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International audience; In advanced propulsion systems such as scramjet engines, endothermic decomposition of onboard large hydrocarbon fuels can be used effectively for cooling and active thermal protection. During the cooling process, large hydrocarbon fuels absorb heat and decompose into small fragments. Since fuel decomposition changes the chemical and transport properties of the reactants, it is expected to affect the combustion afterwards. In this study, forced ignition in quiescent n-decane/air mixtures with fuel decomposition is investigated via a simplified model and transient numerical simulations considering detailed chemistry and transport. The emphasis is placed on assessing the effects of fuel decomposition on ignition kernel development and minimum ignition energy (MIE) in both homogenous and fuel-stratified mixtures. Fuel decomposition is modelled by a homogeneous ignition process in n-decane/air mixture at constant atmospheric pressure and with an initial temperature of 1300 K. Small fragments appear during the pyrolysis process. The partially-reacted mixture is frozen and cooled to a lower temperature and used as the initial mixture in forced ignition. For homogeneous mixtures, fuel decomposition can greatly promote ignition for fuel-lean decane/air mixtures while it has little effect for the stoichiometric case. Fuel decomposition also affects the duration of unsteady ignition kernel transition. Besides, fuel decomposition and fuel stratification are combined to further promote forced ignition. New flame regimes are observed and an optimum stratification radius is identified. In order to promote the forced ignition, only the fuel within the optimum stratification radius needs to be decomposed. Furthermore, laser and spark ignition experiments are conducted to measure the MIE of n-decane/ethylene/air mixtures with different equivalence ratios and ethylene blending ratios. The MIE measured in experiments cannot be directly compared with simulation results since the simulation model for forced-ignition is simplified. Nevertheless, the experimental results are consistent with simulation results and thus validate some conclusions mentioned above. The present results provide useful guidance to the fundamental understanding of forced ignition in a mixture with fuel decomposition.

Measurement on the minimum ignition energy (MIE) of n-decane with pyrolysis is investigated experimentally. Certain decomposition rate n-decane vapor is simulated by blending certain proportion of ethylene into n-decane vapor. Two equivalence ratios (1 and 0.7) and five blending ratios (0,0.2 0.4,0.6,0.7) of ethylene are, respectively, designed to represent different decomposition rates and equivalence ratios. The effect of different decomposition rates and equivalence ratios on the MIE of n-decane is investigated in following experiments. Two experimental methods are used for measurement. Technique of high-speed schlieren system is adopted to obtain images of the flame kernel by which the relationship between flame kernel radius and flame speed is calculated. An ignition probability model based on the logic regression theory is established to obtain the ignition probability curve. Results indicate that MIE is sensitive to the blending ratio of ethylene when equivalence ratio is 0.7. With elevated blending ratio of ethylene, MIE decreases rapidly firstly and with the continuous increasing of the blending ratio, the decreasing of MIE is lagging which indicates that there is a limit to the effect of the blending ratio of ethylene on the MIE. It is also found that values of MIE are closed when ethylene blending ratio is big enough whatever the equivalence ratio is.

Shock wave focus is a phenomenon where energy is rapidly converged in a small area of the medium through the interaction of shock waves, resulting in extremely high temperature and pressure near the aerodynamic focus. Experimental and numerical studies are in progress using the high energy area generated by shock wave focus to induce detonation to study shock wave focus phenomenon, the experiments of shock wave focus in the hemispherical concave cavity was carried out. The emphasis is placed on the effect of ring vent width and inlet pressure. By comparing the peak pressure at the bottom of the concave cavity under different ring vent widths, it was found that the reflection of the incident shock wave formed in the channel decreased with the width of the ring vent as well as the intensity of the shock wave increased. The greater the intensity, the more likely it is to produce the focusing of the shock wave. When the subsonic speed airflow flowed into the cavity through the ring vent, it was found that under the influence of the reflection of the concave cavity and complex motion of shock wave, a local high temperature and high pressure area was formed. By comparing the pressure spectrum at the bottom of the concave cavity under different flow pressures, it was found that with the increase of the flow pressure, there were two whistling modes on the spectrum map, C1 and D1 modes, respectively. It was also found that the pressure pulsation in the concave cavity was more disordered and the magnitude was smaller when the inlet pressure decreased, which means the shock waves were not well focused. It can be concluded that in ignition experiments, inlet pressure is significant to shock wave focusing phenomenon and there is a prompt rehydration ratio which lead to better shock wave focusing.