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Rock 1
EXPERIMENTAL STUDY OF A STRUT INJECTOR FOR
CIRCULAR SCRAMJET COMBUSTORS
Christopher Rock
Graduate Research Assistant
and
Joseph A. Schetz
Advisor, Holder of the Fred D. Durham Chair
Department of Aerospace and Ocean Engineering
Virginia Tech, Blacksburg, VA, 24061
Supersonic combustion is a major challenge in scramjet engine design. Supersonic fuel injection
and mixing research contributes to the effort to make the scramjet a viable option to power
hypersonic aircraft, economical and reusable launch vehicles, and hypersonic missiles. An
experimental study of a strut injector configuration was performed for application to high-Mach-
number scramjets with circular combustion chambers. The strut injector has sixteen inclined, round,
sonic injectors distributed across four struts within a circular duct. The struts are slender, inclined at
a low angle to minimize drag, and have two injectors on each side. The strut injector was
experimentally studied under Mach 4, cold-flow conditions using two different molecular weight
injectants, helium (molecular weight = 4) and air (molecular weight = 28.97). The primary goal of
this study is the refinement of turbulence models for these complex mixing flows. Furthermore,
injectant molecular weight has been identified as a parameter of critical importance in the
development of the turbulence model upgrades. Experimental data such as presented here will be
used to guide the continuing upgrade of turbulence modeling in a closely integrated program.
Nomenclature
A* = plume area of stoichiometric mixture
Cd = discharge coefficient
d = jet diameter
deq = equivalent diameter
Gj = injectant mass flow rate
M = Mach number
P = static pressure
P0 = stagnation pressure
q = jet-to-free-stream momentum flux ratio
R = resistance
U = velocity
w* = plume width
y = vertical distance from the duct centerline
y* = vertical distance to injectant center of
mass
α = mass fraction of injectant
ρ = density
γ = ratio of specific heats
Subscripts
j = jet exit property
∞ = freestream property
I. Introduction
In view of the very high freestream velocity of
scramjets reaching Mach 10, fuel residence time is
on the order of milliseconds1 and supersonic
combustion presents an interesting challenge in
scramjet engines. It is, therefore, desirable to
enhance penetration and mixing of the fuel plume
in order to accomplish rapid combustion leading to
a reduction of the required combustor length,
reducing the skin-friction drag and heat transfer
and increasing the net thrust. To improve the
overall engine efficiency, the injection process
must also induce low total pressure losses. Jet
injector mixing enhancement in high-speed flows
also has applications in other fields such as thermal
protection systems and vehicle control by jet
thrusters.
Many injector configurations have been
studied by various groups in an attempt to produce
enhanced mixing and penetration. Some of these
configurations can be seen in Figure 1 including
Rock 2
wall jets, struts, and swept ramps. Extensive
reviews of injector mixing characteristics are given
in Schetz et al2 and Kutschenreuter3. Flush-walled
injectors are often preferred over in-stream
injectors because they minimize total pressure
losses and heating, but some configurations can
require the use of in-stream injectors in order to
obtain adequate distribution of the fuel across the
combustor. A circular combustor cross-section is
one example where struts might be attractive. Of
course, one has to reckon with the drag of the struts
in assessing engine performance3.
Figure 1: Examples of various injector configurations (from Kutschenreuter3)
Very few of the detailed, high-speed mixing studies
available in the literature concern injection and
mixing in confined ducts representative of
combustors, and one can expect that the effects of
such confinement are very large. This is especially
true for struts protruding into the flow. There are
also bow shocks from the injection process itself.
The purpose of the present research is to
investigate the effectiveness of a four-strut injector
configuration with multiple round, sonic injectors
on each strut in a circular duct for application to
high-Mach-number scramjets with circular
combustion chambers. The nominal Mach 4 air
flow simulates conditions a scramjet combustor
would encounter in Mach 10 flight. The general
goals of cold-flow studies of injection and mixing
in simulated scramjet combustors are first to
determine if the penetration and mixing patterns
observed are in agreement with those used for the
injector design. Second, the experimental data can
be used to gauge the uncertainty in computational
predictions of such flows. The computational tools
can then be used to design and analyze for hot-flow
conditions with known uncertainty. The third and
primary goal of this study is the refinement of
turbulence models for these complex mixing flows.
