Noise mechanisms in a radial fan without volute

Abstract

The noise emission of a large radial fan with 7 backward curved blades is investigated. When decreasing the flow rate, the broadband levels below 1 kHz increase and the tones decrease, and a broad hump appears at about 80% of the BPF. The present numerical investigation aims at identifying the mechanism responsible for the broadband hump in the spectra. The configuration is simulated using the Lattice Boltzmann PowerFLOW solver for four flow-rate conditions. The low flow-rate condition is run longer to achieve statistical convergence at low frequencies. The simulations capture reasonably well the acoustic spectra and in particular the broadband hump at lower flow rate. The flow in the fan shows an unsteady flow detachment in some blade passages shed from the leading edge of the blade, while other blade passages experienced a fully detached flow along the whole blade chord. A spectral proper orthogonal decomposition is applied to identify the modal structures of the flow responsible for this mechanism. Several azimuthal modes are obtained from the decomposition for the different frequencies in the acoustic spectra. In addition an axial acoustic mode in the fan bowl is found at a frequency in the broadband hump.