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Yang, Jun (2015) Flow Patterns Causing Saddle Instability in the Performance Curve of a Centrifugal Pump with Vaned Diffuser. [Ph.D. thesis]

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Abstract (english)

In the present study, a reduced scale model of a low-pressure stage of a two-stage pump turbine was investigated when it operates in pump mode, which is as a centrifugal pump with vaned diffuser. An instability region was found in its Q-H curve. This instability region, centered into the flow rate interval between 0.45 to 0.7 QDes, restricts the stable operating range of the test pump.
The object of this study was to analyze the origins and development of the unsteadiness inner pump during this saddle instability region, and to find the causes of this saddle, which could guide the design, and optimization of the test pump.
Experimental and numerical approaches have been adopted to contribute the understanding on the highly complex flow interactions inside the test pump. The analyses pointed out that the instability of Q-H curve during this flow rate interval was mainly affected by the flow field unsteadiness into the diffuser. Severe flow separation occurred into the diffuser, affecting and/or affected by the unsteadiness from the impeller and return channel remarkably.
Both at full and part load, due to the U shape stay vanes in the return system, a 3-dimentional complex flow was found on the suction side of stay vane near the U turn corner. This flow caused a periodic unforced pressure disturbance with frequency St=0.6625 in the diffuser. A rotating stall with 5 cells into the diffuser was also highlighted by the experimental analyses and the numerical results validated that this disturbance was mainly due to the unsteady flow patterns in the return channel.
With the reduction of flow rate, the location of this unsteadiness in return channel move closer to the outlet of diffuser, and the unsteadiness made the flow separation near the stay vane leading edge on the suction side more severe and it impacted the flow in diffuser flow field in a more severe way.
At the same time, two types of unsteadiness were detected in the impeller in the saddle instability region. They were the flow separation with the frequency St=0.6625 at the impeller blade suction side and the wake-jet near the trailing edge of impeller blade with the frequency St=0.335, respectively. The wake-jet caused a periodic disturbance with a frequency of St=0.335 in diffuser as well. The experimental results indicated that it propagate circumferentially with 2 cells.
At the beginning of this saddle instability, in diffuser, the intensity of the fluctuations at St=0.335 and St=0.6625 both increase with the falling flow rate. With the development of these unsteady patterns, they mixed and caused a blockage in the gap between impeller and diffuser. This blockage caused the increase of the amplitude at BPF, which has been detected both by experimental and numerical results. At the same time, the further development of the blockage weakened the intensity of the two disturbances with frequency of St=0.335 and St=0.6625 in the blockage region. The blockage fully developed near the critical flow rate of 0.6 QDes. At this condition, the intensity of rotor and stator interaction increased and energy loss also increased which caused the drop of head and mean pressure at the exit of impeller. Furthermore, the disturbance in diffuser which caused by wake-jet of impeller lost the periodic character with frequency of St=0.335 at this condition. This is the reason why the non-linear component St=0.335 disappeared in diffuser around 0.6 QDes.

Abstract (italian)

Il campo di moto all’interno di uno stadio a bassa pressione di una macchina reversibile pompa-turbina bistadio è stato analizzato in dettaglio nel funzionamento da pompa. E’ stato identificata una regione di instabilità funzionale centrata in un intervallo di portata comprese fra 0,45-,7 QDes, che limita il campo di funzionamento della turbomacchina sotto test.
Le origini, lo sviluppo e la dinamica delle instabilità che si ingenerano all’interno di questo campo di esercizio sono state analizzate per cercare di identificare le cause della caduta di prevalenza e poter guidare successivamente la progettazione e l'ottimizzazione della pompa di prova.
Approcci sperimentali e numerici sono state adottati per contribuire alla comprensione delle interazioni di flusso estremamente complessi all'interno della pompa di prova. Le analisi hanno evidenziato che l'instabilità della curva Q-H in questo intervallo di portate è principalmente conseguente all'instabilità campo di flusso nel diffusore. Separazioni marcate si verificano nel diffusore, conseguenza o amplificate dalla instabilità della girante o dei canale di ritorno.
Nei canali di ritorno, sia a pieno carico che ai carichi parziali, è stato riscontrato una zona di ricircolo in prossimità del gomito e vicino alla superficie in depressione. Questo ricircolo causa un disturbo con frequenza St = 0,6625 nel diffusore. Nella zona di instabilità alla stessa frequenza è stato individuato uno stallo rotante con 5 celle nel diffusore sia dalle analisi sperimentali che dai risultati numerici.
Al ridursi della portata, la posizione di questa instabilità nel canale di ritorno trasla verso la sezione di uscita del diffusore rendendo il campo di moto nel diffusore più instabile.
Per queste portate sono state identificate due sorgenti di instabilità nella girante. La prima con la frequenza St = 0,6625 sul lato in depressione delle pale e il secondo nella zona di jet vicino alla sezione di uscita della girante con la frequenza St = 0,335. La zona di jet causa anche un disturbo periodico con una frequenza di St = 0,335 nel diffusore. I risultati sperimentali hanno evidenziato che questa instabilità genera una pulsazione di pressione con 2 cellule.
In corrispondenza della zona di instabilità, l'intensità delle fluttuazioni a St = 0,335 e St = 0,6625 aumenta al diminuire della portata. Contemporaneamente ampie zone del traferro tra girante e diffusore risultano bloccate. Questo blocco causa un'amplificazione delle pulsazioni alla frequenza di passaggio delle pale BPF rilevato sia da risultati sperimentali che numerici. Allo stesso tempo, l'ulteriore sviluppo del blocco indebolito l'intensità delle due fluttuazioni di pressioni a frequenza di St = 0,335 e St = 0,6625. Tale sviluppo raggiunge l'apice per una portata di circa 0,6 QDes. A questa condizione, l'intensità dell'interazione rotore e statore aumentata e la perdita di energia aumentata causando la caduta di prevalenza all'uscita della girante. Inoltre, l'instabilità nel diffusore perde il suo carattere periodico con frequenza di St = 0,335.

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EPrint type:Ph.D. thesis
Tutor:Pavesi, Giorgio
Ph.D. course:Ciclo 27 > scuole 27 > INGEGNERIA INDUSTRIALE > INGEGNERIA DELL' ENERGIA
Data di deposito della tesi:02 March 2015
Anno di Pubblicazione:28 February 2015
Key Words:Centrifugal pump with vaned diffuser, saddle instability, unforced unsteadiness, numerical simulation, experiment
Settori scientifico-disciplinari MIUR:Area 09 - Ingegneria industriale e dell'informazione > ING-IND/08 Macchine a fluido
Struttura di riferimento:Dipartimenti > Dipartimento di Ingegneria Industriale
Codice ID:8076
Depositato il:10 Nov 2015 12:34
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