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JSAE 20119146
SAE 2011-01-1981
Z-type Schlieren Setup and its Application to High-Speed
Imaging of Gasoline Sprays
Sanghoon Kook, Minh Khoi Le, Srinivas Padala, and Evatt R. Hawkes
University of New South Wales
ABSTRACT
Schlieren and shadowgraph imaging have been used
for many years to identify refractive index gradients in
various applications. For evaporating fuel sprays,
these techniques can differentiate the boundary
between spray regions and background ambient
gases. Valuable information such as the penetration
rate, spreading angle, spray structure, and spray
pattern can be obtained using schlieren diagnostics.
In this study, we present details of a z-type schlieren
system setup and its application to port-fuel-injection
gasoline sprays. The schlieren high-speed movies
were used to obtain time histories of the spray
penetration and spreading angle. Later, these global
parameters were compared to specifications provided
by the injector manufacturer. Also, diagnostic
parameters such as the proportion of light cut-off at
the focal point and the orientation of knife-edge
(schlieren-stop) used to achieve the cut-off were
examined. From the experiment, it was found that a
light cut-off of approximately 60% performed the best
to image the internal pattern of the gasoline sprays.
An interesting finding from the knife-edge orientation
study was that a vertically oriented stop increased the
contrast of the spray pattern in the vertical direction.
Similarly, a horizontal stop showed higher contrast
and more turbulent spray structures in the horizontal
direction. A combination of a horizontal and a vertical
stop therefore unveiled the most of the gasoline spray
pattern and structure. The light cut-off proportion and
knife-edge orientation, however, did not affect the
refractive-index gradient of the spray border and
therefore no significant variations in the tip penetration
and spreading angle were measured.
INTRODUCTION
Schlieren and shadowgraph imaging are powerful
tools to study turbulent multiphase flows. Despite the
fact that limited quantitative data can be obtained,
schlieren imaging can visualize internal structures of
the gas-phase fluid flow, which is invisible without a
proper optics setup. Also, it is relatively simple and
cheap to build compared to costly planar-laser-based
diagnostics. As a result, the schlieren setup has
become very popular and is widely available.
Details of the theory and diagnostic considerations of
the schlieren technique are readily available in many
fine textbooks. For example, presenting a wide range
of example images, Settles [1] has already discussed
almost all parameters of concern in the schlieren
setup. However, the discussions are for simple
example cases and lack the details needed to study
more complex fuel spray behavior. This study,
therefore, aims to bridge the gap between
fundamentals of the schlieren setup and application
details for fuel spray imaging.
In automotive applications, schlieren imaging has
been widely used to study fuel sprays [2-7] or gaseous
fuel jets [8-9] in engine environments. Often an optical
chamber is used to decouple fuel sprays/jets from
turbulent in-cylinder flow [2-4, 8]. Also, schlieren
imaging has been attempted in more challenging
environments such as optical engines [5-7] or a rapid
compression machine [9]. There are three types of
schlieren systems that are commonly used in
automotive research: (i) lens systems (e.g. [2, 3]), (ii)
double-pass mirror systems (e.g. [7, 8]), and (iii)
z-type mirror systems (e.g. [4-6, 8, 9]).
Lens systems are generally simple and easy to set up
as they follow the “straight” layout as displayed in Fig.
1. For example, evaporating diesel sprays [2] or
gasoline/ethanol sprays [3] have been studied using
lens systems. Light was generated from either a laser
or projection lamp, passed through a pin hole and was
then collimated by a magnifying lens. The simplicity of
such lens systems offers a great advantage as there
is no off-axis aberration, which is the biggest
challenge in mirror systems (to be discussed in
greater detail later in the paper). Despite this
advantage, the lenses in this type of schlieren system
are required to be of very high quality, which leads to
high cost and high maintenance level. Also, lenses
are restricted in size: their diameters cannot be as
large as parabolic mirrors, meaning that these
systems cannot be used to observe a large region of
interest. Due to its straight layout, the lens system is
also longer in length (requiring more space for the
total arrangement) than mirror-based systems.
Copyright © 2011 Society of Automotive Engineers of Japan, Inc. and Copyright © 2011 SAE International
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The double-pass mirror system uses only one mirror,
missing out on the advantages of collimating light rays.
This arrangement is also known as the single-mirror
coincident system. As illustrated in Fig. 1, a single
spherical field mirror with the light source on axis at
the radius of curvature is used. Alternatively, a
parabolic mirror with a corrector lens after the light
source can be used. The schlieren object is positioned
in front of the mirror. The principle is that, as the
diverging light beam passes through the schlieren
object, it hits the mirror and returns along a coincident
path, forming an image. Afterward, a reflecting
knife-edge or beam splitter separates the returning
rays from the source rays. These returning rays then
carry the schlieren image to a camera.
In this system, since the light passes through the
schlieren object twice, the deflection of light rays
occurs twice resulting in increased sensitivity. This in
fact increases the sensitivity as the refraction angle is
small and the schlieren object is close to the mirror
where light is more uniformly dissipating. If the setup
is in perfect coincident on-axis, off-axis aberrations
are completely eliminated. However, the double-pass
setup requires a rather large, high-quality mirror which
drastically increases the cost. Also, the light rays are
not propagating parallel to each other and the
schlieren object is close to the mirror, which is a
disadvantage in some applications. These limitations
are well demonstrated in Ref. [7]: in an
optical-accessible engine, a double pass schlieren
system was used to visualize evaporating diesel
sprays. A metal mirror attached on the injector-mount
plane was used as the main mirror in their system.
