Numerical investigation and cause-and-effect analysis of cyclic fluctuations and their effect on auto-ignition in hydrogen combustion

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In funding period 1, scale-resolved simulations (LES) were used to investigate the influence of cyclic fluctuations on the development of knocking combustion for hydrocarbon-based fuels. It was shown that the complex kinetics of ignition of multicomponent fuels must be taken into account for modeling turbulent auto-ignition. In particular, the strongly nonlinear dependence of the ignition delay time on temperature in the Negative Temperature Coefficient (NTC) regime has a direct impact on auto-ignition under engine conditions. The two-stage auto-ignition that is characteristic of the NTC regime could be modeled using a more advanced auto-ignition model. After validation using data from a direct numerical simulation (DNS) from TP 2, the implementation for the engine LES was carried out. Using a multi-cycle LES, it was shown that there is a strong interaction between the auto-ignition chemistry in the NTC regime and the cyclic combustion fluctuations that affect local auto-ignition processes. Rapid combustion with large pressure rise rates and strong compression of the end gas resulted in earlier knock onset. LES further showed a strong correlation between auto-igniting mass and experimentally observed knock amplitudes. In the second funding period, the occurrence of abnormal combustion phenomena in hydrogen-fueled engines will be investigated. Both knocking and pre-ignition can be attributed to auto-ignition in turbulent flows and at hot surfaces. Under engine conditions, hydrogen exhibits very complex ignition behavior, which is influenced not only by kinetics but also by strong differential diffusion effects. In the few studies on hydrogen engines, mainly global variables have been investigated so far; in this project, the focus is on the analysis of the local ignition processes under engine conditions. A better understanding of the local interaction of transport, hydrogen chemistry and, in the case of surface ignition, the wall will be achieved. The models of funding period 1 will be extended for hydrogen ignition, and validation will initially be performed on generic flames and DNS data from TP 2. The engine application will be investigated in close cooperation with TP 6. Ignition will be considered first in a high-pressure chamber with temperature-controlled surfaces, then in the hydrogen engine. In a cause-and-effect chain analysis, the interaction of local auto-ignition with cyclic variations of the flow, the residual gas fraction and the combustion will be investigated.