Ignition phenomena in shock tubes and the influence of non-ideal effects on measurements of combustion chemistry: Experiments, modeling, and simulations

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Shock tubes provide an excellent platform for the development of chemical kinetics mechanisms because they enable the isolated investigation of high-temperature reactions without interference by mixing or transport processes. At intermediate temperatures that are relevant for practically-important low-temperature combustion processes, several inherent phenomena can induce spatial and temporal non-uniformities. Such non-uniformities lead to early localized reactions and therefore prevent a straight-forward interpretation of the observed process. Such “non-ideal” processes include interaction of the shock wave with boundary layers, finite diaphragm opening times, shock-wave bifurcation, pre-ignition heat release, and ignition by hot particles. The overall target of the proposal is to investigate non-uniform ignition in a high-pressure shock tube to determine the best-suited experimental configurations that enable straight-forward or model-supported data interpretation. The combination of experiment and simulation will also help to reject such reaction conditions, where a concatenation of multiple effects makes data interpretation impossible. In the first project period, non-ideal facility-dependent effects were investigated. Through combination of data from various facilities and the support of simulation, understanding of these effects was achieved and correlations between measured quantities such as the gradual increase in pressure as a function of experimental conditions were deduced. The second period will now go one important step further and include chemistry-dependent effects of selected fuels with high propensity to pre-ignition, which amplify non-uniformity through feedback between gas dynamics and reaction heat release. The focus will be on (i) experiments with long test times using fuels and fuel mixtures with tunable low reactivities (i.e., CH4, H2 and C3H6), on the (ii) mitigation of non-ideal effects using a new shock-tube design (constrained reaction volume, CRV) and inserts, and finallyon (iii) investigations of complex fuels (occurrence of mild ignition; negative temperature coefficients, NTC) such as n-heptane, which are especially sensitive to non-uniform conditions.These phenomena will be investigated in a high-pressure shock tube with a newly designed CRV test section that suppresses remote ignition. Extended diagnostics capabilities will be applied, focusing on non-ideal phenomena by measuring pressure, temperature, and localized ignition with high spatial and temporal resolution. Simulations in 0D, 1D, 2D, and 3D simulations will be performed to support the fundamental understanding of the underlying effects. The ignition of fuels with pronounced NTC behavior will be simulated by considering the change of temperature and pressure due to boundary layer effects.