SCREENING METHODS FOR NEW FIRE SUPPRESSION TECHNOLOGIES

William L. Grosshandler, Jiann C. Yang, and Thomas G. Cleary

Building and Fire Research Laboratory

National Institute of Standards and Technology

Gaithersburg, Maryland 20899 U.S.A.

Introduction

The search for alternatives to halons for fire suppression applications has identified not only new

compounds (e.g., hydrofluorocarbons) which have physical properties similar to the

bromocWorofluorocarbon family, but also inert gaseous agents that are released from a solid state (e.g.,

solid propellant gas generators or SPGGs) and condensed phase agents that may be misted or generated

pyrotechnically. Industry is already investigating innovative ways that these multiple technologies can

be blended or hybridized to create an optimum fire fighting agent/release mechanism for specific

applications. The traditional cup burner method is unable to evaluate these not-in-kind replacement

systems. This paper examines two new concepts for testing liquid aerosol and SPGG fire suppression

technologies. The first concept is for a bench-scale suppression screen suitable to compare the ability

of dispersed fluids with differing chemical and physical properties to extinguish a laboratory flame; the

second is for a facility to test SPGG-based agents and release mechanisms. The following desirable

attributes have been kept foremost in mind during concept formulation:

• applicability - Results of bench-scale tests must transfer to pilot-scale simulators of actual field

situations.

• amenability to analysis - Initial and boundary conditions of the flame and flow system and

characteristics of the agent must be controlled to permit analysis of the facility, interpretation of

the results, and scientifically sound explanations of anomalous behavior.

• repeatability - Replicate tests must produce outcomes similar enough to minimize uncertainty

limits.

• flexibility - Different types of chemicals, aerosols and discharge principles must be accommodated in a consistent manner.

• operability - The apparatus and operating procedure must be such that data can be acquired

safely, efficiently and with a minimum of agent, enabling duplication of the facilities at other

government and industrial laboratories.

Dispersed Liquid Agent Fire Suppression Screening

Since the objective of this study is to design a bench-scale apparatus suitable for evaluating fire

, suppression efficiencies of new advanced liquid agents, it is imperative that a laboratory burner and a

liquid/aerosol delivery system be carefully selected. The rationale for the selection of these two major

components are discussed bellow.

Burner Counterflow diffusion flames have been used extensively to study relative effectiveness

of various gaseous and powder flame inhibitors (see e.g., Tsuji, 1982). There are basically two

configurations of counterflow diffusion flames: (1) a diffusion flame established between two opposed

gaseous jets, one being fuel and the other being oxidant, and (2) a diffusion flame formed in the forward

stagnation region of a porous spherical or cylindrical burner placed in a uniform oxidizer flow, with fuel
principle, may be superior to water. The amount of water required is estimated based on the assumption

that the limiting oxygen concentration to sustain combustion is approximately 15 % (by volume). In the

estimation, water is treated as an inert diluent vapor. The thermal effect due to water droplet evaporation

is not taken into account; hence, the calculation is considered very conservative.

The total volumetric flow (Q~ is equal to the duct cross-sectional area (L.W) times the average velocity

(V), which is also equal to the sum of the volumetric oxygen (Qo), nitrogen (QN) and water vapor (Qw)

flows. Using the burner dimensions and flow velocity mentioned above, and noting that Qo = 0.15 QtotQ/,

and QN = (0.79/0.21) Qo, then the required value for Qw can be calculated to be 0.447 Lis. Assuming

an ideal gas mixture, the water mass flow rate (mw) is then equal to 0.359 g/s. According to the design

by Takahashi et aI., (1994) the equivalent liquid water mass flow can be achieved by using a piezoelectric

droplet generator with 20 orifice openings, each with a diameter of 34 p.m.

Past studies with gaseous agents have shown the importance of uniform agent/air mixing and the velocity

gradient at the flame front on the extinction process. Aerosol uniformity measurements will be made

within the burner using either a PDPA (phase Doppler Particle Analyzer) or stroboscopic microphotography, and the air and fuel flows will be precisely monitored to ensure the above effects can be

separated from the physical/chemical effects of the different aerosols.

Operation The focus of the initial effort will be on fluids (pure or in mixtures) which can be

formed into well characterized aerosols at conditions close to room temperature and pressure. They will

be suspended in air streams at modest velocities and transported to the propane diffusion flame. A stable,

stagnation point flame will first be established in a steady flow of air, the desired droplets/aerosol will

be introduced at a fixed concentration, and the strain rate (2V/R, where V is the external air flow and R

is the radius of the burner), will be increased until the flame is blown off of the stagnation point. If a

stable turbulent wake flame is established, the agent concentration will be increased further until the flame

is totally extinguished. The maximum concentration of the suppressant as a function of strain rite at a

fixed fuel ejection velocity can then be determined.

