Exploring the Role of Stereochemistry in JP-10 Ignition.
Conference Proceeding
Overview
abstract
Although cyclic molecules are a critical component of automobile and aviation fuels, the impact of their stereochemical effects on combustion kinetics has had limited exploration. Stereochemistry determines what populations of transition states are accessible due to geometric restrictions, and subsequently the intermediate product population coming from the fuel. The intermediate product population including radicals like OH and HO2 determines the mixture of reactivity properties such as ignition. Recently there has been more attention on stereochemistry, like effects of diastereomer reactants and transition states on overall reaction rate constants and influence of stereochemistry on reaction network and reactivity. This study addresses the underexplored effects of spatial arrangements (conformational diastereomers) in cyclic molecules and their impact on critical combustion intermediates. Additionally, we extend the understanding of stereochemistry to the extreme conditions applicable to aviation engines, such as high-altitude, low-pressure re-ignition processes. Using JP-10 as a model fuel, we examine how axial and equatorial hydrogen positions influence the formation of peroxy radicals (ROO) and subsequent reaction pathways. JP-10, represented by its major component exo-tetrahydrodicyclopentadiene, provides a structurally rigid system that is computationally simpler and ideal for exploring these stereochemical effects. We analyzed axial and equatorial arrangement, considered ten unique ROO radicals and their subsequent reaction pathways. We identified the key reaction pathways using the PESs computed at composite methods and refined with ab-initio methods. Additionally, we examined the important hydroperoxylalkyl radicals (QOOH) species formed from ROO isomerization, which are crucial for accurate product distribution calculations and reaction network completeness. Two critical abstraction sites were identified: the terminal carbon in the five-membered ring, which may primarily form HO2 radicals irrespective of stereochemical configuration, and the carbon opposite the bridgehead in the six-membered ring, which may exhibit distinct behavior based on stereochemistry, axial configurations forming HO2 and equatorial configurations favoring OH radicals. Hence, ignoring stereochemical effects at the first site may not significantly impact product distribution or combustion behavior. However, at the second site, accounting for stereochemical effects may be crucial, due to the production of competing radicals, OH and HO2, which are formed at different rates depending on which stereochemical isomer is involved. This difference in competition between OH and HO2 radicals can have direct effects on ignition delay times and negative temperature coefficient (NTC) behavior.