The authors found that by modifying illumination angle,
image patterns would change. For instance, higher
angles filtered out the information about the gas flow
structure, which resulted in the spray being presented
with a higher contrast in the image. However, there
were some limitations such as the need for filtering the
reflections on the optical access window surfaces and
requirements for a high quality mirror appropriate for
high pressure and temperature conditions inside the
combustion chamber. Petersen and Ghandhi [8] used
a simil ar system with the inclusion of a large circular
mirror with a hole to accommodate the fuel injector tip.
They used a parabolic mirror to make light rays
coming from the light source shine on this circular
mirror. This parabolic mirror was also tilted in a way
such that the refracted beam was separated from the
original light path. This decreased the difficulty in
setup and avoided the use of beam splitter; however,
it introduced some off-axis aberration.
The z-type two-mirror system is the most popular
schlieren system setup in practice. This is because
the system all ows a larger test r egion without needing
to increase the size of the mirrors. As shown in Fig. 1,
light generated from a light source focuses on a
condenser lens and goes through a slit. It is then
directed to a parabolic mirror and collimated, resulting
in uniform propagation through the test region. The
collimated light is incident on another parabolic mirror,
which refocuses the light rays. The light passes
through a cut-off (an edge or filter) and is refocused
onto a screen or an image sensor of a camera. In
general, light rays travel in a z-shaped, hence the title.
Due to the parallel light shone through the schlieren
image and the space between the two mirrors, this set
up is particularly useful for imaging a two-dimensional
schlieren object. For example, Pickett et al. [4] used
this system to visualize reacting diesel jets, providing
insight into the time sequence of diesel ignition and
combustion. Also, useful tips for the z-type schlieren
system were found: including a small 5° angle for low
astigmatism and the fact that the shadowgraph
technique (no light cut-off) was more than sufficient to
detect the edge of the jet, possibly due to high
refractive index gradients that existed in the
high-density environment. In-cylinder phenomena of
gasoline engines have also been studied using the
z-type schlieren system [5, 6]. Due to the curvature in
the cylinder liner, the setup of schlieren imaging in an
optical engine is more challenging. By manipulating
the outer shape of the cylinder, however, it was found
that light could be collimated through the region inside
the engine cylinder. The importance of a well-defined,
point-like light source in the z-type system is well
demonstrated in Ref. [8] where a f/1.4 condenser lens
was employed (allowing a large amount of light to
pass through) together with a 1 mm pin hole.
While schlieren diagnostics are well-developed and
do not fall into the category of “advanced” diagnostics,
there is still room to improve them, particularly for
automotive applications. For example, schlieren-stops
with a different orientation may improve the schlieren
image quality in the case of fuel sprays. Diagnostic
Screen
Knife -edg e
Lamp
Condenser lens
Spray Lens
Lens system
y
x
Screen
Knife- edge
Lamp
Parabolic
mirror
Spray
Z-type two-mi rror system
Double-pass single-mirror system
Scree n
Knife-edge
Lamp
Condenser
Lens
Spray
Spherical mi rro r
Figure 1: Sketch of optics and light paths for various
schlieren setups.
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details such as selection of the light source and the
technique to create collimated light for the best
uniformity can also affect clarity of the fuel spray
significantly. Limitations imposed by the
space-requirement of the setup are quite common in
automotive experiments and must be considered.
However, how these numerous setup parameters
affect the quality of schlieren signals tends to stay
in-house and has not been shared in the open
literature.
One of the primary objectives of this study is to
present a z-type schlieren imaging system with the
ultimate goal of improving the schlieren signal of
gasoline sprays. From our survey of the literature [4-6,
8, 9], the z-type schlieren system has been identified
as the most common setup in engine applications. It
has certain advantages of simplicity and sensitivity
that make it attractive in comparison to other setups.
Also, to image gasoline sprays, in which geometric
and structural information is of particular interest,
space is required to accommodate a fueling system,
and an optical chamber is required to achieve relevant
ambient conditions. Therefore, we concluded that a
z-type system is the most suitable to study gasoline
sprays.
High-speed schlieren imaging [2-9] also offers room
for improvement. The continuing advancement in
high-speed camera technologies increases the
potential to understand the transient behavior of
sprays. Compared to film-based high-speed cameras
or earlier version digital high-speed cameras, new
cameras offer superior image quality, high framing
rates, and very fast data processing. This may enable
us to understand transient nature of fuel sprays, which
was previously inaccessible due to the long gating
time of older cameras.
In this study, the details of the z-type schlieren setup
are discussed: including mirrors, a light source, a
camera, and schlieren stop. We have applied the
developed technique to gasoline spray imaging.
Specifically, a high-speed imaging at 11,527 frames
per second was performed to uncover the transient
behavior of gasoline sprays. Details of spray image
processing are also discussed. Finally, we have
examined diagnostic parameters such as the
proportion of the light cut-off at the focal point and
using a combination of a horizontal and a vertical
cut-off to optimize the schlieren signal.
