  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.
