Laminar flame speeds of cyclic hydrocarbon fuels: Effect of unsaturation

Laminar flame speeds of five cyclic hydrocarbon fuels (cyclohexane, cyclohexene, 1,3-cyclohexadiene, 1,4-cyclohexadiene, benzene) were determined with the spherically propagating flame at a wide equivalence ratio range, 403 K, two initial pressures of 0.1 MPa and 0.5 MPa. The differences among the five cyclic fuels were identified. It is found that laminar flame speeds of the five cyclic fuels all achieve their peaks around 1.1. At the same condition, laminar flame speed decreases in the order of cyclohexadiene, benzene≈cyclohexene and cyclohexane. To understand the inherent reasons causing the laminar flame speed difference, the thermal, diffusional and chemical effects were discussed. Calculation results indicate that at the same condition, the adiabatic flame temperature slightly decreases in the order of 1,4-cyclohexadiene, 1,3-cyclohexadiene, and benzene. Cyclohexene is apparently lower than benzene, while cyclohexane is much lower. The thermal diffusivity of benzene is much higher than the other fuels while that of cyclohexane is the lowest. These all contribute the fast laminar flame speeds of cyclohexadiene and benzene and the low speed of cyclohexane. It also suggests that the thermal and diffusional factors play a dominant role in causing laminar flame speed difference. Regarding chemical effect, the JetSurf 1.1 model was refined and completed. The refined model yields good performance in predicting laminar flame speeds of all cyclic fuels except benzene. To further clarify the chemical effect, a quantitative comparison was implemented among the mole fractions of combined H, O, OH in cyclohexadiene, cyclohexene and cyclohexane flames. It was found the mole fractions of H + O + OH decreases in the order of 1,4-cyclohexadiene, 1,3-cyclohexadiene, cyclohexene and cyclohexane. This result is roughly consistent with the conclusion of laminar flame speed, and could explain the different reactivities of the four cyclic fuels.