistical covariance of first order 1D-analysis parameters such as the mixing loss of the impeller secondary flow, the slip factor, impeller flow incidence is analyzed, thereby showing strong correlation with the design and off-design point efficiency and pressure slope. Finally, guide lines are derived in order to achieve either optimized design point efficiency or maximum negative pressure slope characteristics towards impeller stall operation.
Analysis of the Mixing and Emission Characteristics in a Model Combustor
Authors: Li Shan , Zhang Shanshan , Hou Lingyun , Ren Zhuyin
Published: August 2018

Modern gas turbines in power systems employ lean premixed combustion to lower flame temperature and thus achieve low NOx emissions. The fuel/air mixing process and its impacts on emissions are of paramount importance to combustor performance. In this study, the mixing process in a methane-fired model combustor was studied through an integrated experimental and numerical study. The experimental results show that at the dump location, the time-averaged fuel/air unmixedness is less than 10% over a wide range of testing conditions, demonstrating the good mixing performance of the specific premixer on the time-averaged level. A study of the effects of turbulent Schmidt number on the unmixedness prediction shows that for the complex flow field involved, it is challenging for Reynolds-Averaged Navier-Stokes (RANS) simulations with constant turbulent Schmidt number to accurately predict the mixing process throughout the combustor. Further analysis reveals that the production and scalar dissipation are the key physical processes controlling the fuel/air mixing. Finally, the NOx formation in this model combustor was analyzed and modelled through a flamelet-based approach, in which NOx formation is characterized through flame-front NOx and its post-flame formation rate obtained from one-dimensional laminar premixed flames. The effect of fuel/air unmixedness on NOx formation is accounted for through the presumed probability density functions (PDF) of mixture fraction. Results show that the measured NOx in the model combustor are bounded by the model predictions with the fuel/air unmixedness being 3% and 5% of the maximum unmixedness. In the context of RANS, the accuracy in NOx prediction depends on the unmixedness prediction which is sensitive to turbulent Schmidt number.
Heavy Duty Gas Turbine Performance and Endurance Testing: The NovaLT™16 Experience
Authors: Asti Antonio , Gamberi Francesco , Del Vescovo Giuseppe , Carta Riccardo , Giannini Nicola , Ignesti Mirko , Orazi Claudio , Pieroni Nicola
Published: August 2018

The NovaLT™16 gas turbine recently developed in Baker Hughes, a GE company (BHGE), is part of a larger class of gas turbines (LT class) aiming at covering a wide space in the small power range segment and at introducing in the market a state of the art technology engine for what concerns performance, emissions, operability, durability and maintainability.

The main purpose of this paper is to describe the entire validation campaign that was performed at BHGE facilities. This campaign can be divided into 3 different phases.

The first phase focused on measuring engine performance in a new, clean and unaltered configuration.

The second phase focused on emissions, vibration, thermal distribution, auxiliary system performances and the like, in order to validate the design assumption and calculation results across the full operational range. In this phase, more than 2000 sensors were installed across the entire engine, covering all modules, and all functional tests were performed (inside and outside of design space) to guarantee reliable engine behavior. At the end of this test phase, a full engine teardown was performed to allow a detailed parts inspection that confirmed the achievement of the design intent.

The standard maintenance plan of the engine requires 35Kh continuous running. Therefore, the third part of the test aimed at validating engine durability with a full endurance test that allowed the identification and correction of any possible remaining operation problem. In this phase, the engine was still equipped with more than 1000 sensors, and was operated continuously following a well-defined operating profile in order to simulate both mechanical drive and power generation modes. This campaign successfully allowed to fine tune several engine control logic details, to monitor emissions behavior across a wide range of ambient temperature and load condition (the test spans from hot to cold day), to analyze trends of standard engine parameters and special instrumentation and, through planned borescope inspection, to evaluate individual component status versus selected operating profile.

Data reported in this paper represent a summary of all the data acquired and post processing results, and illustrate how an endurance test can help tuning machine performance predictions in a wide operating range.
Numerical Investigation on Wide-Chord Fan Blade Forced Response due to Vortex Ingestion
Authors: Wang Zhonglin , Chen Yong , Ouyang Hua , Wang Anjenq
Published: August 2018

When a turbofan engine is taxing or taking-off, a vortex can form between ground surface and the intake. As the diameters of engines increase, intakes are closer to the ground and as a result the possibility of vortex ingestion is increasing. The vortex starts from the ground surface and enters the inlet at high rotating speed. It is likely to draw in hard material or dust from the ground, which leads to blade erosion or impact damage. This is harmful to the engine durability and safety. Besides the vortex, inlet flow separation could induce high level of blade vibration, or aerodynamic instability, such as rotating stall. Cross wind may also lead to both vortex and flow distortion, which is more challenging for engine stability. Therefore, vibration characteristics and forced response under vortex ingestion should be evaluated to ensure the stability and safety of the engine in design phase.

