Fire Safety Journal 41 (2006) 580–604

PIV measurements of water mist transport in a homogeneous

turbulent flow past an obstacle

Cary Pressera,, George Papadopoulosb, John F. Widmannc

a

National Institute of Standards and Technology, 100 Bureau Drive, Stop 8360, Gaithersburg, MD 20899-8360, USA b

ATK GASL, 77 Raynor Avenue, Ronkonkoma, NY 11779, USA

c

Fluent Inc., 10 Cavendish Court, Lebanon, NH 03766, USA

Received 26 August 2005; received in revised form 20 April 2006; accepted 30 May 2006

Available online 11 September 2006

Abstract

To obtain a better understanding of the physical processes involved in liquid suppressant transport in cluttered spaces, particle image

velocimetry (PIV) measurements were carried out in the droplet-laden, grid-generated, homogeneous turbulent flow over both an

unheated and heated cylinder, and a body-centered cube (BCC) arrangement of spheres. Transport of both water droplets and aerosol

particles was characterized upstream and downstream of these obstacles. Data were recorded for the cylinder at ambient and elevated

temperatures (at 423 K) to estimate the effects of the hot cylinder surface on droplet transport. The results indicated that smaller droplets

are entrained into the recirculation region behind the cylinder while the larger droplets impact the cylinder surface, accumulate and drip

off, and/or rebound off the surface and disperse into the free stream. The flow over the heated cylinder resulted in the formation of a

vapor layer on the downstream side of the cylinder in the shear region between the recirculation zone and free stream. Thus, vaporization

of larger droplets impinging on the heated cylinder surface suggests an increased probability of vapor. For the BCC (with a blockage

ratio of about 64%), there was 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.

Published by Elsevier Ltd.

Keywords: Droplet-laden flows; Droplet transport; Fire suppression; Grid-generated turbulence; Heated cylinder; High boiling point suppressants;

Homogeneous turbulent flow; Obstructed spaces; Particle image velocimetry; Spray impingement

1. Introduction

Fires within an aircraft engine nacelle pose significant

challenges for suppression owing to the presence of wires,

fuel, hydraulic, and electrical lines, ribs and other objects

(e.g. mounting brackets), which can obstruct transport of a

suppression agent to the fire source (e.g. broken fuel line).

The complex arrangement of components inside an aircraft

engine nacelle creates an environment for spray transport

that is characterized by a high degree of turbulence, high

blockage ratios, and significant liquid impingement onto

surfaces. Non-ozone-depleting halogen alternatives developed for such applications include chemical suppressants

that have high boiling point temperatures (Tb4330 K), and

exist in the liquid phase after high-pressure release and

under ambient conditions. Liquid suppressants are of

particular interest because of their higher absorption of

heat per unit volume, compared to gaseous suppressants,

which results in a greater reduction of flame and surface

temperatures, as well as their ease of storage which is

important for aerospace applications [1]. Release of these

agents in a confined, cluttered space results in the dispersal

of droplets that may impact upon solid surfaces and reduce

the fire suppression effectiveness of the agent. For this

investigation, a droplet-laden, homogeneous turbulent flow

field was used to simulate the turbulence experienced by

sprays for a representative fire-suppression scenario in an

aircraft engine nacelle at reported airflow conditions [2].

The choice of the above conditions was motivated by

recent efforts to develop and validate the subgrid droplet

impact model of the VULCAN computational fluid

ARTICLE IN PRESS

www.elsevier.com/locate/firesaf

0379-7112/$ - see front matter Published by Elsevier Ltd.

doi:10.1016/j.firesaf.2006.05.003

Corresponding author. Tel.: +1 301 975 2612; fax: +1 301 869 5924.

E-mail address: cpresser@nist.gov (C. Presser).
dynamics (CFD) fire physics code for spray-clutter interactive environments [3].

A large body of literature exists on specific segments of

the problem investigated here, however, there are few

investigations that address the full complexity of the

problem. For example, literature exists for specific issues

related to grid-generated turbulence, droplet transport in

vortices, and droplet/spray impingement on surfaces. Past

research includes single isolated droplet impingement on

flat surfaces (e.g. Ref. [4]) and cylinders (e.g. Ref. [5]),

dripping liquid droplets off cylinders (e.g. Ref. [6,7]), spray

impingement on unheated and heated flat surfaces in

different boiling regimes (e.g. Refs. [8–10]), droplet

interactions with shear layers (e.g. Ref. [11]), and droplet

entrainment in free turbulent flows (without impingement)

to determine the influence of the flow on droplet

trajectories (e.g. Ref. [12]).

Regarding quantitative measurements for validating

multiphase CFD codes, particle image velocimetry (PIV)

is a diagnostic method that has been used extensively in

fluid flow characterization, and has recently attracted

attention for spray characterization. PIV has been used

to obtain two-dimensional spatial maps of the three

components of velocity, as well as turbulence information

such as Reynolds stresses, in a variety of fluid dynamic

studies over different body configurations. Several PIV

studies of sprays have dealt with fire and combustion

applications (e.g. Refs. [13–15]), and direct injection and

diesel engines (e.g. Refs. [16–20]) in which one can either

measure the droplet velocity field, or introduce seed

particles (assumed to follow the fluid stream) to the spray

field and try to segregate between the continuous and

discrete phases. Fire examples include Gorman and

Widmann [21] who used PIV to characterize the velocity

field in a residential fire sprinkler water spray. They

observed that the polydispersity of the droplet sizes has

an effect on the measured velocity field. Widmann et al.

[22] compared the droplet velocity measurements obtained

in a water spray produced by a residential fire sprinkler

using PIV with those obtained using phase Doppler

interferometry. The data showed good agreement between

the two measurement techniques in regions of the spray

where the droplet size distribution was heavily weighted

towards the larger droplets. Along the sprinkler axis,

however, there was significant disagreement between the

two methods. This was attributed to several factors,

including possible biasing of the PIV measurements

towards the velocities of the larger droplets. Sheppard

et al. [23,24] measured the mean velocity field of the water

sprays produced by fire sprinklers using PIV. The data

were used to provide input for the Fire Dynamics

Simulator, a large eddy simulation model developed at

the National Institute of Standards and Technology to

predict the dynamics of large-scale fires [25]. The data of

Sheppard and Lueptow [26] and Sheppard [27] provide the

necessary input required to simulate the interaction of fires

and water sprays.

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. Baseline PIV measurements of the gas and droplet phase velocities with and

without obstacles were carried out to study droplet

dispersion of a liquid agent around cylinders, the cylinder

representing the classical obstruction to be modeled. The

cylinder diameters were chosen to be smaller than, similar to,

and larger than the characteristic length scale of turbulence,

which would allow investigation of the effects of turbulence

intensity on clutter size. Also, PIV measurements were

obtained of the discrete (spray) and continuous (gas) phase

velocities for two representative obstacles—a cylinder

(of diameter larger than the characteristic length-scale of

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Nomenclature

kt total turbulence kinetic energy

n number of samples

N total number of velocity vectors

s standard deviation of the mean

Ti local mean turbulence intensity

U spanwise (across the cylinder) out-of-plane

component of velocity

V cross-stream component of velocity

W streamwise component of velocity

W0 nominal streamwise velocity

Ui ( 1⁄4 U, V, W) mean of the components of the

individual velocity vectors

ui ( 1⁄4 u, v, w) components of the individual velocity

vectors

UiUj ( 1⁄4 UV, UW, VW) components of the velocity

correlation coefficients

uc combined standard uncertainty

Dx, Dy, Dz true particle displacements

Z streamwise position

Greek symbols

Dw, DZ pair of two-dimensional displacements

e(U), e(V), e(W) calibration uncertainties for the three

components of velocity

rij velocity correlation coefficients

si root mean square of the fluctuating components of velocity

Subscripts

i 1⁄4 U, V, W index for the components of velocity

ij 1⁄4 UV, UW, VW index for the velocity correlation

coefficients

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 581
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.

