Exploration of Temporal and Time-Averaged Two-Phase Flow ... · Exploration of Temporal and...

ILASS Americas, 29th Annual Conference on Liquid Atomization and Spray Systems, Atlanta, GA, May 15-18, 2017

Exploration of Temporal and Time-Averaged Two-Phase Flow Structures Using

X-Ray Diagnostics

Kuo-Cheng Lin*

Taitech, Inc.

Beavercreek, Ohio 45430

Alan Kastengren

Argonne National Laboratory

Argonne, Illinois 60439

Campbell Carter

Air Force Research Laboratory, Aerospace Systems Directorate

Wright-Patterson AFB, Ohio 45433

Abstract

Time-averaged and temporal structures of the two-phase flows within the nozzle of an aerated-liquid injector were

experimentally explored with synchrotron x-ray diagnostics, including pathlength-integrated and confocal x-ray fluo-

rescence and x-ray high-speed imaging. A beryllium nozzle with a constant passage diameter was fabricated to mate

with an aerated-liquid injector featuring the outside-in aerating scheme. Water and nitrogen were doped with x-ray

fluorescent elements at low concentrations to facilitate the x-ray diagnostics. The present study shows that the two-

phase flows inside the nozzle section exhibit an annular-like flow pattern, while the aerating gas is distributed in a

Gaussian-like pattern. Axial distributions of the average liquid density, gas density, and liquid velocity were also

explored by integrating the line-of-sight properties over several cross-sectional areas and axial locations within the

nozzle section. For the first time on these sprays, a confocal x-ray fluorescence measurement was carried out with a

polycapillary x-ray optic to spatially resolve the cross-sectional mass distributions within the near field of various

discharged plumes. Plume separation phenomena were readily observed in both liquid and gas plumes. High-speed

imaging shows that large-scale structures are mainly present in the plenum section and can be aerodynamically

stretched into fine structures near the nozzle exit.

__________________________________________ * Corresponding author, [email protected]


INTRODUCTION

Liquid aeration, also called ‘effervescence’ or

‘barbotage,’ is a plausible approach to enhance liquid

atomization in several applications.1-5 The working

principle involves the mixing of a relatively small

amount of aerating gas (by mass) with liquid for the

creation of a two-phase mixture inside the injector prior

to discharge. For well-designed injector and injection

conditions, a well-dispersed plume can be generated for

effective mixing with the ambient air. With the

application of optical-based disgnostics, such as

shadowgraph and phase Doppler particle analyzer, it has

also been shown that the liquid aeration technique can

generate a spray that penetrates deeply into high-speed

crossflows and produces favorable plume/droplet

properties, such as a high-concentration smaller droplets

distributed over a larger plume cross section.6-8

Recently, efforts to characterize the near-field

structures of aerated-liquid jets were carried out with

various x-ray diagnostics, including x-ray phase contrast

imaging (PCI), x-ray radiography, and x-ray

fluorescence.9-14 For instance, the x-ray PCI technique is

capable of providing qualitative visualization of

droplets, ligaments, and even bubbles within the near

field of the aerated-liquid jets.9-11 Droplet diameter,

bubble diameter, and bubble film thickness within the

periphery of the spray could also be measured from the

x-ray PCI images. The studies with pathlength-inte-

grated x-ray radiography and fluorescence techniques

provide quantitative and time-averaged characterization

of liquid and mass distributions within the near fields of

aerated-liquid jets.12-15 Both techniques were also used to

quantitatively characterize the two-phase flow structures

inside an aerated-liquid injector, which was made out of

beryllium and features the outside-in aeration scheme.16

Beryllium has fairly low x-ray absorption and, therefore,

allows for high transmittance of incident, transmitted,

and emitted x-ray photons.

The objective of the present study is to characterize

both time-averaged and temporal two-phase flow struc-

tures generated from an aerated-liquid injector featuring

the outside-in aeration scheme. For this aeration scheme,

the aerating gas flows through an annular passage before

entering the central liquid stream for mixing. Time-aver-

aged flow properties within the beryllium nozzle and

also the near field of discharged plumes were character-

ized with pathlength-integrated and confocal x-ray fluo-

rescence techniques. In addition, high-speed imaging

with “white beam” x-rays was utilized temporally re-

solve qualitative flow evolution and even quantitative

flow velocity in this study.

