Cavitation and Hydraulic Flip in the Outward-Opening GDi ... - Delphi

Copyright © 2009 SAE International
ABSTRACT
Experimental (optical imaging) and CFD investigations of
the cavitating flow in a transparent large-scale model of a
GDi outward-opening injector, with a swirler-type valvegroup,
has been performed. The objective is to elucidate
the flow structure within the valve group and assess a
Reynolds-Averaged Navier-Stokes (RANS) multi-fluid
simulation method.
The optical imaging data show evidence of cavitation
inception in the valve-group swirler-channels - caused by
the rapid flow acceleration - and multiple cavitations
within the conical nozzle region.
The comparison of simulations with data shows that the
CFD method reproduces the fluid dynamics of the valvegroup
with good qualitative agreement with the imaging
data (with respect to the cavitation inception and
geometry) and excellent quantitative agreement of the
valve-group pressure drop - flow rate characteristic. In
addition, the simulations reveal that the apparent
cavitation in the conical nozzle region involves a “partial
hydraulic flip” phenomena and entrainment of the
ambient air.
INTRODUCTION
The gasoline direct-injection (GDi) spray-guided
combustion technology has been adopted by the
European automotive industry as the primary gasoline
engine technology progression to meet the economical
and legislative pressures for reduction of the automotive
fuel consumption and combustion emissions[1,2]. It
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the paper.
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*9-2009-01-1483*
Printed in USA
2009-01-1483
Cavitation and Hydraulic Flip in the Outward-Opening GDi Injector Valve-Group
B. Befrui, G. Corbinelli and G. Hoffmann
Delphi Customer Technology Centre , G. – D. Luxembourg
R. J. Andrews and S. R. Sankhalpara
Delphi Technical Centre Gillingham, U.K.
offers the largest potential for reduction of gasoline
engine fuel consumption and enables implementation of
the advanced combustion strategies (controlled auto–
ignition, rapid catalyst warm-up, etc.) envisioned to
reduce combustion emissions.
A key component of the GDi system is the high-pressure
fuel-injection system, specially the fuel injector which
must posses excellent fuel metering, multi-pulse
capability and spray atomization / mixture preparation
characteristics. The injector design of choice for the
close-arranged spray-guided combustion system is the
pintle-type, outward-opening concept (that produces a
hollow conical spray) due to its superior spray
atomization, penetration and deposit robustness
characteristics. A critical stringent requirement, with
respect to the GDi outward-opening injector, is the
structural stability (i.e. stability of the spray cone angle
against increase of ambient pressure and temperature)
of the hollow–cone conical spray, irrespective of the
cylinder thermodynamic conditions. (It is worth noting
that, dependent on the engine operation conditions, the
cylinder pressure at the time of fuel injection could be in
the range 0.04 – 2 MPa.)
The experience with the development of the GDi
outward-opening injector has shown strong dependence
of the spray primary breakup pattern, atomization
characteristics and structural stability on the subtle
features of the valve-group design [3 - 5]. It has
underscored the intricate influence of the flow
characteristics (streamwise velocity profile, turbulence,
vorticity, etc.) at the nozzle exit on the critical spray
characteristics. In this respect, the implications of
inception of flow cavitation in the injector valve-group,

caused by inevitable variations of the fuel temperature
and/or composition, that could simultaneously affect the
static flow rate and engender major alterations of the
nozzle exit velocity characteristics, are of serious
concern.
There is a need for development of a fundamental
knowledge of the high-pressure, hollow-cone sprays of
the GDi injectors, with respect to the spray atomization
characteristics and its dependence on the valve-group
design and the fuel system operation parameters. An
integral element of this activity is the development,
validation and application of computational methods to
assist analyses of the relationship between the fluid
dynamics of the injector valve-group and the associated
spray atomization characteristics. Therefore, joint optical
/ laser diagnostics experimental and advanced
computational activities have been undertaken at the
Delphi Customer Technology Center Luxembourg to
address the dual objectives for development/validation of
experimental/analytical methods and establishment of a
broad knowledge of the GDi hollow-cone spray
atomization characteristics necessary to meet the
customer-specific requirements for a robust and stable
spray formation [5 , 6].
