bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of Density and Sound Speed in Mixtures Relevant to sCO2 Power Cycles
Authors: Park Suhyeon , Urso Justin , Manikantachari K. R. V. (Raghu) , Hosangadi Ashvin , Zambon Andrea , Vasu Subith S.
Published: January 2021

The objective of this research is to validate properties of mixtures relevant to supercritical carbon dioxide (sCO2) power cycles. Direct fired sCO2 cycles are promising technology for the future power generation systems. The working fluid of sCO2 cycles will be near and above critical point of CO2. One of the challenges is that the simulation of mixtures should consider real gas behavior. Expected operating conditions of Allam cycles reach up to 300 bar and 1000 °C. Characterizing the mixtures at the extreme conditions is an important issue in current researches and industrial applications. Thermophysical properties of mixtures may be beyond the valid range of the widely used database such as NIST REFPROP. Experimental data of mixtures properties in the literature is limited which is necessary to develop high-fidelity design tools for sCO2 power cycles.

We measured density and sound speed of several multi-component mixtures. A temperature-controlled high-pressure test cell was used for the density measurements. Sound speed was measured by resonant frequency detection using an external speaker and a piezoelectric pressure sensor. Mixtures studied in this work includes carbon dioxide, methane, oxygen and water vapor. Properties of pure CO2 were measured to show the validity of our technique. Compositions were selected to be close to frozen mixtures at the inlet, mid-progress and exhaust conditions of a model sCO2 combustor in the previous numerical simulation work. Corresponding reaction progress variables (RPV) were RPV = 0, 0.5, and 1. Temperature and pressure conditions of experiments are 310–450 K, and 0–150 bar. In our study, density and sound speed from NIST REFPROP database agree with experimental measurements within the range of our measurement uncertainties.
Non-Axisymmetric Endwall Contouring in a Linear Compressor Cascade
Authors: Ma Yuchen , Teng Jinfang , Zhu Mingmin , Qiang Xiaoqing
Published: January 2021

Modern axial compressors are designed to be highly loaded in terms of aerodynamics, which can lead to challenges of increasing the compressor efficiency. Losses associated with secondary flow effects are well known to be the major limiting factor of improving the compressor performance. In this study, non-axisymmetric endwall contouring in a linear compressor cascade was generated through the optimization process. Combined with numerical simulation, wind tunnel tests on linear cascades with flat and contoured endwall were performed with various measurement techniques at the design and off-design conditions.

The simulation results show that optimal endwall design can provide 3.08% reduction of the total pressure loss at the design condition. The reduction of pressure loss obtained is mainly below 24%span with the size of the high loss region being effectively reduced. At off-design condition, the numerical benefit of the endwall contouring is found less pronounced. The discrepancy is spotted between simulation and experiments. The experimental pressure loss reduction is mainly below 18% at ADP. And the pressure loss for the CEW increases greatly at offdesign condition in experiments.

Flow patterns revealed by numerical simulations show that the separation on the blade surface is mitigated with focus point disappearing, and reverse flow on the endwall near the suction side corner is moved away from the blade surface. CFD analysis indicates that the altered pressure distribution on the endwall accelerates the flow at the suction side corner and moves the reverse flow core further downstream. The weakened interaction between the corner vortex and tornado-like vortex from the endwall near the suction side corner is the main control mechanism of the CEW. The performance improvement in the linear compressor is mainly gained from it.
Higher-Order Conceptual Model for Labyrinth Seal Flutter
Authors: Corral Roque , Greco Michele , Vega Almudena
Published: January 2021

A simple non-dimensional model to describe the flutter onset of two-fin straight labyrinth seals [1] is extended to account for non-isentropic flow perturbations. The isentropic relationship is replaced by the more general integral energy equation of the inter-fin cavity. A new expression for the Corral & Vega stability criterion is derived which is very consistent with the previous model in the whole design space of the seal but for torsion centers located in the high-pressure side close to the seal. The new model formally depends on more dimensionless parameters since the existing parameter grouping of the previous model does not hold anymore, but this dependency is weak in relative terms. The model blends the limit where the discharge time of the inter-fin cavity is much longer than the vibration period, and the flow is nearly isentropic, and the opposite limit, where the perturbations are isothermic, gracefully. A few numerical examples obtained using a three-dimensional linearized frequency domain solver are included to support the model and show that the trends are correct but the body of the numerical work will be presented in a separated article. The matching between the work-per-cycle obtained with the model and frequency domain solver is good. It is shown that some weird trends obtained using linearized unsteady simulations are qualitatively consistent with the current model but not with the previous one [1]. The largest differences between the new and the previous model are seen when the seal is supported at the high-pressure side.
“I-Spots” and Emmons’ Spot Production Rate
Authors: Mayle R. E. , Stripf M.
Published: January 2021

