1,720,965 research outputs found
Identification of nonlinear dynamic parameters of thrust bearings in rotating machines: Modelling of axial Sub-Synchronous vibration and experimental verification
Full text linkAxial Sub-Synchronous Vibration (SSV) is an uncommon phenomenon that occurs in turbomachines. Specifically in compressors and large gas turbines equipped with double-sided, oil-lubricated thrust bearings. This phenomenon manifests as high-amplitude vibration which extends throughout the entire machine axial clearance. Consequently, the structural integrity of the machinery can be compromised, as the SSV can lead to detrimental effects such as fatigue cracks, seal abrasion, and fretting on bearings. This vibration develops unpredictably, with sudden amplitude jumps related to operational parameters such as active power, axial clearance adjustments, and lubricating oil temperature. While the literature extensively discusses potential causes, often linking vibration levels to fluid film instabilities on the reverse side of thrust bearings, this theory does not account for all observed cases. To understand the problem, the nonlinear behaviour of the oil film in the thrust bearing is analysed by solving the well-known Reynolds equation. The thrust force on the bearing collar is calculated and used to create a simplified axial model of the machine. Time integration and Harmonic balance methods are used to study the nonlinear dynamics, to identify the bifurcation region, and to perform sensitivity analyses by varying operational parameters. This phenomenon usually amplifies the vibration induced by the coupling between axial-lateral vibration, related to the thrust bearing collar. The proposed model, applied to a well-known example in the literature, offers insights into mitigating the adverse effects of axial SSV and improving the reliability of turbomachines
Prediction of a Crack Propagation in a Steam Turbine Rotor
No full textThe early detection of shaft cracks in rotating machines is
essential to avoid catastrophic damage, long outages and high
maintenance costs. An increasing number of transverse and
circumferential cracks caused by thermal fatigue have been
detected in steam turbines, in recent years. Because of usual
long-time intervals between two subsequent planned inspections
of the rotors, it is very important to identify the presence of a
shaft crack by the online analysis of the harmonic content of the
machine vibrations measured by Condition Monitoring Systems.
As the symptoms caused by shaft cracks in the system response
are often masked by the effects induced by other common
malfunctions, it is very useful to study the sensitivity of the
dynamic behavior of turbine shafts to the propagation of
transverse cracks and to predict the lowest value of the crack
depth that can cause detectable symptoms in the machine
vibration. This paper shows the results provided by a numerical
approach, aimed at identifying the propagation of shaft cracks,
which has been applied to the analysis of a real intermediatepressure
steam turbine in which a first crack initiation had been
detected
Large Axial Vibrations in Turbomachines: Non-Linear Behaviour of Double Sided Lubricated Thrust Bearing
No full textEven if not a common phenomenon, large axial vibrations
are a critical concern in turbomachinery, particularly when
dealing with double-sided, oil-lubricated thrust bearings in
compressors and gas turbines. This phenomenon, known as axial
Sub-Synchronous Vibration (SSV), manifests as high-amplitude
vibrations that can extend throughout the entire machine
clearance. The high level of vibrations is a main concern for the
structural integrity of the machines, and it can lead to the onset
of a trip condition. The development of the vibration is often
unpredictable, marked by sudden jumps in vibration amplitude
due to various operational parameters of the machine, such as
active power, clearance adjustments, oil temperature, and flow
rate. To understand the problem, the non-linear behaviour of the
oil film of the thrust bearing is analysed by solving the wellknown
Reynolds equation. The integral force on the bearing
collar is calculated and then used to create a simplified axial
model of the machine. Harmonic Balance is used to study the
non-linear dynamics of the system, the bifurcation region is
found, and a sensitivity analysis is performed by changing the
most important operational parameters of the bearing. This
phenomenon usually amplifies the vibrations induced by the
coupling between axial-lateral vibrations, related to the thrust
collar. Numerical results are compared to experimental data
available in the literature
Squeeze Film Dampers Design and Application in High-Speed Radial Outflow Turbines for Small Size Organic Rankine Cycles
No full textTemperature Profile at Pad’s Leading Edge of a Titling-Pad Journal Bearing: CFD Modeling and Experimental Validation
