bluuebunny/crossref_metadata_2025_split huggingface dataset:
Development and Application of a Reversible PAH Formation Model for Soot Prediction in Complex Fuel CFD Applications
Authors: Eigentler Florian , Gerlinger Peter , Aigner Manfred , Eggels Ruud 
Published: January 2021

Abstract
An improved modelling approach for polycyclic aromatic hydrocarbons (PAHs) and soot formation in complex fuels is presented. The introduction of PAH radicals allows a reversible growth by hydrogen abstraction and carbon addition. Emphasis is placed on the model’s general validity with respect to fuel flexibility and operating condition using one set of model constants. A detailed gas phase mechanism describes the decomposition of fuel species as well as the formation and growth of PAHs and soot precursors. PAHs and PAH radicals are described by a sectional approach. Soot particle dynamics are modeled either by a two-equation model or alternatively by a sectional approach. All models take the processes of growth, collision, oxidation and agglomeration into account. The introduction of a temperature-dependent collision coefficient enhances the PAH and soot interaction. The differences between the two-equation model and the sectional approach are investigated. An extensive set of shock tube experiments is examined to verify the developed PAH and soot model over a wide range of temperatures, pressures, fuels and mixing-ratios. Thereby, the pyrolysis and oxidation of ethylene, benzene, kerosene and its major components are examined. In addition, ignition delay times and laminar diffusion flames are used for further validation. The overall agreement to experimental data demonstrates the applicability of the presented PAH and soot model even for complex fuels.
Integration of Machine Learning and Computational Fluid Dynamics to Develop Turbulence Models for Improved Turbine Wake Mixing Prediction
Authors: Akolekar Harshal D. , Zhao Yaomin , Sandberg Richard D. , Pacciani Roberto 
Published: January 2021

Abstract
This paper presents development of accurate turbulence closures for wake mixing prediction by integrating a machine-learning approach with Reynolds Averaged Navier-Stokes (RANS)-based computational fluid dynamics (CFD). The data-driven modeling framework is based on the gene expression programming (GEP) approach previously shown to generate non-linear RANS models with good accuracy. To further improve the performance and robustness of the data-driven closures, here we exploit that GEP produces tangible models to integrate RANS in the closure development process. Specifically, rather than using as cost function a comparison of the GEP-based closure terms with a frozen high-fidelity dataset, each GEP model is instead automatically implemented into a RANS solver and the subsequent calculation results compared with reference data. By first using a canonical turbine wake with inlet conditions prescribed based on high-fidelity data, we demonstrate that the CFD-driven machine-learning approach produces non-linear turbulence closures that are physically correct, i.e. predict the right downstream wake development and maintain an accurate peak wake loss throughout the domain. We then extend our analysis to full turbine-blade cases and show that the model development is sensitive to the training region due to the presence of deterministic unsteadiness in the near wake region. Models developed including this region have artificially large diffusion coefficients to over-compensate for the vortex shedding steady RANS cannot capture. In contrast, excluding the near wake region in the model development produces the correct physical model behavior, but predictive accuracy in the near-wake remains unsatisfactory. We show that this can be remedied by using the physically consistent models in unsteady RANS, implying that the non-linear closure producing the best predictive accuracy depends on whether it will be deployed in RANS or unsteady RANS calculations. Overall, the models developed with the CFD-assisted machine learning approach were found to be robust and capture the correct physical behavior across different operating conditions.
Modeling of Pollutant Formation in Gas Turbine Combustors Using Fast Reactor Network Model
Authors: Thakre Piyush , Veljkovic Ivana , Lister Vincent , Goldin Graham 
Published: January 2021

