energies
Article
Spray Formation of a Liquid Carbon Dioxide-Water
Mixture at Elevated Pressures
Hakduck Kim, Changyeon Kim, Heechang Lim and Juhun Song *
School of Mechanical Engineering, Pusan National University, Busan 46241, Korea;
[email protected] (H.K.); [email protected] (C.K.); [email protected] (H.L.)
* Correspondence: [email protected]; Tel.: +82-51-510-7330; Fax: +82-51-512-5236
Academic Editor: Mehrdad Massoudi
Received: 6 September 2016; Accepted: 8 November 2016; Published: 14 November 2016
Abstract: Liquid carbon dioxide-assisted (LCO2 -assisted) atomization can be used in coal-water
slurry gasification plants to prevent the agglomeration of coal particles. It is essential to understand
the atomization behavior of the water-LCO2 mixture leaving the injector nozzle under various
conditions, including the CO2 blending ratio, injection pressure, and chamber pressure. In this study,
the flash-atomization behavior of a water-LCO2 mixture was evaluated with regard to the spray angle
and penetration length during a throttling process. The injector nozzle was mounted downstream
of a high-pressure spray-visualization system. Based on the results, the optimal condition for the
effective transport of coal particles was proposed.
Keywords: coal gasification; flash atomization; liquid carbon dioxide (LCO2 ); water-LCO2
mixture; solubility
1. Introduction
Pressurized entrained flow gasification is a promising technology for the conversion of low-grade
fuels to high-quality synthetic fuels, e.g., hydrogen and synthetic natural gas. High gasification
efficiencies using liquid fuels or slurries can only be achieved through the generation of a fine
homogenous spray at a high chamber pressure [1].
During coal-water slurry (CWS) gasification, there is a tendency for the coal particles to
agglomerate within individual droplets while the water is evaporating; the agglomerates are then
dried out and undergo thermal decomposition in the flame. The resulting coal-particle size distribution
(p.s.d.) is determined to a greater degree by the size distribution of the atomized fuel spray than by the
initial particle size of the coal. Large particles formed through agglomeration have increased burnout
times and produce large fly ash particles [2]. A potential method for obtaining finer spray droplets
is CO2 -assisted atomization. The effect of disruptive or flash atomization in reducing the p.s.d. of
droplets was evaluated. The CO2 is dissolved into the fuel by injection into the line between the pump
and the fuel nozzle. During the pressure release in the atomizing nozzle, the dissolved CO2 changes to
the gaseous phase and disrupts the coal-water mixture droplets.
Pure liquid CO2 (LCO2 ) has a different impact on the atomization at the injector nozzle when a
LCO2 slurry is sprayed [3]. Compared with the conventional spray pattern of H2 O, LCO2 produces
effective breakup, leading to smaller liquid droplets, because of its lower viscosity and higher
momentum flux. This helps to transport coal particles more uniformly inside the gasifier. This feature
of better dispersion is attractive because it requires less heat release from partial combustion and thus
less oxygen supply to the coal gasifiers. However, the effect of the blending ratio of LCO2 in the water
on the atomization behavior has not been examined.
In a study on the spray formation of pure LCO2 [4], CO2 was expanded through a nozzle from a
high pressure to an atmospheric pressure. The flash atomization and breakup mechanisms of LCO2
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Energies 2016, 9, 948
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were examined using different liquid contents controlled by the degree of superheat. Under certain
conditions, the LCO2 experienced a sudden pressure drop below the saturation pressure. LCO2 with
Energies 2016, 9, 948
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different
levels of superheat produced different behaviors of flashing sprays and snow-particle
formation. Lee et al. found that sprays in flash-boiling conditions were atomized by bubble growth,
were examined using different liquid contents controlled by the degree of superheat. Under certain
resulting
in a smaller
droplet
size than that generated by aerodynamic force alone [5]. Liu et al. used
conditions,
the LCO
2 experienced a sudden pressure drop below the saturation pressure. LCO2 with
the laser-diffraction
method
to determine
p.s.d. behaviors
of the spray
[6].
different levels of
superheat
produced the
different
of flashing
sprays and snow-particle
Engelmeier
et
al.
[7]
used
a
pure
carbon
dioxide
jet
for
cutting
applications
where
carbon
dioxide
formation. Lee et al. found that sprays in flash-boiling conditions were atomized by
bubble
growth,
resulting inand
a smaller
droplet
size
than that generated
aerodynamic
forceis
alone
[5]. Liudownstream
et al. used of
is compressed
expanded
via
a sapphire
nozzle. Aby
pressure
chamber
installed
the laser-diffraction
method to determine
the p.s.d.
of thethe
spray
[6].
the orifice
to keep the post-expansion
conditions
above
triple-point
pressure of carbon dioxide.
Engelmeier that
et al. carbon
[7] used dioxide
a pure carbon
dioxide
jet for cutting
where
carbon dioxide
They demonstrated
has to
be expanded
to anapplications
environment
of increased
pressure
is
compressed
and
expanded
via
a
sapphire
nozzle.
A
pressure
chamber
is
installed
downstream
of
and thus increase the liquid fraction to improve the impact of the jet.
the orifice to keep the post-expansion conditions above the triple-point pressure of carbon dioxide.
Reverchon et al. reported that the solubilization of amount of supercritical CO2 in liquid solution
They demonstrated that carbon dioxide has to be expanded to an environment of increased pressure
could produce various powders with controllable size and distributions that are useful in several
and thus increase the liquid fraction to improve the impact of the jet.
fine-particle
production
processes
Supercriticalof dissolved-gas
atomization
is an solution
atomization
Reverchon
et al. reported
that[8].
the solubilization
amount of supercritical
CO2 in liquid
process
in
which
carbon
dioxide
at
a
temperature
and
pressure
above
its
critical
point
is
used as the
could produce various powders with controllable size and distributions that are useful in several
atomizing
gas [9].production
It relies onprocesses
dissolved[8].
COSupercritical
from water
to form bubbles.
