THE TRANSPORT OF WATER SPRAYS PAST GENERIC CLUTTER 

ELEMENTS 

FOUND WITHIN ENGINE NACELLES†

Peter J. Disimile and James R. Tucker 

USAF 46 Test Wing, Aerospace Survivability and Safety Flight 

2700 D Street, Bldg 1661 

Wright-Patterson AFB, Ohio 45433-7605 

Tel: (937) 255-6302 ext 212: Fax: (937) 255-2237 

E-mail: Peter.Disimile@wpafb.af.mil

Brian Croswell and John Davis 

Dept of Aerospace Engineering 

UC-FEST Research Group 

University of Cincinnati 

Cincinnati, Ohio 45221-0070 

The current work is presented as part of an effort to develop a spray transport model that would 

be used within the computational fire code currently under development by Sandia National 

Laboratory. As part of a halon replacement research program, new high-boiling-point chemical 

suppressants have been identified. These agents would discharge in a liquid state and initially 

result in the transport of liquid droplets through a portion of the nacelle, impinging on various 

objects prior to reaching the fire zone. The goal of this research effort is to enhance the 

fundamental knowledge of spray interactions with clutter (e.g., obstacles representing fuel and 

hydraulic lines, electrical wire bundles, etc). This paper reports an investigation into the effect of 

generic cylindrical clutter elements on the performance of a suppressant spray impinging on 

various clutter densities and porosities. Specifically the amount of agent (water/air spray) that is 

able to pass through a generic clutter configuration is presented as a function of clutter spacing 

and surrounding air flow (0.5 – 6.5 m/s) at nominal turbulence levels of 10%. 

INTRODUCTION 

Aircraft engine nacelles represent a unique environment for fires and fire protection. Many 

nacelles are characterized by high air exchange rates and a strong degree of nonuniformity and 

turbulence within the airflow. In addition, nacelles contain large amounts of plumbing, wiring, 

avionics and mechanical components mounted on the engine or along the nacelle walls. Some 

nacelles also have large ribs for structural purposes. This combination of a highly turbulent 

airflow, flammable fluids, and numerous ignition sources make aircraft engine nacelles a 

difficult fire zone to protect. 

For the past several decades, halogenated agents, notably halon 1301 have protected engine 

nacelles. Since the production ban on halon, scientists and engineers in the public and private 

sectors have been working on replacement agents and new technologies that attempt to replicate 

halon 1301’s beneficial attributes. Thus far none have succeeded completely. Of the 

technologies that have been deemed acceptable, based on environmental friendliness, toxicity, 
materials compatibility, etc. all lack fire-suppression efficiency as measured by weight and/or 

volume. 

To improve the fire-suppression efficiency of these candidate technologies, one area of focus is 

suppressant distribution. Since halon 1301 was so efficient, research into understanding the 

engine nacelle airflow offered little payoff. Today, however, a better understanding of the 

nacelle airflow and how it impacts fires within a nacelle, as well as suppression distribution, can 

have a large payoff in reducing the design time and/or weight of a halon replacement system. 

An additional complication is due to the fact that some halon replacements are high-boiling point 

fluids when compared to halon 1301. These can prove to be especially challenging to distribute 

in an aircraft engine nacelle. To understand how these systems would perform and thereby 

optimize a design, engineers must understand better how a cloud of liquid droplets behaves when 

moving within a highly turbulent airflow through a cluttered engine nacelle. To this end the 

present study focuses the ability of water drops to pass through a series of cylindrical obstacles, 

representing generic clutter, while moving in a turbulent coflow. 

SUPPRESSANT SPRAY FLOW FACILITY (SF)2

A low-speed flow facility with the test section air speed ranging from 0.0 to 12 m/s (0 to 40 ft/s) 

was modified for the current program. The major components of this facility include an inlet 

contraction, turbulence generator, test section, clutter section, and the return and separation 

plenum. A general overview of these components is provided in the following text. 

Air drawn from a large laboratory is accelerated towards the test section by a 3.5:1 inlet 

contraction that serves as the coflow supply. To minimize outside disturbances from propagating 

directly to the test section, a flow-conditioning unit was placed over the inlet entrance. At the 

exit of this contraction, a turbulence generation section was also attached. As the coflow exits 

the turbulence generation section, it enters the test section. The test section was divided into 

two zones, A and B. Figure 1 is a digital image of the upstream portion of the facility test 

section (zone A). 