For this study, two different injectants were used,
helium (molecular weight = 4) and air (molecular
weight = 28.97), since injectant molecular weight
has been identified as a parameter of critical
importance in the development of the turbulence
model upgrades.
II. Experimental Methods
A. Test Facility
These experiments were conducted in the Virginia
Tech blow-down type high-speed wind tunnel
shown in Figure 2, which operates at speeds
ranging from Mach 2 to 7. The blow-down type
wind tunnel offers run times on the order of a few
seconds at high Mach numbers with relatively
steady flow conditions. This facility was obtained
through our close and long-term collaborations
with the Institute of Theoretical and Applied
Mechanics of the Russian Academy of Sciences in
Novosibirsk, Russia. Air (or other working gas) is
supplied from a compressor to charge the storage
bottles visible within the frame at the bottom. A
special fast-acting control valve initiates flow into
the plenum chamber. The flow then passes through
Rock 3
a contoured, converging-diverging nozzle and out
through the diffuser. Due to the working principle
of the tunnel and the fast-acting control valve, there
is a gradual decrease in total pressure during the
run. The variation of the total pressure during the
run is in the range of approximately 10%. For
Mach numbers above 4, an electric heater raises the
total temperature up to 800 K to prevent
liquefaction. The nozzle exit diameter is 100 mm.
The test cabin arrangement permits the use of
relatively large in-stream models, especially at the
higher Mach numbers.
The wind tunnel setup for these experiments
used a converging-diverging nozzle to achieve a
nominal Mach 4 flow in the test section. Nominal
flow conditions involve total pressure and
temperature in the plenum chamber of 1317 kPa
and 295 K. However, there is a weak oblique shock
observed at the end of the nozzle, where the
injector model attaches resulting in actual inflow
conditions of Mach number, M∞ = 3.9, total
pressure 1311 kPa and total temperature 295 K.
Figure 2: Layout of the Virginia Tech high-speed wind tunnel
B. Injection System
The injector setup investigated in this project
resembles the combustion chamber of a scramjet
engine. In a real scramjet, the combustor is situated
downstream of the inlet and an isolator which
compress the ingested air. In the experimental
setup, the ducted strut injector model is mounted
downstream of the convergent-divergent Laval
nozzle of the high-speed wind tunnel. The injector
model consists of a total of 16 injectors distributed
over four struts within a circular duct and the
necessary connections for the injectant supply.
Downstream of the injection position, a cylindrical
flange connects the injector duct to the test cabin.
A traversing system is installed on top of the test
cabin, which positions flow measurement probes
within the cabin. The injectant is supplied from a
group of commercial gas bottles. The mass flow
rate of the injectant is controlled using a system of
two Teledyne-Hastings model HFC-D-307 digital
mass flow controllers. Each mass flow controller
uses a proportional–integral–derivative (PID)
control valve.
C. Ducted Strut Injector Model
The ducted strut injector model is based on a
circular duct extension of the tunnel nozzle. It
contains four struts with 16 circular injection
nozzles. Figure 3 shows a picture of the strut
injector model. The struts have a width of 8.2 mm,
they start at the front of the extension duct (i.e. the
end of the tunnel nozzle), and they extend 148 mm
in the flow direction with an inclination of 10°.
Two 1.52 mm circular nozzles on each lateral side
of each strut create jets that penetrate into the
tunnel crossflow at an angle of 30° relative to the
streamwise axis of the duct. The centers of the
injectors are located 92 mm from the leading edge
of each strut. The number, shape, and size of the
struts were based on drag considerations and
previous experience. The number, size, and
location of the injectors were based on CFD
studies.