BRIEF SUMMARY OF SCHLIEREN AND
SHADOWGRAPH THEORY
A schlieren signal is a result of refractive-index
gradient. In fuel sprays, temperature or density
variations at the boundary between evaporating fuel
and ambient gases lead to disturbances that refract
light, and these can be detected with the proper setup.
An example of schlieren imaging for evaporating
gasoline spray (i.e. schlieren projected on a screen) is
shown in the left side of Fig. 2. Defining the x
coordinate as an axis of the light rays, the schlieren
image is shown on a y-z plane. It is clear that the
evaporating gasoline spray remains visible above the
background with a more uniform structure.
In the geometric theory of refraction [1], the curvature
of a refracted light ray is a function of the
refractive-index gradient through which it passes i.e.
y
n
nx
y
∂
∂
=
∂
∂1
2
2
, (1)
where
n
is the refractive index. To apply this formula
to the gasoline sprays, the simple case of a positive
vertical refractive-index gradient ( yn ∂∂ >0) at the
spray border (annotated as “A” in Fig. 2) is assumed,
while no gradient is assumed to exist in the xor z
directions.
The sketch in the right side of Fig. 2 illustrates that 2
n
is higher than 1
n
and therefore the light ray is turned
to a counter-clockwise angle
ε
Δ
following Huygens’
principle. The light is collimated prior to the spray
region and is hence initially normal to y-axis upon
passing through x1. As the light ray propagates
through the spray region from x1to x2for a diff erential
time Δt, it is refracted through the differential angle
ε
Δ
.
Using the light speed c and the speed of light in a
vacuum c0, this angle can be expressed as:
t
y
ncnc Δ
Δ
−
=Δ )//(
2010
ε
(2)
Substituting
0
/)( cynxt ⋅Δ=Δ
, adopting
21
)( nnyn ≈≈
as
0
→
Δ
y
, and finally letting all the
finite differences approach zero, it is obtained that:
dy
dn
ndx
d1
=
ε
(3)
From Eq. 3, assuming
ε
is very small and
hence dxdy=
ε
, the curvature of the refracted ray
can be expressed as:
dy
dn
ndx
yd 1
2
2
=
(4)
Note that by writing the total derivatives as partials to
account for other refractive-index gradients, Eq. 1 is
obtained.
Figure 2: An example of schlieren image (left) and
simplified sketch of refracted beam path near the
spray tip (right).
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Furthermore, by integrating Eq. 3 for the surrounding
medium of refractive index
0
n
and a two-dimensional
schlieren object of extent
L
along the optical axis,
the angular ray deflection
ε
in the y- and z- direction
can be expressed as:
y
n
n
L
y
∂
∂
=
0
ε
, and
z
n
n
L
z
∂
∂
=
0
ε
(5)
Above equations provide a theoretical basis for
differentiating schlieren and shadowgraph imaging
techniques. In shadowgraph imaging, a disturbance,
i.e. a deflection angle gradient ( y
y∂∂
ε
or
z
z
∂∂
ε
)is
observed in the form of ray displacement. Using Eq. 5,
this means that the second derivative of the refractive
index (
2
2yn ∂∂
or
2
2zn ∂∂
) is visualized in the
shadowgraph imaging. Equation 5 also shows that the
shadowgraph technique cannot image fluid flows if the
presented refractive-index gradient is uniform (i.e.
yn ∂∂
and
zn ∂∂
is constant). This uniform gradient
shifts the entire light rays undisturbed (or
y
ε
and
z
ε
is
constant in Eq. 5) and hence no shadowgraph may be
imaged. By contrast, the schlieren technique can
visualize flows with a uniform refractive-index gradient.
By placing a knife-edge at the focal point (see Fig. 1),
some of the deflected rays are blocked (or “disturbed”)
and therefore a phase difference is created. This
phase difference then can be projected on the screen
or can be detected by the camera sensor. Therefore,
the schlieren signal is proportional to the first
derivative of the refractive index (
yn ∂∂
or
zn ∂∂
).
This results in higher contrast and sharper edges for
schlieren images than those for shadowgraph images.
On the other hand, the use of a schlieren stop makes
the images darker. This trade-off must be considered
to decide whether to place the stop (schlieren) or not
(shadowgraph).
SCHLEREN OBJECT
OPTICAL CHAMBER AND GASOLINE SPRAYS
Measurements were obtained in an optically
accessible, constant-volume spray chamber in which
tailored ambient temperature and pressure conditions
corresponding to typical gasoline engine intake
systems can be provided. Schematics of the chamber
and schlieren measurement setups are shown in Fig.
3. Sight-glass windows are located in three sides of
the chamber to allow line-of-sight and orthogonal
imaging of the injected fuel spray. The ambient
pressure and temperature conditions are achieved in
the chamber using an air compressor and heater. For
example, the chamber is capable of simulating intake
boosting of up to 200 kPa. Since experiments in this
study were performed at standard laboratory ambient
conditions, however, there was no need of running a
compressor and heater.
A port-fuel-injection (PFI) injector was used to study
gasoline sprays. While PFI injectors have been
available for more than two decades and more
advanced direct-injection injectors are rapidly
penetrating the market, PFI sprays are of particular
interest because hardware and control systems still
have economic benefits and superior reliability.
Considering these, we selected a conventional PFI
injector to demonstrate our schlieren imaging system.
