Resonance tone generated by low pressure axial flow fans. Effects of rod location, rotational frequency and blade tip profile on resonance tone.

Abstract

Resonance tone is minimized by reducing the tip clearance, say less than about 1 mm. Even in these conditions it occurs with a sufficiently high level if a rod is attached near the leading edge on the suction surface of the blade, as well as if a part of the blade tip is cut off. The fundamental resonance frequency, f, is about 60 70% of the blade passing frequency and is correlated by a Strauhal number as f = St. WD, where St = 0.2 and W is the relative velocity. D is the characteristic length, which is without exception equal to the integer multiple of 0.0078 t, where t is the blade pitch.