THE EFFECT OF LENGTH ON THE ACOUSTIC ATTENUATION PERFORMANCE OF CONCENTRIC EXPANSION CHAMBERS - AN ANALYTICAL, COMPUTATIONAL AND EXPERIMENTAL INVESTIGATION

Due to their desirable broadband noise attenuation characteristics, ex pansion chambers are widely used in the ducting systems for pulsating flows, including the breathing systems of engines and reciprocating tu rbomachinery. The present study investigates in detail the effect of t he length on the acoustic attenuation performance of concentric expans ion chambers. Three approaches are employed to determine the transmiss ion loss: (1) a two-dimensional, axisymmetric analytical solution; (2) a three-dimensional computational solution based on the boundary elem ent method; and (3) experiments on an extended impedance tube set-up w ith nine expansion chambers fabricated with fixed inlet and outlet duc ts, fixed chamber diameters and varying chamber length to diameter rat ios from l/d = 0.2 to 3.5. The results from all three approaches are s hown to agree well. The effect of multi-dimensional propagation is dis cussed in comparison with the classical treatment for the breakdown of planar waves. The study also provides a simple relation for the numbe r of repeating attenuation domes prior to the domination of higher ord er modes in terms of the l/d ratio of the expansion chamber. (C) 1997 Academic Press Limited.