Direct numerical simulation of a high-pressure hydrogen micromix combustor: flame structure and stabilisation mechanism

TL;DR

This study uses direct numerical simulation to analyze the flame structure and stabilization mechanisms in a high-pressure hydrogen micromix combustor, revealing ignition-driven stabilization and detailed flame zones.

Contribution

It provides a detailed numerical analysis of flame stabilization in a hydrogen micromix combustor, highlighting ignition events and turbulence effects, which are novel insights for high-pressure combustion.

Findings

Core flame burns over 85% of fuel

Ignition events driven by shear-induced vortices

Stabilization due to turbulent mixing and ignition

Abstract

A high-pressure hydrogen micromix combustor has been investigated using direct numerical simulation with detailed chemistry to examine the flame structure and stabilisation mechanism. The configuration of the combustor was based on the design by Schefer [1], using numerical periodicity to mimic a large square array. A precursor simulation of an opposed jet-in-crossflow was first conducted to generate appropriate partially-premixed inflow boundary conditions for the subsequent reacting simulation. The resulting flame can be described as a predominantly-lean inhomogeneously-premixed lifted jet flame. Five main zones were identified: a jet mixing region, a core flame, a peripheral flame, a recirculation zone, and combustion products. The core flame, situated over the jet mixing region, was found to burn as a thin reaction front, responsible for over 85% of the total fuel consumption. The peripheral flame shrouded the core flame, had low mean flow with high turbulence, and burned at very lean conditions (in the distributed burning regime). It was shown that turbulent premixed flame propagation was an order-of-magnitude too slow to stabilise the flame at these conditions. Stabilisation was identified to be due to ignition events resulting from turbulent mixing of fuel from the jet into mean recirculation of very lean hot products. Ignition events were found to correlate with shear-driven Kelvin-Helmholtz vortices, and increased in likelihood with streamwise distance. At the flame base, isolated events were observed, which developed into rapidly burning flame kernels that were blown downstream. Further downstream, near-simultaneous spatially-distributed ignition events were observed, which appeared more like ignition sheets. The paper concludes with a broader discussion that considers generalising from the conditions considered here.