EXTINCTION STUDIES OF PROPANE / AIR COUNTERFZOW DIFFUSION FLAMES : THE EFFECTIVENESS OF AEROSOLS *

We have examined the fire suppression effectiveness of solid aerosols as suitable halon replacements. Experiments were performed in a counterflow diffusion burner, consisting of two I-cm i.d. tubes separated by 1 cm. Aerosols were delivered to propane/air flames in the air flow. Both sodium bicarbonate (NaHC03) and potassium bicarbonate (KHCO3) powders were examined. The NaHCOl and KHCO? powders were separated into various size groupings (< 38 pm, 3845 pm, 45-53 pm, 53-63 pm, and 63-75 pm) using a commercial sieving system. The agent delivery system incorporated a variable orifice for gross adjustment of the delivery rate, and a variable frequency vibration unit for fine adjustments and to maintain powder flow. Light scattering using a modulated HeNe laser beam and a lock-in amplifier was used to monitor the amount of powder exiting the air tube and entering the flame. Extinction concentrations were determined for each agent sample as a function of the strain rate of the uninhibited flame. In general, suppression effectiveness increased with decreasing particle size or increasing strain rate for the size range studied. Potassium powders were -2.5 times by weight more effective than sodium powders. The suppression effectiveness of these powders as a function of particle size can be interpreted using a simplified model of the behavior of particles in counteflowing streams.