Zone A also contains the suppressant spray nozzle and the clutter package. Zone B represents 

the remaining portion of the test section which is located downstream of the clutter package. 

Both zones have a 609.6 mm x 914.4 mm (24” x 36”) cross sectional area. Zone A is 1220 mm

(4 ft) in streamwise length, while zone B is 2440 mm (8 ft) in streamwise length. As previously 

noted provisions have been made for the insertion of clutter packages at the downstream end of 

zone A. The leading edge of the clutter package is held fixed at 952.5 mm (37.5 inches) from the 

turbulence generation grid. The exit of the suppressant spray nozzle is located within the coflow, 

596.9 mm (23.5 inches) downstream of the turbulence generation grid. Further the nozzle exit is 

positioned along the coflow centerline, located 304.8 mm (12 inches) vertically and 457.2 mm

(18 inches) horizontally from the test section walls. This also corresponds to the nozzle exit 

being located 355.6 mm (14 inches) upstream of the leading edge of the clutter package. 

2
Clutter 

Elements

Nozzle 

Turbulence 

Generator 

Figure 1. (SF)2 Zone A Test Section 

Utilizing constant temperature anemometry, velocity surveys at selected streamwise locations

within the coflow were performed. Specifically, hot film probes were used to measure the 

velocity profile within the (SF)2

 test section in the absence of any turbulence generating grid, 

spray nozzle, or clutter package. Figure 2 displays the results of vertical (y-direction) velocity 

traverses acquired using three hot film probes positioned in zone A, 723.9 mm (28.5 inches) 

downstream of the test section inlet, with the coflow air speed set at 5.0 m/s (16.4 ft/s). These 

probes were positioned on the centerline (z=0), and at transverse locations, z = ±168 mm (± 6.6 

inches). The measurement survey was arranged such that the y = 0 position corresponded to the 

vertical center of the test section. Hot film data were acquired at a sampling rate of 1 kHz for a 

period 20 seconds, providing 20,000 samples for each spatial location traversed. These data 

were analyzed and both the time average and standard deviation computed. Results of these 

surveys indicate that the mean velocity in the streamwise direction over the central portion of the 

test section (vertical height of 508 mm or 20 inches) was 4.75+/-0.17 m/s. Over the available 

span of the probe holder (approximately 16 inches) the two dimensionality of the coflow was 

determined to be ± 2%. Turbulent intensities (T.I.) of approximately 1% were also determined at 

all positions across the (SF)2 test section except close to the walls where boundary layer effects 

are observed. 

TURBULENCE GENERATOR 

A large grid consisting of several 25.4 cm (1 inch) wide, 6.35 mm (0.25 inches) thick, sharpedged flat steel slats were assembled in a checkerboard pattern with open cell dimension between 

the slats of 50.8 mm x 50.8 mm (2 inches x 2 inches). This grid is also visible in Figure 1. 

Velocity surveys using hot film probes were acquired downstream of the turbulence generator, at 

x = 711.2 mm (28 inches), Figure 2. The spatial locations traversed are identical to those used in 

3
the clean test section case. Readily apparent in this figure is the wave-like distribution of the 

streamwise component of the mean velocity with velocities ranging between approximately 3.7 

m/s to 5 m/s (12 ft/s to 16.4 ft/s). Although general symmetry can be observed it appears that the 

lower portion of the test section seemed to have a greater degree of unsteadiness. It is believed 

that additional unsteadiness was a result of smoke generation supply tubes positioned at the 

entrance of the inlet contraction. In a similar manner turbulent intensities are plotted in Figure 3. 

This figure indicates that the large disturbance grid produced T.I. levels in the range of 10 to 

14%. 

1.00

1.50

2.00

2.50

3.00

3.50

4.00

4.50

5.00

5.50

6.00

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

Y Position (mm)

Velocity (m/s)

Probe A Probe B Probe C

Figure 2. Streamwise Velocity Profiles with Large Grid 

Figure 3: Streamwise Turbulent Intensity Profiles with Large Grid 

5.0

6.0

7.0

8.0

9.0

10.0

11.0

12.0

13.0

14.0

15.0

16.0

17.0

18.0

19.0

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250 300

Y Position (mm)

T.I. (%)

Probe A Probe B Probe C

4
SUPPRESSANT SPRAY NOZZLE

The suppressant spray nozzle used in the present study was a dual-fluid, solid cone nozzle 

patented by the Navy as a Liquid Atomizing Nozzle, under the patent number of 5,520,331. 

