An Engineering Model for Water Spray Cooling: Application to a Mechanically-Ventilated Enclosure Fire

The paper presents a simplified engineering model for the prediction of the rate of heat absorption by heat-up and evaporation of water droplets interacting with fire-induced smoke. The algorithm can be easily implemented in the framework of one-zone or two-zone fire models. The general methodology is based on a decoupled time scale analysis for the heating, evaporation and motion of a single droplet. Such analysis allows to determine the heat absorbed by a droplet during its residence time in the smoke layer. Under the assumption of a monodisperse spray, the injected number of droplets per second is calculated and used to estimate the rate of heat absorption (i.e., cooling) by a full spray. The assessment of the model, for single droplet as well as full spray calculations, has been carried out against results obtained with the Fire Dynamics Simulator (FDS 6.7.0). The results show that the model predicts the rate of heat absorption within 15% for droplet diameters between 0.4 mm and 0.8 mm and a surrounding gas temperature below 150°C. Larger deviations are observed under other conditions. The application of the model to a well-confined and mechanically-ventilated compartment fire (a scenario of relevance to nuclear installations and passive houses) allowed to provide a good estimate of the cooling rate of the water spray system and the subsequent average gas temperature and pressure profile within the room.