EXPERIMENTAL METHODS

Experimental Setup The experiment was conducted at the 7-BM beam-

line of the Advanced Photon Source (APS) at Argonne

National Laboratory. For the present study, the injector

assembly and liquid spray were placed in the path of a

small x-ray beam. Water and aerating gas were supplied

into the aerated-liquid injector at the desired flow rates

to form a two-phase mixture inside the injector before

discharge into a quiescent environment. The aerated-liq-

uid injector features the outside-in aeration configuration

with the aerating gas flowing through an annular passage

before entering the central liquid stream for mixing. Fig-

ure 1 shows a schematic of the injection assembly, which

has a stainless steel injector body, an aerating tube, and

an exchangeable beryllium nozzle. The aerating tube has

an internal diameter of 2.2 mm and features 32 small or-

ifices located in 16 rows with two 0.18-mm diameter or-

ifices in each row. Orifices in each row are offset by 90

degrees from the orifices in adjacent rows, resulting in

an orifice distribution pattern in four columns on the aer-

ating tube.

A beryllium nozzle was fabricated for the present

study. Beryllium has high x-ray transmittance compared

to other metals and permits interrogation of the two-

phase flow inside the nozzle passage. Schematics of the

nozzle are shown in Fig. 2. The nozzle features a con-

toured entrance, followed by a cylindrical passage with

a constant diameter (do) of 1.0 mm (0.04 in.) and a pas-

sage length (L) of 10 mm, which lead to L/do of 10. The

nozzle has a short plenum section with a diameter of 2.2

mm to smoothly connect with the aerating tube.

The aerated-liquid jet was vertically discharged into

a collecting bucket with a small opening on the cap to

minimize the probability of stray droplets entering the

beam path. The distance between the nozzle exit and the

bucket cap was kept around 5 mm for probing inside the

beryllium nozzle and 15 mm for probing of the dis-

charged jet, in order to avoid splashing. Both the aerated-

liquid injector and the collecting bucket were rigidly

mounted on a traversing table, which provided move-

ment normal to the x-ray beam.

X-Ray Measurements

7-BM beamline at Argonne National Laboratory is

dedicated to ultrafast x-ray radiography, fluorescence,

and tomography experiments in fuel sprays and associ-

ated phenomena. The x-ray source for the beamline is a

synchrotron bending magnet, which produces nearly col-

limated, broadband x-ray emission. The beamline con-

sists of two radiation enclosures. The first enclosure

(7BM-A) houses a pair of slits to limit the x-ray beam

size and a double multilayer monochromator (1.2 – 4.3%

∆E/E). The monochromatic beam then passes into the

second radiation enclosure (7BM-B), which houses the

experimental equipment. More information regarding

the beamline performance can be found in the study of

Kastengren et al.17

Based on previous success in probing the two-phase

mixtures inside a beryllium nozzle,16 x-ray fluorescence

was used to simultaneously measure gas and liquid con-

centrations. When an element is illuminated with x-rays,

ionization of core shell electrons occurs. To relax from

this highly excited state, atoms can emit an x-ray fluo-

rescence photon. The energy of this photon is character-

istic of the atomic number of the atom emitting the pho-

ton, and the emitted fluorescence flux is proportional to

the amount of fluorescent material in the beam. As such,

x-ray fluorescence is commonly used for nondestructive

elemental analysis. More detail of the x-ray fluorescence

process is given in Refs. [16, 18, 19].

X-ray fluorescence from the liquid and gas phases

can be used to determine the distributions of liquid and

gas independently, by doping the working fluids with

bromine (Br) and krypton (Kr) atoms, respectively. So-

dium bromide (NaBr) was dissolved into the water at a

concentration of 0.13% by mass. This concentration was

monitored during the experiment by measuring the elec-

trical conductivity of the water, which was approxi-

mately 1500 µS/cm. The gas phase was pre-mixed (by

the gas vendor) to a concentration of 3% Kr (and 97%

N2) by volume. The fluorescence signals were measured

by a silicon drift diode detector, which was oriented 90°

to the x-ray beam in order to minimize elastic x-ray scat-

tering. For the current x-ray fluorescence experiments,

the mean x-ray beam photon energy was set at 15 keV; a

photon energy of at least 14.3 keV is required to excite

both Br and Kr. The beam was focused using a pair of

300 mm long Kirkpatrick-Baez focusing mirrors. The

beam focus is approximately 5 6 µm FWHM V H,

located approximately 500 mm from the center of the

horizontal focusing mirror. The effective size of the

beam for the current sprays (which are several mm wide)

is somewhat greater than this minimum focus size. The

experimental setup at the 7-BM beamline is shown in

Fig. 3.

Two setups for the fluorescence detector were used

in the present study. The first setup measured the path-

length-integrated mass distributions within the beryllium

nozzles or within the discharged plumes, as illustrated by

the schematics in Fig. 4(a). This is similar to the setup

used in previous x-ray fluorescence measurements of

aerated sprays.15,16 Obliquely scattered x-rays were mit-

igated using a set of Soller slits. In addition, aluminum

and selenium filters were used to filter out elastically

scattered photons and x-ray fluorescence photons from

impurities (e.g., Fe, Cu) in the beryllium nozzle. The de-

tector area measures 100 mm2, and was positioned ap-

proximately 100 mm from the injector.