2. Experimental arrangement

The experimental arrangement was designed to provide a

well-characterized, homogeneous turbulent flow field

around prescribed obstacles. Two experimental configurations were used in this investigation. The first was a vertical

arrangement in which PIV measurements were carried out

for only the gas-phase turbulent flow field. The PIV

measurements were also carried out in a second horizontal

arrangement in which the flow field was laden with water

droplets.

2.1. Vertical configuration

For the vertical configuration, an octagon-shaped, clear

plastic enclosure (with a wall thickness of 6 mm, height of

610 mm, and major and minor axes of 760 and 560 mm for

the cross section, respectively) was used to define the

boundary conditions (see Fig. 1). A honeycomb layer was

used to laminarize the airflow. The honeycomb was 51 mm

thick with approximately 3 mm (length between opposite

sides) hexagon-shaped cells. A layer of wire mesh screen

(3.2 mm wire thickness and 13 mm size cells) was placed

downstream of the honeycomb to impose grid-generated

turbulence on the air stream. The incoming air was directed

entirely through a selected 125 mm  254 mm rectangular

cross-sectional portion of the honeycomb and then through

the wire mesh screen (placed 25 mm downstream of the

honeycomb), as shown in Fig. 2. The incoming air was

adjusted to provide droplet mean velocities of about 4–5 m/

s near the cylinder. Three cylinders with diameters of 3, 13,

and 32 mm, were chosen to span ranges of clutter sizes

smaller than, on the same order of, and larger than the

integral length scales of turbulence. The obstacles were

placed 100 mm downstream of the honeycomb. Placement

of the grid mesh and obstacles were chosen to be

downstream of any jetting of the airflow from the

honeycomb and grid mesh, respectively, as will be

discussed further in detail. A stepper-motor-driven traversing system was used to translate the entire assembly and

permit measurement of the flow field characteristics at

selected locations around the obstacle. The PIV system was

aligned around the plastic enclosure (see Fig. 2) with the

laser sheet traversing normal to the cylinder, and independent of the traverse.

2.2. Horizontal configuration

For the horizontal configuration, the plastic enclosure

was placed on its side and, along with a front face that

supported the inlet passages for the liquid agent and air,

and a back face that supported the exhaust passage,

served to form a closed system at room temperature. The

experiment was oriented horizontally to enable collection

of liquid agent that dripped 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, which was supplied to the flow field

by means of a 601 hollow-cone, pressure-jet atomizer (with

a nominal flow rate of 3.78 kg/h). For these experiments,

the water spray with coflowing air was introduced to the

grid-generated turbulent flow field. The largest of the three

aforementioned cylinders, which was modified to include a

cartridge heater for preheating of the cylinder, served as the

obstacle towards which the water spray was directed. The

incoming air (supplied at 44070.12 kg/h1 was directed

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Fig. 1. Schematic of the experimental vertical arrangement for the gridgenerated turbulent flow field.

Fig. 2. View of the experimental vertical arrangement with cameras from

the PIV system in the foreground.

1

Estimation of the measurement uncertainty is determined from

statistical analysis of a series of replicated measurements (referred to as

Type A evaluation of uncertainty), and from other means other than

statistical analysis (referred to as Type B evaluation of uncertainty) [28].

Calculated as 2uc (representing a level of confidence of 95%), where uc is

the combined standard uncertainty. The value for uc was estimated

statistically by sn1/2, where s is the standard deviation of the mean and n

582 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
entirely through a distributor plate with steel wool, a

circular cross-sectional area of honeycomb, and then

through a wire mesh screen, as illustrated in Fig. 3. The

honeycomb had a 203 mm outer diameter (i.e. exposed to

the downstream flow). The honeycomb and wire mesh

screen were co-positioned around the atomizer (having a

diameter of 17.8 mm), which resulted in removal of the

central portion of the honeycomb (of an approximate area

of 521.970.19 mm2

). The mean of the inlet air velocity was

estimated by multiplying the honeycomb cell area by the

approximated number of cells (3333 cells) exposed to the

downstream flow field.

Grid-generated turbulence was imposed on the air

stream by placing a square layer of wire mesh screen (with

dimensions of 229 mm width by 330 mm length, 3.2 mm

wire thickness, and 13 mm size cells) 25 mm downstream of

the honeycomb. Note that the grid mesh covered the entire

inlet area (compare the aforementioned honeycomb

diameter to the grid dimensions). A schematic of the grid

mesh pattern relative to the atomizer and incoming airflow

is shown in Fig. 4. The face of the liquid atomizer was

placed flush with the upstream side of the grid mesh, and

centered within one mesh cell so that the liquid spray

would be unimpeded by the grid mesh. Again, the steppermotor-driven traversing system translated the entire

experimental assembly, while the PIV system was aligned

at a stationary location, and independent of the traverse

(see Fig. 5).

Measurements were carried out with two obstacles, an

aluminum cylinder and a body-centered cube (BCC)

arrangement of wooden spheres and connecting posts

(see Figs. 6 and 7, respectively). The cylinder, which was

chosen because its diameter is larger than the integral

length scale of turbulence, was fabricated from a solid

aluminum rod, with an outer diameter of 3270.04 mm and

length of 305 mm (see Fig. 6). Note that the cylinder had a

length that did not span across the entire cross section of

the chamber (to easily adjust its position relative to the

spray nozzle), and a spanwise flow existed along the

cylinder length due to end effects. The spray impinged

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Fig. 3. Schematic of the experimental horizontal arrangement for the droplet-laden, grid-generated turbulent flow field.

Fig. 4. Upstream view and schematic of the inlet for the experimental horizontal configuration, and orientation of the particle image velocimeter (PIV)

with respect to the heated cylinder.

(footnote continued)

is the number of samples (Type A uncertainty, where n 1⁄4 11), and from

the manufacturer uncertainty (Type B uncertainty), if available.

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 583
along the central portion of the cylinder and was estimated

to cover up to approximately 183 mm of the cylinder’s

upstream surface (taking into account the aforementioned

spray angle, cylinder diameter, and atomizer/cylinder

distance). This impingement region was well away from

the cylinder edges. Entrainment of the droplets with the

airflow also reduced the dispersion of the spray. Thus, we

confined our measurements to along the center plane to

minimize any related edge effects. Surface heating was

provided by a 13 mm diameter hole bored through the

center of the cylinder to accommodate a 250 W cartridge

heater (13 mm in diameter and 76 mm in length). The rod

was split along its axis into two halves to allow 1 mm deep

channels to be milled along one segment for placement of

five K-type thermocouples (Inconel sheathed, ungrounded,

0.8 mm in diameter, and 305 mm in length). The thermocouple time response, as specified by the manufacturer was

3 s at a level of confidence of 95%. The cylinder halves were

bolted together and re-machined to produce a seamless

outer diameter. The thermocouples were placed in a cross

pattern (see Fig. 6) in the center of the rod (each separated

by a distance along the surface of 6.4 mm, with the

thermocouple junctions placed about 3.2 mm of the surface

from within bored holes at each location). The central

thermocouple was used for temperature control of the

heater, which was positioned behind the thermocouples.

The expanded uncertainty for the temperature was 7.9 K,

including the Type B uncertainty of 3.1 K. The center of

the cylinder was fixed at a location 18574 mm downstream

of the grid mesh. Note that the far-most upstream position

from the cylinder where data were obtained was 50.8 mm,

which was about 134 mm downstream of the grid mesh. At

this relatively large distance, heat transfer from the hot

cylinder surface did not influence the initial experimental

conditions [29].

