Visualization and Analysis of Spray Impingement Under Cross-Flow Conditions

Abstract

Gasoline Direct Injection (GDI) engines are becoming increasingly popular due to the potential to improve power and fuel consumption. The direct injection of gasoline inside an engine is classified in three types: wall-guided; air-guided; and spray-guided (also designated as air-assisted). In the wall-guided, spray impingement on the piston's bowl head is part of the process to form a stratified charge near the spark plug, as well as to aid in the fuel mixture preparation. In the air-guided (swirl and tumble) and spray-guided, the impingement is undesirable. The interaction between the spray and the surrounding air, as well as the characteristics of the spray, are very important to achieve the benefits of the GDI concept. Although many works have been published which consider the structure of individual sprays, much remains to be known about the interaction between the spray and the wall in the presence of air motion. This is the goal of our work. For that an experimental facility has been built to study the fluid and thermodynamic behavior of a gasoline spray impinging onto a cold/heated surface under cross flow conditions. In a first step of our study we intend to build a fundamental basis on the influence of droplet size and velocity relatively to the cross flow and, therefore, used a PFI injector for easy of experimental analysis. Despite this injector operates at low pressure, images of the spray under quiescent conditions showed a spray quality similar to that of a DI injector, without injection of streams of liquid. Also, measurements obtained with a phase Doppler anemometer showed that droplet sizes range between 10μm and 120μm. The present paper reports the results obtained with Mie scattering and shadowgraph using a high speed CCD camera to analyze the influence of the pressure and duration of injection in the spray/wall interaction, considering the presence of different cross flow velocities.

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SAE TECHNICAL
PAPER SERIES 2002-01-2664
Visualization and Analysis of Spray
Impingement Under Cross-Flow Conditions
Miguel R. Panão and António L. N. Moreira
Instituto Superior Técnico
Reprinted From: Gasoline Direct Injection Engines 2002
(SP-1719)
Powertrain & Fluid Systems
Conference & Exhibition
San Diego, California USA
October 21-24, 2002

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2002-01-2664
Visualization and Analysis of Spray Impingement
Under Cross-Flow Conditions
Miguel R. Panão and António L. N. Moreira
Instituto Superior Técnico
Copyright © 2002 Society of Automotive Engineers, Inc.
ABSTRACT
Gasoline Direct Injection (GDI) engines are becoming
increasingly popular due to the potential to improve
power and fuel consumption. The direct injection of
gasoline inside an engine is classified in three types:
wall-guided; air-guided; and spray-guided (also
designated as air-assisted). In the wall-guided, spray
impingement on the piston’s bowl head is part of the
process to form a stratified charge near the spark plug,
as well as to aid in the fuel mixture preparation. In the
air-guided (swirl and tumble) and spray-guided, the
impingement is undesirable.
The interaction between the spray and the surrounding
air, as well as the characteristics of the spray, are very
important to achieve the benefits of the GDI concept.
Although many works have been published which
consider the structure of individual sprays, much remains
to be known about the interaction between the spray and
the wall in the presence of air motion. This is the goal of
our work. For that an experimental facility has been built
to study the fluid and thermodynamic behavior of a
gasoline spray impinging onto a cold/heated surface
under cross flow conditions. In a first step of our study
we intend to build a fundamental basis on the influence
of droplet size and velocity relatively to the cross flow
and, therefore, used a PFI injector for easy of
experimental analysis. Despite this injector operates at
low pressure, images of the spray under quiescent
conditions showed a spray quality similar to that of a DI
injector, without injection of streams of liquid. Also,
measurements obtained with a phase Doppler
anemometer showed that droplet sizes range between
10µm and 120µm.
The present paper reports the results obtained with Mie
scattering and shadowgraph using a high speed CCD
camera to analyze the influence of the pressure and
duration of injection in the spray/wall interaction,
considering the presence of different cross flow
velocities.
INTRODUCTION
In order to build suitable models to accurately describe
the impingement process it is important to have a
fundamental knowledge on the interaction between the
dispersed (spray) and the continuous phase (air) in a
two-phase flow like a liquid spray impinging onto a
surface in the presence of a cross-flow, namely in what
concerns the fluid-dynamics of the fuel/air mixture. Most
of the experimental studies reported in this field consider
the effects of several operational parameters, like the
injection pressure (pinj), the duration of injection (Dtinj),
the injection frequency (finj), the impingement distance
(Zw), and the angle of the injector with the impinging
surface (α), or the air flow velocity (Vc) when a cross flow
is present.
Studies reported in this area include the analysis of the
spray/wall interaction either in Diesel sprays, as in
Meingast et al.[1], Arcoumanis and Chang [2] and in
Özdemir and Whitelaw [3]; or in gasoline injection
systems, as in Park et al. [4], Kawajiri et al. [5] and
Brehem and Whitelaw [6]. Other studies consider the
development of numerical models aimed at describing
the spray wall impingement based on experimental data,
like in Bai and Gosman [7], Lee and Ryou [8], Lindgren
and Denbratt [9], Mundo et al. [10, 11], Rusche [12], and
Tropea and Roisman [13]. Only a few include a cross-
flow with well defined boundary conditions. Arcoumanis
and Cutter [14] studied a Diesel spray impinging onto a
heated surface. The air stream was simulated using a
cross-flow with a boundary layer confined to 1 mm from
the wall. The authors showed that the spatial distribution
of droplet size and velocity was distorted and the spray
width was reduced in the presence of a cross-flow. The

