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서성현 한국추진공학회 2017 한국추진공학회지 Vol.21 No.6
An increase in acoustic energy in a combustion chamber coupled with heat fluctuations from flame results in the occurrence of combustion instability. The assessment of combustion stability requires the prediction of acoustic energy variation by understanding the acoustical characteristics of flow boundaries in a combustion chamber. The present paper discusses about the characteristics of acoustic impedances at boundaries in terms of Strouhal number and summarizes theoretical analyses on the acoustic characteristics of injector-head-like configurations. Also, the details of the two-microphone measurement technique have been presented. 연소화염 열에너지와 결합된 음향에너지 증가는 연소 불안정성을 유발할 수 있다. 연소 안정성 예측을 위해서는 연소실 내부 유동 경계에서 음향특성을 파악해서 궁극적으로 음향에너지의 증가 여부를 파악하는 것이 필요하다. 본 논문에서는 연소기 헤드와 같은 유동 경계 형상에 대한 음향 특성을 연구한 기존의 주요 분석적 결과를 정리하여, Strouhal 수로 표현되는 경계면 음향 임피던스 특성을 알아보 았다. 또한 이중 마이크로폰을 활용한 실험적 방법을 통해 경계 음향 임피던스 특성에 대한 이론적/해 석적 결과를 검증하기 위한 기법을 조사 정리하였다.
Investigation of Self-Excited Combustion Instabilities in Two Different Combustion Systems
서성현 대한기계학회 2004 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.18 No.7
The objective of this paper is to characterize dynamic pressure traces measured at self-excited combustion instabilities occurring in two combustion systems of different hardware. One system is a model lean premixed gas turbine combustor and the other a fullscale bipropellant liquid rocket thrust chamber. It is commonly observed in both systems that low frequency waves at around 300Hz are first excited at the onset of combustion instabilities and after a short duration, the instability mode becomes coupled to the resonant acoustic modes of the combustion chamber, the first longitudinal mode for the lean premixed combustor and the first tangential mode for the rocket thrust chamber. Low frequency waves seem to get excited at first since flame shows the higher heat release response on the lower frequency perturbations with the smaller phase differences between heat release and pressure fluctuations. Nonlinear time series analysis of pressure traces reveals that even stable combustion might have chaotic behavior with the positive maximum Lyapunov exponent. Also, pressure fluctuations under combustion instabilities reach a limit cycle or quasi-periodic oscillations at the very similar run conditions, which manifests that a selfexcited high frequency instability has strong nonlinear characteristics.
예 혼합 희박 연소기내의 연소 불안정 기구에 관한 연구
서성현 한밭대학교 생산융합기술연구소 2004 생산기반기술연구소 논문집 Vol.4 No.1
Lean premixed combustion has been considered one of promising solutions for the reduction of NOx emissions from gas turbines. However, unstable combustion of lean premixed flow becomes a real challenge on the way to design a reliable, highly efficient dry low NOx gas turbine combustor. Contrary to a conventional diffusion type combustion system, characteristics of premixed combustion significantly depend on a premixing degree of combusting flow. Combustion behavior in terms of stability has been studied in a model gas turbine combustor burning natural gas and air. Incompleteness of premixing is identified as significant perturbation source for inducing unstable combustion. Application of a simple convection time lag theory can only predict instability modes but cannot determine whether instability occurs or not. Low frequency perturbations are observed at the onset of instability and believed to initiate the coupling between heat release rate and pressure fluctuations.