Development of an analytical model to quantify downward smoke displacement caused by a water spray for zone model simulations

• A model to quantify the downward smoke displacement under spray is further developed.
• Entrainment and the heat transfer phenomena are taken into account in the model.
• The over-all quality of the model is illustrated for a range of experimental conditions.
• Some useful conclusions for practice are made by applying the model.

A stand-alone analytical model for downward smoke layer displacement caused by a water spray is developed. The model can be implemented into two-zone model calculations. Smoke flow into and out of the water spray envelope is incorporated, as well as a model for the heat exchange between the smoke and the water droplets. The model input quantities are water flow rate, orifice diameter, droplet diameter, spray angle, initial smoke layer thickness and temperature, and ambient air temperature. The quality of the model predictions is illustrated for a range of experimental conditions. Results are shown to be sensitive to the mean droplet diameter and the spray angle. Then, the combinations of values of these parameters are suggested for the sprinkler considered. A practically important phenomenon of abrupt strong downward smoke layer displacement is explained in that the upward buoyancy flow diminishes due to the entrainment of cool air into the water spray envelope. This only occurs for high enough water flow rates and as long as the smoke layer thickness is below a critical value. Therefore, this is particularly dangerous during the smoke layer build-up phase. Finally, the accuracy of the model for the heat transfer rate from the hot smoke layer to the droplets and the smoke temperature inside the spray region are evaluated by comparison to experimental data.