Therefore, this
fine-particle
dissolved-gas
atomization
is an atomization
2 gas emerging
process
in which
carbon
a temperature
and
pressure
above
its criticalquantities
point is used
the
technique
applies
only
to a dioxide
limitedatrange
of liquids
that
can hold
significant
of as
dissolved
atomizing
gas
[9].mean
It relies
on dissolved
CO2was
gas emerging
from water by
to form
bubbles. release
Therefore,
gas. As
a result,
the
droplet
diameter
mainly influenced
the sudden
of this
the CO2
techniquein
applies
only to a limited range of liquids that can hold significant quantities of dissolved
gas dissolved
the water.
gas. As a result, the mean droplet diameter was mainly influenced by the sudden release of the CO2
In general, the dissolved gas at the exit orifice nucleates to form gas bubbles, as in effervescent
gas dissolved in the water.
atomization, and can rapidly evaporate by flashing as well [10]. The result shows that the mean droplet
In general, the dissolved gas at the exit orifice nucleates to form gas bubbles, as in effervescent
diameter
is mainly influenced by the sudden release of the gas dissolved in the liquid. However, the
atomization, and can rapidly evaporate by flashing as well [10]. The result shows that the mean
behavior
of the
waterisspray
altered
by the
of dissolved
LCOdissolved
2 was not
droplet
diameter
mainly
influenced
byatomization
the sudden release
of the gas
in experimentally
the liquid.
evaluated
in our
That is,
thealtered
spray characteristics
of the
water-LCO
2 mixture
However,
theprevious
behavior work.
of the water
spray
by the atomization
of dissolved
LCO
2 was notmust
be studied.
experimentally evaluated in our previous work. That is, the spray characteristics of the water-LCO2
mixture
must be
studied.
In
this study,
the
spray patterns of a water-LCO2 mixture were examined during a throttling
In
this
study,
the
a water-LCOof
2 mixture
were examined
during a throttling
process. The injector nozzlespray
was patterns
mountedofdownstream
a high-pressure
flashing-spray
system, while
process.
The
injector
nozzle
was
mounted
downstream
of
a
high-pressure
flashing-spray
system,
the downstream pressure was controlled by a back-pressure regulator. A high-speed shadowgraph
while the downstream pressure was controlled by a back-pressure regulator. A high-speed
apparatus was used to evaluate the CO2 -assisted atomization process.
shadowgraph apparatus was used to evaluate the CO2-assisted atomization process.
2. Experimental Setup
2. Experimental Setup
Figure 1 presents a schematic of the high-pressure spray-visualization system used in this study.
Figure 1 presents a schematic of the high-pressure spray-visualization system used in this study.
It consisted
of an
chamber
block,and
and
a downstream
chamber
(vessel).
It consisted
of upstream
an upstream
chamber(vessel),
(vessel), aa middle
middle block,
a downstream
chamber
(vessel).
ThereThere
was was
an optical
quartz
window
on
the
middle
block
and
the
two
chambers,
enabling
an optical quartz window on the middle block and the two chambers, enabling the the
visualization
of
the
behavior
inside
the nozzle
andand
at the
of the
The The
window
visualization offlashing-spray
the flashing-spray
behavior
inside
the nozzle
at exit
the exit
of nozzle.
the nozzle.
◦
window was
designed to
withstand
at a peak temperature
of 80
°C, and a
was designed
to withstand
pressures
uppressures
to 100 barupattoa 100
peakbar
temperature
of 80 C, and
a pressure-relief
was installed
to prevent
pressure
rises
beyond this limit.
valvepressure-relief
was installedvalve
to prevent
pressure
rises beyond
this
limit.
Figure
1.1.Schematic
experimentalsetup.
setup.
Figure
Schematic of
of experimental
Energies 2016, 9, 948
Energies 2016, 9, 948
3 of 11
3 of 11
A
A saturated
saturated mixture
mixture of
of CO
CO22 was
wasachieved
achievedby
by adjusting
adjusting the
the temperature
temperature and
and mass
mass of
of solid
solid CO
CO22
(SCO
(SCO22).).Different
Differentchamber
chambertemperatures
temperatureswere
wereachieved
achievedwith
withthe
theuse
useofofaaconstant-temperature
constant-temperaturebath.
bath.
As
shown in
inFigure
Figure1, 1,
nozzle
installed
between
the middle
block
the downstream
As shown
thethe
nozzle
waswas
installed
between
the middle
block and
theand
downstream
chamber.
chamber.
was long
300 mm
with an internal
of 0.4
mm.
Two different
materials
werefor
used
It was 300Itmm
withlong
an internal
diameterdiameter
of 0.4 mm.
Two
different
materials
were used
the
for
the nozzles.
Somemade
were of
made
of transparent
enabling
the visualization
thepattern
flow pattern
nozzles.
Some were
transparent
Pyrex,Pyrex,
enabling
the visualization
of the of
flow
inside
inside
the nozzle.
This
wasfor
useful
for examining
either action
capillary
action or flash-boiling
behavior.a
the nozzle.
This was
useful
examining
either capillary
or flash-boiling
behavior. However,
However,
a copper
was
used for
purposes
other than
internal visualization.
A data-acquisition
copper nozzle
was nozzle
used for
purposes
other
than internal
visualization.
A data-acquisition
system was
system
was
utilized
to
monitor
the
temperature
and
pressure
inside
the
chambers,
and
a flow
utilized to monitor the temperature and pressure inside the chambers, and a flow controller unit
was
controller
unitthe
was
usedon
to and
turnoff
thepneumatically.
valves on and off pneumatically.
used to turn
valves
Water–LCO
Water–LCO22 mixtures
mixtureswith
withdifferent
different percentages
percentages LCO
LCO22were
wereprepared
prepared for
for blending.
blending. A
Awater
waterisis
first
filled
in
upper
chamber
upto
the
desired
volume,
which
is
determined
by
water
blending
ratio
first filled in upper chamber upto the desired volume, which is determined by water blending ratio
on
on
basis
of volume.
Then,
a solid
2 is
poured
with
desired
volume
percent.
As
a
result,
the
CO
2
thethe
basis
of volume.
Then,
a solid
COCO
is
poured
with
desired
volume
percent.
