Near-limit H$_2$-O$_2$-N$_2$ combustion in nonpremixed counterflow mixing layers

TL;DR

This study uses detailed chemistry simulations to explore various near-limit hydrogen-oxygen-nitrogen combustion modes in counterflow mixing layers, revealing complex flame structures, stability, and hysteresis effects at high dilution levels.

Contribution

It identifies six distinct combustion regimes and characterizes the stability and structure of multiple flame configurations under near-limit conditions.

Findings

Six combustion regimes identified with distinct flame types.

Multiple stable flame configurations exist depending on initial conditions.

Hysteresis observed in flame behavior over certain parameter ranges.

Abstract

Numerical computations employing detailed chemistry are used to characterize the different combustion modes emerging in mixing layers separating nitrogen-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution, resulting in near-limit flames, with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in a plane, having the stoichiometric mixture fraction and normalized strain rate as coordinates, that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, advancing and retreating edge flames, multiple flame tubes, and single isolated flame tubes. Multiple-tube flame configurations vary from small, round, widely separated flame strings at high strain rates to wide, flat, densely packed flame strips, with narrow flame-free gaps between them, at lower strain rates, and they are steady and stable in various arrays over a continuum of tube-separation distances. The observed flame behavior exhibits hysteresis in a certain range of parameters, with the structure that is established depending on the ignition mechanism, as it also does at high strain rates, and a continuum of different stable steady-state flame configurations exists, each accessed from a different initial condition.