Due to the physical obstacle created by the
struts, the formation of shocks at the edges and an
expansion at the rearward facing edge of the struts
can be expected. As a result, high total pressure
losses in such a configuration are unavoidable. As
pointed out before, an optimized injection system
for a scramjet engine should combine good mixing
efficiencies with low total pressure losses. A
system including fixed flow obstacles has to
compensate for this disadvantage by enabling
Rock 4
better mixing in order to remain competitive with
other geometries.
A useful parameter for correlating transverse
jet injection results is called the jet-to-freestream
momentum flux ratio, q , defined as follows
∞∞
=≡)(
)(
)(
)(2
2
2
2
Mp
Mp
U
Uq
jj
γ
γ
ρ
ρ (1)
For this study, two experimental cases were
run using different injectants. One case was run
using helium injection, which safely simulates
hydrogen fuel in a cold-flow, non-combusting
environment. A second experimental case was then
run using air injection. Each case was run with the
same jet-to-freestream momentum flux ratio ( q )
to obtain a similar amount of fuel plume
penetration. For the helium injection case, the total
mass flow rate was set to Gtotal = 22.5 g/s, which
corresponds to q = 3.49 for this injector geometry
and operating conditions. For the air injection case,
the total mass flow rate was set to Gtotal = 62.66 g/s
to match the value of q = 3.49. These values of
q are representative of good practice in strutted
scramjet combustors. For both cases, the injectant
jets are at sonic conditions and are highly under-
expanded. Table 1 summarizes the injection
parameters for the two experimental cases.
Figure 3: Strut injector model Table 1: Injection parameters for the two experimental cases
(for a single injector)
D. Concentration Sampling Probe
In order to analyze the mixing of the helium with
the air freestream, it is crucial to acquire accurate
gas composition measurements. The concentration
is measured in terms of the mass fraction of helium
in the overall gas mixture. To determine this mass
fraction, a special probe is used to simultaneously
sample and analyze the gas mixture at a given
position accurately. The fundamental concept of
the gas analyzer used for this work was developed
at Virginia Tech by Professor Ng4. The
concentration sampling probe is an aspirating type
probe that is attached to a vacuum pump. A picture
and diagram of the concentration probe are shown
in Figure 4. The unit consists of a constant
temperature hot-film sensor operating in a channel
with a choked exit. The housing was designed to fit
around the body of a TSI 1210-20 platinum sensor.
The hot-film has a diameter of 50.8 µm and an
active sensor length of 1.02 mm which is used in
conjunction with a Dantec model 56C17 constant
temperature anemometer (CTA) fitted with a
Dantec model 56C01 CTA bridge. The overheat
ratio of a hot-film sensor is defined as (Rop - R0) /
R0, where Rop is the heated sensor resistance at
operating temperature and R0 is the cold sensor
resistance at ambient temperature. An overheat
ratio of 1.0 was used for the hot-film sensor of the
concentration probe.
The inlet hole at the tip of the probe has the
same diameter as the choked orifice, 0.63 mm.
These diameters were designed to preclude the
occurrence of a standoff shock at the probe tip for
supersonic flow due to the suction of the vacuum
pump through the choked orifice. Schlieren flow
visualization confirmed the absence of a standing
normal shock. The internal probe diameter diverges
from 0.63 mm at the inlet to 3.8 mm at the sensor
plane, causing a normal shock to occur inside the
probe in the diverging channel. By swallowing the
shock into the internal diverging section of the
probe, a stream tube equal in area to the probe
capture area can enter the probe undisturbed and
undistorted. Thus, an isokinetic sampling of the
stream tube is accomplished. Through the
Parameter Unit Helium injection
case,
Gtotal = 22.5 g/s He
Air injection case,
Gtotal = 62.66 g/s
air
Gj [g/s] 1.41 3.92
q [-] 3.49 3.49
Cd [-] 0.70 0.70
Uj / U∞ [-] 1.30 0.47
P0j / P0∞ [-] 0.63 0.70
d [mm] 1.52 1.52
deq = (Cd)1/2d [mm] 1.27 1.27
Rock 5
diverging section and the normal shock inside the
probe, the flow is decelerated to very low
velocities. At Mach numbers around M = 0.05, the
pressure and temperature measured inside the
probe can be approximated to represent the total
pressure and temperature of the fluid.
