Abstract
Gas turbine combustors operating in lean premixed mode are known to be susceptible to flame blowoff due to competing influences of increasing chemical timescales and decreasing flow time scales under these conditions. In this study, combustion stability and the onset of flame blowoff in particular, are characterized in a new swirl burner operated with fully premixed methane (CH4) and air at thermal power of 55 kW, atmospheric combustor inlet pressure, and ambient (∼290 K) combustor inlet temperature. The onset of flame blowoff was shown repeatedly to exhibit high amplitude, low frequency combustion instabilities as a result of periodic flame extinction and reignition events. In addition to detailed isothermal characterization of the burner velocity field using particle image velocimetry, a combination of dynamic pressure sensing and optical combustion diagnostics, including OH∗ chemiluminescence and OH planar laser induced fluorescence, give indication of the combustion rig acoustic response and changes in flame acoustic response, heat release, and flame anchoring location related to the onset and occurrence of blowoff. This analysis shows that the onset of this instability was preceded by a marked reduction in dominant frequency and amplitude until frequency collapse and high amplitudes were observed throughout the burner inlet mixing plenum, burner pilot, combustion chamber, and exhaust ducting. Acoustic and optical signal analysis show potential viability for use in practical applications for precursor indications of lean blowoff. The flame anchoring location within the combustion chamber was shown to detach from the burner exit nozzle and stabilize within the outer and central recirculation zones near the lean blowoff limit, providing evidence of changes to both chemical and flow time scales. Chemical kinetic modelling is used in support of the empirical studies, in particular highlighting the relationship between maximum heat release rate and OH∗ chemiluminescence intensity.
Highlights
With ever-increasing regulatory pressure on gas turbine manufacturers and operators to reduce NOx and CO emissions while maintaining high cycle efficiency, combustion systems for land-based power generation have progressed significantly in recent years towards lean premixed (LPM) modes of operation [1]
While delivering emissions reduction benefits, LPM gas turbine combustors are inherently susceptible to potentially high amplitude, low frequency pressure fluctuations associated with operation near the stability limit of lean flame blowoff [2], resulting in potential structural damage to combustion system components, part-load engine operation, or machine shutdown [3]
While combustion instabilities can be generally categorized as low frequency (f < ∼50 Hz), mid-frequency (∼50 Hz < f < ∼1000 Hz), and high frequency (f > ∼1000 Hz) [1], the particular lean blowoff (LBO) instabilities observed and characterized in this work fall within the low frequency range
Summary
With ever-increasing regulatory pressure on gas turbine manufacturers and operators to reduce NOx and CO emissions while maintaining high cycle efficiency, combustion systems for land-based power generation have progressed significantly in recent years towards lean premixed (LPM) modes of operation [1]. While delivering emissions reduction benefits, LPM gas turbine combustors are inherently susceptible to potentially high amplitude, low frequency pressure fluctuations associated with operation near the stability limit of lean flame blowoff [2], resulting in potential structural damage to combustion system components, part-load engine operation, or machine shutdown [3]. This phenomenon results from increasing chemical timescales, τchem, and decreasing flow timescales, τflow, which can manifest within the combustion chamber as periodic flame extinction and reignition events [4].
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