Implementing pressure gain combustion into a conventional gas turbine is a promising way to achieve an increase in its thermal efficiency.  One concept, among others, that aims for a pressure gain combustion is the Shockless Explosion Combustor (SEC). This concept aims for a periodic process in which a quasi-homogeneous autoignition is achieved inside a combustor. Therefore, a stratified fuel profile is injected into a convecting air flow. This allows for the compensation of differences in residence time by an axial stratification of the equivalence ratio along the combustor. Hence, simultaneous autoignition of multiple ignition sources is achieved leading to a confined volume and therefore a smooth rise in pressure similar to that of a constant volume combustion process. However, reliable detection of the spatially distributed ignition front is challenging.

In order to investigate the homogeneity of autoignition in a continuous flow, a test rig with optical access is designed for atmospheric pressure conditions. By varying the injected fuel trajectories, different modes of autoignition fronts can be observed. In this work, different techniques of detecting the ignition process are investigated. High-speed imaging of the intensity of OH* chemiluminescence is applied to track the axial position of the ignition front during each cycle. Pressure transducers are used to measure the pressure increase due to the combustion. These two methods are combined to characterize the mode of autoignition by the homogeneity of the ignition distribution and the detected pressure amplitudes.

For investigations at application-oriented conditions, e.g. elevated pressure, optical measurement techniques, however, are difficult to apply. Therefore, the time-resolved data from ionization probes at various axial positions inside the combustor are recorded in addition. These data are subsequently compared to the data from OH* intensity imaging and pressure sensors to assess the potential of reliable characterization of the autoignition process by ionization probes and pressure sensors only.