bluuebunny/crossref_metadata_2025 huggingface dataset:
Critical Location Identification for Multi-Mechanistic Damage Modes Using Damage Interaction Charts
Authors: Scaletta Brent , Green Richard
Published: January 2021

Damage in gas turbine engines can accumulate from multiple damage mechanisms including creep, fatigue, and oxidation. Each damage mechanism is influenced by various parameters which typically occur during different portions of engine operation. For instance, fatigue is influenced by large stress/strain ranges that occur during startup and shutdown transient conditions while creep is affected by sustained stress and temperature at dwell conditions. In some cases, the maximum damage location for one mechanism could experience negligible contribution in damage from any other mechanism, but in most cases, there is some degree of influence from two or three mechanisms. In those instances, damage will accumulate at various rates during separate portions of operation under different damage mechanisms. Since some applications require engines to dwell for long periods of time while other applications favor more frequent cycling, every engine will accumulate damage differently at each location. Therefore, it is difficult to estimate which location is critical to durability, rendering it necessary to capture all possible critical locations so that damage can be estimated for each application.

This paper suggests a method by which to visualize and select critical locations based on all possible customer use scenarios. Once critical locations are identified, a Reduced Order Model (ROM) can be generated for each point of interest and damage can be estimated and monitored using data collection. Damage mechanisms can be combined if micromechanistic affects are additive, the material response compounds, or the material properties evolve with time. Examples of each case are demonstrated. In addition, the visual representation of damage interaction allows for uncertainty to be visualized and implemented to rank location criticality.
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

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

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

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

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

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

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

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

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

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

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

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

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

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.
Identification of Dynamic Coefficients of a Five-Pad Tilting Pad Journal Bearing up to Highest Surface Speeds
Authors: Zemella Philipp , Hagemann Thomas , Pfau Bastian , Schwarze Hubert
Published: January 2021

Tilting-pad journal bearings are widely used in turbomachinery industry due to their positive dynamic properties at high rotor speeds. However, the exact description of this dynamic behavior is still part of current research. This paper presents measurement results for a five-pad tilting-pad journal bearing in load between pivot configuration. The bearing is characterized by a nominal diameter of 100 mm, a length of 90 mm, and a pivot offset of 0.6. Investigations include results for surface speeds between 25 and 120 m/s and specific bearing loads ranging from 0.0 to 3.0 MPa.

Results of theoretical predictions are commonly derived from perturbation of stationary operation under static load. Therefore, experimental results for stationary operation including pad deflection under static load are presented first to characterize the investigated bearing. Measured results indicate considerable non-laminar flow in the upper region of the investigated range of rotor speeds. Second, dynamic excitation test are performed with excitation frequencies up to 400 Hz to evaluate dynamic coefficients of a stiffness (K) and damping (C) KC-model, and additionally, a KCM-model using additional virtual mass (M) coefficients. KCM-coefficients are obtained by fitting frequency dependent KC-characteristics to the KCM-model structure using least square approach. The wide range of rotating and excitation frequencies leads to subsynchronous as well as supersynchronous vibrations. Excitation forces are applied with multi-sinus and single-sinus characteristics. The latter one allows evaluation of KC-coefficients at the particular frequency ratio in the time domain. Here, frequency and time domain evaluation algorithms for dynamic coefficients are used in order to assess their special properties and quality. The impact of surface speed, bearing load, and oil flow rate on measured and predicted KCM-coefficients is investigated. Measured and predicted results can be well fitted to a KCM-model and show a significant influence of the ratio between fluid film and pivot support stiffness on the speed dependent characteristic of bearing stiffness coefficients. However, the impact of this ratio on damping coefficients is considerably lower. Further investigations on the impact of oil flow rates indicate that a significant decrease of direct damping coefficients exists below a certain level of starvation. Above this limit, direct damping coefficients are nearly independent of oil flow rate. Results are analyzed in detail and demands on improvements for predictions are derived.
Spectral Analysis of an Aeronautical Lean Direct Injection Burner Through Large Eddy Simulation
Authors: Carreres M. , García-Cuevas L. M. , García-Tíscar J. , Belmar-Gil M.
Published: January 2021

