Classification and lift-off height prediction of non-premixed MILD and autoignitive flames

Abstract

A new non-premixed definition for moderate or intense low oxygen dilution (MILD) com- bustion based on an equivalent activation energy rather than prior assessment of a reference temperature was derived and proved to be consistent with previous experimental observations of gradual ignition. This new definition is a function of initial and final temperatures and the effective activation energy of an equivalent one-step chemical reaction which inherently agrees with the features of the premixed flamelet definition and provides insight to reconcile premixed and non-premixed classifications of MILD combustion. Time-averaged chemiluminescent images of a series of C2H4 flames showed that autoignitive flames exhibit peak temperatures at the flame-base in contrast to the gradual ignition predicted and observed in MILD jet flames. Transient flamelet modeling suggested that the shift in ignition location from regions of low scalar dissipation rate towards the jet shear layer is responsible for the observed non-monotonic trend in flame lift-off heights.