TRANSIENT APPLICATION, RECIRCULATING POOL FIRE, 

AGENT EFFECTIVENESS SCREEN: FINAL REPORT 

William Grosshandler, Anthony Hamins, Jiann Yang, Kevin McGrattan, and Cary Presser 

National Institute of Standards & Technology 

ABSTRACT 

A three-year research effort has been conducted as part of the Next Generation Program (NGP) to develop a 

laboratory screening device suitable for predicting the behavior of halon alternatives in full-scale fire suppression experiments. Most of the work has focused on the transient delivery of gaseous agents and interactions 

with an obstacle-stabilized propane pool tire. The final report of this effort was recently issued as a NlST 

report (NISTIR 6733). Motivation for the study and significant findings are summarized in this paper. 

INTRODUCTION 

The amount of a gaseous agent required to extinguish fires in full-scale engine nacelles varies greatly 

with the geometry ofthe 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 suppression [l]. The superior performance of 

chemically acting agents, such as CF,Br and CFJ, relative to a hydrofluorocarbon alternative, like HFC125, is also accentuated in some of these tests. Full-scale testing carried out by the Navy using two different fixtures, each meant to simulate tires in the F/A-l8 engine nacelle, 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 ofthe fire relative to the discharge nozzle. 

Figure 1 shows a full-scale mock-up of an F-22 engine nacelle built by the Air Force Research Laboratory at Wright-Patterson AFB, which was used to test the suppression effectiveness of HFC-125 and 

SPGG suppressants, as compared to Halon 1301 [2]. When the engine mock-up is slid in on its rails, an 

annular region is formed about the core where flammable vapors can build and leaking fuel can accumulate. Air is brought in through a scoop at the lower right of the structure, it mixes with fuel vapor and/or a 

spray from a simulated leaking hydraulic, fuel or lubricating oil line, and is ignited by a spark or hot surface such as the bleed-air line. A fire can be stabilized anywhere when the fuel and air mixture is within 

its flammable limits and the local velocity is below the flame speed. The many obstructions in the flow 

formed by the nacelle frame, fuel lines, and miscellaneous equipment in the nacelle and on the engine 

core act as flame holders. The fire extinguishing agent is released into the nacelle near the air inlet. 

Depending upon the amount of agent released, the rate of agent addition, the presence of hot surfaces or 

electric arcs, and the location ofthe fire relative to the injection point, the fire may or may not be fully 

extinguished. An engine nacelle simulator, such as the one in Figure I, is useful for demonstrating that a 

particular agent or system can successfully control full-scale tires, but it is not suited for screening the 

suppression effectiveness of new compounds, for ranking alternative technologies, or for gaining understanding of fire suppression physics. 

* W. Leach, personal correspondence, 1998. 

Halon Options Technical Working Conference 24-26 April 2001 I55 
Figure I. Full-scale F-22 EngineNacelle Simulator at WPAFB [3]. 

Agent suppression effectiveness for liquid fuel fires such as those described in NFPA 2001 is determined 

in a laminar diffusion flame using a cup burner [3]. Measurements of suppression effectiveness using a 

counter-flow burner are preferred for revealing the detailed chemical pathways in the process [4]. In 

either of these burners the minimum concentration for suppression is found by increasing the agent flow 

slowly until a critical mole fraction is achieved in the oxidizer stream and flame extinction is observed. 

In practice, however, agents designed to replace CF,Br are discharged rapidly, not quasi-statically. Solid 

propellant gas generators, for example, typically discharge in 10 ms to 500 ms. A robust and repeatable 

means to evaluate the effectiveness of different formulations and application rates requires a non-conventional screening device. 

Hirst and Sutton [5] 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 dep. Hirst et al. [6] 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 fullscale tests done later [7]. 

Experiments by Hamins et al. [8], in cooperation with Walte~Kidde Aerospace, were conducted in a wind 

tunnel scaled down from the earlier work by Hint to examine the performance of HFG125 and HFC-227ea. 

Investigations at the Air Force Research Laboratory [9] as part of the Next Generation Program (NGP) 

sought to determine the detailed structure during suppression of a norrpremixed methanelair flame stabilized 

behind a step. The changing character of the flame with step height and air velocity was etamined, along 

with the amount of Halon 1301 required to suppress the flame as a function of the flow parameters and 

injection interval. 

A turbulent spray burner was designed in earlier work at NIST [IO] to simulate an engine nacelle spray 

fire resulting from a ruptured fuel or hydraulic fluid line. In this apparatus, the agent release and mixing 

processes were well controlled and the air flow was maintained constant with a sonic orifice. This 

156 Halon Options Technical Working Conference 24-26 April 2001 
arrangement allowed the agent to be discharged without disrupting the incoming air. The turbulent spray 

burner was used with both gaseous and powdered agents. Hamins et al. [8] redesigned the burner to 

include a heated disk in the center of the flow downstream of the fuel nozzle. They showed that the 

concentration of nitrogen necessary to extinguish a turbulent propane flame increased substantially with 

surface temperature. The same trend, but to a lesser degree, was observed with hydrofluorocarbons. 

Previous studies have demonstrated the effectiveness of SPGGs and their hybrids in full-scale fire 

suppression experiments [2,11]. Solid propellant gas generators undergo rapid solid-phase reactions 

producing inert gases, principally carbon dioxide, water vapor, and nitrogen, as well as particulates 

composed of inorganic salts. Each component individually behaves as a tire suppressant. For many 

applications, particulate use is unacceptable and the SPGG hardware is typically designed to filter 

particulate mass during deployment. Hybrid generators are particularly attractive for some special 

situations such as inhabited spaces or cold temperature applications [12]. The hybrids use the hot inert 

SPGG exhaust to vaporize and expel a secondary suppressant, typically a liquid, such as water or a low 

boiling point halocarbon, through a nozzle [I I]. 

