FIRE SUPPRESSANT DISTRIBUTION IN AN ENGINE NACELLE1

D. R. Keyser2

INS, Inc. 

Pinehill Technology Park, Bldg. #1 

48015-A Pine Hill Run Road 

Lexington Park, MD 20653 

J. C. Hewson3

Fire Science and Technology 

Sandia National Laboratories 

Albuquerque, NM 87185-1135 

ABSTRACT 

The flow inside an engine nacelle ground test simulator is studied for the purpose of 

understanding and optimizing the distribution of suppressant. One objective of this study is to 

identify conditions for which suppression will and will not be successful in order to test the 

model’s ability to discriminate between various scenarios. To do this, the distribution of 

suppressant during cold flow is studied to identify regions of low concentration. 

Available measurements are coupled with predictions using Sandia National Laboratories’ 

Vulcan fire-field model. Some measurements are required to help design the computational 

model, such as the inlet and outlet flows, which are boundary conditions for the simulation. 

Specific suppressant-concentration profiles as a function of time are helpful in evaluating the 

model’s performance. To create scenarios with insufficient distribution in regions of potential 

fire, the removal or capping of individual agent nozzles is used. The test matrix has been 

constructed by varying the loci and number of discharge nozzles, the mass of suppressant, and 

the air flows. Vulcan’s predictions of suppressant-concentration profiles to date are in 

qualitative agreement with test measurements and show that the suppressant is well-distributed.

Empirical Boundary Conditions

In the autumn of 2002 tests were conducted on the ground in a full-scale ‘iron bird’ nacelle 

simulator to determine the distribution of air flows, without fire, across the boundary of the fire 

test nacelle. [1] (numbers in brackets denote references ). These data were to provide the 

boundary conditions for the Computational Fluid Dynamic (CFD) and Vulcan models of this 

nacelle simulator effected by Sandia National Laboratory. It was found that the distribution of 

 

1

 This work was conducted at Sandia National Laboratories, a multiprogram laboratory operated by Sandia 

Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security 

Administration under Contract DE-AC04-94AL85000. This research is part of the Department of Defense's Next 

Generation Fire Suppression Technology Program, funded by the Department of Defense Strategic Environmental 

Research and Development Program. We also gratefully acknowledge the cooperation of the Naval Air Systems 

Command, Fire Protection Group in sharing test data for the validation of our models. 

2

 Corresponding and presenting author: dkeyser@pax.i-n-s.com. 

3 jchewso@sandia.gov. 
2

effluxes among the vents remained a nearly constant proportion of the inlet flows, and 

consequently the actual mass flows are nearly proportional to the total influx as well. The second 

finding was the rather large variation, proportionately speaking, in the steady-state pressures 

measured inside the nacelle with the range being greater than the mean. The third observation 

was a severe velocity profile across the top aft diamond vent. Most of the flow was exiting from 

the port side of the diamond-shaped area; the velocity out the starboard side was estimated at 

only ~20% of that out the port side. The pitot rake apparatus was designed to observe an overall 

average of that efflux from this vent. 

Nacelle Simulator Test Apparatus This ground test, ‘iron bird’ simulator is designed for 

testing at one flight condition ---- 0.55M, sea level flight, on a standard day. Given this physical 

constraint, three nominal ventilation flows were selected to correspond to three major 

flight conditions: high-speed, high-altitude cruise, loiter, and precision approach. These flows are 

2.1, 1.5, and 1.25 lbm/sec respectively, and were derived from flight tests [2]. The air is supplied 

from a centrifugal compressor which is driven by a gas turbine. The inlet flow is measured by a 

calibrated turbine meter. There is adequate straight pipe, according to ASME Standards [3]; a 

flow straightener is inserted between the compressor and the turbine meter. Likewise, there is 

adequate straight pipe downstream of the turbine meter, and downstream of the 45-deg elbow 

there is an Etoile conditioner in the straight pipe leading to the nacelle to remove swirl. The 

supply air pressure was measured with a water manometer immediately downstream of the 

turbine meter. The air effluxes were measured at each of the outlets separately under steady 

state. The air temperatures in the nacelles were measured with thermocouples at four locations, 

one on each side and near each end of the nacelle. The airflow out the aft diamond vent was 

measured with a pitot rake. The airflow in the other outlets was measured with a calibrated vane 

anemometer and stopwatch. Air pressures in the nacelle were measured at three locations using 

an inclined water manometer. All airflow data were corrected to ambient conditions at the time 

of test in order to determine the mass balance. 