The BCC was 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. 7. As with the cylinder, the central sphere of

the BCC was aligned with the atomizer. 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

surfaces during the optical measurements. The obstacles

were placed nominally 182 mm downstream of the honeycomb, and located along the atomizer centerline.

2.3. Particle image velocimetry

2.3.1. PIV measurement system

The flow field upstream and downstream of the different

obstacles was characterized using a three-dimensional PIV

system manufactured by Dantec Dynamics2 (see Fig. 5).

Particle image velocimetry is a non-intrusive field measuring technique (as opposed to a single-point diagnostic

method) that measures two or three components of

velocity. The 3D stereo PIV system differs from traditional

PIV systems in that two charged-coupled device (CCD)

cameras are used, and three velocity components are

measured [30]. Special camera mounts were utilized 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 non-orthogonal camera alignment [30]. The PIV system

consisted of a dual-cavity, 50 mJ Nd:YAG mini-laser as the

illumination source (with a pulse duration of the laser light

sheet of about 5 ns at a wavelength of 532 nm), two 12 bit

1024 pixels  1280 pixel dual-frame CCD cameras, a data

acquisition and control unit, and software running on a

personal computer for system control, data management,

and data post-analysis. Detailed information on the PIV

system configuration and general performance is provided

in Refs. [31–33].

2.3.2. PIV methodology

The basic relationship of displacement divided by time to

yield velocity is the fundamental principle of the PIV

technique. Although PIV is a non-intrusive measurement

technique, it requires tracer particles to be entrained in the

flow under investigation, so that displacement information

is determined. Hence, PIV measures directly the displacement information of tracer particles, and if these particles

are small and follow the flow, the displacement information of these small ‘‘seeding’’ particles can be used to infer

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Fig. 5. View of the experimental horizontal configuration with the

cameras and laser for the PIV system.

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 the

National Institute of Standards and Technology, nor does it imply that the

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

584 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
the flow velocity. On the other hand, the displacement

information of droplets, and hence their velocity, is directly

measured by the PIV technique.

Illumination of the tracer particles is done using a thin

light sheet, which is pulsed to freeze the particle motion.

The Mie scattering from the imaged tracer particles is

recorded at two instances in time using a digital camera.

The two sequential digital images are then sub-sampled at

particular areas via a prescribed interrogation window, and

a spatial cross-correlation is performed using fast Fourier

transform analysis, as described by Willert and Gharib

[34], which results in a surface function (e.g. see Fig. 8). The

separation time between the light pulses is selected so as to

have particles displace several pixels within the interrogation area (IA), and have most particles remain common to

both images. A high cross-correlation value is determined

for which many particle images match up with their

corresponding spatially shifted partners, and this is

considered to represent the best match of particle images

between the sequential recordings.

The displacement vector of the cross-correlation peak

from the center (origin) of the two-dimensional interrogation window denotes the average distance traveled by the

particles within the interrogation area. Accurate estimation

of the displacement vector to sub-pixel resolution is

performed by locally fitting the two-dimensional array of

correlation values in the vicinity of the peak. The absolute

displacement vector is then calculated from a calibration of

the magnification factor between the pixel domain of the

digital recording device and the physical field of view.

Finally, division of the displacement vector, determined for

each interrogation area along the entire pixel domain, by

the time separation between the two sequential laser pulses

yields the velocity vector field in the physical area under

investigation.

To improve the signal-to-noise ratio and reduce errors in

displacement estimation, a multi-pass adaptive correlation

technique is used whereby the second interrogation area is

displaced dynamically to capture the particle movement

while at the same time being reduced in size. In this way,

the effective probe volume of the PIV is reduced and

stretched to follow along particle path lines, while yielding

good spatial resolution along flow gradients. In addition, a

further sub-pixel refinement step is used to eliminate the

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data acquisition aluminum rod

heater

thermocouples

controller

305 mm

13 mm 32 mm

Fig. 6. Schematic of the cylinder with installed cartridge heater. The thermocouples on the upstream side of the cylinder were arranged in the cross pattern

shown in the callout.

Fig. 7. View of the body-centered cube of spheres.

Fig. 8. Typical spatial cross-correlation function between two sequential

digital PIV images.

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 585
bias error from the displacement estimation and dramatically reduce the random errors, thus achieving higher

accuracy in the PIV signal [35].

Using the above methodology, a planar velocity field is

obtained corresponding to the vector projection in the

illumination plane. By using two cameras to image the

same planar flow field from two separate points of view,

one obtains two distinct planar velocity fields. This is the

foundation of the stereoscopic PIV principle, as illustrated

in Fig. 9. Stereoscopic PIV takes advantage of the parallax

error that is inherent in 2D planar PIV measurements, an

error that comes about from imaging the projected

trajectory of particles in the plane of the laser light sheet,

instead of the actual three-dimensional trajectory. By

imaging particle displacements from two distinctly different viewing angles, distinct projected trajectories for the

same group of particles are determined.

To properly view the plane illuminated by the light sheet

using the stereo-PIV arrangement, the camera back plane

(i.e. the CCD-chip) must be tilted to properly focus the

entire camera field of view. A special optical arrangement is

required to meet the so-called Scheimpflug condition,

which states that the image, lens, and object plane must

be collinear for the camera images to be properly focused

in the entire field of view [36].

The two-dimensional projected trajectories are combined

using a numerical model that describes how each camera

images the flow field to determine the actual threedimensional trajectory of the group of particles within

the laser sheet. The numerical model is determined using a

well-defined calibration target that is used to imitate the

out-of-plane motion. Placing the target in the actual

measurement station offers a robust way of calibrating

the system; one that easily takes into account distortions

caused by index-of-refraction effects, and thus is the

preferred way of calibrating current stereoscopic PIV

measurements.

Construction of the three-dimensional velocity map

means that true particle displacements (Dx, Dy, Dz) are

extracted from a pair of two-dimensional displacements

(Dw, DZ) that correspond to the vector results obtained

from both cameras. This means that a system of four

equations and three unknowns needs to be solved, and

depending on the approach, the equations may be linear or

non-linear, as discussed in Ref. [37].

Conducting PIV measurements in polydisperse sprays

introduces an additional complication when compared to

single-phase measurements. When spray droplets are used

as the ‘‘scatterers’’ for PIV measurements rather than seed

particles, a size bias is introduced. The bias results from the

stronger signal by larger droplets, owing to the greater

intensity of Mie scattered light from these droplets, as

compared to smaller ones. The enhanced scattering leads to

biasing in the cross-correlations, weighting the measured

mean velocity towards the larger droplets. Gorman and

Widmann [21] investigated this effect using Monte Carlo

simulations, and concluded that the mean velocity obtained

from the cross correlations was weighted by the surface

area of the droplets. Thus, the average velocities obtained

from multiphase PIV measurements correspond to surfacearea-weighted mean velocities.

2.3.3. PIV implementation

A photograph of the stereo-PIV setup in the present

investigation is shown in Fig. 2 (vertical configuration) and

Fig. 5 (horizontal configuration). The double-cavity

Nd:YAG laser was mounted above the facility for the

horizontal configuration and a 901 mirror cube was used to

orient the light sheet vertically from above the plastic

chamber and onto a cross section of the obstacle (see

Fig. 5). The two cameras were placed at angles of about 701

and 1101 (vertical configuration), and 951 and 1451

(horizontal configuration), as measured from the forward

direction of propagation of the laser sheet. Bandpass filters

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Displacement

seen from left

Displacement

seen from right

Left

camera

Right

camera

Focal plane =

Centre of

light sheet

True

displacement

L R

Fig. 9. Principle of stereoscopic PIV (displacement enlarged for better visibility).