The present study is concerned with experimental and
computational investigation of the flow in a large-scale
model of a GDi outward-opening valve-group that
incorporates a swirler component upstream of the
conical nozzle. The incorporation of the swiler serves two
purposes: it enables accurate static-flow setting (and
reduces the fuel metering sensitivity to variations of pintle
stroke) and imports tangential momentum onto the flow,
thereby improving the spray penetration and atomization
characteristics. However, it poses concerns with respect
to possible cavitation inception, within the swirler
passages and the conical nozzle, and the consequent
influences on the spray structure. These could engender
circumferential non-uniformity of the liquid sheet
thickness and nozzle-exit velocity, with notable effects on
the spray primary breakup and atomization
characteristics. A second topic of interest is whether a
transition of cavitation to full/partial hydraulic flip will
occur within the conical nozzle, for the fuel pressure
ratios comparable to the GDi fuel system nominal levels.
The cavitation phenomena in the fuel injection nozzles
and its effect on the spray atomization characteristics
has been investigated for the cylindrical nozzles - as the
most common industrial injector nozzle geometry - in
relation to the full-cone sprays of diesel injectors [7 - 11]
and other industrial fields of application [12]. In general a
direct correlation between cavitation inception in the
nozzle-hole and enhancement of the liquid-jet
atomization has been observed. More recently, the
transformation of cavitation to hydraulic flip has become
the focus of attention, due to its causing the irregular
“flipping” of the spray trajectory [13].
The hydraulic flip phenomena is closely – and
predominantly - associated with the cavitation in the
nozzle of high-pressure full-cone liquid jets. It is referred
to as the condition whereby the cavitation bubble
exceeds the “super-critical cavitation” condition , extends
beyond the nozzle domain and merges with the ambient.
Consequently, the ambient air (which normally is at
higher pressure than the vapour saturation pressure) is
drawn into the cavitation domain and forms a “cushion”
between the liquid core and the nozzle walls. This has
remarkable influence on the structure and breakup of the
issuing liquid jet, through (a) inhibition of the liquid-phase
near-wall turbulence production and (b) enhancement of
the liquid-air interface instability dynamics. Also,
dependent on the nozzle geometry and entrance flow
irregularity, there is high probability of asymmetric or
partial hydraulic flips which engender sudden change of
the spray geometry and trajectory [13].
The experimental investigations of the flow in the largescale
models of the GDi outward-opening injector show
evidence of presence of a gaseous-phase within the
nozzle domain (i.e. upstream of the nozzle exit) which is
often interpreted as flow cavitation. However, recent CFD
Large-Eddy Simulations (LES) have revealed occurrence
of “ingestion” (or entrainment) of ambient air into the
injector nozzle domain, with substantial influence on the
nozzle-exit structure of the conical-sheet liquid-jet and
its near-field breakup pattern [5]. In the context of the
present study, partial hydraulic flip is adopted broadly for
an entrainment of the ambient air into the nozzle domain,
that causes formation of an “air-cushion” that separates
the liquid flow from the conical nozzle walls.
The large-scale experimental investigation is
complemented by the CFD multi-fluid Reynoldsaveraged
Navier-Stokes simulations of the two- and
three-fluid flow within the injector valve group and its
neighboring downstream ambient. The objective is to
investigate the capability, the qualitative and the
quantitative accuracy of current CFD method – as
available in commercial CFD codes - for prediction of the
cavitation inception, the cavitation geometry and the
cavitation effect on the valve-group hydrodynamic
parameters. For this purpose, specifically a valve group
design with experimental evidence of multiple cavitation
regions has been selected, because of the specific
interest to assess the capability to accurately predict
distinct, multiple cavitation regions within the simulation
domain (in particular to verify that inception of cavitation
at a flow upstream location does not affect the prediction
of its inception and geometric features at a downstream
location).
EXPERIMENTAL INVESTIGATIONS
The large scale model of a gasoline direct injection (GDi)
injector’s outward-opening valve-group – with alternative
swirler geometries - is constructed and tested in the
Physical Modelling Group at the Delphi Technical Centre

in Gillingham. The experimental methodologies applied
are photographic imaging, in conjunction with
measurements of the pressure and flow rate. The focus
of the study is to obtain basic flow information regarding
the condition of flow within the injector – with the aid of
imaging and pressure measurement at several locations
- and the spray. This information is intended to assist
development of the valve-group design and support
validation of a current state-of-the-art CFD multi-phase
simulation method.
