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MODELING STUDY OF THE BEHAVIOR OF LIQUID FIRE 

SUPPRESSION AGENTS IN A SIMULATED ENGINE NACELLE

James W. Fleming 

Navy Technology Center for Safety and Survivability 

Combustion Dynamics Section, Code 6185, Naval Research Laboratory 

Washington, DC 20375-5342 USA, Tel: 202-767-2065; Fax 202-767-1716 

fleming@code6185.nrl.navy.mil

and 

Jiann Yang

Fire Metrology Group, MS- 8662 

National Institute of Standards and Technology, Gaithersburg, MD 20899-8662 USA 

Tel: 301-975-6662; Fax: 301-975-4052; jiann.yang@nist.gov

INTRODUCTION

This paper is a status report for a modeling study to evaluate the fire suppression potential for 

compounds in protecting engine nacelles. The project, “Vapor Loading and Suppression 

Effectiveness of Two-Phase Fire Suppressant”, is part of the Strategic Environmental Research 

and Development Program (SERDP) funded DoD Next Generation Fire Protection Technology 

Program (NGP) [1]. The project seeks to assess the vapor concentration bounding conditions for 

condensed gases/liquids considering their physical properties and the intended application 

conditions. 

There is a high probability that gases or liquids with high boiling points, likely higher than that 

of Halon 1301, will be required to provide fire protection in engine nacelles. For this 

application, agents will need to be effective both at normal air temperatures near 300 K and also 

at low temperatures present at high altitude. In such a cold, non-trivial flow environment, the 

performance of the suppression agent will depend on a number of parameters including physical 

properties of the agent (heat capacity, boiling point, and heat of vaporization), the application 

temperature, and the flow-imposed time constraints for liquid agent evaporation. There is a need 

to eliminate compounds from consideration that will never be suitable and to identify those 

favorable properties that the successful agents are likely to possess. It is expected that the prequalified list will then undergo further scrutiny, including detailed modeling and experimental 

investigation. 

In this study, transport and evaporation of drops introduced into a simulated engine nacelle [2] 

are treated using the NIST Fire Dynamics Simulator (FDS) version 3 [3]. The computational 

matrix considers the incoming air and engine surface temperature and the quantity and physical 

properties of the suppression agents. A similar drop size distribution is assumed for all of the 

agents. The amount of liquid in this drop distribution will depend on the amount of liquid that 

flash vaporizes when introduced into the nacelle. Evaporation of agent remaining in the form of 

drops after flash vaporization will be evaluated as a function of position in the nacelle using 

FDS3. An estimated suppression effectiveness ranking can then be derived from the 

computational results and related to the physical properties of the compounds in the 

computational matrix. 
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A concern with any computational study, especially one that seeks a predictive capability, is 

validation. There is some limited experimental data applicable to the current study that can be 

used to get a general sense of the degree of validity. A small set of data has been obtained for 

CF3I [4]. More experimental data for this and other agents would be valuable. In this paper, 

computational results for water are presented. In this set of simulation exercises, water was 

selected as a simulant for high boiling point agent. In addition, FDS (v.3) has the inherent 

capability to perform water spray calculations without modifying the code. The latest version of 

FDS (v.4) has the flexibility to change from water to other liquids, if desired. FDS (v.4) will be 

used to carry out calculations using CF3I and CF2Br2. 

FLASH VAPORIZATION VERSUS EVAPORATION

FLASH VAPORIZATION 

When a liquid is released under pressure through an orifice, the pressurized liquid jet will 

experience mechanical breakup leading to the formation of drops and/or vapor. In general the 

release and formation of an aerosol from a pressurized discharge is a very complex process [5]. 

However, some generalizations can help to establish assumptions in order to form a basis for a 

valid comparison of agent performance in the nacelle environment. 

The agent discharge process can be considered to occur in two stages: flash vaporization and 

subsequent evaporation of remaining liquid. Upon release, if the liquid boiling point is lower 

than the ambient temperature, the super-heated liquid agent can immediately change into its 

vapor state (flash vaporize) cooling the environment and leaving some liquid behind which can 

evaporate through thermal exchange with the surround air. Flash vaporization is essentially 

independent of the properties of the compressed liquid; evaporation, however, depends on the 

specifics of the liquid properties in addition to the application conditions, particularly ambient 

temperature and residence time in the nacelle. 

The amount of superheated liquid (i.e., stored at temperature above the boiling point) flash 

vaporized versus the amount of liquid left behind can be estimated from the Jakob Number, Ja. 

