可壓縮流壓降分析及消音器設計策略

Abstract

本文旨在討論消音器幾何形狀與背壓之關係，以求消音器幾何之設計策略。時至今日，使用Head Loss做估計仍是造船業界最常使用的簡單方法。然而此研究做出了兩項革新。一是否定了Head Loss當作排氣背壓的做法，雖然大膽，卻是經過縝密的推論，且有不可不為的理由。二是引入氣體之壓縮性，首見利用R. P. Benedict提出之Generalized Compressible Flow Function做連結，對消音器背壓優化問題重新做出詮釋，給出直觀且能消除Head Loss理論隱含之歧異的結果與解釋。 本文分別從流體力學與熱力學角度切入，藉由討論排氣的熱力學性質、氣流流場特徵，檢視消音器內部流場的壓縮條件；在對質量守恆方程式做出推廣以後，得到了消音器的幾何參數與其內部壓力分布的直接關係。物理模擬軟體COMSOL得出的計算結果，與於摩托車上進行的實驗結果，皆支持理論結果。

This thesis focuses mainly on the relationship between the geometry of a muffler and its back pressure, to obtain a strategy for muffler design. Two revolutions were made in this research. First, head loss analysis, which is still the most frequently used rudimentary method for estimating the performance of mufflers, was denied. Yet audacious, it’s reasonable and inevitable. Second, with compressibility taken into account, the Generalized Compressible Flow Function suggested by R. P. Benedict was employed to reinterpret the muffler design problem. More intuitive results were obtained, also, some discrepancies in head loss analysis were eliminated. In both fluid mechanical and thermodynamical points of view, the internal properties of exhaust gas and the characteristics of the flow were discussed. After generalizing the equation for conservation of mass, a direct relationship between a muffler’s shape and the air pressure distribution was obtained. With help of COMSOL, computational and experimental data both support the theory.