NGP PROJECT 6E: “VERIFICATION OF PRINCIPLES” – REAL SCALE 

TEST APPROACH 

J. Michael Bennett 

Bennettech, LLC 

Hendersonville, TN 37075 USA, Tel: 937-367-5675; Fax 615-264-5990 

bennettech@comcast.net

 INTRODUCTION 

The purpose of the test program comprising Project 6-E of the Next Generation Fire Suppression 

Technology Program (NGP) is to (1) verify the new extinguishing agent distribution guidance

developed during the course of research comprising the NGP∗

 program, (2) verify the relative 

performance of chemicals as a function of their thermophysical properties and test fixture

conditions, and (3) identify any currently unexplained phenomena of the entire extinguishing 

process. These elements will be considered in the configuration and environmental conditions of 

a full-scale, or “real scale” engine nacelle, using a realistically sized test simulator designed to

recreate the conditions of operating aircraft engine nacelles, as well as generating realistic fire 

events with which to perform extinguishing experiments. As such, this testing provides a bridge 

to the laboratory and intermediate-scale agent screening devices devised and used in the duration 

of the NGP research program, to assess their ability to intelligently estimate the influence of real 

nacelle operating parameters on the relative merits of different extinguishing agent candidates, as 

useful, controllable and economical screening tools. 

It is understood that these bench-scale test apparati, such as cup, opposed flow and turbulent 

spray burners, as well as flow and agent condensation simulation devices, have the ability to 

control multiple parameters, exclude or isolate others, and create uniform flow, discharge and

mixing conditions when desired, often in a one or two dimensional fashion, to permit the

development of phenomena characterizations, general design criteria and repeatable 

experimental protocols. However, the “real-scale” environment does not afford such

controllable or simplified, standardized operating conditions, with complex three dimensional 

flows and transient mixing processes, and inherent statistical variances in flame and distribution 

characteristics, due to substantial and varying turbulence, and other process instabilities, most of 

which cannot be controlled even in a test environment. Similarly, physics-based computer

models of the extinguishing process have been developed or enhanced during the course of NGP 

research, incorporating its findings as they have been made available, and validated to date based 

upon results observed in a laboratory environment. The models are constructed so as to also 

permit the simulation of more complex geometries and flows associated with real-scale engine 

nacelle fires, by incorporating the knowledge derived to date. Therefore, both these models and 

the small and intermediate-scale screening apparati will be assessed as to their merits in 

predicting the behavior and performance of extinguishing agents and systems in real scale 

aircraft engine nacelle fires, under various operating conditions, as judged by their correlation to, 

and predictive abilities of the results observed from the real-scale tests. 

 

∗

 The mission of the NGP program is to identify, analyze and demonstrate fire extinguishing chemicals 

(“extinguishants”), technologies or design techniques that can match the efficiencies of Halon 1301 as nearly as 

possible. The program is now focused on engine nacelle fire extinguishing applications. 

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In concert with the general purposes of this task in support of the NGP program, the results from

the preceding research projects of the NGP have also resulted in the observation and

identification of several general principles of the extinguishing process that dictate the

performance of a fire extinguishing agent and system under various conditions. Representatives

of the many investigators and contributors to the NGP program during its execution have 

reviewed these noted principles, and have condensed them into a streamlined set of the most 

significant and profound examples. The interest in verifying these principles as well will also 

influence the structure and content of the test series, with the data also evaluated in light of its 

abeyance with these principles. 

GENERAL PRINCIPLES OF INTEREST TO EVALUATE IN “REAL-SCALE” 

The following is a condensed set of some of the most pertinent general principles of the 

extinguishing process that have been observed and noted as result of NGP research, to be 

verified or modified as a result of planned real-scale engine nacelle experiments: 

Principle 1: Extinguishants with boiling points higher than the local engine nacelle airflow 

and surface temperatures do not disperse sufficiently in time for engine nacelle fire

mitigation, whereas those with boiling points below the prevailing temperature can 

disperse sufficiently if properly applied. 

Rationale: Extinguishants with boiling points above the prevailing ambient temperature in the 

nacelle after discharge will not immediately flash vaporize, relying on the lengthy process of

evaporation to produce sufficient quantities of vapor to extinguish fires before they are entrained

by the ventilation air out of the nacelle outlets. The resulting liquid jet discharged will then

break up into an array of droplets, the size distribution based upon the liquid physical properties 

and flow characteristics. The potential for these droplets to reach the remote fire location,

having been successfully entrained into the air flow streamlines versus making contact with the 

nacelle structure and component surfaces (hence condensing on such surfaces and preventing

their fire mitigation contribution) is dependent upon the resultant droplet sizes and their 

momentum in the particular flow field. Additional droplet evaporation of the liquid reaching the

fire source may occur by entrainment into the flame region, with the degree of evaporation then 

dictated by its residence time in the flame reaction zone. These complexities are largely avoided 

by the flash vaporization of all or a large portion of liquid of low boiling-point extinguishants 

that quickly assume gaseous flow properties, and are generally more “forgiving” of nacelle 

structure and component (commonly known as “clutter”) variations upstream from the fire, 

providing a more uniform flow due to expansion and diffusion, and assuring a greater degree of 

extinguishant vapor present for entrainment near the flame. However, many candidate “next 

generation” replacements for Halons exhibit boiling points higher than the lowest nacelle 

temperatures observed in actual operation. Therefore, the study of the effects of liquid droplet 

evaporation, transport and condensation through realistic nacelle flow geometries, as a function 

of air velocity and other factors, is warranted to determine if special discharge accommodations

or the application of excess extinguishant mass is sufficient to overcome transport and 

entrainment inefficiencies in some cases, or if certain candidates and conditions preclude their 

effective use for nacelle applications. 

