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

• 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