Pyrolysis of 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. These high-pressure conditions can significantly alter their chemical properties and application of existing chemical kinetics models will be significantly limited as they are developed only considering temperature dependence on the reaction rates via simple Arrhenius-type rate laws while neglecting pressure dependence on combustion pathways. This significantly prohibits transferring the models directly from low pressure to high pressure regime. In this presentation, we will demonstrate how the ReaxFF reactive force field method can be used as a valuable tool to study combustion kinetics of fuels and fuel mixtures at supercritical condition. The objective of this study is to investigate how the presence of a highly reactive fuel can alter the properties of a much less reactive fuel during pyrolysis. We will compare our results with continuum simulation results and elaborate why the continuum results fail to capture the phenomenon predicted by the ReaxFF simulations. 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 and the probable reasons behind this characteristic. Our results indicate that, ReaxFF force field based molecular dynamics simulations can provide important atomistic insights on the combustion properties of fuel mixtures at supercritical conditions where the experiments are difficult to perform.