Large-eddy simulation of kerosene spray combustion in a model scramjet chamber

Abstract

Large-eddy simulation (LES) of kerosene spray combustion in a model supersonic combustor with cavity flame holder is carried out. Kerosene is injected through the ceiling of the cavity. The subgrid-scale (SGS) turbulence stress tensor is closed via the Smagorinsky's eddy-viscosity model, chemical source terms are modelled by a finite rate chemistry (FRC) model, and a four-step reduced kerosene combustion kinetic mechanism is adopted. The chamber wall pressure predicted from the LES is validated by experimental data reported in literature. The test case has a cavity length of 77 mm and a depth of 8 mm. After liquid kerosene is injected through the orifice, most of the droplets are loaded with recirculation fluid momentum inside the cavity. Due to lower velocity of the carrier fluid inside the cavity, sufficient atomization and evaporation take place during the process of droplet transportation, resulting in a rich fuel mixture of kerosene vapour accumulating inside the cavity. These rich fuel mixtures are mixed with fresh air by the approaching mixing layer at the front of the cavity and are thus involved in burning accompanied with the approaching boundary layer separation extending towards upstream. The combustion flame in the downstream impinges onto the rear wall of the cavity and is then reflected back to the front of the cavity. During the recirculation of hot flow, heat is compensated for evaporation of droplets. The circulation processes mentioned above provide an efficient flame-holding mechanism to stabilize the flame. Comparisons with results from a shorter length of cavity (cavity length of 45 mm) show that, due to insufficient atomization and evaporation of the droplets in the short distance inside the cavity, parts of the droplets are carried out of the cavity through the boundary layer fluctuation and evaporated in the hot flame layer, thus resulting in incomplete air fuel mixing and worse combustion performance. The flow structures inside the cavity play an important role in the spray distribution, thus determining the combustion performance.