Laminar flame speeds of cyclohexane and mono-alkylated cyclohexanes at elevated pressures

The propagation speeds of expanding spherical flames of cyclohexane, methylcyclohexane and ethylcyclohexane in mixtures of oxygen/inert were measured in a heated, dual-chamber vessel, with the corresponding laminar flame speeds extracted from them through nonlinear extrapolation. Measurements were conducted at atmospheric and elevated pressures up to 20 atm. Computational simulations were conducted using the JetSurF 2.0 mechanism, yielding satisfactory agreement with the present measurements at all pressures, with a slight over-prediction at 1 atm. Measurements reveal the following trend for the flame speeds: cyclohexane > n-hexane > methylcyclohexane ≈ ethylcyclohexane at all pressures, with the maximum difference being approximately 5% at 1 atm and 13% at 10 atm. Examination of the computed flame structure shows that owing to its symmetric ring structure, decomposition of cyclohexane produces more chain-branching 1,3-butadiene and less chain-terminating propene. On the contrary, a more balanced distribution of intermediates is present in the flames of methylcyclohexane and ethylcyclohexane due to substitution of the alkyl group for H.