The transition from fossil fuels to renewable energy hinges on chemical energy carriers, i.e. fuels, that can store, transport, and release renewable energy at scale and on demand. Which molecules, rather than electrons, are suited for which application depends on how they burn. The talk conveys three connected messages, mirrored in its three parts. First, the bigger picture: large-scale renewable storage requires chemical energy carriers, and hydrogen, ammonia, e-fuels, and sustainable aviation fuels each occupy a distinct niche depending on energy density, handling, and infrastructure fit. Second, carbon-free carriers such as hydrogen and ammonia burn fundamentally differently from hydrocarbons. Their fast, unequal diffusion of heat and species destabilizes the flame front, producing behavior that classical combustion models were never built to capture. Direct numerical simulation resolves this physics directly, but only at technically relevant pressure and turbulence levels, which demands exascale computing. The talk shows how GPU-accelerated DNS acts as a numerical microscope, making conditions computable that were previously accessible only in experiment. Third, carbon-based synthetic fuels are considered by the example of aviation, where chemical kinetics governs the formation of carbonaceous nanoparticles, from primary combustion chemistry through spray combustion up to full combustor simulations. These particles do not end at the engine exit plane: their properties determine how they evolve further downstream as atmospheric aerosol, connecting combustion physics directly to atmospheric chemistry — the science in between flame and sky.
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