A new intermittency function flagging the turbulent, becalmed, and non-becalmed regions of transitional flow is introduced. In addition, a method is developed to detect embedded incipient turbulent spots within the non-becalmed regions of the flow, which are then documented, counted, and related to Emmons’ spot production rate. This in turn is used to calculate an intermittency distribution which when compared with that measured displays an excellent agreement. Finally, it is shown that correlations based on Narasimha’s intermittency equation are highly questionable.
Criteria for the Stability Limit Prediction of High Speed Centrifugal Compressors With Vaneless Diffuser: Part I — Flow Structure Analysis
Authors: Cravero Carlo , Marsano Davide
Published: January 2021

High-speed centrifugal compressor requirements include a wide operating range between choking and stall especially for turbocharging applications. The prediction of the stability limit at different speeds is still challenging. In literature, several studies have been published on the phenomena that trigger the compressor instability. However, a comprehensive analysis of criteria that can be used in the first steps of centrifugal compressors design to predict the stability limit is still missing. In previous work the authors have already presented a criterion, so called “Stability Parameter”, to predict the surge line of centrifugal compressors based on a simplified CFD approach that does not require excessive computational resources and that can be efficiently used in the preliminary design phases. The above methodology has demonstrated its accuracy for centrifugal compressors with vaned diffuser, but a lower accuracy has been detected for vaneless diffusers. Before proceeding to identify additional criteria focused on compressors with vaneless diffuser, an in-depth fluid dynamics analysis has been necessary. This analysis has been also carried out through fully 3D unsteady simulations to allow identifying the real phenomena linked to the trigger of the instability of centrifugal compressors. It has been found how these phenomena are strongly related to the rotational speed, in particular have been shown the key role of the volute at high rotational speed.
High Momentum Jet Flames at Elevated Pressure, E: Quantification of Droplet Size Distribution and Transport
Authors: Hampp Fabian , Gounder James Dakshina , Ax Holger , Lükerath Rainer , Lammel Oliver , Hase Matthias , Janus Bertram
Published: January 2021

The present work investigates a model combustor that approximates a full-scale segment of a high momentum jet stabilised combustion concept with the objective to broaden fuel flexibility limits. The rig features optical access for detailed laser based diagnostics. The current experiments are conducted at a pressure of 8 bar, constant jet velocity and adiabatic flame temperature. Three different injection systems are used to inject oil and oil / water blends. Droplet size distributions and turbulent droplet transport are quantified by means of phase Doppler interferometry. The injection system has a noticeable impact on the droplet size distribution. Water addition affects the fuel placement in radial direction significantly with distinct droplet transport into the reaction zone. The jet core and droplet transport is maintained as far as 7 x/d axial downstream from the nozzle exit. The shear layer shows a turbulent intensity of 12% of the main jet bulk velocity that drives the radial droplet transport into the recirculation zone. Mass flow and number density estimations illustrate the spatial distribution of the liquid loading in the combustion chamber. The present findings are complementary to the work in part A–D, support the development of fuel and load flexible high momentum jets based combustion concepts and the development and validation of numerical models.
Impact of the Precessing Vortex Core on NOx Emissions in Premixed Swirl-Stabilized Flames: An Experimental Study
Authors: Lückoff Finn , Sieber Moritz , Paschereit Christian Oliver , Oberleithner Kilian
Published: January 2021

The reduction of polluting NOx emission remains a driving factor in the design process of swirl-stabilized combustion systems, to meet strict legislative restrictions. In reacting swirl flows, hydrodynamic coherent structures, such as periodic large-scale vortices in the shear layer, induce zones with increased heat release rate fluctuations in connection with temperature peaks, which lead to an increase of NOx emissions. Such large-scale vortices can be induced by the helical coherent structure known as precessing vortex core (PVC), which influences the flow and flame dynamics of reacting swirl flows under certain operating conditions. We developed an active flow control system, which allows for a targeted actuation of the PVC, to investigate its impact on important combustion properties. In this study, the direct active flow control is used to actuate a PVC of arbitrary frequency and amplitude, which facilitates a systematic study of the impact of the PVC on NOx emissions. In the course of the present work, a perfectly premixed flame, which slightly damps the PVC, is studied in detail. Since the PVC is slightly damped, it can be precisely excited by means of open-loop flow control. In connection with time-resolved OH*-chemiluminescence and stereoscopic PIV measurements, the flame and flow response to PVC actuation as well as the impact of the actuated PVC on flow and flame dynamics are characterized. It turns out that the PVC rolls up the inner shear layer, which results in an interaction of PVC-induced vortices and flame. This interaction considerably influences the measured level of NOx emissions, which grow with increasing PVC amplitude in a perfectly premixed flame. Nearly the same increase is to be seen for a partially premixed flame. This in contrast to previous studies, where the PVC is assumed to reduce the NOx emissions due to vortex-enhanced mixing.
Physics-Based Thermal Model for Power Gearboxes in Geared Turbofan Engines
Authors: Jafari Soheil , Nikolaidis Theoklis , van Heerden Albert S. J. , Lawson Craig P. , Bosak David
Published: January 2021

