Flow-acoustic lumped element analysis of the side-inlet/outlet combination mufflers and 1-D and FEM validation thereoF

Abstract

A muffler designed to have high insertion loss would be generally complex, involving cross-flow perforated elements as well as absorptive acoustic lining. Side-inlet and/or side-outlet configurations force the acoustic waves as well as mean flow through holes in all directions, which rules out the usual 1-D plane wave analysis. In the low-frequency range of the first few firing frequencies of a typical engine, representing prominent peaks in the unmuffled sound pressure level, distributed acoustic elements can well be approximated as 0-D or lumped elements. However, analysis of such a muffler poses several analytical challenges, like modeling a perforated pipe with bias as well as grazing flow and an acoustically lined cavity as lumped elements. The former requires knowledge of the mean flow distribution which is modeled here by making use of the lumped resistance network theory. The predicted performance (transmission loss) is compared against that predicted using a partial 1-D plane wave analysis and 3-D finite element analysis incorporating appropriate flow-acoustic impedances. The agreement is shown to be reasonable up to about 300 Hz which is good enough for design of mufflers with adequate overall insertion loss (IL) for diesel generator sets running at 1500 rpm. (C) 2017 Institute of Noise Control Engineering.