bluuebunny/crossref_metadata_2025_split huggingface dataset:
Numerical Investigation of Diffuser Rotating Stall and System Instability in a Centrifugal Compressor With Vaned Diffuser
Authors: Zhao Yang , Xi Guang , Zhao Jiayi 
Published: August 2018

The operating range of a centrifugal compressor is often limited by the occurrence of the flow instability, such as diffuser rotating stall or system surge. In the paper, the unsteady numerical simulations are performed on a low-speed centrifugal compressor to investigate the characteristic of the rotating stall in the vaned diffuser. And also, the developed model of lumped parameter is used to predict the system instability.
The flow field in the diffuser is firstly investigated at near stall condition. It is found that the leading-edge vortex and the secondary flow induce the hub-corner separation at the suction side of the diffuser blade. When the mass flow rate is reduced gradually, the fore part of the volute turns to act as a diffuser from a nozzle. Under the influence of the asymmetry induced by the volute, the hub-corner separation firstly develops into rotating stall in the passage with the lowest mass flow rate when at critical stall point. And then the diffuser rotating stall propagates along the circumferential direction at about 7% of the impeller speed.
And also, the model of lumped parameter considering the effect of rotating stall is developed to analyze the system instability of mild surge. The predicted vibration frequency is within 5.8% of the measurement and the predicted transient process in mild surge matches well with the measurement. With different volume of the compressed air, the transient compressor characteristic tends to be stabilized or oscillates in a cycle along the counter-clockwise with different magnitude.
Application of Kriging Metamodels to the Automated Start Value Generation for Gas Turbine Performance Simulations
Authors: Schmeink Jens , Becker Richard-Gregor 
Published: August 2018

This paper presents a process for automated start value generation for gas turbine performance simulations using Kriging metamodels. The metamodels are trained on a small number of pre-selected operating points in the flight envelope. Predictions of the trained metamodels are used as initialization parameters for subsequent performance simulations of arbitrary operating points in order to increase robustness and computational speed of the numerical process. Different approaches for the selection of the training points are evaluated. A comparison to the classical approach of table-based initialization is carried out to highlight the advantages and disadvantages of the new methodology.
Furthermore, the inclusion of supplementary operating points into the training sample of the metamodels is analyzed. Depending on certain criteria, such as the difference of prediction and simulation result, operating points are included into the sample and a retraining of the metamodels is performed. Simulations using the retrained models as guess value generators are compared to the previous Kriging approach. The advantages and disadvantages of the retraining approach are discussed.
Performance of a Semi-Closed Oxy-Fuel Combustion Combined Cycle (SCOC-CC) With an Air Separation Unit (ASU)
Authors: Sammak Majed , Thern Marcus , Genrup Magnus 
Published: August 2018

The objective of this paper is to evaluate the performance of a semi-closed oxy-fuel combustion combined cycle (SCOC-CC) and its power penalties. The power penalties are associated with CO2 compression and high-pressure oxygen production in the air separation unit (ASU). The paper discusses three different methods for high pressure oxygen (O2) production. Method 1 is producing O2 directly at high pressure by compressing the air before the air separation takes place. Method 2 is producing O2 at low pressure and then compressing the separated O2 to the desired pressure with a compressor. Method 3 is alike the second method, except that the separated liquid O2 is pressurized with a liquid oxygen pump to the desired pressure.
The studied SCOC-CC is a dual-pressure level steam cycle due to its comparable efficiency with three pressure level steam cycle and less complexity.
The SCOC-CC, ASU and CO2 compression train are modeled with the commercial heat and mass balance software IPSEpro. The paper analyzed the SCOC-CC performance at different combustion outlet temperatures and pressure ratios. The combustion outlet temperature (COT) varied from 1200 °C to 1550 °C and the pressure ratio varied from 25 to 45.
The study is concerned with mid-sized SCOC-CC with a net power output 100 MW. The calculations were performed at the selected design point which was at 1400°C and pressure ratio at 37. The calculated power consumption of the O2 separation at a purity of 95 % was 719 kJ/kgO2. The power consumption for pressurizing the separated O2 (method 2) was 345 kJ/kgO2 whereas it was 4.4 kJ/kgO2 for pumping liquid O2 to the required pressure (method 3). The calculated power consumption for pressurizing and pumping the CO2-enriched stream was 323 kJ/kgCO2.
The SCOC-CC gross efficiency was 57.6 %. The SCOC-CC net efficiency at method 2 for air separation was 46.7 %. The gross efficiency was reduced by 9 % due to ASU and other 2 % due to CO2 compression. The SCOC-CC net efficiency at method 3 of the air separation was 49.6 %. The ASU reduced the gross efficiency by 6 % and additional 2 % by CO2 compression. Using method 3 for air separation gave a 3 % gain in cycle efficiency.
Influence of Turbocharger Turbine Blade Geometry on Vibratory Blade Stresses
Authors: Naik Pavan , Lehmayr Bernhard , Homeier Stefan , Klaus Michael , Vogt Damian M. 
Published: August 2018