Ultra-High Bypass Ratio Geared (UHBRG) turbofan technology allows a significant reduction in fuel burn, noise and emissions — key metrics for aircraft engine performance. However, one of the main challenges in this technology is the large amount of waste heat generated by the Power Gearbox (PGB). Therefore, having a practical tool for precise prediction of the PGB-generated thermal loads in UHBRGs is becoming a necessity. Such a tool would assist in analyzing engine performance, as well as ensuring that engine physical limitations/restrictions are not breached (e.g. over-temperature in fuel and oil, cocking, etc.). This paper presents a methodological approach to mathematically model the waste heat generated by a PGB on a UHBRG for different points on a typical flight profile. To do this, the total power loss in a PGB system is firstly defined as the summation of load-dependent and load-independent losses. Physics-based equations for each heat loss mechanism are introduced and, through a combination of the associated equations, a simulation model for the thermal loads calculation in PGBs is developed. In addition, the heat losses and efficiency of the PGB has been analyzed across a simulated flight. The developed PGB model calculates the main power losses generated in a gear reduction system of a turbofan engine. It is found that in a typical flight, the heat loss generated by the PGB can reach about 80% of the total waste heat generated by the engine. The values of the mechanical efficiency calculated by the tool at different flight points are above 97% which is in good agreement with publicly available data for planetary gearboxes. This tool is intended to be utilized by engine thermal management system designers to predict and analyze the heat loads generated by the PGB at different flight conditions.
Analysis of Intentional Mistuning on the Aeroelastic Stability of Freestanding Last Stage Blade Rows in Steam Turbines
Authors: Siewert Christian , Pütz Oliver , Eigemann Jonas
Published: January 2021

It is common in the industrial practice of the structural dynamic analysis for a Last Stage Blade (LSB) row to assume that all blades have identical mechanical properties. Nevertheless, the mechanical properties vary from blade to blade due to e.g. manufacturing and assembly tolerances, wear and so forth. These variations influence the vibrational behavior of an LSB row and are known as mistuning.

For freestanding LSB rows, the determination of the margin against the onset of unstalled flutter vibrations is of crucial importance. The margin against the onset of unstalled flutter vibration is determined by a stability analysis, i.e. by solving an aeroelastic eigenvalue problem. The aeroelastic stiffness and damping are, in industrial practice, calculated for each vibration mode of the blade row using Computational Fluid Dynamics (CFD) methods. In these calculations, the complex modal work applied from the fluid to the blade row is determined for each vibration mode of interest. The corresponding stiffness and damping can then be derived from this complex modal work. The damping can be derived either as viscous damping (proportional to the velocity) or as structural damping (proportional to the displacement).

Both approaches are proposed in the literature and are detailed in terms of an unstalled flutter stability analysis for a freestanding and mistuned LSB row in this paper. Mistuning is modelled using the well-known Fundamental Model of Mistuning (FMM) methodology. Even though both approaches result in approximately equal results w.r.t. the determination of the onset of unstalled flutter vibrations, an explanation is given why the viscous damping approach is, in terms of its physical interpretation from the structural dynamics point of view, superior to the structural damping approach.

Mistuning, either natural or intentional, generally increases the margin to the onset of unstalled flutter vibrations. If intentionally mistuning is applied to LSB rows, one question is its interaction with the unavoidable remaining natural mistuning which is random in nature. Even though mistuning is beneficial in terms of the unstalled flutter stability, it has a detrimental effect on forced vibrations because of the amplitude magnification phenomenon which is relevant for start-up and coast-down. Thus, the effect of mistuning on the vibrations of an LSB row needs to be analyzed for both cases, namely unstalled flutter and forced response. A systematic study on the interaction between intentional and natural mistuning is given in this paper for different forms of practical intentional mistuning patterns.
Steam Turbine Improved Operation and Maintenance by Thermal Warming System
Authors: Girezzi Gabriele , Bucciarelli Federico , Checcacci Damaso
Published: January 2021

It is nowadays a common understanding that, due to the uneven availability of renewable energy sources, the operation of traditional power generation plants and especially of combined cycles has to be more flexible and subject to more frequent and, most probably, colder starts than in the past. This phenomenon translates into a negative impact on maintenance intervals and operating costs as resulting by a higher increase rate of equivalent operating hours. In addition, the optimization of start-up time, largely driven by initial components temperature, has become a key performance indicator for the profitability of such plants.

The case study presented in this paper deals with a steam turbine in a combined cycle power plant, commissioned on the early 2000s. The turbine is currently operating daily for half of the year and occasionally for the other half, collecting about 150 warm and 30 cold starts per year.