smaller droplets in the spray tip were entrained by the
cross-flow resulting in a larger overall mean droplet
diameters and velocities at the impinging region.
The mechanism of secondary atomization of the
impinging droplets was altered as droplets from the
approaching spray were entrained by the cross-flow,
reducing the droplet mass flux reaching the wall and
shifting the droplet size distribution towards higher
droplet diameters. High-speed photography showed that
the cross-flow entrains the droplets and shifts the
impinging spray downstream, reducing the influence of
the roll-up vortices formed at the head of the spray.
Arcoumanis et al. [15] considered a gasoline injector,
directed at 20º to the bottom flat surface. The spray was
injected against a dry surface or a film, in the presence
of a cross-flow with a mean velocity of 5 and 15 m/s. The
cross-flow was shown to decrease the thickness of the
liquid film formed at the wall between successive
injections and, hence, the secondary atomization
induced by splash. The authors found that a cloud of
smaller droplets was formed immediately above the
region of impingement, and photographs supported the
conclusion that the average size of the droplets formed
by secondary atomization increases with film thickness,
or alternatively, the number of droplets decreased.
As a result of spray impingement, the liquid film
deposited on the surface of impact is one of the most
important sources of hydrocarbon emissions for GDI and
Port-Fuel Injection (PFI) engines, as stated by Li et
al.[16], Landsberg et al.[17] and Hochgreb, S.[18]. Senda
et al. [19] investigated the characteristics of the wall-
wetted fuel film on a flat wall inside a constant volume
vessel with a laser induced fluorescence (LIF) technique,
simulating the film formed in the intake port of a PFI
engine in quiescent conditions. In their research the
authors included the effect of the injection duration, the
impingement distance and the impingement angle in the
fuel film profile. The authors showed that the adhered
fuel ratio is constant and around 40%, and it is
independent on changes of the injection duration. The
authors also divided the radial spreading of the liquid film
in three phases: the extension of the film with a constant
velocity, the gradual decrease of the spreading velocity,
and a last phase when the fuel stops spreading in the
radial direction.
From the reviewed literature, it is apparent that more
experiments are required to provide a better insight into
the mechanisms of spray-wall interaction in the presence
of a cross flow, in order to improve the performance of
practical engines. The present work is aimed at building
an experimental setup to study the two-phase flow
dynamics, including the effect of a turbulent boundary
layer in the impingement region. In a first stage of our
investigation we consider the use of flow visualization
techniques to analyze the effects of the injection
pressure and duration of injection upon the spray
impingement for different cross-flow velocities and upon
the formation of a liquid film.
EXPERIMENTAL SYSTEM
Figure 1 shows a schematic of the experimental
installation. The cross-flow is supplied by a fan to a low-
speed wind tunnel with a contraction ratio of 10 and a
working section with a cross section of 150 mm by 50
mm and 270 mm long, as shown in Figure 2.
A trip wire is located at the leading edge of the bottom
surface of the working section where the spray impinges
to force the transition of the wall boundary layer. In this
way the impingement occurs within a turbulent boundary
conditions with well defined characteristics.
Figure 1 Schematic of the fuel injection rig
Figure 2 Test Rig. 1) Impinging plate; Perspex
window; 3) PFI Injector; 4) Monorail; 5) Barometer; 6)
Fuel-pressure regulator.
The side walls of the working section are transparent to
provide optical access for flow visualization and the
bottom surface upon which the spray impinges may be,
either of Aluminum (12 mm thick) or of Perspex to allow
visualization of the wetted area.
The fuel (Gasoline, density 737 kg/m3) is supplied by a
Denso pump to a BOSCH injector 280 150 726, triggered

