A Passive Method to Control Combustion Instabilities with Perforated Liner

The effectiveness of perforated liner with bias flow on the control of combustion instability is investigated. Combustion instabilities result from the coupling between acoustic waves and unsteady combustion heat release. Sometimes the phenomenon happens in afterburners of aeroengine and rocket engine, and it always causes damage to flame holders, liner seetions and other engine components. Passive methods, such as perforated liner, are often used to suppress such instabilities in application. In this article, first, a burner testbed is built in order to study the characteristic of this phenomenon. The unstable frequencies and unsta- ble area are investigated experimentally. Then an analytical model, based on "transfer element method", is developed and the numerical results are compared with those from experiments. At last the perforated liner is applied to the burner to suppress the instabilities. The results show that the sound pressure can be greatly reduced by the perforated liner.

An uncertainty investigation for liner impedance eduction methods

Impedance eduction methods have been developed for decades to meet the increasing need for high-quality impedance data in the design and optimization of acoustic liners.To this end,it is important to fully investigate the uncertainty problem,to which only limited attention has been devoted so far.This paper considers the possibility of acoustically-induced structural vibration as a nonnegligible uncertainty or error source in impedance eduction experiments.As the frequency moves away from the resonant frequency,with the increase in the value of cavity reactance,the acoustic particle velocity inside liner orifices possibly decreases to the extent comparable to the vibration velocity of liner facing sheet.Thus,the acoustically-induced vibration,although generally being weak except at the inherent structural frequencies,may considerably affect the impedance eduction results near the anti-resonant frequency where the liner has poor absorption.To demonstrate the effect of structural vibration,the vibration velocity of liner facing sheet is estimated from the experimentally educed admittance of the liner samples whose orifices are sealed with tape.Further,a three-dimensional numerical model is set up,in which normal particle velocity is introduced over the solid portion of liner facing sheet to imitate structural vibration,rather than directly solving the acoustic-structural coupling problem.As shown by the results,the vibration of liner facing sheet,whose velocity is as small as estimated by the experiment,can result in anomalous deviation of the educed impedance from the impedance model near the anti-resonant frequency.The trend that the anomalous deviation varies with frequency is numerically captured.