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Hydrogen flames propagate by forming fractal-like patterns.

Hydrogen flames propagate by forming fractal-like patterns.

By forming fractal-like patterns, hydrogen flames may effectively access new fuel. The two primary processes that explain the experimental results are the combined impact of hydrogen mass diffusivity and severe heat losses.

While using hydrogen as a fuel would help cut CO2 emissions, storing and transporting it has concerns. Researchers have recently demonstrated that the gas may ignite in unexpected places. The researchers examined tiny amounts of gaseous fuels in a several-millimeter-wide fissure and discovered that hydrogen could burn continuously even when its concentration was only 5% of the confined gas. High-speed video recordings revealed that the hydrogen flame fragments into a fractal pattern as it burns, allowing it to effectively acquire fresh fuel.

The initially continuous flame front splits into smaller fronts before reaching the imaged region. Here, the ignition occurred at the top of the cell. The video is slowed down by about 4x compared with real time.

The Carlos III University of Madrid research team, lead by Mario Sánchez-Sanz, filled the small gap between two clear, vertical plates with a combination of air and 5–15 percent hydrogen. They ignited this fuel combination from the bottom or from the top and videotaped the course of the flame front by seeing the trail of condensed water left behind on the plates. A single, continuous flame front spread upward or downward with suitably wide intervals and high fuel percentages. However, for particular cell thickness and fuel-to-air ratio combinations, the flame front split into a fractal pattern of “fingers,” leaving areas of unconsumed fuel between them. 

A slightly different pattern forms when the ignition is at the bottom of the cell. The video is slowed down by about 4× compared with real time.

The researchers conducted identical trials with two heavier fuels that could not stay lit under the same conditions. They explained the unusual survivability of hydrogen flames using simulations as a result of hydrogen’s small weight, which causes it to disperse more quickly than other fuels. As long as the flame was split up into fingers, hydrogen’s fast diffusion was able to continuously feed the flame and maintain the minimal combustion temperature. These findings imply that engineers building hydrogen storage systems must account for hydrogen’s high flammability, especially in small areas.

Unexpected Propagation of Ultra-Lean Hydrogen Flames in Narrow Gaps, Fernando Veiga-López, Mike Kuznetsov, Daniel Martínez-Ruiz, Eduardo Fernández-Tarrazo, Joachim Grune, and Mario Sánchez-Sanz

Published: May, 2020
https://link.aps.org/doi/10.1103/PhysRevLett.124.17450

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