In this paper, a method to influence the vibratory blade stresses of mixed flow turbocharger turbine blade by varying the local blade thickness in spanwise direction is presented. Such variations have an influence on both the static and the vibratory stresses and therefore can be used for optimizing components with respect to High-Cycle Fatigue (HCF) tolerance. Two typical cyclic loadings that are of concern to turbocharger manufacturers have been taken into account. These loadings arise from the centrifugal forces and from blade vibrations.
The objective of optimization in this study is to minimize combined effects of centrifugal and vibratory stresses on turbine blade HCF and moment of inertia. Here, the conventional turbine blade design with trapezoidal thickness profile is taken as baseline design. The thicknesses are varied at four span-wise equally spaced planes and three stream-wise planes to observe their effects on static and vibratory stresses. The summation of both the stresses is referred to as combined stress. In order to ensure comparability among the studied design variants, a generic and constant excitation order dependent pressure field is used at a specific location on blade.
The results show that the locations of static and vibratory stresses, and hence the magnitude of the combined stresses, can be influenced by varying the blade thicknesses while maintaining the same eigenfrequencies. By shifting the maximum vibratory stresses farther away from the maximum static stresses, the combined stresses can be reduced considerably, which leads to improved HCF tolerance.
Twin-Web Turbine Discs: Part 1 — Design and Analysis of Their Efficiency
Authors: Vasilyev Boris , Salnikov Anton , Semenov Artem , Magerramova Liubov 
Published: August 2018

Part 1 of this work discusses the numerical results of the performance efficiency investigation conducted for twin-web (hollow) discs in terms of stress and mass reductions.
Three typical turbine discs and one small-sized turbine blisk are analysed using parameterised finite-element models and the modern methods of multi-criteria optimisation. The discs and blisk have different overall sizes and load conditions (rotation speed [rpm]), thermal state, blade load, etc.). The design of disc variants with a split in the hub is also considered. This option will help to simplify the manufacturing technology and improve design efficiency.
An analysis of the results shows that a twin-web turbine disc design permits increases in turbine disc strength as well as decreases in its mass. The extent of these advantages depends on the overall size of the disc and its rpm: a large-sized disc with a relatively low rpm shows a 4% mass decrease and a 15% increase in cyclic life; a medium-sized disc with an average rpm has an 18% mass decrease and a doubled cyclic life; and a small-sized disc with high rpm has a 30% mass decrease and a 2.8 times longer cyclic life. Additionally, a small-sized blisk with a high rpm shows a 60% mass and a doubled cyclic life.
Rotor Over-Speed Analysis of a Hybrid Solar Gas Turbine
Authors: Kathirgamanathan Darsini , Axelsson Lars-Uno 
Published: August 2018

As the demand for environmentally friendly power production is increasing, the Hybrid Solar Gas Turbine (HSGT) power plant has the potential to provide a reliable electricity production with low carbon foot print by combining the solar heat and the fossil fuel. Compared to the conventional gas turbine system, the HSGT system has an additional heat source and larger volume due to the solar receiver and the ducting system required for guiding the air to and from the solar receiver. The additional heat source and larger volume increase the thermal capacitance which adds complexity to the operational behavior of the gas turbine.
This paper presents an investigation of the transient behavior of a hybrid solar gas turbine. The OP16 gas turbine, rated at 1.85MWe, is integrated into a solar thermal power plant as part of an ongoing R&D project. To analyze the shaft over-speed of the system, a model is created using the commercial cycle analysis software GSP. The model is validated against the measured data for a non-solar mode operation as it is similar to the conventional gas turbine operation on pure fossil fuel.
A rotor over-speed analysis of the OP16-HSGT system during load-shed shows that the rotor over-speed exceeds the acceptable limit due to the thermal capacitance and volumetric effects. An increase in shaft inertia, or re-direction of the air flow from hybrid-solar mode to non-solar mode by using control valves, or the use of blow-off valve to vent out the solar heat can be taken as safety measures to reduce rotor over-speed. It was found that the control valve should act within 0.3s to maintain acceptable rotor over-speed. As a control valve with such characteristics is scarcely available, a concept where the OP16-HSGT system uses a blow-off valve in addition to the control valve was developed. The blow-off valve reduces the rotor over-speed from 111% to 103% by venting out the thermal capacitance from the OP16-HSGT system and by that one can achieve safe operation during a load-shed or an emergency shutdown.
Fleet Monitoring and Diagnostics Framework Based on Digital Twin of Aero-Engines
Authors: Zaccaria Valentina , Stenfelt Mikael , Aslanidou Ioanna , Kyprianidis Konstantinos G. 
Published: August 2018

