Study of n-Heptane Diffusion through Fire Fighting Foams As a Function of Temperature

                                              Study of n-Heptane Diffusion through Firefighting Foams as a Function of Temperature

Katherine Campbell* and Bradley Williams

Code 6185, Combustion Dynamics and Modeling Section

Chemistry Division, Naval Research Laboratory, Washington DC 20375

*Naval Research Enterprise Intern, June-August 2013

Aqueous film-forming foam (AFFF) is used to combat fires above pools of liquid fuel.  AFFF was originally developed to provide fire protection on aircraft carrier flight decks, and has been widely used in both military and civilian applications (for example, airports) for some 50 years.  AFFF and other types of firefighting foam are typically packaged as 1%, 3%, or 6% concentrates.  The number refers to the percentage of the concentrate when mixed with water to form a solution. The concentrate contains surfactants and other additives that promote foam formation and stability. AFFF used for fire protection in the Department of Defense must pass the MIL-F-24385F qualification standard, which requires extinguishment of a 6 foot diameter pool fire of gasoline within 30 seconds when an AFFF solution is sprayed at a rate of 2 gallons/minute.

Aqueous film forming foam relies on fluorinated surfactants to lower the surface tension of dense water and allow a film of the foam solution (10-50 µm) to form on top of the less dense fuel. This film can act as a barrier, even in the absence of foam, preventing the transport of fuel vapor from the pool to the flame above.   The fluorosurfactants, however, pose as an environment concern due to persistence in the environment, and in some cases, tendency to bioaccumulate. Therefore, there is a desire to develop alternative firefighting foams free of environmentally-persistent materials.  Achieving the same performance level as AFFF without fluorosurfactants requires more detailed understanding of the mechanisms by which fire suppression occurs.  The present work investigates one aspect of foams’ interaction with the fuel pool: the mass transport of fuel through the foam layer.  In particular, the diffusion of n-heptane is measured through an AFFF foam and a non-fluorinated (non-filming foam) at temperatures ranging from 16 °C to 50 °C.

We examined two commercially available fire protection foams: Buckeye BFC-3MS (a MIL-F-24385F-qualified 3% AFFF) and Solberg RF6 (a 6% non-fluorinated foam qualified against the ICAO protocol).  Foam was generated using a submerged sparger tube that bubbled nitrogen through the foam solution.  Because this experiment focused on the mass transport effects of heated fuel, the foam generation conditions were kept constant by using a nitrogen flow rate that coincided with a foam expansion ratio between 9 and 10.

n-Heptane was used as the fuel, selected as a pure compound representative of low flash point fuels.  Heptane (mixed isomers) is used as the test fuel in some AFFF qualification standards, and is currently being investigated as a replacement test fuel in the MIL-F-24385F MilSpec AFFF standard, which currently uses gasoline.

To observe fuel transport, we covered a heated pool of fuel in a 5.75 cm inner diameter chamber covered with a 4 cm layer of foam. The fuel was preheated before beginning the experiment, then the lower portion of the chamber was immersed in a temperature-regulated water bath.  The fuel temperature was monitored by a thermocouple immersed in the fuel.  We designed a test apparatus for this experiment to ensure that no fuel was in the chamber above the foam when testing began. This procedure established an initial condition of no fuel concentration within the foam layer; fuel leaving the top surface of the foam had to diffuse through the foam.  Both the steady state flux of fuel through the foam and the time required for the fuel to penetrate the foam are proportional to the diffusivity of the fuel within the foam.

We designed the test apparatus so that it could be quickly disassembled into two pieces.  The lower half of the chamber contained the fuel pool, and when the chamber was disassembled, the foam layer was placed above the fuel before the chamber was reassembled. The top piece had nitrogen flowing through a porous frit toward the upper surface of the foam in a stagnation flow geometry. The nitrogen flow picks us any heptane which has diffused through the foam.   The heptane concentration of the exiting as is determined by a Fourier transform infrared (FTIR) spectrometer; the heptane flux through the foam is calculated from the nitrogen flow rate, the heptane concentration, and the foam surface area.  The chamber was designed for quick assembly to minimize the length of time between foam generation and the beginning of data collection (typically 1-2 minutes).

We evaluated the two foams at fuel temperatures of 16 °C, 35 °C, and 50 °C. For each foam, two data collection runs were performed at 16 °C (ambient temperature) and three runs at each of the elevated temperatures. Data collection for each run lasted for slightly more than 60 minutes, or until the foam layer collapsed to expose the fuel surface.  

For both foams tested, the fuel flux through the foam increased significantly with increasing temperature.  However, the increase in vapor pressure of the fuel, representing the boundary condition at the bottom of the foam in contact with the fuel surface, accounted for almost all of the effect.  The diffusivity of n-heptane through either foam had little variation over the temperature range studied.  The RF6 had a significantly higher conductivity for heptane than did the BFC-3MS AFFF.  Further observations with additional foam formulations are needed to determine if the difference in diffusion is due to the presence of fluorosurfactants, or other factors.  The magnitude of heptane diffusivity through the foams is intermediate, on a logarithmic scale, between its diffusivity in gases and in liquids.