Numerical analysis of a water mist spray : the importance of various numerical and physical parameters, including the drag force

This paper presents a comprehensive set of numerical simulations for the characterization of a water mist spray emerging from a nozzle positioned at 2.2 m from floor level and operating at a pressure of 1.0 MPa. The droplet volume-median diameter is about 90 μm and the spray half-angle is around 42°. The spray shape is visualized using a laser sheet and the water flux density distribution on the ground is measured with a ‘bucket’ test. An initial comprehensive numerical study using the Fire Dynamics Simulator (FDS) has been carried out by varying several numerical and physical models and parameters (e.g., cell size and turbulence modelling). The simulated sprays were very narrow (in comparison to the actual spray), yielding overestimations of the peak water flux density at floor level by about 430% (on average). Subsequently, it was found that there is a significant impact of the drag force modelling because the spray at hand is dense (near the nozzle). An ad-hoc reduction of the drag coefficient to a constant value leads to better results. The current study calls upon new developments for droplet aerodynamic modelling in dense sprays