Investigation of perforated mufflers and plates
Abstract: Mufflers are used in for instance exhaust system of cars or trucks to reduce noise.Perforated pipes are one of the main components of modern mufflers. To study acoustic properties of mufflers with perforated pipe is one of the main targets of this thesis. Furthermore, how flow is distributed along perforated pipes is critical for the acoustic prediction of mufflers since it controls the acoustic impedance of the perforate. Therefore another goal of the thesis is to investigate a simple flow model which can predict the flow distribution in perforated pipe mufflers. So called micro-perforated plates have a large potential to be applied in vehicle applications. To understand the acoustic properties of microperforated plates and their application in mufflers is another purpose of the thesis. A set of typical perforated mufflers, through flow, plug flow and eccentric, were built and tested with and without flow. In this work three issues were studied: i) effects of perforations, ii) effects of higher order modes and iii) effects of the presence of a mean flow. The measurements were compared with numerical simulations using two commercial softwares the 1-D Boost linear acoustics tool and the 3-D FemLab tool. A quasi-steady semi-analytical model is proposed to predict the grazing-bias flow in a perforated-pipe muffler at low Mach numbers for high Reynolds numbers based on the diameter of the perforation. Firstly two one-hole models are presented which are the small hole model and the large hole model. The next is to apply the one-hole model to the whole perforated pipe muffler. This model allows the prediction of the steady flow distribution through a muffler and the pressure drop of the muffler. The model was compared with experimental and numerical data from the literature. Micro-perforated plates (MPP:s) can be subjected to both flow, high temperatures as well as high sound levels and the effect of this on the MPP impedance must be known. In the present work two types of MPP:s have been investigated: i) plates with circular holes and ii) plates with slit-shaped holes. Non-linear effects were investigated with both random noise and pure tone excitation and the results compared with models from the literature. The effect of a grazing mean flow was also investigated and based on the results correction terms for the MPP impedance are proposed. As an application of the work a new type of MPP muffler has been investigated. Based on numerical simulations performed a very efficient through flowmulti-chamber MPP muffler configuration was found. The performance of the new muffler was also verified by experiments.
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