LIQUID AGENT TRANSPORT AROUND SOLID OBSTACLES 

Cary Presser†

, John Widmann†, and George Papadopoulos‡

†

National Institute of Standards and Technology 

Gaithersburg, MD 20899-8360 

‡

Dantec Dynamics 

Mahwah, NJ 07430 

ABSTRACT 

The focus of this effort is to investigate the dispersal of liquid fire suppression agents 

around solid obstacles, and obtain a better understanding of the physical processes of 

droplet transport in cluttered spaces. An experimental investigation is presented to 

examine the flow field dynamics of highly turbulent flow over obstacles, and spray 

transport in such flow fields. The obstacles of interest were a cylinder and body-centered 

cube (BCC) arrangement of spheres. Transport of both water droplets and seed particles 

was characterized upstream and downstream of these obstacles using particle image 

velocimetry (PIV). Data were recorded for the cylinder at ambient temperature and after 

being heated to 423 K to estimate the effects of the hot cylinder surface on droplet 

transport. The results indicate that smaller droplets are entrained into the recirculation 

region behind the cylinder while the larger droplets impact the cylinder surface, 

accumulate, and drip off, or disperse radially away from the surface regardless of their 

interaction with the cylinder. The flow over the heated cylinder indicates the formation 

of layer of vaporizing liquid on the downstream side of the cylinder in the shear region 

between the recirculation zone and free stream. In addition, surface cooling that 

resulted from spray impingement was around 50 % of the preset cylinder temperature. 

For the BCC (with a blockage ratio of about 64 %), there is both transport of droplets 

and seed particles around and through the BCC, as well as significantly more liquid 

accumulation and dripping than for the cylinder. 

INTRODUCTION 

The new generation of Halon replacements include chemical suppressants that have high 

boiling point temperatures (i.e., Tb > 330 K) and exist in liquid phase under high-pressure 

release or in ambient conditions. Release of these agents in a confined space results in 

the dispersal of droplets that either travel along ballistic trajectories, move with the 

convecting flow, or a combination of the two depending on the local Stokes number of 

the droplet. Therefore, an accurate representation of droplet transport is crucial for 

numerical modeling of fire suppression in confined spaces using these agents. To better 

understand the physics of droplet transport around and behind solid objects, an 

experimental arrangement at the NIST spray facility has been modified to impose 

controlled grid-generated turbulence on the air stream. Experimental results from this 

facility provide new experimental data for a well-characterized, homogenous dropletladen turbulent flow field around prescribed obstacles. This experimental effort 

compliments another numerical effort to develop and validate the subgrid model of the 

VULCAN Computational Fluid Dynamics (CFD) fire physics code for spray-clutter 

interactive environments, under development at Sandia National Laboratories. 
 

Baseline measurements of the discrete (spray) and continuous (gas) phase velocities with 

prescribed obstacles are reported in this study. This work is a continuation of a previous

investigation on the measurement and numerical prediction of homogeneous turbulent 

flow over different diameter cylinders [1], which serves as a baseline for this dropletladen flow study. The results were obtained using particle image velocimetry, and 

visualization techniques. Operating flow rates were based on typical engine-nacelle-like

conditions. Liquid agent transport was investigated around two representative obstacles - 

a cylinder (having a diameter larger than the characteristic length-scale of turbulence),

and a body-centered cubic arrangement of spheres. The cylinder represents a classical 

configuration and the body-centered cube represents a more complicated scenario. The

cylinder was also heated to explore thermal effects on droplet vaporization and transport 

around the cylinder. These data will be used for subgrid model validation of the 

VULCAN fire physics code. This paper presents a description of the experimental 

arrangement and diagnostics used to characterize the droplet-laden flow field. The 

experimental results from the nonheated and heated cylinder, and body-centered cube are 

compared and discussed, along with comments on future work. 

