MEASUREMENTS AND NUMERICAL PREDICTIONS OF LIQUID AGENT DISPERSAL 

AROUND SOLID OBSTACLES* 

Cary Presser and John F. Widmann 

National lnstitute of Standards & Technology 

Paul E. DesJardin and Louis A. Gritzo 

Sandia National Laboratories 

ABSTRACT 

The focus of this effort is to investigate the dispersal of liquid agents around solid obstacles and to obtain a better 

understanding of the physical processes of droplet transport in cluttered spaces. A combined experimental/numerical study is presented to examine the flow field dynamics ofhighly turbulent flow over obstacles, and spray transport in such flow fields. Comparison of numerical predictions to the particle imaging velocimetry (PlV) measurements shows good qualitative agreement in spatial distribution of the mean and RMS velocities with a 20% overprediction in mean streamwise velocity and 15% underprediction in RMS. A numerical sensitivity study using HFE7100 agent reveals a strong dependency of droplet penetration on initial droplet size for the flow range and droplet 

sizes of interest in practical agent delivery systems. 

INTRODUCTION 

The new generation of halon replacements includes chemical suppressants that have high boiling point 

temperatures (Le., 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 will 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, 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 dispersion around 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 will provide new experimental data for a well-characterized, homogenous droplet-laden 

turbulent flow field around a prescribed obstacle. Baseline measurements of the single gas-phase velocities with and without obstacles, but without the spray, are reported in this study. Operating conditions are 

based on typical engine nacelle-like conditions, to study the atomization and dispersion of a liquid agent 

around three cylinders chosen as representative obstacles. The sizes of the tubes were chosen such that 

the diameters were smaller, similar to, and larger than the characteristic length scale of turbulence to explore turbulence generation or reduction due to clutter. In addition to the experiments, numerical simulations using the WLCAN Computational Fluid Dynamics (CFD) fire physics code are conducted and 

compared to the experimental measurements of gas mean velocity and turbulent kinetic energy. A sensitivity study is also conducted as part of the CFD calculations to explore the dispersal of liquid droplets 

around the obstacles as a function of initial droplet size. The motivation of this exercise is to understand 

the droplet transport processes that govern agent transport to regions behind solid objects. 

This paper presents a description of the experimental arrangement, diagnostics used to characterize the 

flow field, and the CFD model. The results from both the experimental measurements and numerical 

simulations are compared and discussed, along with comments on future work. 

EXPERIMENTAL ARRANGEMENT 

To explore droplet transport around obstacles, an experimental arrangement is used that provides a wellcharacterized, homogeneous turbulent flow field around prescribed obstacles (Figures I and 2). The 

experimental facility includes an air swirl burner used in the reported experiments to provide nonswirling 

air to the experiment and coflows around the liquid agent injector. 

* The numerical work was performed in part at Sandia National Laboratories, a multiprogram laboratory operated 

by Sandia Corporation, a Lockheed-Martin Co., for US Department of Energy, contract DE-AC04-94AL85000. 

122 Halon Options Technical Working Conference 24-26 April 2001 
VANE CASCADE 

COMBUSTION AIR INLET I 

ARGON 

ION 

LASER 

fa) 

Figure 1. Schematic of (a) the reference spray facility 

mesh-generated air turbulence. 

PDI Transmitter 

56 

i 

11 

~ 

4.7 

(b) 

, and (b) the general facility modifications for 

Figure 2. View of the current experimental arrangement with cameras from the 

PIV system in the foreground. 

Additional details on the design ofthe burner are available elsewhere [I]. Figure la shows the presence 

of a stainless steel eiiclosure to provide improved reproducibility and control of the spray and surrounding airflow field. The chamber height is 1.2 m, the inner diameter is 0.8 m, and several windows provide 

optical access. However, since the current experiments focussed on the gas stream turbulence, only the 

chamber base was used without the enclosure. (The enclosure will be used in the next phase ofthis work 

Halon Options Technical Working Conference 24-26 April 2001 123 
in which liquid agent will be introduced to the turbulent flow field.) In place of the stainless steel enclosure, an octagonal Plexiglas insert (with a wall thickness of 6 mm) was used as a boundary condition 

(generically shown in Figure 1 b). A honeycomb layer (5 1 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 

on the air stream is imposed by placing a layer of wire mesh screen (with 13 mm size cells) downstream 

of the honeycomb. For these experiments, the incoming air was directed entirely through a selected 125 

by 254 cm rectangular cross sectional portion of the honeycomb and then through wire mesh screen 

(placed 25 mm downstream of the honeycomb) (Figure 2). The integral and Kolmogorov length scales of 

turbulence are estimated to be 3 cm and 100 pm, respectively. Three obstacle tubes (diameters of 3, 13, 

and 32 mm) were chosen to span ranges of clutter sizes smaller, on the same order, and larger than the 

integral length scales of turbulence. The obstacles were placed 100 mm downstream of the honeycomb. 

