Experimental and numerical study of the decomposition, product spectrum, and sooting properties of adamantane fuels

This work combined experimental measurements with two theoretical approaches, reactive Molecular Dynamics (MD) simulations and Quantum Mechanics (QM) calculations, to investigate the combustion and sooting properties of three adamantane fuels, adamantane (AD), 1,3-dimethyl-adamantane (DMAD), and 1-ethyl-adamantane (EAD). These fuels were selected since the adamantane fuel family can potentially be used as sustainable aviation fuels and comparing these fuels would reveal the effects of side chain on their combustion and sooting properties. We determined the bond dissociation energies of the three test fuels using QM calculations and found that the functionalized side chains have the weakest bonds and their presence only slightly affects the bond strength in the AD multi-cyclic core. We performed pyrolysis simulations for all three fuels using ReaxFF-based MD simulations and found that DMAD and EAD have higher decomposition rates than AD and also generate more high-molecular-weight products. For all three fuels, these large products were observed to contribute significantly to hydrocarbon growth processes, which lead to large soot nucleating species and even soot-like structures. A higher yield of such soot nucleating compounds was found during the pyrolysis of DMAD and EAD than AD since the decomposition products of DMAD and EAD are more branched and those of AD have mostly straight chains. These theoretical analyses were supported by experimental measurements of yield-based sooting tendencies, which suggest that DMAD and EAD are sootier than AD and that all three fuels are sootier than standard alkane jet fuel surrogates but less sooty than jet fuel aromatic content.