A study of Cool Diffusion Flames utilizing Ignition Delay Characteristics of N-Heptane Autoignition Simulations

Abstract

Gaining deeper insights into cool diffusion flames (CDFs) can significantly enhance engine efficiency and reduce emissions, while also filling in knowledge gaps relating to explosion initiation and the transition from smoldering to flaming fires. While detailed computational fluid dynamic (CFD) models can simulate CDFs, they require substantial computational resources due to the need for detailed chemistry and transport resolution. To circumvent these challenges, this study utilizes an alternative approach using Cantera autoignition simulations, which presumes isobaric, adiabatic conditions. The fuel, n-heptane, is analyzed through six kinetic mechanisms that capture the spectrum of low and high temperature chemistry. The observed ignition process – manifesting as single, two, or three-stage ignition – is observed to vary with initial conditions. Analysis of ignition delay times unveils the Negative Temperature Coefficient (NTC) behavior, crucial for the existence of stable cool flames. The critical transition temperatures, such as the lower and upper turnover and the crossover temperature are also identified, along with the key chemical species produced during the two-stage ignition process. The peak temperature range for stoichiometric n-heptane CDFs is determined to be between 653 and 804 K, aligning favorably with previous experimental measurements. While the first-stage ignition delay time remains nearly constant, the second-stage ignition delay time noticeably decreases as the mixture becomes richer, up to an equivalence ratio of 32. This reduction is attributed to the rapid temperature increase caused by a larger fuel quantity, which accelerates high-temperature chemical reactions. The NTC temperature range is also seen to shorten as the mixture composition gets richer. While the six chemical kinetic models examined concur about the existence of an NTC regime, variations are observed in the threshold temperatures. The insights gained from this study enhance the understanding of CDFs, setting a foundation for future research into different fuels and varying conditions.