The large-scale modelling uses the principles of
geometric and dynamic similarity for steady state flow
conditions. In this respect, the two most relevant nondimensional
groups are the Reynolds number and
cavitation number, to match the model operation
condition with that of the actual-size counterpart. The
Reynolds number similarity takes account of the different
properties of the actual-size fluid (i.e. n-Heptane) and the
large-scale working fluid (i.e. ISO4113 calibration fluid).
Table 1 presents the correspondence between the largescale
model tests and the actual-size injector operation.
Actual size Model (x35)
Liquid Properties
Working Fluid n - Heptane
ISO 4113
calibration
fluid
Viscosity [mm2/s] 0.575 2.75
Density [kg/m3] 680 817
Vapour Pressure [bar] 0.0413 0.002
System Pressure [MPa]
5 0.112
10 0.224
15 0.337
20 0.449
Ambient Pressure [MPa]
0.4 0.009
Table 1. Correspondence of the Large-scale model
tests and actual-size injector operation
EXPERIMENTAL FLOW RIG - Figure 1 shows the flow
rig and the large-scale model in situ. The static
pressures at the tappings are measured with a
Scanivalve unit using a calibrated transducer, and
processed / stored via a personal computer. The flow
rate is continuously monitored with a vane-type flow
meter. In order to create the correct flow cavitation
number, the GDi valve -group model discharges into a
vacuum chamber.
LARGE-SCALE VALVE-GROUP MODEL - The model is
shown in Figure 2. It is modular in construction to allow
design changes to be easily implemented. The main
housing and the swirler are manufactured from acrylic
material to allow viewing of the cavitations regions in the
fluid flow. The pintle can be adjusted to be concentric or
eccentric in the guide bore. A leakage path orifice is
fitted into the swirler plate to model leakage flow through
the pintle - guide clearance annulus.
Figure 1. The in-situ assembled model and the test
apparatus
Figure 2 : construction of the large-scale model and
arrangement of the pressure tappings (the pintle is
shown seated)

OPTICAL IMAGING RESULTS - The experimental test
facility was utilized to investigate alternative valve-group
design concepts, various geometric design parameters
and the influence of fuel system pressure modulation.
Herein, a sample of data that exhibits the salient features
of the complex cavitating flow in the valve-group is
presented.
The present investigations pertain to a swirler design
with five “concentric” radial swirler-arm pattern (that bydesign
does not generate flow rotation in the conical
nozzle section) and zero pintle – guide leakage flow rate
condition. The objective of this valve-group design is to
focus on the cavitation formation in the swirler channels
and in the conical nozzle section, without the additional
flow complexities associated with the flow rotation.
These additional complexities include (1) the effect of the
centripetal pressure gradient and (2) the complex flow
streamline that would make identification of the sources
of flow cavitation within the conical nozzle difficult.
Several measures have been taken to ensure that the
cavitation is representative of actual conditions: (1) the
test-rig has a closed-loop recirculating fuel system (2)
the model is always run for a length of time before image
capture, in order to purge the air and ensure there is no
air trapped/entrained in the flow, (3) dissolved air is kept
to a minimum by maintaining the fuel tank under vacuum
conditions, even when the system is not running (this
includes overnight). The imaging data represents the
steady-state flow condition.
Figure 3 presents the optical imaging results of the flow
within the large-scale valve-group model pertinent to the
fuel pressure of 5 MPa and ambient (i.e vacuum
chamber) pressure of 0.4 MPa.
Cavitation in the
swirler channels
Nozzle entrance
seat-edge
“Ring” cavitation
at nozzle
entrance
Separate
cavitation zones
Nozzle exit
seat-edge
Figure 3 : Large-scale model imaging data for fuel
pressure of 5 MPa (pressure values correspond to
the actual-size injection conditions)
Figure 4 illustrates the influence of fuel system pressure:
it presents optical imaging results of the flow within the
large-scale valve-group model, for conditions
corresponding to fuel pressures of 5, 10, 15 and 20 MPa
and ambient (i.e vacuum chamber) pressure of 0.4 MPa.