Monitoring aircraft performance in a fleet is fundamental to ensure optimal operation and promptly detect anomalies that can increase fuel consumption or compromise flight safety. Accurate failure detection and life prediction methods also result in reduced maintenance costs. The major challenges in fleet monitoring are the great amount of collected data that need to be processed and the variability between engines of the fleet, which requires adaptive models.
In this paper, a framework for monitoring, diagnostics, and health management of a fleet of aircrafts is proposed. The framework consists of a multi-level approach: starting from thresholds exceedance monitoring, problematic engines are isolated, on which a fault detection system is then applied. Different methods for fault isolation, identification, and quantification are presented and compared, and the related challenges and opportunities are discussed. This conceptual strategy is tested on fleet data generated through a performance model of a turbofan engine, considering engine-to-engine and flight-to-flight variations and uncertainties in sensor measurements. Limitations of physics-based methods and machine learning techniques are investigated and the needs for fleet diagnostics are highlighted.
Comparison of DDES and URANS for Unsteady Tip Leakage Flow in an Axial Compressor Rotor
Authors: Liu Yangwei , Zhong Luyang , Lu Lipeng 
Published: August 2018

Tip leakage flow (TLF) has a large impact on compressor performance and should be accurately predicted by CFD methods. New approaches to turbulence modelling, such as Delayed Detached Eddy Simulation (DDES) have been proposed which allow for greater accuracy of the numerical predictions while the mesh size as well as the central processing unit (CPU) time can be tremendously reduced. In this paper, the numerical simulation of the rotor in a low-speed large-scale axial compressor based on DDES and unsteady Reynolds-Averaged Navier-Stokes (URANS) turbulence models is performed subject to different working conditions of different tip gap sizes, to improve our understanding about the tip leakage vortex (TLV) dynamic mechanisms and discrepancy of these two methods.
The rotor simulations are carried out on the same mesh based on the DDES requirements, and compared to experimental stereoscopic particle image velocimetry (SPIV) data before flow physics investigation. The simulation results capture the location of TLV and show qualitatively good agreement with experiments. DDES method brings improvements in time-averaged results over the Reynolds-Averaged Navier-Stokes (RANS) method. Firstly, the time-averaged and instantaneous results are compared to divide the TLV into three parts. Then the anisotropy of the Reynold stress of these two methods are analyzed in the tip leakage flow through Lumley triangle, to see the difference between the way to get the Reynold stress, using Boussinesq-type approximation in URANS and unsteady velocity fluctuation in DDES. The turbulent anisotropy can be derived from the nondimensional form of the anisotropy tensor. In the rotor simulation, the anisotropy varies along the TLV. The anisotropy of RANS method is weak because the eddy viscosity model is isotropic. Last, the velocity fluctuation data along the TLV core at different chord length are transformed to velocity spectra using fast Fourier transformation (FFT) method to discuss the TLV unsteadiness. The high frequency amplitude becomes obvious downstream the TLV because of losing stability. The result of URANS also has a conspicuous frequency but lower than DDES. The amplitude of URANS is much smaller. From this investigation it is determined that URANS can only predict the large-scale low-frequency dominant unsteady features. While the scale of dissipation and unsteadiness of TLV is small, the Reynolds averaging of URANS may neglect this unsteady phenomenon.
Applying Dynamic Programming Algorithms to the Energy Management of Hybrid Electric Aircraft
Authors: Donateo Teresa , Ficarella Antonio , Spedicato Luigi 
Published: August 2018

To explore the application of dynamic programming (DP) to the energy management strategies of hybrid electric aircraft, a hybrid powertrain for a lightweight rotorcraft is introduced and its dynamic control model is designed. The model is conceived for the Agusta-Westland A109 helicopter, a twin-engine rotorcraft used in various roles, such as light transport, search-and-rescue and military roles. The turboshaft single spool engines are modeled with the use of performance maps that allow part load specific fuel consumption to be calculated as a function of actual power request and flight conditions. The state-of-the-art lithium polymer batteries are used for the hybridization and their behavior is evaluated by the Sheperd-Peukert model. The control problem is modeled through a graph structure where a node is obtained from the intersection between a time value, representing the starting of a phase of flight, and a splitting factor, representing the percentage of propulsive power required to the battery in such a phase. The edge connecting two nodes concerns with the state transition and the weight of the edge refers to the transition cost. The goal is to find an optimal splitting sequence to minimize the total cost over the whole mission, that is given with regard to speed and altitude. The Dijkstra algorithm, which allows the shortest energy path to be found between nodes in a graph, is used to look for the optimum. A local optimum is achieved when the cost is defined as the fuel consumption whereas the global optimum can be attained when the model is enhanced to include the effect of the battery usage into the cost.
The results are compared with the original non-hybrid case and the engine efficiency was suitable evaluated. The applicability to other mission data is suitably evaluated so as to deduce the concept of similarity of mission.
Experimental Analysis of a NACA 0021 Airfoil Under Dynamic Angle of Attack Variation and Low Reynolds Numbers
Authors: Holst D. , Church B. , Wegner F. , Pechlivanoglou G. , Nayeri C. N. , Paschereit C. O. 
Published: August 2018

