Jannaf 35 Th Combustion Subcommittee and 17 Th Propulsion Systems Hazards Subcommittee Meeting Joint Sessions

Planar Laser Induced Fluorescence (pLIF) is used to measure OH concentration profiles in an atmospheric pressure, opposed flow, propane (C~)/air flame. Flame inhibiting agents CF~r, N2, Fe(CO)s, C~7H, C~6H2, CH~(O)(OCH3h, and P~~6 were added to the flame and relative OH concentration profiles were measured as each flame was extinguished. The OH profiles illustrate that addition of N2, C~6H2, and C~7H, to the flame produced smaller changes in OH concentrations relative to CF~r implying these agents have chemical inhibition capacities less than CF~r. However, the addition of CH~(0)(OCH3h and Fe(CO)s to the flame demonstrated chemical inhibition capabilities greater than CF~r with larger changes in OH concentrations. INTRODUCTION Fire protection on military platforms, including ground fighting vehicles, is being challenged by the impending loss of the ubiquitous fire fighting agent halon 1301 (CF~r) due to environmental concerns related to the destruction of the stratospheric ozone layer. Replacement:fire extinguishment agents need to be found that will satisfy numerous criteria including: fast fire suppression, minimum production of toxic gases when used, low toxicity, compatibility with storage materials and environmental friendliness. The U.S. Army's search for halon replacement agents has largely involved an empirical approach of testing and evaluation of commercially available compounds/systems. An alternative approach is to study the fundamental physical and chemical mechanisms responsible for flame inhibition with the hope that such studies will uncover differences in the flame inhibition mechanisms which will lead to new chemicals for further consideration and testing. To this end, we have recently initiated planar laser induced fluorescence (pLIF) measurements of the OH radical species as flame extinction was approached in a non-premixed, atmospheric pressure, opposed flow propane/air flame inhibited by halon 1301 [CF;J13r], N2, Fe(CO)s, FM-200 IC~7H], FE-36 [C~6H2], DMMP [CH~(0)(OCH3h], PN [P3N~6]. Presented here are preliminaIy results from this sfudy of compounds which represent distinctly different chemical families in order to understand the differences between each agent's inhibition mechanism. BACKGROUND Chemical inhibition in a flame arises from the lowering of the radical concentrations due to scavenging reactions. In general, effective inhibition mechanisms contain two types of reactions: a) radical scavenging reactions, and b) reactions regenerating inhibitor species that participate in the inhibition cycle. As an example, for CF~r inhibition a free bromine from decomposed CF~r forms HBr which chemically reacts with a hydrogen atom and reduces the flame's hydrogen concentration. The consequence of hydrogen recombination is the overall available radical concentrations (H, 0, OH) and the rate of chain-branching reactions are reduced [1,2,3,4] while regeneration ofHBr and Br2 occurs carrying on the inhibition cycle. The chemicals Fe(CO)s, D~ ~~. PN investigated in our laboratory flame system were chosen based on a comprehensive evaluation [5] offire inhibitors that are more effective than CF;J13r. The inhibition mechanisms for Fe(CO)s, DMMP, and PN are believed to be generally similar to the HBr mechanism. For these postulated mechanisms, each agent decomposes during combustion into inhibition cycle scavenging species, e.g. FeO, FeOH, Fe(OH)2 for Fe(CO)s addition, [6] and HOPO and HOP02 for DMMP and PN addition [7]. In the reaction zone of flames, these scavenging species proceed to behave much like HBr in scavenging hydrogen atoms. FM-200 and FE-36 were studied here due to their popularity as candidate halon replacement agents. FM-200 and FE-36 are refrigerants and it is assumed that their primary fire inhibition capabilities are due to their physical properties of high heat capacities with some chemical reactivity due to CF3radical [8]. Approved for public release, distribution is unlimited.