The concentration sampling probe was
calibrated to measure the helium molar fraction
uniquely related to a given pressure, temperature
and rate of heat transfer sensed at the hot-film. The
hot-film responds to local mass flux variations.
From the known helium molar fraction of the
sample, the mass fraction can be calculated. The
measurement uncertainty of the probe was found to
be approximately +/- 0.005 for helium mass
fraction measurements.
Figure 4: Picture of the concentration sampling probe with an integrated cone-static probe (left) and
diagram of the concentration probe (right)
E. Cone-Static Pressure Probe
The concentration probe has an integrated cone-
static probe (see Figure 4). The cone-static probe is
not required to determine the mixture composition,
but the use of the cone-static probe allows for the
determination of other quantities of interest for a
given flow field using a multiple probe survey
method. The cone-static probe was attached to the
concentration probe to allow simultaneous
concentration and cone-static measurements to be
made. The cone-static probe consists of a 1.59 mm
outer diameter pipe capped with a 10° half-angle
cone. Four small pressure ports are located at 90°
spacing around the surface of the cone. The cone-
static probe is positioned in a location that is
always outside of the oblique shock generated by
the tip of the concentration probe.
F. Miniature Five-Hole Probe
A miniature, fast-response, conical, five-hole
pressure probe is used to measure local values of
Mach number, total pressure, and flow angularity.
A picture and diagram of the five-hole probe are
shown in Figure 5. The probe uses five miniature
piezoresistive pressure transducers, which are
located directly in its tip. The tip of the probe is a
45° half-angle cone with an outer diameter of 1.65
mm. Each pressure port has a diameter of 0.25 mm.
The response time of the probe to a step input is
about 11 ms. Two separate calibrations were
performed to allow for the determination of Mach
number and flow angularity. First, the five-hole
probe was calibrated to determine the Mach
number of an airstream as a function of its port
pressures. The calibration of the five-hole probe to
determine Mach number is necessary due to the
geometry of the probe. The probe has a blunt tip
where the center (Pitot) port is located, which is
surrounded by four peripheral ports. Downstream
of the blunt tip, flow expansion occurs resulting in
a region of lower pressure behind the tip in
comparison to a sharp cone with the same half-
angle. Beyond the region affected by flow
expansion, the pressure distribution will quickly
recover to that of a sharp cone. However, the
peripheral ports for the probe used in the current
study are located in the region affected by flow
expansion. Sharp cone theory cannot be used to
predict the readings for these ports and therefore, it
is necessary to calibrate the probe to determine
Mach number. The calibration was performed over
a Mach number range of 1.6 – 3.9 and includes a
total of 37 data points. This Mach number
calibration is only valid for use of the probe in air,
since mixture composition influences this
calibration curve.
The five-hole probe was also calibrated to
determine the flow angularity of an airstream as a
function of its port pressures. The calibration was
performed at Mach 3.1 with an angularity range of
+18° to -18° of pitch and 0-360° of roll. The
angular calibration of the five-hole probe includes
a total of 795 data points. The angular calibration is
valid over a wide range of Mach numbers and it is
also valid for use in both air and air-helium
Cone-Static
Probe
Concentration Probe
Rock 6
mixtures according to the work of Swalley5. Using
a 40° half-angle cone in experiments run at a Mach
number of 3.55 in air and 21 in helium, Swalley
confirmed the theory that only one calibration
curve is required to determine flow angularity over
a wide Mach number range in either air or helium
for this type of instrument. In addition, Centolanzi
calibrated a 20° half-angle cone to determine the
flow angularity of an airstream at Mach numbers of
1.72, 1.95, and 2.46 and also concluded that the
effects of Mach number on the calibration map are
either negligible or small6.
Figure 5: Picture of the five-hole probe (left) and drawing of the probe tip (right). All dimensions are
in millimeters.
