Experimental investigations of ion current in liquid-fuelled gas turbine combustors

Abstract

This work covers investigations of the static and dynamic behaviour of a confined, co-swirled and liquid-fuelled airblast injection system. The focus lies on the application of ion current sensors for the qualitative measurement of the heat release rate or for flame monitoring purposes in complex technical combustion processes. The ion current sensor is to operate in a feedback control loop in order to react on combustion dynamics in real time. The first part of the work analyses experimental data, which were obtained with different techniques, e.g. dynamic pressure, chemiluminescence, fine-wire thermocouples and ion current. The results show that the thermo-acoustic instability and the precessing vortex core generate an interaction mode. The frequency of this interaction mode is the difference of the other two modes. This has not yet been observed for partially premixed and liquid-fuelled injection systems before and also was not detected by the chemiluminescence of the flame. The ion current measurement technique is able to detect the helical mode of the precessing vortex core as well as the interaction frequency, leading to the conclusion that the chemical reactions are influenced by this helical structure. Contour maps of the frequencies reveal this influence in the outer shear layer. The second part of the study focused on the ion current probe as a method to predict static combustion instabilities, such as lean blowout. According to the results, the ion current is a fast responding method to detect lean blowout, provided that the detector is mounted at a suitable position. Measurements at different positions in the flame were compared with phase-locked chemiluminescence measurements. Precursors in the ion current signal for lean-blowout prediction were found using a statistical approach, which is based on ion peak distance. The precursor events allow for the use of this approach with a feedback control loop in future applications.