Dependence of ignition delay time on its definition - A case study on methane ignition

Ignition delay time (IDT, tau) is the time period from the onset of a given temperature and pressure of a combustible gas mixture to the time of ignition. The time point of ignition is defined as reaching a given condition of pressure, temperature, or one of the species concentrations. In measurements, ignition of hydrocarbons and oxygenates is usually detected based on monitoring the change of pressure or the concentrations of species OH*, CH*, or CO2. In simulations, ignition is also detected based on the temperature profile. Simulated ignition of CH4/O-2/N-2 mixtures was investigated in a wide range of initial temperature (700-3000 K), pressure (0.05-500 atm), and equivalence ratio (phi=0.03-8.0) at two diluent-to-oxygen ratios: 3.76 (corresponding to air), 24.0 (corresponding to the conditions of many shock tube measurements). The agreement between tau(p) and tau(OH*) was good below initial temperature 2500 K and 0.5<phi<2.0, while the agreement between tau(p) and values of tau(CH*), tau(CO2) and tau(T) was much more than 10 % for most of the conditions. The tau(OH) vs. tau(OH*) agreement is poor above phi=3.0 for most conditions. The tau(CH) vs. tau(CH*) agreement is good in the range of phi = 0.4 to 1.5, below 2200 K and 100 atm. The experimental determinations of IDTs should consistently be reproduced by simulations using exactly the same IDT definition. When different IDT definitions are used in various measurements or simulations, the tau values are expected to be in good agreement only in restricted ranges of conditions.