bluuebunny/crossref_metadata_2025_split huggingface dataset:
Prediction in Rotor-Stator Cavities at High Reynolds Numbers up to Re ≈ 2·108: Towards a New Parametric Model
Authors: Schröder Tilman Raphael , Schuster Sebastian , Brillert Dieter 
Published: January 2021

Abstract
Side chambers of centrifugal turbomachinery resemble rotor–stator cavities. The flow in these cavities develops complex patterns which substantially influence the axial thrust on the shaft and the frictional torque on the rotor. Axial thrust caused by the flow pattern in side chambers accumulates in multistage single shaft radial compressors where it is often balanced by a single axial bearing. Miscalculation of axial thrust may lead to axial loads significantly higher than predicted or even undefined load situations which may cause early bearing failure. Likewise, a wrong prediction of friction losses may lead to lower efficiency than originally intended. Current models for axial thrust and friction torque are limited to circumferential Reynolds numbers of Re ≤ 107. New models are needed for modern high-pressure centrifugal compressors which reach circumferential Reynolds numbers up to Re = 109.
The rotor–stator cavity flow model by Kurokawa and Sakuma [16] for merged boundary layers is analysed. It is based on the assumptions of axisymmetric and time invariant flow. Functional forms of the mean tangential and radial velocity and the surface stress vectors on the rotor and stator are assumed. Reynolds averaging is applied to consider turbulence effects in the model. The modelling assumptions are compared with detailed RANS CFD analyses at Reynolds numbers of 4 · 106 ≤ Re ≤ 2 · 108 to investigate their accuracy. Based on these CFD results, a way towards a high Reynolds number model is presented, providing prediction of disc torque, radial pressure distribution and axial thrust in rotor–stator cavities.
Microstructure and Oxidation Behaviour of NGB and WGB Joints With Boron/Silicon Free Nickel Base Braze Alloys
Authors: Huang Xiao 
Published: January 2021

Abstract
In this study, the microstructure and solidus and liquidus of several Ni-Co-Hf-Zr-Ti-Al braze alloys were first examined with the objective to develop a B and Si free low melting braze alloy for narrow gap (NGB) and wide gap brazing (WGB) and turbine component repair applications. Among various alloys examined, DSC was used to measure the solidus and liquidus during heating and cooling cycles. Following the measurements of liquidus and solidus, the microstructure was evaluated using SEM. Equations for calculating solidus and liquidus based on alloy’s compositions were established and the functions of each elements on these two characteristic temperatures were discussed. One selected alloy with a liquidus of 1201 °C was further employed for NGB and WGB experiments. The results showed that it was able join CMSX-4 at 1240°C without interfacial voids; and with the use of externally applied pressure and extended homogenization treatment the interfacial intermetallic compounds were substantially removed. Furthermore, the same braze alloy was used to fill a large artificial cavity in a WGB scheme at a reduced temperature of 1200°C. The braze alloy was able to fully bond the filler powder alloy in addition to join the two alloys to a IN 738 substrate. Finally, oxidation test was conducted at 1050°C (isothermal in static air) for 100 hours after NGB of CMSX-4 and WGB of IN 738. The results showed that the oxide formed on the standalone braze alloy is very dense and there is no sign of spallation. It contained primarily NiO (+CoO) with no other elements measured. For the NGB joints, large amount of scale spallation was observed on base alloy CMSX-4 while the NGB joint remained spallation free. The oxide formed on the NGB was NiO with partitions of Co, Al, Ti, Cr, and W. The WGB joint region in IN 738 showed oxide scale spallation on the IN 738 substrate side, leaving behind steps and depression on the sample surface. In the WGB joint itself, there were three notable phases after oxidation test, however, no scale spallation could be found. For the majority part of the surface, a Ni-rich oxide covered the surface. There were areas of smaller oxide particles with higher Cr content. Overall, the new boron/silicon free braze alloy was found to be able to join several superalloys in both WGB and NGB schemes without occurrence of defects and the oxidation resistance was superior to both substrate alloys examined in this study.
The Effect of Sand Erosion on a Compressor Blade and its Modal Properties
Authors: Weber Robby , Kühhorn Arnold , Klauke Thomas , Schrape Sven 
Published: January 2021

