Experimental and numerical study of water spray system for a fire event in a confined and mechanically ventilated compartment

The present work addresses the application of a water spray system in case of a fire event in large-scale experiments for nuclear safety issues. It focuses on the interaction between a water spray system and a stratified smoke layer due to a pool fire in a mechanically ventilated enclosure. This study is supported by a set of four large-scale tests and one numerical simulation with a 3D CFD software, named CALIF(3)S/ISIS, and developed by the French Institut de Radioprotection et de Surete Nucleaire (IRSN). The modelling used in this paper is based on an Eulerian-Lagrangian approach. The fire tests are performed in a 165 -m(3) mechanically ventilated single room. The fire is a lubricant oil pool fire of about 400 kW. The ventilation flow rate is 2550 m(3).h(-1) and corresponds to a renewal rate of 15.5 h(-1). The spray nozzles are deluge and sprinkler type. The test parameters are the water flow rate, the time of activation, and the duration of activation. Based on the large-scale experiments and the numerical simulation, four typical physical mechanisms have been enlightened. The first one corresponds to the cooling of the gas phase that is the straightforward consequence of the heat transfer exchange between the water droplets and the surrounding gas. The second effect is the process of gas mixing and homogenization induced by the water spraying system. The gas concentrations (O-2, CO2) in the upper and lower parts of the room tend to the same level. The third effect is the significant increase of the fire heat release rate (HRR), up to 25 %, when the water spray is activated. Then, the last noteworthy effect is the occurrence of gas pressure peaks when the water spray is activated or shut off, consequence of the sudden change of the gas temperature. The processes of gas cooling and fire HRR increase are showed to be the main causes of these variations of gas pressure.