The application to the steam turbine of a Thermal Warming System (TWS) is analyzed in detail by assessing casing and rotor temperature distribution, in transient operation. The control algorithms, that allows maximizing the system effectiveness while safeguarding against possible issues due to uneven temperature distributions, are also discussed. The resulting increase in average starting temperature, as evidenced by online rotor temperature calculation, is then considered with respect to its benefits for maintenance optimization and plant profitability.
The Impact of Excitation and Mode Shape on Non-Linear Blade Vibrations
Authors: Linhard Johannes , Hartung Andreas , Schwarz Stefan , Hackenberg Hans-Peter , Sienko Mateusz
Published: January 2021

Recently, reliable non-linear dynamic solvers for the analysis of frictionally coupled turbine blades have been developed which are based on either Higher Harmonic Balance Method or Non-linear Modal Analysis. One of these tools is OrAgL which was developed by Institute of Dynamics of Vibrations (Leibniz University of Hannover) and Institute of Aircraft Propulsion Systems (University of Stuttgart). In [1], the rig and engine validation results of with OrAgL performed forced response analyses have been published: The main aim of this paper was the comparison of non-linear numerical predictions (amplitude, frequencies) with the blade-to-blade averaged values of optical measurement results obtained using MTU’s non-contact vibration measurement system for shrouded turbine blades (BSSM-T).

Detailed analyses and validations performed over the last two years showed several novel aspects of validation such as the comparison with strain gauge measurements. Moreover, a better understanding of the impact of excitation (magnitude and load distribution over the airfoil) as well as of the impact of the mode shape on the formation of saturation regimes is now possible. The results obtained from the analyses of real turbine blades are presented in this work.
Effect of Scratches on a Tilting-Pad Journal Bearing
Authors: Chatterton Steven , Pennacchi Paolo , Vania Andrea , Hassini Mohamed Amine , Kuczkowiak Antoine
Published: January 2021

Many industrial rotating machines are equipped with hydrodynamic journal bearings, such as centrifugal compressors, steam turbines, pumps and motors.

After some time from the installation, however, the surface of the bearings often presents imperfections and slight damages mainly caused by the presence of harder particles in the lubricant during start-ups and shut-downs, when the hydrodynamic mechanism is not well developed and the mixed lubrication can occur. The presence of scratches on a bearing can lead to variations of the oil film thickness which, in turn, causes significant degradation of the bearing hydrodynamic performance. For example, the reduction of the minimum oil-film thickness can lead to the increase in the local temperature, to local pressure peaks and, finally, to the failure of the bearing.

Experimental data relating to scratches on journal bearings are extremely limited in the literature especially for tilting-pad journal bearings (TPJBs). An experimental activity was carried out to study the effect of artificial scratches on pads on the static and dynamic behaviors of a TPJB. The number of scratches, the depth and the axial position have been investigated and the dynamic coefficients have been estimated as well. The experimental results confirmed a degradation of the dynamic performance of the bearing in case of scratches, that it has has been also confirmed by means of numerical simulations.
Evaluation of a Turbulent Jet Flame Flashback Correlation Applied to Annular Flow Configurations With and Without Swirl
Authors: Sullivan Lewis E. , McDonell Vincent G. , Kalantari Alireza , Saxena Priyank
Published: January 2021

The adaptation of high hydrogen content fuels for low emissions gas turbines represents a potential opportunity to reduce the carbon footprint of these devices. The high flame speed of hydrogen air mixtures combined with the small quenching distances poses a challenge for using these fuels in situations where significant premixing is desired. In particular flashback in either the core flow or along the walls (i.e., boundary layer flashback) can be exacerbated with high hydrogen content fuels. In the present work, the ability of a flashback correlation previously developed for round jet flames is evaluated for its ability to predict flashback in an annular flow. As a first step, an annular flow is generated with a centerbody located at the centerline of the original round jet flame. Next, various levels of axial swirl is added to that annular flow. Additional flashback data are obtained for various mixtures of hydrogen and methane and hydrogen and carbon monoxide for all-the annular flow configurations. Pressures from 3–8 bar are tested with mixture temperatures up to 750 K. Flashback is induced by slowly increasing the equivalence ratio. The results obtained show that the same form of the correlation developed for round jet flames can be used to correlate flashback behavior for the annular flow case with and without swirl despite the presence of the centerbody. Adjustments to some of the constants in the original model were made to obtain the best fit, but in general, the correlation is quite similar to that developed for the round jet flame. A significant difference with the annular flow configurations is the determination of the appropriate gradient at the wall, which in the present case is determined using a cold flow CFD simulation.
On the Mechanism of Spike Stall Inception and Near Stall Non-Synchronous Vibration in an Axial Compressor
Authors: Möller Daniel , Schiffer Heinz-Peter
Published: January 2021

The aerodynamic and aeroelastic behavior of an engine-like transonic compressor stage is investigated in this paper. Simulations were carried out for the transonic compressor at the Technical University of Darmstadt using the AU3D flow solver. A comparison with previous experimental investigations shows close agreement for both steady and transient data. The simulations enable a detailed flow analysis during spike stall inception and non-synchronous vibrations (NSV) at operating points close to the stall limit.

