ILASS Americas 26th Annual Conference on Liquid Atomization and Spray Systems, Portland, OR, May 2014

An Approach to Spray Characterization by Combination of Measurement Techniques

D. Bissell*, W. Lai, M. Stegmeir, D. Troolin, S. Pothos TSI Incorporated

500 Cardigan Road

Shoreview MN, 55126

C. Lengsfeld Department of Mechanical & Materials Engineering

University of Denver

2390 S. York St.

Denver, CO 80208

Abstract

A single orifice flat spray nozzle was evaluated with three techniques to characterize various aspects of the resulting

spray. The three techniques were high speed and high resolution flow visualization, particle image velocimetry

(PIV) and Phase Doppler Particle Analysis (PDPA). A high speed CMOS camera was used for flow visualization at

25,000 captures/s, and provided time-resolved analysis of wave instabilities leading to primary sheet breakup. Addi-

tionally a CCD camera with 29 million pixels was used to capture shadowgraph images to realize ligament for-

mation and collapse as well as droplet interaction. This camera was then applied with a PIV system to evaluate the

overall velocity field of the spray, from nozzle exit to droplet discharge. PIV images were further post-processed to

determine the inclusion angle of the spray. Based on the aforementioned results, the PDPA was used to investigate a

series of strategic locations within the spray to obtain droplet size information. The objective of investigating the

spray from the structure and droplet evolution, to the quantitative analysis of the structure using a high resolution

CCD camera, followed by the measurements of the droplet size at strategic location of the spray was accomplished.

The results from those investigations provided significant quantitative understanding of the spray structure with

time-resolved information.

________________________________ *Corresponding author: daniel.bissell@tsi.com

.

ILASS Americas 26th Annual Conference on Liquid Atomization and Spray Systems, Portland, OR, May 2014

Introduction

The characteristics associated with spray for-

mation are of great importance to many in the spray

community. Sprays are applied in numerous circum-

stances, from internal combustion engines, pharmaceu-

tical processes, paint application, agricultural fertilizers

and pesticides, fire suppression, and a multitude of oth-

er applications. Spray performance has significant im-

pacts on daily life. Subsequently, continual research of

spray diagnostics has been carried on for the past sever-

al decades to explore methods to achieve a complete

understanding of spray characteristics.

As such, there has been a continual evolution

in analytical tools and methods which evaluate spray

attributes. A very comprehensive analysis of the vari-

ous spray diagnostics given by [W.D. Bachalo] dis-

cussed the capabilities and deficiencies of those diag-

nostic techniques. The surveyed techniques included

(1) small-angle light–scatter detection, (2) Imaging sys-

tems, (3) Particle Imaging and Particle Tracking Veloc-

imetry, (4) Phase Doppler Interferometry, and (5) Opti-

cal patternators. It was concluded that a combination of

results from those diagnostics techniques, compliment-

ed with numerical models, were all necessary to

achieve the goal of completely understanding the spray.

Additional investigations of the volume flux measure-

ments using the Phase Doppler Interferometry [K.M.

Bade and R.J. Schick] provided detailed information on

the parametric settings of the technique to allow meas-

urements with high sensitivity. The employment of

four detectors in the Phase Doppler Interferometry

[C.M. Sipperley and W.D. Bachalo] showed the im-

provement of the results yielded by the three independ-

ent measurements of phase shift between detectors.

Subsequently, the technique showed improvement in

the measurements of dense spray with enhanced phase

discrimination. Methods still continue to evolve to bet-

ter characterize spray systems.

This investigation is offered to advance spray

analysis methodology. Spray measurements of a single

orifice flat spray nozzle were collected through a varie-

ty of techniques to characterize the attributes of sprays.

As highlighted by [W.D. Bachalo], the combination of

techniques was necessary to fully diagnose the for-

mation and physics of the spray. As such, this case-

study offers an approach to spray characterization, pro-

gressing from qualitative flow visualization, to highly

accurate quantitative results. In this way, the spray

attributes from the nozzle exit, and down-stream, were

fully realized.

Initially, the single orifice flat spray nozzle

was diagnosed with high-speed flow visualization. Ap-

plying such a technique offered temporally resolved

qualitative analysis of wave instabilities within the

sheet region of the spray. This flow visualization sys-

tem was comprised of a high speed CMOS camera and

a continuous LED illumination module. A capture rate

of 25,000 fps was necessary to qualify the primary

sheet breakup. Resultantly, the camera resolution was

significantly reduced to accommodate the high capture

rate. Limited analysis was completed on these low reso-

lution images. With this approach, wave instabilities

within the liquid sheet were assessed.

