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In this study, noise generation is investigated for a generic volutelesscentrifugal HVAC fan at an off-design operation point where tonal noiseincreases. The simulations are performed by coupling IDDES with the FW-Hacoustic analogy, and the experiments are conducted in a rig consisting of aplenum chamber and a reverberation room. In contrast to typical tonal noisesources induced by the fan blades, we find out that another predominant sourceis the turbulence stemming from the gap between the fan shroud and the inletduct. The turbulence evolves along with the shroud and is swept downstream tointeract with the top side of the blade leading edge. The interaction accountsfor uneven surface pressure distribution on the blades. Moreover, the pressureis significantly unsteady near the shroud. The power spectral density (PSD) ofthe noise shows obvious tones at 273Hz that is approximately equal to thedifference of the blade passing frequency (BPF0) and the fan rotationfrequency. By coarsening the mesh resolution near the inlet gap and shroud, weartificially deactivate the gap turbulence in the numerical simulations and,consequently, detect that the tone at 273Hz disappears completely. At thisfrequency, the PSD contours of surface pressure fluctuations are found potentat 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 interactionbetween the gap turbulence and blades. As the gap turbulence exists near theshroud wall upstream of the blades, the rotating wall introduces rotationalmomentum into the turbulence due to the wall friction. Hence the tonalfrequency of the interaction is smaller than BPF0 with a decrement of the fanrotation frequency. To the authors' knowledge, it is the first time thatvoluteless centrifugal fans are studied for the noise generation from the gapturbulence.

Tonal Noise of Voluteless Centrifugal Fan Generated by Turbulence Stemming from Upstream Inlet Gap