bluuebunny/crossref_metadata_2025 huggingface dataset:
Support Vector Machines Model of the Nonlinear Hydrodynamics of Fixed Cylinders
Authors: Ma Yu , Sclavounos Paul D.
Published: January 2021

Data-driven modeling is considered as a prospective approach for many conventional physical problems including ocean applications. Among various machine learning techniques, support vector machine stands out as one of the most widely used algorithms to establish models connecting pertinent features to physical quantities of interest. This paper takes the experimental data for a fixed cylinder in shallow water as the baseline data set and explores the modeling of nonlinear wave loads by the support vector machine (SVM) regression method. Different feature and target selections are studied in this paper to establish the nonlinear mapping relations from ambient wave elevations and kinematics to nonlinear wave loads. The performance of the SVM regression model is discussed and compared with nonlinear potential flow theory focusing on the overall statistics (standard deviation and kurtosis), which is critical for fatigue and extreme statistics analysis.
Scalar Diffusion Equation-Based Model to Predict 2-Dimensional Film Cooling Effectiveness of a Shaped Hole
Authors: Chen Ziyu , Li Yifei , Su Xinrong , Yuan Xin
Published: January 2021

One-dimensional laterally averaged adiabatic film cooling effectiveness η¯lat-based correlations have been widely employed in the cooling design of the modern gas turbine and aero-engine; however, the flow field of the discrete film cooling is fully three dimensional, and thus, the cooling effectiveness distribution on the solid surface is two dimensional. Accurate prediction of the cooling effectiveness distribution in the lateral direction would help to optimize the film cooling design, but few paid attention to this issue in the literature. In this study, a simple yet accurate scalar diffusion equation based model is proposed to extend the one-dimensional correlation into two dimensional. The model is proved to be accurate and efficient. According to the accuracy analysis, the R2 value is larger than 0.95 for the two-dimensional prediction and over 0.93 along the centerline. With given input parameters, the calculation cost for solving a certain case is in the magnitude of 1 × 10−3s in time using the space-marching method. There is only the effective diffusion coefficient left to be modeled in the control equation. It represents the balance between the diffusion and the passive transportation by the main flow. Analyses conducted within the typical experimental range show that κ~eff is only dependent on the velocity ratio and the main-flow turbulence.
On the Importance of Transition Control at Transonic Compressor Blades
Authors: Hergt Alexander , Klinner J. , Grund S. , Willert C. , Steinert W. , Beversdorff M.
Published: January 2021

The flow through a transonic compressor cascade is characterized by high unsteadiness and a high loss level. This results from the shock waves in the blade cascade and their interaction with the blade suction side boundary layer. In the case of a laminar shock wave boundary layer interaction, the loss level is higher due to the occurrence of a laminar separation bubble below the shock wave compared to the shock wave interaction with a turbulent boundary layer. In addition, the oscillation of the shock position in both cases influences the working range concerning the point of stall onset as well as leading to an unsteady interaction with the blade called buffeting. The reduction of losses and of unsteadiness in the shock wave oscillation, connected to a decrease of the blade buffeting effect, is the aim of the current investigation. Therefore, experimental investigations using a roughness patch as well as air jet vortex generators in order to control the transition in a transonic compressor cascade have been conducted at the transonic cascade wind tunnel of the German Aerospace Center (DLR) at Cologne. At an inflow Mach number of 1.21, a loss reduction for both transition control cases is achieved. In spite of a nearly uninfluenced fluctuation range of the passage shock wave compared to the reference cascade, the oscillation spectra of the transition control cases show a reduction of the shock movement amplitude at a frequency below 500 Hz and above 1 kHz. In the closing section of the paper, a detailed discussion on the reasons for the resulting flow behavior based on particle image velocimetry and high-speed shadowgraphy data is given. The resulting conclusion of the study is that the consideration of transition control at transonic compressor blades is very important in order to reduce losses and flow unsteadiness that directly influences blade buffeting and the numerical prediction quality of the stall onset.
Enhancement of Cu Nanoparticles Dispersion in Nanolubricants by Magnetron Sputtering Deposition and Its Influence on the Tribological Behavior
Authors: Trajano M. F. , Franceschini D. F. , Silva Edimilson F. , Correa M. Assolin , Bohn F. , Alves S. M.
Published: January 2021

