An Experimental Validation of Heat Release Rate Fluctuation Measurements in Technically Premixed Flames

Abstract

Understanding the effects of inlet velocity and inlet equivalence ratio fluctuations on heat release rate fluctuations in lean premixed gas turbine combustors is essential for predicting combustor instability characteristics. This information is typically obtained from independent velocity-forced and fuel-forced flame transfer function measurements, where the global chemiluminescence intensity is used as a measure of the flame's overall rate of heat release. Current lean premixed combustors operate in a technically premixed mode where the flame is exposed to both velocity and equivalence ratio fluctuations and, as a result, the chemiluminescence intensity does not provide an accurate measure of the flame's rate of heat release. The objective of this work is to experimentally assess the validity of a technique for measuring heat release rate fluctuations in technically premixed flames based on the linear superposition of fuel-forced and velocity-forced flame transfer function measurements. In the absence of a technique for directly measuring heat release rate fluctuations in technically premixed flames, the heat release rate reconstruction is validated indirectly by comparing measured and reconstructed chemiluminescence intensity fluctuations. The results are reported for a range of operating conditions and forcing frequencies which demonstrate the capabilities and limitations of the heat release rate reconstruction technique. A variation of this technique, referred to as a reverse reconstruction, is also proposed, which does not require a measurement of the fuel-forced flame transfer function. The results obtained using the reverse reconstruction technique are presented and compared to the results from the direct reconstruction technique.

Thermoacoustic Instability Characteristics and Flame/Flow Dynamics in a Multinozzle Lean Premixed Gas Turbine Model Combustor Operated with High Carbon Number Hydrocarbon Fuels

“…This is not consistent with the practical OH* CL distributions shown in Figure , in which the OH* intensity of the n -decane flame is the lowest. A close inspection of this inconsistency indicates that the actual flame OH* CL intensity in the turbulent combustion system was not only determined by the fuel’s kinetic mechanism but also affected by the stretch rate, the local equivalence ratio, etc. , Moreover, although a narrow bandpass OH* filter (305–330 nm) was used, OH* chemiluminescence imaging still inevitably contained chemiluminescence signals emitted from other species. Consequently, it is hard to interpret why the discrepancy exists regarding the flame OH* CL intensities of the test fuels simply based on their chemical kinetics, and more complex models coupled with computational fluid dynamics (CFD) are required for future studies.…”

Interaction of equivalence ratio fluctuations and flow fluctuations in acoustically forced swirl flames

“…The study confirmed that equivalence ratio fluctuations are convected to the flame at the bulk flow velocity. In a follow-up study, the same group investigated the response of a partially premixed flame to both velocity and equivalence ratio fluctuations based on chemiluminescence measurements 29 . They proposed a linear reconstruction technique to separate both effects, but were not able to validate their approach since direct measurements of heat release rate fluctuations in a partially premixed flame were not available.…”