No full textTilting-pad journal bearing (TPJB) is amechanical component that utilizes the hydrodynamic lubrication principle to support a rotating shaft in which each pad can tilt to accommodate the motion of the shaft in its equilibrium position. Within the groove, which is the space between two consecutive pads, the cold oil from the supply system mixes with the hot oil from the upstream pad. This mixing strongly influences the temperature distribution in the bearing. In the paper, a computational fluid dynamics model (CFD) is introduced to simulate the mixing process in the groove. The CFD model is integrated in a Reynolds-based thermos-hydrodynamic (THD) model of the entire bearing to simulate both the static and dynamic behavior of the bearing. The resulting temperature in the groove obtained from the coupled model is compared to experimental measurements of the temperature at the leading edge of a pad of a TPJB installed in a test-rig. The temperature is obtained by an array of temperature probes installed in the leading edge of the pad
Cooled pads with bioinspired gyroid lattice for tilting pad journal bearings: Experimental validation of numerical model for heat transfer
Full text linkHydrodynamic journal bearings are essential components for industrial rotating machineries. Continuously growing specific power allows more compact and efficient machines to be obtained, by reducing the environmental footprint of production plants. The aim of this work is to provide a new design of an innovative pad for Tilting Pad Journal Bearings (TPJBs) with an embedded cooling circuit, able to limit the oil film temperature. As a result, specific load can be increased leading to a possible downsizing of the bearing and reduction of lubricant quantity. The heat exchange in the cooling circuit of the pad has been enhanced using bioinspired gyroid lattice. The thermo-mechanical study of the pad has been performed with both numerical and experimental analysis. The resulting thermal and mechanical performances have been calculated and discussed. The tested components have been manufactured in stainless steel by using a metal 3D printing technology, based on polymer-metal feedstock extrusion. A performance analysis is conducted to catch differences between the nominal and the printed geometry. The prototype is able to dissipate the generated heat with a higher efficiency, and the pressure drop inside the cooling circuit can be estimated with the numerical model here proposed
Gyroid lattice for 3D printed pad of tilting pad journal bearings : modeling and experiments
No full textLAUREA MAGISTRALEI cuscinetti fluidodinamici sono componenti di fondamentale importanza per le macchine rotanti in campo industriale. La continua crescita di potenza specifica permette la costruzione di macchine sempre più compatte ed efficienti, che riducono l’impatto ambientale delle linee di produzione. Nei cuscinetti radiali, il carico è sostenuto dal campo di pressione generato nel meato d’olio tra due superfici in movimento relativo, quella dell’albero e quella delle pastiglie. Questa tecnica è detta lubrificazione idrodinamica e permette di ridurre considerabilmente le dissipazioni di energia; tuttavia, gli forzi di taglio che si generano nel sottile strato d’olio producono calore, che scalda l’olio stesso e riduce il carico massimo ammissibile.
Scopo di questo progetto di tesi è la progettazione di una innovativa pastiglia per cuscinetti, con circuito di raffreddamento integrato, per ridurre la temperatura del film d’olio. L’asportazione di calore può essere aumentata notevolmente attraverso l’utilizzo di lattici di ispirazione naturale come le giroidi. Un’attenta analisi comparativa è condotta variando i parametri significativi per la geometria del reticolo. Prove numeriche e sperimentali sono condotte per ottenere un’adeguata caratterizzazione termo-meccanica del componente.
Per la realizzazione di una geometria così complessa, la stampa 3D in metallo, e in particolare l’innovativa tecnologia di “Bound Metal Deposition”, è scelta per la realizzazione della pastiglia. La manufatturabilità è indagata attraverso il controllo e la quantificazione dei limiti di stampa, dei difetti locali e degli errori macro e micro geometrici.
Infine, questa attività di ricerca apre nuove opportunità interessanti: il reticolo a giroide può essere utilizzato per l’alleggerimento topologico dei componenti strutturali e l’utilizzo del rame può aumentare considerevolmente l’efficienza di scambio termico.Hydrodynamic journal bearings are essential power transmission elements for industrial rotating machineries. Continuously growing of specific power allows more compact and efficient machines reducing the environmental footprint of production plants. Radial load is sustained by the pressure field generated in the oil film wedge between relative moving surfaces, due to hydrodynamic lubrication phenomenon, but shear stress heats up the lubricant, limiting the maximum allowable load.