G. Multiple Probe Survey Method and Data
Analysis Procedure
1. Outside of the Region of the Fuel Jet
Outside of the region of the fuel plume, the gas
composition is known to be entirely air, so the
concentration probe is not used in this region. The
five-hole probe Mach number calibration is valid
and the angular calibration is most accurate in this
region. Therefore, the five-hole probe alone can be
used outside of the fuel plume to determine local
values of Mach number, total pressure, and flow
angularity. The data reduction process needed to
convert the port pressures into incoming flow
properties follows that of Centolanzi6. The
measurement uncertainty of the five-hole probe
was found to be approximately +/- 1.5% for Mach
number, +/- 3% for total pressure, and +/- 1° for
flow angularity in this region.
2. Within the Region of the Fuel Jet
Inside the region of the fuel plume, the
properties of the mixture must be accounted for and
the data analysis procedure is more complex. First,
a concentration probe survey is used to determine
the local gas composition. The measurement
uncertainty for the concentration probe is
approximately +/- 0.005 for helium mass fraction
measurements. Next, a method is needed to solve
for the Mach number in the region of the fuel
plume, since the five-hole probe Mach number
calibration is not valid for gas mixtures. To
determine the Mach number in this region, a
multiple probe survey method is used.
Corresponding data points are taken with the
concentration probe, the cone-static probe, and the
five-hole probe. The helium concentration data is
then used with a combination of the Rayleigh-Pitot
formula and a numerical solution of the Taylor-
Maccoll equation for the local gas composition to
determine the local Mach number at each
measurement location. Once the Mach number at
each measurement location is known, the total
pressure and flow angularity can be solved for
using the method of Centolanzi6 as the five-hole
probe flow angularity calibration is valid for air-
helium mixtures. The measurement uncertainty is
approximately +/- 2% for Mach number, +/- 5% for
total pressure, and +/- 1° for flow angularity in the
region of the fuel plume.
III. Experimental Results
A plane 178.3 mm (1.8 duct diameters)
downstream of the circular injector centers was
selected for data measurement purposes. At this
measurement plane, which is about 2 mm beyond
the end of the duct where the flow enters the test
cabin, the flow field downstream of one half of one
strut was surveyed. For the helium injection case,
helium concentration, Mach number, and total
pressure values were measured at the data
measurement plane. For the air injection case,
Mach number and total pressure values were
measured. To check for symmetry, data points
were also measured on the opposite side of the
strut. The symmetry plane for the fuel plume was
found to be shifted approximately 1 to 2 mm
laterally relative to the centerline of the strut. This
slight 1-2 mm shift of the fuel plume over a length
of 178.3 mm is most likely attributed to a small,
but undetected misalignment of the experimental
hardware.
Rock 7
A. Helium Concentration Results
Results for the helium distribution presented as
mass fraction contours across a section of the duct
are shown in Figure 6. These mass fraction
contours were determined from 127 experimental
data points distributed across the fuel plume in the
radial and peripheral directions. The projected
outlines of the strut and the circular injectors are
shown for reference. An examination of Figure 6
shows that at 1.8 duct diameters downstream, the
injectant achieved good penetration across the
combustor cross-section. However, the individual
jets merged into a single large plume and the rate
of mixing was somewhat slow.
Calculations over the measurement grid
provide parameters that characterize the plume and
the mixing behavior, which are summarized in
Table 2. Here, y* is the location of the center of
mass of the injectant in the plume below the duct
wall, w* is the maximum width of the equivalent
stoichiometric hydrogen/air concentration contour,
A* is the plume area within that contour, αmax is the
maximum injectant mass fraction, and yα,max is the
distance from the duct wall to the location of αmax.
B. Mach Number and Total Pressure Results
Figures 7 and 8 show contour plots of Mach
number and total pressure at the measurement
plane for the helium injection case compared to the
air injection case. The projected outlines of the
strut and the circular injectors are shown for
reference. For the helium injection case, these plots
were generated using the concentration data
combined with 109 experimental data points taken
with the cone-static probe and 186 data points
taken with the five-hole probe according to the data
analysis procedure described in Section II-G. For
the air injection case, the contour plots were
generated using 176 experimental data points taken
with the five-hole probe.