EXPERIMENTAL ARRANGEMENT 

In order to explore droplet transport around obstacles, an experimental arrangement is 

used that provides a well-characterized homogeneous turbulent flow field around 

prescribed obstacles, and is shown in Fig. 1. The experimental setup includes a 

nonswirling air that coflows around the liquid agent injector. The experiment is oriented 

horizontally, as opposed to the original vertical arrangement (see Ref. 1), to enable 

collection of liquid agent that drips off of the obstacle, and prevent liquid droplets 

downstream of the obstacle from falling back upstream into the oncoming stream. The 

agent used in this study was water that was supplied to the flow field with a 60o

 hollowcone pressure-jet atomizer (with a nominal flow rate of 3.78 kg/h). An octagon-shaped 

Plexiglas insert (with a wall thickness of 6 mm, length of 610 mm, and major and minor 

axes of 760 mm and 560 mm) was used as a boundary condition. The Plexiglas insert,

along with the front face that supported the inlet passages for the liquid agent and air, and 

back face that supported the exhaust passage, served to form a closed system. A 

honeycomb layer (51 mm thick with 3 mm size cells) is used to straighten the airflow, 

which is co-positioned around an injector for the agent. Grid-generated turbulence is 

imposed on the air stream by placing a square layer of wire mesh screen (with dimensions 

of 229 mm by 330 mm, and 13 mm size cells) downstream of the honeycomb. For these 

experiments, the incoming air (supplied from a 7 L/min compressor) was directed entirely 

through a distributor plate with steel wool, circular cross sectional area of the honeycomb 

(203 mm in diameter), and then through wire mesh screen (placed 25 mm downstream of 

the honeycomb), as shown in Fig. 1. The face of the liquid atomizer was placed flush with

the upstream side of the grid, and centered within one mesh cell so that the liquid spray

would be unimpeded by the grid. The integral and Kolmogorov length scales of 

turbulence are estimated to be 3 cm and 100 μm, respectively. A stepper-motor-driven 

traversing system translates the entire assembly, and permits measurements of the flow 

field properties at selected locations downstream of the injector. 

2
 

air

liquid

agent

honeycomb

distributor grid

spray

heated obstacle

plexiglas chamber

collector

collector (pan)

exhaust

seeding

agent

atomizer

air

liquid

agent

honeycomb

distributor grid

spray

heated obstacle

plexiglas chamber

collector

collector (pan)

exhaust

seeding

agent

atomizer

air

liquid

agent

honeycomb

distributor grid

spray

heated obstacle

plexiglas chamber

collector

collector (pan)

exhaust

seeding

agent

atomizer

air

liquid

agent

honeycomb

distributor grid

spray

heated obstacle

plexiglas chamber

collector

collector (pan)

exhaust

seeding

agent

atomizer

air

liquid

agent

honeycomb

distributor grid

spray

heated obstacle

plexiglas chamber

collector

collector (pan)

exhaust

seeding

agent

atomizer

air

liquid

agent

honeycomb

distributor grid

spray

heated obstacle

plexiglas chamber

collector

collector (pan)

exhaust

seeding

agent

air

liquid

agent

honeycomb

distributor grid

spray

heated obstacle

plexiglas chamber

collector

collector (pan)

exhaust

seeding

agent

atomizer

Figure 1. Schematic of the experimental arrangement for the droplet laden grid-generated 

 turbulent flow field. 

Measurements were carried out with two obstacles, an aluminum cylinder and a bodycentered cube arrangement of wooden spheres (see Figs. 2 and 3, respectively). The

cylinder used in this study has a diameter of 32 mm, which was chosen because its 

diameter is larger than the integral length scale of turbulence (see Ref. 1). The cylinder is 

an aluminum rod with a length of 305 mm (see Fig. 2). A hole (13 mm in diameter) was 

bored through the center for placement of a 250 W cartridge heater (13 mm in diameter

and 76 mm in length). The rod was also cut into two halves and 1 mm deep channels 

bored along one segment for placement of five K-type thermocouples. The thermocouples 

have an inconel sheath, ungrounded, and 0.8 mm in diameter (305 mm long). The

thermocouples were placed in a cross pattern in the center of the rod (each separated by a 

distance along the surface of 6.4 mm, with their tips (thermocouple junctions) placed

about 3.2 mm of the surface with bored holes at each location). The central thermocouple

was used for temperature control of the heater, which was positioned behind the 

thermocouples. The BCC is composed of nine wooden spheres with a nominal diameter

of 28 mm, all interconnected with posts (with a nominal length of 24 mm and width of 

3.2 mm) as shown in Fig. 3. (The posts connecting the center sphere were nominally 17 

mm in length.) The blockage ratio, or obstructed cross-sectional area for an equivalent 

area encompassing a face of the BCC, was about 64 %. The two obstacles were coated

with flat black paint to reduce reflections of laser light off their surface for any optical 

measurements. The obstacles were placed nominally 182 mm downstream of the 

honeycomb, and centered with the atomizer centerline. Choice of location of the screen 

and obstacles is discussed further in Ref. 1. 