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Principle 2: The measured “cup burner” extinguishing concentration of a candidate 

extinguishant is an effective predictor of the critical flame suppression concentrations 

required within the flame recirculation zone of an engine nacelle fire. 

Rationale: Bench-top “cup burner” devices meter precise flow rates of vaporized extinguishants 

at set volumetric concentrations in air into a vertical glass chamber, in which a small cup of 

liquid fuel is placed and lit. The volumetric concentration is then gradually increased until 

extinguishment is observed. This test has become an international standard in determining the 

critical concentration required to achieve the extinguishment of flames with various 

extinguishants. Due to the steady state nature of extinguishant introduction into the region 

surrounding the flame, it is understood to provide an in essence infinite residence time of 

extinguishant in contact with a flame to extract heat or inhibit flame-sustaining chemical

reactions. The results from this apparatus are highly repeatable by various researchers, due in

part to eliminating the key transport issues associated with extinguishment. 

In a realistic engine nacelle fire, for example, the air flow over obstacles creates air shearing

effects behind obstructions and recirculation vortices. These recirculation zones are locally

stable flow regions that can support and sustain flames that are created due to the contact of

released fuels and ignition sources. Air is entrained from flow over the obstacle to mix with and

feed the flame in a sustained manner. It is difficult to apply extinguishants to such hidden, stable 

flames for several reasons. First, only a portion of the extinguishant applied over the obstacle is 

effectively entrained into the recirculation zone, particularly if it comprises some highmomentum droplets to some extent, due to the momentum of the flow and its tendency to follow 

its original direction of flow due to inertia. Secondly, the extinguishant will only reside within 

the recirculation zone for a limited period of time, before it is ejected and released downstream 

of the fire. It is theorized that a critical volumetric concentration of extinguishant, equivalent to 

the cup burner concentration, must be entrained into the flame zone and reside for some

minimum period of time, mixing with the reacting flow until extinguishment is completed, to 

correlate to the infinite residence times associated with cup burner data. Typical engine nacelle

fire extinguishing systems exploit a high rate of discharge to minimize dilution by the incoming 

ventilation airflow, and to minimize the overall mass of extinguishant discharged to optimize the 

required system weight. This approach generates a transient concentration profile that 

approximates a more nearly Gaussian profile in time (with a peak concentration at some point in 

the middle of the discharge period), with significant variations of the profile under various 

system and operational conditions, and at various locations in the nacelle. The theory proposes 

that the time period that the critical cup burner concentration (for the extinguishant in question)

is held directly within the recirculation zone of the flame at a minimum as observed in the local 

transient concentration profile will dictate the success or failure in extinguishment. 

 

Principle 3: The “quality” or degree of uniformity and effective transport of various

extinguishants can be sufficiently predicted using CFD codes upgraded during the research 

of the NGP program. 

3
Rationale: Numerous investigators and contributors to the NGP research program have devised 

methodologies and algorithms, or fundamental data to support their development, that have been 

used by specialists in upgrading existing computer fluid dynamics/heat transfer codes to 

incorporate such knowledge for the process of extinguishant flow and dispersion after release. 

New data and models of droplet development and flow through clutter, for example, have been 

incorporated into these models. A nacelle structural model using the codes will be made of the 

test fixture, with selected operational scenarios planned for tests will be recreated using the code, 

with the results both used as a predictive tool to assist in testing, and with the test results also 

providing a means of validating the capabilities of the codes to date, or suggesting further

improvements based upon observed test results. 

Principle 4: Increasing the number of local extinguishant discharge sites increases the 

dispersion “quality” in terms of balanced concentration profiles, up to some practical

limits. 

Rationale: By providing multiple, presumably dispersed remote sites for which to independently 

discharge extinguishant into the nacelle, the discharged extinguishant at each site has a shorter

distance and volume to spread and mix with air before it encounters expanding extinguishant 

mixtures from adjacent discharge regions. Therefore, there is less distance to establish gradients

in concentration, and multiple points of origin assure minimum variance in concentration within

smaller regions. Additionally, this time period to reach relative homogeneity as the expanding 

extinguishant vapor/air mixtures meet at the boundaries of adjacent discharge regions occurs

much faster due to the shorter discharge paths required, further reducing the relative effects of 

ventilation airflow dilution as it sweeps through the nacelle and carries extinguishant from the 

nacelle through its outlets. Therefore, relative concentration uniformity is reached on a time

scale that can be relatively fast compared to the air exchange rate due to incoming ventilation

airflow, which can potentially result in extinguishment anywhere a fire is present before airflow

dilution degrades these conditions. However, as the number of remote discharge sites is further

increased, diminishing returns are observed, as the relative mixing efficiency improvements 

become more modest as the discharge periods required for mixing uniformity become

progressively shorter. Since an increased number of discharge sites results in more discharge 

tubing, brackets and possibly nozzles, the additional increase in aircraft weight suggests the

existence of an optimal number of release sites for a given nacelle configuration with respect to

weight optimization. 