We investigated the chemical, physical, and tribological properties of nanolubricants consisting of epoxidized sunflower oil with Cu nanoparticles as additive. These latter are produced by magnetron sputtering at distinct current levels in the deposition, to improve the development of nanolubricants by enhancing the nanoparticles dispersion. The nanolubricants are here characterized by Fourier transform infrared spectroscopy, zeta potential, ultraviolet–visible absorbance spectroscopy, small-angle X-ray scattering, and scanning electron microscopy. After all, the tribological properties of the bionanolubricants are investigated using a high-frequency reciprocating rig equipment, scanning electron microscopy, and energy dispersive spectroscopy. Our results disclose the nanolubricants produced using the magnetron sputtering technique have excellent nanoparticle dispersion, as well as good tribological performance.
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

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

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

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

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

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

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

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

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

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

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

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

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.
Loss Predictions in the High-Pressure Film-Cooled Turbine Vane of the FACTOR Project by Mean of Wall-Modeled Large Eddy Simulation
Authors: Harnieh Mael , Odier Nicolas , Dombard Jérôme , Duchaine Florent , Gicquel Laurent
Published: January 2021

The use of numerical simulations to design and optimize turbine vane cooling requires precise prediction of the fluid mechanics and film cooling effectiveness. This results in the need to numerically identify and assess the various origins of the losses taking place in such systems and if possible in engine representative conditions. Large-Eddy Simulation (LES) has shown recently its ability to predict turbomachinery flows in well mastered academic cases such as compressor or turbine cascades. When it comes to industrial representative configurations, the geometrical complexities, high Reynolds and Mach numbers as well as boundary condition setup lead to an important increase of CPU cost of the simulations. To evaluate the capacity of LES to predict film cooling effectiveness as well as to investigate the loss generation mechanisms in a turbine vane in engine representative conditions, a wall-modeled LES of the FACTOR film-cooled nozzle is performed. After the comparison of integrated values to validate the operating point of the vanes, the mean flow structure is investigated. In the coolant film, a strong turbulent mixing process between coolant and hot flows is observed. As a result, the spatial distribution of time-averaged vane surface temperature is highly heterogeneous. Comparisons with the experiment show that the LES prediction fairly reproduces the spatial distribution of the adiabatic film effectiveness. The loss generation in the configuration is then investigated. To do so, two methodologies, i.e, performing balance of total pressure in the vanes wakes as mainly used in the literature and Second Law Analysis (SLA) are evaluated. Balance of total pressure without the contribution of thermal effects only highlights the losses generated by the wakes and secondary flows. To overcome this limitation, SLA is adopted by investigating loss maps. Thanks to this approach, mixing losses are shown to dominate in the coolant film while aerodynamic losses dominate in the coolant pipe region.
Application of Scale Adaptive Simulation Model to Studying Cooling Characteristics of a High Pressure Turbine Blade Cutback Trailing Edge With Different Cooling Configurations
Authors: Li Yuefeng , Xu Huazhao , Wang Jianhua , Song Wei , Wang Ming , Liu Taiqiu , Wang Xu
Published: January 2021

Effective cooling structure design in the trailing edge (TE) of a high pressure turbine (HPT) blade is essential to increase turbine efficiency and maintain structural integrity. To obtain efficient cooling structures and understand clearly cooling mechanism, this paper adopted numerical simulation methods to investigate fluid flow and cooling characteristics in detail downstream of a HPT blade TE cutback region. The effects of typical TE configurations on cutback cooling performance are investigated including three types of internal turbulators (cylindrical pin fins and elliptic pin fins arranged in streamwise and spanwise orientations), the cutback with/without land extensions and three kinds of ejection lip profiles (one straight lip shape marked as “A” and two rounded lip shapes marked as “B” and “C”, respectively). The Scale Adaptive Simulation (SAS) is implemented to study the complex unsteady mixing process downstream of the cutback under operating condition of blowing ratio M = 0.65. The results from the Shear Stress Transport (SST) k-ω model are compared as well.

