Experimental Investigation of Hydrogen-Air Flame Propagation in Fire Extinguishing Foam

An important element of modern firefighting is sometimes the use of foam. After the use of extinguishing foam on vehicles or machinery operated by compressed gases, it is conceivable that masses of foam were enriched by escaping fuel gas. Furthermore, new foam creation, enriched with a high level of fuel gas, from the deposed foam solution becomes theoretically possible. The aim of this study was to carry out basic experimental investigations on the combustion of water-based H2/air foam. Ignition tests were carried out in a transparent and vertically oriented cylindrical tube (d = 0.09 m; 1.5 m length) and a rectangular thin layer channel (0.02 m x 0.2 m; 2 m length). Additionally, results from larger scale tests performed inside a pool (0.30 m x 1 m x 2 m) are presented. All ducts are semi-confined and a foam generator fills the ducts from below with the defined foam. The foams vary in type and concentration of the foaming agent and hydrogen concentration. The expansion ratio of the combustible foam is in the range of 20 to 50 and the investigated H2-concentrations vary from 8 to 70 % H2 in air. High-speed imaging is used to observe the combustion and determine flame velocities. The study shows that foam is flammable over a wide range of H2-concentrations from 9 to 65 % H2 in air. For certain H2/air-mixtures, an abrupt flame acceleration is observed. The velocity of combustion increases rapidly by an order of magnitude and reaches velocities of up to 80 m/s.