Computational Modeling of Turbulent Spray Combustion Process Using RANS and Large-Eddy Simulations

Abstract

Computational fluid dynamics is applied to reproduce the characteristics of the liquid methanol burner presented by the National Institute of Standards and Technology (NIST). Reynolds average Navier-Stokes (RANS) and large-eddy simulations (LES) are employed, along with the steady nonadiabatic flamelets combustion model (using an extended reaction mechanism). The spray is not directly simulated, but instead, the linearized instability sheet atomization (LISA) model is implemented. The results obtained with RANS are used to estimate the scales of turbulence and design a mesh suitable for LES. The velocity field, spray characteristics, temperature, and combustion products are compared against the experimental data reported in the literature. Both simulations show similar results, differing mainly in the spray characteristics (size of the injected droplets). This seems to be related to the parameters of the Rosin-Rammler distribution used by the LISA model. Although a fraction of the spray evaporates downstream of the reaction zone, the fraction of unburned fuel is underestimated, which is expected considering the assumption of infinitely fast reaction. There is no formation of a vortex breakdown nor strong recirculation zone in the flow (due to the relatively low swirl number); nevertheless, some coherent structures are reproduced, showing the capacity of LES to capture the bigger scales of turbulence.