Choice of location of the screen and obstacles is discussed further in the next section. A stepper-motordriven traversing system translates the entire assembly, and permits measurements of the flow field 

properties at selected locations downstream of the injector. 

The mean gas velocities were initially estimated with a five-hole pitot probe and two-component phase 

Doppler interferometer. Although the overall results (as opposed to the detailed measurement profiles) 

are mentioned later in the text, a brief description of these two diagnostics is given here for completeness. 

The five-hole pitot probe has a hemispherical head that is 1.7 mm in diameter. The head is attached to a 

cylindrical body for a length of 26 mm, and then the diameter of the probe increases smoothly from 1.7 to 

3.2 mm, which is the diameter of the main body of the probe. The probe has one hole on the head of the 

probe tip for total pressure measurements, and four holes located 13 mm from the head and 90 deg apart 

on the side of the probe for static pressure measurements. The average of the four velocity measurements 

is reported as the mean velocity. Mean gas velocities were also obtained at selected points by seeding the 

air flow field with 1 pm particles, generated with a pencil fogger, and using a two-component phase 

Doppler interferometer (PDI) from TSI, Inc.* This system will be used more extensively in the next 

phase of this work, in which liquid agent will be introduced to the flow field, to obtain spatial profiles of 

the droplet size and velocity distributions, and droplet number density. The receiving optics were aligned 

at a 25-deg scattering angle measured from the direction of propagation of the laser beams, and the 

transmitting and receiving optics were aligned at the same elevation. A 5 W argon ion laser operating in 

multiline mode was used as the illumination source. The blue (wavelength = 488 nm) and green (wavelength = 514.5 nm) lines of the argon ion laser are separated and focused by the transmitting optics to 

intersect and form the probe volume. 

The flow field was characterized fully using a three-dimensional particle image velocimetry (PIV) system 

from Dantec Dynamics, Inc. PIV is a non-intrusive field measuring technique (as opposed to a singlepoint 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 is measured, and the velocity is deduced. The seed particles generated by a pencil fogger 

were 1 pm in diameter. The images are obtained in a plane corresponding to the laser light sheet, and the 

time between the images is determined by the time between laser pulses. The spatial resolution of the 

measurements depends directly upon the pixel resolution, field of view, and laser light sheet thickness, 

and indirectly on the seeding density. For the measurements presented here, the spatial resolution is 

approximately 600 pn. 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]. 

* 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 NIST, nor does it imply that 

the materials or equipment are necessarily the best available for this purpose. 

124 Halon Options Technical Working Conference 24-26 April 2001 
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; the wavelength ofthe 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 used to permit the rotation of the camera body with respect to the camera lens 

so that the Scheimpflug condition was satisfied, permitting the laser sheet to be in focus despite the nonorthogonal camera alignment [2]. Bandpass filters (center wavelength = 532 nm, acceptance window = 

30 um) were used to reject the broadband white light from the room. The two cameras were placed at 

angles of about 70 and 110 deg, as measured from the direction of propagation of the laser pulse. The 

processed results are presented as a composite mapping of individual planar regions that are about 40 mm 

in height and 50 mm in width. Statistics were obtained from about 500-700 individual records. Each 

field of view represents about 9000 vectors. Measurements of the flow field were carried out along the 

centerline of the measurement area, and at two planes, + 25 mm from the centerline location. 