Principle 5: The extinguishant concentration established in the recirculation zone is a 

function of the injection time and mixing time behind the obstacle (“clutter”) that stabilizes 

the flame. 

Rationale: Prior research by Takahashi, Hamins, Hewson and others in the NGP program, as 

well as other researchers, have proposed and demonstrated that the relative quantity of 

extinguishant entrained from the free stream over a bluff body that anchors a flame into the

flame’s recirculation zone is a function of the ratio of the free stream velocity over the clutter to

the height of the clutter itself, also with influences of the injection time on the total time the

entrained extinguishant resides in the recirculation zone. These conditions and structures will 

determine if the extinguishant mixed in the free air stream is sufficiently entrained into the

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recirculation zone, and held for a sufficient time with at least cup burner concentrations

sufficient to extinguish the flame. 

Principle 6: The observed behavior by several NGP researchers of extinguishant passage 

through clutter of various configurations will be verified under full scale, three dimensional 

conditions. 

Rationale: Prior NGP research efforts have characterized the degree of flow interruption, 

pressure drop and re-direction through clutter of various idealized configurations, including any 

condensation of liquid portions of mixed-phase extinguishant flows on the clutter arrays. Most 

of these studies were performed on idealized, two dimensional clutter representations, and 

computer models of this behavior were also derived. The complex three-dimensional flows and 

clutter arrangements in realistic nacelles may challenge these simplified representations. 

Principle 7: Assuming that a mix of extinguishant vapor and liquid aerosol reach the flame 

reaction zone, the sufficient vaporization of liquid particles to enhance extinguishment will 

be dependent upon the aerosol particle sizes, the liquid’s heat transfer characteristics,

residence time in the recirculation zone, and other related parameters. 

Rationale: If some mixture of liquid aerosol particles is interspersed with the extinguishant 

vapor as it enters the recirculation zone of the flame, the ability of the particles to evaporate and

increase the extinguishant vapor loading in the flame to facilitate extinguishment is dependent 

upon its vaporization characteristics and residence time in the flame. The evaporation rate is 

based upon the common heat transfer properties known for these extinguishants, and the droplet 

diameters (or characterization of their distribution). 

Principle 8: The effectiveness of bromine in extinguishing flames, at least on a per unit 

mass basis, is independent of the type of molecule in which it is bound. 

Rationale: It is thought that the dominant effects observed by liberated bromine atoms in 

inhibiting self-sustaining flame reactions far supercede secondary effects of other species and 

radicals formed by the breakup of the extinguishant molecule itself. As such, for brominated, 

chemically active extinguishants, they serve in essence as a bromine delivery mechanism. Their

efficiency would then be dictated by the relative mass of bromine present to the overall mass of

the molecule (this presumes that the molecule will be sufficiently disassociated to fully release

all its bromine content in the flame). This general analogy greatly simplifies the search for some

chemically-active extinguishant alternatives, if this observation and hypothesis can be shown to

be verified by full-scale tests. 

Principle 9: Hybrid gas generator fire extinguishers (HFEs) are a more effective way of

suppressing fires when using extinguishants with high boiling points. 

Rationale: Many of the most promising candidate extinguishants identified by NGP research, in 

terms of very low required cup burner concentration values, feature very high boiling points, in 

comparison to the Halon 1301 currently used, as well as many of the first-generation Halon 

replacements. It is anticipated that much of this high efficiency will be lost due to condensation 

5
on nacelle clutter elements prior to reaching the flame zone, or to difficulty in entraining into the 

recirculation zone. This difficulty may be alleviated by applying the heating and propulsive 

power of gas generator extinguishers. These devices, which pyrotechnically convert solid 

materials into gaseous reaction products with the generation of much heat within fractions of a 

second, function similarly to automobile airbag inflators and similar devices. They have been 

shown in recent years to be highly effective in their own right in generating copious quantities of 

inert gas in a weight and space efficient manner. One derivative of this approach is to use a

small gas generator cartridge to heat, vaporize and propel highly efficient extinguishant fluids. 

Such a device can rapidly heat a high boiling point liquid to discharge it as a superheated gas, 

with the inherent flow effectiveness associated with lower boiling point extinguishants. 

However, since the boiling point of these extinguishants will still be higher than the nacelle and

air flows of interest, the potential exists for rapid cooling and condensation before the expanding, 

pre-heated gas reaches the fire zone. The attempt to quantify these competing effects under 

realistic nacelle geometries and operating conditions has not been performed in a controlled 

manner to date, and is of interest to assess this means as a way of exploiting these promising 

candidates. Any efficiencies observed (and resultant system size reductions as well) will be 

compared to increases in system complexity and size when incorporating a gas generator unit 

with a separate extinguishant chamber. The ability to virtually eliminate the large vapor space 

above the liquid in the bottle, since the extinguishant’s vapor pressure will be small and a large

space for compressed nitrogen gas will be unnecessary (the pressurization being alternatively 

supplied by the small solid propellent gas generator), can result in further system size reduction. 