The aerodynamic structure of shear layer fluctuations in the rotor tip region during the initial phase of spike stall inception is investigated. Already before the stall limit is reached such fluctuations can occur. In this case, the compressor continues to operate on the aerodynamically stable characteristic and NSV might be excited due to the unsteady blade force. This paper concludes with a joint analytical interpretation of the shear layer fluctuations observed at the beginning of spike stall and during near stall NSV by basic flow principles.
Effects of Non-Axisymmetric Volute on Rotating Stall in the Vaneless Diffuser of a Centrifugal Compressor
Authors: Niu Zitian , Sun Zhenzhong , Wang Baotong , Zheng Xinqian
Published: January 2021

Rotating stall is an important unstable flow phenomenon that leads to performance degradation and limits the stability boundary in centrifugal compressors. The volute is one of the sources to induce the non-axisymmetric flow in a centrifugal compressor, which has an important effect on the performance of compressors. However, the influence of volute on rotating stall is not clear. Therefore, the effects of volute on rotating stall by experimental and numerical simulation have been explored in this paper. It’s shown that one rotating stall cell generates in a specific location and disappears in another specific location of the vaneless diffuser as a result of the distorted flow field caused by the volute. Also, the cells cannot stably rotate in a whole circle. The frequency related to rotating stall captured in the experiment is 43.9% of the impeller passing frequency (IPF), while it is 44.7% of IPF captured by three-dimensional unsteady numerical simulation, which proves the accuracy of the numerical method in this study. The numerical simulation further reveals that the stall cell initialized in a specific location can be split into several cells during the evolution process. The reason for this is that the blockage in the vaneless diffuser induced by rotating stall is weakened by the mainstream from the impeller exit to make one initialized cell disperse into several ones. The volute has an important influence on the generation and evolution process of the rotating stall cells of compressors. By optimizing volute geometry to reduce the distortion of the flow field, it is expected that rotating stall can be weakened or suppressed, which is helpful to widen the operating range of centrifugal compressors.
Detection of Cracks in Turbomachinery Blades by Online Monitoring
Authors: Kumar Manish , Heinig Roger , Cottrell Mark , Siewert Christian , Almstedt Henning , Feiner Drew , Griffin Jerry
Published: January 2021

The presence of a crack in a blade can change the natural frequencies of that blade. It has long been a goal to detect blade cracks by assessing the change in a measured vibration frequency of the blade over time. It has been found that prior frequency assessment methods can be less accurate than is desirable to reliably detect the relatively small frequency changes that are typically associated with blade crack sizes of practical interest.

This paper describes a method in which potential temporal changes in the frequencies of individual blades are assessed by periodically analyzing complete rows of blades using mistuning analysis techniques that treat the blade rows as coupled systems, in contrast to other techniques that consider each blade individually in turn. This method, while computationally complicated and challenging, has been found to be capable of detecting blade root cracks that are much smaller than those that can be detected using other techniques. Moreover, this method has been demonstrated to detect cracks that are much smaller than the critical size for mechanical separation of the blade from the rotor.

This improved frequency assessment technique has been used to identify more than 30 blades with frequency changes that were considered to be potential indicators of blade cracks. Subsequent inspections verified indications in all of those blades.

In addition to providing operational guidance, the frequency change data were used to infer the time periods during which crack growth had occurred.
Vortex Lattice Method for the Calculation of the Tip Leakage Flow: Evaluation on a Single Blade
Authors: Montsarrat Christophe , Deveaux Benjamin , Boudet Jérôme , Marty Julien , Lippinois Eric
Published: January 2021

This paper investigates the use of a Vortex Lattice Method to simulate tip-leakage flow with small computational effort. The module PyLiSuite presented in the present paper has been completely developed from scratch and is validated on two-dimensional and three-dimensional basic cases. The validation lies on the estimate of the lift coefficient computed from the circulation given by the module. The capabilities of PyLiSuite regarding tip-leakage flow are gauged in comparison with novel experimental measurements on a single blade with an adjustable gap. The results show a good prediction of the shape and size of the tip-leakage vortex for large tip gaps. Differences in the position and the deficit of static pressure in the core of the vortex are noted. The future improvements on the module concern the influence of viscosity to be accounted for and the computation time which could be shortened.
Numerical Simulation of a Radial Turbine at Off-Design Conditions in Presence of Choked Flow
Authors: Tiseira Andrés , García-Cuevas Luis Miguel , Inhestern Lukas Benjamin , Echavarría Juan David
Published: January 2021

