Extinction phenomenon of premixed flames with alkali metal compounds

Abstract In order to investigate the flame suppression mechanism of alkali metal salts, ultrafine powders of these salts with diameters less than 1 μm were introduced into premixed flames in the form of mists of their water solutions. Premixed flames inhibited by the mist suddenly blew offif the mist density was increased beyond a critical value. This sudden extinction of flames was peculiar to alkali metal salts and it was not observed for other inhibitors. From the existence of the extinction condition, the time necessary for the salts to have a chemical inhibition effect was found to be 10 ms at 1200K and 0.5 ms at 1800K. This critical time was found to correspond to the decomposition time of the salts. The result shows that alkali metal compounds must stay in the flames longer than this critical time in order to work as chemical flame suppressants. If the residence time in flames is less than the critical time, the inhibition effect of the powders will be purely thermal, especially for powders with larger diameters. This model for flame suppression of alkali metal compounds can explain many contradictory results in experimental studies reported previously.