The aim of the thesis is to design an innovative pad with an embedded cooling circuit to limit oil film temperature. Heat exchange can be relevantly increased using bioinspired gyroid lattice. A comparative analysis is performed changing relevant parameters for lattice geometry; numerical and experimental tests are performed for a proper thermo-mechanical design of the component.
Metal 3D printing is able to produce such a complicated shape; Bound Metal Deposition technology is chosen for pad production and a manufacturability study is performed. Printability limits, local defects and macro geometrical errors are investigated and quantified.
Finally, this research activity opens new opportunities: gyroid lattice can be used for light weighting of structural components and the use of copper could increase significantly heat exchange efficiency
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Nonlinear rotordynamic model of a gas turbine: coupling effects of the thrust bearing on axial vibrations
Full text linkLAUREA MAGISTRALELe vibrazioni sub-sincrone (SSV) nella direzione assiale si osservano spesso sia nelle grandi turbine a gas per la produzione di energia, sia nei piccoli turbocompressori automobilistici. Tali vibrazioni, che si manifestano a frequenze inferiori rispetto alla velocità di rotazione dell’albero, possono raggiungere ampiezze elevate e compromettere l’integrità strutturale del sistema. Le SSV assiali possono essere innescate da vibrazioni laterali, dando origine a una dinamica accoppiata assiale-laterale, governata dal comportamento del cuscinetto reggispinta idrodinamico, che genera forze e coppie non lineari secondo l’equazione di Reynolds.
Questo tesi ha l’obiettivo di analizzare le vibrazioni SSV assiali e sviluppare un modello rotodinamico per una turbina a gas di media potenza (circa 100 MW), capace di rappresentare accuratamente l’accoppiamento non lineare tra le dinamiche assiali e laterali.
Considerando l’elevato costo computazionale della soluzione diretta del modello basato sull’equazione di Reynolds, vengono proposte due strategie alternative per il calcolo delle forze e delle coppie generate dal cuscinetto: una rete neurale artificiale di tipo feed-forward e un interpolatore, entrambi addestrati su un insieme di dati ottenuto da simulazioni realizzate separatamente in MATLAB. Si eseguono anche simulazioni numeriche nel dominio del tempo, considerando le reali forze sviluppate dal meato d’olio, per essere confrontate con il modello semplificato.
I modelli lineare e non lineare vengono quindi confrontati in termini di accuratezza e prestazioni computazionali. Dalle simulazioni numeriche di forzamento assiale sinusoidale, si osservano effetti non lineari, quali le frequenze di intermodulazione. La frequenza propria assiale dipende dal carico assiale statico. A basso carico assiale statico, rigidezza e smorzamento ridotti causano SSV assiali di ampiezza elevata. L’effetto di accoppiamento indotto dal cuscinetto assiale si intensifica quando la ralla si avvicina ai lati del cuscinetto.
Il modello sviluppato costituisce una base solida per sviluppi futuri, finalizzati a includere ulteriori effetti rilevanti non considerati in questo studio.Sub-synchronous vibrations (SSV) in the axial direction can be observed in both large gas turbines for power generation and small automotive turbochargers. Such vibrations, occurring at frequencies lower than the shaft’s rotational speed, can reach significant amplitudes and pose a risk to structural integrity. Axial SSV can be triggered by lateral vibrations, resulting in coupled axial-lateral dynamics usually governed by the hydrodynamic thrust bearing. This component generates nonlinear forces and moments according to the Reynolds equation.
This work aims to investigate axial SSV and develops a rotordynamic model for a medium-sized (~100 MW) gas turbine capable of capturing the nonlinear coupling between axial and lateral dynamics. Due to the high computational cost of directly solving the Reynolds model, two approaches are proposed to compute bearing forces and moments: a feed-forward artificial neural network and an interpolator, both trained on datasets generated via offline MATLAB simulations. Time-domain numerical simulations with real oil film forces are also performed and compared to simplified model.
The linearized and nonlinear models are compared in terms of accuracy and computational efficiency. From numerical simulations of sinusoidal axial forcing of the unbalanced shaft, nonlinear effects are observed, such as intermodulation frequencies. Axial eigenfrequency depends on the static axial load. At low magnitude static axial load, reduced stiffness and damping cause large axial SSV. Coupling effects induced by the thrust bearing intensify when the collar approaches the bearing sides.
The resulting framework provides a foundation for further extensions to account for additional relevant effects beyond the scope of this study
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