Experimental Flow Characterization and Computational Model Development of Aqueous Film Forming Foam Firefighting Jets
Over the past few decades, aircraft rescue and firefighting (ARFF) research has made technical strides on multiple fronts. Continuing efforts have helped develop computer-aided engineering tools to quantify risk assessment for a variety of ARFF aspects such as aircraft pool fire combustion and dynamic crash-related events. A study was conducted to characterize firefighting agent application behavior and to quantify the flow characteristics that differentiate water and aqueous film forming foam (AFFF) jets.
An aqueous firefighting agent application laboratory was specially constructed to carry out experiments on firefighting jets ranging from 1 to 11 MPa (150 to 1550 lb⋅in.-2) and 4 to 25 l⋅min-1 (1 to 6.4 gal⋅min-1) at AFFF concentration levels ranging from 0% (pure water) to 12% by volume. Experimental flow characterization consisted of flow visualization, agent ground pattern distribution analysis, and two-dimensional phase Doppler particle analysis (PDPA). Flow visualization results depicted minimal qualitative differences in terms of overall jet structure between AFFF and water jets. However, PDPA results showed AFFF enhanced jet breakup and generated droplet sizes 7% to 38% less in diameter compared to water jets with AFFF jets lagging water jet velocities by as much as 10% in certain cases. Agent ground pattern results confirmed flow performance factors, such as ground coverage area, reach, and maximum span, all benefit from an increase in nozzle pressure-flow rate.
An Euler-Lagrange, large eddy simulation computational fluid dynamic (CFD) strategy accounting for droplet collision and breakup was employed to predict firefighting jet flow dynamics with and without the addition of AFFF. AFFF influence was handled computationally via material property variation from pure water in terms of density, viscosity, and surface tension effects. CFD model results were agreeable with flow visualization and phase Doppler data as they reproduced global trends in both droplet velocity and size data, particularly with respect to the influence of AFFF. However, oversimplified nozzle injection conditions led to greater differences than expected. CFD model result errors were difficult to quantify entirely due to PDPA upper particle size range limitations and complexities associated with direct comparisons to data.
This work describes the first known, comprehensive effort to quantify flow characteristics and properties that differentiate water and AFFF firefighting jets using high-fidelity experimental techniques. This work also includes the first known iteration of a firefighting agent application CFD model designed for use in the ARFF industry that takes into account the influence of AFFF.
DOT/FAA/TC-15/21
Authors: Christopher P. Menchini, Gary J. Morris, and Wade W. Huebsch