Additional imaging-based analysis of the spray

was completed with high resolution images. The se-

cond system employed was the particle image veloci-

metry (PIV) system consisting of a CCD camera with

29 million pixel resolution and dual-pulsed laser light

source. The PIV system was setup in a shadowgraph

configuration with an illuminated diffusion surface lo-

cated directly opposite of the camera and behind the

spray. The super-high resolution shadowgraph images

offered new insight into ligament formation and col-

lapse, as well as droplet interaction. The illumination

was also re-arranged into a traditional orthogonal light-

sheet configuration to achieve velocity field measure-

ments of the spray. These spray images were further

post-processed to derive the spray angle as well.

The third and final technique applied to the spray

was the Phase Doppler Particle Analysis (PDPA) sys-

tem. Strategic locations were identified by analysis of

the high resolution PIV results. The PDPA system of-

fered highly accurate and well resolved droplet size

measurements. These measurements were carried out at

specific locations, below the nozzle exit consistent with

past analysis, as well as PIV results.

It was realized that the employment of all analyti-

cal techniques (high speed flow visualization, Shadow-

graphy, super-high resolution PIV, and PDPA) provid-

ed a unique and insightful approach to spray characteri-

zation. Conducting measurements in such a sequence

offered an evolving understanding of the spray, which

aided in isolating the critical regions in the spray, and

best suited method of interrogation. With the quantita-

tive measurements performed in those critical regions,

sufficient information was obtained to provide repro-

ducible characterization of the spray behavior.

Background in Measurement Techniques

As previously mentioned, several measure-

ment techniques were applied to the flat spray to fully

characterize the spray system. In completeness, the fol-

lowing is a broader discussion of the measurement

techniques applied in this analysis.

The shadowgraph imaging technique is often

considered to be one of the most approachable spray

analysis methods as it requires few resources to imple-

ment. Typically, backlit shadowgraphy involves an

illumination source placed behind a light diffuser, and a

camera positioned opposite of the diffuser plane. Gen-

erally, the spray experiment is situated between the

diffuser and camera. Fundamentally, shadowgraph im-

.

aging relies on the re-direction of light due to refrac-

tion. The light traveling toward the direction of the

camera, away from the diffuser, will be perturbed (re-

fracted/reflected) due to the change in refractive index

of the sprayed liquid relative to the spray environment.

As such, un-reflected/refracted light rays will fill the

image sensor. Light rays diverted by the spray will

travel elsewhere causing a lack of photonic energy on

the image sensor, translating to a dark portion of the

image thus illustrating a ‘shadow’. The lack of light

indicates the presence of the spray medium.

With the evolution in image-based diagnostics,

there exist various approaches to obtaining and analyz-

ing shadowgraph results. Primarily, implementation of

back-lit shadowgraphy separates into two distinct fami-

lies; that of 1) low-resolution but fast image capture and

2) high-fidelity non-consecutive still images. Both ap-

proaches apply the same fundamental principles for

shadowgraph imaging, although nuances exist in the

hardware. Distinctly, the former technique lends well

to transient analysis such as understanding the evolu-

tion of wave instability leading to sheet breakup at the

start of the spray. The latter, however, illustrates in

great detail features like primary and secondary sheet

breakup, formation and collapse of ligaments, and drop-

let formation and coalescence. In both cases, advance-

ments in software analysis offer quantifiable infor-

mation about the spray characteristics. Both approach-

es, however, offer unique and necessary insight towards

full characterization of a spray.

Keeping with spray visualization, Particle Image

Velocimetry (or PIV) is one of the most well-

established techniques to obtain instantaneous velocity-

related measurements and related properties of fluids.

In general the fluid flow (either air or liquid) is seeded

with tracer particles that are very small and are assumed

to faithfully follow the fluid flow. In a spray context,

the droplets serve as the tracer particles. These tracers

are typically illuminated by a planar, short duration

high powered lightsheet, generated by a dual-pulsed

laser source. Each light pulse scatters off of the tracers

and collected on either a CCD or CMOS camera. The

scattered light, registered as intensity in the pixels of

these cameras, identifies particle locations at each time

instant. The measured particle displacement, either in-

dividual or specially averaged, of the resulting image

pairs and time separating the laser pulses determine

speed and direction of the velocity field of the flow

being studied. More information on the technique can

be found in the following references [Adrian (1991),

Raffel et al. (2007)].