Abstract
The wear and damage of High-Pressure Compressor (HPC) blades due to erosion or Foreign Object Damage (FOD) have a significant influence on HPC aerodynamic performance, vibration resistance against High-Cycle Fatigue (HCF) and thus component lifetime. The changes in airfoil geometry reduce the overall engine efficiency. Furthermore extended off-wing engine maintenances due to blade failures are increasing the cost of ownership. The safe operation of every engine within a reduced number of shop visits requires a reliable prediction of future deterioration. This enables the optimization of services and off-wing time. One contribution to this is a better understanding of the component’s dynamics and based on this providing an improved wear modeling to reliably predict the remaining lifetime and the decreased efficiency.
This contribution determines the material removal of HPC blades due to sand erosion. Originally, this stage was built as a blisk (Blade Integrated Disk). After sand erosion test completion, the blisk was cut into segments containing one airfoil only. First, the material removal is determined for ten blades of one exemplary rotor. A blue light fringe projector is employed to identify the geometrical differences between the eroded blades and the nominal design. Second, realistic finite element models are generated to enable comparable modal analyses of eroded blades. This procedure suffers from unavoidable and mostly random imperfections due to the manufacturing process, which significantly affects the blade surface before the erosion test can be conducted. Therefore, an already published approach is implemented in the third step to predict the blade surface after erosion based on nominal blade design. The investigation is completed by comparing measured and predicted surfaces. Finally, the aforementioned tool is employed to predict the locations and intensities of the material losses and the accompanying change in modal properties of this compressor blade concerning operational time.
Experimental Research on the Characteristics of Thermal Bow in an Aeroengine HP Spool
Authors: Yu Xiangyu , Liu Zhansheng , Zhou Zhaohua , Feng Jigang , He Peng 
Published: January 2021

Abstract
During the cooling process after shut down for aeroengines, internal hot air rises and cold air drops due to natural convection, which makes uneven temperature distribution in the casing and creates temperature difference in radial and axial directions, causing uneven deformation of rotor. Once aeroengine starts after a little time of cooling, thermal bow forms more easily, causing excessive vibration. In some cases, the thermal bow can be so severe that the engine will be unable to start. Based on the rotor for one certain type of aeroengine, the paper describes an experiment of thermal bow failure, which is divided into a static one and a dynamic one, both having simulated the uneven temperature field. Firstly, the static experiment measures temperature difference and deformation of rotor in different temperature environments and dissimilar cold blowing conditions. Results show temperature difference of each cross section increases with the growth of casing temperature. And cold blowing can quickly and effectively eliminate uneven temperature distribution and structural deformation. Secondly, the dynamic experiment produces the results that the vibration amplitude increases obviously when rotating frequency approaches critical speeds (2365r/min and 2892r/min). As the cooling time increases, the amplitude decreases until normal, which is the most important feature different from that in failure of initial mass imbalance. Thermal bow mainly influences the fundamental frequency vibration. Cold blowing can quickly and effectively reduce vibration amplitude. The conclusions obtained from the dynamic experiments are consistent with the known regulations from engineering experience.
A Non-Uniformity Factor to Assess Fuel/Air Mixing Quality
Authors: Jiang Lei-Yong 
Published: January 2021

Abstract
A practical method to assess quantitatively the uniformity of fuel/air mixing is essential for research and development of advanced low-emission combustion systems. A non-uniformity factor, based on the fuel/air equivalence ratio, is proposed and demonstrated in a simple methane/air venturi mixer. It has two advantages over the unmixedness parameter and uniformity index approaches: it is correlated with the fuel/air mixture combustion property, and the maximum temperature deviation from the designed value caused by fuel/air non-uniformity can be estimated. Because of these, it can be used as a criterion to check fuel/air mixing quality, or as a target for fuel/air mixer design with acceptable maximum temperature deviation.
Test Results for the Static and Rotordynamic Characteristics of a Long (L/D = 0.75) Smooth Seal in Two-Phase (Mainly Gas) Conditions With a 62-Bar Inlet Pressure
Authors: Tran Dung L. , Childs Dara W. , Shrestha Hari , Zhang Min 
Published: January 2021

