Abstract:

Methyl pentanoate (MPE, C₆H₁₂O₂) is one of the intermediate produced species during the combustion of biodiesel and long-chain methyl esters. Until now, experimental results for MPE ignition are not available in literature, hence, a study on the ignition characteristics of MPE is necessary. In the present work, the ignition delay times for the gas phase of MPE/air and MPE/4%O₂/Ar were measured behind reflected shock waves. Experimental conditions included temperatures of 1050–1350 K, pressures of 1.5 × 10⁵ and 16 × 10⁵ Pa, and equivalence ratios of 0.5, 1, and 2 for MPE/air, and temperatures of 1210–1410 K, pressures of 3.5 × 10⁵ and 7 × 10⁵ Pa, and equivalence ratios of 0.75 and 1.25 for MPE/4%O₂/Ar. Ignition delay time was determined using the arrival of a reflected shock wave and the excited CH radical emission at the observation location, which is at a distance of 15 mm from the shock tube end wall. The obtained results indicate that as temperature or pressure increases, the ignition delay time of MPE/air and MPE/4%O₂/Ar mixtures decreases definitely. However, the effect of the equivalence ratio on the ignition delay time of MPE/air is different at high and low pressures, (16 × 10⁵ Pa: τ_ign = 5.43 × 10⁻⁶Ф^−0.411exp(1.73 × 10²/RT), 1.5 × 10⁵ Pa: τ_ign = 7.58 × 10⁻⁷Ф^0.193exp(2.11 × 10²/RT). In addition, an ignition delay correlation, i.e., τ_ign = 2.80 × 10⁻⁵(10⁻⁵P)^{−0.446±0.032}Ф^0.246±0.044exp((1.88 ± 0.03) × 10²/RT)), is developed for MPE/4%O₂/Ar in a pressure range of 3.5 × 10⁵–7 × 10⁵ Pa. This correlation shows that ignition delay depends on temperature, pressure, and the equivalence ratio, and it is useful for scaling experimental data to certain conditions for comparison purposes. In addition, the ignition delay times of MPE/air are considerably lower than that of MPE/4%O₂/Ar because the fuel concentration of MPE/air is significantly higher than that of MPE/4%O₂/Ar in the present ignition conditions. The ignition delay time behaviors of MPE were compared with those of other long-chain methyl esters, and results indicate that the ignition delay times of MPE are longer than those of long-chain methyl esters at lower temperatures (under 1200 K for MPE/air, under 1280 K for MPE/4%O₂/Ar). The two available chemical kinetic mechanisms of MPE cannot predict current measured data accurately, a further refinement of the mechanisms of MPE ignition is required. Sensitivity analyses indicate that the chain-branching reaction, H + O₂ = O + OH, plays the most promoting role in the high temperature ignition of MPE. To the best of the authors’ knowledge, this is the first study to report the high temperature experimental ignition delay data of MPE. The results of this study are useful for understanding the ignition characteristics of MPE and for providing measured data to refine the chemical kinetic mechanism of MPE.

Key words: Ignition delay time, Methyl pentenoate, Shock tube, Sensitivity analysis, Kinetic mechanism