586 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
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Fig. 10. Schematic of the measurement plane and position of the interrogation areas.

300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb) With Grid Obstacle 1(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb)

With Grid Obstacle 1

(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb)

With Grid Obstacle 1

(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

5.000

4.750

4.500

4.250

4.000

3.750

3.500

3.250

3.000

2.750

2.500

2.250

2.000

1.750

1.500

1.250

1.000

0.750

0.500

0.250

0.000

-0.250

-0.500

-0.750

-1.000

1.600

1.400

1.200

1.000

0.800

0.600

0.400

0.200

0.000

-0.200

-0.400

-0.600

-0.800

-1.000

-1.200

-1.400

-1.600

1.200

1.100

1.000

0.900

0.800

0.700

0.600

0.500

0.400

0.300

0.200

0.100

0.000

-0.100

-0.200

-0.300

-0.400

-0.500

-0.600

-0.700

-0.800

-0.900

-1.000

-1.100

-1.200

W

U

V

W (m/s) V (m/s)

U (m/s)

Fig. 11. Variation of mean streamwise (W) and cross-stream (U, V) velocities with streamwise position (Z).

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 587
(center wavelength 1⁄4 532 nm, acceptance window 1⁄4

30 nm) were used to reject the broadband white light from

the room. These filters were mounted in front of Nikon

f 2.8, 105 mm lenses used to image the flow. 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 (vertical configuration) and about 33 mm in height

and 53 mm in width (horizontal configuration), i.e. field of

view for the common overlap region of both cameras.

Statistics were obtained from about 500 to 700 individual

image pairs, which were acquired at a rate of approximately 3 Hz. Hence, the processed results are presented

as a composite mapping of individual planar regions, as

shown in Fig. 10. Each field of view represented about

9000 vectors (vertical configuration) and 9900 vectors

(horizontal configuration) for the seeded-only flow, and

4950 vectors for the spray-only case (horizontal configuration). The spatial resolution of the measurements depends

directly upon the pixel interrogation area (i.e. 16

pixels  16 pixels, with a pixel size/resolution of 6.7 mm),

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 600–700 mm in the plane of

the laser sheet, and with only the spray droplets, the

resolution was about 1.5 mm. For the latter, because of the

lower droplet concentration and larger droplet sizes a final

IA of 32 pixels  32 pixels was used. An adaptive

correlation routine was used to process the image maps

with a final IA of 16 pixels  16 pixels (flow with seed),

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300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb)

With Grid Obstacle 1

(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

TU

2.000

1.570

1.232

0.967

0.759

0.595

0.457

0.367

0.288

0.226

0.177

0.139

0.109

0.086

0.067

0.053

0.041

0.032

0.025

0.020

300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb)

With Grid Obstacle 1

(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb)

With Grid Obstacle 1

(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

W

U

V

TV

2.000

1.570

1.232

0.967

0.759

0.595

0.457

0.367

0.288

0.226

0.177

0.139

0.109

0.086

0.067

0.053

0.041

0.032

0.025

0.020

TW

2.000

1.570

1.232

0.967

0.759

0.595

0.457

0.367

0.288

0.226

0.177

0.139

0.109

0.086

0.067

0.053

0.041

0.032

0.025

0.020

Fig. 12. Variation of mean streamwise (TW) and cross-stream (TU, TV) turbulence intensities with streamwise position (Z).

588 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
while the initial IA was 64 pixels  64 pixels. A 50%

overlap was used to best take advantage of the image

information. The velocity resolution was approximately

5 mm/s given the typical particle displacement within the

interrogation area and additional sub-pixel refinement for

correlation peak estimation (timing between frames was set

to 70 ms, resulting in particle displacements of about

20–25% of the typical 32 pixel  32 pixel interrogation

area). Calibration uncertainties for the three components

of velocity, U, V, and W, were estimated to be

e(W) 1⁄4 0.1 mm/s, e(V) 1⁄4 0.1 mm/s, and e(U)/U 1⁄4 4%.3

Note that W (streamwise) and V (cross-stream) are the

components in the measurement plane, while U (spanwise

across the cylinder) is the out-of-plane component, as

determined from the stereoscopic PIV reconstruction.

A pencil fogger was used to generate water-based aerosol

particles approximately 1 mm in diameter. A separate

supply of air (negligible with regard to the total air flow

supplied for the experiment) was used to transport the

aerosol from an enclosed aluminum box containing

the fogger into the main air stream downstream of the

distributor plate (see Fig. 1 for the vertical configuration

and Fig. 3 for the horizontal configuration). A tube with

several holes was stretched across the passage to enable

dispersion of the aerosol across the measurement area. The

PIV images were obtained in the plane illuminated by the

laser light sheet, which was oriented normal to a cross

section of the obstacle. The time between the images was

determined by the time between laser pulses.

Measurements of the flow field were carried out along

the cylinder centerline at the measurement areas shown in

Fig. 10. After images were obtained for one measurement

ARTICLE IN PRESS

300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb)

With Grid Obstacle 1

(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb)

With Grid Obstacle 1

(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

300

250

200

150

100

50

0

Z (mm)

Baseline

(Honeycomb)

With Grid Obstacle 1

(3 mm Dia.)

Obstacle 2

(13 mm Dia.)

Obstacle 3

(32 mm Dia.)

W

U

V

ρWU

0.700

0.600

0.500

0.400

0.300

0.200

0.100

0.000

-0.100

-0.200

-0.300

-0.400

-0.500

-0.600

-0.700

ρWV

0.700

0.600

0.500

0.400

0.300

0.200

0.100

0.000

-0.100

-0.200

-0.300

-0.400

-0.500

-0.600

-0.700

ρUV

0.700

0.600

0.500

0.400

0.300

0.200

0.100

0.000

-0.100

-0.200

-0.300

-0.400

-0.500

-0.600

-0.700

Fig. 13. Variation of turbulence correlation coefficients (rWU, rWV, rUV) with streamwise position (Z).

3

Uncertainties are at 95% confidence, unless otherwise stated.

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 589
area, the traverse was used to reposition the experiment at

the next adjacent region (see Fig. 10), and in this way

spatial profiles were obtained after joining the images. For

the vertical configuration, PIV measurements were also

carried out along two additional off-axis planes of

725 mm from the centerline location. Three cylinders of

varying diameter were investigated with the vertical

configuration. Three cases were recorded with the horizontal configuration using the largest diameter cylinder:

(1) with seed only, (2) with droplets only, and (3) with both

seed and droplets, in order to isolate the effects of both the

droplets and particles on the flow field dynamics. These

cases were repeated for both unheated and heated cylinder

conditions. Measurements of the flow field were also

carried out along the centerline of the BCC. Two

additional off-axis planes were interrogated to map out

one quadrant of the BCC: (1) between the center and two

outer spheres, and (2) slightly outside of the center of the

two outer spheres.

3. Results and discussion

3.1. Vertical experimental configuration

Particle image velocimetry was used to obtain instantaneous two-dimensional images of the 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) particle mean

velocities (see Fig. 11), local mean turbulence intensity (Ti,

where i 1⁄4 U, V, W, see Fig. 12), and velocity correlation

coefficients (rij, where ij 1⁄4 UV, UW, VW, and from which

the Reynolds stresses can be derived, see Fig. 13), both

upstream and downstream of the cylinder, and along the

central plane of the honeycomb rectangular cross-sectional

area.

The local mean turbulence intensity for each component

is defined as

Ti 1⁄4 si ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

U2 þ V2 þ W2 p , (1)

where si is the root mean square of the fluctuating

components of velocity

si 1⁄4 1

N 1

X

N

k1⁄41

ð Þ uik Ui

2

" #1=2

, (2)

where N is the total number of velocity vectors, and Ui

( 1⁄4 U, V, W) is the mean of the components of the

individual velocity vectors, ui ( 1⁄4 u, v, w).

