Enlarge /. Laboratory technician lights a ball with a hydrogen blowtorch.
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Hydrogen is not your friend. This was the first lesson I learned when I sent a PhD student to study hydrogen reactions on a surface. Hydrogen is explosive in a wide range of concentrations, making even the smallest leak an invitation to explore the delights of high-speed stainless steel with an additional bonus of third-degree burns. I have now learned that the situation is actually worse than I thought because hydrogen can burn even in the smallest of spaces.
Fire needs three things: fuel, oxygen and heat. If you have a well-mixed fuel-oxygen combination, the first two are not a problem. So all you have to do is add heat. When the ignition is triggered, fuel and oxygen are consumed locally quickly, so that a combustion front expands outward from the ignition point and thereby consumes fuel and oxygen. In order for this expansion to take place, the heat generated during combustion must be transferred to the outside with the flame front, otherwise the gas is not hot enough to ignite.
In a large room this is not a problem as gases do not require a lot of energy to heat up. In a confined space, however, the walls begin to play a role. Energy will flow into the heating of the walls, but the wall temperature must never exceed the ignition temperature of the gas. If the walls are close enough, a spark will not lead to a spreading flame front. Instead, the flame dies locally. However, the story is somewhat more complicated for hydrogen.
To investigate this, a team of researchers set up a burning sandwich. Two large glass plates that could be spaced one to six millimeters apart were used to enclose a hydrogen-air mixture. Once the mix concentrations were selected, the researcher lit either the top or the bottom of the sandwich. The team then traced the invisible flame by watching the water droplets that formed as a result of the hydrogen-oxygen combustion.
Fractal branched flame that moves upwards. Courtesy of Fernando Veiga-López and colleagues.
At large distances (3 mm or more) and high hydrogen concentrations (just over 10 percent) a normal flame front would occur and spread outwards from the ignition point as shown above.
However, if the gap were reduced to 2 mm, the flame would decay into a branched network of smaller flame channels. The branching followed a pattern similar to that of bacteria growing in a medium without sufficient food (the branches also resembled the airways in your lungs). The researchers analyzed the branching pattern and showed that it was fractal in nature.
Two-track flame that moves down. Courtesy of Fernando Veiga-López and colleagues.
However, this is not the end of the story. A tiny decrease in the hydrogen concentration stops the formation in a fractal flame front. Instead, a single or a pair of narrow traces of flame are formed. The tracks do not expand or branch. Instead, they simply follow a smooth (but not straight) trajectory until they suck themselves against the edge of the plate. Gravity also plays a role: the same behavior is observed with downward and upward flames, but with different hydrogen concentrations.
Similar experiments with flame-retardant molecules do not lead to the same behavior. Heavier molecules (such as methane) either have a continuous flame front or die at the ignition source.
Several single traces of flame move upwards. Courtesy of Fernando Veiga-López and colleagues.
To understand why this was the case, the researchers modeled their burning gas. They found that hydrogen can withstand the heat loss from the walls because it moves a lot more (hydrogen can diffuse further compared to heavier molecules). This enables a narrow trace of flame to be formed, which the researchers also observed in their models. However, these models don't seem to reproduce the fractal flame front, which I find somewhat surprising.
The researchers also claim that these fractal and trace-like flame fronts have not been observed, indicating that a completely new set of flame dynamics needs to be explored. I look forward to a follow-up that explains the fractal observed. In terms of safety and handling of gases, this research doesn't really change anything, since our rules in general (and for good reason) are extremely careful. There is no reason why someone who works safely could create the right conditions for the formation of a fractal flame.
Physical Review Letters, 2020, DOI: 10.1103 / PhysRevLett.124.174501 (About DOIs)