Comprehensive analysis of the impact of a novel droplet volume fraction-based drag reduction correlation in a numerical study on water sprays with different levels of density

Multiple sets of CFD simulations of water sprays have been carried out with FDS 6.7.6 with a novel expression for the drag reduction model (representing the influence of the surrounding droplets on the drag when compared to the situation for isolated droplets). The sprays are representative for use in fire suppression, but have been characterized in the absence of a fire-driven flow, which is key for a step-wise approach. Previous simulations have shown the importance of the drag reduction model, indicating that for a dense spray, a much stronger significant drag reduction seemed necessary than what is commonly applied, in order to reproduce the measured width of the spray envelope and the subsequent water flux distribution at floor level in the CFD simulations. In the present study, the drag reduction is calculated locally, depending solely on the local droplet volume fraction. It is thus only invoked in the dense region of the spray, typically near the injection. The results reveal very good agreement of the water flux distribution on the ground for the dense spray, with deviations from the experimental results that are less than 4% in the center of spray for the initial mesh. However, the simulation results are sensitive to the mesh size. The novel expression for the drag reduction model has also been tested against two other sprays to evaluate its impact in less dense sprays. The initial set of parameters leads to acceptable simulation results for less dense sprays