As
a
result,
the
CO
2
2 in
in
liquid
vapor
phase
is made
with water
as it reaches
temperature
at equilibrium.
liquid
andand
vapor
phase
is made
with water
as it reaches
ambientambient
temperature
at equilibrium.
Before the
Before
test, isthe
mixture
is agitated
stirringanunit.
Therefore,
an efficient,
continuous
test, thethe
mixture
agitated
by stirring
unit. by
Therefore,
efficient,
continuous
solubilization
of CO2
solubilization
of CO2 in
liquidThe
solution
allowed.
The
liquid
CO
2 exists
as
layer
above
the
water
in the liquid solution
is the
allowed.
liquidisCO
exists
as
layer
above
the
water
layer,
indicating
low
2
layer,
indicating
low
miscibility
of
CO
2
with
water.
This
separation
is
clearly
observed
in
the
inset
of
miscibility of CO2 with water. This separation is clearly observed in the inset of Figure 1. In addition,
Figure
In addition,
droplets
COwater.
2 are found
in the
During
this process,
neither1.droplets
nor neither
particles
of CO2nor
areparticles
found inofthe
During
thiswater.
process,
the total
volume
the
total
volumeremained
of the mixture
remained
constant.
effect ratio
of thewas
blending
ratio was
investigated
of the
mixture
constant.
The effect
of theThe
blending
investigated
at two
different
at
two different
the
upstream and
downstream
and 65/40gas
bar.
Nitrogen
pressures
of the pressures
upstream of
and
downstream
chambers:
85/60chambers:
and 65/4085/60
bar. Nitrogen
was
used as
gas
waswhen
used aaspressure
a filler when
a pressure
beyond
the saturation
pressure was necessary.
a filler
increase
beyondincrease
the saturation
pressure
was necessary.
Figure
a schematic
p-T p-T
diagram
of theof
global
multiphase
behaviorbehavior
of carbonofdioxideFigure22illustrates
illustrates
a schematic
diagram
the global
multiphase
carbon
water
between
273
and
310
K
as
can
be
determined
from
the
literature
data
[11,12].
In the figure,
dioxide-water between 273 and 310 K as can be determined from the literature data [11,12].
In the
three-phase
line connects
all p, T coordinates
where two
liquid
phases
(aphases
water-rich
liquid L1 and
a
figure, three-phase
line connects
all p, T coordinates
where
two
liquid
(a water-rich
liquid
carbon
dioxide-rich
liquid
L2)
coexist
with
a
carbon
dioxide-rich
vapor
V.
The
pressure
at
L1L2V
L1 and a carbon dioxide-rich liquid L2) coexist with a carbon dioxide-rich vapor V. The pressure at
equilibrium
is close to
line, the liquid
and
vaporand
curve
of pure
CO
but always
below
the
L1L2V equilibrium
is two-phase
close to two-phase
line, the
liquid
vapor
curve
of2, pure
CO2 , but
always
vapor
pure CO
Because
this similarity
of the three-phase
line andline
two-phase
line at
below pressure
the vaporofpressure
of2.pure
CO2 .ofBecause
of this similarity
of the three-phase
and two-phase
◦
tested
ranges
of
pressure
and
temperature
(25
°C),
pressure
release
from
85
to
60
bar
has
higher
line at tested ranges of pressure and temperature (25 C), pressure release from 85 to 60 bar has higher
presence
presence of
of CO
CO22-rich
-richliquid(L2)
liquid(L2)than
thanCO
CO22-rich
-richvapor(V)
vapor(V)since
sinceititisislocated
locatedwell
wellover
overthe
the three-phase
three-phase
equilibrium
line.
In
contrast,
pressure
release
from
65
to
40
bar
experiences
a
phase
transition
equilibrium line. In contrast, pressure release from 65 to 40 bar experiences a phase transition from
from
the
the CO
CO22-rich
-rich liquid(L2)
liquid(L2) to
to the
the CO
CO22-rich
-rich vapor(V)
vapor(V) phase.
phase. The
Thedetailed
detailedproperties
propertiesofofthe
the two
two pure
pure
components,
components, water
water and
and LCO
LCO2,2,are
arelisted
listedin
inTable
Table 1.
1.
Figure
Figure2.
2.Schematic
Schematicof
ofspray
spraytest
testcondition
conditionmapping
mappingin
in P–T
P–T diagram
diagram of
of CO
CO22-water
-watermixture
mixture[12].
[12].
Energies 2016, 9, 948
4 of 11
Table 1. Properties of CO2 and water used in the experiment.
Fluids
Water
CO2
Vapor pressure at 298 K (kPa)
Density (mol/L) at 298 K and 64 bar
Viscosity (mPa·s) at 298 K and 64 bar
3
55.5
0.89
6400
16.2 (liquid)
0.057 (liquid)
The constant pressure of the lower chamber was attained using a backpressure regulator mounted
downstream of the lower chamber. Without the regulator, the pressure in the lower chamber
would have increased as the liquid flow filled the lower chamber, thereby compressing the gas.
The dimensions of the high-pressure flashing-spray system are listed in Table 2. The visualization
region in the lower chamber was provided by an optical window with a diameter of 5 cm. It was
desirable to fit most of the spray penetration into this visualization region for all the pressure conditions
tested in this study. Thus, wetting of the chamber bottom by spray plumes should be prevented. For this
reason, we selected a nozzle with a relatively high ratio of length to diameter, as previously mentioned.
Table 2. Dimensions of high-pressure flashing-spray system.
Parameters
Description
chamber shape
chamber diameter/length
chamber volume
capillary nozzle inner diameter
capillary nozzle length
Cylinder
5 cm/13.4 cm
260 cm3
0.4 mm
300 mm
A shadowgraph technique was employed to examine the flashing-spray patterns of the LCO2 and
its mixture with water. During the throttling process across the nozzle, a high-speed camera was used
to record consecutive images at a frame rate of 1/40,000 frames per second. Meanwhile, the pressure
difference was measured to verify the downstream pressure controlled by the back-pressure regulator.