A Bosch PFI injector (MPI Model EV-6) equipped with
six orifices (“director plate” multi-orifice) was used for
this study. According to the injector manufacturer’s
specifications, the spreading angle (α80%)was 70°: by
Bosch’s definition, 80% mass of sprays was within
α80%. The f ueling rate measured, at 300 kPa of orifi ce
pressure drop, was 382 cm3/min. Physical properties
of the gasoline used in this study are also given in
Table 1.
SCHLEREN OPTICS AND SETUP
OFF-AXIS ABERRATIONS AND MIRRORS
The z-type schlieren system involves some
challenges, mostly due to the fact that light goes off
axis in its path. Namely, off-axis aberrations are
created when the captured light is not on the axis from
which the light was generated. Two most common
off-axis aberrations effects are “coma” and
astigmatism.
Comatic aberration (or coma in short) occurs when
light is reflected from the mirror on an angle. The
image of a point is focused at sequentially differing
heights, producing a series of asymmetrical spot
shapes of increasing size that result in a comet-like
structure [10]. However, the coma can be corrected by
using a combination of lenses that are positioned
symmetrically around a central stop. This is
particularly useful in z-type schlieren systems, where
combinations of two identical mirrors are used and
hence the tilt angles of the mirrors could be arranged
to be symmetrical.
Astigmatism is the failure of focusing a point to a point,
and the image is therefore not properly focused. Rays
that propagate in two perpendicular planes have
different foci, which means that, as light travels in two
waves, horizontal and vertical, horizontal lines will be
focused at a different place compared to vertical lines.
Astigmatism is shown to be proportional to the square
power of the off-axis angle [11] hence it can be
reduced by reducing the size of the off-axis angle. The
longer focal length obtainable with parabolic mirrors
can also help reduce astigmatism. In the present
setup, the angle created by the illuminator path
(between light source and first mirror) and the
collimated path (between the two mirrors) was twice
the off-axis angle, so any reduction in this angle would
only count as half the reduction in the off-axis angle.
We chose quality mirrors with high surface accuracy
of λ/8 and enhanced aluminium coating. To minimise
the coma, the focal length and f-numbers of the
mirrors were maximised. At the same time, limitations
of the test space had to be considered. As a result, a
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mirror with the aperture of f/4.1 with a focal length of
444.5 mm was chosen.
For the positioning, there should be at least a distance
of twice the focal length between the two mirrors to
provide space for the test area; therefore, a minimum
distance of 890 mm was required. Considering the
size of the spray chamber (260 mm in the optical path),
the minimum distance is increased to 1150 mm. In the
actual set up, the distance between the mirrors was
1450 mm.
LIGHT SOURCE
As the positions of the mirrors were now fixed, the
illuminator path had to be moved. This required the
positioning of the light source to be as close to the
collimating path as possible as shown in Fig. 3. As the
collimating path crossed over the spray chamber, it
acted as a constraint in the positioning of the light
source in this experiment. Hence, the light source
should be as close as possible to the chamber.
Obviously, the distance between the slit source and
first mirror is to be at least the focal length of the
mirrors. In the present setup, the distance on the
illuminator path was 450mm, slightly longer than a
mirror focal length of 444.5 mm.
Others have used continuous-wave lasers [5-6, 9] as
a light source because of a constant supply of high
power. However, a modern Xe-Arc lamp performs well
for the same purpose [4, 8]. We used a 150-W
(electrical power) mercury-xenon arc lamp as a white
light source in this experiment. To collect radiation off
the back to the lamp and direct it through the optical
system, a reflector was also placed behind the lamp.
For the focusing, a f/1 condensing lens was used and
the light was spatially-filtered through a 1-mm
aperture.
HIGH-SPEED CAMERA
Using the second parabolic mirror, which was
nominally identical to the first mirror, the collimated
beam was re-focused into a high-speed camera
(VisionResearch Phantom v7.3). Containing a CMOS
sensor with size of 17.6 mm by 13.2 mm (22 µm pixel
size) and quantum efficiency of 31% at 530 nm, this
camera is capable of taking up to 500,000 frames per
second. However, these frame rates were only
available at very low resolutions of 32-by-32 pixels,
which was not enough to capture the gasoline sprays
of this study. Therefore, we selected 512-by-512
pixels at the maximum framing rate of 11,527 frames
per second.
The lens used for this camera was a 50-mm Nikon
Nikkor lens. With a wide aperture of f/1.4, the lens
allowed a maximum amount of light reaching the
sensor of the camera, which was essential for the
schlieren imaging. A focal length of 50 mm provided a
suitable field of view, with horizontal field of view
(FOV) angle of 39.60, vertical FOV angle of 270and
diagonal FOV angle of 46.80. The exposure time was
fixed at 2 μs, which was the minimum value of the
camera, to avoid saturation of the bright background
as well as blurring of the fast-moving sprays. For the
shadowgraph imaging setup with no schlieren stop,
however, the saturation was unavoidable due to a
high power Xe-Arc lamp and therefore a neutral
density filter with an optical density of 0.5 was used.