During the last decades, many efforts have been invested by the scientific community in minimising exhaust emissions from aeronautical gas turbine engines. In this context, many advanced ultra-low NOx combustion concepts, such as the Lean Direct Injection treated in the present study, are being developed to abide by future regulations. Numerical simulations of these devices are usually computationally expensive since they imply a multi-scale problem. In this work, a non-reactive Large Eddy Simulation of a gaseous-fuelled, radial-swirled Lean-Direct Injection (LDI) combustor has been carried out through the OpenFOAM Computational Fluid Dynamics (CFD) code by solving the complete inlet flow path through the swirl vanes and the combustor. The geometry considered is the gaseous configuration of the CORIA LDI combustor, for which detailed measurements are available. Macroscopical analysis of the main turbulent features related to the swirling flow and the generated Central Recirculation Zone (CRZ) are well established in the literature. Nevertheless, a more in-depth characterization is still required in this area of active research since theory and experimental data are not yet able to predict which unstable mode dominates the flow. This work aims at using Large Eddy Simulation for a complete characterisation of the unsteady flow structures generated within the combustion chamber of a gaseous methane injection immersed in a strong non-reactive swirling flow field. To do so, a spectral analysis of the flow field is performed to identify the frequency, intensity and instabilities associated to the phenomena occurring at the swirler outlet region. A coherent structure known as Precessing Vortex Core (PVC) is identified both at the inner and the outer shear layers, resulting in a periodic disturbance of the pressure and velocity fields. The pressure and velocity fluctuations predicted by the CFD code are used to compute the spectral signatures through the Sound Pressure Level (SPL) amplitude at multiple locations. This allows investigating both the complex behaviour of the PVC and its associated acoustic phenomena. The acoustic characteristics computed by the numerical model are first validated qualitatively by comparing the spectrum with available experimental data. In this way, the use of dimensionless numbers to characterise the most energetic structures is coherent with the experimental observations and the characteristics of the PVC. Then, the numerical identification of the main acoustic modes in the chamber through Dynamic Mode Decomposition (DMD) allows overcoming the Fast Fourier Transform (FFT) shortcomings and better understanding the propagation of the hydrodynamic instability perturbations. This investigation on the main non-reacting swirling flow structures inside the combustor provides a suitable background for further studies on combustion instability mechanisms.
Data-Driven Identification of Nonlinear Flame Models
Authors: Ghani Abdulla , Boxx Isaac , Noren Carrie
Published: January 2021

This paper presents a data-driven identification framework with the objective to retrieve a flame model from the nonlinear limit cycle. The motivation is to identify a flame model for configurations, which do not allow the determination of the flame dynamics: that is commonly for industrial applications where (i) optical access for non-intrusive measurements of velocity and heat release fluctuations are not feasible and (ii) unstable combustion is monitored via multiple pressure recordings.

To demonstrate the usefulness of the method, we chose three test cases: (i) a classical Rijke tube; (ii) an experiment of a laminar flame (EM2C case) (iii) and a high-pressure, turbulent premixed flame (DLR case). The procedure is as follows: First, acoustic network models for the three cases are generated for which the in-house software taX is employed. Next, the acoustic network models are embedded in an optimization framework with the objective to identify flame parameters that match the experimental limit cycle data: based on the instability frequency and pressure amplitudes, we formulate physical constraints and an objective function in order to identify the flame model parameters gain nopt and time delay τopt in the nonlinear regime. We demonstrate for the three cases that the identified flame parameters reproduce the unstable combustion processes and highlight the usefulness of the method for control purposes.
Effects of Casing Relative Motion on Aerodynamic Performance With Modified Designs of Winglet and Squealer Tip
Authors: Garg Anmol , Singh Rawat Sachin , Prasad B. V. S. S. S.
Published: January 2021

