Fuel Surface Cooling by Aqueous Foam: A Pool Fire Suppression Mechanism

Abstract

Aqueous foams are generally thought to suppress pool fires by forming a transport barrier (either an aqueous film or the foam itself) that prevents fuel vapor transport from the hot pool surface into the fire above. The present work is aimed at evaluating a different potential suppression mechanism wherein the fuel vapor pressure is reduced due to pool surface cooling that occurs when a room-temperature foam is brought in direct contact with the hot pool surface. We present a model to predict the sudden decrease in pool surface temperature when aqueous foam is applied instantaneously and uniformly onto a shallow, burning, heptane fuel pool. Conduction is assumed to dominate heat transfer at short time scales (a few seconds) due to the steep temperature gradient at the interface. The model describes the time evolution of the temperature profile by numerically solving a transient, one-dimensional, heat-conduction equation in the liquid pool and in the foam layer. We also obtained an analytical solution that is valid immediately after contact between fuel and foam. Model predictions show a significant decrease in fuel surface temperature in less than a second after the foam layer is placed on top of a hot liquid pool surface, causing a decrease in the fuel vapor pressure of over 75%. The predictions indicate that surface cooling could be an important mechanism of fire suppression by aqueous foams.