报告题目：Towards divide and conquer for combustion instability: conversion of low-frequency instability wave to high-frequency radiation
报告摘要：Thermoacoustic instability is a well-known and difficult technology challenge. Unsteady heat release leads to sound waves, which are reflected by the small opening of the combustion chamber and coupled to the turbulent combustion dynamics. This leads to energy conversion from heat release to pressure and velocity oscillations and ultimately to structural damage. Traditional control by Helmholtz sound absorbers targets dominant instability modes, which are normally low-frequency, and require large resonator volume. This talk explores the alternative method of dealing with low-frequency wave energy. Instead of absorbing it by a resonator, a time-varying material property by the resonator converts part of the energy to high frequencies, which is then easily radiated out of the combustion chamber.
The talk will begin with the basic physics behind the coupling between sound waves and electromagnetic forces which can be easily manipulated by analogue circuits. The performance of the first design using dynamic circuit switching is presented. When the switching follows a modulation frequency of fm, an incident sound of frequency fs is mainly scattered to fs+fm and fs-fm. The modulation action is independent of the incident wave; the device remains passive and preferred by engineering applications. The higher acoustic radiation efficiency of the fs+fm component contributes to the suppression of thermoacoustic instability. Experimental evidence is presented for the basic frequency conversion phenomena, but only essential numerical simulation is presented for the application in thermoacoustics. The talk is focused on the physics of the energy conversion process.。