This paper presents a computational study of the forced response of a wide-chord fan blade under vortex ingestion. A finite element model was built, and modal analysis was conducted to characterize the vibrating characteristics of the fan blade with a corresponding Campbell diagram. Transient simulations of vortex passing over the fan blade were conducted with and without the blade pre-vibration at the natural frequency of the first bending mode. The forced response level was evaluated under various conditions, including different hitting time and increasing intensity of vortex. Results showed that the ingested vortex is able to amplify the displacement and vibratory response to a significant level of 18% at most. Linear relation between vortex intensity and blade response was found. The results give a comprehensive prediction of forced response for a better blade design against vortex ingestion.
Influence of Non-Axisymmetric Confinement on the Hydrodynamic Stability of Multi-Nozzle Swirl Flows
Authors: Subramanian Harish G. , Manoharan Kiran , Hemchandra Santosh
Published: August 2018

Interaction between coherent flow oscillations and the pre-mixed flame sheet in combustors can result in coherent unsteadiness in the global heat release response. These coherent flow oscillations can either be self-excited (eg. the Precessing Vortex Core) or result from the hydrodynamic response of the flow field to acoustic forcing. Recent work has focused on understanding the various instability modes and fundamental mechanisms that control hydrodynamic instability in single nozzle swirl flows. However, the effect of multiple closely spaced nozzles as well as the non-axisymmetric nature of the confinement imposed by the combustor liner on swirl nozzle flows remains as yet unexplored. We study the influence of inter-nozzle spacing and non-axisymmetric confinement on the local temporal and spatiotemporal stability characteristics of multi-nozzle flows in this paper. The base flow model for the multi nozzle case is constructed by superposing contributions from a base flow model for each individual nozzle. The influence of the flame is captured by specifying a spatially varying base flow density field. The non-axisymmetric local stability problem is posed in terms of a parallel base flow with spatial variations in the two directions perpendicular to the streamwise direction. We investigate the case of a single nozzle and three nozzles arranged in a straight line within a rectangular combustor. The results show that geometric confinement imposed by the combustor walls has a quantitative impact on the eigenvalues of the hydrodynamic modes. Decreasing nozzle spacing for a given geometric confinement configuration makes the flow more unstable. The presence of an inner shear layer stabilized flame results in an overall stabilization of the flow instability. We also discuss qualitatively, the underlying vorticity dynamics mechanisms that influence the characteristics of instability modes in triple nozzle flows.
Second Law Analysis of Condensing Steam Flows
Authors: Grübel Marius , Schatz Markus , Vogt Damian M.
Published: August 2018

A numerical second law analysis is performed to determine the entropy production due to irreversibilities in condensing steam flows. In the present work the classical approach to calculate entropy production rates in turbulent flows based on velocity and temperature gradients is extended to two-phase condensing flows modeled within an Eulerian-Eulerian framework. This requires some modifications of the general approach and the inclusion of additional models to account for thermodynamic and kinematic relaxation processes.

With this approach, the entropy production within each mesh element is obtained. In addition to the quantification of thermodynamic and kinematic wetness losses, a breakdown of aerodynamic losses is possible to allow for a detailed loss analysis. The aerodynamic losses are classified into wake mixing, boundary layer and shock losses.

The application of the method is demonstrated by means of the flow through a well known steam turbine cascade test case. Predicted variations of loss coefficients for different operating conditions can be confirmed by experimental observations. For the investigated test cases, the thermodynamic relaxation contributes the most to the total losses and the losses due to droplet inertia are only of minor importance. The variation of the predicted aerodynamic losses for different operating conditions is as expected and demonstrates the suitability of the approach.
Measurement and Evaluation of Shaft Torsional Vibrations Using Shaft Instantaneous Angular Velocity
Authors: Liska Jindrich , Jakl Jan , Kunkel Sven
Published: August 2018

