Abstract
Moderate or intense low oxygen dilution (MILD) combustion has been the subject of numerous studies in recent years. An issue remains, however, in the definition of the boundaries of the MILD combustion regime with respect to non-premixed configurations without predefined reference temperatures. A flamelet definition is applied to non-premixed configurations to better understand the MILD combustion regime and classify previous experimental investigations. Through a simplified analysis, a new definition for the non-premixed MILD combustion regime is derived. This new definition is a function of initial and final temperatures, and the effective activation energy of an equivalent one-step chemical reaction. This inherently agrees with the features of the premixed flamelet definition and provides insight into previous attempts to reconcile premixed and non-premixed classifications of MILD combustion. Previously studied turbulent flames are classified using the new definition of MILD combustion and are compared to experimental observations. The new definition of MILD combustion is subsequently compared to the ignition characteristics of opposed-flow ethylene flames, showing good agreement. Finally, transient flamelets are solved for a modelled flow-field to successfully reproduce non-monotonic trends in the lift-off that is observed experimentally in a series of MILD and autoignitive, turbulent ethylene flames in hot, diluted coflows.
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