The design of centrifugal compressors that offer high efficiency and high-pressure ratio during service represents a complex work that involves a high level of fluid dynamics understanding. In order to facilitate the design, Computational Fluid Dynamics (CFD), along with experimental campaigns, are widely used. The validation of the numerical simulations must be done to obtain accurate outcomes. The present work is focused on validating the experimental results of a high-pressure ratio centrifugal compressor designed by the National Aeronautics and Space Administration (NASA) in 1982. The centrifugal compressor is compounded by an impeller with 15 main and splitter blades and a vaned diffuser with 24 vanes. Ansys applications such as Bladegen, Turbogrid, DesignModeler Geometry, Mesh, and CFX were used to perform the simulations and study the evolution of the flow. Different configurations of the compressor were considered to assess the performance of the compressor, including those of the impeller coupled to a vaneless and a wedge-vaned diffuser. The results showed a good agreement in the prediction of the performance curves for all the cases being the impeller with a vaneless diffuser the case that presented the highest discrepancies for the efficiency and pressure ratio at nominal condition about 1.24% and 7.14% respectively. For the case of the impeller with a vaned diffuser, the difference in efficiency and pressure ratio was 0.95% and 0.58% while the case with fillet on the vanes was 0.673% and 1%. In the case of the impeller with a vaneless diffuser, the surge limits were not reached, and the choking condition occurred at the impeller inlet. On the contrary, in the case of the impeller with a vaned diffuser, the choking condition was set by the diffuser. In order to approach the simulation results with the experimental results, fillets were implemented at the hub and shroud of the vanes. The study of flow development showed that near the inlet of the diffuser, the boundary layer at the hub was more affected by the fillet than at the shroud, however, this effect on the global performance of the centrifugal compressor was not remarkable having a difference in the efficiency and pressure ratio with respect to the case without fillet about 0.16% and 0.42%, respectively. Finally, the mixing plane and frozen rotor approach were used to model the interface rotor-stator. The outcomes showed that the implementation of the mixing plane affects the flow downstream of the impeller neglecting the effect of the flow separation at the inlet of the diffuser. On the other hand, the frozen rotor model considers such instabilities but is strongly affected by the selection of the relative position of the rotor with respect to the stator obtaining different results for each position. The difference with respect to the experiments for the efficiency oscillates between 0.59% and 7.59% while for the pressure ratio between 0.13% and 3.95%.
La progettazione di compressori centrifughi che offrano un'alta efficienza e un elevato rapporto di pressione durante il servizio rappresenta un lavoro complesso che implica un alto livello di comprensione della fluidodinamica. Per facilitare la progettazione, si fa largo uso della Computational Fluid Dynamics (CFD), insieme a campagne sperimentali. La validazione delle simulazioni numeriche deve essere effettuata per ottenere risultati accurati. Il presente lavoro è incentrato sulla validazione dei risultati sperimentali di un compressore centrifugo ad alto rapporto di pressione progettato dalla National Aeronautics and Space Administration (NASA) nel 1982. Il compressore centrifugo è composto da una girante con 15 pale principali e 15 pale splitter e da un diffusore con 24 pale. Per eseguire le simulazioni e studiare l'evoluzione del flusso sono state utilizzate applicazioni Ansys come Bladegen, Turbogrid, DesignModeler Geometry, Mesh e CFX. Per valutare le prestazioni del compressore sono state prese in considerazione diverse configurazioni, ovvero la girante accoppiata a un diffusore non-palettato e a un diffusore palettato. I risultati si sono rivelati in accordo nella previsione delle curve di prestazione per tutti i casi, con la configurazione con diffusore non palettato che presenta le maggiori discrepanze nell’efficienza e nel rapporto di compressione in condizioni nominali, con una differenza del 1.24% e 7.14% rispettivamente. Per il caso con diffusore palettato invece, le differenze in efficienza e rapporto di compressione sono dello 0.95% e 0.58%, mentre nel caso con pale raccordate sono rispettivamente dello 0.673% e