Fuel similarity for laminar flames of C₅ to C₈ n-alkanes

Previous experiments have demonstrated the fuel similarity for C5 to C8 n-alkanes, i.e., the laminar flame speeds and Markstein lengths of these fuels are very close to each other at various pressures. The possible explanation is that large hydrocarbon fuels always rapidly decompose into smaller fuel fragments, whose subsequent oxidation and transport would control the bulk of the flame structure and propagation rate. Such an understanding would significantly simplify/generalize the description of the combustion of large hydrocarbon fuels with similar molecular structures. The present work computationally investigated the fuel similarity for C₅ to C₈ n-alkanes systematically in various flame configurations, namely the 1D planar premixed flames, 1D stretched premixed flames, 1D diffusion flames, and 1D unsteady stretched premixed flames. The thermal and chemical structures of these flames, effects of pressure and stretch as well as the large fuel cracking time scale are studied to quantify the criterion for fuel similarity. In addition, for the case of ignition and extinction in nonpremixed systems such as the counterflow and the stagnation flow, for which the ignition/extinction responses seem to be fuel dependent, computation shows that the cause is due to the difference in the diffusive transport such that kinetic similarity is retrieved when the diffusive influence is removed.