3
 

aluminum rod heater

thermocouples

controller

data acquisition

305 mm

13 mm 32 mm

aluminum rod heater

thermocouples

controller

data acquisition

305 mm

13 mm 32 mm

aluminum rod heater

thermocouples

controller

data acquisition

305 mm

13 mm 32 mm

aluminum rod heater

thermocouples

controller

data acquisition

305 mm

32 mm

aluminum rod heater

thermocouples

controller

data acquisition

305 mm

13 mm 32 mm

a

heater wires 

thermocouple for set 

point temperature 

controller

b 

Figure 2. Experimental arrangement for the heated cylinder: a) schematic and b) front

view. 

The flow field was characterized using a three-dimensional particle image velocimetry 

(PIV) system from Dantec Dynamics* (see Fig. 4). PIV is a non-intrusive field measuring 

technique (as opposed to a single-point diagnostic method) that can measure two or three 

components of velocity. The method requires seed particles to be added to the flow, 

which are then assumed to follow the streamlines and act as tracers. The spatial 

displacement of the seed particles, corresponding to two images separated by a known 

time period (the time between laser pulses was 70 μs), is measured and the velocity is

deduced. The pencil fogger generates water-based seed particles approximately 1 μm in

diameter. A separate supply of air (negligible with regard to the total compressor air flow 

supplied to the experiment) was used to carry the fog aerosol from an enclosed aluminum 

4

 

*Certain commercial equipment or materials are identified in this publication to specify adequately the

experimental procedure. Such identification does not imply recommendation or endorsement by the 

National Institute of Standards and Technology, nor does it imply that the materials or equipment are 

necessarily the best available for this purpose. 
 

5

streamwise flow

direction

streamwise flow

direction

m

Figure 3. View of the body-centered cube of spheres.

 24 m 24m

 28 mm 28

box containing the fogger into the main air stream downstream of the distributor plate 

(see Fig. 1). A tube with several holes was stretched across the passage to enable 

dispersion of the fog seed across the measurement area. The PIV images are obtained in

a plane corresponding to the laser light sheet, which is oriented downward from above 

the Plexiglas chamber and onto a cross section of the obstacle. 

The 3-D stereo PIV system differs from traditional PIV systems in that two CCD cameras 

are used, and three velocity components are measured [2]. A 50 mJ Nd:YAG laser was 

used as the illumination source for the PIV measurements. The pulse duration of the

laser light sheet was about 5 ns, and the wavelength of the light was 532 nm. A pair of 

12-bit double-frame CCD cameras with a resolution of 1024 x 1280 pixels was used to 

obtain the images. Special camera mounts were utilized to permit the rotation of the

camera body with respect to the camera lens so the Scheimpflug condition was satisfied, 

permitting the laser sheet to be in focus despite the non-orthogonal camera alignment [2]. 

Bandpass filters (center wavelength = 532 nm, acceptance window = 30 nm) were used to 

reject the broadband white light from the room. The two cameras were placed at angles of 

about 95o

 and 145o from the vertical orientation of the laser beam, i.e., as measured from

the forward direction of propagation of the laser sheet. The processed results are 

presented as a composite mapping of individual planar regions that are about 33 mm in 

height and 53 mm in width (i.e., field of view for common overlap region of both 

cameras). Statistics were obtained from about 500 - 700 individual records (obtained at a 

rate of approximately 3 Hz). Each field of view represents about 9900 vectors for the 

seeded-only flow, and 4950 vectors for the spray-only case. The spatial resolution of the 

measurements depends directly upon the pixel interrogation area (i.e., 16 x 16 pixels, 

with a pixel size/resolution of 6.7 μm), resulting field of view, total number of pixels, and

laser light sheet thickness (i.e., 2 – 3 mm), and indirectly on the seeding density. Using 

only seed particles, the spatial resolution was approximately 700 μm in the plane of the 
 

