NCAD2008-73039 NEAR-FIELD AEROACOUSTIC EXPERIMENTAL INVESTIGATION IN LOW SPEED AXIAL FANS

ABSTRACT This paper describes a technique for the investigation of noise sources correlated to tip clearance flows in a low-speed axial fan. A detailed experimental acoustic study is carried out examining the chord-wise evolution of a rotor flow field in the proximity of the blade tip in a low-solidity impeller. The experiment is performed by keeping the rotor "frozen" inside an anechoic chamber. The Mach number, Reynolds number, and blade incidence angle are set in the static frame of reference, reproducing the flow field in the rotating frame. The 'frozen' fan rotor is mounted in an anechoic chamber. The near-field pressure perturbations are measured using a chord-traversed microphone. Near-field pressure data is then compared with theoretical predictions, experimental data, and numerical simulations. In this way the validity of the developed experimental scheme is assessed. The purpose of the present program of work is to identify the change in near-field noise as a result of the chord-wise turbulent structures that are located close to the blade surface at the tip. The objective is to study the evolution of turbulent flow structure paths along the chord, and thus to provide insights into their acoustic significance. The present program of work is facilitated by the existence of a detailed pre-existing experimental database on the fan studied. INTRODUCTION The minimization of fan noise presents the designer with a challenge, understanding how the fluid flow behaviour in the blade passage contributes to rotor acoustic emissions. For a low-solidity fan, noise arises from sources in the near-field as a consequence of sound pressures associated with turbulence and swirling flows. To mitigate the effects of aerodynamic induced noise, the designer requires a method for quantifying the acoustic implications of rotor flow structures, thus enabling th