SAMPE Journal - March/April 2024 - 17

the material sample. Another note from these
simulations is the clear chemistry freezing in the
nozzle, and also temperature freezing is observed
in preliminary 2-temperature CFD simulations
performed using Park's 2-temperature model, but
not presented here.[10,11]
on any material sample placed in the nozzles due
to the different chemical reactions occurring.
Figure 3. Drawing of the long conical nozzle. Positions of temperature and pressure
sensors along the nozzle wall are highlighted. Note the visible cooling channel.
3. EXPERIMENTAL CHARACTERIZATION
In this section, the experimental setup
and measurement techniques used for the
characterization of both, the long conical and
semi-elliptical nozzles are briefly described.
3.1 Experimental Setup
The Plasmatron test chamber, with an internal
diameter of 1.4 m and a length of 2.5 m, is equipped
with a chamber static pressure pch
This will affect the heat flux
port measured by
an absolute transducer (DI200, Leybold Vacuum,
CH) with an accuracy of 1 % of the reading and a
range between 0.1 and 220 mbar.
The reservoir pressure at the nozzle inlet is
(a) Temperature
˙
(b) Pressure
Figure 4. (a) Temperature and (b) Pressure fields from CFD of centerline slice from the
3D semi-elliptical simulation (top) and axisymmetrical long nozzle (bottom).
reaches roughly 2000 K for both cases. For the
axisymmetrical nozzle, a clear under-expanded jet
flow structure forms with the barrel shock at the
nozzle exit and reflecting each other and creating
reflection shocks, and the start of the familiar
diamond pattern.[8]
Due to the great variation of
flow parameters along the centerline, great care
should be taken when determining the distance a
material sample should be placed concerning the
nozzle exit. The temperature contour of the 3D
semi-elliptical nozzle is taken from the symmetry
plane of the nozzle. For the semi-elliptical nozzle,
the barrel shock hits the flat plate and induces
flow separation, seen as a bubble just before the
area where a sample would be installed ends. The
separation bubble creates a flow obstacle and
consequently an oblique shock wave at the front.
This phenomenon is well known as illustrated by
Yao et al.[9]
The flow separation should be further
studied to ensure a known heat flux profile across
m [ g/s ]
.
6
8
10
12
16
20
pres [ hPa ]
127.4
152.7
176.2
201.2
256.9
278.9
pch [ hPa ]
2.36
2.80
3.43
4.10
6.01
8.70
Table 1. Reference experimental conditions for a constant electrical power
within the range 400- 420 kW (for the long conical nozzle).
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measured by another absolute transducer (DI2000,
Leybold Vacuum, CH) with an accuracy of 1 % of
the reading and a range between 1 and 2000 mbar.
Before and/or after each experimental run with the
axisymmetric nozzles, the plasma condition set in
the Plasmatron test chamber can be characterized
by two intrusive probes injected into the plasma
stream. A mechanical system allows injecting them
into the plasma flow when needed. The probes
are a water-cooled calorimeter for measuring the
heat flux level provided by the high-temperature
gas, as well as a pitot pressure measurement from
which the total pressure is obtained. Mostly used
are two hemispherical probes of 50 mm diameter
but different sizes and shapes exist. In addition,
the test chamber hosts a static pressure port
outside the flow field to record the test chamber's
static pressure
during the experiment.
The
long conical nozzle is instrumented with three
pressure tabs. Validyne DP45 very low differential
pressure transducers have been chosen for the
measurements.
Their measurement principle
relies on the pressure difference between the wall
pressure and the chamber pressure (Dp = pw
pch). Their operating range was decided based on
−
values given by preliminary CFD simulations at
20 g/s (respectively, from nozzle throat to exit, the
Validyne ranges are Dp ∈[0, 14000], [0, 2200], and
[0, 350] Pa). A margin of at least twice the expected
pressure has been taken into account. They are
placed inside a vacuum box to be in a similar
environment to the test chamber. Indeed, this aims
at preventing the apparition of an offset in their
measurements (related to the piping used) when
the chamber pressure is close to vacuum.
Test samples are instrumented with several
type-K thermocouples and various optical
MARCH APRIL 2024
|
SAMPE JOURNAL |
17
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SAMPE Journal - March/April 2024

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