The Control of Hydrogen-Air Ignition Dynamics to Prevent Flashback

A potential strategy to safely ignite a hydrogen-air mixture is investigated using high-fidelity large eddy simulations (LESs) on an academic burner with a chamber back pressure to mimic the flow blockage characteristic of a turbine cascade. Indeed, the high heat release rate of hydrogen mixtures, compared to conventional fuels, could promote a flashback event due to higher overpressure that arises inside the combustion chamber after the ignition, thereby altering the dynamic response of the rig. Nevertheless, the wider flammability range of hydrogen permits the ignition of the mixture at a lower equivalence ratio, reducing the amplitude of the overpressure and thus mitigating the risk of a flashback scenario. Consequently, once the flame stabilizes on the burner, the equivalence ratio can be gradually increased to reach the nominal value, which would otherwise cause a flashback if directly ignited. This procedure is numerically investigated by performing LES on a perfectly premixed hydrogen-air mixture at two equivalence ratios that either trigger flashback (φ=0.43) or ensure regular ignition (φ=0.30). Subsequently, starting from the stabilized flame at φ=0.30, a single step on the fuel concentration at the inlet is applied to reproduce the possible worst-case scenario, and the calculation is conducted until the flame stabilizes on the new operating conditions. The results demonstrate the feasibility of this approach to achieve stable combustion at a higher equivalence ratio while preventing the flashback occurrence.