Extinction experiments will be conducted with and without obstacles (e.g., uniform screens or cylinders)

in the air stream just ahead of the flame. This will further complicate the flow but may be necessary to

more closely emulate a cluttered environment. In order to characterize and calibrate the performance of

the burner, gaseous agents will also be used. This will allow comparisons of the performances of gaseous

agents with those of aerosols.

SPGG-Based Fire Suppression Screening

The test fixture envisioned for evaluating solid propellant gas generators and hybrid extinguishers

represents an intermediate-scale screening method. It differs from the Tsuji opposed flow flame burner

described above in that, not only is its physically larger, it is intended to screen fully functional

extinguishers.

Potential applications of SPGGs and hybrids include the following: engine nacelles and military aircraft

dry bays, locations not adversely affected by hot effluent such as paint spray booths, restaurant kitchen

ranges, total flooding in un-occupied spaces like aircraft cargo compartments, protection of machinery

with a directed discharge, etc. The fire environments can be broken into forced flows (e.g., engine

nacelles, spray booths, kitchen range ducting) and otherwise quiescent situations (e.g., cargo

compartments, directed discharges) In the forced flow scenario, the flow assists in the mixing and

transport of agent to the fire location, but it also carries the agent away from the fire. In the quiescent
environment, the mixing and transport of the agent to the fire zone depends almost entirely on the

momentum of the effluent with obstacles and wall boundaries impacting the flow.

Standards exist for prescribing an inert or chemically active agent in enclosures (National Fire Protection

Association, 1996) where design concentrations and holding times (the minimum time period the design

concentration must be maintained) are specified for different agents and fire scenarios. Two facets

separate typical SPGGs from the "clean agents" addressed in NFPA 2001: the elevated temperature and

the composition of the effluent. In addition, hybrids could contain solid or liquid particulates. These

factors pose problems regarding the use either of the cup-burner or of an explosion flask for obtaining

inerting concentration. A recognized equivalent measure of extinction and/or inerting concentration is

needed for total flooding design.

If the only application of the SPGG or hybrid system were a total flooding scenario, a suitable test fixture

could be a fire established in a small enclosure, but blocked from direct impact of the discharge. Scaling

based on the volume of the enclosure to be protected would provide the design criterion. However,

applications such as engine nacelle protection where an air flow is always present are quite different.

There is no accepted NFPA test for evaluating agent effectiveness for a fire established in a forced flow.

The NIST turbulent spray burner and the Walter Kidde Aerospace baffle stabilized pool fire configuration

(both described by Hamins et ai., 1995) are forced flow apparatuses that do provide measures of

effectiveness. With these systems, the effects of flow velocity, agent concentration, and discharge

duration can be explored. The results may be generalized to allow for comparisons to different scenarios.

Of these two facilities, the baffle-stabilized pool fire is more attractive because it appears to be more

difficult to extinguish. The mixing time scale is longer and the minimum agent concentration is higher

than for the turbulent spray flame.

A wind tunnel configuration is proposed to provide the forced flow and to facilitate the mixing of the

SPGG discharge. Figure 2 is a schematic. The SPGG is discharged into a plenum and mixed with

incoming air. The air will be provided from a compressed air bottle farm, or compressor at sufficient

delivery pressure so that the SPGG discharge, itself, does not effect the air flow. From the plenum, the

flow then moves into a duct that holds the test section. The duct will have a movable floor to control

the cross sectional area which will readily change the flow velocity at the test section for a given

volumetric flow. Temperature and axial velocity measurements will be made at a number of locations

in the duct. The duct will have windows at the test section location for visible observation of flame

extinction.

The baffle-stabilized pool fire will follow the design of Hirst and Sutton (1961) with some modifications.

In their design, the flame is stabilized in the recirculation zone in the wake of a bluff body (the baffle).

