The unsteady low-frequency characteristics are investigated for a voluteless centrifugal fan
for HVAC systems. The study is motivated by findings in a previous study by the present
authors, where different surface pressure distributions among the blades and a periodic low-
frequency rotation in relation to the fan rotation were observed.
At the gap entry, the velocity magnitudes are unevenly distributed with clear low and high-
velocity regions. The tangential velocity component along the fan rotation plays a dominant
role in this effect, whereas the velocity component in the fan axial direction is turbulent. An
interrelation is found between the velocity at the gap entry and the pressure inside the fan. As
the velocity magnitude is low, high pressure commences near the pressure side of the blade's
trailing edge. Intensive turbulent flow structures are triggered from the low-velocity region at
the gap entry and are swept along the intersection between the blade and shroud. Furthermore, the flow is distorted around the locations of the high pressure. But this effect decays as the spanwise distance to the shroud increases. Relatively even pressure distribution is found near the fan backplate.
Unsteadiness with an ultralong period of 20T is identified for the fluctuations of the velocity
at the gap entry and the pressure inside the fan. Here T is one fan rotation period. In previous
acoustic measurements of this fan, tonal infrasound at 2.4 Hz was identified in the noise. This frequency is in line with the ultralong period of the flow unsteadiness. Therefore, it is deduced that the low-frequency unsteadiness from the gap entry is responsible for the tonal infrasound generation.