Abstract
Recent multiphase-pump developments encountered several rotordynamic issues with smooth balance-piston seals, creating a need to better understand the performance of annular seals under multiphase-flow operation. This paper presents measurements of static and dynamic characteristics of a long smooth seal (L/D = 0.75, D = 114.686 mm, and Cr = 0.200 mm) operating under pure- and mainly air condition in which air is mixed with silicone oil (PSF-5cSt). Tests are performed at a supply pressure of 62.1 bars-a, three rotation speeds (5, 10, 15 krpm), three pressure ratios (PRs) (0.6, 0.5, 0.4), for a range of inlet liquid volume fraction (LVFi) from 0% to 8%. The results are then compared to: (1) the previous test reported by Zhang et al. (2017, “Experimental Study of the Static and Dynamic Characteristics of a Long Smooth Seal with Two-Phase, Mainly-air Mixtures,” J. Eng. Gas Turbines Power, 139(12), p. 122504) with similar testing condition but a different seal geometry (L/D = 0.65, D = 89.306 mm, and Cr = 0.188 mm) and (2) the predictions from a bulk-flow model developed by San Andrés (2012, “Rotordynamic Force Coefficients of Bubbly Mixture Annular Pressure Seals,” ASME J. Eng. Gas Turbines Power, 134(2), p. 022503).
Results show a significant increase of direct dynamic stiffness KΩ as LVFi increases, especially at low PR. Test results reported by Zhang et al. (2017) has an opposite tendency of KΩ as an impact of increasing LVFi.
Concerning cross-coupled dynamic stiffness kΩ and cross-coupled damping c, the results from Zhang et al. (2017) and the present results agree to the effects of changing speed, PR, and LVFi under pure- and mainly air conditions.
As LVFi increases, direct damping C increases while test results reported by Zhang et al. (2017) showed no significant increase.
Except for the direct dynamic stiffness and the impact of changing LVFi on the cross-coupled dynamic stiffness, the bulk-flow model of San Andrés (2012) predicts decently the tendencies and magnitudes of the rotordynamic coefficients.
Analysis of Aeroacoustic Phenomena in Centrifugal Compressors Using a Lattice Boltzmann Flow Simulation Approach
Authors: Phogat Rahul , González Díez Néstor , Smeulers Jan , Casalino Damiano , Avallone Francesco 
Published: January 2021

Abstract
Impeller rotation, vortex shedding, secondary flows or a combination of these phenomena can lead to the generation of acoustic waves in the compressor cascade causing dynamic pressure loading on the impeller. When the eigenfrequency and eigenmode shape of the acoustic mode match with the structural ones of the impeller, high fatigue stresses and vibrations occur, which can lead to structural failure. It is well known that cavities enclosing shrouded impellers may strongly amplify the acoustic excitation of the impeller by means of Tyler-Sofrin modes; however, little knowledge is available about the physics of flow-induced noise and resonance mechanisms. In this research, a Lattice Boltzmann Method based approach is employed to predict the origin and amplitude of pressure loading responsible for the strong impeller trailing edge vibrations measured in experiments. The results reveal that this is caused by the acoustic mode generated from the interaction of upstream vane wakes with the impeller that is reflected by the return channel vanes. This research highlights the importance of accounting for aeroacoustic mechanisms in the design of centrifugal compressor stages and paves the way towards the numerical assessment of unsteady flow and resonance phenomena.
Modeling Variants of Heat Loss in RANS Based Simulation of Jet-and-Recirculation Stabilized Combustion
Authors: Grimm Felix , Spoerl Konstantinos , Drude Aurelia , Lingstädt Timo , Kutne Peter , Aigner Manfred 
Published: January 2021

