The Effect of Length on the Acoustic Attenuation Performance of Concentric Expansion Chambers: An Analytical, Computational, and Experimental Investigation

Abstract

Abstract Due to their desirable broadband noise attenuation characteristics, expansion chambers are widely used in the ducting systems for pulsating flows, including the breathing systems of engines and reciprocating turbomachinery. The present study investigates in detail the effect of the length on the acoustic attenuation performance of concentric expansion chambers. Three approaches are employed to determine the transmission loss: (1) a two-dimensional, axisymmetric analytical solution; (2) a three-dimensional computational solution based on the boundary element method; and (3) experiments on an extended impedance tube set-up with nine expansion chambers fabricated with fixed inlet and outlet ducts, fixed chamber diameters and varying chamber length to diameter ratios from l / d =0·2 to 3·5. The results from all three approaches are shown to agree well. The effect of multi-dimensional propagation is discussed in comparison with the classical treatment for the breakdown of planar waves. The study also provides a simple relation for the number of repeating attenuation domes prior to the domination of higher order modes in terms of the l / d ratio of the expansion chamber.