Leakage flow noise and related flow pattern in a low-speed axial fan with rotating shroud

The effect of rotational speed and pressure rise on the leakage flow noise radiated by a low-speed axial fan, provided with rotating shroud, has been systematically investigated. The leakage flow noise generally increases with the blade loading, with a trend which is qualitatively independent from the rotational speed but non-monotonic, as its growth is interrupted by local minima. As the loading increases, the SPL spectrum shows important modifications, since the characteristic frequency of the subharmonic narrowband humps related to the leakage noise decreases. The flow in the gap region has been studied by means of PIV measurements taken in the meridional plane. At low blade loading, the leakage flow is restrained close to the rotor ring and, at higher loading, it forms a wide recirculation zone. In the latter conditions, an unsteady flow separation likely takes place in the blade tip region which may be observed in the instantaneous flow field only. Possibly, it is responsible for the observed frequency shift of the humps

Instantaneous PIV data related to the leakage flow of a low-speed axial-flow fan with rotating shroud

The present paper is companion to Large-scale unsteady flow structures in the leakage flow of a low-speed axial fan with rotating shroud, Canepa et al., 2019. Two-dimensional PIV has been used in order to investigate the leakage flow in a low-speed fan with rotating shroud at three operating conditions. The reported data are constituted by about 3000 instantaneous meridional velocity fields, which are statistically independent. Each velocity field contains 41 7 55 velocity values deployed on a rectangular grid. In order to allow taking ensemble averages of the data, each velocity field has been assigned to a 4-deg bin in the rotor reference. The data are particularly valuable, since no data of this kind and detail have been made available to the scientific community yet

Leakage flow flutter in a low-speed axial-flow fan with shrouded blades

A large-amplitude axial vibration of a rotor fan with shrouded blades has been experimentally observed. The analysis of the Campbell diagram shows that the vibration is related to a backward-whirl vibrational mode of the rotor which is always present, with different amplitudes depending on the operating conditions and configuration. Modifications of the shroud roughness and insertion of small obstacles in the gap region have independently shown that leakage flow fluctuations constitute the excitation of the large-amplitude vibration. This indicates that the phenomenon is likely a flutter, as it is also suggested by the observed intermittency and aerodynamic stiffening. The feedback of the vibration on the flow is likely due to the variations of the gap size

OTTIMIZZAZIONE AEROACUSTICA DI PALETTATURE DI VENTILATORI ASSIALI

Nel presente lavoro vengono dapprima descritte le criticit\ue0 esistenti nel progetto di nuovi ventilatori assiali: soddisfacimento delle prestazioni aerodinamiche richieste e dei parametri acustici limite definiti per lo spettro. Vengono poi analizzati i principali contributi al rumore generato: componenti tonali (turbolenza di grande scala all\u2019ingresso e interazione rotore statore), componenti broadband (turbolenza di piccola scala), e loro fluttuazioni durante le rampe di avviamento e di cambio del punto di funzionamento (effetti di propagazione). Infine, viene mostrato come, nella fase progettuale, disegno aerodinamico della pala, metodi di previsione del rumore generato e calcoli CFD debbano alternarsi prima di giungere alla realizzazione di un prototipo da provare in camera anecoica e al banco prova portata. Al fine di realizzare un ventilatore ottimizzato aerodinamicamente e aeroacusticamente, \ue8 necessario introdurre accorgimenti progettuali consistenti in un opportuno disegno aerodinamico delle pale, nella scelta della loro spaziatura circonferenziale ed in un\u2019adeguata disegnazione dei componenti fissi dell\u2019elettroventilatore. Un ventilatore progettato con questo approccio, sommariamente descritto nel presente lavoro, \ue8 attualmente in produzione