The effect of air convection on the performance of water mist fire suppression systems

Experimental results obtained from a full-scale test series of water mist systems using various ventilation conditions are reported in this paper. The full-scale test series was conducted under fire scenarios with different fire sizes, types and locations in an empty enclosure and in a simulated machinery space. The ventilation conditions in the tests varied from the door being closed to the door open (natural ventilation) and with the door open combined with an exhaust fan running (forced ventilation). Two types of water mist systems (single-fluid and twin-fluid) were used in the tests. The effect of air convection on the extinguishment performance of the water mist system was examined by an analysis of extinguishment times and the distribution of room temperatures and gas concentrations in the compartment. The test results and corresponding analysis indicated that the water mist discharge quickly cooled the compartment and reduced the mass exchange, between the room and its surroundings, caused by the temperature difference. Also, the strong dynamic mixing created by the water mist discharge restricted the penetration of air convection into the depths of the compartment and reduced the impact of air convection on fire suppression by water mist. The water mist systems effectively extinguished fires under natural ventilation conditions. The effect of natural convection on the performance of the water mist system was dependent on the fire location in the compartment and the characteristics of the water mist system used. Under forced ventilation conditions, the extinguishment capability of the water mist system was reduced due to the strong mass exchange between the room and its surroundings.