Principle 10: The suppression effectiveness of a non-catalytic extinguishant in a full-scale 

engine nacelle is determined by its ability to absorb heat as it enters the flame zone. 

Rationale: Studies conducted by NGP researchers Sheinson and Pitts on the importance of 

thermal effects in simple flame systems indicate that the key property of a non-catalytic 

extinguishant is its ability to absorb heat starting upstream of the flame zone. The heat absorbed

may be sensible, or it may involve a phase change or decomposition. The degree to which the

latent heat can be used to decrease the amount of extinguishant necessary to suppress a fire in a 

full-scale engine nacelle needs to be verified in real-scale conditions, as seen in intermediatescale experimental apparati. 

 

AIRCRAFT ENGINE NACELLE TEST FACILITY (AENTF) FEATURES AND 

CAPABILITIES 

FACILITY SPECIFICATIONS 

The Aircraft Engine Nacelle Test Facility (AENTF) at Wright-Patterson Air Force Base was

constructed expressly to realistically recreate the environment experienced within a wide range 

of aircraft engine nacelles, with the capability to conduct repeated, full-scale nacelle fire tests 

under those conditions. An overall diagram of the key mechanisms of the facility is shown in 

Figure 1. 

 

6
Figure 1. AENTF Primary Mechanical Components. 

A primary feature of the facility is the capability to recreate the airflow conditions within the

nacelle volume, with respect to both airflow rates and temperatures. An atmospheric blower and 

associated high-pressure bottle farm is used to provide up to 11.0 lbs. per second of airflow, 

which is more than adequate for the types of aviation platforms currently in operation. An 

industrial chiller is used to chill the rapidly flowing air, which subsequent reheats due to 

insulation limitations between the chiller and the nacelle section, resulting in a nacelle internal 

temperature of -30 F (-34.4 C) or less. A remote fuel supply cart is used supply heated 

flammable fluids, such as JP-8 jet fuel or various aviation hydraulic fluids, preheated and 

pressurized to their appropriate operating values before being introduced into the nacelle section.

A separate extinguisher unit is used that permits heating or cooling of the extinguisher contents, 

with cooling provided by an immersion bath of dry ice, regulated by a heat tape wrap, to a

container temperature of down to -65 F (-53.9 C) for cold temperature tests. A remote control 

room is used to monitor internal nacelle conditions of airflow rate, temperature and pressure, as 

well as fuel and extinguishant conditions, before and during each test. The operation of the fuel 

release, airflow control, fire and extinguisher initiation, and shutdown procedures are all 

controlled by automated, computerized controls at this site, and video monitoring. 

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“Generic” Real-Scale Nacelle Test Fixture 

The test fixture recommended and planned for use in this test series is a “generic”,

reconfigurable engine fixture nacelle previously used in the National Halon Replacement for

Aviation, back in the mid-90s, as well as more recent Halon replacement testing for the U.S. 

Army. It features the capability for extensive reconfiguration to replicate a variety of engine 

nacelle configurations and operating conditions. The outer nacelle case of the test fixture is a 

cylinder 4 feet in diameter, and fifteen feet in length, as an effective test section. For this

program, the three foot diameter internal cylindrical simulated engine core section is planned for 

use. Rather than changing core diameters and the “clearance” area between the nacelle and 

engine core, the airflow patterns (including local recirculation) will be changed by changes in the

clutter (rib) height. The outer nacelle has ports in two locations to mount fuel nozzles and spark 

ignitors to produce two downstream fire locations. The entire nacelle assembly can be rotated to 

represent “top”, “side” or “bottom” fire locations. Similarly, multiple ports for the fire 

extinguisher assembly are present downstream to effectively shorten or lengthen the effective 

test section, as well as “top”, “side” and “bottom” discharge ports, where the extinguishant can

be discharged in-line, 90 degrees off center, or 180 degrees off center from the site of the fire, to 

evaluate the ability of the extinguishant to wrap around the nacelle volume to remote sites before 

it is exhausted from the nacelle. Annular ribs can also be mounted on the inside of the outer

nacelle and on the inner core of heights varying from one inch to three inches, to simulate 

structural ribs, supports and nacelle components (they alternate in frequency, from the nacelle 

surface extending down, to the engine core extending up, every 12 inches downstream). They 

also serve to realistically block the flow of extinguishant and to serve as “bluff bodies” that 

shield the base of the fire and create recirculation zones that make fires stable and resistant to

blowout. In the flame region, ribs parallel to the flow are also present to simulate clutter with 

surfaces that extend parallel to the flow, and to realistically inhibit the lateral “wrapping” of the 

extinguishant flow around the engine core surface, requiring entrainment over these lengthwise 

obstructions to enter the flame’s recirculation zone. A photograph of this test fixture (installed) 

is shown in Figure 2. 

 

Figure 2. Universal Test Fixture Installed in Facility. 

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Figure 3 is a close up illustration of the region of the fixture near the origin of the fire site, and 

the configuration of components in proximity (dimensions are in inches). 

