SAMPE Journal - March/April 2024 - 18

FEATURE / SUPERSONIC NOZZLES
instruments are used to measure surface
temperature. An overview schematic of the test
chamber highlighting the intrusive and nonintrusive
techniques with the conical nozzle is
presented in Figure 5. This setup was used to
characterize the conical nozzle and obtain an
experimental test envelope. A similar setup was used
for the test campaigns discussed in Section 4. The
setup for the semi-elliptical is similar to that used in
the conical configuration with some modifications
Figure 6. A water-cooled injection system is used to
hold and inject the tested material with or without
any angle of attack.
3.2 Experimental Results
3.2.1 Conical Nozzle Experimental Results
The long conical nozzle experimental campaign
has been conducted
Figure 5. Schematic of the conical Plasmatron experimental setup. P is the pitot probe
and HF is the heat flux probe.
considering a constant
electrical power within the range 400-420 kW to
generate the plasma. The chamber pressure has
been set up as low as possible. The corresponding
experimental reservoir and chamber conditions
are summarized in Table 1.
Wall pressure measurements: Table 2 compares
these experimental measurements to numerical
simulations at the exact same reservoir conditions
(less than 1% difference in terms of Pres
). It shows
the difference in percentage, between experiments
and simulations. A good agreement between CFD
and experimental values is achieved, the numerical
values being within the uncertainty range of the
experimental ones. The 8 g/s simulation shows the
largest discrepancies with the experimental data,
from 3.1 to 5.1% higher than the expected values.
This case corresponds to the largest difference in
reservoir pressure: 154.14 hPa for the simulation
instead of 152.7 hPa (0.9% discrepancy). It is
therefore important to meticulously set the
same Pres
Figure 6. Schematic of the semi-elliptical Plasmatron experimental setup, CaF2 window
was mounted on top of the Plasmatron chamber, above the sample location, and used
to obtain optical access with normal angle to the test plate.
in the simulations as it could lead to
considerable differences in the flow through the
nozzle. One single measurement is considerably
outside the expected range (for the 20 g/s case,
Pw3
is 19% lower than the experimental value). It
may imply a problem that had not been noticed
during the test (most likely, a disconnection of
pressure transducer piping). Figure 7a displays
a visual comparison between the experimental
measurements and the pressure distribution along
the nozzle wall obtained with CFD for cases m = 8
and 16 g/s. The experimental measurements and
their respective uncertainties fit well the numerical
pressure distribution.
Stagnation pressure and heat flux: A first
stagnation pressure and heat flux measurement has
been performed with a distance probe-to-nozzle
exit of d = 42.9 mm (slightly higher for the 20 g/s
case which was conducted during another test, d =
46.5 mm) and is presented in Table 3. The highest
stagnation pressures are logically reached for high
reservoir pressures (i.e., high mass flows since the
electrical power is maintained quasi-constant
from one case to another). The first evaluation of
the stagnation heat flux gives a magnitude from 1.4
to 3.5 MW/m2
in function of the mass flow applied.
Since the static enthalpy contained in the flow is
greater for low mass flow rates, higher heat fluxes
are achieved for mass flows of 6, 8, 10, and 12 g/s
than high mass flows (16 and 20 g/s) for which a
significant decrease is noticeable.
3.2.2 Semi-elliptical Nozzle Experimental
Results
The semi-elliptical characterization campaign
has been conducted at a constant electrical power
of 500k±10kW. The chamber pressure has been
set as low as possible, the same as for the conical
nozzle. The reservoir and chamber pressure testing
conditions are presented in Table 4 and their
measurement accuracy is ±1% (for any nozzle).
18 | SAMPE JOURNAL | MARCH APRIL 2024
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SAMPE Journal - March/April 2024

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