📄 Showing the "Simple" version — "Popular" is not ready yet. Add it to favorites to help prioritize it. Ask for the full read

Someone starts crumpling a flat sheet of paper. At first, a few creases appear, then they spread across the entire surface, and the sheet is no longer flat but rough. A hydrogen flame behaves similarly: in a calm gas flow, it is smooth and tilted at a single angle, but changing the mixture composition causes the surface to break into cells and wrinkles. This phenomenon is called thermodiffusive instability: light hydrogen quickly escapes the combustion zone, and heat propagates differently than fuel molecules, so the planar front spontaneously begins to curve.

The scientists anchored the flame on a thin rod to keep it from blowing away, and directed a flow of premixed hydrogen and air onto it. While the mixture is lean (little hydrogen), the flame remains smooth, like taut fabric. But adding a bit more hydrogen causes it to abruptly transition downstream to a crumpled state, just like paper crumpled from one edge. From the flame's tilt, one can calculate how much faster it began to burn: the angle relative to the flow increases, visible in photographs.

To determine how many times the burning speed increased, measuring the tilt is not enough — one must also account for how much the flame's surface area grew due to wrinkles. Three different methods were used to estimate the area, all giving similar results. It turned out that for a very lean mixture (fuel-to-air ratio 0.35), the speed increases by a factor of 1.1–1.3, while for a slightly richer one (0.40), it actually drops to 0.8–0.9 of the original. This matches theoretical predictions and computer simulations.

Such precise measurement is important for future engines: hydrogen burns without soot or carbon dioxide, but its flame is prone to instability. If we learn to control this wrinkling, we can make burners more efficient and safer. For now, the flame on the rod is like a laboratory sheet of paper, showing every crease.