For unshrouded high pressure turbines, the design of rotor blade tips is a dynamic multi-purpose process. The model should have least aerodynamic and heat transfer losses caused by the flow through the tip gap and the blade. Recent studies show that by the modification of the blade tip, there is some improvement in efficiency and reduced tip gap flow. These include mainly parametric study of cavity tip and winglet tip modifications. Previous studies confirm the benefit of overhang in reducing tip leakage loss and heat transfer by changing the location of the tip leakage vortex away from the blade. But, to the best of authors’ knowledge none of the study reports the effect of casing relative motion on modified winglet and squealer tip. In the present study, novel modified tip blade geometry is introduced named as Top Squealer with Bottom Winglet (TSBW). Tip gap physics and loss generation has been investigated on three other different designs of squealer and winglet geometries and compared with the novel design. These designs are named as Flat Winglet, Cavity Squealer, Top Squealer with Bottom Winglet (new design) and Top Winglet with Bottom Squealer (TWBS). The flat tip rotor blade is considered as the base case for comparison. Three-dimensional computational study using ANSYS CFX 18.2 has been performed in order to examine the effect of casing relative motion on various designs of winglet and squealer tip. Structured mesh is created using ANSYS ICEM 18.2. At the downstream of trailing edge, distinct regions of momentum deficits named as the tip leakage vortex (LV), tip passage vortex (TPV), wakes and hub passage vortex (HPV) has been observed. Wakes formed due to the interaction of scraping vortex (SV), Tip passage vortex (TPV) and the leakage vortex. It has been found that Cavity Squealer and Top Squealer with Bottom Winglet (TSBW)gave the lowest total pressure loss coefficient and lowest tip leakage flow rate. But in casing relative motion case, cavity squealer with bottom wingletout runs the cavity squealer aerodynamic performance. This is due to the enlarged cavity at the tip. The incoming pass over flow got blocked because of enhanced interaction of LV and Scraping vortex (SV) in the tip cavity. As a result, tip leakage losses and tip leakage mass flow rate decreased.
Long Duration Uniform Crystal Temperature Sensor Application in Industrial Gas Turbine for Cooling Design Validation
Authors: Vogel Gregory , Thomas Anastasia , Ginzbursky Edward , Torkaman Alex
Published: January 2021

This paper describes an in-situ validation of advanced cooling designs for turbine airfoils using high accuracy Uniform Crystal Temperature Sensors (UCTS). This was achieved by running a rainbow test of UCTS instrumented parts with different cooling designs in an industrial gas turbine in commercial operation for a period exceeding 6 months. Test results provided valuable information on the cooling performance benefits of the improved design, as well as on different thermal barrier coatings tested as part of the rainbow configuration. This paper presents the comparison between empirical and analytical results.

The real engine environment presented obvious challenges to the validation effort, which assumes high accuracy experimental data from a well-controlled test as the basis for comparison. To get as close as possible to this high standard, sources of variation and uncertainties were identified and accounted for in the preparation of the test campaign. This paper describes these in some detail, providing a useful model to practicing engineers.

Survival and timeliness of the results were also a concern. Expert installation and removal of the micro-sized crystal sensors was a key contributing factor to the sensor survival. Automated, quality controlled post processing, which correlated changes in the crystal structure and the time history of testing with temperature, enabled accurate reporting of the maximum temperature experienced during the full length of the test for each crystal.

