Unexpected Propagation of Ultra-Lean Hydrogen Flames in Narrow Gaps

Very lean hydrogen flames were thought to quench in narrow confined geometries. We show for the first time how flames with very low fuel concentration undergo an unprecedented propagation in narrow gaps: ${\mathrm{H}}_2$-air flames can survive very adverse conditions by breaking the reaction front into isolated flame cells that travel steadily in straight lines or split to perform a fractal-like propagation that resembles the pathway of starving fungi or bacteria. The combined effect of hydrogen mass diffusivity and intense heat losses act as the two main mechanisms that explain the experimental observations.

Figure 1

Schematic of the experimental setup. $\mathsf{Z}$-shape Schlieren system used for image acquisition. The dimensions of the cell are $950\times 200\times 6–1\mathrm{mm}$ ($L\times W\times h$). The black arrows at the top end of the chamber represent the unobstructed release of the combustion products.

Figure 2

Downward-propagating hydrogen flames and their different propagation modes. Image and scheme of (a-a1) continuous flame front propagation, (b-b1) splitting cells that propagate forming fractal patterns, and (c-c1) several two-headed isolated steady flame cells. The Supplemental Material [26] includes a video illustrating the three propagation regimes described in the figure.

Figure 3

Upward-propagating hydrogen flames and their different propagation modes. Image and scheme of (a-a1) continuous flame front propagation, (b-b1) fractal-like propagation mode, and (c-c1) several one-headed isolated steady flame cells. The Supplemental Material [26] includes a video illustrating the three propagation regimes described in the figures.

Figure 4

Flame propagation modes in the $h\text{−}{\%\mathrm{H}}_{\mathbf{2}}$ parametric space for both (a) downward- and (b) upward-propagating flames. The solid lines separate the different propagation modes, and the symbols represent the cases actually measured. Every experiment is repeated at least three times to reduce the uncertainty of the measurements. The mixture composition was obtained with an approximate error of $\pm 0.04\%{\mathrm{H}}_2$, and the gap thickness $h$ has an average error of $\pm 0.114\mathrm{mm}$. (c) and (d) detail the single- and double-headed flame cells $\mathrm{\Omega }$ and the dimensionless temperature field $T=(T^{\prime }-T_u)/(T_b-T_u)$ obtained from our nonbuoyant numerical computations in the limit of very narrow channels. $T^{\prime }$ is the local temperature, and $T_b$ represents the adiabatic (equilibrium) flame temperature of the simulated mixtures (Supplemental Material [26]) with $T_u=298\mathrm{K}$ being the ambient temperature.