Online evaluation of possible failures of a turbine is a key factor for a successful long-term turbine operation. Fluctuations of generator air-gap torque, caused for example by non-stationary conditions of electrical power grid, influence shaft torsional vibrations as well as rotating blades vibrations. Symptoms of shaft torsional vibrations are not measurable by normally used relative shaft vibrations sensors, so special measurement must be used. The direct consequence of shaft torsional vibrations is local acceleration or deceleration of shaft circumference when it is measured by a stationary sensor. This article deals with a measurement method using an optical probe measuring the passage of black and white stripes of a zebra tape which is stuck on the rotor. The shaft torsional vibrations manifest themselves as a phase modulation of the optical probe output signal, so the sampling rate influence the achievable resolution of the calculated shaft vibrations. The presented method for the calculation of the shaft torsional vibrations is based on the evaluation of shaft instantaneous angular velocity. The advantage of this method is a direct compensation of possible non-regular geometry of the zebra tape. The analysis of shaft torsional vibrations evaluation using this method is supplemented by two case studies from the authors’ current work.
Vibration Isolation of Rotating Machinery by Parameters Switched Magnetic Bearing and High Static Low Dynamic Stiffness Supports
Authors: Ma Yanhui , Su Yixin , Yu Suyuan
Published: August 2018

Vibrations in rotating machinery cause many problems such as excessive noise and the vibration transmission to the supporting structure. In addition, the vibration isolation of the machinery with a wide range of rotational speed is a vital requirement and different to realize in practical engineering. As one of the key components in rotating machinery, bearing is not only the major vibration excitation source, but also the main vibration transmission path. Therefore, the mechanical property of the bearing has a significant effect on the vibration of the rotating machinery. Compared with traditional mechanical bearing, the equivalent stiffness and damping of magnetic bearing can be changed by adjusting its control parameters. On this basis, a novel design strategy of parameters switched magnetic bearing and high static low dynamic stiffness (HSLDS) supports for the vibration isolation of rotating machinery is proposed in this article. The design changes the equivalent linear stiffness and damping of the magnetic bearing by adjusting the control parameters according to the rotor speed, which will improve the vibration isolation capability of magnetic bearing at different rotor speed. Meanwhile, to further reduce the vibration transmission, the rotating machinery was suspended by nonlinear isolator with high static low dynamic stiffness characteristic. As an example, a rotating machinery system consisting of a rigid rotor supported by parameters switched magnetic bearings and high static low dynamic stiffness supports is studied. In this model, the rotor is excited by the unbalance force with different rotational speed. The amplitude of force acted on the base and the maximum relative displacement between the journal and the bearing are used to judge the isolation performance. Simulation results show that the demonstrated the effectiveness of this design strategy and the force transmission is significantly reduced with a wide rotational speed range.
An Optimization Design Platform for Fir-Tree Root and Groove for Steam Turbine
Authors: Yu Deqi , Zhang Xiaojun , Yang Jiandao , Cheng Kai , Shu Weilin , Lv Kai , Li Ming
Published: August 2018

Fir-tree root and groove profiles are widely used in gas turbine and steam turbine. Normally, the fir-tree root and groove are characterized with straight line, arc or even elliptic fillet and splines, then the parameters of these features were defined as design variables to perform root profile optimization. In ultra-long blades of CCPP and nuclear steam turbines and high-speed blades of industrial steam turbine blades, both the root and groove strength are the key challenges during the design process. Especially, in industrial steam turbines, the geometry of blade is very small but the operation velocity is very high and the blade suffers stress concentration severely. In this paper, two methods for geometry configuration and relevant optimization programs are described. The first one is feature-based using straight lines and arcs to configure the fir-tree root and groove geometry and genetic algorithm for optimization. This method is quite fit for wholly new root and groove design. And the second local optimization method is based on B-splines to configure the geometry where the local stress concentration occurs and the relevant optimization algorithm is used for optimization. Also, several cases are studied as comparison by using the optimization design platform. It can be used not only in steam turbines but also in gas turbines.
A Comparative Study of Data and Physically Based Gas Turbine Modeling for Long-Term Monitoring Scenarios: Part II — Emission Prediction Utilizing Different Levels of Design Information
Authors: Lipperheide Moritz , Bexten Thomas , Wirsum Manfred , Gassner Martin , Bernero Stefano
Published: August 2018

Reliable engine and emission models allow for an online monitoring of commercial gas turbine operation and help the plant operator and the original equipment manufacturer (OEM) to ensure emission compliance of the aging engine. However, model development and validation require fine-tuning on the particular engines, which may differ in a fleet of a single design type by production, assembly and aging status.

