Shockless explosion combustion - Controlled autoignition in stratified mixtures for pressure gain combustion
This work investigates the reliable generation of a homogeneous autoignition in a reactive mixture flow as a pressure gain combustion approach. Pressure gain combustion represents a promising concept to achieve an increase in the thermal efficiency of gas turbine applications compared to conventional constant pressure combustion. The concept is based on a pulsating operation with high frequency injection of a defined mixture profile into a continuous air flow that undergoes homogeneous autoignition. The injected fuel profile has been tailored to compensate for the gradient in residence time and hence, enable the simultaneous ignition of the entire combustor volume leading to an aerodynamic confinement which ultimately results in an increase in pressure. This pressure rise is highly dependent on the homogeneity of the autoignition which can be characterized by the number of quasi-simultaneous ignitions occurring inside the combustor volume. An increased homogeneity results in a greater pressure rise and simultaneously minimized the occurrence of shock waves. This type of combustion is termed as shockless explosion combustion (SEC). The main objective of this work is investigate the SEC process experimentally and enable a repeatable and reliable operation which has been realized within the frame of four publications. First, an initial test rig was modified to allow for the precise injection of a desired fuel profile into a continuous air flow and subsequently observe autoignition within a desired combustor section. Secondly, optical measurement techniques were applied to quantify the successful injection of the desired fuel profile which stays largely preserved during convection in the combustor. As a next step, the correlation between three model injection profiles and the resulting autoignition was investigated. A significant and reproducible influence of the fuel injection on the ignition distribution is observed. These observations are subsequently used to apply an extremum seeking control algorithm which controls the cycle–averaged formation of different autoignition modes by optimizing the fuel injection profile. Optical and pressure measurements reveal a complex interaction between heat release and pressure waves influenced by low and high temperature chemistry of the applied fuel. Four different modes of ignition have been identified which are classified, namely: turbulent deflagration, subsonic autoignition, supersonic autoignition, and aerodynamic confinement by multiple simultaneous ignition fronts. The results presented in this work, demonstrate the experimental feasibility of a shockless explosion combustion. It was shown that autoignition, which is primarily driven by chemical kinetics, and thus, highly sensitive to perturbations, can be greatly affected by the injected fuel profile. The amplitude of the pressure rise was found to viii strongly correlate with the autoignition homogeneity. Moreover, it was found that the autoignition modes observed, are impacted by the applied fuel which exhibits multi-stage ignition behavior. These fuel characteristics can be exploited to trigger different modes of autoignition by the proper adjustment of the injected fuel trajectory. By this, the probability of the occurrence of distinct autoignition modes can be greatly impacted, which was shown by the successful application of a closed-loop control algorithm.
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