Abstract
We present a robust, fast, and highly-automated Reactor Network model within a single simulation framework in Simcenter STAR-CCM+. An industrial gas turbine combustor operating at 3 bar is numerically investigated using the GRI 3.0 chemical mechanism. A baseline CFD solution with RANS and Flamelet Generated Manifold combustion model was used to create the network of reactors. A number of model variations have been investigated, such as the use of constant pressure vs. perfectly stirred reactors. Two options for the temperature solution are considered, namely temperature mapped from the CFD solution and temperature computed from an equation of state. Different numbers of reactors are investigated to understand the overall sensitivity on the key combustion results. It was found that with appropriate clustering variables, using a few thousand reactors provide a reasonable representation of the species fields. The simulation results are compared with the available experimental data for the combustor. The NOx and CO emissions predictions with the Reactor Network model perform better than the baseline CFD model. The Reactor Network model was about ∼3 orders of magnitude faster than a detailed chemistry CFD of the same combustor.
Dimensional Characterizations Using SEM and Surface Improvement With Electrochemical Polishing of Additively Manufactured Microchannels
Authors: Min Zheng , Wu Yingjie , Yang Kailai , Xu Jin , Parbat Sarwesh Narayan , Chyu Minking K. 
Published: January 2021

Abstract
MicroChannel manufacturing is one of the fastest-growing areas in advanced manufacturing with numerous applications, including turbine blade cooling structures, compact microchannel heat exchangers, and electronic cooling devices. Recent development of metallic additive manufacturing based on direct metal laser sintering technology is capable of fabricating micro-scale structures with high complexity and design flexibility. However, powder bed laser sintering process produces rough surface characteristics caused by hatch overlaps and particle attachments, leading to channel size reductions and rough surfaces. In this paper, dimensional metrology of cross-sectional views of multi-row microchannels made by additive manufacturing was conducted by a scanning electron microscope at different locations along the printing direction. Channel size reduction, surface roughness and circularity tolerance of the as-printed channels were analyzed based on micrographs captured by SEM. Results showed that both channel sizes and hole pitches affected the printing qualities of microchannels. The as-printed channel sizes reduced by more than 15% compared to the designed values. Two approaches were made in this paper to improve printing qualities. The first one was to redesign channel size in CAD model to make the as-printed channel sizes closer to the objective values. Electrochemical polishing was then applied as a second way using sulfuric acid solutions. Surface roughness value was reduced by more than 40% after the ECP process.
Modification of DDES Based on SST kω Model for Tip Leakage Flow in Turbomachinery
Authors: Gao Yanfei , Liu Yangwei 
Published: January 2021

Abstract
Both LES and DDES are conducted in a low-Reynolds number tip leakage flow model. The DDES uses the SST kω model and employs the same grid with the LES, but the turbulence field diverges from the LES result. Referring to the comparison between LES and DDES, a modification of the zonal function in the DDES model is proposed, which enhances the dissipation of the modeled turbulence thus promote the transition to fully LES in the tip region when the mesh is fine enough. It can generate much finer vortex structure than the original model, including the primary streamwise vortex, induced vortices and the vortex fragments after breakdown. The modification fixes the underestimation of the vorticity and pressure drop at the formation stage of the tip leakage vortex, and generates more reasonable turbulence field and energy spectra. The modified model is introduced to a real rotor simulation at engineering Reynolds number. Compared with the original model on both mean flow field and turbulence field, the modified model shows favorable agreements with the measurements. The study also gives a practical example of using the tip leakage flow model in turbulence modeling.
Numerical Investigation on Aerodynamic Characteristics of Variable Geometry Turbine Vane Cascade for Marine Gas Turbines
Authors: Gao Jie , Huo Dongchen 
Published: January 2021

Abstract
Variable geometry turbines for marine gas turbines typically use variable vane technology to regulate turbine performance under variable operating conditions, but the variable geometry turbine produces additional losses as compared to the fixed geometry turbine. The method of combining experiment and numerical calculations was adopted to investigating the variable vane tip leakage loss at different vane turning angles, and its influence on the vane aerodynamic characteristics. The numerical calculations were performed using the ANSYS CFX 18.0 numerical prediction code, adopting the SST k-ω turbulence model to investigate the aerodynamic parameter distribution downstream of the variable vane under five different vane turning angles (−6°, −3°, 0°, +5° and +10°) and three different Mach Numbers (0.3, 0.5 and 0.6). The results showed that the tip leakage is the main source of aerodynamic loss of variable vanes. The tip leakage vortex and passage vortex show strong mixing characteristics in the downstream of variable vanes, especially at the 0.3Mach condition. The change of the vane turning angle alters not only the incidence angle to the vane itself, but also the outflow angle downstream of the vane. There is a linear relationship between the downstream outflow angle and the turning angle of the vane. The total pressure loss coefficient and energy loss coefficient decrease as the Mach number increases, and the changes of energy loss coefficient value from 0.3Mach to 0.5Mach are most obvious. Results from this investigation are well presented and discussed in this paper.
Development of ODS Coating for Critical Turbine Components Using DED Additive Manufacturing
Authors: Kang Bruce S. , Ma Changyu 
Published: January 2021