For this purpose, Artificial Neural Networks (ANN) offer a good and fast alternative to traditional physically-based engine modeling, because the model creation and adaption is merely an automatized process in commercially available software environments. However, ANN performance depends strongly on the availability of suitable data and a-priori data processing. The present work investigates the impact of specific engine information from the OEM’s design tools on ANN performance. As an alternative to a strictly data-based benchmark approach, engine characteristics were incorporated into ANNs by a pre-processing of the raw measurements with a simplified engine model. The resulting ‘virtual’ measurements, i.e. hot gas temperatures, then served as inputs to ANN training and application during long-term gas turbine operation. When processed input parameters were used for ANNs, overall long-term NOx prediction improved by 55%, and CO prediction by 16% in terms of RMSE, yielding comparable overall RMSE values to the physically-based model.
High Fidelity Modeling of the Acceleration of a Turboshaft Engine During a Restart
Authors: Ferrand Antoine , Bellenoue Marc , Bertin Yves , Cirligeanu Radu , Marconi Patrick , Mercier-Calvairac Fabien
Published: August 2018

In order to decrease the fuel consumption, a new flight mode is being considered for twin-engine helicopters, in which one engine is put into sleeping mode (a mode in which the gas generator is kept at a stabilized, sub-idle speed by means of an electric motor, with no combustion), while the remaining engine operates at nominal load. The restart of the engine in sleeping mode is therefore deemed critical for safety reasons. This efficient new flight mode has raised the interest in the modeling of the restart of a turboshaft engine.

In this context, the initial conditions of the simulations are better known relative to a ground start, in particular the air flow through the gas generator is constant, the fuel and oil system states are known and temperatures of the casings are equal to ambient. During the restart phase of the engine, the gas generator speed is kept at constant speed until the light-up is detected by a rise in inter-turbine temperature, then the starter torque increases, accelerating the engine towards idle speed. In this paper, the modeling of the acceleration of the gas generator from light-up to idle and above idle speeds is presented. Details on the light-up process are not addressed here.

The study is based on the high-fidelity aero-thermodynamic restart model that is currently being developed for a 2000 horse power, free turbine turboshaft. In this case, the term high-fidelity refers not only to the modeling of the flow path components but it also includes all the subsystems, secondary air flows and controls with a high level of detail.

The physical phenomena governing the acceleration of the turboshaft engine following a restart — mainly the transient evolution of the combustion efficiency and the power loss by heat soakage — are discussed in this paper and modeling solutions are presented. The results of the simulations are compared to engine test data, highlighting that the studied phenomena have an impact on the acceleration of the turboshaft engine and that the model is able to correctly predict acceleration trends.
An Investigation on the Loss Generation Mechanisms Inside Different Centrifugal Compressor Volutes for Turbochargers
Authors: Tanganelli Andrea , Balduzzi Francesco , Bianchini Alessandro , Ferrara Giovanni , Cencherle Francesco , De Luca Michele , Marmorini Luca
Published: August 2018

In centrifugal compressor design, the volute plays a key role in defining the overall efficiency and operating range of the stage. The flow at the impeller outlet is indeed characterized by a high kinetic energy content, which is first converted to potential energy in the diffuser downstream. The compressed gas is then collected by the volute at the cylindrical outlet section of the diffuser and directed to the intake piping, possibly with a further pressure recovery to enhance the stage performance. Due to the high flow speed at the volute inlet, the capability of ensuring the lowest amount of total pressure loss is pivotal to prevent a detriment of the machine efficiency. Moreover, the flow conditions change when the volute operates far from its design point: at mass flow rates lower than the design one, the flow becomes diffusive, while at higher mass flow rates the fluid is accelerated, thus leading to different loss-generation mechanisms. These phenomena are particularly relevant in turbocharger applications, where the compressor needs to cover a wide functioning range; moreover, in these applications the definition of the volute shape is often driven also by space limitations imposed by the vehicle layout, leading to a variety of volute types.

The present paper reports an analysis on the sources of thermodynamic irreversibilities occurring inside different volutes applied to a centrifugal compressor for turbocharging applications. Three demonstrative geometrical configurations are analyzed by means of 3D numerical simulations using common boundary conditions to assess the overall volute performance and the different loss mechanisms, which are evaluated in terms of the local entropy generation rate. The modification of the loss mechanisms in off-design conditions is also accounted for by investigating different mass flow rates.