Based on preliminary PDA measurements it was decided to reverse the gas liquid entrance 

locations as opposed to the specifications listed in the patent. The nozzle was installed at the y-z 

center of Zone A of the test section. This resulted in the centerline of the spray nozzle being 

positioned tangent to the centerline of the test section. However, the spray nozzle centerline was 

off center, ejecting its spray at a slight downward angle to the test-section centerline. This 

caused the spray at the center of the flow to have a nonzero velocity in the V and W directions. 

From a visual inspection, it can also be seen that the nozzle is slightly oriented in the negative y 

and positive z directions. Although slight, this misalignment was also confirmed during an 

examination of the velocity data. 

CLUTTER PACKAGE

The clutter package used in the current test series can be seen installed within zone A of the 

(SF)2

 test section (Figure 1). The present clutter package consists of 16 two-dimensional 

elements, spanning the width of the test section. These elements were cylinders and were made 

from sections of PVC plastic tubing. Elements were assembled to form three separate arrays 

with equally spaced rows of 5, 6, and 5 tube elements, separated vertically by 42.875 mm (1.75 

inches) between each cylindrical element. The elements have an outer diameter of 50.8 mm (2 

inches). The streamwise spacing between each clutter array is variable, ranging from 13.27 mm

(0.5 inches) to 101.6 mm (4 inches) with an error of ±3.175 mm (0.125 inches). In terms of 

element diameter, the spacing ranges from 0.25 D to 2 D with an error of 0.0625 D ( Figure 4). 

U

12D

Clutter 

D = 50.8 mm OD 

PVC tubes 

(not to scale)

2D

1.875D

0.94D

Spray

Nozzle

Coflow

Return & 

Separation

Plenum

7D

Turbulence 

grid

18.75D

 Figure 4. Sideview Schematic of SF2

 Test Section 

5
RETURN AND SEPARATION PLENUM 

Once the combined coflow air and suppressant mixture exit zone B of the test section, the liquid 

component must be separated. The separation of the liquid was achieved by first redirecting the 

flow downward into a recovery pan using a series of turning vanes (see Figure 5). Upon 

approaching the liquid pool, already present in the recovery pan the flow is required to make a 

second, 180 degree turn. Due to the combined mass and high momentum possessed by the liquid 

drops, the majority of drops are unable to negotiate the tight 180-degree turn, thus impacting the 

liquid pool. To further minimize suppressant from exiting the return plenum and leaving the 

flow facility, a 10 micron filter was installed. 

PHASE DOPPLER ANEMOMETRY SYSTEM

A three-dimensional Phase Doppler Anemometry (PDA) was used to acquire velocity 

components as well as the diameter of the liquid drops exiting the suppressant spray nozzle. To 

position the measuring volume in the center of the test section 1000 mm lenses were used on 

both the transmitting and receiving optics. This measurement configuration also utilized the 

largest spatial filter (Mask A) in the receiving optics. In this configuration the maximum drop 

diameter capable of being measured was 810.8 μm. 

= Water 

Particle

Filter

Drain

= Air Stream

48”

6”

24”

16”

Air Outlet

1”

1”

Collection Pan

=Turning 

vanes

Figure 5. Side view Schematic of SF2

 

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EXPERIMENTAL STRATEGY 

The present experimental program utilized a dual-fluid nozzle emitting an air/water spray. This 

spray was injected into the coflow and allowed to impinge directly on the first array of 

cylindrical clutter elements. This program was divided into two phases: The first phase 

documented the transport behavior of a liquid suppressant(water) and its ability to pass through a 

generic clutter package. The second phase focussed on the measurement of water drop velocities 

and diameters using the PDA system. 

GENERAL STRATEGY 

The spray nozzle water flow rate was set and maintained at 17.1 ± 0.4 L/min (4.5± 0.1 gal/min) 

at a corresponding nozzle water pressure of 158.0 ± 13.7 kPa (23 ± 2.0 psi). The water flow was 

monitored using a turbine type flow meter that was positioned directly upstream of a pressure 

gage. The incoming air was regulated to a pressure of 171.8 ± 3.4 kPa (25 ± 0.5 psi). Five 

different clutter package densities were evaluated during both phases of experimentation. These 

densities were varied by changes in the streamwise spacing between individual clutter arrays. 