Abstract
The challenge for design and manufacturing of future advanced gas turbine systems is to meet the requirement of increasing turbine inlet temperature (TIT), which is higher than the substrate melting temperature. Increasing high thermal load also causes severe oxidation and corrosion of base alloy in gas turbine. Current approach is coating the inlet turbine blade with thermal barrier coating (TBC) combined with internal cooling channel in the substrate. However, neither the ceramic coating layer nor the metallic bond coat in the TBC system can provide structural loading support to house the internal cooling channels. Development of structural bond coat with embedded cooing channels can be one of the key technologies for future advanced turbine systems.
In this research, high temperature protective structural coating on top of a superalloy substrate (popular for making gas turbine component) by additive manufacturing (AM) technique using oxide dispersion strengthening (ODS) powder is presented. A novel combined mechanochemical bonding (MCB) plus ball milling (BM) process is utilized to produce ODS powders suitable for AM applications. AM-processed ODS coating by direct energy deposition (DED) method on MAR-247 substrate were carried out. The ODS coated samples were then subjected to cyclic thermal loadings for over 4000 cycles (each cycle consists of alternating between 45 minutes at 1100 °C and 45 minutes at room temperature). SEM and EDX were applied for oxide formation analyses of the ODS coating at selected thermal cycles. In particular, stability of gamma prime phase in the ODS coating at different thermal cycles is analyzed. Test results revealed a thin continuous durable alumina oxide layer on ODS coating surface after over 4,000 thermal cycles. Test results also showed relatively stable substrate microstructures due to the protective alumina surface oxide layer and strong bonding at ODS coating/substrate interface is maintained. Oxidation weight gain of a AM-processed ODS sample is conducted and the results compared favorably with those literature available alumina forming alloys (AFA) under similar testing conditions.
The Experimental Rotordynamic Stability Evaluation Method Using Magnetic Excitation System for an Integrally Geared Compressor
Authors: Arihara Hirotoshi , Baba Yoshitaka , Morinaka Shunsuke , Kameyama Yuki 
Published: January 2021

Abstract
In Turbomachinery the rotordynamic design is highly important to keep lower shaft vibration in the high speed rotating machinery for personal safety & machine protection. To resolve same, the authors have developed a direct rotor excitation technique using the magnetic exciters designed in-house. The development of compact magnetic exciters enables to be incorporated into the casing and can be an effective method of exciting rotor vibration modes. Four magnetic exciters are installed facing the back side of an integrally geared compressor impeller in the circumferential direction. Test procedure implemented a rotational excitation method to excite forward mode and backward mode independently based on phase controlled magnet currents. In this case rotor has totally ten shaft vibration sensors which detect the rotor vibration modes excited by magnetic excitation technique. The directional frequency response functions (dFRFs) are separate results of forward mode component and backward mode component included in FRF. It achieves more accurate modal analysis to obtain natural frequency and log decrement. The dFRFs with rotational excitation denotes enough amplitude response and thus the magnetic excitation method proves to be more effective. This study successfully carry’s out the measurement of natural frequencies and log decrements of operating rotor. The comparison test results of tilting pad journal bearing (TPJB) with and without the integral squeeze film damper (ISFD), that show the ISFD reduces natural frequency and considerably increases log decrements. These test results demonstrated the effectiveness of the developed rotor excitation method for integrally geared compressor pinion rotors.
An Investigation Into the Effect of Clearance Aspect Ratio on the Performance of a Variable Geometry Vaned Diffuser for Automotive Turbocharger Application
Authors: Gibson Lee , Spence Stephen , Kim Sung In , Stuart Charles , Schwitzke Martin , Starke Andre , Filsinger Dietmar 
Published: January 2021

