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

CFD STUDY OF THE LEAKAGE FLOW IN LOW-SPEED AXIAL-FAN WITH ROTATING SHROUD

The paper reports a study of the effect of the rotational speed on the leakage flow development in a low-speed shrouded axial fan. As the rotor deforms due to centrifugal force and aerodynamic loading, a static structural analysis has been operated at five rotational speeds ranging from 500 to 3000 rev/min, with the pressure distributions on the rotor surfaces derived from RANS-based CFD simulations alternating with FEM ones. Once convergence has been reached, both the structural and aerodynamic results have been validated through available experimental data. Then, the deformed geometries related to two rotational speeds, 2400 rev/min and 3000 rev/min respectively, have been used to perform URANS CFD simulations and then to analyze the effects of the different gap geometries on the leakage flow behavior. The results have allowed interpreting the behavior of the leakage flow evolving in the gap between the rotating shroud and the stationary casing

A general procedure to determine the transfer matrix of Francis turbine conduits in presence of cavitating and periodically detaching flow

In present paper the equations are determined which describe small harmonic oscillation of velocity, pressure and velocity potential for low celerity flow in straight ducts with cross section arbitrarily varying in both time and space. The obtained equations allow for the modelling of phenomena as cavitation and pulsating flow detachment. Eventually a general procedure is given which allows for the numerical determination of the duct transfer matrix in the general case

A Transfer Matrix Model for a Francis Turbine Group Interacting with Hydraulic Circuit and Electrical Network

An analytical transfer matrix model for a hydropower group composed by a Francis turbine, alternator, speed governor, excitation system and transmission line is presented. The model assumptions are clearly stated and the fully modular structure allows for an easy change of submodels

CHARACTERIZATION OF THE UNSTEADY LEAKAGE FLOW IN AN AXIAL FAN

The present work reports an experimental study of the leakage flow in a low-speed ring fan. Existing 2D Particle Image Velocimetry (PIV) measurements taken in a meridional plane in front of the rotor gap have been further processed and analyzed by means of the Proper Orthogonal Decomposition (POD). Three values of the dimensionless pressure rise across the rotor have been investigated. Namely, attention has been focused on the intermediate case, the one for which a strong radial oscillation in the leakage flow has been observed: the POD has shown that, at this condition, the leakage flow has a periodic character that is not correlated to the periodic blade passing. Moreover, such coherent motions have been found to promote turbulence transport at different radial positions, a behavior that was not observed for the periodic fluctuations related to the rotor. The present POD procedure can be therefore generally applied to turbomachinery flows to characterize their unsteady behavior beside the classical phase-averaging methods based on rotor related quantities. The present approach is novel for the leakage flow study

Analysis of the SPL Spectra Generated by Axial Flow Fans Working under Similarity Conditions

In the present paper a technique based on the similarity theory is shown which is aimed to study the trailing edge noise generated by axial flow fans. Given a set of SPL data measured on rotors of different diameter, once the analitycal structure of the SPL power spectrum is assumed, the technique allows to determine the dependence of the spectrum on the rotational speed and on the diameter. Such a technique has a general usefulness since it allows to verify the main features of measured and predicted SPL spectra. In the present case it has been developed to test trailing edge noise prediction methods and, possibly, to develop new semi-empirical correlations. After a description of the basic theory, the technique is applied to preliminary measurements taken in a hemi-anechoic chamber. The application shows that, for the present rotors, the measured OASPL follows the expected scaling law and slightly decreases as the Reynolds number is increased. Such a result needs to be confirmed by further investigations taken under more controlled inflow conditions