Supersonic reaction front propagation initiated by a hot spot in n-heptane/air mixture with multistage ignition

For large hydrocarbon fuels such as n-heptane, multistage ignition occurs at low initial temperature. Therefore, multiple pressure pulses produced by multistage ignition and complicated reaction-pressure wave interactions are expected to happen during autoignition and reaction front propagation initiated by a hot spot. In this study, 1D simulations are conducted for n-heptane/air mixture with three ignition stages respectively caused by low-, intermediate- and high-temperature chemistries. Multiple pressure waves, shock waves, and detonation waves are identified and they are found to be generated by heat release from different ignition stages and reaction-pressure wave interactions. The thermal states of flow particles at different initial locations are tracked and analyzed; and the mechanism for the development of multiple shock waves and detonation waves is discussed. With the change of temperature gradient inside the hot spot or the hot spot size, such interactions can be strengthened or weakened and thereby the mode of supersonic reaction front propagation changes. Furthermore, both planar and spherical configurations are considered and the curvature effects are examined. It is found that in spherical configuration, the pressure wave caused by intermediate-temperature ignition is not strong enough to induce a second detonation wave as that in planar configuration.