Burning branches

Molecular hydrogen is highly flammable, even when it’s mixed with air in concentrations as low as 4%. That property makes it an attractive alternative energy source to fossil fuels. But compared with other hydrocarbons, hydrogen requires one-tenth the energy to ignite, which makes it more prone to dangerous explosions, especially in locations with poor ventilation. To better investigate how hydrogen flames propagate through small spaces, Fernando Veiga-López and Mario Sánchez-Sanz, both with the University Carlos III of Madrid in Spain, collaborated with Mike Kuznetsov of the Karlsruhe Institute of Technology in Germany and other colleagues.

The researchers piped a mixture of hydrogen and air between two transparent plates separated by a few millimeters. The image here shows the various paths the flames took to travel downward before the hydrogen fuel was depleted. The flames themselves are hard to see; the visible branching pattern is the result of the condensed water trail that formed as combustion oxidized the hydrogen. In two additional experiments, the flames fueled by methane and by dimethyl ether, both mixed with air, quenched more quickly than hydrogen. The lightest element’s exceptional mass diffusivity explains why its flames persist: In narrow spaces, hydrogen diffuses quickly enough to counteract the conductive heat losses and sustain the temperature necessary for continued combustion. (F. Veiga-López et al., Phys. Rev. Lett. 124, 174501, 2020, doi:10.1103/PhysRevLett.124.174501 ; image courtesy of Fernando Veiga-López.)