Acoustic Performance Analysis And Optimization Of Air Shaft Muffler Based On Acoustic Modes

To reduce noise of air shaft in one coal mine,the hybrid muffler composed of expansion chamber and honeycomb muffler was designed for large outlet whose section size is 3.2m×6.4m.Acoustic simulation and analysis on resistant part of muffler have been done using LMS Virtual.Lab,and a method of paralleling expansion chambers has been put forward to solve higher order wave problem in large section muffler.At the same time,the structure of chamber was optimized based on results of simulation and analysis.In the end,paralleling 4 square section chambers with insertion tube was adopted,silence frequency band was broadened and acoustic performance was improved later.The corresponding hybrid muffler was designed to reduce the wide-band noise of axial flow fan.The paralleling expansion chambers was adopted to reduce low and medium frequency noise of fan,4 different types of chambers were paralleled in interlaced method considering the size of simple paralleling chambers is too huge.The resistive honeycomb muffler was adopted to reduce medium and high frequency noise.To make the best of transition elbow between resistant part and resistive part of muffler,honeycomb elbow was designed to replace original rigid elbow.This measure not only increases noise reduction but also shortens the length of vertical honeycomb muffler.Higher order wave in large section muffler is an important element that influents the acoustic performance of muffler.For considering the influence of higher order wave adequately,the acoustic FEM simulation based on acoustic modes of ducts was adopted in this paper to calculate acoustic modes,inner acoustic field,as well as transmission loss of expansion chambers.At the same time,acoustic performance analysis and structural optimization were done based on the above-mentioned results.First,we optimized the shape of paralleling chambers' section.Paralleling chambers with circular section,semicircular section and square section were compared,and better acoustic performance and structural performance were appeared in paralleling chambers with square section.Then,we optimized the number of paralleling chambers.Acoustic performance of paralleling 2 chambers and paralleling 4chambers with square section were analyzed and compared.Paralleling 4 chambers' every higher order wave cut-off frequency is higher,and plane wave cut-off frequency exceeds 500 Hz.The silence frequency band is also extended to1200 Hz.Obviously,acoustic performance of 4 chambers is more ideal.Afterwards,we optimized the length and locations of insertion tubes.The configuration was adopted that the length of offset inlet and outlet insertion tube is respectively 1/2l and 3/4l as well the second expansion chambers were added to eliminate passing frequencies,finally.In the end,absorbent material was added to the inner wall of expansion chambers to improve the acoustic performance of muffler in high frequency.The performance of muffler from 10 Hz to 3000 Hz is up to standard after optimization.