The second setup used confocal x-ray fluorescence

to examine only a small part of the beam path through

the spray, as illustrated by the schematics in Fig. 4(b). A

polycapillary x-ray optic with a focal length of 100 mm

was placed in front of the fluorescence detector.20 The

optic only allowed x-rays from a region approximately

300 m in width to reach the fluorescence detector. The

polycapillary and detector were aligned with the x-ray

beam. Distributions of liquid and gas mass resolved in

three dimensions can be established by moving the spray

normal to the x-ray beam and the detector and polycapil-

lary together along the x-ray beam. This setup was only

used to interrogate the plume structures outside the in-

jector, in order to avoid the absorption of the scattered

photons by the beryllium body. Given that only a small

portion of the beam path was sampled in these measure-

ments, the NaBr concentration was increased to 0.45%

by mass for these measurements.

The first parameter derived from the processed flu-

orescence signals is the equivalence path length (EPL),

which is the equivalent length for continuous distribution

of the media of interest, such as liquid or gas phase, to

generate identical fluorescence intensity measured from

the two-phase mixture along the x-ray beam path. The

schematics in Fig. 5 conceptually illustrate the EPL

measurement from the present x-ray diagnostics. With

the successful measurement of EPLs for both liquid and

gas phases, densities of liquid and gas phases and veloc-

ity of the liquid phase can be averaged over the cross

section of the interrogation plane, if the boundary of the

cross section can be reasonably defined.13 Note that only

time-averaged properties are measured by the present x-

ray fluorescence diagnostics.

In order to temporally resolve the two-phase flow

structures, high-speed images of these flowfields were

also performed. For these measurements, the raw x-ray

emission from the bending magnet source (termed

“white beam”) was used to illuminate the sample with no

filtering (other than that inherent in the 750 µm of beryl-

lium windows and 50 µm of polyimide in the beamline

and the air gaps in the experimental setup). The x-ray

beam, measuring about 3 mm (H) × 8 mm (W), was

transmitted through the injector and then impinged on a

LuAG:Ce scintillator crystal (100 µm thick) to convert

the x-ray image to visible light. For this purpose, a Pho-

tron SA-Z camera was operated at a framing rate of

271,554 fps (equaling the synchrotron ring frequency)

and a shutter speed of 1.0 s. At this framing rate, the

maximum frame size for the camera is 144 (H) × 256

(W) pixels. The field was then imaged 1:1, using a Ni-

kon 105-mm f/2.8 macro lens, giving a spatial resolution

of 20 m/pixel and a field of view of approximately 5.0

mm (H) × 2.8 mm (V). Two approaches were utilized to

enhance the image contrast ratio. First, water was doped

with 10% potassium iodide (KI) and 0.45% NaBr by

mass, in order to increase the absorption coefficient of

the liquid phase under the x-ray. Second, each of the ob-

tained images was normalized by an average reference

image (a so-called flatfield image) at each imaging loca-

tion and then saved as a 32-bit floating-point image using

the ImageJ software package.21 Image acquisition was

carried out at several locations, in order to cover the en-

tire x-ray accessible region within the beryllium nozzle

and a small portion of the discharged plume. A total of

20,000 images were acquired at each location. For the

present study, however, only the first 2,000 images were

selected for analysis. A commercially available software

package (DaVis, Version 8.1.4, LaVision, Inc.), which is

mainly used for data analysis of particle image veloci-

metry (PIV), was utilized to derive the liquid phase ve-

locity, using a cross-correlation algorithm. Multi-pass

cross correlation iterations were carried out with three

passes with initial window size of 64 pixels × 64 pixels

and two passes with a final window size of 4 pixels × 4

pixels with 50% overlap between windows. Post pro-

cessing was done with one pass of smoothing with a cell

size of 3 pixels × 3 pixels.

RESULTS AND DISCUSSION

Time-Averaged Flow Characterization

Pathlength-Integrated Fluorescence Measurements

Contours of line-of-sight EPL distributions for both

liquid and gas phases inside the nozzle are shown in Fig.

6. The injection condition has a liquid flow rate of 18.2

g/s and a gas-to-liquid mass ratio (GLR) of 4%. Please

note that the discontinuities in these contours result from

the limited number of measurement locations in the axial

direction. The injector body and flange, which are made

out of stainless steel, prevent x-ray access at the further

upstream locations in the plenum section. Also, the re-

gion with contoured transition from plenum to constant-

diameter passage was ignored. Nonetheless, the results

show that the two-phase flow inside the plenum region

is not uniform. Less liquid is distributed to the right of

the plenum section. Also, several localized regions of

high liquid concentration are clearly visible near the end

of the plenum section. The non-uniformity in liquid mass

distribution may come from imperfections in fabricating

or positioning the aerating tube inside the injector body,

or the distribution pattern of the aerating orifices on the

aerating tube. The corresponding gas EPL contours also

exhibit non-uniformity in gas phase distribution, as ex-

pected. In the nozzle passage region, however, non-uni-

formity in both liquid and gas distributions diminishes

substantially. Values of both water and gas EPLs are

smaller in the passage region, due to a reduction in pas-

sage diameter and the expansion of the two-phase mix-

ture toward the nozzle exit, which will be discussed be-

low.

