Abstract
Modern, lean and premixed gas turbine combustion concepts for low NOx emissions are prone to combustion instabilities. In a previous work it was shown that laser interferometric vibrometry (LIV) can be used to record global as well as local heat release fluctuations in swirl-stabilized premixed methane flames quantitatively, if other effects influencing density are small. In this work a newly developed camera-based full-field LIV system (CLIV) was applied to a lean, confined, premixed and swirl-stabilized methane flame under atmospheric conditions. Instead of time-consuming pointwise scanning of the flame, CLIV records full-field line-of-sight density fluctuations with high spatio-temporal resolution. With a recording rate of 200 kHz, CLIV enables the visualization of highly unsteady processes in fluid dynamics and combustion research. As an example for an unsteady process, the propagation of the flame front through a lean, premixed gas volume is visualized during an ignition process. A discussion of algorithms and assumptions necessary to calculate heat release oscillations from density oscillations is presented and applied to phase-averaged data recorded with CLIV for this type of flame. As reference, OH* chemiluminescence data were recorded simultaneously. While density gradients travelling with the flow are recorded by LIV and CLIV, chemiluminescence imaging will show nothing in the absence of chemical reaction.Graphic abstracta Time-averaged density gradient within the combustor in lateral direction. b Density fluctuations along line-of-sight 7 ms after ignition. c Phase-averaged and local heat release fluctuations at 225 Hz perturbation frequency
Highlights
Due to reduced core mass flow and damping capabilities, modern low NOx combustion concepts, operating at lean and premixed conditions, are more vulnerable to instabilities (Dowling and Yasser 2015)
The results presented are intended to follow the chronological structure of data evaluation visualized in Fig. 6, beginning with instantaneous recordings of density fluctuations, to local phase-averaged data and to global oscillation amplitudes
In a first step unsteady data of an ignition process are presented to demonstrate the potential of camera-based full-field LIV system (CLIV) to capture dynamical processes by spatio-temporal correlations
Summary
Due to reduced core mass flow and damping capabilities, modern low NOx combustion concepts, operating at lean and premixed conditions, are more vulnerable to instabilities (Dowling and Yasser 2015). Pressure fluctuations caused by unsteady heat release are a source for combustion noise, but in a worst case can lead to malfunction or even damage of gas turbines. The complex interaction between highly turbulent and unsteady swirled flow fields, chemical reaction kinetics and acoustics in a field of strong temperature gradients still requires experimental investigations to quantify the stability behavior of a combustion system. Using the Gladstone–Dale relation, refractive index oscillations can be linked to oscillations in density. In the paragraphs a short overview is given, how fluctuations in density, heat release and pressure are linked to each other.
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