Rotor-Stator Interaction in an Axial Flow Blower

Abstract

Measurements have been made on a ducted, single-stage blower, 12 in. in diameter, rotating at 4500 rpm. The duct includes a venturi, enabling mechanical prover and efficiency to be measured simultaneously with SPL's. Recently, Lowson [“Reduction of Compressor-Noise Radiation,” J. Acoust. Soc. Am. 40, 1248(A) (1966)] surveyed various possible noise-reduction methods. The experiments support qualitatively some of his estimates, though some quantitative differences also occur. As a function of axial spacing between rotor and stator, the SPL (both the pure-tone spectrum at the blade-passage frequency, and broad-band noise with the pure tones removed) shows a break in slope at a spacing rougly equal to the chord of a rotor blade. At smaller spacing the slope lies between 3 and 10 dB per doubling of spacing—depending on stator blade pitch, number of stator blades, and other parameters—while at larger spacing the sound levels decrease between 0 and 2 dB per doubling of spacing. Mechanical efficiency is independent of spacing up to about one chord length; at greater spacings it usually decreases slightly—typically it is reduced 2%–3% at a spacing of three or four chord lengths.