Ja can be evaluated from the enthalpy change required to take the liquid from its normal boiling 

point to the ambient temperature (sensible enthalpy) and the heat of vaporization of the liquid, 

∆Hvap. Expressed in terms of the heat capacity at constant pressure for the liquid, CpL, the 

temperature change ∆T = Tamb – Tb between the ambient temperature, Tamb and the boiling point, 

Tb, Ja may be written as 

 

vap

pL

H

C T

Ja

∆

∆ = (1) 

In general, CpL will exhibit some dependence on temperature. In this case, the term CpL∆T in 

Equation 1 must be integrated from the boiling point temperature to the ambient temperature in 

order to determine the sensible enthalpy. 
3

 ∫

amb

b

T

T

CpL ( (2) T)dT

Ja is thus not a constant but depends on the temperature of the air into which the agent is 

discharged. For those agents discharged into air that is at a higher temperature than the boiling 

point (at the ambient pressure), Ja, which represents the fraction of liquid flashed to vapor in an 

adiabatic expansion process, will be between 0 and 1. A Ja of one implies that the entire agent 

vaporized immediately upon discharge. The heat content in the superheated liquid vaporizes the 

liquid. A Ja of zero or less applies to compounds discharged into air at a temperature equal to or 

lower than their boiling point. For compounds and conditions where Ja ≤ 0, all of the agent 

released into the nacelle will be in the liquid state. Liquid evaporation in this case must rely on 

the much slower process of extracting heat from the surrounding air. Examination of Equation 1 

indicates that for a given ambient temperature, a higher fraction of the original liquid will be in 

the vapor phase for compounds with a lower boiling point, larger heat capacity, and a smaller 

heat of evaporation. However, compounds and conditions presenting a low Jakob number 

cannot necessarily be eliminated from consideration without further examination of the agent 

evaporation behavior. 

EVAPORATION 

The immediate vaporization of the super-heated liquid upon release, controlled by the Jakob 

number, occurs on a fast time scale for the adiabatic vaporization (~ 10–6 to 10–9 sec). Any liquid 

remaining that must subsequently evaporate to get into the vapor phase does so on a much slower 

time scale (~ 10–3 sec to several seconds depending on drop size and ambient conditions). For 

compounds with boiling points higher than the inlet air temperature (Ja < 0), the evaporation 

time scale will determine the agent vapor concentration in the nacelle. Evaporation of liquid 

agent will depend on the physical properties of the agent including heat capacity, boiling point 

temperature, enthalpy of vaporization, and transport properties (thermal conductivity and/or mass 

diffusivity). 

Several models exist for describing the evaporation of drops. One model suitable for the 

conditions under consideration here is the d-square law [6] 

 D t D K t = o − 2 2 ( ) (3) 

where K, the evaporation constant, is given by 

 ln (1 )

8

B

C

K

pg

g +

ρ

λ = (4)

With the mass transfer number, B, expressed as 

 

vap

pg amb b

H H

C T T

B

∆

− = ( )

 (5) 
4

or 

 

s

s a

L Y

Y Y

B −

− = 1 (6) 

Equation 3 predicts the drop diameter, D, at time, t, in terms of the original drop diameter, Do. 

BH is normally used at ambient temperatures above the boiling point; BL is used at ambient 

temperatures below the boiling point. The evaporation constant depends on the temperature, T; 

the thermal conductivity, λg, of the surrounding gas; mass fraction of agent in the ambient 

surrounding air, Ya; mass fraction of the agent at the drop surface, Ys (assessed from the 

temperature dependent vapor pressure of the liquid); the heat capacity of the gas at constant 

pressure, Cpg; the liquid density, ρ; the enthalpy of evaporation at its boiling point, ∆Ηvap; and the 

boiling point, Tb. The time required for complete evaporation can be expressed as 

 K

D

t o

vap

2

= (7) 

Higher agent mass will exist as vapor in the nacelle if tvap is small compared to the residence 

time in the nacelle. In general, properties that lead to small tvap or large K are low boiling point, 

small heat of vaporization for the liquid and low heat capacity. Recall that in general for the 

super-heated liquid, more of the agent will start out in the vapor phase (flash vaporize) for those 

compounds with high heat capacity relative to its heat of vaporization. As a final consideration, 

better thermal agent suppression performance will be exhibited for those compounds that 

contribute a larger thermal load per mass of agent (high sensible enthalpy, high heat capacity) to 

the air stream. The determination for compound effectiveness in suppressing engine nacelle fires 

obviously lies in a trade-off between the agent properties and the application conditions. A 

detailed treatment of the transport and evaporation behavior of the dispersed agent, considering 

the agent specific properties, is necessary in the nacelle environment. 

FIRE DYNAMICS SIMULATOR

The Fire Dynamics Simulator (FDS) is a computational fluid dynamics (CFD) computer code 

developed by NIST to study fire dynamics, smoke movement, and sprinkler-fire interaction [3]. 

