The relevance of wall roughness modeling for simulation of powder flows in laser metal deposition nozzles

Point-particle large-eddy simulations and high-speed imaging are used to investigate effects of wall roughness and particle size on characteristics of a powder flow issuing from a vertical, round nozzle. Wall roughness effects on dynamics of particles can be characterized by the standard deviation of the roughness angle distribution Δ γ, which is a hybrid roughness parameter that represents a combination of amplitude and spacing roughness parameters. We optically scan the inner nozzle surface to obtain the two-dimensional roughness profile, using which Δ γ is estimated. We adopt a stochastic approach in the numerical simulations to model the wall roughness. We find that this modeling is essential to obtain a good agreement between simulation and experimental results. The wall roughness is found to enhance the transverse dispersion of particles and to eliminate the preferential accumulation of particles in the near-wall region, giving rise to reduction of the mean particle velocity within the nozzle and to clustering of the particles in the nozzle core. Results also reveal that an increase in the particle size (characterized by Stokes number) and in the wall roughness leads to a reduction of the particle velocity and to an enhancement of the particle scattering throughout the jet region. However, a saturation behavior is observed in the particle scattering with the Stokes number. All this dependence is rationalized by the fact that the Stokes number and the wall roughness determine the correlation between two phases and the inelastic particle-wall collision frequency, respectively.