The most noteworthy features of the visible flow
cavitations are:
• Inception and development of a super-critical
cavitation region within the swirler channels, due to
the sudden flow acceleration at entrance to the
swirler channels.
• Insensitivity of the super-critical cavitation region in
the swirler channels to the increase in the fuel
pressure from 5 MPa to 20 MPa.
( a ) Injection pressure =5 MPa
Ambient pressure = 0.4 MPa
( b ) Injection pressure =10 MPa
Ambient pressure = 0.4 MPa
( c ) Injection Injection pressure =15 MPa
Ambient Ambient pressure = 0.4 MPa
( d ) Injection pressure =20 MPa
Ambient pressure = 0.4 MPa
Cavitation in the
Swirler channels
Cavitations in the
conical nozzle
Figure 4 : Large-scale model imaging data for fuel
pressures of 5 – 20 MPa (pressure values correspond
to the actual-size injection conditions)

• Formation of a cavitation ring, with irregular inception
locations around the circumference, at the entrance
to the conical nozzle domain of the valve-group
• Additional downstream separate cavitation regions
(patches) with limited circumferential spread, within
the conical nozzle domain (At 5 MPa fuel pressure, it
appears as if the cavitation is caused by the
presence of the pressure tapping holes. This is not
the case, as evidenced by absence of a
correspondence at higher fuel system pressures).
• Enhancement of the intensity and increase in the
number of the cavitation zones (patches) in the
conical nozzle, with the increase of the fuel pressure
from 5 MPa to 20 MPa.
Although the imaging data provides valuable information,
the limitations of the optical arrangement does not permit
distinction between cavitation inception on the pintle or
the nozzle-seat walls. This is considered an important
information with respect to the velocity-profile of the
liquid jet at the nozzle exit and its potential influence on
the deviations of the spray cone-angle from that intended
by the nozzle design. Another issue of interest that the
experimental arrangement – and the imaging data –
does not allow is to discriminate between cavitation and
partial hydraulic flip: i.e. whether the “apparent” cavitation
in the conical nozzle domain is indeed cavitation (i.e.
liquid to vapour phase-transfer) or is a visual
consequence of the entrainment of ambient air into the
nozzle domain. (This would require injection into a liquidfilled
ambient, which would inhibit visualization of the
conical-sheet breakup structure.)
COMPUTATIONAL INVESTIGATIONS
The computational studies are undertaken in an attempt
to assess the capability of current CFD methodology to
elucidate the aforementioned uncertainties with regards
the imaging results, as well as to establish the qualitative
and quantitative accuracy of the method to support the
valve-group design optimization process.
The commercial CFD code AVL-FIRE [14] is used for the
simulation of the multi-fluid (liquid, vapour, ambient air)
flow within the injector and its near-field ambient
environment. The computational method adopts an
incompressible multifluid approach that is based on the
finite-volume discretization and numerical solution of
independent (but coupled) sets of conservation
equations for the liquid and the gaseous (fuel vapour, air)
phases [15]. The set of conservation equations comprise
the Reynolds-averaged equations of the conservation of
mass, momentum (Navier-Stokes equations) and the
total enthalpy of the components. In addition, transport
equations for the k-epsilon turbulence model are solved
to account for the stochastic effects of turbulence on the
transport process. The simultaneous solution of the
three equation system enables determination of the fielddistributions
of the liquid, vapour and air volume
fractions.
In the present publication, the results of iso-thermal twofluid
(liquid, vapour) and three-fluid (liquid, vapour,
ambient air) simulations are presented, in order to
underscore the significant difference in the simulation
results - and interpretation of the physical flow
phenomena within injector valve group - between the two
simulation cases.
COMPUTATIONAL DOMAIN AND MESH - Figure 5
presents the solution domain and the computational
mesh for calculation of the two-fluid (Fig. 5.a ) and threefluid
(Fig. 5.b) flows. The computational domain for the
two-fluid calculations is limited to the injector internal
space (it is bounded by the nozzle exit surface), while
the computational domain for three-fluid calculations
extends into the ambient air.