Abstract
The current state-of-the-art in radial compressor design for automotive turbocharger applications utilize impellers with a high trailing edge backsweep angle and a vaneless diffuser to provide a high boost pressure over a wide operating range. A unique feature of this type of design is that the peak efficiency island is typically located near the choke side of the compressor map. As such, the compressor efficiency is generally satisfactory when the engine is operating at high speed, such as the rated power condition. However, at low speeds the engine operating line is located close to the compressor surge line where the efficiency is generally modest. Thus, there is a need to improve the compressor efficiency at low engine speeds without compromising performance near the choke side of the map or the overall map width.
Variable geometry devices have shown good potential to improve the compressor performance without a compromise in map width. In general, variability is achieved by moving walls or rotating vanes to best suit the flow conditions for a given mass flow rate. In order for this to be practically realised, a clearance or gap is required between the stationary and moving parts. This ultimately gives rise to leakage flows within the compressor stage and generally results in a lower achievable efficiency relative to the fixed geometry configuration.
A study by the authors on an on/off type variable geometry vaned diffuser identified significant loss mechanisms due to the clearances required for the vanes to slide in to and out of the main flow path. Moreover, the endwall position of the clearance was found to have a marked impact on the compressor stability and peak efficiency. This paper assesses the effect of the clearance depth to width ratio (or aspect ratio) at different endwall positions with the aim of identifying an appropriate geometry and position to approach an optimised design.
Steady-state Reynolds-Averaged Navier-Stokes (RANS) simulations were performed using ANSYS CFX at three operating speeds to obtain a broad sense of the effect of the clearance aspect ratio on the compressor performance. It was found that a high value of aspect ratio enabled the formation of large vortical structures in the vaned diffuser. The mixing between the core flow and the vortical structures resulted in significant losses in the vaned diffuser and affected the compressor map width differently depending on the endwall position.
Assimilation of Experimental Data to Create a Quantitatively-Accurate Reduced Order Thermoacoustic Model
Authors: Garita Francesco , Yu Hans , Juniper Matthew P. 
Published: January 2021

Abstract
We combine a thermoacoustic experiment with a thermoacoustic reduced order model using Bayesian inference to accurately learn the parameters of the model, rendering it predictive. The experiment is a vertical Rijke tube containing an electric heater. The heater drives a base flow via natural convection, and thermoacoustic oscillations via velocity-driven heat release fluctuations. The decay rates and frequencies of these oscillations are measured every few seconds by acoustically forcing the system via a loudspeaker placed at the bottom of the tube. More than 320,000 temperature measurements are used to compute state and parameters of the base flow model using the Ensemble Kalman Filter. A wave-based network model is then used to describe the acoustics inside the tube. We balance momentum and energy at the boundary between two adjacent elements, and model the viscous and thermal dissipation mechanisms in the boundary layer and at the heater and thermocouple locations. Finally, we tune the parameters of two different thermoacoustic models on an experimental dataset that comprises more than 40,000 experiments. This study shows that, with thorough Bayesian inference, a qualitative model can become quantitatively accurate, without overfitting, as long as it contains the most influencial physical phenomena.
Unsteady Flow Simulation Through Stator-Rotor Blade Rows in Intermediate-Pressure Steam Turbines With Cutback Blades
Authors: Miyazawa Hironori , Uemura Akihiro , Furusawa Takashi , Yamamoto Satoru , Yonezawa Koichi , Umezawa Shuichi , Ohmori Shuichi , Suzuki Takeshi 
Published: January 2021