In the application of turbocharging and organic Rankine cycles (ORCs) the radial turbine can operate at high pressure ratio leading to the presence of choking inside the turbine. Information in this map region is needed to estimate the overall machine performance and to achieve an optimum matching of engine-turbocharger or an integration into ORCs. As a preliminar study before doing experimental tests in a turbocharger gas stand and before starting modeling activities, 3D CFD simulations were run for this paper. The occurrence of choking in the stator passage, in the vaneless space and in the rotor passage is categorized and analyzed under different operating conditions and VGT openings. The appearance of the shock wave depends strongly of the stator vane position and pressure ratio. In the analyzed turbocharger VGT turbine no choking has been identified in the throat of the stator vanes. In technically relevant closed VGT position choking was identified in the vaneless space. Whereas at open vane position the shock wave appears at the rotor throat and the Mach number can increase from the blade tip to the hub or from the hub to the tip depending on the rotational speed. Furthermore, tip leakage flow plays an important role, being responsible of subsonic regions in the rotor outlet although a big percentage of the section is choked.
Design and CFD Simulation of a Micro Gas Turbine Combustor Fuelled With Low LHV Producer Gas
Authors: Nicolosi Francesco F. , Renzi Massimiliano
Published: January 2021

In this paper, the authors analyze the feasibility of fuelling a small-scale 3.2 kWe MGT, manufactured by the Dutch company MTT, with a low LHV fuel produced via a gasification process. In particular, a CFD analysis on the combustor of the MGT is carried out in order to assess the behaviour of the component when it is fuelled with a traditional fuel (natural gas) and with a producer gas coming from a gasification process. The operating conditions of the combustor, used as boundary conditions for the simulations, are obtained by analyzing the characteristic performance curves of the turbo-machines used in the MGT. The simulation of the combustion process with methane has been validated using the temperature output from experimental tests and the NOX emissions. A RANS simulation using the Non-Adiabatic Non-Premixed Combustion Model Approach has been adopted. NOX formation has been simulated by the adoption of the extended Zel’dovich mechanism. Both nominal and part load simulations have been performed. This simplified modelling strategy allows to assess the main issues and figures of the combustion process with a reasonable computational effort. The CFD simulations showed that the combustion with a low LHV fuel are feasible but some modifications of the present configuration of the combustor are required, with specific attention to the fuel injection system. Results showed that, with Natural Gas, the average temperature of the exhaust mass flow is 1297 K, the level of CO and NOX referred to the 15% of O2 are respectively less than 1 ppm and 30.365 ppm, respectively. With S the original design of the injector proved to be non-adequate for a proper air and fuel mixing; therefore, a modified design has been proposed with an increased injection section. In the novel design for syngas, a better temperature distribution and lower emissions have been found: an average temperature of the flue gas at the combustor discharge of 1249 K is obtained, and the level of CO and NOX are both less than 1 ppm. The lower operating temperature is determined by the higher fuel flow rate and, in particular, by the high share of inert gases in the fuel. Additional simulations have been run at part load operation to assess the viability of the proposed design also in off-design conditions.
Detailed Velocity and Heat Transfer Measurements in an Advanced Gas Turbine Vane Insert Using MRV and IR Thermometry
Authors: Benson Michael J. , Bindon David , Cooper Mattias , Davidson F. Todd , Duhaime Benjamin , Helmer David , Woodings Robert , Van Poppel Bret P. , Elkins Christopher J. , Clark John P.
Published: January 2021

This work reports the results of paired experiments for a complex internal cooling flow within a gas turbine vane using Magnetic Resonance Velocimetry (MRV) and steady-state Infrared (IR) thermometry. A scaled model of the leading edge insert for a gas turbine vane with multi-pass impingement was designed, built using stereolithography (SLA) fabrication methods, and tested using MRV techniques to collect a three-dimensional, three-component velocity field data set for a fully turbulent test case. Stagnation and recirculation zones were identified and assessed in terms of impact on potential cooling performance. A paired experiment employed an IR camera to measure the temperature profile data of a thin, heated stainless steel impingement surface modeling the inside turbine blade wall cooled by the impingement from the vane cooling insert, providing complementary data sets. The temperature data allow for the calculation of wall heat transfer characteristics, including the Nusselt number distribution for cooling performance analysis to inform design and validate computational models. Quantitative and qualitative comparisons of the paired results show that the flow velocity and cooling performance are highly coupled. Module-to-module variation in the surface Nusselt number distributions are evident, attributable to the complex interaction between transverse and impinging flows within the apparatus. Finally, a comparison with internal heat transfer correlations is conducted using the data from Florschuetz [1]. Measurement uncertainty was assessed and estimated to be approximately +7% for velocity and ranging from +3% to +10% for Nusselt number.
Auto-Ignition of Gas Turbine Lubricating Oils in a Shock Tube Using Spray Injection
Authors: Cooper Sean P. , Browne Zachary K. , Alturaifi Sulaiman A. , Mathieu Olivier , Petersen Eric L.
Published: January 2021

