TOWARDS SIMULATING PRIMARY ATOMIZATION OF FIRE SPRINKLERS

The atomization of water by a fire sprinkler is of great interest for fire-suppression research. The resulting droplet velocity, diameter, and liquid volume flux largely determine the suppression effectiveness. Traditionally, atomization in fire sprinklers has been studied experimentally. These measurements have been used to better understand the atomization process and to develop spray injection models for use within fire suppression modeling. The goal of this work is to demonstrate the feasibility of using VOF modeling to adequately capture key aspects of sprinkler atomization in an idealized sprinkler geometry, shown in Figure 1, and to provide a path forward to subsequently simulate realistic sprinkler geometries. Accurately resolving the key flow features in the near-field, such as film thickness and sheet breakup length, is critical to enable eventual simulation of the fully atomized spray. Comparisons of the predicted flow features are made with a previously characterized idealized fire sprinkler geometry [1]. A mesh refinement study is performed for better understanding of the required resolution necessary to enable accurate representation of the liquid surface and the details of the subsequent breakup. The experiment of Zhou and Yu [1] was used for model comparison and validation. The idealized sprinkler used in this experiment consists of a horizontal disk placed beneath a vertical, cylindrical nozzle. Being idealized, there are no slots, tines, frame arms, or boss elements that typically are present in a realistic sprinkler. Three disk diameters were considered: 25.4 mm, 38.1 mm, and 50.8 mm. The nozzle was placed 20 mm above the disk top surface and had an inner diameter of 9.5 mm. Figure 1. Comparison of a) typical sprinkler geometry showing the frame arms, the boss, and deflector; and b) idealized sprinkler geometry showing the nozzle and disk. Water discharge pressures ranging from 0.034 bar to 0.83 bar were used to investigate the spray formation as affected by sprinkler geometry and operating pressure using a laser-based shadow imaging system. The water film thickness (only measured for the lower end of the tested range of discharge pressures), sheet breakup distance, and drop size distributions were measured. The sprinkler simulations utilized a VOF solver, navalFoam, implemented in foam-extend [2], a community-driven fork of the OpenFOAM [3] CFD software. The equations solved in navalFoam have been adequately documented elsewhere [4-6]. Rather than using typical interface compression schemes [7] to try to maintain a sharp interface, navalFoam includes the isoAdvector scheme [6] for approximating a geometric reconstruction of the interface. This scheme explicitly reconstructs a phase interface in each computational cell where 0 <