Direct Numerical Simulation of MILD Combustion: Mixing and Autoignition from Non-Premixed Streams

TL;DR

This study uses direct numerical simulation to analyze how mixing and autoignition processes in MILD combustion are influenced by different mixing scales and conditions, revealing the dominant role of hot product mixing in ignition.

Contribution

It provides a novel DNS dataset of a three-stream mixing layer to investigate the detailed dynamics of MILD combustion and the influence of mixing on ignition modes.

Findings

Higher dilution levels lead to MILD combustion regime.

Ignition in MILD conditions is mainly driven by hot product mixing.

In MILD cases, ignition occurs mainly through premixed-autoignition mode.

Abstract

Moderate or intense low-oxygen dilution (MILD) combustion is achieved by strongly diluting and preheating the reactants through mixing with hot combustion products before ignition. To better understand how fuel/air/product mixing and interaction govern MILD combustion dynamics, a novel direct numerical simulation (DNS) dataset of a temporally evolving three-stream mixing layer consisting of fuel, air, and hot combustion products has been performed. In this configuration, both fuel-air and air-hot products mixing processes are considered with varying time scales, through four carefully designed DNS cases, to assess how their combined interaction controls ignition under MILD conditions. It is observed that the cases with higher dilution levels fall within the MILD combustion regime, whereas those with lower dilution correspond to non-MILD conditions. The results show that, as long as MILD conditions are observed, ignition is mainly driven by mixing with hot products. Flame index (FI) combined with chemical explosive mode analysis (CEMA) further identifies the local combustion mode: in MILD cases, ignition occurs predominantly through a premixed-autoignition mode, while in non-MILD scenarios, the premixed-deflagrative contribution to the heat release rate is more substantial. Conditional analysis of scalar dissipation rates shows that the combustion modes in MILD conditions are sensitive to mixing by both the fuel and hot products, whereas the combustion modes in non-MILD conditions are mainly influenced by the mixing of the fuel with the surrounding gases.