In choosing the lubricating oil for a gas turbine system, properties such as viscosity, viscosity index, corrosion prevention, and thermal stability are chosen to optimize turbine longevity and efficiency. Another property that needs to be considered is the lubricant’s reactivity, as the lubricant’s ability to resist combustion during turbine operation is highly desirable. In evaluating a method to define reactivity, the extremely low vapor pressure of these lubricants makes conventional vaporization by heating impractical due to the high temperatures and fuel cracking as well as issues with preferential vaporization. To this end, a new experiment was designed and tested to evaluate the reactivity of lubricating oils using an existing shock-tube facility at Texas A&M University equipped with an automotive fuel injector. This experiment disperses a pre-measured amount of lubricant into a region of high-temperature air to study auto-ignition. To ensure proper dispersal, a laser extinction diagnostic was used to measure the lubricant particles behind the reflected shock as they are dispersed and vaporized. An OH* chemiluminescence diagnostic measuring light emitted during combustion at around 306 nm was used to determine ignition delay time. Pressure was also measured at the sidewall and endwall positions for test repeatability and exothermicity of the experiments. The methods were validated by conducting experiments with ethanol and comparing the results to previous heated shock-tube experiments conducted in the same facility. Using this method, various 32-, 36-, and 46-weight lubricants identified as widely used in the gas turbine industry were tested. Experiments were conducted in post-reflected shock conditions around 1370K (2006 °F) and 1.2 atm, where ignition delay time, peak OH* emission and time-to-peak values were recorded and compared. Ignition was observed for all but one of the lubricants at these conditions, and mild to strong ignition was observed for the other lubricants with varying ignition delay times.
Damage Detection of Composite Materials Using Data Fusion With Deep Neural Networks
Authors: Dabetwar Shweta , Ekwaro-Osire Stephen , Dias João Paulo
Published: January 2021

Composite materials have enormous applications in various fields. Thus, it is important to have an efficient damage detection method to avoid catastrophic failures. Due to the existence of multiple damage modes and the availability of data in different formats, it is important to employ efficient techniques to consider all the types of damage. Deep neural networks were seen to exhibit the ability to address similar complex problems. The research question in this work is ‘Can data fusion improve damage classification using the convolutional neural network?’ The specific aims developed were to 1) assess the performance of image encoding algorithms, 2) classify the damage using data from separate experimental coupons, and 3) classify the damage using mixed data from multiple experimental coupons. Two different experimental measurements were taken from NASA Ames Prognostic Repository for Carbon Fiber Reinforced polymer. To use data fusion, the piezoelectric signals were converted into images using Gramian Angular Field (GAF) and Markov Transition Field. Using data fusion techniques, the input dataset was created for a convolutional neural network with three hidden layers to determine the damage states. The accuracies of all the image encoding algorithms were compared. The analysis showed that data fusion provided better results as it contained more information on the damages modes that occur in composite materials. Additionally, GAF was shown to perform the best. Thus, the combination of data fusion and deep neural network techniques provides an efficient method for damage detection of composite materials.
Experimental Study on the Effect of Clocking on the Propagation of Inflow Pressure Distortions in a Low Pressure Turbine Stage
Authors: Simonassi L. , Zenz M. , Bruckner P. , Pramstrahler S. , Heitmeir F. , Marn A.
Published: January 2021

The design of modern aero engines enhances the interaction between components and facilitates the propagation of circumferential distortions of total pressure and temperature. As a consequence, the inlet conditions of a real turbine have significant spatial non-uniformities, which have direct consequences on both its aerodynamic and vibration characteristics.

This work presents the results of an experimental study on the effects of different inlet total pressure distortion-stator clocking positions on the propagation of total pressure inflow disturbances through a low pressure turbine stage, with a particular focus on both the aerodynamic and aeroelastic performance. Measurements at a stable engine relevant operating condition and during transient operation were carried out in a one and a half stage subsonic turbine test facility at the Institute of Thermal Turbomachinery and Machine Dynamics at Graz University of Technology. A localised total pressure distortion was generated upstream of the stage in three different azimuthal positions relative to the stator vanes. The locations were chosen in order to align the distortion directly with a vane leading edge, suction side and pressure side. Additionally, a setup with clean inflow was used as reference. Steady and unsteady aerodynamic measurements were taken downstream of the investigated stage by means of a five-hole-probe (5HP) and a fast response aerodynamic pressure probe (FRAPP) respectively. Strain gauges applied on different blades were used in combination with a telemetry system to acquire the rotor vibration data.