by a TTL pulse from an Injector Control Driver controlled
with a function generator computer board, NI5411, from
National Instruments, allowing full control of the duration
and frequency of injection. Downstream the injector, a
fuel-pressure regulator was modified to allow external
control of the injection pressure. At this stage of our work
the values for the injection pressure are characteristic of
PFI engines. The impingement angle is 90º and the
impingement distance was set to 50 mm.
Visualization of the spray/wall interaction is made based
on macroscopic Mie scattering images collected with a
CCD camera. The system makes use of a 9W Argon-Ion
laser, with a beam diameter of 2 mm, which is expanded
into a thin sheet (1 mm) using a cylindrical lens. The
CCD camera is a Kodak SR-Series. All the movies were
recorded with an image acquisition rate of 2000 Frames
per second, a resolution of 256x120 (pixel2) (which is the
maximum resolution provided by the camera) and an
exposure time of 0.1 ms. The same TTL pulse provided
by the injection system to control the aperture of the
injector is used to trigger the CCD camera. Figure 3
shows the configuration of the measuring system.
Figure 3 Schematic diagram of the optical system
used for Mie scattering visualization.
The laser light sheet enters the working section
perpendicularly to the side transparent walls and crosses
the spray through its axis. In order to visualize the wetted
area at the impinging surface, the laser light sheet was
directed horizontally, near the wall, as shown in Figure 4.
The area of impact was then filmed through a “Perspex”
surface using a mirror with an angle (θ) of 45º.
The Mie scattering technique was combined with a
shadowgraph technique to visualize the liquid and the
vapor phase spray plume propagating in the working
section. For that, the laser beam was shifted, expanded
with a magnifying lens and collimated with a plane-
convex lens with an appropriate focal distance. The
collimated laser beam goes through the working section
and is projected upon a white paper sheet, as shown in
Figure 5. Images were recorded with the same CCD
camera used with the Mie scattering technique, with a
viewing area of 52 x 24 mm (W x H).
Perspex
surface
Mirror
High-speed camera
θ
Laser light
sheet
Figure 4 Layout of the Mie scattering system used to
visualize the liquid film at the surface
Figure 5 Setup of shadowgraph system
The flow conditions considered throughout the work are
summarized in Table 1.
Table 1. Experimental conditions
Category Values
Tair (ºC) 25ºC (± 2ºC)
Continuous
Phase Vc (m/s) 5.3, 11.5, 17.3
Fuel Gasoline
Pinj (bar) 2, 3, 4.5
Disperse phase Dtinj (ms) 3, 5, 7, 10, 20
Zw (mm) 50
Impinging
Surface αimpact ( º ) 90
RESULTS AND DISCUSSION
EFFECT OF INJECTION PRESSURE AND CROSS-
FLOW VELOCITY
Figure 6 shows the structure of the free spray in
quiescent surroundings for an injection pressure of 4.5
bar. Despite this injector operates at a lower pressure,
the image in Figure 6 shows a spray quality similar to
that of a DI injector, without injection of streams of liquid.
The spray spreads with an angle of 28º, which is similar
to a narrow-cone DI swirl injector (20º), and
measurements obtained with a phase Doppler
anemometer but not reported here, showed that droplet
sizes range between 10µm and 120µm.
Cross-Flow