Array spacing was selected to be 0.25 D, 0.5 D, 1.0 D, 1.5 D, and 2.0 D, where D is the diameter 

of an individual tube. In the current study D = 50.8 mm or 2 inches. 

SUPPRESSANT VOLUME RECOVERY STRATEGY 

Experiments were initiated to determine the volume of water transmitted through the clutter 

package as a function of the streamwise spacing of the clutter array and coflow speed. Thirteen 

air speeds were investigated ranging from 0.5 m/s to 6.5 m/s (1.5 ft/s to 19.5 ft/s) in increments 

of 0.5 m/s (1.6 ft/s). To measure the volume of water passing through the clutter configuration 

accurately, a repeatable procedure for measuring the liquid water volume was followed. This 

included the water supply tanks being filled to the same initial starting volume of 303 L (80 gal), 

the coflow set to the desired speed, and the nozzle air pressure set. A run-time clock was then 

energized when the water pump was powered on such that the water flow through the nozzle was 

set to 17.1 ± 0.4 L (4.5 ± 0.1 gal/min). Due to gravity, water drops that wetted the clutter pooled 

on the individual elements and dripped down to a collection pan located beneath the clutter. 

Water droplets that passed through the clutter into zone B were deposited within the return 

plenum. The volume of water that is collected directly under the clutter package was carefully 

measured, as was the water collected in the return plenum by using graduated containers. 

PDA STRATEGY 

PDA measurements were also acquired upstream of the clutter package. At a location in the 

center of the Zone A test section (z = 0) and 2D (101.6 mm or 4 inches) upstream from the 

leading edge of the clutter package, velocity and diameter traverses were performed. 

Specifically, the vertical upstream traverses involved PDA measurements at 37 positions (in the 

y-direction @ z = 0) through the center of the water spray. At each location 20,000 data points 

were acquired. Each vertical location was spatially separated by 3.155 ± 0.022 mm (0.124 

inches ± 0.001 inches). The center of the spray nozzle was identified as the location where U, 

the streamwise component of the mean velocity, reached a maximum, while the lowest Sauter 

and arithmetic mean diameters were observed. Therefore, the starting and ending position of the 

traverse could be slightly different depending on this offset, which was within approximately 

3.175 to 9.525 mm (0.125 inches to 0.375 inches). The coflow air speed was maintained at a 

7
constant value of 5.0 m/s ± 0.5 m/s (16.4 ft/s ± 1.6 ft/s) during PDA. Optimal PDA data 

acquisition was achieved with a laser power setting of 300 mW. 

SUPPRESSANT VOLUME RESULTS AND DISCUSSION 

Figure 6 shows the percentage of water recovered directly under the clutter package as a function 

of element spacing and air speed.. The general trend of the data suggests that the volume of the 

liquid collected decreases with increasing coflow speed. In fact above 4.5 m/s (~15 ft/s) less 

than 5% of the liquid is captured and the differences between the five clutter densities diminish. 

However, with the coflow speed set to 0.5 m/s (1.6 ft/s) the amount of water collected under the 

clutter ranged from 61.25% (at 0.25 D spacing) to 37.50% (at 2.0 D). As the spacing between 

the individual clutter arrays is reduced, the amount of water captured increases. Specifically, as 

the streamwise spacing was reduced from 2.0D to 0.25D, thereby increasing the clutter package 

density, the water collected increases to over 50%. Data regression using a 3rddegree polynomial 

was also applied to each clutter-spacing data set and excellent correlation resulted. 

0

10

20

30

40

50

60

70

0 1 2345 6 7

Cofow Airspeed (m/s)

Clutter Volume (% of Initial Supply)

0.25 D 

0.5 D

1.0 D

1.5 D

2.0 D

0.25 D

0.5 D

1.0 D

1.5 D

2.0 D

Figure 6. Clutter Recovery vs. Airspeed 

Likewise, Figure 7 indicates the amount of water collected in the return plenum, as function of 

the coflow speed and clutter spacing. All five clutter densities are represented in this figure. The 

volume in Figure 7 is also represented as a percentage of the total starting supply volume. 

8
20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6

Coflow Airspeed (m/s)

Plenum Recovery Volume 

(% of Initial Supply)

0.25 D

0.5 D

1.0 D

1.5 D

2.0 D

7

Figure 7. Plenum Recovery vs. Airspeed 

The trends shown in Figure 7 are the inverse of those seen in Figure 6. The volume collected in 

the plenum increases as the coflow airspeed increases. The change in clutter density also has the 

opposite effect on the plenum recovery volume compared to that of the clutter recovery volume. 