560 mm

760 mm

PIV laser/mirror

PIV camera

PIV camera

Plexiglas chamber

obstacle

grid / atomizer /

honeycomb

θ

560 mm

760 mm

PIV laser/mirror

PIV camera

PIV camera

Plexiglas chamber

obstacle

grid / atomizer /

honeycomb

560 mm

760 mm

PIV laser/mirror

PIV camera

PIV camera

560 mm

760 mm

PIV laser/mirror

PIV camera

PIV camera

Plexiglas chamber

obstacle

grid / atomizer /

honeycomb

θ

 laser/mirror 

 cameras

 experiment a b

Figure 4. View and schematic of the experimental arrangement with the cameras and laser 

 from the PIV system. 

laser sheet (for the spray only, the resolution was about 1.5 mm due to the lower droplet 

concentration and larger droplet sizes). The velocity resolution was about 5 mm/s. 

Measurements of the flow field were carried out along the centerline of the measurement

area for the cylinder and BCC. Also, two additional off-axis planes were interrogated to 

map out one quadrant of the BCC, i.e., 1) between the center and two outer spheres, and 

2) slightly outside of the center of the two outer spheres. For the cylinder, three cases

were recorded, i.e., 1) with seed only, 2) with droplets only, and 3) with both seed and 

droplets, in order to try to isolate the effects of the droplets/particles on the flow field 

dynamics. 

RESULTS AND DISCUSSIONS 

FLOW FIELD VISUALIZATION 

The droplet-laden flow field over each obstacle was recorded with a digital movie camera 

at 9 frames/s. Examples of the observed droplet/particle transport processes are shown in 

Fig. 5 for a) seeded only and b) droplet only flow over the unheated cylinder, and c) 

combined droplet and seeded flow over the BCC. The seed is entrained in the turbulent 

flow field (see Fig. 5a) and a relatively high concentration of particles is observed in the 

wake behind the cylinder [3]. On the other hand, the droplets in the center of the spray 

are found to impinge on the surface of the cylinder while those droplets at larger radial 

positions flow around or past the cylinder with an increased radial component of velocity 

(see Fig. 5b). Few droplets are observed behind the cylinder in its wake. Droplets that 

impinge and wet the cylinder surface drip off at a rate of approximately one drop every

5 s. The seed (fog smoke) was observed to leave a residue on the upstream side of the

cylinder. There was no evidence of secondary breakup of the droplets, which was

expected because the Weber number (We) was too low (We ≅ 140 for a 500 μm diameter 

water droplet, whereas the critical Weber number for droplet shattering is approximately 

6
 

7

a

b

Figure 5. Photographs of the a) seed/droplet-laden flow field around the unheated

cylinder, b) droplet-laden flow field around the unheated cylinder, and

c) seed/droplet-laden flow field around the body-centered cube of spheres. 

Flow direction is from right to left. 

c 

two orders of magnitude larger) [4]. When the cylinder was heated to 423 K, the spray 

cooled the cylinder down to 351 K. The droplet-laden flow over the cylinder appeared to 

be similar to the unheated case except along the shear layer downstream of the cylinder.

In this region, a vapor layer formed which was presumed to be the result of vaporization 

of the liquid that wet the hot surface.

The transport of droplets through the BCC (see Fig. 5c) is interesting in that both the 

spheres and connecting rods (that act like cylinders) block the droplets, while the flow

field traversing through the obstacle will also provide a path for the entrained droplets 

(recall that the nominal blockage ratio is about 64 %). Dripping is observed from each 
 

sphere at a rate of approximately 1 droplet/s. If one assumes that droplets fall off each 

sphere at this rate, one can determine that this liquid represents approximately 4.5 % of 

the inlet water flow (assuming a dripped droplet diameter of 8.5 mm, which was an 

estimated largest droplet size observed from digital movies). Although the BCC has 

more dripping of liquid than the cylinder, the major portion of the spray is still able to 

traverse the obstacle. 

