bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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

Abstract
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.