3. Results and Discussion
3.1. Effect of Water-Blending Ratio in Water-LCO2 Mixture
In a previous work [3], spray patterns under three different levels of upstream pressure (pressure
difference) were observed using a high-speed camera: 65/40, 75/50, and 85/60 bar. At 65/40 bar,
the internal flashing due to bubble formation caused a wider spray angle and shallower penetration
length among three sprays. This is sometimes called a “bowl spray” and involves a high level of
flash-atomization, which is an effective breakup mechanism of the liquid column, due to a burst of
bubbles. A phenomenon similar to the bowl spray was observed with an increasing degree of superheat
by Lin et al. [4] and Park et al. [13]. In contrast, a narrower spray angle and deeper penetration were
observed at 85/60 bar, where the flash boiling is absent because of the liquid abundance. This is
considered a normal jet spray. This difference is caused by the higher velocity of the liquid flow inside
the nozzle orifice.
At the same pressure of 65/40 bar, the same nozzle length and diameter were used. The spray
of H2 O was not fully developed compared with that of LCO2 . Such weak atomization is expected
because of the low velocity and higher surface tension occurring when the H2 O flows with a high
viscosity inside the nozzle. This breakup mode is similar to the Rayleigh-type breakup mode observed
in the experiment of Dechelette et al. [14], which is the main atomization zone of high-viscosity
CWSs. However, the behavior of water sprays altered by the addition of LCO2 was not experimentally
evaluated in our previous work.
Energies 2016, 9, 948
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Energies 2016, 9, 948
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Figure 3 shows the effect of the water fraction blended in the water-LCO2 mixture on the spray
pattern atFigure
65/403bar.
Thethe
amount
water
the mixture
was
while
the total
shows
effect ofofthe
waterinfraction
blended
in increased
the water-LCO
2 mixture
on volume
the sprayof the
pattern
65/40 bar. The
amount
of water
the mixture
increased while
the total
of thewater,
mixture
wasatmaintained.
Water
contents
of in
0 and
100 volwas
% correspond
to pure
LCOvolume
2 and pure
mixture was
Water
contents
of conditions
0 and 100 volare
% correspond
to pure
LCO2 and pure
respectively.
Themaintained.
spray patterns
under
both
similar to past
observations,
as water,
previously
respectively.
The
spraycontent
patternsincreased
under both
are similar
past observations,
previously
mentioned.
As the
water
toconditions
75%, the spray
angletoincreased
slightly,asfrom
8.5◦ to 9.8◦
mentioned.
As
the
water
content
increased
to
75%,
the
spray
angle
increased
slightly,
from
8.5°
9.8° was
◦
and then to 12.3 . Subsequently, it decreased to zero when the water content reached 100%.toThis
and then to 12.3°. Subsequently, it decreased to zero when the water content reached 100%. This was
because of the increased dissolution of LCO2 when water content in the mixture is high. The high
because of the increased dissolution of LCO2 when water content in the mixture is high. The high
content
of dissolved CO2 can
evolve gaseous CO2 and thereby create the flash or disruptive boiling
content of dissolved CO
2 can evolve gaseous CO2 and thereby create the flash or disruptive boiling
during
a
pressure
release.
during a pressure release.
(a)
(b)
(c)
(d)
(e)
Figure 3. Effect of water fraction blended in water-LCO2 mixture on spray pattern at 65/40 bar: (a) 0%;
(b) 5%; (c) 25%; (d) 75%; and (e) 100% of water fraction.
Energies 2016,
2016, 9,
9, 948
948
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11
66 of
Figure 3. Effect of water fraction blended in water-LCO2 mixture on spray pattern at 65/40 bar: (a) 0%;
(b) 5%;
5%; (c)
(c) 25%;
25%; (d)
(d) 75%;
75%; and
and (e)
(e) 100%
100% of
of water
water fraction.
fraction.
(b)
Figure
3. 948
Effect of water fraction blended in water-LCO2 mixture on spray pattern at 65/40 bar: (a) 0%;6 of 11
Energies
2016, 9,
A similar
similartrend
trendwas
wasobserved
observedwhen
when
dissolved
CO
2 content
increased
in the
the
water-CO
2 mixture
A
dissolved
CO
increased
in the
water-CO
[2].
2 content
2 mixture
A
similar
trend
was
observed
when
dissolved
CO
2 content
increased
in
water-CO
2 mixture
[2].
Better
atomization
was
achieved
by
the
increased
flash
boiling
of
CO
2dissolved
dissolved
in
mixture.
Better
atomization
was
achieved
by
the
increased
flash
boiling
of
CO
in
the
[2]. Better atomization was achieved by the increased flash boiling of CO2 2 dissolved in the mixture.
The solubility
solubility of
of
CO
ininwater
water
was
fixed
once
pressure
and
temperature
are are
given
in the
the
data.
For
of CO
CO222in
waterwas
was
fixed
once
pressure
and
temperature
given
in data.
the data.
The
fixed
once
pressure
and
temperature
are
given
in
For
◦
example,
2.5 mol
mol
%
is obtained
obtained
for constant
constant
solubility
at 65
65 bar
bar
and
25 °C
°C
in 25
the predicted
predicted
results of
of
For
example,
2.5 %
mol
% is obtained
for constant
solubility
atand
65 bar
and
C in the predicted
example,
2.5
is
for
solubility
at
25
in
the
results
Larryn
W.
Diamond
et
al.
[11].
When
water
makes
up
75%
of
the
mixture,
absolute
amounts
soluble
results
of
Larryn
W.
Diamond
et
al.
[11].
When
water
makes
up
75%
of
the
mixture,
absolute
amounts
Larryn W. Diamond et al. [11]. When water makes up 75% of the mixture, absolute amounts soluble
in the
the water
water
become
higher
compared
to the
the case
case
of
25%
despite
having
thehaving
same solubility.
solubility.
Therefore,
soluble
in the
water higher
become
higher compared
toof
the
case
of 25%
despite
the same Therefore,
solubility.
in
become
compared
to
25%
despite
having
the
same
higher soluble
soluble
CO22soluble
amountCO
contribute
tocontribute
higher flash
flash
spray
asflash
waterspray
content
increases
in the
theincreases
mixture.