The velocity correlation coefficients are given by

rij 1⁄4 CovfUiUjg

sisj

1prijp þ 1

, (3)

where UiUj 1⁄4 UV, UW, VW and

Cov UiUj

  1⁄4 1

N 1

X

N

k1⁄41

ð Þ uik Ui ujk Uj

 , (4)

where uj is the component of the individual velocity vectors

corresponding to Uj.

Each figure consists of five spatial profiles with the

baseline (honeycomb only) profile presented on the left side

of the graph, followed by a profile with the grid mesh in

ARTICLE IN PRESS

Fig. 14. Photographs of the (A) aerosol seeded flow field around the unheated cylinder, (B) droplet-laden flow field around the unheated cylinder, (C)

droplet-laden flow field around the heated cylinder (arrow indicated presence of vapor layer), and (D) seed/droplet-laden flow field around the bodycentered cube of spheres.

590 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
place, and then with the three cylinders. Downstream of

the honeycomb (baseline profile), the flow field was found

to be relatively uniform throughout the measurement field,

as shown in Figs. 11 and 12 (maximum gas streamwise

component of velocity, Wffi4.5 m/s, and maximum turbulence intensity, TWffi13%), except immediately downstream of the honeycomb. The local jetting downstream

of the honeycomb exit decays with increasing streamwise

distance, and becomes negligible at about 25 mm. Based on

this observation, the grid mesh that is used to generate

turbulence was placed at this location.

The flow downstream of the grid mesh was obtained

along the central plane of the measurement region. Due to

the presence of the mesh, flow nonuniformities formed

downstream of the mesh and relaxed to roughly a

homogeneous state of 9% turbulence intensity in each

direction at about 100 mm downstream of the honeycomb

(Wffi4.0 m/s). The obstacles were placed at this location,

shown in Figs. 11–13 as black circles, to study the

evolution of the turbulent flow around the cylinders in

this environment. Fig. 11 indicated that the free stream air

velocity reached about 5.0 m/s around the side of the

cylinders, and the flow behind each cylinder formed a

recirculation region (Wffi1.0 m/s). The reverse flow

behind the cylinders appears to extend about two cylinder

diameters downstream. The turbulence intensity increased

behind the cylinders (TWffi20% in a banded region behind

the obstacle). There was 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 was reached for the correlation coefficient,

see Fig. 13). This result is an indication of the shear

between the free stream and recirculation region behind the

cylinder. The flow was also found to decelerate to a

stagnation region near the centerline of the upstream face

of each cylinder. The overall flow characteristics around

ARTICLE IN PRESS

U (m/s)

-0.5 -0.3 -0.1 0.1 0.3 0.5

210

200

190

180

170

160

150

z (mm)

-30 -20 -10 0 10 20

y (mm)

TU

210

200

190

180

170

160

150

z (mm)

-30 -20 -10 0 10 20

y (mm)

V (m/s)

-0.5 -0.3 -0.1 0.1 0.3 0.5

210

200

190

180

170

160

150

z (mm)

-30 -20 -10 0 10 20

y (mm)

W (m/s)

2.0 2.5 3.0 3.5 4.0 4.5

210

200

190

180

170

160

150

z (mm)

-30 -20 -10 0 10 20

y (mm)

0.00 0.02 0.04 0.06 0.08 0.10 0.12

TV

210

200

190

180

170

160

150

z (mm)

-30 -20 -10 0 10 20

y (mm)

0.00 0.02 0.04 0.06 0.08 0.10 0.12

TW

210

200

190

180

170

160

150

z (mm)

-30 -20 -10 0 10 20

y (mm)

0.00 0.02 0.04 0.06 0.08 0.10 0.12

(A)

(B)

Fig. 15. Variation of the components of the (A) free stream mean velocity (U, V, W), and (B) turbulence intensity (TU, TV, TW) with spatial position.

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 591
each cylinder appeared to be similar, except that the

features grew proportional to the cylinder diameter.

3.2. Horizontal experimental configuration

3.2.1. Flow visualization

The droplet-laden flow field was recorded with a highresolution digital camera, which provided both still images

and movies at 9 frames/s, and the planar laser light from

the PIV system. Examples of the observed droplet/particle

transport processes are shown in Fig. 14 for (A) aerosol

seeded only and (B) droplet only flow over the unheated

cylinder, (C) droplet-laden flow over the heated cylinder,

and (D) combined droplet and seeded flow over the BCC.

For the seed only case, entrainment into the turbulent flow

field (see Fig. 14A) resulted in a relatively high concentration of particles observed in the cylinder wake, similar to

the findings of Wang et al. [38]. On the other hand, for the

spray only case (see Fig. 14B), droplets in the center of the

spray impinged on the cylinder surface, while those

droplets at larger radial positions were transported around

and past the cylinder. Few larger size water droplets (as

defined by the relative light intensity noted in the images)

were observed in the cylinder’s wake, but this region was

abundant with smaller size droplets. There was no visual

evidence of secondary breakup of the droplets (i.e.

splashing) [29]. It was observed that water droplets dripped

off the cylinder (not evident from Fig. 14B) at a rate of

approximately 6.5 mL/min for the unheated cylinder.

Using digital high-speed photography, impinging droplets

were also observed to rebound off the surface back into the

free stream. When the cylinder was heated to 423 K, there

was significant cooling of the upstream face of the cylinder

to about 351 K. The droplet-laden flow over the cylinder

appeared to be qualitatively similar to the unheated case

except along the shear layer downstream of the cylinder. In

this region, a vapor layer formed (see arrow in Fig. 14C),

which was presumed to be the result of vaporization of the

liquid that wetted the hot surface. Dripping of water

droplets was not observed for the heated cylinder

presumably due to droplet vaporization.

The transport of droplets through the BCC (see

Fig. 14D) was interesting in that both the spheres and

connecting rods (that act like cylinders) impede droplet

transport, while the flow field traversing through the

obstacle (i.e. between the spheres and rods) provided a

relatively unobstructed path for the entrained droplets

(recall that the nominal blockage ratio was about 64%).

Dripping was 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 had more dripping of

liquid than the cylinder, the majority of the spray was still

able to traverse the obstacle.

3.2.2. Free stream conditions

Particle image velocimetry was used to obtain the three

components of velocity for both the atomizer-generated

droplets and fogger-generated aerosol particles. The spatial

profiles were reconstructed using the same coordinate

system as for the vertical configuration, but now referenced

by the position of the grid mesh and atomizer face. Little

difference was found in the characteristics of the homogeneous turbulent flow field, although the orientation of

the plastic chamber was changed and an atomizer was

incorporated into this configuration.

Characterization of the flow field without the obstacle

offered a baseline condition, which was important for

comparison of experimental results, or input/validation of

any modeling effort. Hence, measurements were carried

out at planar locations A and B (see Fig. 10). Results for

the air mean velocities and corresponding turbulence

intensities are shown in Fig. 15. The flow was essentially

unidirectional (i.e. in the streamwise direction, W), with a

uniform streamwise velocity upstream of the cylinder. The

PIV results indicated that the ‘nominal’ streamwise

velocity, W0, was approximately 3.870.3 m/s, which

ARTICLE IN PRESS

Fig. 16. Comparison of the free stream total turbulence kinetic energy

from the vertical and horizontal configurations.

