Turbulent liquid spray mixing and combustion–fundamental simulations

Abstract

Fundamental simulations are used to investigate the ignition process of turbulent nheptane liquid fuel spray jets. A DNS quality Eulerian method is used to solve the carrier gas flow field, while a Lagrangian method is used to track the liquid fuel droplets. Two-way coupling between the phases is included through the exchange of mass, momentum and energy. A detailed mechanism with 33 species and 64 reactions is used to describe the chemical reactions. The simulation approach allows studies of larger scale interaction of sprays and turbulence, including evaporation, mixing, and detailed chemical reaction. Both time developing and spatially developing liquid spray jets are studied. The initial carrier gas temperature was 1500 K. Several cases were simulated with different droplet radii (from 10 microns to 30 microns) and two initial velocities (100 m/s and 150 m/s). In the time developing case it was found that evaporative cooling and turbulence mixing play important roles in the ignition process of liquid fuel spray jets. Ignition first occurs at the edges of the jets where the fuel mixture is lean, and the scalar dissipation rate and vorticity magnitude are low. For smaller droplets, ignition occurs later than larger droplets due to increased evaporative cooling. Higher initial droplet velocity enhances turbulence mixing and evaporative cooling. For smaller droplets, higher initial droplet velocity causes the ignition to occur earlier, whereas for larger droplets, higher initial droplet velocity delays the ignition time. In the spatially developing liquid jets, ignition and flame lift-off characteristics similar to diesel sprays are observed. Near the injector, combustion development progresses very rapidly along the stoichiometric surface. In the downstream region of the spray, combustion develops with steep temperature fronts in a flamelet mode.