Therefore,
higher
tospray
higher
as increases
water
content
2 amount
higher
CO
amount
contribute
to
higher
as
water
content
in
mixture.
However,
this
trend
is
not
consistent
with
the
results
of
supercritical
dissolved-gas
atomization
[9].
in the mixture.
However,
trend iswith
not the
consistent
the results dissolved-gas
of supercritical
dissolved-gas
However,
this trend
is notthis
consistent
results with
of supercritical
atomization
[9].
The cone
cone half-angle
half-angle
was
reported
for aawas
wide
range of
offor
gas-to-liquid
ratios
(GLRs). The
The cone
cone
half
angle
atomization
[9]. Thewas
cone
half-angle
reported
a wide range
of (GLRs).
gas-to-liquid
ratios
(GLRs).
The
reported
for
wide
range
gas-to-liquid
ratios
half
angle
◦decreases
◦increased.
is
approximately
13°
at
a
low
GLR
and
to
4°
when
the
GLR
is
This
is
attributed
The
cone
half
angle
is
approximately
13
at
a
low
GLR
and
decreases
to
4
when
the
GLR
is
increased.
is approximately 13° at a low GLR and decreases to 4° when the GLR is increased. This is attributed
to differences
differences
in the
the
experimental
apparatus
and method.
method.
This
is attributed
to differences
in the
experimental
apparatus and method.
to
in
experimental
apparatus
and
Figures444and
and
5
show
the
change
in
the
spray
angle
and penetration
penetration
length, respectively,
respectively,
Figures
5
show
the
change
in
the
spray
angle
penetration
length, respectively,
according
and 5 show the change in the spray and
angle
and
length,
according
to
the
water
fraction.
The
information
was
derived
from
the
images
of
Figure
3. image,
In the
the
to
the
water
fraction.
The
information
was
derived
from
the
images
of
Figure
3.
In
the
spray
according to the water fraction. The information was derived from the images of Figure 3.
In
spray
image,
two
lines
are
drawn
and
intersected
at
the
nozzle
tip.
The
angle
between
two
lines
is
two lines
are drawn
and are
intersected
at theintersected
nozzle tip. at
The
between
twoangle
lines between
is carefully
measured
spray
image,
two lines
drawn and
theangle
nozzle
tip. The
two
lines is
carefully
measured
for spray
spray horizontal
angle. We
We line
identified
horizontal
line
of spray
sprayline
tip and
normal
to vertical
vertical
line
for
spray angle.
We identified
of spray
tip normal
to of
vertical
measured
distance
carefully
measured
for
angle.
identified
horizontal
line
tip
normal
to
line
and
measured
distance
from
nozzle
tip
to
this
line.
This
is
used
for
penetration
length.
The
data
at aa
frommeasured
nozzle tipdistance
to this line.
is used
penetration
The
data at a water
content
of zero
and
fromThis
nozzle
tip tofor
this
line. This islength.
used for
penetration
length.
The data
at
water
contentin
ofthese
zero are
are
excluded
in
thesecontent
plots. As
As
the water
waterthe
content
increased,
the
spray angle
angle
and
are
excluded
plots.
As thein
water
increased,
sprayincreased,
angle andthe
penetration
length
water
content
of
zero
excluded
these
plots.
the
content
spray
and
penetration
length
increased
to
75
wt
%
and
then
decreased.
At
75%
water
blend
condition,
higher
increased
to
75
wt
%
and
then
decreased.
At
75%
water
blend
condition,
higher
penetration
length
penetration length increased to 75 wt % and then decreased. At 75% water blend condition, higher
penetration
length
and angle
angle
were observed,
observed,
indicating
a higher
higher
degree
ofcase
flashof
spray.
Inflash
the case
case
of
and
angle were
observed,
indicating
a higherindicating
degree of aflash
spray.
In the
higher
spray,
penetration
length
and
were
degree
of
flash
spray.
In
the
of
higher flash
flash
spray, droplet
droplet
size
becomes
smallerTherefore,
and more
more droplet
uniform.
Therefore,quickly
dropletand
evaporates
droplet
size becomes
smaller
and
more uniform.
evaporates
particle
higher
spray,
size
becomes
smaller
and
uniform.
Therefore,
droplet
evaporates
quickly
and
particle
would
agglomerate
less.
Based
on
this
observation,
a
water
content
of
75
wt %
%
would
Based
on this observation,
water
of 75 wt
% appears
to of
be 75
optimal
quicklyagglomerate
and particleless.
would
agglomerate
less. Basedaon
this content
observation,
a water
content
wt
appears
to
be
optimal
for
the
effective
transport
of
coal
particles
by
improving
the
spray
quality.
for the effective
transport
of coal
particles
by improving
the spraybyquality.
appears
to be optimal
for the
effective
transport
of coal particles
improving the spray quality.
Figure 4.
4. Change
Change in
in spray
spray angle
angle with
with respect
respectto
towater
waterfraction
fractionat
at65/40
65/40 bar.
bar.
Figure
bar.
fraction
at
65/40
Figure 5.
5. Change in
in penetration
penetration length
length with
with respect
respect to
to water
water fraction
fraction at
at 65/40
65/40 bar.
bar.
Figure
Figure
5. Change
Change in penetration
length with
respect to
water fraction
at 65/40
bar.
Energies 2016, 9, 948
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Energies 2016, 9, 948
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7 of 11
3.2. Effect of Injection Pressure
3.2. Effect of Injection Pressure
The spray characteristics were observed
observed for
for aa higher
higher injection
injection pressure
pressure of
of 85/60
85/60 bar. Figure 6
The spray characteristics were observed for a higher injection pressure of 85/60 bar. Figure 6
water-blending ratio
ratio of
of the
the water-CO
water-CO22 mixture on the spray angle at 85/60
85/60 bar.
shows the effect of the water-blending
bar.
shows
the effect
water-blending
ratio oftothe
2 mixture on the spray angle at 85/60 bar.
