Full-field laser vibrometry for combustion diagnostics

In order to meet the stringent pollutant regulations set by the governments, low-emission concepts have been developed for gas turbines in power plants. Unfortunately, these combustors have a strong tendency towards combustion instabilities. Such unsteady heat release will cause sound radiation and might damage the machine. Thus, a characterization of the flame is important, already during design and construction procedures. Such a characterization is also necessary for turbomachinery in aero-engines. Research in the field of turbomachinery always is energy related research, with a strong impact on society. The hypothesis of this project claims that such a characterization of flames can be done laser- optically, non-intrusive and as full-field recording using modern high-speed sensor arrays. The laser-optical sensor developed in this project comes without probes and without the need for disturbing tracer particles, so common in the recording of flow velocities. This approach in tracer-free velocity recording also breaks new ground in fluid mechanics. Due to the full-field and high-speed recording of combustion phenomena from one or two directions, a spatio- temporal signal correlation is possible for volumetric data sets, also novel to combustion diagnostics. Based on the experience gathered at TU Graz and TU Dresden in previous projects fundedby theAustrian ScienceFund FWF andthe Deutsche Forschungsgemeinschaft DFG, the basics of such a sensor will be developed in this international project between TU Graz (J. Woisetschläger) and TU Dresden (J. Czarske, A. Fischer). This includes complex algorithms for signal processing based in the principles of thermo-acoustics as well as a fast optoelectronic sensor technique based on modern signal processing, spatio-temporal correlation techniques and high-speed camera technology. A benchmarking of this novel technique is performed using conventional laser-vibrometers (at TU Graz) and an ultra-fast velocity recording (TU Dresden). Three European research institutes are willing to participate in the test phase of such a sensor or provide access to a variety of industry relevant combustors, namely TU Munich, DLR Cologne and PTB Braunschweig.

Within the framework of the project and through the cross-border cooperation of both institutions, we proofed the main hypothesis that full-field vibrometry can be implemented for the simultaneous and tracerless detection of heat release rates and flow velocities in swirl-stabilized. It was also shown that this technique can provide flame transfer functions, as well as measurements of different physical quantities, such as density or pressure, with a high resolution in time and space. The theoretical and experimental analysis for the use of laser vibrometers to record flame transfer functions provides a fundamentally new approach in the areas of combustion diagnostics and thermoacoustics without the need for tracer particles. The combination of this new approach with modern high-speed cameras in this project showed the great potential of this technology in the recording of locally resolved flow velocities. The camera-based laser vibrometer developed in this project was experimentally validated by reference measurements on known density fields, as well as swirl-stabilized flames. Due to the high sampling rate of 200 kHz and a local resolution of 0.47 mm, the system also enables the detection of highly transient processes, such as ignition processes. Furthermore, it was shown that correlating the detected density structures during combustion, not only provides the mean flow velocity at the burner outlet, but also enables imaging of the 2D / 2C velocity fields, comparable to the procedure used for "Particle Image Velocimetry". This camera-based laser vibrometer for tracerless velocity measurement offers potential for use in many different areas of fluid mechanics of fluid flows, in which both the detection of the density and the velocity are often of interest. The findings of this project support the new measurement approach to be used in extensive series of experiments on various test stands, whereby the new camera-based laser vibrometer enables time-efficient measurements with high data rates and thus extensive parameter studies, which can lead to the development and improvement of novel physical models. Especially in the area of combustion diagnostics, these models play a crucial role in the further development of technical combustion, not only of hydrocarbons, and are therefore of great relevance for society and especially for environmental protection.