CFD SPRAY MODELING 

As a complement to the experimental measurements, a general-purpose fire simulation code, VULCAN, 

was used to conduct a parametric study of spray transport around the largest of the cylinders considered 

in the experiment. Specific details on numerical methods and modeling used in VULCAN, as well as the 

spray model, can be found in DesJardin et al. [3]. A 3D mesh was generated for the entire experimental 

facility with a total grid size of 26 x 5 1 x 80 = 106,080 cells. The initial conditions of the calculation are 

chosen to best match the experiment by setting the mean streamwise velocity, turbulence kinetic energy 

and its dissipation to values of 4.5 m/s, 0.304 m2/s’ and 0.0212 m’/s’, respectively. The agent considered 

in this parameter study is HFE-7100 (C,F,OCH,) [4]. Many ofthe thermodynamic parameters of HFE7100 needed for input to the spray model are available from the manufacturer and summarized in Table 

1. However, the gas-phase enthalpy of HFE-7100 is not readily obtainable, and so only one-way coupling of the gas phase onto the spray transport is explored in this study. Future studies will consider twoway coupling effects. 

TABLE 1. THERMODYNAMIC PARAMETERS OF HFE-7100 

ProDertv Value 

Molecular Weight 250 ghol 

Boiling Temperature 334 K 

Critical Temperature 468.3 K 

Heat of Vaporization @ B.P. 

Specific Heat @ room temperature 1 183 Jkg 

Density @ room temperature 1520 kg/m’ 

11 1.6 kJ/kg 

- ~ 

RESULTS AND DISCUSSION 

Preliminary characterization of the flow field uniformity was determined by means of planar Mie scattering flow visualization, pitot probe, and a two-component phase Doppler interferometer for different airflow speeds. The results indicated a flow mean velocity of 4.5 m/s, corresponding to a Reynolds number 

of 3700 (based on the grid cell size), provide a homogenous distribution of turbulence across the measurement area. This flow velocity is also representative of airflow speeds through aircraft engine nacelles. 

Particle image velocimetry (PIV) was used to obtain instantaneous two-dimensional images of the airflow 

field velocity (Le., three components of velocity). These images are then used to construct the streamwise (w) and two cross-stream (Uand v) mean and root mean square (RMS) profiles, both upstream and 

downstream of the cylinder, and along the centerline location. Downstream of the honeycomb, the baseline flow field was found to be relatively uniform throughout the measurement field (Figures 3 and 4) 

Halon Options Technical Working Conference 24-26 April 2001 125 
re W 

Wt v 

Figure 3. Variation of mean streamwise and cross-stream velocities with downstream distance. 

(gas streamwise component of velocity, vws 4.5 ds, and turbulence intensity, zws 13%), except 

immediately downstream of the honeycomb. The local jetting downstream of the honeycomb exit decays 

with increasing streamwise distance and becomes negligible after 25 mm. Based on this observation, the 

wire grid mesh to generate turbulence is placed at this location. The flow downstream of the wire mesh 

was obtained along the centerline of the measurement region. Nonuniformities of the flow due to the 

mesh are created downstream of the mesh and relaxed to roughly a homogeneous state of 9% turbulence 

intensity in each direction at 100 mm downstream of the honeycomb (vw E 4.0 ds). The obstacles are 

placed at this location, shown with black circles, to study the effects of the turbulence and eventually 

spray transport around the cylinders in this well-controlled turbulent environment. The velocity accelerates to vw 5 5.0 m/s around the side of the cylinders and forms a recirculation region behind the cylinders 

(vw E -1.0 ds). The turbulence intensity increases behind the cylinder (7, G 20% in a banded region 

behind the obstacle). The flow also decelerates to a stagnation region at the centerline, upstream face of 

the cylinders. 

126 Halon Options Technical Working Conference 24-26 April 2001 
Figure 4. Variation of mean streamwise and cross-stream turbulence intensities with 

downstream distance. 

Figures 3 and 4 indicate that for all three cylinders a strong recirculation zone is present on the downstream side of the cylinder with a large increase in tui-bulent kinetic energy. Comparisons of the CFD 

predictions (Fipres 5 and 6) to the PlV measurements indicate that qualitatively the location and size of 

the recirculation zone for the largest cylinder case are well captured. A two-dimensional slice of the 

mesh (Figure 5a) shows local grid refinement in regions near the cylinder to resolve the shear layer properly. Quantitative comparison indicates that the numerical results overpredict the streamwise mean velocity by 20% and underpredict the RMS in strcamwise velocity by 15%. These differences indicate that 

the numerical predictions underestimate the extent of turbulent mixing behind the cylinder, which may be 

attributed to the use of the k-E model, which is known to perform poorly in highly recirculating flows. 

llalon Options Technical Workine Conference 24-26 April 2001 127 
0.3 

0.X 

0.2 

. 0.15 

0.1 

0.05 

-0.15 -0.1 &.E 0 0.05 0.1 E 

X (m) 

(a) (b) 

Figure 5. Cross-sectional views of CFD mesh showing (a) mesh refinement around the cylinder and 

(b) mean streamwise velocity. 