The application of the PIV to spray analysis is

straight-forward since there is no need to apply tracer

particles to follow the flow. The typical droplet diame-

ters found in sprays that are in the range of few to hun-

dreds for microns act as particles and the scattered light

is used to extract information about the spray velocity.

Within the liquid sheet of the spray, wave fronts and

voids offer unique characteristics within the image ap-

plicable for velocity analysis. Post processing of spray-

PIV images offer numerous analysis approaches from

understanding surface shear tension to spray inclusion

angle.

Separate from imaging-bases analysis, Phase Dop-

pler Particle Analysis (PDPA) offers further quantifia-

ble analysis of the spray. PDPA is an interferometric

laser diagnostic technique that provides insight into

several important spray properties, including drop size

and velocity, number-density, flux, time-of-arrival sta-

tistics, and gas-phase velocity. Crossed laser beams

and slit aperture define a cylindrical measurement vol-

ume (typically 10s to 100s of microns in each dimen-

sion) with obliquely-angled ends. Within this meas-

urement volume the crossing laser beams interact and

generate an interference “fringe” pattern. Drops pass-

ing through this volume scatter light which is collected

by an off-axis receiver. Scattered fringe spacing and

frequency is dependent on drop size and velocity and is

measured by three detection regions in the receiver. An

additional validation of drop size is provided by the

measuring the intensity of the scattered light. By way of

this technique, highly accurate droplet attributes are

quantified.

Experimental Setup

The aforementioned spray analysis techniques

were applied to a single-orifice flat spray nozzle for

sake of full characterization. As described, three exper-

imental setups were used for this investigation, includ-

ing high speed flow visualization, a PIV system ar-

ranged for shadowgraphy as well as velocity measure-

ments, and a PDPA system. A description of the exper-

imental arrangement applying the different systems is

provided in the subsequent section.

The spray nozzle used in the investigation was

a single orifice hydraulic flat spray nozzle from Spray-

ing Systems Company, model TPU650050-TC. The

nozzle has been the subject of previous investigations

[K.N. Bade and R.J. Schick] and served as a baseline

for the investigation. The nozzle operated under a liquid

pressure of 4 bar, with an estimated flow rate of 225

mL/min. Liquid water at room temperature was sprayed

from the nozzle under atmospheric conditions. The flat

spray nozzle offered a symmetric volume distribution

as well as a simple spray plume shape. Further infor-

mation regarding the geometry of the spray was report-

ed by [K.N. Bade and R.J. Schick].

Although configurable, the various systems

were not used simultaneously. The experiment operated

under steady state conditions, thus each measurement

technique was applied separately. Systems were ar-

ranged together so that the measurements were taken

with minimal disturbance. This experimental configura-

tion helped to maintain the steady state nature of the

spray. Figure 1.1 and 1.2 illustrates the equipment ar-

rangement of the PIV and PDPA systems. The PIV

system pictured was configured in the high-resolution

shadowgraph setup.

Figure 1.2. PDPA measurement volume.

Both the high speed flow visualization system

and the PIV offered the planar measurement of the flat

spray nozzle, giving the result of the entire spray with a

single capture. The PDPA, however, was a single point

measurement. Hence the nozzle was mounted on a trav-

erse mechanism to allow the nozzle to be moved hori-

zontally and vertically so that the measurement at the

specific locations of the spray could be performed

without the moving of the PDPA transmission probe

and the receiving optics.

The high speed flow visualization system was

employed to achieve a qualitative understanding of the

time resolution of the flow structure inducing ligament

formation. The high speed system included a Phantom

M340 high speed camera with 4 million pixel resolution

and a LED illumination module with 40 W power. The

illumination of the spray was arranged from the back of

the spray to allow the shadow of the spray structure to

be captured. Such arrangement was beneficial because

the distribution of the illumination could be very uni-

form with the arrangement of a “diffuser” plate in the

path of the illumination. Uniform illumination was

important such that the light scattering from the differ-

ent structures (wave fronts, ligament, large and small

droplets) in the spray could be captured. In addition,

the forward scattering arrangement by the camera pro-

vided the highest sensitivity of the capture. Figure 1.3

shows the uniform illumination pattern for the flow

visualization arrangement. The high speed camera was

operated at 25,000 fps at 256 and 256 pixel resolution

to capture the flow evolution of the spray. At such

frame rate, the time resolution was sufficient to see the

development of the ligament, droplet formation and

breakup. A sequence of greater than 5000 frames was

captured.

Figure 1.3. Back illumination of shadowgraph

configuration.