The circumferential cyclic-symmetry and the planar
symmetry of the injector valve-group geometry is
exploited to limit the solution domain to a 1/10th segment
of the complete geometry.
Swirler channel
Swirler cavity
Pintle
Conical nozzle
Seat
(a) Two-fluid flow
simulation
(b) Three-fluid flow
simulation
Figure 5 : Computational flow domain and mesh of the
valve-group for two-fluid (liquid + vapour) and threefluid
(liquid + vapour + ambient air) flow simulations
INITIAL AND BOUNDARY CONDITIONS - The
methodology for simulation of the flow and cavitation
requires prescription of the liquid- and the gaseousphase(s)
thermo-physical conditions, for the initial
conditions and at the boundaries of the solution domain.
With respect the gaseous phase(s), this entails
additional description of the physical conditions (c.f.
bubble number density) [14].
The initial conditions for all calculations are the injector
internal flow domain filled with liquid, at rest, with
nominal-zero (1. e-6) volume-fraction(s) of the gaseous
phase(s). In the case of three-fluid calculations, the

ambient domain contains air at rest (with relevant
thermodynamic conditions).
In the present calculations, the flow boundary conditions
at the inlet are prescribed static pressure, nominal-zero
vapour and air (for three-fluid cases) volume fractions
and physical conditions (bubble number density). The
boundary condition at outlet is prescribed static pressure.
MULTI-PHASE FLOW SIMULATION RESULTS - Isothermal,
two-fluid and three-fluid simulations have been
performed for fuel system pressure variation between 5 –
20MPa and the quantitative (pressure, flow rate) and
flow field results were checked against the
measurements and the experimental imaging data. In
the present article, selected simulation flow-field results
for the injection conditions of 5 and 20 MPa fuel pressure
and 0.4 MPa ambient pressure (for the fuel and ambient
temperature of 23º C) are presented, analyzed and
compared with the imaging data. These results are
representative of the complexity of the liquid flow,
cavitation and partial hydraulic flip in the valve group and
the level of agreement with the experimental data.
Global Flow Parameters - Figure 6 presents the
predicted and measured variations of the injector (liquid)
flow rate as a function of the fuel system pressure in the
range 5 – 20 MPa. The “theoretical” curve corresponds
to a single-phase liquid flow with a valve-group discharge
coefficient Cd =1. The predictions represent both the
two- and the three-fluid simulations, as the predicted
pressure drops and the flow rates are indistinguishable
for the two simulation approaches. It is notable that , in
spite of the super-critical cavitation formation in the
swriler, the valve-group flow rate does not exhibit the
chocked-flow condition. The comparison indicates good
quantitative accuracy, especially in view of the complex
flow - that will be discussed in the succeeding sections.
Flow rate [cm3/s]
25.00
20.00
15.00
10.00
5.00
0.00
Theoretical
Measurement w/out Leak
Simulation
0 20 40 60 80 100 120 140 160 180 200
Pressure Drop [bar]
Figure 6 : Comparison of the predicted and measured
valve-group liquid flow rate, as function of the fuel
system pressure (ambient pressure = 0.4 MPa)
Figure 7 presents the measurement and simulation
results of the static pressure, at the selected “pressure
tapping” locations within the valve-group, for the nominal
fuel system pressure of 20 MPa. The results show
excellent quantitative agreement of the predicted and
measured pressure drop within the valve group. The
significant pressure drop across the swirler is notable: it
is caused by the combination of conversion of potential
to kinetic energy, due to the flow acceleration at entrance
to the swirler (with remarkable influence with respect to
the cavitation inception) and the associated pressure
losses. The pressure variations indicate that the swirler
geometry plays a dominant role in control of the injector
flow rate.
P ressure [bar]
240
220
200
180
160
140
120
100
80
60
40
20
Pressure tapping #1
Pressure tapping #2
Pressure tapping #3
Pressure tapping #4
Measurement
Simulation
0
0 1 2 3 4 5
Pressure Tapping Streamwise Position (No.)