The aerodynamic interactions between the stator and rotor rows and the circumferential perturbation were studied through the identification of the main structures constituting the flow field. This showed that the steady and unsteady alterations created by the distortion in the flow field lead to modifications of the rotor vibration characteristics. Moreover, the importance of the impact that the pressure distortion azimuthal position has on the LPT stage aerodynamics and vibrations was highlighted.
Experimental Assessment of a Sliding-Blade Inside-Out Ceramic Turbine
Authors: Thibault D. , Dubois P. K. , Picard B. , Landry-Blais A. , Plante J.-S. , Picard M.
Published: January 2021

In order to reach 40% efficiency, sub-MW turbines must operate in a recuperated gas Brayton cycle at a turbine inlet temperature (TIT) above 1300°C. Current sub-MW turbines have material-related operating temperature limits. Still to this day, there is no cost-effective rotor design which operates at such high temperatures. This paper introduces a novel, sliding-blade, inside-out ceramic turbine (ICT) wheel configuration, which could enable high-efficiency sub-MW recuperated engines to be achieved with cheap monolithic ceramic blades.

The inside-out configuration uses a rotating structural hoop, or shroud, to convert centrifugal forces into compressive blade loading. The sliding-blade architecture uses a hub with angled planes on which ceramic blades slide up and down, allowing to match the radial expansion of the structural shroud. This configuration generates low stress values in both ceramic and metallic components and can achieve high tip speeds.

A prototype is designed and its reliability is calculated using CARES software. The result is a design which has a single blade probability of failure (Pf) of 0.1% for 1000 h of steady operation. Analyses also demonstrate that reliability is greatly dependent on friction at ceramic-to-metal interfaces. Low friction could lead to acceptable reliability levels for engine applications. The prototype was successfully tested in a laboratory turbine environment at a tip speed of 350 m/s and a TIT of 1100 °C without any damage.

These achievements demonstrate the robustness of the sliding-blade ICT configuration. Further research and development will focus on increasing tip speed and TIT to higher values.
FEM Considerations to Simulate Interlocked Bladed Disks With Lagrange Multipliers
Authors: Córdoba Oscar
Published: January 2021

A few structural simulation issues with constrained Finite Elements using Lagrange multipliers are described. The main difficulties associated to interlocked bladed disks are:

1. Contacts. The structural state must be calculated by the FEM solver accounting the centrifugal, thermal and pressure loads. The interlock surfaces may slide, some area will be in contact and some others will be separated. This is a nonlinear condition that needs solver iterations.

2. Interference. The blade surfaces susceptible to contact are not necessarily periodic. In many cases pretwist is needed during assembly to couple the blades. The solution for surfaces in contact require a cyclic symmetry boundary condition. Different issues related to coordinates systems will be discussed.

3. Overconstraint. The model mesh nodes may accommodate more than one boundary condition. Interlock nodes are affected for both conditions, possibility of contact and cyclic symmetry.

The use of Lagrange multipliers may cope with these problems. This is a well-known method although some FEM methodology peculiarities will be derived from scratch, either in the static and modal simulations. Pros and cons of this approach will be discussed related to classical solutions to the aforementioned problems. Every algorithm described has been already implemented in a FEM code than runs with successful results.
Low Rank Education of Cascade Loss Sensitivity to Unsteady Parameters by Proper Orthogonal Decomposition
Authors: Lengani D. , Simoni D. , Yepmo V. , Ubaldi M. , Zunino P. , Bertini F.
Published: January 2021

In the present work, Proper Orthogonal Decomposition (POD) has been applied to a large dataset describing the profile losses of Low Pressure Turbine (LPT) cascades, thus allowing: i) the identification of the most influencing parameters that affect the loss generation; ii) the identification of the minimum number of requested conditions useful to educate a model with a reduced number of data.

The dataset is constituted by the total pressure loss coefficient distributions in the pitchwise direction. The experiments have been conducted varying the flow Reynolds number, the reduced frequency and the flow coefficient. Two cascades are considered: the first for tuning the procedure and identifying the number of really requested tests, and the second for the verification of the proposed model. They are characterized by the same axial chord but different pitch-to-chord ratio and different flow angles, hence two Zweifel numbers.

The POD mode distributions indicate the spatial region where losses occur, the POD eigenvectors provide how such losses vary for the different design conditions and the POD eigenvalues provide the rank of the approximation. Since the POD space shows an optimal basis describing the overall process with a low rank representation (LRR), a smooth kernel is educated by means of Least-Squares method (LSM) on the POD eigenvectors. Particularly, only a subset of data (equal to the rank of the problem) has been used to generate the POD modes and related coefficients. Thanks to the LRR of the problem in the POD space, predictors are low order polynomials of the independent variables (Re, f+ and ϕ). It will be shown that the smooth kernel adequately estimates the loss distribution in points that do not participate to the education. Additionally, keeping the same steps for the education of the kernel on another cascade, loss distribution and magnitude are still well captured. Thus, analysis show that the rank of the problem is much lower than the tested conditions, and consequently a reduced number of tests are really necessary. This could be useful to reduce the number of hi-fidelity simulations or detailed experiments in the future, thus further contributing to optimize LPT blades.