SCHLIEREN STOP (KNIFE-EDGE)
Intuitively, the successful and correct positioning of
the schlieren stop will uniformly darken the image
captured. In other words, if the schlieren stop was
incorrectly positioned, it would be easily noticeable
because of the partial darkening of the image and the
apparent difference in brightness. Figure 4 shows
some examples. The image at the top-left shows a
case with no knife-edge. Below this image, a vertical
knife-edge was introduced from the left side of the
camera and was incrementally moved along the light
path axis until the knife-edge was at the vertical focus
point. From there, it could be incremented toward the
right side of the camera, perpendicularly to the
analyser axis. The horizontal knife-edge was also put
Circular slit
?
Parabolic
mirror (f/4.1)
Vertical
knife-edge
Horizontal
knife-edge
High-speed
CMOScamera
?
Parabolic
mirror (f/4.1)
Ref le ct or
Xe- ar c lamp
(150 W)
Condensing
lens ( f/1 )
Air in
Air + gasoline out
Gasoline injector Glass
window
xz
Nozzle Configuration
(Bottom View)
Figure 3: Schematic of schlieren optics setup and
light path.
Table 1: Operating conditions
Ambient air
Pressure [kPa, gauge]
0
Temperature [K]
295
Fuel injector
Injector type
Bosch MPI EV
-
6
Number of holes
6
Injection pressure [kPa]
250
Spreading angle [
°
,
α
80%
]
70
Gasoline propert
ies
Vapor
pressure [kPa
, absolute
]
45~90
Density [kg/m
3
]
720 (15
°C)
Viscosity [Pa
-
s]
0.00042
Surface tension [N/m]
0.0189
Flash point [°C]
-
43
Heat of vaporization [kJ/kg]
310
Boiling point [°C]
30~200
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through the same ‘tuning’ process. This cut-off was
introduced from below the light path. It was also
moved along the axis until the correct position was
found. From that position, the amount of light cut-off
was controlled by the knife-edge movement in the
plane perpendicular to the light path axis. The result
with a correct knife-edge position is shown at the
bottom-right.
SCHLIEREN IMAGE PROCESSING
BACKGROUND CORRECTION AND SPRAY
BOUNDARY DETECTION
An example of schlieren images with no schlieren stop
(i.e. shadowgraph) is shown in Fig. 5. In the first
column of the figure, still images obtained from a
movie of gasoline sprays are shown. The time after
the start of injection (aSOI) is stamped at the each
image top-left.
The image at 1 ms aSOI shows that soon after the
injection starts, evaporating sprays penetrate
downstream and are clearly visible above the
near-uniform background. The spray droplets
continue to travel across the chamber while
maintaining the overall structure during the injection
period as demonstrated in the image at 3 ms aSOI. At
5 ms aSOI, when the injection stopped, small droplets
are seen in the near-nozzle region.
In Fig. 5, white solid lines are overlaid on the spray
images to annotate the spreading angle (α
80%
= 70°).
By Bosch’s definition, 80% of fuel mass is within α
80%
when the injector is tested using a mechanical
patternator [12]. Since schlieren images are
light-of-sight integrated, a direct comparison between
α
80%
and measured spreading angle from the images
is not possible. However, a visual inspection indicates
that majority of fuel droplets present within α
80%
and
the spreading angles show a reasonable match. Later
in the following section, we will propose a method to
determine the spreading angle, which fits α
80%
well
regardless of the schlieren sensitivity.
As discussed earlier in the section on Schlieren and
Shadowgraph Theory, the spray images are a result
of the refractive-index gradient created by both liquid-
and vapor-phase fuel. Due to this principle, the liquid
spray with stronger density gradients (and hence
refractive-index gradient) is clearer than the vapor
fuels in the schlieren images. For example, vapor
fuels near the spray border are transparent and hard
to identify. To address this issue, a background
correction was conducted. The idea was that the
transparent vapor regions would become much
clearer by subtracting the background image.
The second column of Fig. 5 shows a result of the
background-corrected images (IcA) where the raw
image (In) was subtracted by the relatively stable
background obtained prior to the fuel injection (I0)–
method A. An offset is also added so that the images
are shown with more contrast to highlight features of
the spray. This is why the spray appears darker, for
example. The background correction appears to work
well as the ambient now appears uniform, though not
perfectly so. The correction scheme capitalizes on
one of the advantages of high-speed imaging in that
the background immediately before injection is
recorded. Background images acquired at a different
time, or time-averaged images, would not be
sufficient.
With the background correction, more interesting
characteristics of the spray penetration are now visible.
Indeed, IcA unveils that vapor regions are larger than In
particularly near nozzle and spray boundaries. For the
background-corrected images, a threshold-based
boundary-detection was also implemented. The
“gray-thresh” function used in this study is Otsu’s
method [13], which is one of the built-in models in the
Matlab software. This method chooses the threshold
to minimize the intra-class variance of the black and
white pixels and hence is usually a more reasonable
choice than an arbitrary choice of the threshold value.
Indeed, as is demonstrated in the third column of Fig.
5, boundaries of the spray droplets are successfully
obtained using Otsu’s method for both liquid- and
vapor-phase fuel regions. To illustrate, we also
overlaid the detected boundaries on the original image
as shown in the last column of Fig. 5.
Although IcA was effective as demonstrated in Fig. 5,
the structure in the background of the shadowgraph
actually does change slightly during the course of
injection. Therefore, we al so attempted another
background correction method that used the
background from a preceding image for the correction
similar to Refs. [4, 14] – method B. By subtracting
successive images, variations in the background can
be eliminated. For example, the imaging interval was
87 µs in which the background did not change
significantly while the droplets travelled about 10
Figure 4: Schlieren images of the chamber prior to
fuel injection (i.e. background image) during the
knife-edge setup.