Figure 7 : Comparison of the predicted and
measured variation of the static pressure in the
valve-group (fuel pressure = 20 MPa , ambient
pressure = 0.4 MPa)
Three-Dimensional Flow Fields : Two-Fluid Calculations-
A common practice in calculation of multi-phase injector
internal flow is to confine the solution domain to the
injector internal volume. This enables simulation of the
liquid phase and the phase transfer caused by the
inception of cavitation due to localized reduction of the
static pressure (owing to flow acceleration or pressure
loss) to a level below the thermodynamic fuel vapour
saturation pressure. This practice, however, suffers from
the risk and limitations that : (1) the simulation of the
cavitation in the nozzle can be influenced by the
proximity of the outlet boundary condition, and (2) it
does not enable investigation of the growth of the
cavitation until it merges to the ambient. In such a
circumstance, it is expected that the ambient air
(normally at higher absolute pressure than the fuel
vapour saturation pressure) will cause formation of a
partial/full hydraulic flip phenomenon.
In the present publication, the simulation results for the
two schemes are discussed in conjunction with the
imaging data.

Fuel System Pressure = 5 MPa - Figure 8 provides a
description of the flow and cavitation in the nozzle, with
the aid of the field plots of the pressure, liquid-phase
velocity and the vapour-phase volume-fraction. The
results show rapid flow acceleration at entrance to the
swirler, accompanied with major static pressure drop in
the swirler channel. There is indication of a ring
cavitation at entrance to the swirler, with an uneven
growth to form a super-critical cavitation bubble (on the
flow-approach circumference of the swirler channel wall).
The static pressure field-plot shows a static pressure
recovery in the swirler cavity (downstream of the
channels) and formation of a stagnation region due to
swirler flow – pintle interaction (shown by the flow
streamline plot). Evidently, this prohibits the extension of
the super-critical cavition bubble into the swirler cavity.
The simulations predict inception of a cavitation ring on
the seat edge at the entrance to the conical nozzle
section, that extends to the nozzle-exit circumference in
the form of a cavitation annulus adjacent to the nozzle
seat surface.
Figure 8: Two-fluid simulation results for fuel pressure
of 5 MPa (ambient pressure = 0.4 MPa)
Fuel System Pressure = 20 MPa - Figure 9 presents the
simulation results for the 20 MPa fuel system pressure.
These show close similarity of the liquid phase flow and
the cavitation field to the counterparts for the 5 MPa fuel
system pressure. The most noteworthy aspect of the
results is the insensitivity of the super-critical cavitation in
the swirler channels to the increase of the fuel system
pressure. The cavitation annulus in the conical nozzle
region is also similar to that of 5 MPa case: the notable
difference is the partial cavitation (the region
downstream of the swirler channel), that indicates a
circumferential non-uniformity of the static pressure
within the conical nozzle region. Notably, examination of
the flow field at the nozzle exit shows good
circumferential uniformity of the velocity profile, which
confirms the capability of the nozzle design to fulfill this
requirement, without swirl assistance.
Figure 9: Two-fluid simulation results for fuel pressure
of 20 MPa (ambient pressure = 0.4 MPa)
The predicted features of the cavitation in the swirler
channels, for instance the inception location, supercritical
geometry, insensitivity to the fuel system
pressure, are all in agreement with the imaging data in
Figure 3. The predicted inception of an annular cavitation
region at the seat edge appears to be in broad
agreement with the imaging data (e.g. note the irregular
circumferential separation region at the entrance to the
conical nozzle section, in Figures 3(a) – 3(d)). However,

Figure 10: Three-fluid (liquid, vapour, air) simulation results of the stationary flow field
( fuel pressure = 5 MPa, ambient pressure = 0.4 MPa)
Figure 11: Three-fluid simulation results of transient developments of cavitation and partial hydraulic flip
( fuel pressure = 5 MPa , ambient pressure = 0.4 MPa)
as underscored previously, the imaging data does not
permit unambiguous conclusions with regards to the
cavitation inception location (on the seat and/or pintle
surfaces) and geometry.
Three-Dimensional Flow Fields : Three-Fluid
Calculations – The following provides three-fluid
simulation results to underscore the influence of
inclusion of the ambient into the calculations.