592 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
corresponds to a Reynolds number of about 3134 (based

on the grid cell size and an air kinematic viscosity of

0.154 cm2 s

1

), still in the turbulence regime for gridgenerated turbulence [39]. The ‘nominal’ velocity is defined

as the free stream mean air velocity obtained over the full

measurement domain. The local mean turbulence intensity

for each component was 7.8%, 6.7% and 7.0% across the

flow field for U, V and W, respectively. The variation

between each component was small and for all practical

purposes, the assumption of homogeneous turbulence was

a valid one. To further characterize the turbulent flow at

the location where the cylinder was to be placed, the free

stream total turbulence kinetic energy for the horizontal

and vertical configurations is shown in Fig. 16. The figure

presents kt, given as

kt 1⁄4 1

2 s2

U þ s2V þ s2W

  (5)

and indicates the development of turbulence downstream

of the grid mesh. These measurements were somewhat

smaller than the values obtained for the vertical configuration because the cylinder was placed further downstream at approximately Z 1⁄4 185 mm (as opposed to

Z 1⁄4 75 mm for the vertical configuration).

3.2.3. Air flow field

Since the spray influences the flow around the cylinder, it

was necessary to examine under droplet-laden conditions

the state of the airflow by trying to isolate the aerosol seed

velocity. Results from the stereoscopic PIV measurements

are presented in Figs. 17 and 18, for the unheated and

heated cylinder, respectively. The figures include air mean

and root mean square velocities, all normalized by the

nominal streamwise velocity of W0 1⁄4 3:8 m=s.

The information presented in the figures is velocity

determined under droplet-laden flow conditions, and with

the airflow seeded with aerosol particles. The sparseness of

the droplets, as well as their larger relative image size,

created the opportunity to determine the velocity of the

seeded airflow, mostly unbiased by the presence of the

droplets. While ideally it would be desirable to optically

ARTICLE IN PRESS

U/W0 V/W0 W/W0

-0.25 -0.15 -0.05 0.05 0.15 0.25

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

z (mm) z (mm)

y (mm) y (mm) y (mm)

y (mm) y (mm) y (mm)

-0.50 -0.30 -0.10 0.10 0.30 0.50 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

σu/W0 σv/W0 σw/W0

0.00 0.10 0.20 0.30 0.40 0.00 0.10 0.20 0.30 0.40 0.00 0.10 0.20 0.30 0.40

(A)

(B)

Fig. 17. Variation of the normalized air (A) mean velocity (U/W0, V/W0, W/W0) and (B) root mean square velocity (sU/W0, sV/W0, sW/W0) with spatial

position for droplet-laden flow around the unheated cylinder (note that contour levels are different for the mean velocities).

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 593
ARTICLE IN PRESS

U/W0 V/W0 W/W0

-0.25 -0.15 -0.05 0.05 0.15 0.25

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

z (mm) z (mm)

y (mm) y (mm) y (mm)

y (mm) y (mm) y (mm)

-0.50 -0.30 -0.10 0.10 0.30 0.50 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

σu/W0 σv/W0 σw/W0

0.00 0.10 0.20 0.30 0.40 0.00 0.10 0.20 0.30 0.40 0.00 0.10 0.20 0.30 0.40

(A)

(B)

Fig. 18. Variation of the normalized air (A) mean velocity (U/W0, V/W0, W/W0) and (B) root mean square velocity (sU/W0, sV/W0, sW/W0) with spatial

position for droplet-laden flow around the heated cylinder (note that contour levels are different for the mean velocities).

U/W0 V/W0 W/W0

-0.25 -0.15 -0.05 0.05 0.15 0.25

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

z (mm)

y (mm) y (mm) y (mm)

-0.50 -0.30 -0.10 0.10 0.30 0.50 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Fig. 19. Variation of the normalized air mean velocity components (U/W0, V/W0, W/W0) with spatial position for the seeded flow only (in the absence of

the spray) around the unheated cylinder.

594 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
de-couple the two image signals, and thus obtain independent velocity estimates for the aerosol particles (air flow)

and spray droplets, and thus the local droplet/air relative

velocity, dealing with the difficulty of implementing an

optical discrimination methodology was outside of the

scope of this study.

With the seeding density typically much higher than the

droplet density, especially in the wake region, a final

interrogation area of 16 pixels  16 pixels was used to

determine the correlation function between interrogation

areas in each image pair. While the PIV correlation

technique estimates the average displacement of particles

within an interrogation area, the validity of the estimate is

a strong function of the number of particles that correlate

well within the interrogation area (as shown in Ref. [40]).

The smallest possible interrogation area that still detected

the signal of the aerosol particles was chosen to reduce the

bias towards larger droplets. It was assumed that one or

two larger droplets would not bias strongly the signal from

20 to 30 smaller aerosol droplets. Also, a 12-bit camera was

used with better intensity resolution to enhance the crosscorrelation signal-to-noise ratio. Therefore, since the

number count of the aerosol particles was always higher

than that of the spray droplets (as a result of the aperture

effect of the chosen smaller interrogation area), the

displacement estimate was biased in favor of the seeded

flow.

It was not possible to examine absolutely the above

reasoning in the present flow, since the spray influences the

airflow around the cylinder. Even so, measurements of the

seeded flow without the spray were also obtained, and

results of the mean velocity components are presented in

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350

325

300

275

250

225

200

175

150

Z (mm)

seed spray/seed spray

W (m/s)

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

350

325

300

275

250

225

200

175

150

Z (mm)

seed spray/seed spray

350

325

300

275

250

225

200

175

150

Z (mm)

seed spray/seed spray

V (m/s)

1.5

1.2

0.9

0.6

0.3

0.0

-0.3

-0.6

-0.9

-1.2

-1.5

W

V

U

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

U (m/s)

Fig. 20. Variation of the droplet mean streamwise (W) and cross-stream (U, V) velocities with streamwise position (Z) for the unheated cylinder. Contours

are the stream traces of the in-plane vectors obtained from the streamwise and V-cross-stream components of velocity.

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 595
Fig. 19. Comparison with similar results obtained for the

droplet-laden flow over the unheated cylinder (see Fig. 17)

indicates that the spray influences the flow, as expected.

The influence is modest, especially upstream of the cylinder

for the U- and V-components of velocity. The latter

component compares well in both cases (i.e. seeded airflow

only and droplet-laden seeded airflow), providing some

level of confidence in the post-processing approach used to

determine the airflow velocity for images containing both

seed and droplets. As will be discussed for the velocity of

the spray only case, the V-component of velocity immediately upstream of the cylinder was significantly smaller

than that for the seed only case, owing to the high

streamwise momentum of the droplets along the spray

centerline.

Comparison of the airflow velocity fields presented in

Figs. 17 and 18 indicates that the air velocity reached a

maximum streamwise value as the flow accelerated around

the cylinder, which is 15–20% higher than the nominal

streamwise value. The shear layer that forms past the

cylinder is quite evident in the streamwise root mean

square velocity figures, for both the unheated and heated

cylinder cases. Although, the velocity in the shear layer

indicates little magnitude difference between the two cases,

the mean streamwise velocity maps show evidence that for

the heated cylinder case the shear layer stretches further

downstream past the cylinder. The V-component of the

air velocity increases significantly around the side of

the cylinder, reaching a value of approximately 50% of

the nominal streamwise velocity. In the shear layer region,

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350

325

300

275

250

225

200

175

150

Z (mm)

spray/seed spray

W (m/s)

4.5

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

0.0

-0.5

-1.0

350

325

300

275

250

225

200

175

150

Z (mm)

spray/seed spray

350

325

300

275

250

225

200

175

150

Z (mm)

spray/seed spray

V (m/s)

1.5

1.2

0.9

0.6

0.3

0.0

-0.3

-0.6

-0.9

-1.2

-1.5

W

V

U

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

U (m/s)

Fig. 21. Variation of the droplet mean streamwise (W) and cross-stream (U, V) velocities with streamwise position (Z) for the heated cylinder. Contours

are the stream traces of the in-plane vectors obtained from the streamwise and V-cross-stream components of velocity.

