Hydrogen is the simplest fuel, yet it has very different characteristics compared to common hydrocarbons: (a) high energy release per unit mass, (b) very high diffusivity, and (c) high reactivity. These three factors result in high flame speeds, which peak at around ten times those of hydrocarbons, and extremely wide flammability limits, from 3 to 95 percent in air. Hydrogen also has a propensity to form unstable flame surfaces owing to thermo-diffusive instabilities associated with the very light nature of hydrogen molecules, which form long finger-like leading edges, and very thick reaction zones, which means that the way in which we describe the physics of flames for other hydrocarbons does not work well for hydrogen. In this project we aim to develop simulations and experiments that will unveil quantitatively how these instabilities affect the reaction rate and local species formation, allowing the development of models that can be used in new carbon-free engines and gas turbines.

The project will use direct numerical simulations and experiments of a stabilised hydrogen flame at atmospheric pressure and temperature, for a range of hydrogen/oxygen ratios and dilution. The experimental database will for the first time generate reconstructed 3D flame surfaces and velocities, joint two-dimensional temperature, OH radical measurements and one-dimensional hydrogen species concentrations. The numerical database will produce simulations overlapping with the experiments, as well as an extension of conditions inaccessible to experiments to higher pressures of up to 5 times atmospheric. The combination of matched experimental and numerical data will enable direct comparison, to explore the instability behaviour and dependence on reactant conditions, confirm numerical predictions, and use more complete DNS data to extrapolate from lower-fidelity experimental data.
The particular issues of thermodiffusive instabilities are also relevant to other potential reactive mixtures, and some of the findings may be generalisable to other physical situations.

More immediately, the research is also supported by industrial partners at the leading edge of development of hydrogen-based land and air propulsion, and findings from the proposed research will be immediately incorporated into models for turbulent combustion used at the collaborating facilities.