Simulations of laminar non-premixed flames of methane with hot combustion products as oxidiser

Abstract

A numerical study has been performed to investigate the behaviour of laminar strained non-premixed flames of CH4 with hot combustion products as an oxidiser with a focus on various characteristics of MILD combustion. The oxidiser stream was hot combustion products from a Φ = 0.6, 0.7, 0.8, and 0.9 laminar premixed flame, therefore having temperatures above 1650 K and oxygen content, XO2, below 8%, placing the flames studied in the MILD combustion regime. The simulations used the GRI-Mech 3.0 chemical scheme with a chemiluminescence kinetic sub-mechanism. Heat release rate profiles in MILD flames were spread over a wide region in mixture fraction space. The location of this region and its peak is discussed in comparison to conventional experimental markers for heat release such as the OH* chemiluminescence and pixel-by-pixel product of OH- and CH2O-PLIF signals. These experimental markers cannot predict the location of peak heat release rate in MILD flames as accurately as in conventional flames due to the extended nature of MILD heat release rate, in contrast to the thin regions of both XOH* and XOH, presented in both real and mixture fraction space. The effect of strain rate on temperature rise during combustion, ΔT, XOH*,XOH, and heat release rate is discussed with an analysis of extinction behaviour. MILD flames produced NO through both thermal and prompt NO mechanisms, with both the high temperatures and initial NO concentrations in the oxidiser significantly affecting reaction rate behaviour. The simulations provide insights to some experimental observations usually associated with MILD combustion.