When the spray impacts onto the wall, a three
dimensional wall-jet vortex develops from the
impingement region with a donut shape.
Secondary atomization is seen to occur as reported by
Arcoumanis and Cutter [14], Arcoumanis et al. [15] and
Özdemir and Whitelaw [3], due to splashing and rebound
of the incoming droplets, as well as film stripping.
Incoming
Spray
Wall - Jet
Vortex
Droplets from
Secondary
Atomization
Incoming
Spray
Wall - Jet
Vortex
Droplets from
Secondary
Atomization
Figure 6 Spray pattern under quiescent conditions at
4.5 bar (duration of injection = 10 ms)
The results of Mie scattering are compared with
shadowgraphs in Figure 7 to 9 for several flow
conditions. The frequency of injection was set at 10 Hz,
which corresponds to a moderate velocity in typical SI
engines and the duration of injection was set at 10 ms.
An initial injection was made to assure the presence of a
liquid film, simulating cold start conditions. The horizontal
line in the shadowgraphs represents the boundary layer
thickness as obtained from velocity measurements made
for each cross-flow condition and the vertical line
represents the axis of the injector. The frames shown in
the Figures were chosen from the entire films in order to
characterize the behavior of impingement: at spray
impact (t = 3 ms); at half the duration of injection (t = 5
ms); before the end of injection (t = 7 ms); right at the
end of injection (t = 10 ms); and at an instant just after
the end of injection (t = 12 ms). An arrow at the top right
of each image indicates the direction of the cross flow.
A dark region in the shadowgraphs at the surface
indicates the presence of liquid droplets and dimples
indicate the presence of the vapor phase. In the following
analysis, shadowgraphs are compared with Mie
scattering results, which shows light scattered by
droplets. Here, a lower intensity of the scattered light is
associated with a lower concentration of droplets and/or
smaller droplets
The influence of the cross flow on the structure of the
spray is analyzed for cross flow velocities between 5.3
m/s and 17.3 m/s. Although the main goal of our study is
a GDI application, we first consider the use of a PFI
injector in order to build a fundamental basis on the
influence of droplet size and velocity relatively to the
cross flow. In this context, the velocities of the cross flow
were chosen so the interaction with the spray could be
noticed (lower limit) and occurs within the length of the
working section (upper limit).
Figure 7 shows the structure of the spray for a cross-flow
velocity of 5.3 m/s. The results show an increase of the
cone angle of the spray with increasing injection
pressure, as expected due to the fact that a higher
differential pressure enhances droplet breakup at the
nozzle. Right before the end of injection (t = 7ms in
Figure 7) a small roll-up vortex is seen upstream the
impingement region, suggesting the presence of a wall
jet, as also observed by Arcoumanis and Cutter [14] with
a Diesel spray and by Arcoumanis et al. [15] with a
gasoline spray. Therefore, for the lower cross flow
velocity (Vc = 5.3 m/s), two stagnation points are
identified: the first at the impingement point of the
incoming spray; the second as a result of the
deceleration exerted by the cross-flow upon the liquid
film spreading in the opposite direction. The image at t =
7 ms in Figure 7 still shows another roll-up vortex
downstream the impingement region, far from the
impingement point.
The shadowgraphs show the vapor phase around the
incoming spray, which may be convected by the airflow
further downstream before impingement occurs. The
images show a dark layer at the surface in the region
where the spray impinges. This layer is associated, not
only with the formation of a liquid film, but also with the
occurrence of secondary atomization. Therefore, the
vapor layer, which emanates from the surface, is due to
volatilization of the secondary droplets and also of the
liquid film. The interface between this vapor and that due
to volatilization of the pre-impinging droplets forms a
vapor shear layer. This is indicated in Figure 7 (3 bar,
at 5 ms) by a blue line referred as “shear layer”.

