Detailed simulations on the mixed modes of premixed and diffusion combustion in supersonic hydrogen flames

Abstract

Combustion in scramjets generally proceeds in diffusion mode due to the independent injection of fuel and air streams. However, premixed combustion is also important especially in the recirculation zones for overall flame stabilization. Flame dynamics and statistics of mixed modes of premixed and diffusion combustion under varied fuel injector number, injection pressure, and temperature (denoted as Nj, pH2, and TH2, respectively) in a strut–based, hydrogen-fueled model supersonic combustor are numerically investigated. The overall heat release rate, combustion efficiency, and premixed flame liftoff distance are calculated. Three spanwise-averaged fractions for the premixed flow region, premixed combustion region, and heat release rate from the premixed combustion, respectively, are compared to identify the mixed combustion modes. The spatial probability distributions of premixed and diffusion combustion modes are analyzed based on multiple instantaneous numerical snapshots. The supersonic combustion cases with changed Nj and pH2 exhibit typical characteristics of triplet lifted jet flames. An upstream premixed flame reservoir beneficial to downstream flame propagation is essential for the overall flame stabilization in these cases. With increased TH2, the combustion field shows a propensity of lifted autoignition flames after the upstream forced ignition. The flame base monotonically moves toward the strut base with increased Nj, pH2, and TH2. However, the premixed flame liftoff distance indicates different oscillation modes when increasing the above qualities. They include the dispersive, lifting, stable, attaching, oscillating, and steady modes under various conditions.