When does hydrogen burn in oxygen

Hydrogen burns fractally

Geometric flames: As an experiment reveals, hydrogen unfolds an amazing ability when it burns. If you hardly leave any room for the gas fire, the flames form a fractal pattern - branched, self-similar structures. This enables the hydrogen to continue to burn even in cracks as thin as a millimeter and with low hydrogen concentrations. However, this physically exciting phenomenon makes the gas even more flammable.

Hydrogen is not only the lightest chemical element, the gas is also considered to be the climate-friendly energy supplier of the future. Because H2 can be used to generate electricity, for heating and as fuel for drives - and only water is produced during combustion. But there is a risk: hydrogen is highly flammable and can cause an explosion if it reacts in an uncontrolled manner with oxygen. It is all the more important to know exactly how it burns.

High-speed view of burning hydrogen

But this is exactly where hydrogen is causing a surprise. “Up until now, it was thought that very thin hydrogen flames would extinguish themselves in cramped conditions,” explain Fernando Veiga-Lopez from the Carlos III University in Madrid and his colleagues. Because typically gas fires in extremely narrow cracks or in high dilution simply run out of supplies of fuel.

This is not the case with hydrogen, as the researchers' experiment demonstrated. To do this, they passed hydrogen gas of various dilutions between two transparent plates that were only a few millimeters apart. Then they lit the gas on the edge of the plate and observed with the help of a high-resolution high-speed camera whether and how the hydrogen fire was spreading. The gas burns with an invisible flame, but the camera was able to follow the condensation trail of the water that formed during the burning process.

Flames follow a fractal pattern

The surprising result: Despite the gap that was only millimeter wide, the burning hydrogen spread between the plates without any problems. Even with the lowest concentration of only five percent hydrogen in the air mixture, the fire persisted. "We are showing for the first time that H2 flames can spread unprecedentedly even in very narrow gaps - even at very low concentrations," say Veiga-Lopez and his team.

But even more amazing: under such extreme conditions, the flame front breaks into a fractal pattern. The burning gas forms geometric, self-similarly branching flame fingers that move forward through the gap. "This fractal spread is similar to the patterns that fungi or bacteria create," the researchers say. Such behavior is not known from other heavier gases under such conditions.

How do the fractals come about?

But how does the fractal pattern come about? Veiga and his colleagues have investigated this more closely with the help of simulations. These showed that the low molecular weight of hydrogen allows it to diffuse much faster than heavier gases. This means that enough fuel can flow in for the flames even in gaps that are as small as millimeters.

The separation of the flame front into the fractal "fingers" also promotes replenishment because fresh gas can then flow in from the sides. The intense heat radiation of the burning gas ensures the necessary branching of the flame front, as the researchers explain. It displaces the immediately neighboring flames and thus leads to the formation of the flame fingers.

More flammable than expected

This means that hydrogen not only exhibits a previously unique burning behavior, it is also more flammable than previously thought. Because the experiment demonstrates that the gas burns at far lower concentrations and does not go out even in the narrowest of gaps and gaps. For hydrogen storage facilities and tanks, this must now be taken into account in the future, according to Veiga and his colleagues. (Physical Review Letters, 2020; doi: 10.1103 / PhysRevLett.124.174501)

Source: Universidad Carlos III de Madrid, American Physical Society (APS)

May 14, 2020

- Nadja Podbregar