596 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
the difference between the unheated and heated cases

is that the heated cylinder has a slightly higher crossstream velocity over a longer distance downstream

of the cylinder. On the other hand, a magnitude difference

of the values of sW in the shear layer is noted between

the two cases. This turbulence is lower for the heated

case, especially immediately downstream of the cylinder.

Lower values of sW indicate less diffusion, which would

support the apparent longer shear length observed

for the heated case. A recirculation zone also develops

behind the cylinder, with reverse velocities reaching about

20% of the nominal streamwise velocity. The reverse flow

appears to extend about two cylinder diameters downstream, and no significant length difference is apparent

between the two cylinder scenarios (i.e. unheated and

heated).

3.2.4. Spray characteristics—droplet velocity

Comparison of the velocity fields obtained with water

droplets and aerosol seed indicated that dispersion of

droplets/seed around the obstacle is dependent on the

droplet/seed size. A more detailed analysis of the dependence of size on transport of droplets around the cylinder is

given in Ref. [29]. For example, the three components of

droplet velocity for the droplet-laden flow over the

unheated cylinder are presented in Fig. 20. In this 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. (Note the change of

contour color scale in Fig. 20 and subsequent figures.) The

seed only case compares with the results from the vertical

configuration (see Fig. 11), and was discussed in detail

earlier. Comparison of the two cases with spray to the seedonly case indicates that the recirculation zone is somewhat

larger for the spray cases. Larger size droplets require a

longer transit time to interact with the turbulent flow field

and reduce their initially higher momentum to be entrained

into the recirculation zone, if at all.

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U/W0 V/W0 W/W0

-0.25 -0.15 -0.05 0.05 0.15 0.25

275

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175

150

275

250

225

200

175

150

275

250

225

200

175

150

z (mm)

y (mm) y (mm) y (mm)

-0.50 -0.30 -0.10 0.10 0.30 0.50 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

(A)

U/W0 V/W0 W/W0

-0.25 -0.15 -0.05 0.05 0.15 0.25

275

250

225

200

175

150

275

250

225

200

175

150

275

250

225

200

175

150

z (mm)

y (mm) y (mm) y (mm)

-0.50 -0.30 -0.10 0.10 0.30 0.50 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

(B)

Fig. 22. Variation of the droplet normalized mean velocity components (U/W0, V/W0, W/W0) with spatial position around the (A) unheated and (B)

heated cylinders.

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 597
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 along the upstream

side 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) [41].

The droplet-laden flow over the heated cylinder,

presented in Fig. 21, is similar to the unheated cases (only

the spray and combined spray/seed cases are presented in

the figure since the seed only case was not expected to

change from that of the unheated cylinder). When the

heated cylinder is compared to the unheated cylinder

the following differences 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 U-component crossstream flow extends further downstream of the cylinder.

These results are consistent with that of a heated flow field,

in which the reduction in gas density (and concomitant

reduction on droplet drag) results in increased streamwise

droplet-laden flow around the cylinder.

Normalized mean velocity contours with only the spray

droplets (in the absence of aerosol seed), are shown in

Fig. 22 for both the unheated and heated cylinders. In light

of the droplet sparseness during each image pair acquisition, especially in the wake region, a moving ensemble

averaging technique was implemented, whereby several

image pairs were initially averaged (separately for Images

A and B) to produce a composite image pair prior to

implementing the PIV cross-correlation methodology. The

ensemble size chosen was three, and thus N2 composite

image pairs were created for each planar acquisition.

Consequently, the vector maps determined from each

composite image pair were used for further analysis.

Comparison of the normalized mean values for the

unheated cylinder indicates that the spray droplets entrain

into the wake region, with their U- and W-components of

velocity comparable to the air flow velocity (shown in

Fig. 17). A difference is, however, noted for the

V-component of velocity near and around the surface of

the cylinder. While the air stream velocities indicate strong

flow redirection due to the presence of the obstacle, the

droplet velocities in the same region are 40–50% less than

the air cross-stream velocity at the same location. This

indicates that the droplets fail to follow the flow around the

cylinder due to their larger streamwise momentum, leading

to collisions with the cylinder for droplets within the core

region of the spray. The core spray region is defined by

estimating which droplets are destined to impact directly

with the cylinder, based on the velocity field upstream of

the cylinder, as shown in Fig. 23. The figure presents colorcoded streamlines, coded by the normalized magnitude of

the total velocity, for the case of droplet-laden airflow over

the unheated cylinder. In addition to these color-coded

streamlines, a solid white line estimates the division

streamline between the flow transported around the

ARTICLE IN PRESS

0 0.2 0.4 0.6 0.8 1 1.2

0 10 20

Fig. 23. Color-coded streamlines for droplet-laden air flow past the

unheated cylinder (normalized to the magnitude of the total velocity);

white lines indicate the dividing streamlines separating the flow that

impacts the cylinder to that transported around it: solid—droplet-laden air

flow with the unheated cylinder; dash—droplets with the unheated

cylinder; and dash-dot—droplets with the heated cylinder (dimensions

are in mm).

598 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
cylinder (on the right of the line) and the flow impacting

with the cylinder (on the left). The solid line is the estimate

for the droplet-laden flow around the unheated cylinder

(i.e. droplets and seed). In a similar fashion, the dashed

white line was constructed for the spray flow past the

unheated cylinder, based on the planar velocity components that exist upstream of the cylinder. Note that the

contribution of the entrained seed in the droplet-laden

airflow case results in the solid white streamline positioned

closer to the center of the flow than in the droplet only case

(dashed line). This result further supports the methodology

used to separate airflow velocity from droplet velocity in

the droplet-laden flow cases, as described earlier.

The results for the spray flow around the heated cylinder

are similar to that of the unheated cylinder (see Fig. 22).

However, the V- and W-components of velocity are

noticeably smaller. An estimate on the dividing streamline

is also shown in Fig. 23, by the white dash-dot line. The

reduced magnitude of the cross-stream velocity components results in an increase in the width of the spray core

region. Thus, the results indicate greater droplet impingement on the cylinder surface than for the unheated

cylinder, and less droplet entrainment into the shear layer.

In the wake region, there is less droplet entrainment into

the recirculation region for the heated cylinder, as

compared to the unheated cylinder. Also, the surface

temperature along the downstream side of the cylinder is

presumed to be higher than on the upstream side, since it is

not quenched by droplet impingement and wetting of the

surface, and may contribute to the diminished presence of

droplets in this region.

The root mean square velocities for the ambient seeded

case ambient spray/seed case, and heated spray/seed case

(assuming the heated seed only case is similar to the

corresponding ambient case) are presented in Fig. 24. The

root mean square velocity increases behind the cylinder,

reaching a maximum value of about 1.4 m/s in the shear

layer region for sW and sV of the heated cylinder case.

There appears to be little difference when the contribution

of the spray is considered, however, for the heated cylinder

ARTICLE IN PRESS

350

325

300

275

250

225

200

175

150

Z (mm)

ambient

seed

ambient

spray/seed

heated

spray/seed

350

325

300

275

250

225

200

175

150

Z (mm)

ambient

seed

ambient

spray/seed

heated

spray/seed

σv (m/s)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

σW (m/s)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

350

325

300

275

250

225

200

175

150

Z (mm)

ambient

seed

ambient

spray/seed

heated

spray/seed

σU (m/s)

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

W

V

U

Fig. 24. Variation of the root mean square velocity (sU, sV, sW) with streamwise position (Z).

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 599
case, the root mean square velocity is significantly higher

along the shear layer region than for the unheated cylinder

case.

