Experimental investigation of axial spacing effect on rotor-stator interaction tonal noise by in-duct measurements

Previous conclusions regarding the effect of axial spacing on the tonal noise of axial-flow fans were inferred from the frequency spectra at the far field or one axial location of the duct, leading to unexpected uncertainty. In this study, the spacing effect was investigated by in-duct measurements for a 0.2 m diameter single-stage fan, covering a parameter space spanning axial spacings from 0.22 to 0.65 rotor hub chords and speeds from 4700 to 5200 rpm. Attentions were paid to controlling the fan speed, tip clearance, and inflow uniformity. The inflow velocities showed negligible variation with spacing, indicating a consistent incident angle for the rotors. Radial mode analysis was employed to separate the outgoing and reflecting waves to remove the influence of non-Tyler-Sofrin modes and duct end reflections. It was found that the sound pressure level (SPL) of the first blade-passing frequency (1BPF) exhibited a linear decline in the range of 0.22 to 0.44 rotor chords. The SPL of 1BPF shows a slight variation if the spacing is further increased. However, both 2BPF and 3BPF decreased continuously across all spacings. For the fixed-order BPF, the same trend was observed for all fan speeds. The uncertainty of the experiments is discussed in detail.