The variation
of of
thethe
spray
angle with respect
thewater-CO
water fraction
as the water content increased was
The variation of the spray angle with respect to the water
fraction as the water content increased was
65/40 bar.
65/40 bar.
similar to that at 65/40
bar. However,
However, the angle was 15°–21°,
15◦ –21◦ , which is far larger than that at 65/40
bar.
similar to that at 65/40 bar. However, the angle was 15°–21°, which is far larger than that at 65/40 bar.
This can
can be
be explained
explained by
by the
the reduced
reduced evolution
evolution of
of gaseous
gaseousCO
CO22 in
in the
the water
water droplet.
droplet. Dissolved
Dissolved CO
CO22
This
This can be explained by the reduced evolution of gaseous CO2 in ◦the water droplet. Dissolved CO2
has aa higher
than vapor
vapor pressure
pressure at
at the
the temperature
temperature of
of 25
25 °C
has
higher pressure
pressure than
C and
and is
is likely
likely to
to remain
remain aa liquid.
liquid.
has a higher pressure than vapor pressure at the temperature of 25 °C and is likely to remain a liquid.
Therefore, the
the addition
addition of
of liquid
liquid CO
CO22 provides
where the
the
Therefore,
provides less
less viscosity
viscosity for
for the
the mixture
mixture than
than for
for water,
water, where
Therefore, the addition of liquid CO2 provides less viscosity for the mixture than for water, where the
velocity and
andthus
thusthe
the
momentum
increase.
can increase
the spray
angle despite
theamount
lower
velocity
momentum
increase.
ThisThis
can increase
the spray
angle despite
the lower
velocity and thus the momentum increase. This can increase the spray angle despite the lower
amount
of flash The
boiling.
The presence
liquid
in the
jetimportant
was also important
the usedioxide
of carbon
of
flash boiling.
presence
of liquid of
in the
jet was
also
for the usefor
of carbon
in
amount
of
flash
boiling.
The
presence
of
liquid
in
the
jet
was
also
important
for
the
use
of
carbon
dioxide applications
in cutting applications
jets containing
andshowed
liquid showed
almost
no
effect
cutting
[7]. While[7].
jetsWhile
containing
only gas only
and gas
liquid
almost no
effect
on
the
dioxide in cutting applications [7]. While jets containing only gas and liquid showed almost no effect
on the selected
samples,
the impact
increased
dramatically
with
the presence
of liquid.
selected
samples,
the impact
increased
dramatically
with the
presence
of liquid.
on the selected samples, the impact increased dramatically with the presence of liquid.
Figure 6.
6. Effect
Effect of
of water-blending
water-blending ratio
ratio in
in water-CO
water-CO2 mixture
mixture on
on spray
spray angle
angle at
at 85/60
85/60 bar.
Figure
bar.
Figure 6. Effect of water-blending ratio in water-CO22 mixture on spray angle at 85/60 bar.
The images in Figure 7 compare the effects of the water fraction and the injection pressure on
The images
images in
in Figure
Figure 77 compare
compare the
the effects
effects of
of the water
water fraction
fraction and
and the
the injection
injection pressure
pressure on
on
The
the spray pattern of the water-CO2 mixture. They confirm that the spray angle increases with the
the
spray
pattern
of
the
water-CO
mixture.
They
confirm
that
the
spray
angle
increases
with
the
the spray pattern
22 mixture. They confirm that the spray angle increases with the
injection pressure and has little dependence on the water-blending ratio. Compared with the 65/40injection pressure
water-blending
ratio.
Compared
with
the the
65/40-bar
injection
pressure and
andhas
haslittle
littledependence
dependenceononthe
the
water-blending
ratio.
Compared
with
65/40bar condition, less flash boiling was apparent at 85/60 bar.
condition,
less
flash
boiling
was
apparent
at
85/60
bar.
bar condition, less flash boiling was apparent at 85/60 bar.
(a)
(a)
(b)
(b)
Figure 7. Cont.
Energies 2016, 9, 948
8 of 11
Energies 2016, 9, 948
Energies 2016, 9, 948
8 of 11
8 of 11
(c)
(c)
(d)
(d)
Figure
of effects
effectsofofwater
waterfraction
fractionand
and
injection
pressure
spray
pattern
of waterFigure 7.
7. Comparison
Comparison of
injection
pressure
on on
spray
pattern
of water-CO
2
Figure 7. Comparison of effects of water fraction and injection pressure on spray pattern of water2 mixture: (a) 25% at 65/40 bar; (b) 75% at 65/40 bar; (c) 25% at 85/60 bar; and (d) 75% at 85/60 bar
CO
mixture: (a) 25% at 65/40 bar; (b) 75% at 65/40 bar; (c) 25% at 85/60 bar; and (d) 75% at 85/60 bar.
CO2 mixture: (a) 25% at 65/40 bar; (b) 75% at 65/40 bar; (c) 25% at 85/60 bar; and (d) 75% at 85/60 bar
Figure 8 shows the dependence of the distribution of the liquid-droplet size on the degree of
Figure
of of
thethe
distribution
of the
liquid-droplet
size on
of flash
Figure88shows
showsthe
thedependence
dependence
distribution
of the
liquid-droplet
sizethe
ondegree
the degree
of
flash boiling. A conventional image-analysis technique was employed to obtain the droplet-size
boiling.
A conventional
image-analysis
techniquetechnique
was employed
to obtain the
flash boiling.
A conventional
image-analysis
was employed
todroplet-size
obtain the distribution.
droplet-size
distribution. The results show that the mean diameter of the liquid spray at 65/40 bar was 150 μm.
The
results show
that theshow
meanthat
diameter
of the
liquidof
spray
at 65/40
baratwas
150bar
µm.
However,
distribution.
The results
the mean
diameter
the liquid
spray
65/40
was
150 μm.
However, the higher pressure of 85/60 bar yielded a larger droplet of 300 μm with a larger deviation
the
higher
pressure
of
85/60
bar
yielded
a
larger
droplet
of
300
µm
with
a
larger
deviation
from
However, the higher pressure of 85/60 bar yielded a larger droplet of 300 μm with a larger deviation
from the mean diameter. A similar trend in the droplet-size distribution was observed by Xiao et al.
the
mean
diameter.