Downloaded from SAE International by University of New South Wales, Monday, February 23, 2015
pixels (=1.85 mm). Figure 6 shows this result (IcB),
which is the intensity of the current image (In) minus
the intensity of the previously acquired image (In-1).
For the purposes of presentation, a grayscale offset
was also added. In the images, turbulent spray
patterns are well captured (the second column) and
the boundary detection was more effective (the third
column). Indeed, new boundaries overlaid on the
original shadowgraph images (the last column) show
that the internal spray pattern is well captured
including small-scale structures, which is not possible
in the simple correction using the intensity of the
background prior to injection (IcA).
How these two different correction methods apply to
more sensitive schlieren images is demonstrated in
Figure 5: Original shadowgraph images and corrected images using background-correction method A. Solid
white lines are drawn using a spreading angle provided by the injector manufacturer (80% mass angle).
Shown at
the image top-left is time after the start of injection.
Figure 6: Original shadowgraph images and corrected images using background-correction method B.
Downloaded from SAE International by University of New South Wales, Monday, February 23, 2015
Fig. 7. Using both IcA and IcB methods, the schlieren
images of gasoline spray were processed and the
detected boundaries were overlaid on the original
schlieren images. It is obvious in Fig. 7 that IcB
outperforms IcA for the schlieren images. In Figs. 5
and 6 for shadowgraph images, both IcA and IcB
detected the entire spray region and the only
difference was found in the internal spray pattern.
However in Fig. 7 for the schlieren image, some spray
regions were missed out when IcA was applied. One
solution for this problem was to adjust the grayscale
threshold until the boundary detection became
successful but this resulted in a serious question of
consistency in the image processing as each image
required a manual selection of the thresholds.
Therefore for the schlieren images in this study, we
used IcB to detect the boundaries of the spray droplets.
SPRAY TIP PENETRATION AND SPREADING
ANGLE
The image processing and boundary detection in Figs.
5 to 7 enable measurement of the penetration and
spreading angle of the spray. Figure 8 shows
definitions of the spray tip penetration and spreading
angle used in this study. The tip penetration was
measured by calculating the distance between the
nozzle and the spray droplet in the farthest
downstream as shown at the figure top-left. A circle
symbol is used to denote this droplet region. Note that
this region can be found either in the left or right side
of the image and our real-time processing software for
the spray movie captured it successfully.
The spreading angle was measured using the farthest
spray droplet from the nozzle: similar to the tip
penetration but in the horizontal direction. In Fig. 8, a
square symbol is shown to denote this region for the
spreading angle measurement. It is worth to noting
that the transient behavior of the spray was well
observed by this method. For example, the spray at
1ms aSOI shows a much wider dispersion than that of
a steady-period of injection (i.e. after 2 ms aSOI). Also,
Fig. 8 shows that our definition of the spreading angle
follows α
80%,
which is drawn as white solid lines.
Note that the same shadowgraph images of Figs. 5
and 6 are used again in Fig. 8. However, Fig. 8 shows
a circular light boundary and dark regions at the
corner. For the rest of the figures, the image pixels
outlined as a dashed line (see the box in the image at
4ms aSOI) are presented by masking out these outer
pixels. However, the tip penetration and spreading
angle measurement were continued until the spray
went out of the camera field of view.
The tip penetration and spreading angle of
shadowgraph images are plotted in Fig. 8. To
compare the effectiveness of IcA and IcB for the tip
penetration and spreading angle measurements, the
shadowgraph images of Figs. 5 and 6 were used
because IcA was not successful for the boundary
detection for the schlieren images (see Fig. 7). One
might argue the difference in the tip penetration and
spreading angle between IcA and IcB is due to a
different internal spray pattern. However, it was not a
concern for the tip penetration and spreading angle
because only the droplets at the spray border were
used as per the definitions of this study.
Figure 7: Spray boundaries overlaid on raw and corrected schlieren images. Both method A (left two columns)
and method B (right two columns) are applied. Solid white lines are drawn using a spreading angle provided by the
injector manufacturer (80% mass angle). Shown at the image top-left is time after the start of injection.
Downloaded from SAE International by University of New South Wales, Monday, February 23, 2015
Figure 9 shows that the spray tip penetration
increases with increasing time after the start of
injection. An immediate conclusion from the figure
may be drawn that the spray tip penetration and
spreading angle are not sensitive to the background
correction methods. The tip penetrations of IcA and IcB
are almost identical and so is the spreading angle.
Once again, this may be expected because the
droplets at the spray border were used for the
measurement and the difference in the spray pattern
did not affect global parameters like the penetration
rate and spreading angle.
There are many details to discuss in Fig. 9. For
instance, sharp spikes are seen for both tip
penetration and spreading angle. These are due to
turbulent fluctuations of the spray since the data was
from an instantaneous cycle. Also, it is interesting that
in the tip penetration a discrete step change may be
observed at about 1.2 ms aSOI. This is also
coincident with the higher spreading angle during the
initial transient. It is likely that a higher injection rate in
the earlier stage of fuel injection caused a higher
penetration and dispersion. Since the fuel injector has
a small volume in the nozzle, fuel left in the previous
injection was injected together with the fuel of the
present injection, which temporarily increased the
injection rate. After this initial transient, the spray tip
penetration increases linearly again as shown in Fig. 9.
