Effects of mixture composition, dilution level and pressure on auto-ignition delay times of propane mixtures

• The ignition process of propane mixtures is investigated at intermediate temperatures.
• The activation energy of the ignition process changes from intermediate to high temperatures.
• Such behavior strongly depends on the mixture dilution levels, stoichiometry and system pressure.
• The kinetics of the ignition process is numerically exploited.
• The competition between oxidative and recombination/pyrolytic routes promotes this behavior.

The auto-ignition process of propane/oxygen mixtures has been widely studied in several facilities. Literature on shock tubes and tubular flow reactors has shown a change in the activation energy of the ignition process in the transition region from intermediate to high-temperature chemistry. Although fuel ignition chemistry has been widely exploited at low and high temperatures, further studies are required at intermediate temperatures. Based on previously published experimental results, this paper aimed to investigate the kinetics responsible for such a phenomenology, through a detailed kinetic analysis of the main pathways involved in the ignition process of propane mixtures. Simulations were performed over a wide range of temperatures (from low to high), changing the dilution levels of the mixture from “air” conditions to 97%, the pressure from 0.1 up to 3 MPa, and the mixture compositions from lean in fuel to rich in fuel. The analysis suggests that the phenomenology strongly depends on the mixture dilution levels and pressure. In particular, the differences in the ignition chemistry between low to intermediate temperatures and between intermediate to high temperatures is more evident for systems that are highly diluted and at low pressure. Such results are supported by data from the literature.This aspect explains why such a behavior is not reported for any experimental configurations and partially justifies the differences among the data obtained in several facilities, which is commonly addressed by the non-idealities of systems.