Line profiles of the line-of-sight liquid densities are

shown in Fig. 7 for three injection conditions with vari-

ation in liquid flow rate, mL, at the same aeration level of

GLR = 4%. In general, the line profiles are not symmetric

with respect to the injector axis. More liquid is distrib-

uted to the left of the injector, due to the non-uniform

liquid mass distribution observed near the end of the ple-

num section. The liquid mass distribution exhibits an an-

nular-like pattern, with more liquid on the nozzle wall

and less liquid in the core flow for all three injection con-

ditions. The injection condition with a low liquid flow

rate of mL = 9.1 g/s has the most distinct annular distri-

bution pattern, with the largest difference in liquid den-

sity being between the region adjacent to the nozzle wall

and the region around the injector axis. As liquid flow

rate increases, more liquid can be distributed in the core

region, resulting in a less distinct annular distribution

pattern. The asymmetric liquid mass distribution is grad-

ually improved as the two-phase mixture moves toward

the nozzle exit, where more liquid is redistributed toward

the core region. The spikes in liquid density at the x = -8

mm location for the injection condition with mL = 9.1 g/s

mainly come from droplets adhering to the exterior of the

nozzle body within the x-ray beam path.

Line profiles of line-of-sight gas density are shown

in Fig. 8. Gas phase is mainly distributed in the core re-

gion of the nozzle passage. The gradual reduction in gas

phase concentration toward the nozzle exit indicates the

expansion process of the two-phase mixture within the

nozzle section. As the pressure gradually drops toward

the nozzle exit, the gas phase density decreases accord-

ingly. For the injection condition with mL = 27.2 g/s in

Fig. 8(c), a quick transition in gas density distribution

profile can be observed between x = -8 mm and -6 mm.

A similar observation can be made in the corresponding

liquid phase distribution profiles in Fig. 7(c). This obser-

vation, along with the phenomena noted above, indicates

that macro non-uniformity in two-phase flow structures

can be quickly filtered out as the flow passes through a

small-orifice nozzle.

Spanwise integration (x direction) of the above line

profiles for each spanwise probing line (y location) can

provide a useful measure of the axial development of the

flowfield. Figure 9 shows the axial distribution profiles

of average liquid density (L), average gas density (G),

and average liquid velocity (uL) for the injection condi-

tions discussed in Figs. 7 and 8. Within the present x-ray

probing range, the average liquid density gradually in-

creases toward the end of the plenum section, resulting

in a corresponding reduction in liquid velocity, due to

continuity. As the flow enters the nozzle passage, the av-

erage liquid density gradually decreases toward the noz-

zle exit. The corresponding liquid velocity exhibits a sig-

nificant increase across the nozzle entrance, due to a

400% reduction in flow cross-sectional area, and a con-

tinuous increase toward the nozzle exit, due to flow ex-

pansion. For a given nozzle geometry, an increase in liq-

uid flow rate leads to an increase in both average liquid

density and average liquid velocity, as expected.

The axial distribution profile for the average gas

density exhibits a trend similar to that of the average liq-

uid density. With the same aeration level of GLR = 4%,

the absolute amount of aerating increases for the injec-

tion condition with a high liquid flow rate. It should be

noted that the gas density levels at the location nearest

the nozzle exit plane are still far higher than the density

at room temperature and pressure. This strongly suggests

that the flow inside the nozzle is underexpanded, and an

abrupt gas expansion should be expected at the exit

plane.

Using the same analysis approaches, the effects of

aeration level on the two-phase flow structures within the

nozzle are characterized in terms of line profiles and av-

erage properties in Figs. 10-12. The liquid flow rate was

kept at mL = 18.2 g/s. With an increase in aeration level

to add more aerating gas into the same liquid flow rate,

the present results show that liquid density decreases and

gas density increases in general, as can be seen in the line

profiles of liquid and gas densities in Figs. 10 and 11,

respectively. Similar to previous observations, the liquid

distribution is mainly in an annular-like pattern, with

more liquid distributed adjacent to the nozzle wall, while

aerating gas is mainly distributed in the core flow. For

the present injector design, it seems that the injection

condition with a high liquid flow rate and a high aeration

level, i.e., mL = 18.2 g/s and GLR = 6% in Figs. 10(c) and

11(c), has a tendency to create asymmetric flow struc-

tures near the nozzle entrance region (x = -8 mm). The

asymmetric flow structures, however, transition quickly

into more symmetric structures as the flow proceeds

downstream. The axial distributions of average proper-

ties in Fig. 12 show that an increase in aeration level

leads to an increase in average gas density (due to in-

creases in both gas flow rate and the required injection

pressure), a reduction in average liquid density (due to

enhanced aeration), and an increase in average liquid ve-

locity (due to continuity).