Fuel System Pressure = 5 MPa - Figure 10 presents a
description of the flow, through the field plots of the
pressure, liquid-phase velocity and the vapour-phase
and air volume-fractions. The general liquid-phase flow
structure, and the cavitation within the swirler component
(upstream of the conical-nozzle region), are almost
identical to the two-fluid model calculations (within the
accuracy of the solution method), as evidenced by the
features of the super-critical cavitation in the swirler

channel. However, the predicted flow within the conical
nozzle is markedly different from the two-fluid
simulations, as illustrated by the results in Figures 10
and 11.
The iso-surface plots of the fuel vapour and air in
Figure 11 reveal a complex transient flow within the
conical nozzle, with a fast time-scale. They portray the
following sequence: (a) formation of a cavitation ring, on
the seat edge, at the nozzle entrance, (b) the growth of
the cavitation ring at the nozzle seat edge into a
cavitation annulus that extends to the nozzle exit, (c)
concurrent flow acceleration and formation of cavitation
on the pintle surface that extends to the nozzle exit,
(d) air entrainment into the cavitation bubbles and
formation of a partial hydraulic flip – extending almost the
full nozzle length – on the pintle surface (concurrent with
the flow re-attachment on the seat surface). The
entrapped air, collocated with the cavitation bubble at
the seat-edge in Figure 11 is a remnant of this transient
process.
The three-fluid simulations of the injector valve-group
provide interesting and significant results that indicate
concurrent formation of cavitation and partial hydraulic
flip in the injector valve group. As noted earlier, the
imaging data do not permit differentiation between
cavitation and partial hydraulic flip, so experimental
confirmation of this results is not readily possible.
However, a careful examination of the imaging data for
the 5 MPa fuel pressure in figure 3 provides indications
in support of the predicted phenomena in Figure 10
(specifically, the irregular cavitation ring at the conical
nozzle entrance that appears distinct from the patches of
intense two-phase interactions downstream of the
pressure tappings, suggesting presence of separate
liquid-gas phase interface regions on the pintle and the
nozzle seat). Overall, the three-fluid simulation are more
physically valid than the two-fluid calculations with
respect to the cavitation – partial hydraulic flip
phenomenon in the nozzle (as evidenced by the imaging
data) and provision of an insight into the potential flow
complexities.
It must be remarked that the methodology, application
range and accuracy of the cavitation simulation method,
the nozzle geometry and the flow boundary conditions
are of critical importance with regards the predicted flow
structure.
It is worth mentioning that although the air-entrainment
phenomenon is similar to that observed in the LES
simulations of the flow within the GDi conical nozzles, the
RANS methodology is incapable of predicting the
associated non-uniformity of flow separation and liquid –
air interface instabilities that are responsible for the
particular “jet string” primary breakup structure of the
conical-sheet liquid-jets [5]. This is due combined
absence of high-resolution simulation of the turbulent
large-scale motions (a feature of the Reynolds-averaged
treatment of the turbulence in the RANS methodology)
and accurate numerical treatment of the air – liquid
interface (e.g. via the VOF interface capturing/tracking
method [5]).
CONCLUSIONS
The experimental imaging data reveal inception of a
super-critical cavitation bubble in the swirler channels,
independent of the fuel system pressure in the range 5 –
20 MPa. They provide evidence of a complex two-phase
flow, with multiple cavitation zones, within the conical
nozzle region.
The computational method provides good quantitative
accuracy in prediction of the static pressure variation in
the valve-group, the flow rate and the effect of flow
cavitation. Hence, it offers a design optimization tool for
the valve-group with regards the hydrodynamic flow
parameters. In the present analyses, the two-fluid and
the three-fluid calculation schemes provide similar level
of predictive accuracy for the flow rate - pressure drop
relationship: this can be explained as a consequence of
the over-riding effect of the swirler geometry - and the
associated super-critical cavitation - on the flow rate.
The predictions of the cavitation structure within the
swirler channels and its insensitivity to the fuel system
pressure are in excellent agreement with the data. With
respect to the complex flow structure in the conical
nozzle, the experimental data is not sufficiently
unambiguous to ascertain the accuracy of the
simulations. Nevertheless, the imaging data provides
sufficient evidence in support of the three-fluid
simulations which point to the simultaneous presence /
conversion of flow cavitation, separation and partial
hydraulic flip within the conical nozzle. In this respect, the
simulations provide insight into the potential flow
complexity and assist interpretation of the imaging data.
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