Figure 6: Contour plot of helium concentration at a plane 1.8 duct diameters downstream of the
circular injector centers
Parameter Unit Value
y* [mm] 16.9
w* [mm] 24
A* [mm2] 408
αmax [-] 0.101
yα,max [mm] 16
Table 2: Strut injector mixing parameters (for an entire plume created by 1 strut with 4 injectors)
Rock 8
Helium Injection Air Injection
Figure 7: Contour plots of Mach number for helium injection vs. air injection at a plane 1.8 duct
diameters downstream of the circular injector centers
Helium Injection Air Injection
Figure 8: Contour plots of total pressure for helium injection vs. air injection at a plane 1.8 duct
diameters downstream of the circular injector centers
A complex shock system forms downstream of the
injector array, which includes oblique shocks from
the struts and bow shocks from the injectors. This
phenomenon reduces the Mach number of the flow
downstream of the strut as shown in Figure 7. Also,
the injectant jets are at sonic conditions, whereas
the freestream is nominally at Mach 4 conditions.
The region of reduced Mach number in the vicinity
of the fuel plume is substantially larger for the
helium injection case compared to the air injection
case. This is due to the properties of the gases such
as molecular weight and specific heat ratio. These
properties influence the sound speed of a gas,
therefore helium has a higher sound speed than air
at the same temperature conditions. Mach number
is inversely proportional to sound speed, so the
Mach number in the region of the fuel plume is
lower for the helium injection case. Both cases
showed good penetration of the injectant across the
combustor cross section, but there is a substantial
total pressure loss downstream of the strut as
shown in Figure 8. The total pressure loss is larger
for the helium injection case than for the air
injection case, which is largely due to Mach
number effects. Another factor that contributes to
the lower total pressure for the helium injection
case is the total pressure in the injector manifold,
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which is lower for the helium injection case than
for the air injection case (see Table 1).
IV. Application of Experimental Research to CFD Turbulence Model Upgrades
The experiments presented here are part of an
integrated experimental and computational study
being conducted by a team of researchers from
Virginia Tech and a small business, CRAFT Tech
(Combustion Research and Flow Technology,
Inc.). The experimental research is being conducted
at Virginia Tech, whereas the computational
research is being conducted at CRAFT Tech. The
primary goal of this study is to upgrade the
turbulence models that are used for CFD
predictions of the flow inside a scramjet
combustor. There are two primary upgrades that
are currently being developed for the k-ε turbulence
model: (1) scalar fluctuation modeling and (2) a
baroclinic turbulence source term correction. Scalar
fluctuation modeling predicts local variations in
turbulent Prandtl and Schmidt numbers. CFD
predictions for the flow inside a scramjet
combustor currently generally use turbulence
models that utilize global estimates for the
turbulent Prandtl and Schmidt numbers. In this type
of flow, the local values of turbulent Prandtl and
Schmidt numbers can vary significantly and this is
believed to be a source of error. The baroclinic
torque term is a modification to the turbulence
model to account for the effects of strong density
gradients, which occur in high-speed mixing flows.
In high-speed mixing flows, the flow conditions
deviate significantly from those used in the
standard k-ε model derivation. Therefore,
refinement of the turbulence model is necessary to
accurately predict these complex mixing flows.
To support the turbulence model upgrades,
experiments are required that parametrically vary
the jet molecular weight and freestream Mach
number, while keeping other basic mixing
parameters the same. The experiments provide a
database of fuel injection and mixing data that is
being used for turbulence model refinement and
CFD code validation. Figure 9 shows an example
of how the experimental data is being used to
upgrade the turbulence modeling. In this figure, the
experimental helium concentration results along
the centerline of the strut injector are compared to
CFD predictions with and without the additional
baroclinic torque term. The baroclinic torque term
was tested and validated for a wide range of
experimental cases consisting of different flush-
wall and in-stream injector designs, varying
molecular weight fuels, and varying freestream
Mach numbers7. From examining Figure 9, it is
evident that the baroclinic torque term significantly
improves the CFD predictions.
