Gas turbines are a popular source of power for aerospace and land-based applications. Nevertheless the importance of particle ingestion and its implication is an often underestimated problem which can lead to the variation of geometry of aerodynamic surfaces, entailing performance degradation and, possibly, a reduction in engine life. The presence of sand and other type of particles can cause erosion or deposition in both the compressor and turbine sections. This problem can occur for land based turbines operating in harsh environment, where the problem can be mitigated (but not eliminated) by installing filtration systems. For what concern the aerospace field, filtration systems cannot be used. Volcanic eruptions and sand dust storms can send particulate to aircraft cruising altitudes. Also, aircraft operating in remote locations or at low altitude can be subjected to particle ingestion, especially in desert environments. A thorough analysis of the deterioration mechanisms of gas turbines, and of their modelling, including erosion, deposit formation and deposit evolution over time has not been presented in the literature so far. In this work, the modelling of all such mechanisms has been performed for both the hot and the cold section. In all the presented approaches, the blade contamination process analysis is based on the Computational Fluid Dynamics (CFD) study, with a Eulerian-Lagrangian approach. The stochastic particle tracking for the trajectory computation has been employed. With this approach, both the transient and the steady state tracking can be performed, and both the methods will be employed: the choice of the time treatment will be made by comparing the time scale of the deposition/erosion phenomena to the time scale of the performance degradation. The boundary variation as a consequence of the particle impact has been implemented, either with a mesh morphing approach or innovative techniques that are proposed to include the particle impact consequences on the flow field. The displacement of the boundary, or the variation of some of its characteristics (e.g. roughness), represents the most important effect of the deposit/erosion and, therefore, particular care should be taken when dealing with it. From this work, some important information regarding the areas which are affected the most by the particles ingestion can be retrieved and the techniques that can be employed for the evaluation of the effects of the particle-wall interaction are explained. Indeed, Thanks to the model here proposed, a preliminary methodology for the robust design under fouling conditions is proposed. Eventually a non-dimensional analysis of the behaviour of particles upon impact is proposed, suggesting that a universal map for the classification of the consequences of the impingement can be outlined. Such map is proposed as a tool for the detection of the effect of the impact. Eventually, starting from the considerations and the models developed, a design-for-fouling oriented optimization is proposed.

Nonostante le turbine a gas siano una comune fonte di energia sia per applicazioni stazionarie che per il campo aeronautico, l’effetto della presenza di particolato nell’aria processata `e un aspetto che `e stato spesso sottostimato nella letteratura. Le particelle presenti nell’aria possono risultare, a seguito dell’impatto con le superfici bagnate dal flusso, in una variazione di geometria e comportare una riduzione delle performance o piuttosto della vita utile della macchina. Sabbia o, in generale, altri tipi di contaminant possono causare erosione o deposizione sia nelle sezioni fredde (compressore) che calde (turbina) della macchina. Questo problema si verifica tipicamente per turbine a gas stazionarie che lavorano in ambienti ostili, aventi alta concentrazione di particolato nell’aria: in tali casi, il problema pu`o essere solo mitigato, ma non completamente evitato, dall’installazione di sistemi di filtrazione all’aspirazione della macchina. Tali filtri non vengono invece utilizzati in caso di turbomacchine per la propulsione aeronautica. Perci`o, a seguito di tempeste di sabbia o eruzioni vulcaniche, possono essere presenti particelle all’altezza di crociera. In alternativa, il decollo, l’atterragio o il volo a bassa quota in localit`a remote possono rappresentare un’ulteriore rischio per l’ingestione di particelle. Un’analisi completa di tutti gli aspetti del deterioramento delle turbine a gas e della loro modellazione numerica, tenendo in considerazione erosione, formazione del deposito ed evoluzione del deposito nel tempo non `e ancora stata presentata in letteratura. In questo lavoro viene presentata la modellazione di tutti questi meccanismi, di entrambe le sezioni della macchina. In particolare, per tutti i fenomeni analizzati, il processo di contaminazione delle pale `e basato sullo studio numerico tramite CFD (Computational Fluid Dynamics), con un approccio Euleriano-Lagrangiano. La traiettoria delle particelle `e calcolata considerando il contributo stocastico derivante dalla turbolenza del moto. Con tale approccio possono essere considerati sia il tracciamento transitorio che quello stazionario: entrambi i metodi saranno utilizzati e la scelta dell’uno piuttosto che dell’altro verr`a giustificata in base al tempo caratteristico del fenomeno che si sta considerando. Infatti la decisione sar`a basata sul confronto del tempo caratteristico del fenomeno di erosione/deposizione con quello relativo alla rapidit`a con cui si manifesta il degrado delle prestazioni a livello di macchina. La modifica delle condizioni al contorno del dominio e, in particolare, delle pareti interessate dall’impatto di particelle costituisce il passaggio fondamentale per rappresentare il loro effetto. L’implementazione presentata in questo lavoro `e stata ottenuta mediante due diversi approcci: mesh mobili (moving mesh) e una serie di tecniche innovative che verranno prersentate nel seguito. Da questo lavoro possono essere ottenute alcune informazioni importanti riguardo le aree che sono maggiormente affette dalla presenza di particelle. Inoltre alcune tecniche che possono essere usate per la valutazione dell’effetto dell’interazione parete-particella vengono illustrate. Infine viene presentata un’analisi non-dimensionale del comportamento della particella all’impatto, suggerendo come pu`o essere realizzata una mappa universale per la classificazione delle conseguenze di tale impatto. Dalle considerazioni e dai modelli sviluppati, viene infine proposta una metodologia preliminare per la progettazione della pala in condizioni di presenza di particelle.