At the same time, the spreading angle becomes
nearly constant and close to α
80%
.
LIGHT CUT-OFF IN SCHLIEREN IMAGING
Two parameters are important in positioning the
schlieren stop: the amount of light cut-off and
orientation of the stop. If a higher proportion of light is
blocked at the focal point, the schlieren sensitivity will
increase while the overall image will be darkened.
Also, if different orientations of the schlieren stop
relative to the flow axis are applied, different flow
patterns will be seen in schlieren images. For instance,
a schlieren image taken with the knife-edge at right
Figure 8: Image processing to determine the spray tip penetration and spreading angle. Spray images and
boundaries from Figs. 5 and 6.
Figure 9: Spray tip penetrations determined from the
spray boundaries using method A (InA) and method B
(InB) in Fig. 8.
Downloaded from SAE International by University of New South Wales, Monday, February 23, 2015
angles to the flow axis will illustrate axial density
gradients in the flow. Likewise, a schlieren image
taken with the knife-edge parallel to the flow axis will
illustrate transverse density gradients in the flow.
Figure 10 shows schlieren (background) images for
various schlieren stops prior to the fuel injection. In the
top row, “darkening” of the image is observed as the
cut-off ratio increases. Compared to a 0 % cut-off ratio
(shadowgraph), 60 % and 80 % cut-off ratios using a
vertical knife-edge appear to be darker throughout the
image. In the experiments, other cut-off ratios were
also tested; however, 60 and 80 % proportions
showed distinct differences in the spray images and
Figure 10: Schlieren images of the chamber prior to fuel injection (i.e. background image) for various cut-off
proportions and orientations tested in this study.
Figure 11: Effect of cut-off proportion on schlieren spray images. A vertical knife-edge is used. Solid white lines are
drawn using a spreading angle provided by the injector manufacturer (80% mass angle). Shown at the image
top-left is time after the start of injection.
Downloaded from SAE International by University of New South Wales, Monday, February 23, 2015
hence were selected. In the bottom row, background
images with three different schlieren stops are shown.
At a fixed cut-off proportion of 60 %, we tested a
vertical knife-edge and a horizontal knife-edge. A
combination of these two was also examined again
while 60 % cut-off ratio was maintained.
PROPORTION OF LIGHT CUT-OFF
When experimentalists setup a schlieren system, they
adjust a proportion of the light cut-off until the
schlieren images are optimized. The presented
images in the literature, therefore, typically have the
best possible quality and clarity. We followed the
same approach and concluded that about 60 % cut-off
performed best for the gasoline sprays of the present
study. This is well demonstrated in Fig. 11. The
shadowgraph images with no light cut-off show dark
sprays. No internal pattern or turbulent structure can
be seen in this shadowgraph. By contrast, higher
cut-off ratios reveal more information in the spray
pattern as shown in the second and third column of
Fig. 11. Note that the schlieren images were
background corrected (i.e. IcB) and a gray-scale off-set
was added, resulting in brighter spray regions than
those of the shadowgraph images. At an 80 % cut-off
ratio, some regions within the spray were too bright
which hindered the image processing as high
schlieren sensitivity was achieved at the expense of
darkening in the raw images. This meant there was an
optimal cut-off proportion and in this study it appeared
to be 60 %.
While issues discussed above discussion were
expected, Fig. 12 shows an unexpected result. Using
the image processing technique of Fig. 8, the spray tip
penetration and spreading angle were determined and
the results are plotted in Fig. 12. The first noticeable
point from the figure is that the tip penetrations and
spreading angles do not vary much for all cut-off
proportions tested. If the initial transient is excluded,
both the tip penetrations and spreading angles are
very similar. Only difference is seen between the start
of injection and about 1.5 ms aSOI. However, this
initial transient is not repeatable and hence cannot be
used for this discussion. In fact, no monotonic trend
was found with increasing cut-off ratios either during
this initial transient or for the steady period of injection.
Therefore, it was concluded that high refractive-index
gradients near the spray border made no difference in
the tip penetration and spreading angle for varying
cut-off ratios. This conclusion is consistent with Ref.
[4]: essentially that the shadowgraph technique was
sufficient to detect of the edge of the diesel jet.
However, we do not discount the value of visual
information obtained from high-sensitivity schlieren
images and therefore 60 % cut-off ratio was used for
the following section.
ORIENTATION OF SCHLIEREN-STOP
While how much of the light was blocked at the focal
point was important for the schlieren sensitivity and in
turn spray images, in what direction the knife-edge
was applied was also important. For instance, if only a
horizontal knife-edge is used, the schlieren imaging
detects only vertical components
yn ∂∂
in the
schlieren object. Refractions parallel to the edge, due
to
xn ∂∂
, move rays along it but not across it and
therefore there is no change in schlieren images. A
signal with purely horizontal gradients will remain
invisible despite the presence of the knife-edge. This
issue was addressed by varying the orientation of the
knife-edges. Figure 13 shows schlieren images of the
gasoline sprays for a vertical, a horizontal, and a
combined knife-edge. The spray images with 60 %
cut-off ratio from Fig. 11 are shown again as an
example for the vertical knife-edge. Next to it, spray
images with a horizontal knife-edge are shown.
