Acoustic Detection of Blowout in Premixed Flames

Abstract

Work to develop a practical, fast diagnostic technique to monitor the proximity of a combustor to blowout using measurements of the flame’s acoustic signature is described. The feasibility of this approach was demonstrated on three combustors with different flame holding mechanisms that are used in most practical combustion devices: pilot, swirl, and bluff-body stabilized flames. Extensive high-speed flame images were obtained and analyzed in conjunction with simultaneous acoustic data. These analyses revealed changes in the low-frequency spectrum and/or the increased presence of time-localized and intermittent events in the acoustic data as the combustor approached blowout. Based on these observations, spectral, statistical, wavelet, and thresholding signal-processing schemes were developed for detecting blowout precursors with varying levels of time response, sensitivity, and robustness. I. Introduction P RACTICAL combustors are required to operate over a wide range of operating conditions with high levels of combustion efficiency. However, blowout is a serious concern in modern, highly loaded land-based and aeroengine combustors, particularly in aircraft engines where the combustion process is ultimately the source of the vehicle’s thrust. It is a particular concern in both military and commercial aircraft during sudden changes in throttle setting. Fo re xample, during rapid decelerations, the fuel flow rate can be reduced very quickly, whereas the slower airflow transient rate is controlled by the rotational inertia of the compressor. 1 When coupled with overall engine system dynamics, flame blowout can result in the inability of an engine to recover from a compressor stall event. 2 Blowout is particularly dangerous in high-altitude vehicles where the stability limits are narrowed, which may necessitate their descent to lower altitudes to relight. Blowout is also a major concern in land-based, industrial systems, where the engines are required to operate economically and reliably over long periods with minimal shutdown time. Emissions legislation has motivated the design of lean, premixed combustors that operate near the blowout limit. Under these lean conditions, the combustion process is vulnerable to small perturbations in combustor conditions, particularly during load changes or because of changes in fuel composition 3 or air temperature and humidity. In land-based gas turbines, such blowout events may require a lengthy (and therefore expensive) system shutdown and restart, which increases maintenance costs and reduces engine life and availability. Currently, blowout is avoided by the operation of the combustor with a wide margin from the somewhat uncertain stability limit. Precise, real-time knowledge of this margin would allow it to be reduced, resulting in lower pollutant emissions and enabling faster engine transients. The ability to sense blowout precursors can, therefore, provide significant payoffs in engine reliability and operability, in enabling optimal performance over extended time periods as an engine ages, in reducing maintenance costs, and in increasing engine life. The objective of this work is to develop a practical diagnostic