Exploration of the spontaneous detonation development mechanisms in a constant volume configuration for hydrogen/n-decane/air mixtures

In an effort to reduce harmful emissions from the transport sector, modern Spark Ignition (SI) engines have been downsized and boosted leading to much higher compression ratios. Moreover, Pressure Gain Combustion (PGC) technologies are a set of technologies with potential efficiency benefits being explored, particularly in the field of propulsion. Amongst the PGC technologies is Constant Volume Combustion (CVC) which is a subsonic implementation of PGC technology. Due to the high temperature and pressures of the end-gas in SI engines, an extreme combustion event termed super-knock has been observed which is characterized by very high-pressure oscillations which can lead to mechanical damages in the engine. One cause of super-knock has been identified as unwanted transitions to detonation resulting from autoignition in the presence of a reactivity gradient. Similar detonation transition mechanisms with similar consequences could potentially arise in CVC applications when used in propulsion-based systems. Therefore, this present dissertation explored the spontaneous detonation transition resulting from autoignition in the presence of a reactivity gradient for hydrogen/n-decane/air mixtures in a constant volume chamber configuration. The ultimate aim of the dissertation was to understand the phenomenon behind the transition mechanisms resulting from autoignition and to suggest potential strategies for suppressing detonation transition in CVC applications when used in propulsion-based systems. Numerical investigations were performed exploring the detonation transition of H2/air hotspots with the aim of understanding the specific heat release/pressure wave interactions governing a transition to detonation from an autoignition wave. It was found that one of the main mechanisms by which detonation transition occurred in the conditions tested was that the leading pressure wave was initially faster than the autoignition wave and ran ahead of the autoignition wave compressing the pockets of gas ahead of the autoignition wave. This compression increased the reactivity of the pockets of gas and consequently accelerated the autoignition wave until the autoignition wave eventually coupled with an induced pressure wave leading to a detonation transition. The hotspot temperature gradient, the thermal diffusion which occurred in the autoignition induction time and the mixture energetic content and enthalpy change during autoignition were identified as determinants as to whether a detonation transition occurred or not. The experimental investigations focused on the detonation transition behavior of H2/n-decane mixtures in a constant volume chamber with a temperature gradient imposed. Of particular interest was the influence of the deflagration properties in detonation transition behavior as the deflagration influence was not included in the numerical model. It was found in the current experimental configuration that both increasing the fuel mass percentage of H2 and the presence of Residual Burned Gas (RBG) suppressed detonation transition. This was due to the deflagration autoignition competition favoring deflagration. This meant that the deflagration consumed so much of the unburnt gas before autoignition that the required detonation transition induction distance exceeded the available chamber length of unburnt gas left for the autoignition wave to propagate into. These results were positive as they implied that the presence of Hydrogen and RBG would not increase the instances of detonation transition, however these results were also found to be highly geometry dependent. When the temperature gradient was varied, it was found that a very strong temperature gradient suppressed detonation transition. At weaker temperature gradients, there were more zones reactive enough to transition to detonation and multiple detonation kernels formed simultaneously via different transition mechanisms. When the initial conditions were varied, it was found that a low-pressure condition and a very rich mixture condition successfully suppressed a transition to detonation. It was also hypothesized that if the initial temperature was decreased further and higher levels of Argon dilution were investigated, these conditions would also suppress detonations. Overall, it was found that in this present study, high hydrogen fuel mass percentages, high levels of RBG dilution, a low initial pressure, a very rich mixture and a strong temperature gradient were all conditions which suppressed detonation transition. The strong temperature gradient was the most effective in suppressing detonation, however, in reality the temperature gradients which may arise in combustion chambers are very difficult to control. The other conditions suppressed detonations because they pushed the autoignition-deflagration competition to favor deflagration. However, the effects of the deflagration-autoignition competition are highly geometry dependent. Therefore, it is recommended that the suggested transition strategies need to be explored for specific chamber geometries