Abstract
Lean premixed (LPM) combustion processes are of increased interest to the gas turbine industry due to their reduction in harmful emissions. These processes are susceptible to thermoacoustic instabilities, which are produced when energy added by an in-phase relationship between unsteady heat release and acoustic pressure is greater than energy dissipated by loss mechanisms. To better study these instabilities, quantitative experimental resolution of heat release is necessary, but it presents a significant challenge. Most combustion systems are partially premixed and therefore will have spatially varying equivalence ratios, resulting in spatially variant heat release rates. For laminar premixed flames, optical diagnostics, such as OH chemiluminescence, are proportionally related to heat release. This is not true for turbulent and partially premixed flames, which are common in commercial combustors. Turbulent eddies effect the strain on flame sheets which alter light emission, such that there is no longer a proportional relationship. In this study, phased, averaged, and spatially varying heat release measurements are performed during a self-excited thermoacoustic instability without and with porous inert media (PIM). Previous studies have shown that PIM can passively mitigate thermoacoustic instabilities, and to the best of the authors’ knowledge, this is the first-time that heat release rates have been quantified for investigating the mechanisms responsible for mitigating instabilities using PIM. Heat release is determined from high-speed PIV and Abel inverted chemiluminescence emission. OH ∗ chemiluminescence is used with a correction factor, computed from a chemical kinetics solver, to calculate heat release. The results and discussion show that along with significant acoustic damping, PIM eliminates the direct path in which heat release regions can be influenced by incoming perturbations, through disruption of the higher energy containing flow structures and improved mixing.
Highlights
Lean premixed (LPM) combustion processes are of increased interest to the gas turbine industry due to their reduction in harmful emissions. ese processes are susceptible to thermoacoustic instabilities, which are produced when energy added by an in-phase relationship between unsteady heat release and acoustic pressure is greater than energy dissipated by loss mechanisms
A Z∗ 0 in the porous inert media (PIM) and no PIM cases correspond to the PIM downstream surface and the swirler exit, respectively. e velocity results have regions with no flow data reported, due to reflections from the quartz cylinder interfering with Particle Image Velocimetry (PIV) measurements at that location. e region outside r/r0 −0.9 is omitted from the results for this reason
Phase averaged equivalence ratios and heat release measurements were performed on an atmospheric, partially premixed combustor. e system was operated at a self-excited thermoacoustic instability and porous inert media (PIM) were to passively mitigate the instability. e addition of PIM caused a significant reduction in peak frequency sound pressure level (SPL) of 38 dB
Summary
Ese processes are susceptible to thermoacoustic instabilities, which are produced when energy added by an in-phase relationship between unsteady heat release and acoustic pressure is greater than energy dissipated by loss mechanisms. Introduction ere is an increased effort to utilize lean premixed (LPM) combustion processes in gas turbine combustors to reduce NOx emissions and fuel consumption These systems are more susceptible to thermoacoustic instabilities [1]. Gas turbine designers continue to look for insightful tools to predict instabilities and assess the effectiveness of mitigation strategies To predict these instabilities, the models must capture multiple physical interactions including, but not limited to, turbulent flame vortex interaction, system damping, and acoustic wave propagation [4].
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