A study of gas phase fire suppression mechanisms in laminar counterflow diffusion flames.

Suppression of unwanted fire is important to protect life and property, hence, the better understanding of suppression mechanisms is essential. In the present study, fire suppression was investigated with the objective to develop an integrated understanding of its mechanisms for various suppressants. A laminar counterflow diffusion flame configuration was used for the investigation with detailed experimental and computational analysis methods. Three different types of suppressants, nitrogen, water and CF$\sb3$Br, were completely studied with two different flame structures. In this work, external flame characteristics, boundary conditions, temperatures and stagnation plane locations were measured to supplement the real flame information for calculations. The numerical results were also verified based on measured major species and OH concentrations. Oxygen concentration was progressively reduced to study the suppression phenomenon with different suppressants. When CF$\sb3$Br was introduced, the measured OH concentrations decreased but soot volume fractions were increased while they were decreased with water substitution. As oxygen concentration was decreased to extinction, the flame temperature, OH radical concentration and soot volume fraction were reduced. The limit flame temperature and oxygen concentration at extinction were the same for the base and water-substituted flames and substantially higher for CF$\sb3$Br flames of the same structure. However, the limit OH concentration near extinction was the same, 0.11% for the present flames, regardless of the flame structure or the type of additive. As a result, the limiting OH concentration is the most general measure of flame extinction conditions. As oxygen concentration was reduced to extinction, the major heat release rate reactions were suppressed. The production of active radical was reduced accompanied by a decrease on the fuel radical formation and oxidation, and final product formations. This consequently reduced both the flame exothermicity and temperature. Extinction is performed when the radical concentration is reduced below a certain level. The CF$\sb3$Br flames had a lower active radical level by the radical consuming inhibition mechanism. As oxygen concentration was reduced, the major heat release rate reactions were decreased and reached a limiting active radical concentration at higher oxygen concentration and flame temperature. For water substituted flames, an enhancement of CO$\sb2$ formation by water was observed along with an increase of OH production.Ph.D.Applied SciencesMechanical engineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/130842/2/9811200.pd