Table 1: Summary of Test Results of Flow Distribution 

Air Inflow Air OutFlows  Air Out Flows are All shown As ratios Leaks 

 lbm/s Aft Top BalPist Stb.2" 

vent 

Center 

3.6"vent 

Port,top 

Vent 

Port,Bottom 

 Vent 

Distribut’n 2.024 0.6651 0.0975 0.0260 0.1424 0.0199 0.0474 0.00127 

 of flow 1.418 0.6397 0.0958 0.0263 0.1658 0.0234 0.0474 0.00122 

 (ratio) 1.152 0.6442 0.0931 0.0264 0.1680 0.0196 0.0472 0.00123

Average Distr. % 64.973 9.5526 2.6275 15.879 2.1024 4.7395 0.12442 

 

As can be seen, there were air leaks, but their cumulative effect was less than 1.3% of the total 

flow. 

In discussing the agreement between model and test in the total flow and average pressures, this 

test was also a calibration of the nacelle simulator. In constructing the model, it was assumed 

that the balance piston vent and aft top vent would behave as parallel orifices and that the 

AMAD vents would behave more like nozzles. A weighted average of these coefficients of 

discharge was predicted to be 0.733. By actual test and calibration, it was determined that the 

effective coefficient of discharge equals 0.614. This value implies that all the vents behave 
3

essentially as sharp-edged orifices. This value of the coefficient of discharge correlates very 

well with published values for the coefficient of discharge of orifices at these Reynolds 

numbers. By correcting these coefficients of discharge in the boundary-flow model, it may be 

used with confidence in setting up future tests. 

Agent Concentration Testing 

Three phases of testing were performed with the ground test nacelle simulator at NAS Patuxent 

River [4]. Phase One measured fire extinguishing agent concentration using an agentdistribution system which had been proven to extinguish live fires in previous Qualification 

testing. Phase Two used various bottle and distribution system configurations to validate 

concentration measurements acquired during previous risk-reduction testing. Phase Three 

utilized various Halonyzer sampling probe configurations to quantify associated effects on agent 

concentration readings. A total of twenty-three test runs were conducted. 

Both HFC-125 and Halon 1301 concentration measurements were observed, but only the HFC125 test results are compared to the Vulcan model. 

Halonyzer II System 

The Halonyzer II (S/N 3), was used to acquire and record agent concentration data. The system 

was manufactured by Pacific Scientific, Inc., HTL Div., of Duarte, California. It is capable of 

measuring concentrations of Halon 1301, HFC-125, and other fire-extinguishing agents. This 

system consists of the following major components: 

• Three ”Quad” units each containing four sensor assemblies for four probes 

• Twelve agent sampling probes 

• Power supply/vacuum pump unit 

• Data acquisition system 

The operation of this instrument is based on a linearized viscosity mixing theory using the 

weighted viscosities of the constituent gases. Since the viscosity of pure air differs from that of 

pure fire-extinguishing agents, readings will show that a mixture of gases is present and the 

”relative” concentration of extinguishing agent will be indicated by the differential pressure 

reading obtained. The processor also converts relative readings to volumetric readings based on 

the unique calibration of each Halonyzer and extinguishing agent. 

The vacuum pump draws the gas samples through the sampling probes into the respective sensor 

assemblies in the “Quads”. In each sensor assembly, the gas passes through filter screens, a heat 

exchanger, a capillary tube differential pressure sensor section, and finally through a sonic flow 

orifice. The heat exchanger section ensures uniformity of monitoring conditions, and tests are 

performed only after thermal equilibrium is achieved. The sonic flow orifice ensures a constant 

flow while the capillary tubes create the pressure drop measured by a transducer. The 

transducer transmits the pressure signal to the processor unit which performs the necessary 

calculations and then records and displays relative concentrations. A scan rate of 10 Hz. was 

used. 

Test Procedures Field calibrations and calibration verifications using 100% agent were 

performed on site prior to testing and whenever agents were changed. 
4

The general step-by-step procedures followed during the tests were: 

• Set engine simulator air flow and allow to stabilize 

• Conduct Halonyzer calibration 

• Initiate NAVAIR data collection 

• Initiate Halonyzer recording and confirm data acquisition 

• Discharge engine fire-extinguishing system 

• Record concentration data for at least 30 seconds 

Figure 1 HFC-125 Transient Injection Test Results 

A total of twenty-three test runs were conducted during the test program. Two test runs were 

conducted for Phase One, with average HFC-125 volumetric concentrations peaking at 

approximately 20%, with all channels exceeding 10% for 0.5 seconds. (These concentrations 

should guarantee extinction.) Thirteen test runs were conducted for Phase Two plus eight test 

runs for Phase Three. Figure 1 shows the test results used for comparison with the Vulcan 

modeling described below. These data show that the rate of injection decreases between 2.70 

and 2.75 seconds, while the bottle discharge data shows a 3 to 4 second injection interval. Note 

also the great variation in agent concentrations observed between all of the 12 sampling tubes—

from 10% to 24% by volume in various parts of the nacelle, simultaneously. These clearly show 

the great nonuniformity in the flow profiles throughout the nacelle caused by four nozzles 

discharging high-momentum jets in contrary directions. These nonuniformities make model 

validation quite difficult near the nozzle discharge loci. 