DROPLET/PARTICLE VELOCITIES MEASUREMENTS 

Particle image velocimetry (PIV) was used to obtain instantaneous two-dimensional 

images of the air flow field velocity (i.e., three components of velocity). These images 

were then used to construct profiles of the streamwise (W) and two cross-stream (U – 

spanwise along the cylinder length, and V – upward vertically from the cylinder) mean

velocities, turbulence intensity (Ti, where i = U, V, W), and velocity correlation 

coefficients (ρ, from which the Reynolds stresses can be derived), both upstream and 

downstream of the cylinder, and along the central plane that is referenced by the position

of the atomizer. Although the orientation of the Plexiglas chamber was different than in

our earlier study [1], and an atomizer was incorporated into this configuration, little 

difference was found in the homogeneous turbulent flow field. The maximum gas 

streamwise component of velocity was W ≅ 4.0 m/s (Reynolds number of about 3300, 

based on the grid cell size), and the maximum turbulence intensity was TW ≅ 7.5 %. Note 

that these values are slightly smaller than reported earlier because obstacles were placed 

approximately 75 mm further downstream than in our previous study in order to reduce 

the influence of the jetting from the grid (see Ref. 1).

The three components of droplet velocity for the flow over the unheated cylinder are 

presented in Fig. 6. In the figure, three cases are presented that represent the flow with 

the seed only, both seed and spray, and spray only. The black circle represents the 

position and size of the cylinder. The black contours lines represent stream traces (i.e., 

direction) of the in-plain V-W velocity vectors. The seed only case compares with our

earlier study of the fluid flow over different size cylinders (see Ref. 1). The flow pattern 

is, as expected similar to the previous study. The free stream air velocity reaches a 

streamwise value of W ≅ 4.5 m/s around the side of the cylinder, and the flow behind 

each cylinder forms a recirculation region (W ≅ -1.0 m/s). The reverse flow behind the 

cylinders appears to extend about two cylinder diameters downstream. The turbulence 

intensity increases behind the cylinders (TW ≅ 30 %) in a shear region behind the obstacle. 

There is a significant correlation of the velocity fluctuations downstream of the cylinder 

between the streamwise and V-cross-stream components (an absolute value of 0.7 is 

reached for the correlation coefficient, similar to what was reported in Ref. 1). This 

result is indication of the shear between the free stream and reverse flow region behind 

the cylinder. The flow also decelerates to a stagnation region along the centerline at the

upstream face of the cylinder. The V-component of velocity increases significantly

around the side of the cylinder indicating the radial displacement and separation of the 

droplets, seed, and fluid flow. Comparison of the two cases with spray to the seed-only 

case indicates that the recirculation zone is somewhat larger for the spray cases. Larger 

8
 

9

Figure 6. Variation of the mean streamwise and cross-stream velocities with

downstream distance for the unheated cylinder. Contours are the stream

traces of the in-plane vectors obtained from the axial and radial components

of velocity. 

size droplets require a longer distance to interact with the turbulent flow field, reduce 

their higher momentum, and be entrained into the recirculation zone. 

The stream trace (contour lines) also shows an unexpected pattern behind the cylinder 

near the stagnation point. Instead of presenting closed loops that indicate a recirculating 

pattern, the contours appear to emanate from the stagnation region. One possible 

explanation is that in the stagnation region the U-component of velocity reaches an

absolute value of nearly 0.8 m/s, indicating a strong spanwise flow along the length of the 

cylinder in both directions. Spanwise flow is also found on the upstream side face of the

cylinder. It appears to be generated by the cylinder and may be related to the finite

cylinder length (with an aspect ratio of about 10:1) [5]. The droplet-laden flow over 
 

Figure 7. Variation of the mean streamwise and cross-stream velocities with

downstream distance for the heated cylinder. Contours are the stream traces

of the in-plane vectors obtained from the axial and radial components of

velocity. 

the heated cylinder, presented in Fig. 7, is similar to the unheated cases (only the spray

and combined spray/seed cases are presented). Differences between the heated and

unheated cases are found for the spray-only case: 1) a stronger streamwise flow and 

weaker reverse flow behind the cylinder (see W-component of velocity), 2) a weaker 

radial flow around the side of the cylinder (see V-component of velocity), and 3) the Ucomponent cross-stream flow is further downstream of the cylinder. The result is that the 

hot cylinder is developing a more streamwise droplet-laden flow near the cylinder.

Figure 8 presents velocity field around the BCC for the combined spray/seed case. 

