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

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.