PROGRAM OBJECTIVES 

The research summarized here was conducted over a three-year period and is reported fully in the final 

report [13]. The specific objectives put forth in the original proposal were to provide a well-characterized 

test fixture for screening the effectiveness of agents to suppress and prevent re-ignition of a turbulent, 

obstructed flame; to allow evaluation of the impact of transient agent delivery on flame extinguishment; 

and to provide a means to screen the effects of new and currently available agents 011 solid and liquid 

fuel surfaces. 

Full funding for the three-year period was to deliver the following products: 

Well-characterized bench-scale suppression screen for comparing the performance of gaseous 

agents and dispersed fluid mists in suppressing and preventing reignition of a turbulent, obstructed 

flame burning either gaseous or condensed fuels 

Documentation on the screen apparatus including detailed design information and experimental 

procedures, with round-robin testing among interested partners to ensure utility of the documentation 

Evaluation of the impact of transient agent delivery on flame extinguishment 

Development of a means to screen the effects of new agents on condensed (solid and liquid) fuel 

surfaces 

The focus of the research deviated a bit from what was originally proposed, in some instances due to 

budget reductions and in others due to discovery. 

Solid fuels were not examined because there was no evidence that they posed a greater threat than 

propane in this scenario. 

Formal round-robin was not established, but work conducted during the same period at the 

University of Maryland [ 141 and the Air Force Research Laboratory [9] helped to support the 

conclusions drawn in the current study. 

Design for the liquid mist agent delivery system was not finalized due to the complexity of the 

process, although the general direction for such a system and its likely capabilities were revealed. 

More emphasis was placed on the deployment of an SPGG suppression effectiveness screen. 

0 

llalon Options Technical Working Conference 24-26 April 2001 I51 
SUMMARY OF IMPORTANT FINDINGS 

The transient application, recirculating pool fire (TARPF) agent effectiveness screen developed in this 

research project features a propane fire stabilized behind an obstruction. The character of the flame and 

impact of the air flow, fuel flow, obstruction geometry, and rate of agent addition on the amount of material 

needed for suppression were evaluated for N?, CF,Br, and a solid propellant gas generator. The importance 

of the injection process on the flow field and the transport ofthe agent downstream was examined, and a 

simple mixing model was used to explain the observed trend of decreasing suppressant mole fraction with 

increasing injection duration, even for agents as different as CF,Br and N,. Direct numerical simulation of 

the suppression event was shown to predict successfully the quantity and rate ofN, required to extinguish 

the flame, based upon a published global reaction rate for premixed propane/air flame propagation. 

The final report [13] describes a comprehensive study utilizing experimentation, advanced numerical 

modeling, and analysis to develop and characterize a well-controlled fire suppression facility. Sample 

experimental results demonstrate the utility of the facility. Fabrication drawings and the operational 

procedures are detailed in the report. The facility provides the capability to test solid-propellant gas 

generators, allowing direct comparison of compressed and solid-propellant generated gases for the first 

time. The capability to test SPGGs under well-controlled laboratory conditions allows evaluation among 

different propellant formulations, particulate yields, and burning rates for various SPGG designs. Results 

showed that the effectiveness of the hybrid SPGG exceeded that of CF,Br and that its effectiveness is 

significantly greater than expected from the model prediction. Further research could lead to significant 

improvement of the SPGG, as well as traditional agents, through enhanced suppressant system design. 

Key findings can be summarized as the following: 

The minimum mole fraction of agent for suppression, normalized by the cup-burner value, 

correlates with [I - exp(- AUT)].', where At is the injection time interval and T characterizes the 

mixing time behind the obstacle in the flow. 

The general character of the flame and its extinction by a thermal gaseous agent is captured by a 

direct numerical simulation of the flow based upon single-step chemistry, and numerical 

experiments have corroborated the simple correlation of the experimental data for N,. 

The measured difference between the decrease in agent storage bottle pressure and the arrival of 

the agent at the fire highlight the importance of determining the agent concentration locally and the 

difficulty in relating changes in bottle pressure to actual mixing conditions. 

For the first time, both compressed and solid-propellant generated gases can be compared side by 

side, and the effect on performance of different formulations, particle loadings, and burning rates 

for various SPGG designs can be unambiguously discriminated. 

When the temperature of a hot surface downstream of the pool is above 800 "C, the flame, 

following suppression, will reignite and stabilize on the hot surface. At a temperature below 

800 "C, the number of reignitions approaches zero. This result is in contrast to when the hot 

surface is located between the stabilizing step and the fuel pool, in which a delayed reignition can 

be observed at temperatures as low as 400 OC [14]. 

The effectiveness of suppression with a liquid agent is dependent upon how well the droplets are 

entrained into the flame zone downstream of the stabilizing obstacle. 

Several other research areas require further investigation. These include the effect of the air flow on the 

steady-state extinction mole fraction of agent, the relationship between agent injection and its concentration history at the flame, and especially the observed differences in the normalized volume fractions of 

CF,Br, N,, and the SPGG for very short injection time intervals. 

158 Halon Options Technical Working Conference 24-26 April 2001 
ACKNOWLEDGMENTS 

This work was supported by the Department of Defense Next Generation Fire Suppression Technology 

Program (NGP), (co-funded) by the DoD Strategic Environmental Research and Development Program 

(SERDP). The authors would like to acknowledge the significant contributions of our NIST colleagues 

Michael Selepak, Michelle Donnelly, S. Rao Charagundla, Erik Johnsson, and John Widmann, and of 

Jose Torero of the University of Maryland. Atlantic Research Corporation provided the hybrid gas generators and assisted with the SPGG experiments. 

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