On the crucial role of mean flow in the design of multiply-connected coaxial perforated element mufflers

Abstract

Efficient muffler design for automotive engine requires adequate overall insertion loss (IL) in dB(A), lower backpressure to ensure low brake specific fuel consumption (BSFC) of the engine, and a low muffler volume to engine capacity ratio. In this study, the role of mean flow on the flow-acoustic performance of four different multiply-connected co-axial mufflers (MCCA) is illustrated. The parametric studies on one of the configurations highlight the importance of the bias and grazing mean flow Mach number. The automotive engines operate over a wide range of engine speeds, which demands a wideband transmission loss (TL) and insertion loss rather than sharp peaks at certain frequencies. In the literature, a higher mean flow Mach number has been associated with more flow-noise as well as backpressure. However, in this study, it is shown that higher mean flow can be used to advantage in the MCCA mufflers by making use of the inline flow-acoustic resistance which raises the TL curve over the entire frequency range. This article presents some design guidelines for the mufflers with higher specific insertion loss (ratio of overall insertion loss (in dB(A)) to the muffler to engine volume ratio) as well as a modest backpressure.