Abstract
Stator and rotor blades in intermediate-pressure steam turbines gradually deteriorate during operation because of solid particle erosion. In addition to that, turbine blades unexpectedly crack because of metal fatigue or thermal stress deformation. As eroded blades increase the aerodynamic losses and cracked blades may induce rupture of the blade, the periodic maintenance, repair, and overhaul of steam turbines is essential. Eroded or cracked blades should be replaced with new ones or repaired for further use. Cutback treatment is one of the repair methods wherein the deteriorated trailing edge on a turbine blade is removed to avoid further cracking and blade fracturing. The use of cutback blades can reduce the replacement cost; however, that may affect the steam flow and the turbine’s performance. In this study, we numerically investigated the effect of the blade deterioration on the performance of a three-stage intermediate-pressure steam turbine using a numerical method that was developed at Tohoku University. Further various cutback lengths were considered for the deteriorated first-stage stator-blade trailing edge. The obtained numerical results indicate that the cutback first-stage stator blades certainly affected the steam flow in the turbine, resulting in a negative influence on the torque obtained from the adjacent rotor blades, which depends on the cutback length. However, the torque decrement can be mitigated by arranging the cutback and non-cutback stator blades alternately in a row.
Additively Manufactured Compliant Hybrid Gas Thrust Bearing for sCO2 Turbomachinery: Design and Proof of Concept Testing
Authors: Ertas Bugra 
Published: January 2021

Abstract
The following paper presents a new type of gas lubricated thrust bearing that utilizes additive manufacturing or also known as direct metal laser melting (DMLM) to fabricate the bearing. The motivation for the new bearing concept is derived from the need for highly efficient supercritical carbon dioxide (sCO2) turbomachinery in the mega-watt power range. The paper provides a review of existing gas thrust bearing technology, outlines the need for the new DMLM concept, and discusses proof of concept testing results. The new concept combines hydrostatic pressurization with individual tilting pads that are flexibly mounted using hermetic squeeze film dampers (HSFD) in the bearing-pad support. This paper describes the thrust bearing concept and discusses the final design approach. Proof-of-concept testing in air for a 6.8” (173mm) outer diameter thrust gas bearing was performed; with thrust loading, up to 1,500 lbs (6.67kN) and a thrust runner speed of 10krpm (91 m/s tip speed). The experiments were performed with a bent shaft resulting in thrust runner axial vibration magnitudes of 2.9mils (74microns) p-p and dynamic thrust loads of 270 lbs (1.2kN) p-p. In addition, force deflection characteristics and stiffness coefficients of the bearing system are presented for an inlet hydrostatic pressure of 380psi (2.62MPa). Results at 10krpm show that the pad support architecture was able to sustain high levels of dynamic misalignment equaling 6 times the nominal film clearance while demonstrating a unit load carrying capacity of 55psi (0.34Mpa). Gas-film force deflection tests portrayed nonlinear behavior like a hardening spring, while the bearing pad support stiffness was measured to be linear and independent of gas film thickness.
Shape Optimization of Thermoacoustic Systems Using a 2D Adjoint Helmholtz Solver
Authors: Falco Stefano , Juniper Matthew P. 
Published: January 2021

Abstract
Thermoacoustic instabilities, which arise due to the interaction between flames and acoustics, are sensitive to small changes to the system parameters. In this paper, we apply adjoint-based shape optimization to a 2D finite element Helmholtz solver to find accurately and inexpensively the shape changes that most stabilise a 2D thermoacoustic system in the linear regime. We examine two cases: a Rijke tube and a turbulent swirl combustor. Both systems exhibit an unstable longitudinal mode and we suppress the instability by slightly modifying the geometry. In the case of the turbulent swirl combustor, the sensitivities are higher in the plenum and in the burner than in the combustion chamber, mainly due to the effect of the mean temperature. In the cooler regions, the local wavelength is shorter, which means that geometry changes of a given distance have more influence than they do where the local wavelength is longer. This is the first time adjoint-based shape optimization is applied to 2D Helmholtz solvers in thermoacoustics, after being previously applied to low-order thermoacoustic networks. But Helmholtz solvers have an intrinsic advantage: they can handle complex geometries. The easy scalability of this method to complex 3D geometries make this tool a strong candidate for the iterative design of thermoacoustically stable combustors.