A similar
trendtrend
in theindroplet-size
distribution
was observed
by Xiao
et al. et
[15],
from
the mean
diameter.
A similar
the droplet-size
distribution
was observed
by Xiao
al.
[15], who demonstrated that the addition of CO2 in diesel spray produced a far more uniform droplet
who
that the
of COof
in
diesel
spray
produced
a
far
more
uniform
droplet
size
[15], demonstrated
who demonstrated
thataddition
the addition
CO
2
in
diesel
spray
produced
a
far
more
uniform
droplet
size because of the flash separation of CO2 2 from the liquid. The finer and more uniform droplet
because
of the of
flash
of CO2 from
the
liquid.the
The
finer and
droplet
confirms
the
size because
theseparation
flash separation
of CO
2 from
liquid.
Themore
fineruniform
and more
uniform
droplet
confirms the strong flash atomization
of the spray at 65/40 bar. This condition is favorable, as small
strong
flash
the spray at of
65/40
bar. This
condition
is favorable,
as is
small
liquid droplets
confirms
theatomization
strong flashofatomization
the spray
at 65/40
bar. This
condition
favorable,
as small
liquid droplets may evaporate quickly and thereby prevent coal agglomeration when coal is present
may
evaporate
quickly
and thereby
prevent
coal agglomeration
when coal is present
[2]. isThe
size
liquid
droplets may
evaporate
quickly
and thereby
prevent coal agglomeration
when coal
present
[2]. The size distribution was measured using the open source software ImageJ. The threshold was
distribution
measuredwas
using
the open
source
ImageJ.
The threshold
was threshold
appropriately
[2]. The sizewas
distribution
measured
using
thesoftware
open source
software
ImageJ. The
was
appropriately chosen to identify the droplet particle from the background. Then, a measuring tool
chosen
to identify
the to
droplet
particle
from the
background.
a measuring
was usedtool
to
appropriately
chosen
identify
the droplet
particle
from the Then,
background.
Then, tool
a measuring
was used to calculate the size of multiple droplets automatically. It is instructive to note the
calculate
of multiple
droplets
automatically.
is instructive to
the limitations
of the
was usedthe
to size
calculate
the size
of multiple
dropletsItautomatically.
It note
is instructive
to note
the
limitations of the shadowgraph technique. Smaller droplets scatter away much less light than large
shadowgraph
Smaller technique.
droplets scatter
away
much scatter
less light
thanmuch
large less
droplets.
Therefore,
limitations of technique.
the shadowgraph
Smaller
droplets
away
light than
large
droplets. Therefore, the smallest droplets in the spray are not detectable with current shadowgraph
the
smallest
droplets the
in the
spray droplets
are not detectable
with
shadowgraph
technique.
However,
droplets.
Therefore,
smallest
in the spray
arecurrent
not detectable
with current
shadowgraph
technique. However, this effect may not be significant when determining any difference in injection
this
effect may
not bethis
significant
when
difference
in injection
pressureinsince
it is
technique.
However,
effect may
notdetermining
be significantany
when
determining
any difference
injection
pressure since it is present at both pressure conditions.
present
both it
pressure
conditions.
pressureatsince
is present
at both pressure conditions.
(a)
(a)
(b)
(b)
Figure 8. Dependence of atomized liquid droplet distribution on degree of flash boiling at (a) 65/40
Figure 8.8. Dependence
Dependence of
of atomized
atomized liquid
liquiddroplet
dropletdistribution
distributionon
ondegree
degreeof
offlash
flashboiling
boilingatat(a)
(a)65/40
65/40
Figure
and
(b) 85/60 bar.
and
(b)
85/60
bar.
and (b) 85/60 bar.
3.3. Effect of Chamber Pressure
3.3. Effect of Chamber Pressure
Figure 9 shows a series of spray events of LCO2-water mixture at different chamber pressures.
Figure 9 shows a series of spray events of LCO2-water mixture at different chamber pressures.
As the chamber pressure increased at the same injection pressure, the flow rate decreased, which
As the chamber pressure increased at the same injection pressure, the flow rate decreased, which
Energies 2016, 9, 948
9 of 11
3.3. Effect of Chamber Pressure
Figure 9 shows a series of spray events of LCO2 -water mixture at different chamber pressures.
Energies 2016, 9, 948
9 of 11
As the chamber pressure increased at the same injection pressure, the flow rate decreased, which
reduced
reduced the
the spray
spray angle
angle as
as well
well as
as the
thepenetration
penetration length.
length. This
This phenomenon
phenomenon was
was observed
observed in
in the
the
visualization
visualization results
results of
of Jakobs
Jakobs et
et al.
al. [1]
[1] and
and Johnson
Johnson et
et al.
al. [16].
[16]. The
The spray
spray angle
angle as
as aa function
function of
of the
the
chamber
pressure
has
been
measured
for
values
between
1
and
20
bar
[1].
An
increase
in
the
chamber
chamber pressure has been measured for values between 1 and 20 bar [1]. An increase in the chamber
◦
◦
pressure
pressurefrom
from11to
to20
20bar
barresults
resultsin
inaadecrease
decreasein
inthe
thewater-spray
water-sprayangle
anglefrom
from36
36°to
to24
24°.. An
An increased
increased
chamber
pressure
appeared
to
contract
the
spray,
yielding
a
more
dense
cloud
of
droplets
compared
chamber pressure appeared to contract the spray, yielding a more dense cloud of droplets compared
with
with the
the corresponding
correspondingatmospheric
atmosphericcase.
case. This
This was
was explained
explained by
by the
the smaller
smaller volume
volume of
of the
the entrained
entrained
air
air due
due to
to the
the increased
increased pressure.
pressure. This
This tendency
tendencymatches
matchesthe
thediesel
dieselspray
sprayreported
reportedin
in[17].
[17].
(a)
(b)
(c)
(d)
Figure 9.