Other applications, including pool fires and tank farm fires, have also been modeled using FDS. 

Details of FDS, its theory and operation manual, can be downloaded from the NIST Building and 

Fire Research Laboratory website [7]. 

SIMULATED ENGINE NACELLE 

The engine nacelle is the region of on an aircraft between the body of the engine and the 

housing. Fuel and hydraulic lines, pumps, and lubrication systems are located within the nacelle. 

Air is vented through the nacelle to prevent any build-up of combustible vapors, and underside 

drain holes are used to mitigate potential pooling of flammable fluids as a result of a leak. 
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For fire suppression performance evaluation in this environment, it is important that a sufficient 

representation of the relevant application conditions be included. The complex geometry within 

the nacelle permits many opportunities for unwanted fires. The convoluted flow pattern resulting 

as air passes through the nacelle favors those agents that do not persist too long in the liquid state 

after release into the nacelle area. Thus, an assessment of suppression performance must 

consider evaporation behavior in this type of flow field. 

Features of a previously developed experimental nacelle [2,4] were used to develop the 

computational grid used in this study. The experimental nacelle simulator consists of two 

concentric cylinders with agent and air flowing through the annulus with baffles and ribs. Figure 

1 is a schematic of the simulator. The annulus of the simulator has an inside diameter of 0.6 m 

and an outside diameter of 0.9 m. The length (2 m) of the simulator is comparable to the 

distance between the agent injection port and the downstream end of a typical small engine 

nacelle. The baffle height was 0.075 m. The baffles are used to mimic a complicated flow path 

for the agent as in the case of a real nacelle. Two longitudinal ribs with the same height as the 

baffles are placed on the outer surface of the inner core of the nacelle between the inner forward 

and aft baffles. The ribs are used as barriers to the agent flowing circumferentially. The agent is 

released through a vertical tee. Agent concentrations are measured at the two locations, termed 

here as front port and aft port. A frequency-controlled, variable-speed blower provided airflow 

through the nacelle simulator. 

To exhaust

Video camera

Baffle

Air from blower

Observation window

2 m

60 cm

Pitot tube

15 cm

Agent bottle

Fiber-optic probes (UV spectrometer)

Agent release location

Simulated

engine core

1

2

3 4

Horizontal rib

Figure 1. NIST nacelle simulator 

The configuration used in the FDS simulation is shown in Figure 2. The grid model has 

geometrical dimensions commensurate with the experimental fixture. The computational grid 

model was previously developed in a separate NGP project [1]. The following conditions were 

used in the simulations. Air speed at the inlet of the nacelle was set at 4.25 m/s or 2.125 m/s and 
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22 °C. Two sprays, one pointing vertically upward and one vertically downward, were used to 

simulate the tee used for agent discharge. Water was discharged at a rate of 416 L/min, which 

was comparable to a high rate discharge used in nacelle fire suppression. The sprays were 

activated at 1.5 s from the start of FDS simulation and then de-activated at 1.8 s. The total 

activation time was 0.3 s. The dispersion process was simulated to 5 s with 50 × 200 × 50 grid 

cells. The drop size distribution used in the simulations was in the form of a RosinRammler/log-normal distribution with a median volumetric diameter of 20 μm with the 

distribution parameter set at 2.4 (default value used in FDS). The minimum spray angle was set 

at 30°, the maximum at 75°, and the drop speed at 20 m/s. Water drop interaction with nacelle 

walls was handled in the way described in FDS. When a water drop impacts a solid horizontal 

surface, it is assigned a random horizontal direction and moves at a fixed horizontal velocity 

component until it reaches the edge, where it drops straight down at the same fixed velocity. 

When the water drop hits a solid surface vertically, it is removed from the computational 

domain. 

Figure2. Grid model of NIST nacelle simulator for FDS calculations.

Two nacelle conditions were examined. The nacelle walls were either at 22 °C or at 100 °C. 

Relative humidity was initially set at zero throughout the computational domain. The initial 

temperature in the annulus region was 22 °C. The dispersion effectiveness of water drops was 

assessed based upon water vapor concentration (mass fraction) measurements at the front and aft 

ports. The intent was to evaluate vapor loading that could be attained at these two locations. 

Results and discussion 

Figures 3 and 4 show the temporal variation of water vapor mass fraction at the two locations 

during the 5 s simulations when the nacelle walls were at 22 °C and 100 °C, respectively. 
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Several pertinent features are revealed in the two figures. There is a delay time (due to transport 

and evaporation) for water vapor to get to the two locations upon activation of the sprays (at t = 

1.5 s). The time-averaged water vapor mass fraction at the front port is higher than that at the 

aft. The time-averaged concentrations at the two locations are higher when the nacelle walls are 

at higher temperature (100 °C vs. 22 °C). Higher nacelle wall temperature results in higher 

ambient temperature in the annulus region, which in turn causes water drops to evaporate faster. 