The wind industry needs reliable and accurate airfoil polars to properly predict wind turbine performance, especially during the initial design phase. Medium- and low-fidelity simulations directly depend on the accuracy of the airfoil data and even more so if e.g. dynamic effects are modeled. This becomes crucial if the blades of a turbine operate under stalled conditions for a significant part of the turbine’s lifetime. In addition, the design process of vertical axis wind turbines (VAWTs) needs data across the full range of angles of attack between 0 and 180 deg.
Lift, drag and surface pressure distributions of a NACA 0021 airfoil equipped with surface pressure taps were investigated based on time-resolved pressure measurements. The present study discusses full range static polars and several dynamic sinusoidal pitching configurations covering two Reynolds numbers Re = 140k and 180 k, and different incidence ranges: near stall, post stall and deep stall. Various bi-stable flow phenomena are discussed based on high frequency measurements revealing large lift-fluctuations in the post and deep stall regime that exceed the maximum lift of the static polars and are not captured by averaged measurements. Detailed surface pressure distributions are discussed to provide further insight into the flow conditions and pressure development during dynamic motion.
The experimental data provided within the present paper is dedicated to the scientific community for calibration and reference purposes, which in the future may lead to higher accuracy in performance predictions during the design process of wind turbines.
Analysis of Nonlinear Modal Damping due to Friction at Blade Roots Using High-Fidelity Modelling
Authors: Chen Junjie , Zang Chaoping , Zhou Biao , Petrov E. P. 
Published: August 2018

In this paper, a methodology is developed for analysis of modal damping in root joints of bladed discs using large finite element models and detailed description of friction contacts at contact interfaces of the joints. The methods allows the analysis of: (i) a single blade vibration and (ii) a bladed-disc assembly for any family of modes (lower and higher modes) calculating the modal damping factors for different levels of vibrations.
Three-dimension solid finite element models are used in the calculations. The analysis is performed in time domain through the transient dynamics analysis. The methodology allows the use of widely available finite element packages and based on the direct calculation of the energy dissipated at root joints due to micro-slip over the multitude of contact elements modelling the surface-to-surface friction contact interactions.
The numerical studies of the dependency of modal damping factors on the vibration amplitudes are performed for simplified and realistic bladed disc models for different blade mode shapes, engine-order excitation numbers and nodal diameter numbers using high-fidelity models.
Implementation of the Multi-Level Multi-Integration Cluster Method to the Treatment of Vortex Particle Interactions for Fast Wind Turbine Wake Simulations
Authors: Saverin Joseph , Marten David , Pechlivanoglou George , Paschereit Christian Oliver , van Garrel Arne 
Published: August 2018

A method for the treatment of the evolution of the wake of aerodynamic bodies has been implemented. A vortex particle method approach has been used whereby the flow field is discretized into numerical volumes which possess a given circulation. A lifting line formulation is used to determine the circulation of the trailing and shed vortex elements. Upon their release vortex particles are allowed to freely convect under the action of the blade, the freestream and other particles. Induced velocities are calculated with a regularized form of the Biot-Savart kernel, adapted for vortex particles. Vortex trajectories are integrated in a Lagrangian sense. Provision is made in the model for the rate of change of the circulation vector and for viscous particle interaction; however these features are not exploited in this work. The validity of the model is tested by comparing results of the numerical simulation to the experimental measurements of the Mexico rotor. A range of tip speed ratios are investigated and the blade loading and induced wake velocities are compared to experiment and finite-volume numerical models.
The computational expense of this method scales quadratically with the number of released wake particles N. This results in an unacceptable computational expense after a limited simulation time. A recently developed multilevel algorithm has been implemented to overcome this computational expense. This method approximates the Biot-Savart kernel in the far field by using polynomial interpolation onto a structured grid node system. The error of this approximation is seen to be arbitrarily controlled by the polynomial order of the interpolation. It is demonstrated that by using this method the computational expense scales linearly. The model’s ability to quickly produce results of comparable accuracy to finite volume simulations is illustrated and emphasizes the opportunity for industry to move from low fidelity, less accurate blade-element-momentum methods towards higher fidelity free vortex wake models while keeping the advantage of short problem turnaround times.