 

F U EL 

N O ZZLE 

P R O P A N E IG N IT E R

T C

H E A TER P L A TE N

C L UTTE R

R IB

3 .3 9 .4 

3 .5 

11.0 

C L UTTE R 

T U BE 

8 .1 

Figure 3. Drawing of Close Up of Fire Site, Nearby Components in Nacelle. 

DESIGN OF EXPERIMENTS METHODOLOGY 

To collect sufficient data while considering all the potential physical parameters (or “factors”) of 

interest, within the cost and schedule constraints of the program, requires employing the use of 

orthogonal matrices in a Design of Experiments (DOX) approach, as was used in the 

aforementioned National Halon Replacement for Aviation. This process can sometimes be used 

most efficiently by separating all the parameters of interest into one or more independent groups

of dependent factors, each with their own customized experimental matrices, to evaluate the 

most factors and interactions of interest within these limits. Splitting a large matrix into submatrices is best accomplished when a global process can be conceptualized as two or more

separate and independent sub-processes, where unique factors are of interest to each (although 

there can be some similarities between the two). This acknowledges that due to the separate 

issues and factors of each sub-process, interactions that extend between factors in different subprocesses are often irrelevant. This permits the scaling of the number of factors for each subprocess to a minimum, with smaller, more efficient matrices with the missing cross-matrix

interactions not of interest. Since the matrices are independent, it permits the selection of a 

different critical response variable for each matrix of a sub-process. If simultaneous

measurements are made of multiple response variables of interest for the individual runs in a 

given matrix (such as observation of mass required to extinguish, required peak concentration or 

duration of residence time), which is possible with each response variable having its own 

independent analysis from the same set of data, then it is often useful to couple the two or more

matrices with at least one common response variable to sustain a global analysis, particularly if

the data should suggest a design and settings of factors in both to optimize both sub-processes 

into an optimal global process. 

This special circumstance appears to be advantageous for the topic of interest in this test 

program. It can be visualized that the extinguishment process comprises two relatively 

9
independent sub-processes – the sub-process of discharging the extinguishant in an appropriate 

quantity and manner uniformly throughout the nacelle in an efficient manner, and the subprocess of determining what quantity and configuration of extinguishant concentration is needed 

to extinguish fires locally at the site of the fire zone, to determine the critical conditions required 

for uniform replication in the nacelle in the first sub-process mentioned. Not only do these two 

sub-processes have different factors that control each process (with some in common), but they

have different required instrumentation needs, and even differ in their need for the presence of 

fire or not! These unique test setup differences suggest further test operation optimization by 

segregating their setup into independent sub-matrices. This optimization is particularly required 

to accommodate the additional number of tests required for each “run” (test configuration and set

of conditions) for replications and the number of tests using varying quantities of extinguishant, 

to determine the threshold quantity required for extinguishment as determined using a preestablished “bracketing” procedure. This procedure, refined in earlier engine nacelle fire test

programs, typically requires approximately twenty tests for each run to confirm five successful 

extinguishments with an approximate 90% degree of confidence, and within a 10% weight

tolerance band, of a final extinguishment mass threshold value for each run. 

 

TEST APPROACH, CONFIGURATION AND PROCEDURE 

Phase I – Local Conditions in Fire Zone Required for Extinguishment 

Purpose: The purpose of this test phase is to determine how factors of the local fire zone 

configuration, operating conditions and extinguishant discharge conditions and composition 

influence the conditions required within the recirculation zone of the fire to result in 

extinguishment. 

Response Variable(s): The response variable to be measured is the minimum duration that the 

cup burner concentration is held in the recirculation zone where the flame is present at the 

threshold of extinguishment, in accordance with the initial hypothesis. The area under the 

concentration-time curve will also be tabulated, if it is observed that changes in the discharge 

rate and concentration-time profile change the required residence time of the extinguishant at cup 

burner concentration, under otherwise identical conditions. 

Factors of Interest and Settings:

(1) Airflow Rate: 1.0 lbm/sec and 3.0 lbm/sec. (local velocity measurements can be made near

the flame region, if needed) 

(2) Clutter (rib) Height: 1.0 inch high and 3.0 inches high 

(3) Fire Type: Spray fire or pool fire 

(4) Relative Air Temperature to Extinguishant Boiling Point, or Revised Temperature 

Difference (note: The aspect of whether the ventilation air temperature (and extinguishant 

storage bottle temperature) is some minimal value either above or below the extinguishant’s

boiling point should control the extent of flash vaporization and additional evaporation during

transport; therefore, rather than keep the air flow temperature and extinguishant boiling points as 

independent factors (which would have differing relative effects on extinguishants with differing 

boiling points if the air temperature settings are kept as uniform), a combined relative air 

temperature (“temperature delta”) of the air temperature to the extinguishant boiling point will be 

considered as a factor. A setting both above and below the boiling point of some set amount will 

10
be used, with the absolute value dependent upon the boiling point of the extinguishant tested.): -

10 C below the extinguishant boiling point, and +60 C above the extinguishant boiling point. 

(5) Presence of Chemical Reactivity and Inhibition (note: chemical reactivity and inhibition in a

flame, presumably by scavenging hydroxyl radicals and other key catalysts that sustain 

combustion reactions, is influenced by different mechanisms and conditions than simple reaction

zone cooling until reaction rates drop below sustainability, as with simple thermal

extinguishants; hence an interest in studying the affects of the various factors on chemicallyactive extinguishant efficiencies versus non-chemically active candidates): a chemically-active 

and non-chemically active extinguishant candidate - CF3I and HFC-227 respectively. 

