Investigation of Tonal Noise Sources from Centrifugal Fan using Detached Eddy Simulation

Sammanfattning: Since we spend the majority of our time indoors, heating, ventilation, and air conditioning (HVAC) systems have become a more and more important part of many people´s life. They provide a sufficient amount of airflow with the correct temperature, quality, and humidity. The negative side is the noise it produces and the dominant one is the tonal noise which is generated by the fan. In the fan, there is a gap between the rotating shroud and the stationary inlet duct. The pressure difference between the fan's inner and outer sides drives air to pass through the gap.      In this thesis, tonal noises at the blade passing frequency (BPF) produced by a centrifugal fan is investigated to be able to understand the generation mechanism and identify their sources. The approach is to use the hybrid method coupling the improved delayed detached eddy simulation with the acoustic analogy Formulation 1A of Farassat. It is found that recirculating flows at the blades are associated with the gap between the shroud and the inlet duct. The turbulence that develops at the gap is swept along the shroud wall and is swept downstream to interact with the top side of the blade leading edge (BLE). The interaction renders uneven surface pressure distributions among the blades that agree with the tonal noise sources from the wall-pressure fluctuations.      Different gap geometries are studied and it is found that the gap designs affect the amplitude of the root mean square (RMS) pressure on the BLE. The spectral analysis shows that the regions with high energy correspond to the high RMS regions at the BLE and that the amplitude of the tonal noise at the BPF differs between the cases. Also, the turbulent structures at the gap are swept downstream along the intersection between the blade and shroud, on the pressure side of the blade. They render uneven high-pressure regions in the blade passage. The high-pressure regions rotate with a speed of approximately 5% of the fan rotation speed.