dell’1%. Nel caso della girante con diffusore non-palettato, i limiti di pompaggio non sono stati raggiunti, mentre la condizione di blocco sonico si è verificata all'ingresso della girante. Al contrario, nel caso della girante con diffusore palettato, la condizione di blocco sonico è stata imposta dal diffusore, motivando l'implementazione di raccordi alla base ed apice pala. Lo studio dello sviluppo del flusso ha mostrato che, in prossimità dell'ingresso del diffusore, lo strato limite alla base della pala è più influenzato dal filetto rispetto all’apice; tuttavia, questo effetto sulle prestazioni globali del compressore centrifugo non è significativo, con una differenza tra i casi con e senza filetto dello 0.16% e dello 0.42% rispettivamente per efficienza e rapporto di compressione. Infine, gli approcci “mixing plane” e “frozen rotor” sono stati utilizzati per modellare l'interfaccia rotore-statore. I risultati hanno mostrato che l'implementazione del “mixing plane” influenza il flusso a valle della girante trascurando l'effetto delle separazione all'ingresso del diffusore. D'altra parte, il modello “frozen-rotor” considera tali separazioni, ma è fortemente influenzato dalla selezione della posizione relativa del rotore rispetto allo statore, ottenendo risultati diversi per ogni posizione. La differenza rispetto ai dati sperimentali per l’efficienza oscilla tra lo 0.59% e il 7.59%, mentre per il rapporto di compressione varia tra lo 0.13% e il 3.95%.
Numerical validation of high-performance 4:1 pressure ratio centrifugal compressor
Gomez Perez, David Fernando
2022/2023
Abstract
The design of centrifugal compressors that offer high efficiency and high-pressure ratio during service represents a complex work that involves a high level of fluid dynamics understanding. In order to facilitate the design, Computational Fluid Dynamics (CFD), along with experimental campaigns, are widely used. The validation of the numerical simulations must be done to obtain accurate outcomes. The present work is focused on validating the experimental results of a high-pressure ratio centrifugal compressor designed by the National Aeronautics and Space Administration (NASA) in 1982. The centrifugal compressor is compounded by an impeller with 15 main and splitter blades and a vaned diffuser with 24 vanes. Ansys applications such as Bladegen, Turbogrid, DesignModeler Geometry, Mesh, and CFX were used to perform the simulations and study the evolution of the flow. Different configurations of the compressor were considered to assess the performance of the compressor, including those of the impeller coupled to a vaneless and a wedge-vaned diffuser. The results showed a good agreement in the prediction of the performance curves for all the cases being the impeller with a vaneless diffuser the case that presented the highest discrepancies for the efficiency and pressure ratio at nominal condition about 1.24% and 7.14% respectively. For the case of the impeller with a vaned diffuser, the difference in efficiency and pressure ratio was 0.95% and 0.58% while the case with fillet on the vanes was 0.673% and 1%. In the case of the impeller with a vaneless diffuser, the surge limits were not reached, and the choking condition occurred at the impeller inlet. On the contrary, in the case of the impeller with a vaned diffuser, the choking condition was set by the diffuser. In order to approach the simulation results with the experimental results, fillets were implemented at the hub and shroud of the vanes. The study of flow development showed that near the inlet of the diffuser, the boundary layer at the hub was more affected by the fillet than at the shroud, however, this effect on the global performance of the centrifugal compressor was not remarkable having a difference in the efficiency and pressure ratio with respect to the case without fillet about 0.16% and 0.42%, respectively. Finally, the mixing plane and frozen rotor approach were used to model the interface rotor-stator. The outcomes showed that the implementation of the mixing plane affects the flow downstream of the impeller neglecting the effect of the flow separation at the inlet of the diffuser. On the other hand, the frozen rotor model considers such instabilities but is strongly affected by the selection of the relative position of the rotor with respect to the stator obtaining different results for each position. The difference with respect to the experiments for the efficiency oscillates between 0.59% and 7.59% while for the pressure ratio between 0.13% and 3.95%.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/210729