Mechanism for Tonal Noise at Blade Passing Frequency from an Isolated Axial Fan Rotor

• Applicant: Professor Dr.-Ing. Thomas Carolus
• Subject Area: Hydraulic and Turbo Engines and Piston Engines
• Term: from 2010 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 137115359

Due to increasingly strict governmental regulations and the growing demand on comfort fac-tors, modern fans have to be both energy-efficient and quiet.The acoustic spectrum of a fan is often dominated by the tonal noise at blade passing fre-quency, whose value is determined by the product of speed and number of blades, and their higher harmonics. While the tonal noise of fans with up- or downstream stators or struts is due to the interaction of the rotor with the disturbed flow, the actual source mechanism of a fan with an obviously undisturbed in- and outflow is still unknown. Based on experiences at the University of Siegen and the open literature, three hypotheses regarding the source mechanism of tonal noise at blade passing frequency have been postulated and investigated: (i) The inflow contains large turbulent eddies that interact with the rotating blades, (ii) the rotating impeller causes upstream vortex type structures and recirculation patches that interact with the blades, (iii) Tip vortex/blade interaction.A fan unit was designed, manufactured and investigated experimentally on the standardized test rigs of the institute. For variation of the tip/diameter ratio the fan unit was equipped with three rotors of different diameter. An extra set of blades have been instrumented with flush mounted pressure transducers to capture the blade pressure fluctuations. A hemispherical inflow control device (ICD) was used to homogenize the natural inflow and hence to vary the inflow conditions. Devices for smoke based flow visualization allowed visualization of the flow at the inlet of the fan. Moreover, RANS-simulations of the fan unit including the large upstream suction chamber were carried out. Measurements proved that tip clearance effects only the broad band portion of the spectrum but not the blade passing frequency related tones. A comparison of the acoustic spectra with natural inflow and with the spherical ICD upstream of the rotor showed that the tones are dramatically reduced by the ICD. Usually the inflow from a large suction chamber to a relatively small fan is assumed to be undisturbed. However, smoke visualization and modal analysis revealed unexpectedly large-scale eddies in the inflow that also cause a signature on the rotating blades in terms of pressure fluctuations. RANS simulations support the existence of an asymmetric inflow to the fan impeller. The question of a relationship between the disturbed inflow and the sound pressure level at blade passing frequency is currently still open. A simulation based prediction of the tonal noise at various inflow conditions is expected to shed some light on this issue. For that it is regarded essential to simulate the coupled system, i.e. fan unit and surrounding large suction chamber.

• DFG Programme: Research Grants