Flame suppression by aerosols derived from aqueous solutions containing phosphorus

Aerosols derived from aqueous solutions containing phosphorus are investigated as possible alternatives to halon-based flame suppressants. Phosphorus-containing compounds (PCCs) have been shown to be effective flame suppressants in the gas phase; a water-based solution would provide a practical means of delivering a condensed-phase PCC to the flame. Flame suppression was characterized by measuring the global extinction strain rate of a counterflow, nonpremixed methane-air flame with and without a PCC additive. An aqueous solution of the additive was introduced as an aerosol into a heated chamber upstream of the air flow tube. A high-efficiency nebulizer produced a polydisperse spray of droplets with a Sauter mean diameter of 8 μm, as measured with a phase-Doppler particle analyzer. The droplet size distribution was nearly independent of the composition and flow rate of the liquid. Externally maintaining the reactant streams at 360 K allowed the water in the aerosol to evaporate prior to reaching the flame. Evaporation of water leaves behind residual particles for solutes that are solids at 360 K, and thus solid particulates, not droplets of solution, enter the flame zone. Comparisons of different solutions with different phase solutes were used to provide an estimate of the physical effect of particles on the flame. Three neat phosphorus-containing compounds, dimethylmethylphosphonate (DMMP), diethylmethylphoshonate (DEMP) and dimethylphosphonate (DMP) and five 1.6% molar aqueous solutions, orthophosphoric acid (OPA), phosphorus acid, phosphonic acid, methylphosphonic acid, and dimethylmethylphosphonate (DMMP), were investigated. The acid solutions (with solid solutes) all displayed similar effectiveness, and were all slightly more effective, on a per mole basis, than the DMMP solution (a liquid solute). The effectiveness of a DMMP/water solution is found to be a weighted average of the effectiveness of its constituents. Per mole of water delivered, a 1.6% molar solution of a PCC is approximately twice as effective in fire suppression compared to neat water vapor. Flame modeling calculations for the extinction condition with added gas-phase phosphorus compounds using a published phosphorus reaction mechanism grossly underpredict the total effectiveness of the compounds investigated.