SAS is capable to reproduce the periodical vortex shedding phenomena and resolve the vortices coherent structures. Compared with the experimental data, SAS provides more accurate predictions in terms of laterally averaged adiabatic cooling effectiveness ηaw and discharge coefficient Cd than the SST k-ω model. On the rear part of the cutback surface, large deterioration in ηaw is predicted by SAS for all configurations, but ηaw is considerably over-predicted by the SST k-ω model except for the case of elliptic pin fins with spanwise orientation. The elliptic pin fins with streamwise orientation significantly improve ηaw at the rear part of the cutback surface over the baseline model with cylindrical pin fins and slightly increase Cd. However, the elliptic pin fins with spanwise orientation drastically reduce the ηaw and Cd. Downstream of the cutback, the coherent structures are strongly disturbed and become chaotic compared to the TE with cylindrical and streamwise oriented elliptic pin fins. The application of land extensions only causes an evident change to the coherent structure immediate downstream of the lip, and slightly improves ηaw and reduces Cd over the baseline model on the rear part of the cutback surface. Rounded lip shapes B and C also show an obvious increase in ηaw on the rear part of the cutback surface but only a minor increase in Cd compared to the straight lip shape A. The rounded lip helps the coolant diffuse into the TE cutback and reduce the intensity of mixing. Due to larger rounding radius of shape B, the cooling effectiveness predicted by shape B is slightly better than shape C.
Experimental and Numerical Analysis of Gas Premix Turbulent Flames Stabilized in a Swirl Burner With Central Bluff Body
Authors: Biagioli Fernando , Innocenti Alessandro , Terhaar Steffen , Marchione Teresa
Published: January 2021

Lean premixed gas turbulent flames stabilized in the flow generated by an industrial swirl burner with a central bluff body are experimentally found to behave bi-stable. This bi-stable behaviour, which can be triggered via a small change in some of the controlling parameters, for example the bulk equivalence ratio, consists in a rather sudden transition of the flame from completely lifted to well attached to the bluff body. This has impact on combustion dynamics, emissions and pressure losses. While several experimental investigations exist on this topic, numerical analysis is limited. The present work is therefore also of numerical nature, with a two-fold scope: a) simulation and validation with experiments of the bi-stable flame behaviour via Computational Fluid Dynamics (CFD) in the form of Large Eddy Simulation (LES) and b) analysis of CFD results to shed light on the flame stabilization properties. LES results, in case of the lifted flame, show that the vortex core is sharply precessing at a given frequency. Phase averaging these results at the frequency of precession clearly indicates a counter-intuitive and unexpected presence of reverse flow going all the way through the flame apex and the bluff body tip. The counter-intuitive presence of a lifted flame is explained here in terms of the phase averaged data which show that the flame apex is not placed at the centre of the spinning reverse flow region. It is instead slightly shifted radially outward where the axial velocity recovers to low positive values of the order of the turbulent burning rate. A simple one-dimensional flame stabilization model is applied to explain this peculiar flame behaviour. This model provides first an estimation of the flame radius of curvature in terms of axial velocity and turbulence quantities. This radius is therefore used to determine the total flux of reactants into the flame, given by an axial convection and a radial diffusion contributions. Subsequently the possibility of the flame positioned at the centre of the vortex is excluded based on the balance between this flux and the turbulent burning rate. A clear explanation of the mechanism leading to the sudden flame jump has instead not been identified and only some hypotheses are provided.
Equivalent Linearization of Bladed Disk Assemblies With Friction Nonlinearities Under Random Excitation
Authors: Förster Alwin , Panning-von Scheidt Lars , Wallaschek Jörg
Published: January 2021

