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

Numerical computations employing detailed chemistry and experiments in a slot-jet counterflow burner are used to characterize the different combustion modes emerging in mixing layers separating N$_2$-diluted counterflowing planar streams of hydrogen and oxygen. Attention is focused on high degrees of dilution resulting in near-critical flames with peak temperatures close to the crossover temperature. A bifurcation diagram is presented in the mixture-fraction vs. strain-rate plane that identifies six different combustion regimes involving four different flame types, namely, diffusion-flame sheets, propagating/retreating edge flames, broken-flame tubes, and isolated flame tubes. While the isolated flame tube is always stationary, the broken flame tubes can be stationary or can propagate with an oscillatory speed. The observed flame behavior exhibits hysteresis in some parametric regions, where the flow that is established depends on the ignition mechanism.