t = 3ms
5 mm
t = 5ms
t = 7ms
t = 15ms
t = 10ms
x = 135 mm
t = 3ms
t = 5ms
t = 7ms
t = 15ms
t = 10ms
t = 3ms
t = 5ms
t = 7ms
t = 15ms
t = 10ms
2 bar 3 bar
4.5 bar
Downstrea m
Vort ex
Upstream
Vortex
Figure 7. Mie Scattering and
Shadowgraph results for different
injection pressures (duration of injection
= 10 ms; cross-flow velocity = 5.3 m/s)
Shear layer

t = 3ms
x = 135 mm
5 mm
t = 5ms
t = 7ms
t =10ms
t =15ms
t = 3ms
t = 5ms
t = 7ms
t =10ms
t =15ms
t = 3ms
t = 5ms
t = 7ms
t =10ms
t =15ms
2 bar 3 bar
4.5 bar
Figure 8. Mie Scattering and
Shadowgraph results for different
injection pressures (duration of injection
= 10 ms; cross-flow velocity = 11.5 m/s)

x = 135 mm
5 mm
t = 3ms
t = 5ms
t = 7ms
t = 12ms
t = 10ms
t = 3ms
t = 5ms
t = 7ms
t = 12ms
t = 10ms
t = 3ms
t = 5ms
t = 7ms
t = 12ms
t = 10ms
2 bar 3 bar
4.5 bar
Figure 9. Mie Scattering and
Shadowgraph results for different
injection pressures (duration of injection
= 10 ms; cross-flow velocity = 17.3 m/s)

The vapor layer in Figure 7 increases with increasing
injection pressure, suggesting that the liquid film is
thicker for higher values of pressure. The presence of a
turbulent boundary layer seems to have a relatively small
influence in the spray/wall interaction for lower cross-flow
velocities compared with higher velocities, which is
visible by the presence of the vapor from the secondary
atomization and liquid film in a region above the
boundary layer.
Increasing the cross-flow velocity, the spray structure is
deflected downstream and the thickness of the vapor
layer decreases. In the shadowgraphs reported in Figure
7 for a higher injection pressure (4.5 bar) the shear
layers are slightly visible downstream suggesting that the
wall boundary layer may have a larger effect on the flow
development. This is clear after the end of injection (10
ms) and at 4.5 bar when the cross flow velocity is larger
(Vc = 11.5 m/s in Figure 8).
An increase of the cross-flow velocity also reduces the
influence of the roll-up vortex upstream of the
impingement region. The downstream vortex is not seen
when Vc = 17.3 m/s (Figure 9), so it either appears
downstream or is even inexistent. The magnitude of the
influence of the cross-flow velocity on the spray structure
reduces as the pressure of injection increases, as shown
in the set of images in Figures 8 and 9. This is in
accordance with the fact that higher injection pressures
give rise to larger droplets with larger velocities and,
therefore, less influenced by the air stream.
Liquid film formation after spray impact
The behavior of spray impact was visualized with and
without cross-flow using Mie scattering. The results are
depicted in Figure 10, where the vertical lines represent
the location of the pintle of the injector nozzle. The
Figure shows that without cross-flow, the area of impact
is circular and increases with the pressure of injection.
With cross-flow, the impingement point is shifted
downstream and the area of impact develops into an
ellipsoidal shape. As the cross-flow velocity increases,
successively larger droplets are dragged by the air
stream, which reduces the width of the impact area. This
is more noticeable in Figure 10 at 4.5 bar and is in
accordance with the results reported by Arcoumanis et
al. [15]. For the cross flow velocities considered here, it
was not observed that droplets were prevented of
reaching the wall.
2
b
ar
3
b
ar
4
.5
b
ar
5.3 m/s
11.5 m/s
17.3 m/s
0 m/s
10 mm
Figure 10 Effect of injection pressure and cross-flow velocity in the impingement upon a wetted surface (duration
of injection = 10 ms)