Confocal Fluorescence Measurements

Figure 13 shows the cross-sectional distributions of

liquid and gas densities at an axial location of 5 mm from

the nozzle exit (x = 5 mm) for three injection conditions.

To the best of our knowledge, this is the first time the

confocal x-ray fluorescence technique has been used to

spatially resolve liquid and gas distributions within the

near field of spray plumes. For the injection condition

with mL = 18.2 g/s and GLR = 1%, the injected liquid is

mainly distributed in an annular ring without any signif-

icant changes in liquid distribution pattern inside the

nozzle (depicted in Fig. 10(a)). The plume diameter is

bigger than the nozzle exit diameter, indicating a further

but limited expansion process after the nozzle exit. The

gas plume preserves the Gaussian-like distribution pat-

tern observed in the internal flow (Fig. 10(b)), with a

plume diameter smaller than that of the liquid plume. As

the aeration level is increased to GLR = 4% at the same

liquid flow rate, the liquid plume is dispersed to a wider

plume with a Gaussian-like liquid distribution pattern.

The line profiles for the same injection condition in Fig.

7(b), however, show that the two-phase flow at the noz-

zle exit is still in the annular-like pattern, indicating a

fairly quick transition in liquid distribution pattern for

this highly-dispersed spray within a short distance. With

more aerating gas added to reach GLR = 4%, the gas

plume is wider than in the case with GLR = 1%. The gas

plume diameter, however, is still significantly smaller

than that of the liquid plume, indicating once again the

separation between liquid and gas plumes within the aer-

ated-liquid jets. The injection condition with mL = 9.1 g/s

and GLR = 4% generates a highly annular-like distribu-

tion pattern for the liquid plume. Recall that the line pro-

files for the line-of-sight liquid density in Fig. 8(a) also

exhibits a highly annular-like distribution pattern within

the nozzle, indicating a fairly limited expansion process

in the plume. The liquid plume diameter for the injection

condition with mL = 9.1 g/s and GLR = 4% is the smallest

among the three injection conditions.

Integration of the spatially-resolved EPLs obtained

by the confocal x-ray fluorescence technique along the

beam path direction (y direction) is expected to give the

line-of-sight EPLs obtained by the pathlength-integrated

x-ray fluorescence technique without the polycapillary

optic. Figure 14 shows comparisons in line-of-sight wa-

ter and gas EPLs using confocal (with polycapillary op-

tic) and pathlength-integrated (without polycapillary op-

tic) x-ray fluorescence measurements for the injection

condition with mL = 18.2 g/s and GLR = 4%. Good agree-

ment between both measurement techniques can be ob-

served. For the line-of-sight gas EPL obtained using the

pathlength-integrated x-ray fluorescence technique in

Fig. 14(b), however, a region outside the gas plume still

exhibits non-zero values, probably due to some spectral

bleed-through from the Br fluorescence in the liquid

phase. The bleed-through should be further explored ex-

perimentally to reduce this minor uncertainty. Nonethe-

less, the present study shows that the confocal x-ray flu-

orescence technique can be used to map out the time-av-

eraged mass distributions within a plume cross section.

Any non-uniformity and asymmetry in spatial mass dis-

tribution can be directly visualized, as can be seen in the

non-uniform liquid density contours within the annular

rings of two injection conditions in Fig. 13. If the path-

length-integrated x-ray fluorescence technique were

used to visualize the spatial mass distribution within a

spray cross section, limited-view tomography algorithms

along with multiple transverse scans at various azimuthal

angles would be required to depict any non-uniformity

features.

