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By Philip Buelow and Steven Smith, Turbine Fuel Technologies,
Goodrich Corporation, West Des Moines, IA
Effective design of high-performance fuel injectors for aircraft and
power-generation gas turbine engines requires a clear understanding of
both the aerodynamic and hydraulic flow fields of the injector. At Goodrich
Corporation’s Turbine Fuel Technologies, FLUENT has been used extensively
for this purpose.
120-degree cut-out of a simplex atomizer showing the liquid fuel (red)
and the air (blue); flow is from left to right 
120-degree cut-out of a pure-airblast atomizer showing the liquid fuel
(red) and the air (blue); flow is from left to right
Injector life and engine performance can
be severely limited by the formation of carbonaceous
deposits within the fuel circuits
and/or on the face of the injector. These deposits
can take the form of varnishes, gums, or soft
or hard carbon, and always form from the fuel.
On internal liquid fuel passageways, they tend
to form if the “wetted wall” temperatures exceed
certain values. Carbon deposits on the face
of the injector are typically due to inadequate
aerodynamic wiping of the face by compressor
discharge air.
CFD simulations using FLUENT have
become the mainstay at Turbine Fuel
Technologies for predicting the likelihood
that a nozzle will form carbon deposits.
Predictions of heat transfer coefficient
have been used effectively to estimate the
wetted wall temperatures within the liquid
fuel circuits in order to design injectors with
a reduced propensity to form deposits. Time
and again, FLUENT has proved to be an invaluable
tool for predicting the presence of flow
field features that are historically related to
carbon formation, and for guiding design
changes to prevent it from happening.
One of the primary functions of a fuel injector
is to atomize the fuel into very small droplets
so that it can adequately mix with air for the
combustion process. Recently, Turbine Fuel
Technologies has used FLUENT’s VOF model
to simulate the formation of the thin liquid
fuel film1, which is a precursor to atomization.
In the simplex atomizer, the fuel enters
the spin chamber through angled spin-slots,
which impart a strong swirling motion to the
flow. As the flow exits the atomizer through
the orifice, it spreads out into a conical sheet.
A key characteristic of such flows is the formation
of an air core along the centerline
of the atomizer. This air-core typically
extends all the way to the back end of the
spin chamber, and is correctly captured by
the FLUENT simulation. Other key characteristics
are the film thickness, film velocities,
and the angle of the conical sheet exiting
the atomizer. These parameters can be taken
from the FLUENT simulation and input into
Turbine Fuel Technologies’ proprietary software to estimate film
break-up lengths and droplet Sauter Mean Diameters (SMDs). In contrast
to simplex atomizers, which utilize high-pressure in the fuel circuit
to drive the atomization process, pure-airblast atomizers use relatively
low-pressure fuel along with high velocity air adjacent to the fuel film
to drive the atomization process. A recent FLUENT simulation modeled a
pure-airblast atomizer under liquid-only operation (i.e. no driving air-pressure)
so that a distinct conical fuel film could be observed. The CFD results
were compared with an experimentally determined cone angle, and yielded
reasonable agreement, with the cone angle underpredicted by only 5.5%.
Further results on the pure-airblast simulations can be found in Reference
1.
Comparison of cone angle between experiment (128°) and CFD (121°)
for a pure air-blast atomizer operating at a mass flowrate of 0.0139 kg/sec
(110 lbm/hr)
Reference
1 Buelow, P.E.O., Mao, C-P., Smith, S., Bretz, D.,
“Application of Two-Phase CFD Analysis to a
Prefilming Pure-Airblast Atomizer,” AIAA Paper
2001-3938, July 2001.
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