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Courtesy of Renault
At the Paris, France base of Renault, many different CFD simulations
are carried out each year.Some of the most novel of these applications
involve the use of FLUENT in paint spraying and drying processes for full
body or individual automotive components.
One such application is the simulation of a paint spray gun whose operation
is augmented by an electric field. The spray gun nozzle consists of six
long cylindrical electrodes in an annular locus around a cylindrical hub
that has a rotating bowl at its tip. The rim of the bowl has a slot for
a thin air curtain to emerge during spraying. Upstream of the bowl is
an annular array of paint jet holes. During normal operation, jets of
paint emerge from the holes and break up into droplets as they hit the
spinning bowl. The resultant spray is directed via the air curtain towards
the target car component or panel.
Paint spray gun nozzle
To generate the electric field, the six electrodes are charged to one
potential and the target component is grounded. The electric field causes
the paint droplets to be charged and focused onto the panel more accurately
than if the field were not used. Several process parameters are of prime
importance to the Renault engineers. These include the shape of the paint
spray, the trajectory of the droplets, and the final thickness of the
deposited paint, which has to be as uniform as possible for a wide range
of component shapes and curvatures. Using the discrete phase model (DPM)
for the paint spray, the engineers created special user-defined functions
(UDFs) to simulate the effects of the electric field on the charged droplets.
This CFD modeling approach was validated against f lat plate deposition
tests and was used to model paint dissolved in both a solvent base and
a water base. The experimental tests showed good agreement with the CFD
model.
A typical CFD prediction of the paint spray gun delivering a spray of
charged droplets (using Lagrangian tracking) that impinge upon a car's
wing panel after exiting the spray gun in an electric field.
A second automotive painting application validated at Renault is the
paint drying cabin, which is used to dry cars that have been positioned
in the cabin and sprayed. In essence the cabin is a ventilated room where
it is desirable to have a homogeneous temperature field. Good flow patterns
in the room and uniform gas and surface temperatures are therefore critical
to its optimal performance. A complicating factor with drying cabins is
that the room air flows and temperatures are periodically cycled. Understanding
how these unsteady conditions impact the performance of the room is one
of the key outputs from the CFD simulations. In a recent validation test,
good agreement between point temperatures measured at specific locations
on a car and time-dependent CFD predictions gave Renault engineers confidence
in the technique.
Ventilation flow velocities in a paint drying cabin
These two examples illustrate how the automotive paint fabrication process
at Renault is being improved by integrating CFD into their design process.
CFD has helped Renault reduce risks, lower production costs, and improve
the quality of the painting process.
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