The advantage of using the confocal x-ray fluores-

cence technique to map out the cross-sectional mass dis-

tributions within an asymmetric spray is further illus-

trated, using injection of an aerated-liquid jet injected at

small angle relative to the horizontal plane. Figure 15

shows the macroscopic spray appearance of an aerated-

liquid jet injected at a shallow angle. The spray structure

is highly asymmetric, with the two-phase mixture mainly

expanding on the acute side of the plume. The plume

edge on the obtuse side is more or less parallel to the in-

clined injector axis. Figure 16 shows the cross-sectional

density distributions at two axial locations of 5 and 10

mm from the nozzle exit. The injection condition has an

injection angle of 30 degrees, a liquid flow rate of 18.2

g/s, and a GLR of 4%. Both liquid and gas plumes are

highly asymmetric. In the liquid plume, early relief of the

high-pressure two-phase mixture on the acute side at the

nozzle exit plane effectively disperses the liquid over a

large cross-sectional area. More gas mass is also concen-

trated on this side of the plume. The two-phase mixture

on the obtuse side of the nozzle exit plane, however, is

bounded by the final passage for a slightly longer dis-

tance, and experiences reduced pressure for limited lat-

eral expansion, resulting in a small region with highly

concentrated liquid. The region with a high liquid con-

centration travels downstream almost along the inclined

injection angle. The gas plume on the obtuse side is

small, since the majority of the aerating gas has been re-

leased on the acute side. As observed previously, the size

of the gas plume is smaller than that of the liquid plume.

High fidelity realization of both liquid and gas cross-sec-

tional structures for this injection condition by the con-

ventional pathlength-integrated x-ray fluorescence tech-

nique can be fairly challenging.

Time-Resolved Flow Characterization

Instantaneous Flow Structures

Figure 17 shows the composite images from uncor-

related individual instantaneous images taken at various

locations to illustrate the two-phase flow structures in-

side the beryllium nozzles at the injection condition of

mL = 18.2 g/s, and GLR = 4%. Please note that the present

camera setup has a spatial resolution of 20 m per pixel.

Any interface features below that spatial resolution can-

not be resolved. The camera shutter speed of 1.0 s cre-

ates additional ambiguity in accurately capturing the pre-

cise interface locations. For instance, an object travelling

at a speed of 50 m/s can travel a distance equal to 2.5

pixels of a 1.0 s span. Nonetheless, the images in Fig.

17 show that large-scale structures with identifiable in-

terfaces are mainly located in the plenum section. The

length scale of the two-phase structures dramatically de-

creases in the nozzle passage section, probably due to

aerodynamic stretch during flow acceleration from the

plenum section to the nozzle section. In the nozzle pas-

sage, it appears that clusters of high-concentration liquid

are present in the upstream section. At the further down-

stream locations near the nozzle exit, the two-phase

flows mainly consist of relatively “fine” structures for all

three nozzles. The obtained images can be compiled to

create movies at a low playback speed to further under-

standing of the temporal evolution of the two-phase

flows within the nozzles. Areas of improvement for the

present high-speed imaging approach include increases

in spatial resolution, camera shutter speed, and the use of

higher potassium iodide concentration to enhance image

contrast ratio.

Velocity Measurements

Despite the observed limitations in the present high-

speed imaging setup, attempts to measure the axial ve-

locity of identifiable interface features was carried out,

using sequential high-speed images within the nozzle

section. Noting that an annular-like flow structure in the

liquid phase inside the nozzle for most of the injection

conditions, a thin layer of liquid is expected to flow

along the nozzle wall. Interfaces on the liquid film are

probably the most identifiable features in the high-speed

images. The measured velocity is, therefore, more repre-

sentative of the liquid film velocity. Figure 18 shows the

contours of average and standard deviation of axial ve-

locity for the two-phase mixture inside the nozzle with

mL = 18.2 g/s and GLR = 4%. The sudden increase in ax-

ial velocity around the nozzle entrance is clearly de-

picted, as observed in the average liquid velocity derived

from x-ray fluorescence measurements at the corre-

sponding injection condition shown in Fig. 9(c). The

flow then gradually accelerates toward the nozzle exit

and reaches an average velocity of around 95 m/s at the

x = -3 mm location. This relatively high velocity is

higher than at the maximum average liquid velocity of

around 75 m/s at the nozzle exit in Fig. 9(c). The meas-

ured velocity near the nozzle exit (x > -1.5 mm), how-

ever, exhibits a reduced average velocity of around 50

m/s, with a high standard deviation. The average liquid

velocity derived from fluorescence measurements exhib-

its a trend of continuous increase up to the nozzle exit.

The factors contributing to this discrepancy are unknown

and should be explored in the future.

SUMMARY

Time-averaged and temporal structures of the two-

phase flows within the nozzle of an aerated-liquid injec-

tor were experimentally explored with synchrotron x-ray

diagnostics, including x-ray fluorescence and x-ray high-

speed imaging, at the Advanced Photon Source at Ar-

gonne National Laboratory. The aerated-liquid injector

features an outside-in aerating scheme with the aerating

gas flowing through aerating orifices from an annular

passage to mix with the core liquid flow. A beryllium

nozzle with a constant passage diameter of 1.0 mm and

a length of 10 mm was fabricated for high x-ray trans-

parency. Water and nitrogen were doped with bromine

and krypton, respectively, at low concentrations, to facil-

itate both confocal and pathlength-integrated x-ray fluo-

rescence measurements. Water was also doped with po-

tassium iodide for high-speed imaging, in order to tem-

porally resolve the two-phase flow evolution.