With the coflow airspeed set to 0.5 m/s the amount of water collected under the clutter ranged 

from 37.5% (at 0.25 D spacing) to 58.75% (at 2.0 D). The differences between the five clutter 

spacings also diminish at speeds of approximately 3.5 to 4.0 m/s and higher. Also it can be seen 

that the volume recovered in the plenum approaches 100% as the test section speed increases. 

During the test evaluation (five clutter spacings and 13 coflow speeds) the percentage of 

unaccounted water volume was determined. In all cases the difference was random and less than 

4%. 

SPRAY DOCUMENTATION 

A velocity vector plot provides an indication of the behavior of the water spray in twodimensional space. Figure 8 displays the results of a typical u,v vector plot prepared by

traversing across the spray in the y-direction at x = 254.00 mm (10 inches) and z = 0 mm, when 

the clutter array x-separation was set to 2D. In general the flow appears to have a symmetric 

spread with the maximum velocity occurring at the jet centerline. Immediately evident as the 

water drops exit the nozzle, the drops decelerate in the streamwise direction and spread in the ydirection. It was determined that the maximum streamwise droplet velocity (U) was 

approximately 20 m/s (65 ft/s) in the center (y=0), decreasing to 9 m/s (30 ft/s) at the edge of the 

spray. The V component of velocity varied between 1 m/s and 2 m/s (3.3 ft/s and 6.6 ft/s), 

however, the W component was found to be approximately symmetric around YZ equal zero, 

varying between –1 m/s and +1 m/s (-3.3 ft/s and +3.3 ft/s). An examination of these profiles 

indicated that the spray centerline was pointed downward 0.8 degrees, relative to the x-z plane, 

which confirmed the visual indication.

9
The Arithmetic mean and Sauter diameter profiles across the center of the jet are shown in 

Figure 9. From this figure it can be seen that both the Arithmetic and Sauter mean diameter 

profiles are parabolic in nature. The smallest particles were measured towards the center of the 

spray. By comparing the diameter and velocity profiles, it can be seen that their diameters are 

inversely related to the drop velocity. Over the extent of the traverse through the spray, Sauter 

diameters varied between 285 microns at the center of the jet to 350 microns at the edge of the 

jet. 

Figure 8. UV Vector Plot 

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Arithmetic & Sauter Mean Diameter Profiles

100

150

200

250

300

350

400

-70 -65 -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70

Y Position (mm)

Mean Diameter (μm)

Arithmetic Mean Sauter Mean

Figure 9. Mean Diameter Profiles 

SUMMARY 

To help provide insight into the transport of fire suppressants with high boiling points, an air/-

water spray was ejected into a highly turbulent coflow of air. Documentation of this coflow at a 

nominal test section setting of approximately 5 m/s (16.4 ft/s) verified the presence of a highly 

turbulent flow. Under these conditions the streamwise TI was estimated between 10 to 14% and 

mean velocity distribution between 3.7 to 5 m/s (12 to 16.4 ft/s). 

Once the nominal boundary conditions were documented, the suppressant volume conservation 

experiments were performed. This evaluation indicated that, at low coflow airspeeds, as much as 

61.25% (at 0.25 D clutter spacing) and 37.50% (at 2.0 D clutter spacing) of the total incoming 

water supply was collected directly under the clutter element. This corresponded to only 37.5% 

(at 0.25 D clutter spacing) and 58.75% (at 2.0 D clutter spacing) of the water volume passing 

through the clutter package. As a measure of repeatability no less than 96% of the total supply 

volume was recovered in the combined clutter and plenum return recovery tanks. 

The results of this experiment provide valuable insight into the transport of liquid agents through 

engine nacelles and provide essential data to develop first-generation computer models to aid in 

the design of fire suppression systems for aircraft engine nacelles. The work however, only 

addresses round elements of a uniform size and distribution. More data will need to be added to 

create a library of information examining effects of different-sized elements, nonuniform

distribution of elements, sharp-edged elements, as well as different levels of turbulence. 

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ACKNOWLEDGEMENTS 

This research is in part supported by the Department of Defense's Next Generation Fire 

Suppression Technology Program, funded by the DoD Strategic Environmental Research and 

Development Program. The authors would like to thank Dr. Dave Keyser acting on behalf of 

Naval Air Systems Command for coordinating this activity as well as providing invaluable 

discussion relating to the experiments 

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