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

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