Separate from the high-speed flow visualiza-

tion configuration, a 29 million pixel CCD camera-

based system (at 4 fps) was also arranged for the spray

so that the fine and detailed structure of the spray could

be captured. Image capture was synchronized with a

Figure 1.1. Experimental setup of spray analysis systems including PIV and PDPA Instrumentation.

Nd:YAG 200 mJ dual-cavity laser system. The laser

light was directed in back illumination against the dif-

fuser plate to capture shadows of the spray structures.

Due to the high pixel resolution of 29 million pixel, the

images of the ligament and droplets in spray were very

quantitative, meaning that the size of those objects

could easily be measured by counting the number of

pixels representing the objects. In addition to still

frames collected for shadowgraph analysis, the laser

source was positioned to illuminate a plane of light for

PIV image capture and velocity analysis. The velocity

profile for the entire flat spray was measured, including

the velocity distribution at the exit of the nozzle.

Lastly, the PDPA system and technique was ap-

plied for measurement of the droplet size within spray.

The system employed for this investigation was a Di-

ode-Pumped Solid State (DPSS) laser based transmis-

sion optical module called the PowerSight. The laser

provided 300 mW of power in the 561 nm wavelength.

The receiving optics employed a three-detector, fiber-

bundle, arrangement to capture the phase difference of

the three Doppler signals. The PDPA system was fixed

while the nozzle was mounted to a traverse stage allow-

ing the measurement at prescribed locations of the

spray. The specific locations were identified based on

the flow visualization results taken by the two other

systems.

Results

The data was analyzed in a way that utilized

the strengths of each of the measurement techniques in

providing information about specific aspects of the

spray. As such, the discussion of the results progress

from a qualitative global analysis to quantitative point

analysis.

Shadowgraph images were taken of the flat

spray for analysis of wave instabilities leading to prima-

ry breakup within the water sheet, ligament formation

and collapse, and droplet interaction. Wave instabilities

within the sheet region were captured at high speeds to

characterize the process of primary sheet breakup. The

high-speed images were captured in a region of interest

approximately 10mm below the nozzle exit. Evolution

of sheet collapse was demonstrated by Figures 2.1 –

2.5. Initially, wave fronts downstream of the nozzle exit

caused thinning of the liquid sheet. The break-up pro-

cesses began with two ‘tears’ at thin portions of the

water stream and grew larger under the pull of the liq-

uid surface tension. As the sheet began to collapse, the

tears merged to create a gap between the liquid sheet

and the downstream portion. Eventually, the two por-

tons of the liquid sheet separated completely, the upper

portion remaining attached to the sheet, the lower por-

tion further collapsing by the pull of the surface tension

to form a ligament segment. In this way, the process of

sheet collapse was characterized.

Figure 2.1-2.5. High speed images depicting primary

sheet breakup process, outlined in red.

High resolution shadowgraph images were an-

alyzed to better characterize general spray attributes.

Although the image was captured with back illumina-

tion, the grey-scale values of the images were inverted

to better highlight attributes of the spray. Specifically,

the sheet breakup length was determined to be

16.55mm below the nozzle exit, by review of the in-

stantaneous image in Figure 2.6. Just below which, both

a ligament formation and collapse were observed.

.

Additionally, the high resolution images depicted larger

droplets at the edges of the spray and smaller toward

the center. This specific observation was consistent

with Phase Doppler results which follow. It was also

clear that droplet interaction was more prevalent in the

center portion of the spray than the outside edges. Addi-

tionally, it was observed that droplets captured in the

shadowgraph images appeared mostly spherical toward

the bottom of the image, although there were some ir-

regularities.

Figure 2.6. High resolution shadowgraph image depict-

ing sheet break-up length, ligament charac-

teristics, and droplet interaction.

The spray was further analyzed using particle

image Velocimetry (PIV) in order to determine the av-

erage velocity on a plane located along the centerline.

A 29 megapixel CCD camera was used to collect 700

image pairs. The total field of view was approximately

90 x 70mm, and the resultant vector pitch was 0.55mm.

Ensemble correlation was used to determine the aver-

age velocity field. Ensemble correlation is a technique

by which the correlation maps at a given location and

across multiple realizations are added to determine the

average displacement at a given location. The ad-

vantage to this technique is that high SNR data is

achievable in regions of relatively low seeding density,

such as very close to the nozzle exit. The ensemble-

averaged vector field can be seen in Figure 2.7. The

velocity field reveals a dual-lobed structure which ex-

hibits a very slight asymmetry in the spray plume, as

the lobe of higher velocity extends further downstream

on the left side than on the right (see, for example, the

yellow contour level). This asymmetry is minimal how-

ever, and the overall spray structure is quite uniform in

terms of velocity. The velocity very near the nozzle

exit is greater than -25 m/s and decreases to a velocity

of -20 m/s at 50mm downstream, and -18 m/s at 70mm

downstream.