From Fig. 13, one may notice that the horizontal
knife-edge enhanced the schlieren sensitivity in the
transverse direction. Indeed, the horizontal knife-edge
shows more detail in the horizontal direction, in
contrast to the vertical knife-edge that shows higher
gradients in the vertical direction. As a result, the
sprays appear more scattered and dispersed for the
horizontal direction while more bold and stretched
sprays are observed for the vertical knife-edge.
Intuitively, a combination of these two should give the
best result, as the spray images would be optimized in
both directions. A quick answer to this question was
yes, as shown in the last column of Fig. 13. The spray
images with both a vertical and a horizontal
knife-edge do show a better structure and pattern than
the other two cases. For example, noise-like
small-structures near the spray border are filtered out
Figure 12: Effect of cut-off proportion on spray tip
penetration and spreading angle determined from
the spray boundaries in Fig. 11.
Downloaded from SAE International by University of New South Wales, Monday, February 23, 2015
and the “boldness” of the sprays is depressed
resulting in both clear boundaries and turbulent
structures.
While the aforementioned information from a visual
inspection is important, it might be worth to measure
the spray tip penetration and spreading angle for
various knife-edge orientations. Figure 14 shows the
results. Interestingly and similar to Fig. 12, it is clear
that both the tip penetration and spreading angle are
similar for all knife-edge orientations tested. The tip
penetrations show an almost linear increase and
spreading angles converge to α
80%
quickly after the
initial transient.
There are two implications from these observations.
First, the better knife-edge orientation for visualization
of the spray structure and pattern appears to be that
with both the vertical and horizontal cut-off. Although
not quantitative, visual information from the schlieren
images are valuable to study gasoline sprays. In this
regard, a combination of the vertical and horizontal
knife-edge provides optimized images. Second, the
spray tip penetration and spreading angle are not
affected by the orientations of the schlieren stop. This
was because a refractive-index gradient, which was
used to determine the tip penetration and spreading
angle, was very strong near the spray border and
therefore was not sensitive to variations of the
schlieren stop.
Figure 13: Effect of cut-off orientation on schlieren spray images. The cut-off proportion is fixed at 60 %. Solid
white lines are drawn using a spreading angle provided by the injector manufacturer (80% mass angle).
Shown at
the image top-left is time after the start of injection.
Figure 14: Effect of cut-off orientation on spray tip
penetration and spread angle determined from the
spray boundaries in Fig. 13.
Downloaded from SAE International by University of New South Wales, Monday, February 23, 2015
CONCLUSION
Gasoline injection into an optical chamber simulating
engine intake conditions was visualized using a
high-speed schlieren system. A z-type schlieren
system was used to investigate the evaporating
gasoline spray structure and pattern as well as the tip
penetration and spreading angle. Types of schlieren
system and their principles based on the geometric
theory of the refraction were summarized using a
gasoline spray example. Also, details of the z-type
schlieren system setup and application were shared
including off-axis aberration issues, the parabolic
mirror setup, the schlieren stop orientation, and the
techniques used for image processing. Major findings
from this study can be summarized as follows:
1. Two background correction methods were used to
help detect spray boundaries: one in which the
raw image was subtracted by the background
obtained prior to the fuel injection and the other
that used the background from a preceding image
for the correction. For shadowgraph images, two
methods exhibited no difference in the boundary
detection. However, for schlieren images, the
former method showed problem in the detection
of some spray regions. Therefore, the latter
correction method was used in this study.
2. The tip penetration was measured by calculating
the distance between the nozzle and a spray
droplet in the farthest downstream while the
spreading angle was measured using the farthest
spray droplet from the nozzle in the horizontal
direction. The spreading angle definition used in
this study matched well with the manufacturer’s
specification.
3. Transient behavior of sprays in the initial stage of
injection was well captured in the present
schlieren system. The tip penetration showed a
discrete step change between this initial transient
and the steady period of injection. At the same
time, much higher spreading angle was measured
during the initial transient. Higher fuelling rate due
to fuel leftover from the previous injection was
likely cause for this behavior.
4. Various schli eren stops were investigat ed and it
was found that approximately 60% cut-off of light
performed best to image the internal pattern of the
gasoline sprays. Also, a vertical knife-edge
enhanced the spray pattern in the vertical
direction while a horizontal cut-off showed higher
contrast and more turbulent spray structures in
the horizontal direction. As a result, a combination
of horizontal and vertical cut-offs was optimal to
unveil the most of the gasoline spray pattern and
structure.
5. Parameters of the schlieren stops including the
cut-off ratio and knife-edge direction did not affect
the tip penetration and spreading angle due to a
high refractive-index gradient near the spray
border.
ACKNOWLEDGMENTS
The experiments were performed at the Engine
Research Laboratory of School of Mechanical and
Manufacturing Engineering at the University of New
South Wales. The authors would like to acknowledge
that Australian Research Council supported this work
via the Linkage Project (LP110100595).
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CONTACT
Dr Sanghoon Kook: s.kook@unsw.edu.au
Lecturer of School of Mechanical Engineering
Academic-in-Charge, Engine Research Laboratory
University of New South Wales
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