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

and V-cross-stream components, as indicated in Fig. 25 for

the seed only case. An absolute value of 0.7 is reached for

the correlation coefficient, similar to the results given in

Fig. 13. This result is indicative of the shear between the

free stream and reverse flow region downstream of the

cylinder (i.e. the fluid flow stagnation point is off the

downstream face of the cylinder). The presence of the spray

appears to have negligible effect on the results when

comparing the seed only and spray/seed cases in Fig. 25.

The flow also decelerates near the stagnation region along

the centerline at the upstream face of the cylinder.

Fig. 26 presents the 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

ARTICLE IN PRESS

350

325

300

275

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225

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150

Z (mm)

seed spray/seed

ρWU

0.5

0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

350

325

300

275

250

225

200

175

150

Z (mm)

seed spray/seed

ρWV

0.5

0.6

0.7

0.8

0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

-0.6

-0.7

-0.8

350

325

300

275

250

225

200

175

150

Z (mm)

seed spray/seed

ρUV

0.5

0.6

0.7

0.4

0.3

0.2

0.1

0.0

-0.1

-0.2

-0.3

-0.4

-0.5

W

V

U

Fig. 25. Variation of the turbulence correlation coefficients (rWU, rWV, rUV) with spatial position (Z).

600 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
there is reverse flow in the wake region. In addition, the

presence of a 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 aerosol particles

through convoluted pathways of this obstacle.

3.2.5. Spray characteristics—droplet size

A semi-quantitative estimate for the relative droplet size

of the spray can be obtained by considering certain

characteristics of the PIV cross-correlation function. As

mentioned earlier, each image pair is broken up into

smaller image areas, called ‘interrogation areas’. For each

one of these interrogation areas a cross-correlation

function is generated according to the PIV methodology.

This cross-correlation function is a surface function, as

presented in Fig. 8, that is characterized by its peak height

and its peak half-width (i.e. width of the function at half its

peak height value). While the location of the peak within

the interrogation area is mostly independent of particle

image size and appearance, the contrary is true about the

width of the correlation function. The broadness of the

correlation function has a strong dependence on how

particles image onto the CCD, with larger apparent

particles contributing to a broader correlation function,

or larger peak half-width values. Thus, the cross-correlation

ARTICLE IN PRESS

W (m/s)

-0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50

V (m/s)

-1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00

V

W

U

U (m/s)

-1.00 -0.80 -0.60 -0.40 -0.20 0.00 0.20 0.40 0.60 0.80 1.00

Fig. 26. Variation of the mean streamwise (W) and cross-stream (U, V) velocities with streamwise position (Z) 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.

C. Presser et al. / Fire Safety Journal 41 (2006) 580–604 601
peak half-width provides a semi-quantitative estimate for

an ‘‘average’’ relative size of the ensemble of spray droplets

within the interrogation area.

With the benefit of having one of the cameras looking at

the flow nearly perpendicular to the illumination plane, an

average correlation function field was generated from

image pairs captured by this camera, providing nearly

distortion-free imaging of the droplets. An average

correlation field was generated using the methodology

proposed by Meinhart et al. [42], whereby the correlation

functions of many image pairs were first averaged prior to

determining correlation peak characteristics. In such a way,

the signal-to-noise ratio of the correlation function is

dramatically improved, which is important in the present

case, where regions exist that are sparsely populated by

droplets, especially in the wake. The extracted information

from three averaged ensembles of more than 150 correlation averaged image pairs is presented in Fig. 27. Shown is

the relative cross-correlation peak half-width, normalized

by the value upstream of the cylinder (i.e. at Z 1⁄4 140 mm

and along the cylinder centerline), for the unheated and

heated cylinder scenarios.

Upstream of the cylinder, the relative droplet pseudosize indicates a 10–15% increase at off-axis locations in

relation to the spray center, which is attributed to the

hollow-cone nozzle design. Downstream of the cylinder,

the presence of larger droplets in the shear layer is

indicated by an increase in the relative peak half-width

value, approximately 80–120% larger than in the spray

core upstream of the unheated cylinder, while it is more

than 200% larger for the heated cylinder. Note that the

larger value obtained for the heated cylinder is to a large

extent biased by the vapor formed due to droplet

evaporation, which tends to broaden the correlation

function due to the vapor’s flat intensity distribution.

Hence, the semi-quantitative results of the current methodology are not expected to capture the correct trend with

regards to droplet size in the shear layer when comparing

the unheated and heated cylinder cases.

In the wake region of the unheated cylinder, the estimate

of the relative pseudo-size indicates the presence of droplet

sizes smaller than those detected in the spray upstream of

the cylinder. Determination of the size distribution in the

recirculation region becomes difficult at positions near the

aft cylinder surface. For the heated cylinder case, however,

the relative pseudo-size estimate indicates that larger

droplets are detected in the recirculation region than for

the unheated cylinder. Again, these results for the heated

cylinder may contain bias due to vapor being entrained

into the wake region.

4. Conclusions

Particle image velocimetry measurements were carried

out for a droplet-laden, homogeneous turbulent flow over

an unheated and heated cylinder, and body-centered cubic

arrangement of spheres. Droplets were observed to impact

the cylinder surface and either wet (leading to dripping) or

rebound off the surface. Coating of an obstacle surface by

a liquid agent, as opposed to gaseous agents, may be

considered a positive event in that the agent will cool the

surface and inhibit reignition. The body-centered cube

results indicated that it takes a relatively high blockage

ratio to influence significantly liquid agent transport past

an obstacle and prevent the transport of agent to a flame.

Comparison of the velocity fields obtained with and

without water droplets indicated that droplet dispersion

around an obstacle is dependent on the droplet size and

velocity.

Velocity results for the droplet-laden airflow and the

droplet-only cases indicated that the airflow is relatively

unaffected by heating of the cylinder, while the heat has a

significant effect on droplet transport. By considering

planar streamlines generated from the droplet velocities

upstream of the cylinder, it was possible to construct a

dividing streamline that marked the boundary between the

spray that impacts the cylinder and the spray that is

transported past the cylinder. The lower cross-stream

velocities measured for the heated cylinder indicated that

a larger core region from the spray was not redirected past

the cylinder, and resulted in less droplet transport,

especially smaller droplets, into the wake region.

Heating of the cylinder surface resulted in vaporization

of impinging droplets, and a visible vapor layer downstream of the cylinder in the shear region. Semi-quantitative sizing of droplets using the peak half-width

information of the PIV cross-correlation calculation

indicated that droplets are entrained in the wake region,

and these droplets are, in general, smaller than those

existing in the spray upstream of the cylinder and in the

shear layer.

Assuming a positive correlation between droplet size and

velocity, larger droplets move ballistically and smaller

ARTICLE IN PRESS

0.8 1.2 1.6 2.0 2.4 2.8 3.2

240

220

200

180

160

140

z (mm)

y (mm)

0.8 1.2 1.6 2.0 2.4 2.8 3.2

240

220

200

180

160

140

z (mm)

y (mm)

Fig. 27. Relative cross-correlation peak half-widths for spray flow around

the unheated (left) and heated (right) cylinders.

602 C. Presser et al. / Fire Safety Journal 41 (2006) 580–604
droplets are entrained into the surrounding aerodynamic

flow field. Thus, different strategies to control and optimize

the spatial dispersion of droplets can be developed by

including a wide range of droplet sizes, e.g. to disperse

droplets far downstream of and/or immediately behind an

obstacle. The atomizer design must also be considered since

it dictates the initial spatial dispersion of droplets. These

points are important for consideration in the development

of fire suppression models and validated simulations. In

addition, the effect of various spray characteristics like

droplet size, velocity, and spatial distribution on transport

and extinguishment of fires is important in developing

different scenarios for fire suppression.

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 for the

discussions and guidance that was provided us, B.S.

Johnson for help with processing of the PIV data, and R.

Fink for his assistance in completing the NIST facility

modifications.

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