It is finally shown that the use of the entropy generation rate for the assessment of the irreversibilities is helpful to understand and localize the sources of loss in relation to the various flow structures.
Novel Shape Parametrization Technique Applied to the Optimization of a Supersonic ORC Turbine Cascade
Authors: Romei Alessandro , Persico Giacomo
Published: August 2018

Shape-optimization techniques are nowadays an essential tool in the design chain of turbomachinery to strike the high-performance targets demanded by modern applications. Although the geometric parametrization may affect significantly the optimization cost and ultimately the optimization outcome, the selection of control points and of the subsequent design space is usually based on heuristic considerations.

This paper proposes a cost-efficient parametrization procedure based on the ANOVA analysis of B-Spline control points. To tackle the extremely large computational burden arisen from the use of several control points, a surrogate strategy is implemented, testing four different methods. The optimization relies on surrogate-assisted evolutionary strategies coupled with an experimentally validated CFD solver.

The technique is applied to a supersonic Organic Rankine Cycle turbine cascade, which features a converging-diverging bladed channel. It is shown that only the diverging section of the suction side as well as the adjacent region of unguided turning have the major impact on the aerodynamic performance. These findings enable to select an optimal distribution of mobile control points in the optimization block, leading to significant savings in computational cost.

Finally, three optimizations are carried out, varying locally the number of control points; results are widely discussed in terms of both optimization outcomes and optimizer robustness.
Secondary Flows in LPT Cascades: Numerical and Experimental Investigation of the Impact of the Inner Part of the Boundary Layer
Authors: Giovannini Matteo , Rubechini Filippo , Marconcini Michele , Simoni Daniele , Yepmo Vianney , Bertini Francesco
Published: August 2018

Due to the low level of profile losses already reached in the design of modern low-pressure turbines for turbofan applications, a renewed interest is devoted to the other sources of loss, and namely to the secondary losses. At the same time, the importance of secondary losses has been reinforced by the current design trend towards high-lift profiles. A great attention, therefore, is dedicated to reliable and effective prediction methods as well as on the correct understanding of the mechanisms that drive the secondary flows.

In this context, a systematic numerical and experimental campaign was carried out focusing on the impact of different inlet boundary layer (BL) profiles and considering a state-of-the-art low-pressure turbine cascade.

Starting from a computational environment representative of a design standard, detailed RANS analyses were carried out in order to establish dependable guidelines for the computational setup. As a major result, such analyses also underlined the importance of the shape of the inlet BL very close to the endwall, hence suggesting tight requirements for the characterization of the experimental environment.

The impact of the inlet BL profile on the secondary flow development was experimentally investigated by varying the profile shape very close to the endwall as well as on the external part with respect to a reference condition. The effects on the cascade performance were evaluated focusing on the intensity of the over-under-turning as well as on the associated losses (intensity and penetration) by measuring the span-wise distributions of flow angle and total pressure losses at the cascade exit plane. For all the inlet conditions, comparisons between CFD and experimental results are discussed.

Besides providing guidelines for a proper numerical and experimental setup, the present paper underlines the importance of a detailed characterization of the inlet BL for an accurate assessment of the secondary flows. From a broader perspective, when aiming at reproducing (numerically or experimentally) a real engine environment, this suggests that an in-depth matching of the inlet profiles is crucial for reliable estimates of the secondary losses.
Experimental and Numerical Investigation of the Flow in a Trailing Edge Ribbed Internal Cooling Passage
Authors: Baek Seungchan , Lee Sangjoon , Hwang Wontae , Park Jung Shin
Published: August 2018

The flow field in a ribbed triangular channel representing the trailing edge internal cooling passage of a gas turbine high pressure turbine blade is investigated via Magnetic Resonance Velocimetry (MRV) and Large Eddy Simulation (LES). Results are compared to a baseline channel with no ribs. LES predictions of the mean velocity fields are validated by the MRV results. In the case of the baseline triangular channel with no ribs, the mean flow and turbulence level at the sharp corner are small, which would correspond to poor heat transfer in an actual trailing edge. For the staggered ribbed channel, turbulent mixing is enhanced, and flow velocity and turbulence intensity at the sharp edge increase. This is due to secondary flow induced by the ribs moving toward the sharp edge in the center of the channel. This effect is expected to enhance internal convective heat transfer for the turbine blade trailing edge.
The Dynamic Load of Inter-Shaft Bearing and its Transmission Characteristic of Complex Rotor System
Authors: Yu Huan , Sun Bo , Ma YanHong , Hong Jie
Published: August 2018