The present study shows that time-averaged mass

distributions of liquid and gas phases within the two-

phase flows of aerated-liquid jets can be quantitatively

characterized by both x-ray fluorescence approaches.

Within the beryllium nozzle, the pathlength-integrated

fluorescence technique was utilized to provide line-of-

sight mass distributions for the two-phase flows inside

the nozzle. It was found that the two-phase flows inside

the nozzle section exhibit an annular-like flow pattern

with more liquid distributed mainly within a thin layer of

liquid film flowing along the nozzle wall. The aerating

gas is distributed in a Gaussian-like pattern. Non-uni-

formity or asymmetry in liquid distributions within the

plenum section can diminish quickly within a short dis-

tance from the nozzle entrance, and the liquid distribu-

tion reaches a more uniform/symmetric pattern near the

nozzle exit. Axial distributions of the average liquid den-

sity, gas density, and liquid velocity were also explored

by integrating the line-of-sight properties over the cross-

sectional areas at several axial locations within the noz-

zle section. For the present nozzle configuration with a

constant-diameter passage, an increase in liquid flow rate

and aeration level increases the liquid velocity toward

the nozzle exit.

For the first time, confocal x-ray fluorescence meas-

urements were carried out with a polycapillary x-ray op-

tic to resolve the time-averaged cross-sectional liquid

and gas distributions in three dimensions within the near

field of discharged plumes. Preservation of the annular-

like flow structures within the discharged plumes was di-

rectly observed for injection conditions with a low liquid

flow rate or a low aeration level. It was found that a quick

transition in flow pattern from annular-like within the

nozzle passage to Gaussian-like within the discharged

plume can be achieved for the injection condition with a

high liquid flow and a high aeration level. Plume separa-

tion phenomena for both liquid and gas plumes were

readily observed using this technique. The capability of

the confocal x-ray fluorescence technique to spatially re-

solve a highly asymmetric flow structure was further

demonstrated, using an injection configuration with an

aerated-liquid jet injected at a shallow angle. It should be

noted that these 3D resolved measurements were made

without resorting to tomographic measurements, which

require rotating the injector with respect to the x-ray

beam. As such, they are more amenable to flowfields

with limited optical access. Comparison of the line-of-

sight mass distributions between focused and unfocused

x-ray fluorescence techniques reaches a fairly good

agreement, providing an enhanced level of confidence in

applying the focused fluorescence technique for two-

phase flow characterization.

High-speed radiographic x-ray imaging shows that

large-scale structures are mainly present in the plenum

section and can be aerodynamically stretched into fine

structures in the nozzle within a short distance. Axial

flow velocity of the two-phase flow was also measured

from the sequential high-speed images and compared

with the average liquid velocity derived from fluores-

cence measurements. Qualitative agreements in axial ve-

locity distribution were partially reached between both

velocity measurement approaches. Improvement to the

present imaging setup is needed, in order to better re-

solve the flow structures both spatially and temporally in

the future.

ACKNOWLEDGEMENTS

This work was sponsored by the AFRL/Aerospace Sys-

tems Directorate under contract number FA8560-14-D-

2316 (Contract monitor: Stephen Smith) and by the Air

Force Office of Scientific Research (AFOSR). Use of the

Advanced Photon Source at Argonne National Labora-

tory was supported by the U. S. Department of Energy,

Office of Science, Office of Basic Energy Sciences, un-

der Contract No. DE-AC02-06CH11357. The authors

would like to thank Travis Tidball, Steve Enneking, Matt

Streby, and Justin Stewart of Taitech, Inc. for their assis-

tance in hardware design, setup, data acquisition, and

data reduction.

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Figure 1. Schematic of the aerated-liquid injector as-

sembly

Figure 2. Schematic of the beryllium nozzle. d0 = 1.0

mm, L = 10 mm

Figure 3. X-ray and injection stand setup at the 7-BM beamline at the Argonne National Laboratory.

(a) (b)

Figure 4. Schematics to illustrate (a) pathlength-integrated and (b) confocal fluorescence setups

Figure 5. Schematics to illustrate the concept of equivalent path length (EPL) in the present x-ray measurement

(a) (b)

Figure 6. Contour plots for line-of-sight EPL of (a) water and (b) nitrogen in the beryllium nozzle. mL=18.2 g/s,

GLR=4%.

(a) (b) (c)

Figure 7. Line-of-sight water density profiles to illustrate the effects of liquid flow rate on two-phase flow structures

in the nozzle section. (a) mL=9.1 g/s, (b) 18.2 g/s, and (c) 27.2 g/s. GLR=4%.

(a) (b) (c)

Figure 8. Line-of-sight nitrogen density profiles to illustrate the effects of liquid flow rate on two-phase flow struc-

tures in the nozzle section. (a) mL=9.1 g/s, (b) 18.2 g/s, and (c) 27.2 g/s. GLR=4%.