Although the configuration is more complicated, there are still similar features to the 

cylinder case. The flow accelerates around the spheres and there is reverse flow in the

wake region. In addition, the presence of cross-stream component of the flow is evident 

around the spheres. This cross flow may be a result of the three-dimensional nature of 

the spheres and flow field, and the transport of entrained droplets and seed through 

convoluted pathways of this obstacle. 

10
 

11

 the axial and radial components of velocity. 

Figure 8. Variation of the mean streamwise and cross-stream velocities with

downstream distance for the body-centered cube of spheres. Contours are the

stream traces of the in-plane vectors obtained from the axial and radial

components of velocity. 

NEXT PHASE 

This experimental work will study next droplet transport processes of another liquid fire 

suppressant agent in the regions upstream and downstream of different obstacle 

configurations. The liquids that will be used are water and HFE-7100 (boiling point of 

334 K) because of the effectiveness of high boiling point liquids to extract heat from a 

flame zone. A two-component phase Doppler interferometry system will be used to 

obtain spatial profiles of the droplet size and velocity distributions, and droplet number 

density, and allow comparison of water and HFE-7100 in order to study the effects of

different physical properties on droplet transport. If needed, PIV measurements will be 

carried out to provide the spray and flow field characteristics upstream and downstream 

of obstacles under different operating conditions. The results will be used to provide data 

for input and validation for the subgrid turbulence model of the VULCAN fire code. 
 

CONCLUSIONS 

Particle image velocimetry measurements were carried out for a droplet-laden, 

homogeneous turbulent flow over an unheated and heated cylinder, and body-centered 

cube arrangement of spheres. Droplets were observed to impact the cylinder surface and 

either wet (leading to dripping) or rebound off the surface. The Weber number was too 

low to lead to droplet shattering. Significant spray cooling of the surface and the 

presence of a vapor stream downstream of the cylinder along the shear layer was 

observed for the heated cylinder. Comparison of the velocity fields obtained with water 

droplets from a pressure-jet atomizer and 1 μm size seed particles formed from a fogging 

device indicate that dispersion of droplets/particles around an obstacle is dependent on its 

size. Future efforts of this work will be directed to investigating other liquid agents (i.e., 

HFE-7100) to determine physical property effects on transport processes. Also, these 

results provide a baseline for future comparison with simulations from the Vulcan fire 

code under development at Sandia National Laboratories. 

ACKNOWLEDGEMENTS 

The authors wish to acknowledge the partial support of this research by the Department 

of Defense Next Generation Fire Suppression Technology Program, funded by the DoD 

Strategic Environmental Research and Development Program. The authors wish to thank 

P. DesJardin, L. Gritzo, D. Keyser, P. Disimile, and J. Tucker of the NGP Project 6A/1 

on development of a spray-clutter interaction model for the discussions and guidance that 

they have provided us, Mr. R. Fink for his technical assistance in completing the NIST 

facility modifications. 

REFERENCES 

1. Presser, C., Widmann, J.F., DesJardin, P.E., and Gritzo, L.A., "Measurements and Numerical

Predictions of Liquid Agent Dispersal Around Solid Obstacles," Proc. Halon Options 

Technical Working Conf. (HOTWC-2001), pp. 122-130, New Mexico Engineering Research

Institute, Albuquerque, NM, 2001. 

2. Flowmap® 3D-PIV System Owner’s Manual, Dantec Measurement Technology, Inc., 

Mahwah, NJ, July 1999. 

3. Wang, G., Bachalo, E.J., Sankar, S.V., and Bachalo, W.D., "An Investigation of Spray 

Interaction with Large-Scale Eddies, " Proc. 5th Annual Conf. on Liquid Atomization and 

Spray Systems (ILASS Americas '92), pp. 53-58, San Ramon, CA, 1992. 

4. DesJardin, P.E., Presser, C., Disimile, P.J., and Tucker, J.R., "A Droplet Impact Model for 

Agent Transport in Engine Nacelles," Proc. Halon Options Technical Working Conf.

(HOTWC-2002), on CD, New Mexico Engineering Research Institute, Albuquerque, NM,

2002. 

5. Williamson, C.H.K., “Oblique and Parallel Modes of Vortex Shedding in the Wake of a 

Circular Cylinder at Low Reynolds Numbers,” J. Fluid Mech., Vol. 206, pp. 579-627 (1989). 

12