Tonal Noise of Voluteless Centrifugal Fan Generated by Turbulence Stemming from Upstream Inlet Gap
In this study, noise generation is investigated for a generic voluteless centrifugal HVAC fan at an off-design operation point where tonal noise increases. The simulations are performed by coupling IDDES with the FW-H acoustic analogy, and the experiments are conducted in a rig consisting of a plenum chamber and a reverberation room. In contrast to typical tonal noise sources induced by the fan blades, we find out that another predominant source is the turbulence stemming from the gap between the fan shroud and the inlet duct. The turbulence evolves along with the shroud and is swept downstream to interact with the top side of the blade leading edge. The interaction accounts for uneven surface pressure distribution on the blades. Moreover, the pressure is significantly unsteady near the shroud. The power spectral density (PSD) of the noise shows obvious tones at 273Hz that is approximately equal to the difference of the blade passing frequency (BPF0) and the fan rotation frequency. By coarsening the mesh resolution near the inlet gap and shroud, we artificially deactivate the gap turbulence in the numerical simulations and, consequently, detect that the tone at 273Hz disappears completely. At this frequency, the PSD contours of surface pressure fluctuations are found potent at the inlet gap and the blade top side only if the gap turbulence is resolved. These findings indicate that the tonal noise source at 273Hz is the interaction between the gap turbulence and blades. As the gap turbulence exists near the shroud wall upstream of the blades, the rotating wall introduces rotational momentum into the turbulence due to the wall friction. Hence the tonal frequency of the interaction is smaller than BPF0 with a decrement of the fan rotation frequency. To the authors' knowledge, it is the first time that voluteless centrifugal fans are studied for the noise generation from the gap turbulence.
voluteless centrifugal fan