Figure 9: Comparison of the mixing results predicted by the CRUNCH CFD code to the
experimental data along the centerline of the strut for the helium injection case
V. Conclusion
This paper presented the results of an experimental
study of a four-strut injector configuration with
multiple round, sonic nozzles on each strut in a 100
mm diameter circular duct under cold-flow
conditions for application to high-Mach-number
scramjets with circular combustion chambers. The
freestream Mach number was nominally 4, which
simulated the conditions a scramjet combustor
would encounter in nominal Mach 10 flight. The
primary goal of this study is the refinement of
turbulence models for these complex mixing flows.
Injectant molecular weight has been identified as a
parameter of critical importance in the
development of the turbulence model upgrades and
the use of two different injectants, helium and air,
Rock 10
allowed the effects of injectant molecular weight to
be studied. For comparison purposes, a constant
jet-to-freestream momentum flux ratio was
maintained to achieve a similar amount of fuel
plume penetration for the two experimental cases.
The main reason for considering an intrusive
injector design for application in such a
challenging thermal environment as a Mach 10
scramjet combustor is the goal of minimizing the
combustor length in the low aspect ratio
combustors currently being considered for such
applications. An in-stream injector inherently
yields better penetration and airstream coverage in
short axial distances than a flush-wall injector. If
adequate mixing can also be achieved in a short
distance, then the drag and thermal load penalties
of the in-stream injector can be overcome.
For the case with helium injection, the use of
helium safely simulates hydrogen fuel in a non-
combusting environment. The experimental results
for the helium injection case obtained at
approximately 1.8 duct diameters downstream
showed good penetration of the injectant across the
combustor cross-section, but the individual jets had
merged into a single large plume and the rate of
mixing was somewhat slow. In addition, a
substantial total pressure loss occurred in the flow
downstream of the strut. There were substantial
regions of fuel-rich (using the stoichiometric ratio
of hydrogen to air, 0.0292, as a metric)
concentrations in the plumes, even though the
overall helium-air mixture in the duct was lean, on
the same basis. The maximum concentration of
helium detected by the experiment was 10.1%. One
might have expected better mixing based on
simple, isolated injector correlations. The ratio of
the measured injectant mass flow to the calculated
air flow (equivalent fuel/air ratio) in the duct was
0.0098 for the helium injection case, which
corresponds to an equivalence ratio of 0.34 in
terms of hydrogen. The area of the helium plume
created by one strut with four circular injectors was
calculated to be 408 mm2 based on the
experimental data. Thus, the overall helium plume
area in the duct covered by a total of 16 jets
distributed across four struts is 1632 mm2, which
accounts for 20.8% of the total duct cross section.
The air injection case exhibited similar features
to the helium injection case including good
penetration of the injectant across the combustor
cross-section, the individual jets merging into a
single large plume, and substantial total pressure
loss in the flow downstream of the strut. However,
the helium injection case had a larger overall total
pressure loss than the air injection case. The
equivalent fuel/air ratio for the air injection case
was 0.0273 in comparison to 0.0098 for the helium
injection case. The increase in the fuel/air ratio for
the helium injection case vs. the air injection case
is similar to the fuel/air ratio increase that would
occur for a scramjet engine operating on a low
molecular weight fuel vs. a high molecular weight
fuel.
The experiments presented here are part of an
integrated experimental and computational study
that is being conducted to improve the turbulence
models that are used for CFD predictions of the
flow inside a scramjet combustor. A database of
fuel injection and mixing data is being built for
turbulence model refinement and CFD code
validation. Certain improvements to the turbulence
modeling have already been achieved as part of this
integrated experimental and computational study.
Nevertheless, it will likely be possible to further
improve the turbulence modeling by obtaining
additional experimental data.
References
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Murthy, Editors), AIAA, New York, 2000.
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