Figure 2.7. Average velocity field obtained from 700

PIV vector fields. The contour quantity

shown is the vertical velocity, with red in-

dicating greater negative velocity and blue

indicating lesser negative velocity

A zoomed-in, single image of the near-nozzle

measurement region can be seen in Figure 2.8, with the

ensemble vector field overlaid. The liquid sheet is visi-

ble very close to the nozzle exit, as well as the liga-

ments before breakup in the downstream portion of the

spray. The ensemble correlation was beneficial in

achieving reliable data in this region of the spray.

Figure 2.8. A zoomed in region of a single image; with

the average vector field overlaid (not all

vectors are shown, for clarity).

Additionally, the spray angle for this experi-

ment was also derived from the PIV images. From the

original PIV images, 100 captures were averaged to

generate a generalized spray pattern. From this, edge-

detection software determined the spray pattern angle.

The software analysis determined a 64.6o spray inclu-

sion angle.

.

0

20

40

60

80

100

120

140

160

180

-60 -40 -20 0 20 40

D3

2 (

um

)

X (mm)

Sauter Mean Diameter (D32)

Figure 2.9. 100 PIV images were averaged and thresh-

old analysis determined spray edges and an-

gle of inclusion.

Lastly, the spray was analyzed using PDPA in

order to determine the drop size distribution along the

centerline of the spray 50 mm downstream of the noz-

zle exit. Data were acquired at 17 points spaced 5 mm

apart along the long axis of the flat spray. At each loca-

tion 30,000 measurements of drop size and velocity

were recorded. Sauter mean diameter D32 was reported

for each location.

Figure 2.10 shows the measured Sauter mean

diameter D32. D32 was observed to reach a minimum

value in the center of the spray, and increase to a max-

imum near the edge of the spray. The left side of the

spray showed a slightly higher maximum value (164

microns) as compared with the right side of the spray,

which showed a maximum D32 of 150 microns.

Conclusions

Three different diagnostic techniques were

used to evaluate the different aspects of the single ori-

fice flat spray nozzle. The high speed shadowgraph

images revealed the evolution of the liquid breakup at

the exit of the nozzle into the formation of ligaments

downstream. High resolution shadowgraphy showed

the quantitative results of ligament formation and col-

lapse, and subsequent droplet formation. The larger

droplets were observed at the edges of the spray and

smaller ones toward the center. The PIV results provid-

ed both the quantitative velocity profile from the nozzle

exit and downstream, as well as spray angle. The

PDPA was used to measure quantitative results such as

droplet size. The distribution of the Sauter mean diam-

eter D32 across the spray was shown.

Further investigation using the imaging tech-

nique for individual droplet size and velocity is planned

to be carried out as the next phase of measurement.

Figure 2.10. Sauter Mean Diameter 50 mm below

nozzle

References

“Spray Diagnostics for the Twenty-first century”, W.D.

Bachalo, Atomization and Sprays, vol. 10, p 439-474,

2000

“Phase Doppler Interferometry volume flux sensitivity

to parametric settings and droplet trajectory”, K.M.

Bade, R.J. Schick, Atomization and Sprays, 21 (7),

p.537-551, 2011

“Particle-imaging techniques for experimental fluid

mechanics”, R. Adrian, Ann Rev Fluid Mech 23, p.

261–304, 1991

“Particle Image Velocimetry, A Practical Guide”, M.

Raffel, C. Willert, S. Wereley, and J. Kompenhans , 2nd

Edition (ISBN 978-3-540-72307-3 Second Edition

Springer Berlin Heidelberg New York), 2007

“Triple Interval Phase Doppler Interferometry : Im-

proved dense sprays measurements and enhanced phase

discrimination”, C.M. Sipperley, W.D. Bachalo, ILASS

America, 25th

Annual conference on Liquid Atomiza-

tion and Spray System, Pittsburgh, PA, May 2013

“Post-Processing of Phase Doppler Interferometry data

for Planar spray characteristics”, K.M. Bade, R.J.

Schick, ILASS America, 25th

Annual conference on

Liquid Atomization and Spray System, Pittsburgh, PA,

May 2013