Vulcan Model Comparisons 

Measurements from the agent concentration tests are compared with simulations in which 3.2 kg 

of HFC-125 is injected over 3 and 4 seconds. Air flow into the nacelle is 1.0 kg/s and 

suppressant mass flow is proportional to the nozzle area for four nozzles. To evaluate the 

predictive capability of the Vulcan simulations in describing the suppressant distribution in the 

nacelle, we have compared HFC concentration-time traces with those measured using a 

Halonyzer II. The locations of Halonyzer II probes are shown in Table 2 below: 
5

Table 2: Nozzle characteristics. 

No. Dia. (cm) X,m Y,m Z,m 

1 0.396 0.09 15.78 1.02

2 0.475 0.09 15.84 1.15

3 0.475 0.19 16.82 .41 

4 0.330 1.03 17.7 .46 

In these simulations, the suppressant has been assumed to enter the nacelle in the vapor phase or 

to vaporize instantly upon entrance. Other simulations, not reported here, indicate that there is a 

finite vaporization time. Actual vaporization times depend on the square of the initial particle 

diameters, assuming a vaporization rate proportional to the surface area. For reasonable droplet 

diameters (100 to 500 micron), estimated evaporation times can vary from 0.1 to 2.5 seconds. 

These estimates assume flash vaporization of roughly a third of the suppressant which then cools 

the remaining liquid suppressant to its boiling point. While relatively few large droplets are 

expected, the droplet size distribution leaving the suppressant nozzles does affect the simulation 

results. The effects of finite-rate evaporation are clear in the qualification test measurements in 

the following figures in which the suppressant concentrations drop off more gradually than 

comparable simulations after the end of the 3 or 4-second injection. 

Table 3. Concentration Sampling Probe Locations 

Probe No. Location: ( fuselage sta.(in), 

o’clock looking fwd., cm aft of bulkhead) 

1 (Sta 604; 7:00, 31.75cm) 

2 (Sta 604; 12:00, 31.75cm) 

3 (Sta 645; 3:00, 136cm) 

4 (Sta 645; 12:00, 136) 

5 (Sta 680; 2:30, 224.8cm) 

6 (Sta 680; 6:00, 224.8cm) 

7 (Sta 680; 10:30, 224.8cm) 

8 (Sta 722; 12:00, 331.5cm) 

9 (Sta 722; 4:30, 331.5cm) 

10 (Sta 722; 7:30, 331.5cm) 

11 (Sta 617; 7:30, 64.75cm) 

12 (Sta 625; 7:30, 85cm) 

Channel 1 is in the forward lower end of the nacelle on the port side. The injection period is 2.75 

seconds, and therefore the more relevant results from Vulcan are those of the 3-second injection. 
6

Figure 2 shows that the rise characteristics follow quite closely up to the peak, but after that the 

calculated decrease is faster than the test data. Most of this can be attributed to the simulation 

method. The injection simulated using Vulcan was a square wave, 3 seconds long, but during 

the actual bottle blow-down the flow tapers off as the pressure in bottle decreases—particularly 

in the latter phase where the discharge is no longer sonic. The response curve calculated for the 

4-second injection illuminates the difference in peak time and its subsequent rate of decrease. 

Channel 3 is almost half way aft located on the starboard side. Results for channel 3 are 

reasonably good, with 3 and 4-second injection periods closely bounding the test measurements. 

Figure 2 HFC-125 mole fractions at Halonyzer II sensor locations 1 and 3. 

Suppressant comes from nozzle 1 (over the top of the engine) and from nozzle 2 (under the 

engine). Sensors are located between nozzles 3 and 4, roughly two-thirds of the way aft. 

Channel 6 has been located below the engine and just in front of one large rib while channel 5 is 

located on the upper starboard side. Simulated results for channels 5 and 6 are relatively 
7

insensitive to the rate of injection and these simulations bound the measurements for those 

channels. 

Figure 3 HFC-125 mole fractions at Halonyzer II sensor locations 5 and 6. 