The present article addresses the vibrational behavior of bladed disk assemblies with nonlinear shroud coupling under random excitation. In order to increase the service life and safety of turbine blades, intense calculations are carried out to predict the vibrational behavior. The use of friction dampers for energy dissipation and suppression of large amplitudes adds a nonlinearity to the mechanical system, which complicates the calculations. Depending on the stage, different types of excitation can occur in a turbine, from stationary to transient, synchronous to asynchronous as well as deterministic to random excitation. Random excitation in combination with the presence of nonlinearities makes the calculation of the vibrational behavior even more complex. So far, this problem has only been dealt with to a limited extent in the literature on turbomachinery. Nevertheless, there are in general different approaches and methods to address this problem most of which are strongly restricted with regard to the number of degrees of freedom. The focus of this paper is the application of an equivalent linearization method to calculate the stochastic response of an academic model of a bladed disk assembly under random excitation. The fundamental idea of the method is to linearize a nonlinear system in such a way that the most suitable equivalent linear system is found taking into account the approximated distribution of the response amplitude. To apply this method to a system with a friction nonlinearity, the linear part of the system is considered in state space and extended with additional nonlinear equations. The nonlinear contact is modelled with a Bouc-Wen formulation to reproduce the hysteretic character of a nonlinearity occurring in the presence of a friction damper. The classical Bouc-Wen formulation is standardized in such a way that the usual parameters can be replaced by physical ones such as the normal force or contact stiffness. The nonlinear force of the friction nonlinearity is linearized regarding the stochastic distribution of the system response. Both the excitation and the response are limited to mean-free, stationary stochastic processes, which means that the stochastic moments do not change over time. However, the spectrum of the excitation is not limited to being constant, as it is the case with Gaussian white noise. The equivalent linearization method could also deal with a narrowband or broadband excitation spectrum. Unlike previous papers on this topic, the calculations are performed on a full bladed disk assembly in which each sector is represented by a reduced order model with several degrees of freedom.
Conjugate Heat Transfer Characteristics of Double Wall Cooling on a Film Plate With Gradient Thickness
Authors: He Juan , Deng Qinghua , Zhou Weilun , He Wei , Gao Tieyu , Feng Zhenping
Published: January 2021

Double wall cooling, consisting of internal impingement cooling and external film cooling, is an advanced cooling method of gas turbines. In this paper, the flow and conjugate heat transfer characteristics of double wall cooling which has a film plate with gradient thickness are analyzed numerically. The detailed overall cooling effectiveness distributions are obtained by solving steady three dimensional Reynolds-averaged Navier-Stokes equations.

In the double wall cooling scheme, seven vertical film holes and six impingement holes are staggered with same diameter (D), and the hole pitch of them are both set to 6D in flow direction and lateral direction. The gradient thickness along the flow direction is realized by setting the angle (α) between the lower surface of the film plate and the horizontal plane at −1.5 deg and 1.5 deg respectively. By comparing the results of four broadly used turbulence models with experimental data, SST k-ω is selected as the optimal turbulence model for double wall cooling analysis in this paper. In addition, the number of grids are finally determined to be 5.2 million by grid sensitivity calculation. The influence of the thickness gradient on the overall cooling effectiveness is revealed by comparing with the constant thickness film plate (Baseline 1 and 2), and all the cases are performed under four various coolant mass flow rates, which correspond to blowing ratios ranging from 0.25 to 1.5.

The calculated results show that the thickening of the film plate downstream is beneficial to improve overall cooling effectiveness at low blowing ratio, which is benefit from two aspects. One is the thicken film plate weakens the flow separation in film hole and velocity of film hole outlet, another is the thicken film plate makes the impingement channels convergence, and impingement cooling is strengthened to some extent. However, with the increase of blowing ratio, the increasing trend gradually weakens due to the jet-off and limited impinge ability. For thickening film plate, the variations of the double wall cooling configurations are considered at initial film plate thickness tf of 2D and 3D, it is found that the ability to improve the overall cooling effectiveness by thickening the film plate downstream decrease as the initial film plate thickness increases, which is due to the increase of heat transfer resistance, and another finding is the cooling effectiveness of downstream thickening film plate with initial thickness of 2D is higher than that of 3D, which will provide a theoretical foundation both for improving cooling performance and reducing turbine blade weight at the same time. The influence of initial impingement gap H is also observed, and the study come to the fact that the best cooling performance occurred in H = 2D.
Axisymmetric Tip Gap Profiling in a Shroudless Axial Turbine
Authors: Abda M. , Rose M. G.
Published: January 2021