Abstract
In combustion systems operated at low-calorific conditions, physical boundary conditions for numerical modeling have to be chosen carefully. In particular, heat-loss mechanisms have to be taken into consideration, especially when they are expected to be in the order of magnitude of combustion power output itself.
Therefore, in the presented paper, different modeling strategies for heat loss are taken into account and results are evaluated against experimental data in order to subsequently complete experimental findings with information on flow field and combustion. The studies are carried out on a low-power, laboratory scale micro gas turbine combustor, operated with off-gas from a solid oxide fuel cell (SOFC).
Several operation conditions are investigated with different formulations of boundary conditions for heat loss, whereas radiation effects are neglected due to low combustion temperatures. Numerical simulations of the application case are carried out with the commercial ANSYS Fluent CFD tool. Investigated modeling variants of heat loss are threefold. Those are isothermal temperatures, heat flux boundaries, where losses are estimated from preceding simplified simulations, and fully coupled modeling.
Averaged flame chemiluminescence measurements serve as validation data for the numerical results. Recommendations for the stable and reliable simulation of fluid-solid coupling are given for RANS based solution strategies. Generality of applicability is tested by applying the established numerical procedures to several base-load and part-load operation conditions. After validation, flow field and combustion properties of the operation points are discussed in view of heat loss influences based on numerical findings, which support the limited experimental data. Finally, advantages and drawbacks of the different approaches are discussed and overall respective heat loss is balanced and compared to overall thermal power of the combustion system.
Numerical Study on Duct Acoustic Modal and Source Flow Structure of Fan Tones With Leaned and Swept Stator
Authors: Liangfeng Wang , Luqin Mao , Kangshen Xiang , Wenhua Duan , Hang Tong , Weiyang Qiao 
Published: January 2021

Abstract
The present study is focused on the fan tone noise reduction with leaned and swept stator blade. A hybrid URANS/Goldstein’s equations method is used to calculate the unsteady flow and tone noise of a high loaded axial-flow fan. The numerical simulation results show that the fan tone noise will be increased with negative angle lean, while it will be reduced with positive lean angle. The higher the harmonic number, the larger the noise reduction with positive lean angle blade. It is found that when the stator blade sweeps back of 30 degrees, the fan tone noise at the fan inlet can be reduced by 5.5 dB, while the fan tone noise at the fan outlet can be reduced by 9.8dB. It is also found that the combined leaned and swept blade has the largest noise reduction. When the fan stator blade lean angle and sweep angle are both 30 degrees, the fan inlet tone noise can be reduced by 8.5 dB, while the fan outlet tone noise can be reduced by 17dB. The numerical simulation results indicate that the influence of blade lean and sweep on fan mass flow is less than 1% within the scope of this study. The negative lean angle of stator blade can improve the aerodynamic performance, but the positive lean angle of the stator blade will reduce the total pressure ratio and isentropic efficiency of the fan.
Design of a Non-Reactive Warm Rig With Real Lean-Premix Combustor Swirlers and Film-Cooled First Stage Nozzles
Authors: Cubeda Simone , Bacci Tommaso , Mazzei Lorenzo , Salvadori Simone , Facchini Bruno , Fiorineschi Lorenzo , Volpe Yary 
Published: January 2021

