A Theoretical and Experimental Study of Extinguishing Compressed Air Foam on an N-Heptane Storage Tank Fire with Variable Fuel Thickness

Compressed air foam (CAF) is an environment friendly fire extinguishing technique being used around the world since the ban of halogen-based agents. In this work, its extinguishing behavior on a radiation-controlled storage tank fire was firstly investigated based on the controlling mechanism of burning rate. A formula of burning rate after foam discharge is deduced. It was found that the oleophobic effect of fluorocarbon surfactants, the increased foam layer thickness and lower diffusion coefficient had coupled chemical and physical effects on extinguishing, which outweighed the cooling effect of foam evaporation. Due to the properties of CAF, a special phenomenon of the sudden increase in the flame temperature and radiation after foam discharge was analyzed. The increase in foam spreading length with time was found to still abide by a power law under fire conditions. A concept of effective and total extinguishing time was suggested, both of which reduce with a rising foam flow rate, but the reduction rate decreases at higher flow rate. In addition, the foam dosage with flow rate had a U-shaped curve at total extinction. For the case of lower fuel thickness, its burning rate was lower and the extinguishing time was longer. (C) 2020 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.