Numerical Simulation of a Model Spray Flame under MILD conditions using Stochastic Upstream Flow and Temperature Forcing

Numerical simulations of the Delft Spray in Hot Co-flow (DSHC) flame are presented, in order to aid the understanding of reacting multiphase flows under moderate or low-oxygen dilution (MILD) conditions. The test case consists of a single swirled pressure atomizer installed in the center of a cylindrical hot co-flow, operated with ethanol fuel. A large variety of experimental data is available for the burner’s MILD combustion configuration. Here, the particular H-II case is studied. Three different modelling approaches are employed, an unsteady RANS simulation and two scale-resolving methods, namely LES (Large Eddy Simulation) and SAS-SST (Scale Adaptive Simulation) in combination with a Shear Stress Transport turbulence model. Here, for the scale resolving SAS and LES, transient inflow boundary conditions are necessary in order to propagate turbulent flow and temperature structures into the computational domain, supporting the evolution of a full turbulent energy cascade. However, preliminary simulations have shown that due to the low Reynolds number of the co-flow, artificially imposed spatial and temporal turbulent fluctuations of temperature and velocity field are subject to strong artificial decay, prior to reaching the actual combustion zone. Therefore, a simplified stochastic forcing approach based on a first order Langevin model is adopted, reducing the boundary condition to a time dependent function which generates time-coherent structures featuring turbulent decay in time. The accurate implementation of the methodology is verified by means of analytical solutions and validated with the Delft Spray flame test case

Derivation and Numerical Study of Spray Boundary Conditions for a Pressure Swirl Atomizer Issuing into Co-Flowing Air

Newly measured experimental data from detailed measurements of a hollow cone pressure swirl atomizer using simultaneous Phase Doppler Anemometry (PDA) and Laser Doppler Anemometry (LDA) serves as a database for the derivation of spray boundary conditions for multiphase flow simulations. In the experiments the spray is characterized in both quiescent and co-flowing air with an air velocity of ug=36m/s. Based on the results, a reconstruction strategy for the upstream characteristics of the spray is defined. The resulting spray boundary condition is then investigated by means of Reynolds Averaged Navier Stokes (RANS) simulations and a global sensitivity analysis method. As a result, the necessary calibration factors for the spray boundary condition are reduced and calibrated against the experimental data in quiescent air. Finally, the calibrated boundary condition is used to simulate the experiments in co-flowing air. This approach is found to accurately describe downstream spatial distribution of droplet size and velocity