The results show excellent agreement between the UCTS reported temperature and the Conjugate Heat Transfer (CHT) model predictions. Hence the experimental results validated the advanced cooling design configuration. Additionally, this case study demonstrates that UCTS is a well suited non-intrusive, accurate and practical technique applicable for long duration in-situ temperature measurements of turbine components in the conditions most representative of the engine.
Entropy Generation and Efficiency of a Transonic Rotor With Water Injection: A Numerical Study
Authors: Zawadzki Natan , Szymanski Artur , Block Novelo David Alejandro , Igie Uyioghosa
Published: January 2021

The application of compressor water injection in aeroengines is of renewed interest in the civil aviation industry. Water due to its unprecedented heat capacity has the potential to cool the engine air through evaporation and thus reduce the NOx emissions formed in a combustion process. It is well known that the evaporative cooling increases thermodynamic cycle efficiency and thus improves the fuel economy. A relatively unexplored area, however, is the entropy generation due to water phase change as well as the balance between the corresponding entropy yield and the savings from the cooling of the core compressor flow. Hence, little consensus in the literature exists on the ultimate effect of water injection on compressor efficiency. In this study, a numerical analysis of water injection on an axial transonic rotor was carried out. The compressor model was tested at near-peak efficiency conditions with and without water injection. The flow was analysed using the Eulerian-Lagrangian approach with two-way coupling and the k-ω Shear Stress Transport turbulence model with Reattachment Modification. A universal, second thermodynamic law approach to quantify the entropy generation is proposed and used to evaluate the compressor flow. Results show that evaporation can facilitate the compression process and does not impair the compressor efficiency if applied at favourable conditions. The entropy generation in droplet-laden flow scales according to the gains from cooling effect and losses due to the evaporation and increased friction in the fluid. Some of the discrepancies in the public domain could be addressed, showing that the observed improvement in compressor efficiency is highly sensitive to the entropy flux measurement location. Most benefits from water injection were observed at the rotor tip proving the case for part-span injection from an entropy balance perspective.
Static Performance of a Hydrostatic Thrust Foil Bearing for Large Scale Oil-Free Turbomachines
Authors: LaTray Nguyen , Kim Daejong , Song Myongsok
Published: January 2021

This work presents a novel design of a hydrostatic thrust foil bearing (HSTFB) with an outer diameter of 154mm along with simulation and test results up to specific load capacity of 223kPa (32.3psi). The HSTFB incorporates a high pressure air/gas injection to the thrust foil bearing with a uniform clearance. This bearing has high load capacity, low power loss, and no friction/wear during startup and shutdown. In addition, the HSTFB allows for bidirectional operation. The paper also presents an advanced simulation model which adopts the exact locations of a tangentially arranged bumps to a cylindrical two-dimensional plate model of the top foil. This method predicts top foil deflection with better accuracy than the traditional independent elastic foundation model which distributes the bump locations over the nodal points in the cylindrical coordinates, and with less computational resource than the finite element method applied to the entire bump/top foils. The presented HSTFB, was designed for Organic Rankine Cycle (ORC) generators, but its performance was predicted and measured using air in this paper. The bearing static performance is compared analytically against the rigid counterpart, and presented at different supply pressures, speeds, and minimum film thicknesses. Experimental verification is conducted at 10, 15 and 20krpm. The measured load capacity and frictional loss agree well with the prediction. The measured film thickness also agrees with the prediction after the structural deflection of the thrust runner disc is compensated. Overall, the novel HSTFB demonstrates an excellent static performance and shows good potential for adoption to the intended ORC generators and other large oil-free turbomachines.
Analytical Formulation of the Performance of the Allam Power Cycle
Authors: Haseli Yousef
Published: January 2021

