Pyrolysis of binary fuel mixtures at supercritical conditions: A ReaxFF molecular dynamics study

Combustion devices such as rocket engines, gas turbines and HCCI engines frequently operate at a pressure higher than the critical pressure of the fuel or the oxidizer. This significantly limits the transferability of existing chemical kinetics models as they are developed and validated at low pressure/temperature conditions, considering only temperature dependence on the reaction rates while neglecting pressure dependence on combustion pathways. Since the experiments are difficult to perform at the supercritical region, in this study, we demonstrate the capability of ReaxFF reactive force field method simulations to study combustion kinetics of fuels and fuel mixtures at these conditions with an objective to investigate how the presence of a highly reactive fuel can alter the properties of a much less reactive fuel during pyrolysis. We consider two different fuel mixtures, namely JP-10/toluene and n-dodecane/toluene and find that they behave differently at different mixing conditions and densities. We also compare our results with continuum simulation results using a detailed chemical kinetic model and elaborate why the continuum results fail to capture the phenomena predicted by the ReaxFF simulations. Lastly, with the help of product distribution of decomposition of different fuel components, we explain the reasons behind the observed behavior. Additionally, this study enables us to identify the pressure/temperature regime and the mixing conditions where the simple first order kinetics and Arrhenius type relations do not prevail. This study reveals that the overall pyrolysis characteristics of a fuel mixture do not only depend on the activation energy of its most reactive component. In fact, the product of pyrolysis plays a more significant role in helping other, less reactive, molecules to decompose. Overall, ReaxFF force field based molecular dynamics simulations can provide important atomistic insights on the pyrolysis properties of fuel mixtures at supercritical conditions.