CO2 and H2O effect on propane auto-ignition delay times under mild combustion operative conditions

The auto-ignition process of propane/oxygen mixtures was experimentally and numerically studied over a range of temperatures (850-1250K) and mixture compositions (from fuel-ultra-lean to fuel-rich conditions) under MILD operative conditions. The mixtures were diluted in CO2 or H2O from 90 up to 97%. The experimental tests were realized in a Tubular Flow Reactor (TFR) at atmospheric pressure.Several combustion regimes were identified as a function of the mixture composition and inlet temperature. The experimental results showed that CO2 and H2O significantly alter the ignition process. In particular, a significant slowing of the system reactivity was observed with respect to the mixtures that were diluted in nitrogen.Numerical simulations were performed by commercial codes and detailed kinetic mechanisms. Comparisons between experimental and numerical results pointed out that kinetic models are not able to correctly reproduce system behaviors in all the experimental conditions.For CO2-diluted mixtures a good agreement between experimental and numerical analysis was obtained for fuel lean mixtures, whereas for stoichiometric and fuel-rich mixtures conditions the consistency of predicted data was less satisfactory.In the case of steam-diluted systems, the discrepancy between the experimental data and the predictions is about one order of magnitude for any mixture composition, but the model can reproduce the slight dependence of the ignition data on the mixture compositions.Further numerical analyses were performed to identify the reactions controlling the ignition process under MILD operative conditions in presence of CO2 and H2O. Results suggested that steam and carbon dioxide drastically alter the main branching mechanisms as third molecular species in termolecular reactions and/or by decomposition reactions.