EFFECT OF THE DURATION OF INJECTION
Figure 11 shows shadowgraphs recorded right at the end
of injection for each period of injection. With a higher
duration, more fuel is injected and the adhered fuel area
increases, as also shown by Senda et al. [19]. The figure
shows that increasing the duration of injection, the vapor
layer increases in height. In the near-wall region, where
strong fluid shear is induced, the magnitude of the
Saffman lift force may be significant relative to the drag
force acting upon the impinging droplets (Bai, 1996 [20]).
Also, it is known that fuel volatility affects spray formation
and fuel/air mixing through its effect on drop size (e.g.,
Tong et al. [21]). Therefore, the observed layer growth
suggests that these phenomena, associated with the
mixture capabilities of the turbulent boundary layer, may
influence the fuel/air mixture preparation during cold start
conditions.
Senda et al. [19] showed that the percentage of fuel
which adheres to the surface is independent of the
injection duration. The recorded pictures of the spray
impact verify this statement. Figure 12 shows the
evolution of the shape of the liquid film with the injection
duration, and for the period of 20 ms, two dark areas are
visible indicating that a larger droplet concentration
reaches the wall.
Dt = 3 ms
inj
Dt = 5 ms
inj
Dt = 7 ms
inj
Dt = 10 ms
inj
Dt = 20 ms
inj
5 mm
Figure 11 Effect of the duration of injection on the impingement (injection pressure = 3 bar; cross-flow velocity =
5.3 m/s)
Dt = 3 ms
inj
Dt = 5 ms
inj
Dt = 7 ms
inj
Dt = 10 ms
inj
Dt = 20 ms
inj
10 mm
Figure 12 Effect of the duration of injection on the impingement (injection pressure = 3 bar; cross-flow velocity =
11.3 m/s)

SUMMARY
This paper reports macroscopic visualization of an
impinging gasoline spray under cross flow conditions
using a combination of Mie scattering with shadowgraph
to discern between the liquid and vapor phases
qualitatively. Images of the flow were recorded with a
high speed camera and analyzed in terms of the effects
of cross flow velocity, pressure and duration of injection
on the interaction between the spray, wall and air motion.
The following is a summary of the main observations:
1. The spray angle increases with the injection
pressure and the cross-flow shifts the spray
downstream.
2. Two stagnation points are identified: one at the
impingement point; and another upstream due to
deceleration of the wall liquid jet by the cross
flow.
3. Shadowgraphs show fuel vapor released by the
pre-impinging droplets and released from the
wall liquid film and secondary droplets. At the
interface a shear layer of vapor is observed.
4. The thickness of the vapor shear layer
decreases with increasing cross-flow velocity.
Also, for higher velocities the results suggest the
likely influence of the wall boundary layer at the
impinging plate on vapor dispersion.
5. The liquid film formed at the wall is stretched
between the shear forces exerted by the cross-
flow and the shear forces at the wall. The cross-
flow reduces the spray width and, therefore the
width of the impacting area.
6. The results further suggest the likely influence of
the wall boundary layer on fuel/air mixing either
due to the influence on droplet dispersion, but
also on the dispersion of fuel vapor.
Further developments require a detailed analysis to
account for the influence of size and velocity of the
individual droplets in the poly-dispersion. This may be
done based upon simultaneous measurements of droplet
size, velocity and concentration.
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