DGV flow field analysis and OH* chemiluminescence imaging in an industrial combustor operating under thermo-acoustic combustion oscillations

Kurzfassung

On the way towards a low emission combustion a possible solution seems to be the use of leaner fuel mixtures. But the main problem occurring at full power conditions are the thermo-acoustic combustion oscillations which can not be tolerated. In order to examine the reasons for that problem we applied two different measurement techniques – DGV and OH* imaging – to an industrial sized combustion chamber operating at atmospheric pressure conditions. For the DGV measurements in hot environments the biggest challenge is to overcome the strong background luminosity. Therefore we applied a self-developed pulse laser system with suitable DGV characteristics such as high power (~1mJ), narrow bandwidth (<1MHz) and frequency stability (single-mode operation) together with a gated camera system. To record phase resolved measurements we extracted a trigger signal from the output of a piezo pressure transducer placed in front of the burner outlet. Another important aspect of applying the DGV technique is the addition of seeding particles to the flow. Therefore we used AlO-powder in a solid particle seeding generator which contains a sintered glass filter and a stirring device. Three light sheets were guided through small slits from the sidewalls into the measurement area. A cooled endoscope was mounted inside the combustor looking directly onto the burner head. With this arrangement we obtained all three velocity components for stationary conditions and for different phase angles during an oscillation cycle. The results show that under stationary conditions the flow field is nearly rotational symmetrically with a distinct recirculation zone. In contrast to the stationary flow the phase resolved measurements revealed a drastic change from a combustion with recirculation to a complete forward directed flow depending on the phase angle. In addition to this flow field measurements we recorded heat release images. The underlying principle of this technique is the use of the chemical excitation of the OH-radical as an indicator of the heat release. This is a simple imaging technique which uses only an intensified CCD-camera together with a wavelength filter permeable for OH* fluorescence. We examined the transition from stationary conditions to combustion oscillations with the help of those heat release images. During combustion oscillations the maximum of the heat release is connected to the development of a maximum velocity gradient between the recirculation zone and the outer swirled flow. The minimum of the heat release corresponds to a complete forward directed flow with a vanishing recirculation zone.