Aiming at the dynamic design of dual rotor of aero-engine, the dynamic model of dual rotor system was established. And then based on the simulation results of the dynamic model established above, the unbalance distribution of dual rotor and the strain energy distribution of the flexural deformation were analyzed. The results showed that the dynamic load on inter-shaft bearing is determined by the rotor vibration mode and the rotor vibration response at the position of bearing. The flexural deformation can also affect the dynamic load on inter-shaft bearing. Under the influence of elastic restoring force generated by the flexural deformation of rotors, the dynamic load on the inter-shaft bearing under flexural mode is higher than that under the rigid mode. It was found that large dynamic load may act on inter-shaft bearing even though the rotor is not operating in the resonance state. And due to the internal damping of the rotor, the large dynamic load on the inter-shaft bearing can be obviously reduced when the load transmit to other bearings via the rotor with flexural deformation.
Blade Fissure Determination by Means of Blade Tip-Timing System
Authors: Mileshin Victor , Fateev Victor , Stepanov Alexander
Published: August 2018

Since the turbomachinery came into existence there is control issue of rotor blade stress condition and blade health monitoring both aero engines in service and in process of impeller machines refinement. This issue relevance is to rotor blade failure lead to serious damage of compressor and aero engines in whole. The timely detection rotor blade failure such as a fissure can prevent emergency and enabled to avoid costly repair. One of the techniques for determination of rotor blades vibration behavior is the blade tip-timing method. Of late years this method find use for turbo machines testing and it has already considerable theoretical and practical experience of application. This work presents the application of blade tip-timing system for measurements of rotor blades vibration parameters in order to detect of rotor blade fissures. The paper presents the instance of blade fissure detection in model fan C180-2 with blade number equaled 18 and circumferential speed 400 m/s by means of non-contact vibration measurement system during the input non-uniformity test. Also data processing of tests was carried out for rotor blade fissure detection in compressor stage. Some assessment criteria of rotor blade condition are considered for blade health monitoring. The purpose of this work is to estimate as a rule famous methodologies of rotor blade failure determination in turbo machines by means blade tip-timing system during the test.
Flow Physics of Diffused-Exit Film Cooling Holes Fed by Internal Crossflow
Authors: McClintic John W. , Fox Dale W. , Jones Fraser B. , Bogard David G. , Dyson Thomas E. , Webster Zachary D.
Published: August 2018

Internal crossflow, or internal flow that is perpendicular to the overflowing mainstream, reduces film cooling effectiveness by disrupting the diffusion of coolant at the exit of axial shaped holes. Previous experimental investigations have shown that internal crossflow causes the coolant to bias toward one side of the diffuser and that the severity of the biasing scales with the inlet velocity ratio, VRi, or the ratio of crossflow velocity to the jet velocity in the metering section of the hole. It has been hypothesized and computationally predicted that internal crossflow produces an asymmetric swirling flow within the hole that causes the coolant to bias in the diffuser and that biasing contributes to ingestion of hot mainstream gas into the hole, which is undesirable. However, there are no experimental measurements as of yet to confirm these predictions. In the present study, in and near-hole flow field and thermal field measurements were performed to investigate the flow structures and mainstream ingestion for a standard axial shaped hole fed by internal crossflow. Three different inlet velocity ratios of VRi = 0.24, 0.36, and 0.71 were tested at varying injection rate. Measurements were made in planes normal to the nominal direction of coolant flow at the outlet plane of the hole and at two downstream locations — x/d = 0 and 5. The predicted swirling structure was observed for the highest inlet velocity ratio and flow within the hole was shown to scale with VRi. Ingestion within the diffuser was significant and also scaled with VRi. Downstream flow and thermal fields showed that increased biasing contributed to more severe jet detachment and coolant dispersion away from the surface.
RANS Simulation of a Radial Compressor With Supercritical CO2 Fluid for External Loss Model Development
Authors: Kim Seong Gu , Cho Seong Kuk , Lee Jeong Ik , Lee Jekyoung , Lee Si Woo , Addad Yacine , Cha Jae Eun
Published: August 2018

From the observation of previous experimental results, it was recognized that the amount of external losses at the external flow paths can be substantial. Understanding the external loss mechanisms, and reducing the external losses are crucial to the design of a high performance S-CO2 compressor. Therefore, this study first focused on verifying the existing external loss models by estimating the internal loss accurately through a high fidelity CFD calculation and then substituting from the total losses. By analyzing the CFD result and energy balance data obtained from the experiment, the authors are suggesting the best combination of the external loss models for the S-CO2 compressor. This is followed by the investigation of the condensation effect in the impeller by utilizing a two-phase VOF model with metastable properties. As a result, it is re-confirmed that there is a negligible amount of condensation effect.