The inevitable gap between the rotor tips and the casing promotes flow leakage driven by the pressure difference between the pressure side and suction side of the blade. Axisymmetric tip gap profiling was applied at the blade tip and the casing endwall to reduce the tip leakage maintaining the same gap clearance. The investigation was held on a shroudless single stage axial turbine designed in ETH Zurich University named LISA D. The numerical calculation showed that axisymmetric tip gap profiling reduced the tip leakage flow and improved the efficiency by 0.65% and 0.1% respectively. However, the stage mass flow increased and as a result so did the rotor capacity. When the stage mass flow was reduced to the design value to maintain the design capacity, the effect of the axisymmetric tip gap profiling further improved, due to a reduction in the entropy generation rate of the tip leakage and passage vortices. The tip mass flow reduced by 2.39% and the efficiency improved significantly by 0.6%. It was observed that the tip profiling increased the size of the separation bubble in the PS/tip junction, which increased blockage effect in the gap. Hence, reduced the leaking flow to the SS, which results in weaker tip leakage vortex and its associated losses.
Behavior of a Variable Geometry Turbine Wheel to High Cycle Fatigue
Authors: Avola Calogero , Racca Alberto , Montanino Angelo , Williams Carnell E. , Renella Alfonso , Belluscio Michele
Published: January 2021

Maximization of the turbocharger efficiency is fundamental to the reduction of the internal combustion engine back-pressure. Specifically, in turbochargers with a variable geometry turbine (VGT), energy losses can be induced by the aerodynamic profile of both the nozzle vanes and the turbine blades. Although appropriate considerations on material limits and structural performance of the turbine wheel are monitored in the design and aero-mechanical optimization phases, in these stages, fatigue phenomena might be ignored. Fatigue occurrence in VGT wheels can be categorized into low and high cycle behaviors. The former would be induced by the change in turbine rotational speed in time, while the latter would be caused by the interaction between the aerodynamic excitation and blades resonating modes.

In this paper, an optimized turbine stage, including unique nozzle vanes design and turbine blades profile, has been assessed for high cycle fatigue (HCF) behavior. To estimate the robustness of the turbine wheel under several powertrain operations, a procedure to evaluate HCF behavior has been developed. Specifically, the HCF procedure tries to identify the possible resonances between the turbine blades frequency of vibrations and the excitation order induced by the number of variable vanes. Moreover, the method evaluates the turbine design robustness by checking the stress levels in the component against the limits imposed by the Goodman law of the material selected for the turbine wheel. In conclusion, both the VGT design and the HCF approach are experimentally assessed.
Volumetric Velocimetry Measurements of Purge-Mainstream Interaction in a 1-Stage Turbine
Authors: Carvalho Figueiredo A. J. , Schreiner B. D. J. , Mesny A. W. , Pountney O. J. , Scobie J. A. , Li Y. S. , Cleaver D. J. , Sangan C. M.
Published: January 2021

Air-cooled gas turbines employ bleed air from the compressor to cool vulnerable components in the turbine. The cooling flow, commonly known as purge air, is introduced at low radius, before exiting through the rim-seal at the periphery of the turbine discs. The purge flow interacts with the mainstream gas path, creating an unsteady and complex flow-field. Of particular interest to the designer is the effect of purge on the secondary flow structures within the blade passage, the extent of which directly affects the aerodynamic loss in the stage.

This paper presents a combined experimental and Computational Fluid Dynamics (CFD) investigation into the effect of purge flow on the secondary flows in the blade passage of an optically-accessible 1-stage turbine rig. The experimental campaign was conducted using Volumetric Velocimetry (VV) measurements to assess the three-dimensional inter-blade velocity field; the complementary CFD campaign was carried out using URANS computations. The implementation of VV within a rotating environment is a world first and offers an unparalleled level of experimental detail.

The baseline flow-field, in the absence of purge flow, demonstrated a classical secondary flow-field: the roll-up of a horseshoe-vortex, with subsequent downstream convection of a pressure-side and suction-side leg, the former transitioning in to the passage vortex. The introduction of purge, at 1.7% of the mainstream flow-rate, was shown to modify the secondary flow field by enhancing the passage vortex, both in strength and span-wise migration. The computational predictions were in agreement with the enhancement revealed by the experiments.