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Suppressant Performance Evaluation in a Baffle-Stabilized Pool Fire.


pdf icon Suppressant Performance Evaluation in a Baffle-Stabilized Pool Fire. (977 K)
Grosshandler, W. L.; Donnelly, M. K.; Charagundla, S. R.; Presser, C.

Halon Options Technical Working Conference. Proceedings. HOTWC 1999. April 27-29, 1999, Albuquerque, NM, 105-116 pp, 1999.

Sponsor:

Department of Defense, Washington, DC

Keywords:

halon alternatives; pool fires; fire suppression; aircraft fires; fire tests

Abstract:

The amount of a gaseous agent required to extinguish fires in full-scale engine nacelles varies greatly with the geometry of the test fixture and the manner in which the flame is stabilized. It has been observed that if the test is designed to allow fuel to collect behind obstacles in the vicinity of a hot surface, a significantly higher mass of agent is necessary for sustained suppresssion. The superior performance of chemically acting agents, such as CF3Br and CF3I, relative to a hydrofluorocarbon alternative like HFC-125 is also accentuated in some of these tests. Full-scale testing carried out by the Navy using two different fixtures, each meant to simulate fires in the F/A-18 engine nacelle, has led to different conclusions regarding the amount and relative performance of both HFC-125 and solid propellant gas generator (SPGG) fire suppressants. The complexity and unpredictability of full-scale tests can be traced to two factors: flame stabilization and agent mixing. Flame stability is governed by local geometry, surface temperature, and fuel and air flow patterns. Flame extinction will occur if the agent is entrained into the flame zone in sufficient concentration, if the fuel and air flows are disrupted enough by the agent discharge process, or by a combination of the two effects. Entrainment and localized flame stretch are, in turn, controlled by the way the fire suppression system is designed and by the location of the fire relative to the discharge nozzle. Hirst and Sutton developed a wind tunnel to explore the impact of step height, air flow, and pressure on the blow-out of a jet fuel pool fire stabilized behind a backward facing step. Hirst et al. studied the suppression of these types of fires using various halons, and concluded that a liquid pool burning in a flow behind an obstacle is the most difficult fire to extinguish. This was born out in full-scale tests done later. Experiments by Hamins et al., in cooperation with Walter-Kidde Aerospace, were conducted in a wind tunnel scaled down from the earlier work by Hirst to examine the performance of HFC-125 and HFC-227ea. Investigations at the Air Force Research Laboratory as part of the Next-Generation Program (NGP) sought to determine the detailed structure, during suppression, of a non-premixed methane/air flame stabilized behind a step. The changing character of the flame with step height and air velocity was examined, along with the amount of Halon 1301 required to suppress the flame as a function of the flow parameters and injection interval.