Deflagration/Autoignition/Detonation Transition Induced by Flame Propagation in an N-Decane/O₂ /Ar Mixture

The aim of this study is to experimentally investigate the combustion regime transitions of a single-component kerosene surrogate (n-decane). For this purpose, a deflagration is generated by a spark in a constant-volume vessel with a length-to-width aspect ratio of 4.3. By consuming the unburnt gas, the flame behaves as a piston that compresses the end gas. The pressure and temperature of the end gas increase with the flame propagation until autoignition conditions are reached. Ultrafast schlieren visualizations are set up to monitor the dynamics of the reactive processes, whereas the time-pressure evolution associated with non-dimensional (0D) numerical models is used to characterize the unburnt gas thermodynamic conditions. Once the thermodynamic conditions needed to initiate the autoignition reactions are reached in the end gas, a transition between deflagration (flame speed of approximately ten m/s) and autoignition fronts is observed (propagation velocity ˜200 m/s). This transition occurs for various fuel equivalence ratio values (0.75 – 1). For the strongest thermodynamic conditions, once the velocity of this autoignition front reaches the speed of sound, a new reactive front propagating up to 1800 m/s is observed, thus indicating the transition to detonation combustion mode. Parametric studies indicated that the occurrence of both transitions was a function of the pressure, temperature and equivalence ratio for n-decane fuel. A small initial temperature variation (approximately 40 K) could change the phenomenology of the constant-volume combustion from deflagration to detonation through the autoignition process, even for lean mixtures. The results obtained using our experimental setup show that the transition between autoignition and detonation is due to the acceleration of the autoignition front towards the speed of sound in the unburnt gas.