Nacelle walls @ 22 oC

Time (s)

012345

Water vapor mass fraction

0.000

0.001

0.002

0.003

0.004

0.005

0.006

Front port

Aft port

Figure 3. Temporal variation of water vapor at the two measurement locations resulting 

from the injection of 416 L/min of liquid water at 1.5 s into a 22 oC inlet air flow with a 

speed of 4.25 m/s. The nacelle walls were at 22 oC.

Figures 5 and 6 show the effect of nacelle airflow on the water vapor loading at the two 

measurement locations. The reduction in airflow increases the transport time of water vapor to 

the two measurement locations. The time-averaged concentrations do not appear to be 

significantly different under the two flow conditions at the same location. This could be 

attributed to the combined effect of dilution due to airflow and evaporation. Higher airflow 

results in faster evaporation but more dilution, and conversely for a lower flow. 
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Nacelle walls @ 100 oC

Time (s)

012345

Water vapor mass fraction

0.000

0.002

0.004

0.006

0.008

0.010

0.012

Front port

Aft port

Figure 4. Temporal variation of water vapor concentration at the two measurement 

locations resulting from the injection of 416 L/min of liquid water at 1.5 s into a 22 oC inlet 

air flow with a speed of 4.25 m/s. The nacelle walls were at 100 o

C. 

Aft port

Nacelle walls @ 22o

C

Time (s)

012345

Water vapor mass fraction

0.000

0.001

0.002

0.003

0.004

0.005

Airflow @ 4.25 m/s

Airflow @ 2.125 m/s

Figure 5. Effect of airflow at front port.
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Front port

Nacelle walls @ 22 o

C

Tim e (s)

012345

Water vapor mass fraction

0.000

0.001

0.002

0.003

0.004

0.005

0.006

Airflow @ 4.25 m /s

Airflow @ 2.125 m /s

Figure 6. Effect of airflow at aft port.

CONCLUSIONS 

This paper reports progress on a computational study to evaluate the fire suppression potential of 

compounds for engine nacelle fire protection. Drop transport and evaporation behavior of 

dispersed liquids (or gases under pressure dispersed as liquids) in a simulated engine nacelle is 

modeled using the NIST Fire Dynamics Simulator (version 3). Results for water, an example of 

a high boiling point liquid with excellent thermal properties, entrained in 22 oC air and 

interacting with cold (22 oC) and hot (100 o

C) engine walls within the nacelle are presented. The 

results show the importance of the application conditions. Studies are underway to model the 

behavior of CF3I at very low inlet air temperature conditions to simulate a low boiling, low 

temperature application. The ultimate goal of this project will be a determination of those 

compound properties and application conditions that should exhibit good fire suppression 

potential using the nacelle simulator as a test bed. This understanding will provide needed 

guidance for further development of two-phase fire suppression systems for engine nacelle 

applications. 

ACKNOWLEDGMENTS 

We would like to thank Dr. Kevin McGrattan for his assistance with the FDS code. This 

research is part of the Department of Defense’s Next Generation Fire Suppression Technology 

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

Program (SERDP). Dr. Richard G. Gann of the National Institute of Standards and Technology 

(NIST) is the technical program manager. Additional support has also been received from the 

Office of Naval Research through the Naval Research Laboratory. 
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REFERENCES 

1. For the NGP Website see http://www.bfrl.nist.gov/866/NGP/ 

2. Yang, J.C., Manzello, S.L., and Nyden, M.R., Discharge of CF3I in a Cold Simulated Aircraft 

Engine Nacelle, Proceedings of HOTWC 2002, April 2002. 

3. Fire Dynamics Simulator (Version 3): McGrattan, K. B.; Forney, G. P.; Floyd, J. E.; 

Hostikka, S.; Prasad, K., NISTIR 6748 November 2002. 

4. Yang, J. C., Nyden, M.R, and Manzello, S.L., “Cold Discharge of CF3I in a Simulated 

Aircraft Engine Nacelle”, Proceedings of Halon Options Technical Working Conference, 2003. 

5. Witlox, H.W.M, and Bowen, P.J., “Flashing liquid jets and two-phase dispersion: A review”, 

Contract Research Report 403/2002, HSE Books, 2002. 

6. Fleming, J.W., Williams, B.A., and Sheinson, R.S., “Suppression Effectiveness of Aerosols: 

The Effect of Size and Flame Type”, Proceedings of the Halon Options Technical Working 

Conference 2002. 

7. For the NIST Building and Fire Research Laboratory (BFRL) website see www.bfrl.nist.gov