Such a flame will persist in that location at a free stream velocity much higher than the blow off velocity

of a boundary layer flame. The design proposed here is a rear-facing step configuration. In this case,

the flame is stabilized by the recirculation zone in the wake produced by the step. The step height will

be adjustable. The benefits of this design are: (1) the flow is less complicated, (2) fewer solid or liquid

particles will be lost since the forward stagnation zone is eliminated, and (3) it allows for the test section

to be narrower in the vertical direction which will reduce the duct blockage. Figure 3 is a schematic of

the test section. The step, between 5 mm and 25 mm high, is imbedded in an airfoil with an elliptical

nose that extends the width of the duct (about 300 mm). The forward base of the step is the location of

the pool, proposed to be about 120 mm square. The pool may actually be a flat sintered metal burner

with gaseous or liquid fuel fed at a specified rate. This would eliminate fuel being blown out of a pan,

and the heat release rate of the fire could be controlled. In addition, orientation could be changed without

fuel spillage.
Velocity & temperature

probes

• .- Exhaust

Window

x

x x

Plenum Compressed

air

SPGGor

distribution tube

Test section

Variable

cross-section duct

Figure 2. Schematic of the proposed SPGG wind tunnel test apparatus.

The test fixture will be put through a series of tests with inert compressed gases (such as nitrogen) with

tighter flow control to probe sensitivities to the operating parameters such as fire scenario (including fuel

and step height), velocity, duct cross-section, inert concentration and discharge duration. The results will

be used to characterize the dynamics of the recirculation/fire zone in terms of a characteristic mixing time

for flame extinction. Hamins et ai. (1995) proposed the following equation for extinction of spray flames

and baffle-stabilized pool fires:

(1)

where X"" is the agent concentration at extinction for very long injection time (the minimum agent

concentration), Xc is the agent concentration for a time duration of I::.t that puts the fire out, and Tf is the

characteristic mixing time for flame extinction. Each fire scenario will have a different characteristic time

which is strongly influenced be the step height, flow velocity, and duct cross-section. It may also be a

function of fuel and heat release rate.

For a SPGG, the number of moles and temperature of the effluent, and the discharge time are important

and can be tailored to some extent. While propellent formulations may be proprietary, the effluent most

likely will be specified by the manufacturers. Consider two "fictitious" SPGG canisters, each with a

propellant weight of 454 g. The effluent composition for the first is 60% N2, 30% CO2, and 10% water,

and for the second it is 80% N2, 10% CO2, and 10% water. Inerting volumetric concentrations for these

components rank as N2 > H20> CO2, The number of moles produced is 14.4 moles for the first case

and 16.0 moles for the second. The bum time for the first canister is 1 s and for the second is 1.5 s,

thus the molar flow is 14.4 molests for the first and 10.7 molests for the second. The temperature of the

effluent will affect the extinction process. Thus, the chemical constituents, mass flow, volumetric flow,

and effluent temperature will impact the extinction process of a test fire. The test fixture proposed will

account for these effects such that comparisons can be made on a common basis.
.

•

Burner

.-

Air

Flow Airfoil

Fire

Figure 3.

Duct Floor

SPGG test section.

Assume, now, that a number of canisters of each formulation are available to test, and each burns such

that the molar flow is constant over the bum time. For the first formulation, a number of tests are

performed and the critical air flow is found below which the flame will be extinguished during a canister

discharge. The characteristic mixing time (Tf) in Equation (1) is a function of velocity, and since its

functionality will be determined with the compressed gas experiments, it is known. The agent

concentration is known and discharge duration is fixed. Thus, X"" (the minimum agent concentration)

can be calculated. For the second canister, the molar flow is lower, but the discharge duration is longer.

The critical air flow below which the flame is extinguished is obtained after a number of discharges.

Again, given the velocity, the characteristic mixing time is obtained, and with the agent concentration and

discharge duration, the minimum agent concentration is calculated. For this simple case, it is expected

that the minimum agent concentration for the first canister is lower than the minimum agent concentration

for the second canister. If the canister discharge rate is not constant (which it is likely not to be),.~he

analysis is somewhat more complex.

Evaluations of SPGGs for directed discharge applications must proceed on an ad hoc basis. The nature

of the effluent flow is as important (or more so) than the chemical constituents. Proper discharge nozzle

design, which depends on the application, in addition to sufficient agent mass flow are required for such

cases. The test fixture proposed here will still be valuable in screening particular formulations for their

efficacy once they reach the fire zone.

Closure

There remains a risk that the aerosol and SPGG screening facilities developed in this research may be

incompatible with a particular agent or discharge design. Because the field application may be so

specialized or complex, there is also the risk that the results of the screen may not properly predict fullscale performance. These risks do not argue against the development of smaller-scale screens, but in fact
support them, since what is learned in the laboratory experiments can be applied towards the design of

meaningful intermediate-scale facilities and full-scale test programs.

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