Turbulent combustion, flame acceleration and detonation

6 problems

• Origin of the bending of turbulent burning velocity at high intensity

  Contested

  Stirring a premixed flame harder makes it burn faster, but at strong stirring the gain levels off or even reverses. Is this caused by stretching of the flame surface, local blow-out of the flame, unequal diffusion of fuel and heat, or by how burning speed is defined in each experiment?

• First-principles prediction of detonation cell size from mixture chemistry

  Open

  The front of a gas detonation is a pattern of crossing shock waves that scratches a fish-scale pattern on a sooted wall, and the width of these cells decides whether a detonation can pass through a pipe or an opening. At present the width can only be estimated from chemistry to within a factor of a few.

• What makes detonation cells regular in some mixtures and irregular in others?

  Open

  Detonations in mixtures diluted with argon leave neat, regular cells, while in fuel-air mixtures the pattern is chaotic and pockets of unburned gas appear behind the front. What decides this, and does the irregularity change how the detonation survives?

• Predicting run-up distance to detonation in smooth and obstructed tubes

  Open

  In a pipe filled with a fuel-air mixture and lit at the closed end, the flame speeds up until it becomes a detonation. How far it travels before that happens decides the safety of pipelines, mines and reactors, and it cannot yet be computed reliably.

• Asymptotic acceleration exponent of large self-wrinkling expanding flames

  Contested

  A large spherical flame wrinkles by itself because the hot burned gas expands, and its radius then grows faster than steadily in time. Does this growth settle into a universal power law, and with what exponent?

• Can turbulence alone turn an unconfined premixed flame into detonation?

  Contested

  A flame (deflagration, burning that spreads slower than sound) usually needs walls or obstacles to build up into a detonation. Simulations suggest that strong turbulence alone can do it in open space; does this happen in real gas mixtures, and by what route?