Novel insights into mild combustion processes through analyses of hysteresis behavior

Abstract

In many numerical scientific papers, MILD combustion is defined on the basis of the disappearance of extinction phenomena while varying mixture dilution levels, through the identification of the condition where ignition and extinction collapse in an unique event. Albeit with numerous contributions in elementary reactor configurations (Continuous Stirred Flow Reactors, Opposed-Diffusion flames), operating conditions to achieve this noticeable circumstance do not properly match with experimental evidences, since they lead to extreme mixture dilution levels (higher than 99% for CH₄). In addition, many works suggest the occurrence of extinctions phenomena or instabilities under MILD conditions, symptomatic of extinction/re-ignition phenomena. Simulations in an adiabatic CSTR for CH₄/O₂/N₂ mixtures, varying the N₂ content, were realized with the focus to analyze the system behavior through hysteresis. Results show different hysteresis behaviors from “air” to diluted conditions, demonstrating the occurrence of the condition T_ext=T_ign is not a strict constraint for MILD combustion systems. In addition, the “unstable” branch has characteristic temperatures lower than the ones related to both the ignition and extinction events, enlarging the opportunity to stabilize MILD combustion processes. These results pave the way to the reconsideration of peculiar aspects of MILD combustion processes.