Modeling approaches for gas turbine deterioration analysis

CASARI, Nicola
2019

Abstract

Gas turbines are a popular source of power for aerospace and land-based applications. Nevertheless the importance of particle ingestion and its implication is an often underestimated problem which can lead to the variation of geometry of aerodynamic surfaces, entailing performance degradation and, possibly, a reduction in engine life. The presence of sand and other type of particles can cause erosion or deposition in both the compressor and turbine sections. This problem can occur for land based turbines operating in harsh environment, where the problem can be mitigated (but not eliminated) by installing filtration systems. For what concern the aerospace field, filtration systems cannot be used. Volcanic eruptions and sand dust storms can send particulate to aircraft cruising altitudes. Also, aircraft operating in remote locations or at low altitude can be subjected to particle ingestion, especially in desert environments. A thorough analysis of the deterioration mechanisms of gas turbines, and of their modelling, including erosion, deposit formation and deposit evolution over time has not been presented in the literature so far. In this work, the modelling of all such mechanisms has been performed for both the hot and the cold section. In all the presented approaches, the blade contamination process analysis is based on the Computational Fluid Dynamics (CFD) study, with a Eulerian-Lagrangian approach. The stochastic particle tracking for the trajectory computation has been employed. With this approach, both the transient and the steady state tracking can be performed, and both the methods will be employed: the choice of the time treatment will be made by comparing the time scale of the deposition/erosion phenomena to the time scale of the performance degradation. The boundary variation as a consequence of the particle impact has been implemented, either with a mesh morphing approach or innovative techniques that are proposed to include the particle impact consequences on the flow field. The displacement of the boundary, or the variation of some of its characteristics (e.g. roughness), represents the most important effect of the deposit/erosion and, therefore, particular care should be taken when dealing with it. From this work, some important information regarding the areas which are affected the most by the particles ingestion can be retrieved and the techniques that can be employed for the evaluation of the effects of the particle-wall interaction are explained. Indeed, Thanks to the model here proposed, a preliminary methodology for the robust design under fouling conditions is proposed. Eventually a non-dimensional analysis of the behaviour of particles upon impact is proposed, suggesting that a universal map for the classification of the consequences of the impingement can be outlined. Such map is proposed as a tool for the detection of the effect of the impact. Eventually, starting from the considerations and the models developed, a design-for-fouling oriented optimization is proposed.
PINELLI, Michele
TRILLO, Stefano
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2478784
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