9. Series
Series of
of spray
spray events
events of
of LCO
LCO2-water
-water mixture
mixture reported
reported at
at different
different chamber
chamber pressures
pressures with
with
Figure
2
same
injection
pressure
(75
bar):
(a)
70
bar;
(b)
60
bar;
(c)
50
bar;
and
(d)
30
bar.
same injection pressure (75 bar): (a) 70 bar; (b) 60 bar; (c) 50 bar; and (d) 30 bar.
Figure 10 shows the temporal variation of two different pressures between the chambers during
Figure 10 shows the temporal variation of two different pressures between the chambers during
the throttling process. One is the upstream pressure (injection pressure), and the other is the
the throttling process. One is the upstream pressure (injection pressure), and the other is the
downstream pressure (chamber pressure). The chamber pressure remained constant, indicating the
downstream pressure (chamber pressure). The chamber pressure remained constant, indicating
proper operation of the back-pressure regulator. The injection pressure in the upper chamber
the proper operation of the back-pressure regulator. The injection pressure in the upper chamber
experienced a continuous and smooth decay during the throttling process.
experienced a continuous and smooth decay during the throttling process.
Energies 2016, 9, 948
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10 of 11
Figure 10. Temporal variation of differential pressure between chambers during the throttling process
Figure 10. Temporal variation of differential pressure between chambers during the throttling process
at 75/70 bar.
bar.
at 75/70
75/70 bar.
Figure 11 plots the spray angle against the pressure difference. When the chamber pressure is
pressure difference.
difference. When the chamber pressure is
Figure 11 plots the spray angle against the pressure
high, the pressure difference is low, which indicates a reduced flow rate. The spray angle remains
low, which
which indicates
indicates aa reduced
reduced flow
flow rate.
rate. The spray angle remains
high, the pressure difference is low,
low at 13.5°.
However, when the chamber pressure decreases, i.e., the pressure difference increases,
◦
13.5°.. However, when the chamber
low at 13.5
chamber pressure decreases,
decreases, i.e., the pressure difference increases,
increases,
the angle is increased to 20°◦ and afterwards it is not changed substantially. The latter is attributed to
20° and afterwards it is not changed substantially. The latter is attributed to
the angle is increased to 20
choke flow, where vapor is prevalent in the mixture and therefore the flow rate is limited.
flow, where
where vapor
vapor is
is prevalent
prevalentin
inthe
themixture
mixtureand
andtherefore
thereforethe
theflow
flowrate
rateisislimited.
limited.
choke flow,
Figure 11. Variation of spray angle with respect to pressure difference.
Figure
with respect
respect to
to pressure
pressure difference.
difference.
Figure 11.
11. Variation
Variation of
of spray
spray angle
angle with
4. Conclusions
4. Conclusions
4. Conclusions
We investigated the spray characteristics of a mixture of LCO2 and water leaving an injector
We investigated the spray characteristics of a mixture of LCO2 and water leaving an injector
We
investigated
the spray
characteristics
of awas
mixture
of LCO
water
leaving
an injector
2 and
nozzle. The
optimal amount
of CO
2 in the mixture
reported
for the
spray
angle
and length.
This
nozzle. The optimal amount of CO2 in the mixture was reported for the spray angle and length. This
nozzle.
Thedetermined
optimal amount
CO2 in the
mixture
was reported
thewater-LCO
spray angle
and length.
value was
by theof
difference
in the
solubility
of CO2 inforthe
2 mixture.
The
value was determined by the difference in the solubility of CO2 in the water-LCO2 mixture. The
This
value
was
determined
by
the
difference
in
the
solubility
of
CO
in
the
water-LCO
mixture.
2
2
smaller droplets obtained at 65/40 bar indicate the strong flash atomization of the spray compared
smaller droplets obtained at 65/40 bar indicate the strong flash atomization of the spray compared
The
droplets
obtained
at chamber
65/40 barpressure
indicate the
strong
flash atomization
of the
spray
compared
withsmaller
85/60 bar.
Changes
in the
have
no significant
influence
on the
spray
angle,
with 85/60 bar. Changes in the chamber pressure have no significant influence on the spray angle,
with
85/60
Changes
the chamber
pressure have no significant influence on the spray angle,
except
in thebar.
case
of a highinchamber
pressure.
except in the case of a high chamber pressure.
except in the case of a high chamber pressure.
Acknowledgments: The authors thank the Korean government (MKE) for financial support from the manpower
Acknowledgments: The
The authors
thank
the
government
(MKE) for
support from
the
manpower
Acknowledgments:
authors This
thankwork
the Korean
Korean
government
for financial
financial
from of
theKorea
manpower
program (No. 20144010200780).
was supported
by a(MKE)
National
Researchsupport
Foundation
grant
program (No. 20144010200780).
20144010200780). This
This work
work was
was supported
supported by
by a National Research Foundation of Korea grant
(NRF-2016M1A3A1A02005033) funded by the Korean government (MEST).
(NRF-2016M1A3A1A02005033)
(NRF-2016M1A3A1A02005033) funded
funded by
by the
the Korean
Korean government
government (MEST).
(MEST).
Author
Contributions:
Hakduck
Kim
and
Changyeon
Kim
performed
the spray
experiment
under
the
advice
Author
experiment
under
thethe
advice
of
Author Contributions:
Contributions: Hakduck
Hakduck Kim
Kimand
andChangyeon
ChangyeonKim
Kimperformed
performedthe
thespray
spray
experiment
under
advice
of Juhun
Song.
Heechang
Lim
reviewed
the
experimentaldata
dataand
andrevised
revisedaamanuscript.
manuscript.Juhun
JuhunSong
Song supervised
supervised
Juhun
Song.
Heechang
Lim
reviewed
the
experimental
of Juhun Song. Heechang Lim reviewed the experimental data and revised a manuscript. Juhun Song supervised
the research
research and
and wrote
wrote the
the manuscript.
manuscript.
the research and wrote the manuscript.
Conflicts
Conflicts of
of Interest:
Interest: The
The authors
authors declare
declare no
noconflict
conflictof
ofinterest.
interest.
Conflicts of Interest: The authors declare no conflict of interest.
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