Comparative study on the laminar flame speeds of methylcyclohexane-methanol and toluene-methanol blends at elevated temperatures

Laminar flame speeds of methylcyclohexane (MCH)-methanol and toluene-methanol blends were experimentally determined with spherically expanding flame method in a constant volume bomb at atmospheric pressure, initial temperatures of 393 and 433 K, covering wide equivalence ratio range. The blending ratio of methanol in liquid volume varies as 0%, 20%, 40%, 60%, 80%, 100%. Nonlinear methodology was employed to remove the stretch effect in the data processing. Experimental results show that MCH-air flame propagates faster than toluene at the same condition. The addition of methanol into MCH and toluene results in acceleration of laminar flame speed especially at the rich mixtures. Since the published model suffers difficulty in reproducing experimental data, model refinements were carried out and the refined model yields better performance. Comprehensive analyses were developed regarding thermal and chemical kinetic properties. For MCH has lower adiabatic flame temperature than toluene and methanol addition into the two cyclic fuels decreases the adiabatic flame temperature, the thermal effect on laminar flame speed difference is negligible. Therefore, the effect of chemical kinetics was specifically discussed. MCH and toluene have different ring structures. The disintegration of aromatic ring plays as the limiting step in the high-temperature oxidation of toluene, resulting in low concentration of active radical pool and overall reactivity. However, the ring opening reaction of MCH occurs easily after the initial H-abstraction reaction, which favors the production of active intermediates and the enhancement of flame propagation. For the blending fuels, the analyses show that the laminar flame speed variation of the blends are primarily caused by the methanol substitution and the disturbance of reaction pathway through affecting the generation of important intermediates.