5 

(a) (b) 

Figure 6: CFD predictions of (a) mean cross-stream velocity and (b) estimate of cross-stream fluctuation. 

The next experimental phase of this work is to inject a liquid fire suppressant agent and to study the 

change in droplet transport as the spray interacts with the obstacles. One possible choice is HFE-7100 

(boiling point of 334 K) since it has shown to be effective at extracting heat from a flame zone [4]. Preliminary modeling results of this arrangement are shown in Figures 7 and 8. The spray is modeled as a 

stream of droplets with a mass load rate of 1.598 p/s and zero slip for the droplets (i.e., a droplet streamwise velocity of 4.5 ds). Figures 7 and 8 present instantaneous snapshots of the spray distribution and 

velocity for three assumed droplet sizes of 10 pm, 50 pm, and 100 pm, These droplet sizes, chosen to 

span the range of credible scenarios during agent release, illustrate some interesting behavior. Figures 7a 

and 7b present results for the extreme 10 pm and 100 pm droplet cases. In both cases, the droplets are 

128 Halon Options Technical Working Conference 24-26 April 2001 
Figure 7. Instantaneous snapshots ol'spray impacting the 32 inin cylinder for (a) 10 pm, 

and 100 pm droplets 

tlalon Ontions Technical Working Confcrencc 24-26 April 2001 129 
unable to penetrate beyond the cylinder because they are either too small and evaporate before reaching 

the cylinder (Le., the 10 pm case) or too large and follow a ballistic trajectory that results in direct impact 

into the cylinder (i.e., the 100 pm case). However, the 50 pm case is able to penetrate beyond the cylinder (Figure 8). In this case, the droplets are large enough not to evaporate too quickly, yet small enough 

to follow the streamlines and accelerate around the cylinder. Future efforts will focus on exploring this 

scenario experimentally, as well as investigating other realistic liquid agent release conditions. 

CONCLUSIONS 

PIV measurements and numerical predictions of a turbulent flow over cylindrical obstacles are presented. 

Comparisons of the flow field CFD predictions to the measurements show reasonable agreement. A 

numerical sensitivity study of initial droplet size reveals that droplet penetration around obstacles is 

dependent on droplet size. Future efforts of this work are to better understand these sensitivities for 

optimization of liquid suppressant delivery systems on larger scale experimental platforms. 

ACKNOWLEDGMENTS 

The authors wish to acknowledge the partial support of this work by the Department of Defense Next 

Generation Fire Suppression Technology Program, co-funded by the DoD Strategic Environmental Research Development Program. The authors also wish to thank Dr. G. Papadopoulos of Dantec Dynamics, 

Inc. for his assistance in recording and post processing the PIV results, and Mr. R. Fink for his assistance 

in completing the NIST facility modifications. 

1. 

2. 

3. 

4. 

REFERENCES 

Widmann, J.F., Charagundla, S.R., Presser, C., and Heckert, A,, Benchmark Experimental Database for Multiphase Combustion Model Input and Validation: Baseline Case, Internal Report No. 

6286, National Institute of Standards and Technology, Gaithersburg, MD, 1999. 

Flowmap@’ 3D-PIVSystem Owner’s Manual, Dantec Measurement Technology, Inc., Mahwah, NJ, 

July 1999. 

DesJardin, P.E., Gritzo, L.A., and Tieszen, S.R., “Modeling the Effect of Water Spray Suppression 

on Large-Scale Pool Fires,” Proceedings, Halon Options Technical Working Conference, Albuquerque, NM, pp. 262-273,2000, 

Yang, J.C., Bryant, R.A., Huber, M.L., and Pitts, W.M., “Experimental Investigation of Extinguishment of Laminar Diffusion Flames by Thermal Agents,” Proceedings, Halon Options Technical 

Working Conference, Albuquerque, NM, pp. 433-446,2000. 

130 Halon Options Technical Working Conference 24-26 April 2001 

__~~