Study of De-stratification and Optical Effects Observed During Smoke/Mist Interactions

Abstract

This work focuses on interactions between a water mist and a smoke layer generated by a fire. It aims at highlighting and quantifying the mist impact on smoke stratification, investigating both thermal and optical effects. The study is based on large-scale experiments and numerical reconstruction with the computational code fire dynamics simulator (FDS v.6). The use of FDS allows understanding the complicated flow generated by the water mist and the fire, which yields specific gas temperature and opacity conditions within the medium composed of soot and water droplets in suspension. There are two main results. As expected, a de-stratification occurs as a consequence of mist application. A 1 m high smoke-free layer was observed in the present case before mist activation, which disappears due to the mist action. However, consecutive effects on optical and thermal properties are not identical. In the presented tests, water mist application homogenized the gas temperature downward the nozzle location to a value close to 50°C due to both mixing and cooling effects (while temperature was varying between ambient temperature near the floor and 150°C below the ceiling before mist activation). On the contrary opacity measurements revealed remaining discrepancies, in soot and droplet concentrations. Transmissivity decreases after mist activation, but significant variations are still seen in the smoke layer. As a consequence, characterization of the smoke layer with one single technique or the other (namely: opacity measurement or temperature measurement) would lead to different conclusions on the smoke homogeneity. It appears that both measurements should be involved simultaneously for a complete characterization of the environment.