Fire Suppression And Re-ignition Prevention Ina Full-scale Engine Nacelle Simulator

Abstract

An analysis is presented on the full-scale fire suppression experiments conducted on the F-22 engine nacelle simulator at Wright Patterson Air Force Base. Experiments investigated the relative effectiveness of halogenated agents and solid propellant gas generators (SPGG) in suppressing a series of spray fires with and without a fuel re-ignition source. Several agents were tested including CF3Br (halon 1301), C2HF5 (HFC-125), and two basic types of SPGG, including one that produced inert gases in conjunction with a fine solid particulate composed of K2CO3 and one that produced inert gases only. The measured agent effectiveness was compared to the predicted effectiveness based on results from cup burner suppression experiments. Estimates of the suppression effectiveness of the SPGG were based on the effectiveness of the components of its effluent. The mass fraction of SPGG effluent required to extinguish heptane cup burner flames was estimated as 0.15 and 0.29 as compared to previously measured values of 0.14 and 0.28 for CF3Br and C2HF5, respectively. The predicted suppression requirements (relative to CF3Br) agreed with the full-scale measurements within 35% for the halogenated compounds, whereas the SPGG performed as much as a factor of 3.3 better than predicted. This difference suggests that a large fraction of the SPGG performance may have been related to its fast deployment, which enhances flame straining and thereby reduces agent mass suppression requirements. The SPGG effluent that contained a significant percentage of K2CO3 particulate was particularly effective for re-ignition protection, a scenario that dominates agent mass requirements for the compressed halogenated liquids. In this case, the SPGG required 27 times less mass than CF3Br. INTRODUCTION Halon 1301, or trifluorobromomethane (CF3Br), has been used as a fire extinguishing agent in many applications because of its many positive attributes. Due to its high ozone depletion potential, however, its production has been terminated. In the search for a suitable replacement, novel types of agents are under development. One class of such devices is the solid propellant gas generator (SPGG), a device in which solid-phase combustion yields products which can be used for fire suppression [1]. The key parameters that affect flame stability, control flame extinction and the prevention of re-ignition are agent effectiveness and flow field dynamics. Whereas there is a general understanding of the suppression mechanisms of gaseous agents, little has been published on SPGG effectiveness. Recently, the relative suppression effectiveness of a SPGG has been Copyright © International Association for Fire Safety Science 704 measured in a well-controlled bench-scale experiments [2]. There are many types of SPGG, each with its own composition, packaging geometry, and ignition sequencing. It is expected that gas specie and particulate yields, particulate size distributions, exhaust temperatures, and the rate and duration of effluent generation will vary from one SPGG to another, and each of these parameters may impact SPGG effectiveness. Table 1 shows the species composition of the SPGG effluent for the formulations tested here. For interest, a hypothetical mixture of unfiltered effluent from SPGG 1 is also included. Several SPGG types from Manufacturer 1 were tested and all were assumed to have the same effluent composition [3]. The hot exhaust products were principally gaseous N2, CO2, and H2O vapor and salt particulates. Table 1 shows that there was approximately one-third as much CO2, more than twice as much H2O, and almost two-thirds as much N2 produced in the SPGG 2 effluent than in the filtered SPGG 1 effluent. The SPGG 2 effluent is assumed to be unfiltered, so that it contains K2CO3(s), whereas the filtered SPGG 1 effluent is assumed to contain trace amounts of particulate only. Particle size can influence suppression effectiveness [4]. The majority of the SPGG particulate is very small and it is assumed that the particulate is smaller than the critical diameter [4]. To investigate the suppression effectiveness of SPGG in a simulated F-22 engine nacelle, a series of full-scale fire experiments was undertaken utilizing the Wright Patterson Aircraft Engine Nacelle Test Facility. The experiments described here investigated the relative effectiveness of halogenated agents and SPGG in suppressing spray fires with and without a re-ignition source. Several SPGG types were tested, including one that produced inert gases and a fine solid particulate, and one with the particulate filtered, such that only inert gases were emitted. Table 1 SPGG mole based fractional effluent composition. Species filtered SPGG 1 Unfiltered SPGG 1 SPGG 2 CO2 0.313 0.264 0.0917 H2O 0.219 0.183 0.559 N2 0.468 0.395 0.325 K2CO3 0* 0.158 0.0246 * assumed to be trace amounts only. _______________________________ * Certain commercial materials and equipment are identified in this report in order to specify adequately the procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material or equipment is necessarily the best for the purpose.