Abstract
Modern industrial gas turbines typically employ lean-premix combustors, which can limit pollutant emissions thanks to premixed flames, while sustaining high turbine inlet temperatures that increase the single-cycle thermal efficiency. As such, gas-turbine first stage nozzles can be characterized by a highly-swirled and temperature-distorted inlet flow field. However, due to several sources of uncertainty during the design phase, wide safety margins are commonly adopted, having a direct impact on engine performance and efficiency. Therefore, aiming at increasing the knowledge on combustor-turbine interaction and improving standard design practices, a non-reactive test rig composed of real hardware was assembled at the University of Florence, Italy.
The rig, accommodating three lean-premix swirlers within a combustion chamber and two first stage film-cooled nozzles of a Baker Hughes heavy-duty gas turbine, is operated in similitude conditions. The rig has been designed to reproduce the real engine periodic flow field on the central vane channel, also allowing for measurements far enough from the lateral walls. The periodicity condition on the central sector was achieved by the proper design of both the angular profile and pitch value of the tailboards with respect to the vanes, which was carried out in a preliminary phase via a Design of Experiments procedure. In addition, circular ducts needed to be installed at the injectors outlet section to preserve the non-reactive swirling flow down to the nozzles’ inlet plane.
The combustor-turbine interface section has been experimentally characterized in nominal operating conditions as per the temperature, velocity and pressure fields by means of a five-hole pressure probe provided with a thermocouple, installed on an automatic traverse system. To study the evolution of the combustor outlet flow through the vanes and its interaction with the film-cooling flow, such measurements have been replicated also downstream of the vanes’ trailing edge.
This work allowed for designing and providing preliminary data on a combustor simulator capable of equipping and testing real hardware film-cooled nozzles of a heavy-duty gas turbine. Ultimately, the activity sets the basis for an extensive test campaign aimed at characterizing the metal temperature, film effectiveness and heat transfer coefficient at realistic aerothermal conditions.
In addition, and by leveraging experimental data, this activity paves the way for a detailed validation of current design practices as well as more advanced numerical methodologies such as Scale-Adaptive Simulations of the integrated combustor-turbine domain.
A Model Reduction Method for Bladed Disks With Large Geometric Mistuning Using a Partially Reduced Intermediate System Model
Authors: Schwerdt Lukas , Panning-von Scheidt Lars , Wallaschek Jörg 
Published: January 2021

Abstract
Reduced order models (ROMs) are widely used to enable efficient simulation of mistuned bladed disks. ROMs based on projecting the system dynamics into a subspace spanned by the modes of the tuned structure work well for small amounts of mistuning. When presented with large mistuning, including changes of geometry and number of finite element mesh nodes, advanced methods such as the the pristine-rogue-interface modal expansion (PRIME) are necessary. PRIME builds a reduced model from two full cyclic symmetric analyses, one for the nominal and one for the modified type of sector. In this paper a new reduced order model suitable for large mistuning with arbitrary mesh modifications is presented. It achieves higher accuracy than PRIME, while saving approximately 25% computational effort during the reduction process, when using the same number of cyclic modes. The new method gains its efficiency by recognizing that large modifications from damage or repair are unlikely to be exactly the same for multiple blades. It works by building a partially reduced intermediate model: All nominal sectors are reduced using cyclic modes of the tuned structure. The single modified sector is kept as the full model. For this reason, the new reduction method is called Partially Reduced Intermediate System Model (PRISM) method. The accuracy of the PRISM method is demonstrated on an axial compressor blisk and an academic blisk geometry.
Complex Energy Networks Optimization: Part II — Software Application to a Case Study
Authors: Ancona M. A. , Bianchi M. , Branchini L. , De Pascale A. , Melino F. , Peretto A. , Rosati J. 
Published: January 2021

Abstract
In order to increase the exploitation of the renewable energy sources, the diffusion of the distributed generation systems is grown, leading to an increase in the complexity of the electrical, thermal, cooling and fuel energy distribution networks. With the main purpose of improving the overall energy conversion efficiency and reducing the greenhouse gas emissions associated to fossil fuel based production systems, the design and the management of these complex energy grids play a key role.
In this context, an in-house developed software, called COMBO, presented and validated in the Part I of this study, has been applied to a case study in order to define the optimal scheduling of each generation system connected to a complex energy network. The software is based on a non-heuristic technique which considers all the possible combination of solutions, elaborating the optimal scheduling for each energy system by minimizing an objective function based on the evaluation of the total energy production cost and energy systems environmental impact.
In particular, the software COMBO is applied to a case study represented by an existing small-scale complex energy network, with the main objective of optimizing the energy production mix and the complex energy networks yearly operation depending on the energy demand of the users. The electrical, thermal and cooling needs of the users are satisfied with a centralized energy production, by means of internal combustion engines, natural gas boilers, heat pumps, compression and absorption chillers. The optimal energy systems operation evaluated by the software COMBO will be compared to a Reference Case, representative of the current energy systems set-up, in order to highlight the environmental and economic benefits achievable with the proposed strategy.