Uncertainty and Sensitivity Estimates 

Boundary Condition Flow Model When comparing the calibrated model of the influx-efflux 

tests to the means of these test results, given the flow, the mean nacelle pressures agree within 

+4.5%. If given the supply pressure, the total flow agrees within +2.3%. As a check on the 

overall accuracy of these flow measurements, a mass balance was performed as well. In the 

mass balance calculations of the means, the difference between the inflow and the sum of all 

effluxes ranges from 0 to 1.11% in the worst case. Almost 2/3 the total flow exited from the aft, 

upper diamond vent. Before calibrating the discharge coefficients of the several vents, the 

influx-efflux model predicted about 6.5% more flow leaving via the Balance Piston Valve and 

8.5% more by the four AMAD vents in the front bulkhead than was observed. After calibration, 

the above uncertainties were obtained. The Halonyzer II instrumentation tends to smooth the 

concentration data because there is ~ 6 m of 6mm tubing leading to the ‘quad’ sensors (which 

also causes a transport delay in the data), and the length of capillary sensing tubes within them 

further mix and smooth the viscosity variations of the incoming gas mixtures. Since the flow of 
8

sampled gases was not reported, further estimates of uncertainty from this source are not 

possible. 

Agent Concentration Testing There is an unknown uncertainty associated with the Halonyzer II 

concentration measurements, both in terms of the absolute concentration measured and in the 

locations where the measurements were taken. The indications from the agent concentration test 

report [ref] are that the measurements are fairly repeatable so that significant errors are most 

likely of the systematic type. The probe locations are not specified in detail in the report; for 

example, the probe corresponding to channel 1 is identified as being 604 inches aft of the aircraft 

nose and at 7:00. This places it roughly in front of the upper port edge of the gearbox assembly, 

but there is some latitude on the exact placement. The stated locations and the coordinates used 

in the simulations are indicated in Table 3. Each probe location within the Vulcan simulation 

averages over a cubic inch of volume. 

Vulcan Sensitivity Analyses and Observations Model sensitivities in the areas of the 

suppressant spray momentum vector and magnitude relative to the air inflow and the rate of 

suppressant vaporization indicate the effect of the relative momentum. Simulations are 

conducted with the 3.2 kg of HFC-125 injected in the gas phase over either a 3 or 4-second 

injection period; this difference in the period effects a 30% change in the jet momentum. It is 

evident that this modest change in the suppressant momentum can lead to as much as a 50% 

change in the concentration predicted at a specific point because the high-concentration 

suppressant jets either hit a measurement cell or are shifted by ~10 cm away from it. A typical 

example of these sensitivities is shown in Figure 4, which highlights the model’s sensitivity both 

to the spray momentum and to the Halonyzer II probe locations. Because of this enormous 

sensitivity, there is more of a qualitative significance to the agreement or disagreement in many 

of these detailed results. 
9

Figure 4: Typical Variations caused by Sensitivity to Jet Momentum and Turbulence 

Because the turbulence generated by the suppressant injection is not known very well, this 

parameter was varied to determine its sensitivity. The concentrations measured by probe 

channels 4 and 11 do show some sensitivity to turbulence levels generated by the spray nozzles. 

There are two orders of magnitude between the levels of turbulence caused by 3 vice 4-second 

injection, which bracket the concentration profiles for channels 4 and 11 in fair agreement with 

the Halonyzer II measurements. There is a correlation here: whenever a channel exhibits a high 

sensitivity to jet momentum, it also shows similar sensitivity to high turbulence. We conclude 

that the sensitivity to the injection momentum and the turbulence generated as the suppressant 

mixes is more substantial than the sensitivity to the turbulence imposed as a boundary condition. 
10

Conclusions 

For the boundary flow model and testing, it was found that the distribution of effluxes among the 

vents remained a nearly constant proportion of the inlet flows, and consequently the actual mass 

flows are nearly proportional to the total influx as well. Almost 2/3 of the total flow exited via 

the aft, top diamond vent, which was not expected. The reason for this most likely was the 

discovery that the screen over diamond vent opposite it on the bottom of the nacelle was virtually 

closed by particles resulting from the combustion products. 

The discrepancies noted between Vulcan calculations and the measured concentrations do not 

indicate a change in final results from where suppression is expected to where suppression is not 

expected, or vice versa. When the Vulcan simulations and the Halonyzer II probe measurements 

differ noticeably, one shows a concentration that is sufficient to suppress a fire while the other 

shows a markedly higher concentration, but still sufficiently concentrated when it reaches that 

probe location to extinguish the test fires. 

References 

[1] Dolinar, J.,Hudgins, D, and Keyser, D. “F/A – 18 E/F Nacelle Simulator Input/Ouput 

Boundary Condition Flows” NAWCADPAX /SUM-2002/171 

[2] Boeing Technology Services Contract # 6-1171-10A-6847, “NAVAIR F/A 18 E/F Fire 

Extinguishing Test,” dated May 8, 2000 

[3] ASME Performance Test Code 19.5, “Flow Measurement –2004 “, ASME, Three Park Ave. 

NY, NY 10016 

[4] Northrop Grumman Test Work Description (TWD) LNV12.01-013, "F/A-18 E/F (HFC-125) 

Fire Protection System Qualification Test", dated February 29, 2000