Thermal power plants operating on fossil fuels emit a considerable amount of polluting gases including carbon dioxide and nitrogen oxides. Several technologies have been developed or under development to avoid the emissions of, mainly, CO2 that are formed as a result of air-fuel combustion. While post-combustion capture methods are viable solutions for reduction of CO2 in the existing power plants, implementation of the concept of oxyfuel combustion in future power cycles appears to be a promising technique for clean power generation from fossil fuels. A novel power cycle that employs oxyfuel combustion method has been developed by NET Power. Known as the Allam cycle, it includes a turbine, an air separation unit (ASU), a combustor, a recuperator, a water separator, CO2 compression with intercooling and CO2 pump. (Over 90% of the supercritical CO2 flow is recycled back to the cycle as the working fluid, and the rest is extracted for further processing and storage. The present paper introduces a simplified thermodynamic analysis of the Allam power cycle. Analytical expressions are derived for the net power output, optimum turbine inlet temperature (TIT), and the molar flowrate of the recycled CO2 flow. The study aims to provide a theoretical framework to help understand the functional relationships between the various operating parameters of the cycle. The optimum TIT predicted by the presented expression is 1473 K which is fairly close to that reported by the cycle developers.
Design of a Compact Magnetically Levitated Blower for Space Applications
Authors: Hawkins Larry , Filatov Alexei , Khatri Rasish , DellaCorte Chris , Howard S. Adam
Published: January 2021

NASA is leading the design and development of a next-generation CO2 removal system, the Four Bed Carbon Dioxide Scrubber (4BCO2), and intends to use the International Space Station (ISS) as its testbed. A key component of the system is the blower that provides the airflow through the CO2 sorbent beds. To improve performance and reliability, magnetic levitation (magnetic bearings) will be used in lieu of more conventional bearings (e.g. ball bearings or air bearings) to improve resistance to contaminants and enable extensibility with regards to blower speed, pressure rise and mass flow rate. The blower will pull air from the ISS through an adsorbing desiccant bed and push it through a CO2 sorbent bed and desorbing desiccant bed. The 4BCO2 blower features an overhung permanent magnet motor, a centrally located five-axis, active magnetic bearing system, backup bearings, and an overhung centrifugal impeller in a very compact package. Magnetic bearings are a natural choice for this application due to low power consumption, low transmitted vibration and oil free operation. This paper describes the design considerations and design selections for the blower system with a focus on the magnetic bearings. Magnetic FEA of the actuator/sensor system, rotordynamics/controls analysis, and backup bearing drop simulations are discussed in detail. It is expected that the successful implementation of magnetic bearings for this space application will encourage the more widespread adoption in other space applications (e.g., fluid pumps, reaction wheels) that challenge conventional bearing technologies.
Aerodynamic Performance Investigation of Turbine in the Event of One Blade Primary Fracture Failure
Authors: Zhang Shenghui , Ding Shuiting , Qiu Tian
Published: January 2021

One of major safety requirements from current airworthiness regulations is that the probability of hazardous engine effects should not occur exceed 10−7 per engine flight hour even in the event of component failure. Service experience of aeroengines indicates that turbine blade fracture is a common fault whose probability is far more than 10−7 per engine flight hour. It is obvious that overall engine system will be affected by blade failure. So, aerodynamic performance investigation in the event of one blade fracture failure has been assessed in the current study. With ANSYS-CFX, numerical model of GE-E3 (Energy Efficient Engine) high pressure turbine was established according to literature data. By comparing surface Mach number distribution at mid-span of vane in the first stage obtained numerically and experimentally, the most efficient turbulence model, i.e., the SST k-ω model, was identified. Based on the model, the 3-dimensional flow simulations under two configurations, full wheel geometry GE-E3 high pressure turbine without and with one blade fracture failure have been achieved. The following conclusions were drawn from 3-dimensional simulations: firstly, as for GE-E3 high pressure turbine, the effect of single turbine blade failure on turbine characteristics is slight; secondly, with blade loading coefficient as a criterion which is used for judging whether blade is affected, five blades which are significantly affected can be identified, and the surface pressure distributions of these five affected blades alter to varying degrees, accord-ingly, these film outflow static pressure characteristics alter as well; thirdly, after turbine blade fails, airflow accelerates violently along the suction side of downstream blade closest to failed blade and separates, however, air flow can not expand efficiently along the pressure side of upstream blade nearest to failed blade.