(6) Extinguishant Flow Rate: 3.0 lbm/sec (-1) and 10.0 lbm/sec 

 An L-16 (16 run configuration) test matrix is proposed for these six factors at two levels. 

Instrumentation: 

Since the focus of this test phase is to study the detailed mechanisms and processes within the 

flame region itself, it is proposed to install an array of Halonyzer concentration measurement 

probes within the region of the flame, to characterize the local concentration gradients within this 

region, and to identify any local inhomogeneities that might exist. Figure 4 is an illustration of

an anticipated layout of these Halonyzer probes. They function by siphoning a local sample of 

the atmosphere in the proximity of the probe opening, heating and vaporizing the mixture, and 

measuring densities of the mixture and comparing to calibrated mixtures of the extinguishant in

question and air. This device is typically used in sets of twelve probes, and when such probes 

are placed in a dispersed pattern throughout the nacelle, they serve as an FAA-certified method 

of verifying a uniform concentration of discharged extinguishant in nacelle systems to confirm 

the design of a sufficiently balanced discharge system. In this series three probes will be placed 

just above the rib shielding the flame, half the distance between the top of the rib and the top of

the nacelle, to measure the free stream concentration of extinguishant as it enters the flame

region. One probe will be mounted in-line with the fuel nozzle and the presumed base of the 

flame, and one probe on either side, approximately one rib height apart, to measure 

inhomogeneities in the mixture as it flows past the rib. Three similarly spaced probes will be

placed to sample near the nacelle core, behind but as close to the rib as possible and yet be just 

downstream of the fuel nozzle, to be in a region that is normally immersed in the flame and in 

the recirculation zone. The adjacent probes are also intended to measure lateral inhomogeneities 

in the concentration within the recirculation zone. A similar trio of probes will be mounted 

further aft of this set but still within the expected recirculation zone, to measure any lengthwise 

inhomogeneities. Next to the center probe behind the rib will be two additional probes on both

sides, facing laterally (to the left and right). Since it is uncertain how sensitive these probes and

their measurements are to the direction in which they are oriented (it should depend upon the 

relative momentum of the flow and local pressure in comparison to the negative pressure suction 

of the probe), these probes will be better able to directly measure lateral flows resulting from 

helical flows that sweep tangentially into the flame region, versus over the upstream rib. Any 

measurement difference between these probes and the upstream-facing central probe will suggest 

an orientation sensitivity that must be accounted for. A twelfth probe will measure just outside 

the discharge port of the extinguisher unit, to compare its timing and concentration patterns to 

that measured over the downstream rib. A fiber optic probe may also measure sprays behind the 

rib. 

11
 Downstream Airflow Direction 

 Free Stream Probes Over Rib 

 Probes Behind Clutter 

 Lateral Probe Site 

Recirculation Zone/Flame Region Extinguisher 

Rib Probe 

 

Figure 4. Halonyzer Probe Mounting in Flame Region. 

A pitot tube can be used above the downstream rib to measure a local velocity value. 

Thermocouples will be used to measure the temperature profile in the recirculation zone as well 

during the extinguishment process. Pressure readings inside the extinguisher will not only 

measure the steady state conditions to assure proper conditions for the test, but also transient 

pressure drops during discharge that indicate the extent of evacuation and mass flow rate. 

Approach: 

1. A pan will be used to hold the fuel for the pool fires, residing in the bottom of the nacelle.

The pan elevation is lower than the height of the ribs, which serve as a bluff body to stabilize a 

flame on the fuel pan pool. An illustration of the pan, seated in the nacelle, is shown in Figure 5. 

Engine Core 

Nacelle Rib

9.7 in.

 

Figure 5. Pool Fire Pan in Nacelle.

2. Following the active fire tests, the extinguisher mass charges at the threshold of 

extinguishment for each run will be repeated in a no-fire condition, to collect Halonyzer trace 

12
data for those conditions, using the previously described probe matrix. If possible, one 

Halonyzer probe may be configured to collect liquid entrained in the probe, at the point just

exterior to the nacelle, before it enters a heated section. Three repeats will be done for each run, 

and the data averaged to minimize test instabilities. 

Analysis:

DOX Approach – The test data will be analyzed using ANOVA (analysis of variance) and other

established protocols for orthogonal arrays. The data will first be analyzed with response to the 

duration in the fire zone that the cup burner concentration is retained, as a minimum. If this is

found to be sensitive to the mass flow rate (and hence the profile of the concentration-time

curve), then the data will also be analyzed with respect to the area under the concentration-time

curve to see if this criteria eliminates the variance with the mass flow rate, or some other similar

criteria. Note that other factors may also influence this required residence time, and will be

noted in the analysis, as well as factors that do not offer appreciable or statistically significant 

sources of variance. 

