Methane and hydrogen are different fuels. They burn at different temperatures, different speeds, with different chemical pathways. Hydrogen flames propagate roughly eight times faster than methane flames. Their chemistry involves different radical species, different chain-branching mechanisms, different ignition behaviors. Everything about them as fuels is different.
Yet turbulent premixed jet flames of methane and hydrogen follow the same scaling laws. When properly nondimensionalized — flame height, width, and blowoff limits expressed in terms of the relevant flame speed and turbulent intensity — the two fuels collapse onto universal curves. The fuel-specific chemistry drops out. What remains is the turbulence.
This means the structure of a turbulent premixed flame is governed primarily by the interaction between the turbulent flow field and the flame front, not by the detailed chemistry happening at the front. The chemistry sets the flame speed, which enters as a parameter. But once you account for that parameter, the spatial organization of the flame — how it wrinkles, stretches, and extinguishes — is determined by the fluid mechanics alone.
The universality has a practical implication: if scaling laws transfer across fuels, then turbulent combustion models calibrated on methane (cheap, well-studied) can predict hydrogen flame behavior (expensive, poorly characterized at scale). The chemistry doesn't need to be resolved if the turbulence-flame interaction is captured correctly. The universal flame says the physics is in the flow, not the reaction.