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Imaging measurements of transient flow behavior in harsh environments are a demanding task in industrial development, e.g. regarding the reduction of pollutant and sound emissions of jet engines by means of lean combustion. Such processes can exhibit instabilities based on flame oscillations due to thermo-acoustic fluctuations of pressure and heat release rate, which potentially result in failure of the device. We present a characterization of the combustion by non-invasive measurements of the heat release rate inside the flame and the flow velocity at the combustor outlet. Additionally, measurements of local sound pressure fluctuations are presented.
Simultaneous, seedingless, optical measurements of these quantities can be achieved using multiple laser-vibrometers and signal correlation [1, 2]. The measurement is based on the linear relation between the heat release rate or sound pressure and the change of the refractive index, which can be detected integral along the laser beam. Using multiple laser-vibrometers it is furthermore possible to measure the flow velocity due to the phase delay between two vibrometer signals and the known distance between the measurement positions. However, simultaneous field measurements have to be accomplished in order to detect the transient spatio-temporal flow behavior of the combustion and to enable tomographic reconstruction of the instable process.
In order to overcome these limitations, a high-speed camera-based laser-vibrometer (CLIV) is designed for non-invasive seedingless measurements of the flow velocity and heat release rate inside premixed flames. The novel imaging CLIV offers single pixel resolution with measurement rates up to 40 kHz at an image resolution of 110 x 110 px, resulting in a high data rate of 0.5 GHz. A discussion of the traceability to SI units and the measurement uncertainty is presented. Spatially resolved simultaneous measurements of the heat release rate and its temporal fluctuations inside a swirl-stabilized flame are demonstrated. Subsequently, the calculation of the flow velocity field inside the flame is performed by means of signal correlation between different pixel data. Additionally, local sound pressure fluctuations are detected using the novel system in combination with tomographic reconstruction.
References
[1] J. Gürtler, F. Greiffenhagen, J. Peterleithner, J. Woisetschläger, D. Haufe, J. W. Czarske, “Noninvasive seedingless measurements of the flame transfer function using high-speed camera-based laser vibrometry”, SPIE Optical Metrology, Optical Measurement Systems for Industrial Inspection, Proceedings pp. [10329-66], Munich, Germany, 26.06. – 29.06.2017
[2] S. Köberl, F. Fontaneto, F. Giuliani, J. Woisetschläger, “Frequency resolved interferometric measurement of local density fluctuations for turbulent combustion analysis”, Meas. Sci. Technol., 21:035302 (10pp), 2010.
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