Modeling – The test nacelle will be modeled in VULCAN, and the same conditions run to

determine if agent concentration profiles, and the gross mass capacities that produce local cup 

burner residence times which result in extinguishment, are matched by the code, for selected runs

of the test matrix. Specific probe readings that suggest local inhomogeneities, or significant

lateral entrainment, will also be useful in understanding the realism of the model. 

Analysis - Other computational analysis will be performed on the fundamental conditions

observed for extinguishment, based upon the key dimensionless parameters and other criterion 

established earlier in the NGP program. The predicted amount entrained into the recirculation 

zone, for example, for a given rib height and air flow rate can be compared to differences in the 

concentration traces of the probes above the rib in the free stream versus that immersed in the 

recirculation zone (providing data to assess the mixing time for the clutter, which may correlate

to the Strouhal number). If the recirculation zone is shown to be inhomogeneous, then some

averaging technique or representative equivalent may need to be derived and compared to the

physical models that presume homogeneous conditions. The effects of several key cited 

dimensionless numbers will be able to be estimated using calculations with the actual test data.

These could include the ratios of the rate of extinguishant mass injection to the air mass flow

rate, and the ratio of the product of the rate of extinguishant mass injection and the injection 

period, divided by the total mass of air and extinguishant in the nacelle at any one time. 

Phase II – Assessment of Extinguishant Dispersion Optimization 

Purpose: The purpose of this test phase is to determine how factors pertaining to an engine 

nacelle’s operating conditions and extinguishant system overall design parameters control the

uniformity of the extinguishant discharge characteristics, as evidenced by the transient 

extinguishant concentration profiles measured simultaneously throughout the nacelle. Another 

purpose is to determine the system capacity and design requirements necessary to provide a 

sufficient supply of extinguishant to any location where a fire might be present, and of a duration 

deemed sufficient for fires experienced under those operating conditions. 

Response Variable(s): The first response variable to be measured is the average and standard 

deviation of the extinguishant concentration duration at cup burner values for all twelve 

measured remote locations in the engine nacelle (or a substituted parameter such as area under 

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the concentration-time curve if so determined from Phase I), for a given mass of extinguishant. 

Four different extinguishant mass charges will be tested for each run, sufficiently high such that 

the lowest concentration measured of any of the probes is sufficient to extinguish all the fires

presented by the conditions in all of the runs. A curve fit procedure will be performed for the 

four points in each run to determine a relationship between the mass and the average and 

standard deviation for the probes, and the mass at which all probes meet the minimum 

concentration for each run. Another response variable will be the change in concentration from 

one location to the different adjacent probe locations under cold conditions in comparison to the 

hot air flow conditions and otherwise identical configurations, to indirectly estimate the amount 

of extinguishant lost due to condensation in the transport from one site to the next. 

Factors of Interest:

(1) Airflow Rate: 1.0 lbm/sec and 3.0 lbm/sec. 

(2) Clutter (rib) Height: 1.0 inch high and 3.0 inches high 

(3) Number of Injection Sites of Extinguishant: One setting will involve the use of a single

extinguisher and discharge port, in the side of the nacelle as in the Phase I testing. The other 

setting will entail the use of two independent extinguishers (although fired simultaneously), each

directed through a low-flow restriction T-section and tubing conduit to two opposite ports in the 

nacelle (if two opposing ports do not currently exist in the test fixture, the T-section may be 

mounted inside the nacelle). The layout for this variant is shown in Figure 6. 

Distribution Tubes

Extinguishers 

Distribution Tubes

Discharge Ports 

Nacelle

Figure 6. Layout for Multi-Port Discharge. 

(4) Relative Air Temperature to Extinguishant Boiling Point, or Revised Temperature 

Difference: -10 C below the extinguishant boiling point, and +60 C above the extinguishant 

boiling point. 

(5) Nozzle Exploitation (Note: Nozzles can be used to break up the liquid flow into much smaller 

droplets, of a controlled diameter distribution. A smaller droplet configuration promotes faster 

14
evaporation by means of an increased surface area interface with the ambient area through which

evaporation can take place at a faster rate. A smaller droplet will also completely evaporate to

pure gas state much quicker. These traits are thought to be significant enhancements at cold 

operating temperatures, when flash vaporization is not possible, and the faster evaporation can

continue until the surrounding air reaches the vapor pressure associated with that extinguishant at

that temperature (if it ever does), with a much better chance of achieving this state before it is 

transported out of the nacelle.): The settings will be either the presence or lack of nozzles. 

(6) Extinguishant Flow Rate: Currently flow rates are planned of 3.0 lbm/sec and 10.0 lbm/sec. 

A 16 run, L-16 test matrix is also proposed for this phase, for the six factors at two levels. 

Instrumentation: 

For this series of tests the Halonyzer probes will be spread out into twelve separate “zones” to 

measure the uniformity of concentration in the nacelle during and after discharge. Each of three 

downstream positions will be split up into four quadrants, thus comprising twelve separate zones 

of the nacelle. The layout of the zones is illustrated in Figure 7. 

12

11

10

9

8

7

6

5

3

2

4

1

Figure 7. Nacelle Concentration Measurement Quadrants (numbered at upstream edge of 

each zone). 