(a) (b) (c)

Figure 9. Averaged properties of the two-phase flow structures in the nozzle section at various liquid flow rates. (a)

Water density, (b) nitrogen density, and (c) liquid velocity. GLR=4%.

-1.2 -0.8 -0.4 0 0.4 0.8 1.2y (mm)

0

200

400

600

800

1000

L (

Kg

/m3)

mL=9.1 g/s, GLR=4%, Config. #1, L/d0=10

Line x (mm)

-8

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-4

-2

-0.5

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/m3)


Line x (mm)

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Line x (mm)

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-12 -10 -8 -6 -4 -2 0x (mm)

0

200

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L(

kg

/m3)

Config. #1, d0= 1.0 mm, L/d0=10

GLR=4%

Nozzle

Passage

SYM. mL (g/s)

9.1

18.2

27.2

-12 -10 -8 -6 -4 -2 0x (mm)

0

4

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G (

kg

/m3)

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GLR=4%

Nozzle

Passage

SYM. mL (g/s)

9.1

18.2

27.2

-12 -10 -8 -6 -4 -2 0x (mm)

0

20

40

60

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100

uL (

m/s

)

Config. #1, d0= 1.0 mm, L/d0=10

GLR=4%

Nozzle

Passage

SYM. mL (g/s)

9.1

18.2

27.2

(a) (b) (c)

Figure 10. Line-of-sight water density profiles to illustrate the effects of aeration level on two-phase flow structures

in the nozzle section. (a) GLR=1%, (b) 2%, and (c) 6%. mL=18.2 g/s.

(a) (b) (c)

Figure 11. Line-of-sight nitrogen density profiles to illustrate the effects of aeration level on two-phase flow struc-

tures in the nozzle section. (a) GLR=1%, (b) 2%, and (c) 6%. mL=18.2 g/s.

(a) (b) (c)

Figure 12. Averaged properties of the two-phase flow structures in the nozzle section at various aeration levels. (a)

Water density, (b) nitrogen density, and (c) liquid velocity. mL=18.2 g/s.

-1.2 -0.8 -0.4 0 0.4 0.8 1.2y (mm)

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kg

/m3)

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mL=18.2 g/s

Nozzle

Passage

SYM. GLR (%)

1

2

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6

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0

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G (

kg

/m3)

Config. #1, d0= 1.0 mm, L/d0=10

mL=18.2 g/s

Nozzle

Passage

SYM. GLR (%)

1

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4

6

-12 -10 -8 -6 -4 -2 0x (mm)

0

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100

uL (

m/s

)

Config. #1, d0= 1.0 mm, L/d0=10

mL=18.2 g/s

Nozzle

Passage

SYM. GLR (%)

1

2

4

6

(a) (b)

Figure 13. Contour plots for cross-sectional density distributions of (a) water and (b) nitrogen at various injection

conditions. 5.0 mm from the nozzle exit (x=5.0 mm).

(a) (b)

Figure 14. Line-of-sight EPL comparison between confocal and pathlength-integrated x-ray fluorescence measure-

ments. (a) Water and (b) nitrogen. mL=18.2 g/s, GLR=4%, 5.0 mm from the nozzle exit (x=5.0 mm).

Figure 15. Photograph of an aeraed-liquid jet injected at a shadow angle to illustrate the spray structure.

-8 -4 0 4 8y (mm)

0

0.01

0.02

0.03

0.04

0.05

0.06L

ine

-of-

Sig

ht

Wa

ter E

PL

(m

m)

Config. #1

L/d0=10

mL=18.2 g/s

GLR=4%

x=5 mm

SYM. Fluorescence Diagnostics

Pathlength-Integrated

Confocal

-8 -4 0 4 8y (mm)

0

1

2

3

Lin

e-o

f-S

igh

t N

itro

ge

n E

PL

(m

m)

Config. #1

L/d0=10

mL=18.2 g/s

GLR=4%

x=5 mm

SYM. Fluorescence Diagnostics

Pathlength-Integrated

Confocal

Figure 16. Cross-sectional contour plots for water and nitrogen densities at two axial locations for the aerated-liquid

jet injected at a 30 angle. d0=1.0 mm, L/d0=10, mL=18.2 g/s, GLR=4%. Injector orifice at (Y, Z) = (0, 0).

Figure 17. Uncorrelated composite instantaneous images from high-speed camera to illustrate the two-phase flow

structures inside the beryllium nozzle. mL=18.2 g/s, GLR=4%.

(a) (b)

Figure 18. Axial velocity contours for the two-phase mixtures inside the beryllium nozzle. (a) Average velocity and

(b) standard deviation velocity. mL=18.2 g/s, GLR=4%.

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