The same measurement approach will be used within the extinguishers as with Phase I, including 

pressure transducers in the extinguishers to estimate transient flow rates. 

Approach: 

It is planned to use HFC-227ea as the representative extinguishant – by adjusting the relative 

temperatures to its boiling point, its dispersion characteristics should be representative of all 

clean extinguishants. 

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Analysis:

DOX Approach – The test data will be analyzed using ANOVA (analysis of variance) and other 

established protocols for orthogonal arrays. The data will first be analyzed with response to the 

average and standard deviation of the extinguishant concentration duration at cup burner values 

for all twelve measured remote locations in the engine nacelle (or a substituted parameter such as

area under the concentration-time curve if so determined from Phase I), for a given mass of 

extinguishant for all runs. An analysis will also be performed using the curves developed using 

the four mass values for each run that define the relationship between the mass and the average 

and standard deviation for the probes, and the mass at which all probes meet the minimum 

extinguishing concentration, for each run. The analysis of multiple discharge mass levels will 

reveal how the variability in average and standard deviation with factor changes varies according 

to the mass of the extinguishant charge. The determination of which factors influence these 

response variables, as opposed to factors that do not offer appreciable or statistically significant 

sources of variance, will be a significant outcome of this analysis. 

Modeling – The test nacelle will be modeled in VULCAN or other CFD codes, and some of the 

same conditions run to determine if agent concentration profiles are similar to that seen in test, 

and if estimates of droplet distributions in the nacelle and local estimates of condensation from

the instrument readings are consistent with computational codes. 

Analysis - Other computational analysis will be performed, based upon the key dimensionless 

parameters and other criterion established earlier in the NGP program, related to distribution and 

transport of gaseous and mixed phase extinguishants. This includes studies on the degree of 

condensation or pressure drop measured or predicted through various types of clutter and 

obstacles. The results of the required flame zone concentrations and durations for various 

operational conditions in Phase I, coupled with the extinguisher system requirements to create 

identical conditions everywhere in the nacelle from Phase II data, will allow an integrated

analysis to determine which factors most affect the entire multi-faceted process in total, and a

means of optimizing the extinguisher system conditions for different operating conditions to 

achieve the best overall performance. The ability of the DOX process to estimate effects for

factor settings that haven’t even been tested, in support of proposing a “paper champion” most 

optimal design that may comprise factor combinations as well as settings that would not have

been tested, for a range of different operating conditions (or the best overall design to support a 

range of conditions within one flight operating envelope), provides the capability to develop 

formulations to support the design of the most optimal system for any set of conditions. 

Phase III – Follow-On Tests and Special Topics 

Data reviews with the various NGP contributors will be held at the end of Phase I and II, to 

review the data and subsequent analyses, and to plan the results for the next phase. It is intended

that a significant portion of remaining test facility time will be available at the end of Phase II to

address a number of special topics that do not fit acceptably within the strict DOX protocol, or 

perform follow-on tests to clarify data collected from Phases I and II (although the results from

Phases I and II will comprise a complete set of information successfully fulfilling the original 

intent and mission of this project). These areas of interest will need to be prioritized by the

review team, based upon the results at the end of Phase II, to focus the remaining test time on 

subject areas of greatest interest. Some of the areas expected to be of interest at that time to

consider include the following: 

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(1) Additional testing to “un-confound” confounded two-factor interactions shown to be

significant in Phase I and II. 

(2) Repeats or specially modified tests to resolve poorly understood results or conditions 

where the test results differ significantly from predictions. 

(3) Multiple additional settings of significant factors, where additional settings are 

meaningful, to provide quadratic or higher order relationships, and maxima or minima. 

(4) Additional special measurements for a few select test conditions, using laser diagnostics 

or other means where desirable and practical. 

(5) Additional data with “fixed” dimensionless number values, to better compare to 

fundamental analyses and supplement the existing data. 

(6) Data with extinguishants or varying quantities of bromine, to determine if extinguishing 

efficiency of chemically-active extinguishants is based predominantly upon the relative 

amount of “bromine loading” per unit mass, regardless of the structure of the molecule. 

(7) Testing of pure inert or hybrid gas generators, to determine if the heating of the stored 

extinguishant can overcome cooling once it is discharged. 

(8) Different clutter arrangements to perform tests consistent with conditions evaluated by 

Disimile and Presser. 

(9) Test a “next generation” Halon replacement with chemical activity and performance 

similar to Halon 1301, in a select number of comparative tests. 

(10) Small “witness fires” could be placed in various sites in the nacelle, to confirm that a

proven “balanced” (with respect to concentration uniformity) system can indeed 

extinguish actual fires anywhere in a nacelle. 

(11) The “witness fire” approach would facilitate the evaluation of condensed phase

extinguishant systems, such as water mist, dry chemical and chemically active gas 

generator or aerosol units. This new approach, in a manner consistent with Underwriters 

Laboratory tests of enclosed space fire protection systems, would offer a means for the 

aerospace community to take advantage of the performance enhancements of these types 

of systems, many of which have shown promise in the NGP program. 

ACKNOWLEDGEMENTS 

The author would like to thank the Strategic Environmental Research and Development Program 

(SERDP), sponsor of the overall NGP research effort. 

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