Plasma wind tunnel: Temperature jump effect of testing material samples

G. Petraconi*, A.M. Essiptchouk, C. Otani*, H.S. Maciel*, A.Marotta,
I. Spassovska
Instituto de Fisica “Gleb Wataghin”, Universidade Estadual de Campinas, Campinas, SP, Brazil
*Technological Institute of Aeronautics, Department of Physics – Plasma and Processes Laboratory
(LPP), 12228-900, ITA – CTA , São José dos Campos, SP, Brazil.
Abstract. The study of the ablation properties and of microstructural behavior of carbon based
materials under ablative conditions of plasma jet are presented. It was observed that during the
test a sudden and unexpected jump of the surface temperature occurs before its saturation on
constant level, while the mass loss per unit area is approximately proportional to the exposure
time and depends strongly on the temperature of material surface.
Keywords: Temperature jump; erosion; TPS, ablative properties
PACS: 52.40.Hf
INTRODUCTION
The velocity of spacecraft, whish entering to atmosphere of the earth or other
planets attain several km/s and stay exposed to severe heat loads by aerodynamic
heating. For the space shuttle entries the earth at the velocity of 7 km/s (that
corresponds M>20) the gas temperature in front of its nose is heated up to 7000K. The
need for an efficient Thermal Protection System (TPS) material to protect the reusable
transportation system during the re-entry phase directly calls for a need to test the
candidate TPS samples in a similar environment. Long-duration testing of samples in
flows with temperatures of several thousands of degrees can be achieved by
immersion in plasma flows, generated by electric arc or induction. Thus the problems
under investigation include supersonic plasma flow, temperature and heat flux
measurements and testing of TSP materials.
Arc jet facilities produce jets with gas enthalpies comparable to those encountered
during atmospheric reentry. These facilities are used to evaluate components of a
reentry vehicle’s thermal protection system (TPS) and to better understand the heating
and flow conditions encountered during reentry. An effective utilization of thermal
plasma sources requires a thorough understanding of the heat transfer mechanism from
the jet flow to materials, which is a critical problem in most plasma-material
processing, such as plasma spray and materials surface modification.
The accurate simulation of the stagnation point flow on a re-entry vehicle can be
achieved if the total enthalpy, pressure and velocity gradient at the boundary layer
edge are properly simulated, provided that the chemical composition is identical.

EXPERIMENT
High-energy air plasma bombardment was carried out in an ambient atmosphere. A
linear plasma torch with “hot” cathode and stepped anode was used as the reactive
plasma source. DC arc plasma system, which is shown schematically in Figure 1, was
designed for continuous working at power up to 50 kW, but currently the available
power supply is limited to 135 A and 306 V. In order to carried out the plasma jet
diagnostic the working parameters were adjusted at current 135 A and arc voltage 300
V, which limits the maximum achievable power to about 41 kW. In these operation
conditions, the net power in the plasma jet is about 30 kW giving a plasma enthalpy of
about 5.5 MJ/kg. The plasma flow rate of air was 4.5 ×10-3 kg/s.
The heat flux density from plasma jet was measured as function of radial and axial
position by using a water-cooled calorimetric probe, fabricated of a copper tube (outer
diameter 3 mm, inner diameter 2 mm) and is supposed to be fully catalitical [1]. The
water flow rate was adjusted for obtain maximal sensitivity of calorimeter. The probe
was installed perpendicular to plasma jet and can be moved in axial and radial
direction. The temperature increase of the cooling water was measured using chromelalumel
thermocouples (diameter 0.1 mm).
Two different materials were used as target material: graphite and Carbon/Carbon
composite. As graphite target materials, a high-density (1.83×106 kg/m3) isotropic
graphite was used. C/C composite used in this experiment (density: 1.75×106 kg/m3)
was manufactured by GOUP-SNPE, France. As the specimens, the materials were cut
into cylindrical geometry, diameter of 0.016 m with thickness approximately 0.012 m.
The thickness of the specimens is not so important because the region to be eroded by
air plasma is restricted to the near-surface region. Thus, the exposure area of the
surfaces to be bombarded by reactive plasma was kept fixed at 2.04×10-6 m2.

The surface temperature of the target material was controlled by varying the
irradiation distance and was measured by an optical pyrometer (model IR-AH 3SUChino).
The distance (d) between a nozzle tip of a plasma gun and the front surface of
the specimen was varied in the range of (0.06 – 0.14) m, corresponding to stead state
surface temperature in the range of (1700 – 2000) K. The erosion rate of target
material was calculated by dividing the specimen thickness or the weight change
before and after the test, during an exposure time for each specimen within the range
(40 – 180) s. The average values were taken from the results obtained by repeating
several times the test with the same specimen.
RESULTS AND DISCUSSION
The typical surface temperature behavior with the exposure time for different
distances from plasma torch is shown in Figure 2. At low heat flux densities the
behavior of temperature for both graphite and C/C target are similar: increase during
initial faze of heating with subsequent saturation at certain temperature which depends
on the value of heat flux density. A sudden and unexpected jump of surface
temperature occurs for C/C composite at small distances from plasma torch nozzle.
Figure 3 show the distribution of the heat flux density on the target surface in the
assumption of the total catalytic of target material. No uniformity of the heat flux
density attain more that 30% with maximal value of 2.82×106 W/m2 on the target
center and 1.9×106 W/m2 on the border. The mean value qmean=2.37×106 W/m2.
The strong phenomenon of temperature jump was related in [2] where a target of
C/C-SiC specimen was treated in plasma wind tunnel PWK2 (the Institut für
Raumfahrtsysteme of the University of Stuttgart). This plasma wind tunnel is
equipped with a magnetoplasmadynamic plasma generator designed for a power level
up to 1 MW and for specific enthalpies ranging from 5 MJ/kg up to 150 MJ/kg in
order to simulate the first phase of the reentry, where the specific enthalpy is high and
the temperature is at its maximum. Moreover, the pressure in the chamber was 61 hPa.
The condition in [2] differ very strongly from our, atmospheric pressure tests.

Thus, we can assume that the effect really exists and is not a matter of wrong test
conditions or measurements. The understanding of the mechanism of the temperature
jump is very important because it may be presents natural limitations of the material
use or open new possibilities.
In spite of the temperature jump the mass lost rate as function of time not shows
any deviation from linearity, see Figure 4. Thus we can conclude that the phenomenon
is related to the boundary “plasma – target”.
The obtained results are important for the heating tests of TSP.
CONCLUSION
This work presents a stationary experiment performed to study the variation of the
surface temperature of carbon-based materials as function of exposure time in ablative
conditions generated by a reactive air plasma torch. In the experiment, graphite and
C/C composite are chosen as the target materials. The experiments show that the mass
loss per unit area is approximately proportional to the exposure time and depends
strongly on the temperature of material surface. It was shown that temperature jump
effect can be observed at atmospheric pressure and it is related to the boundary
“plasma-target”.
ACKNOWLEDGMENTS
The authors thank to the Brazilian Space Agency and the FAPESP.
REFERENCES
1. A.M. Essiptchouk, G. Petraconi, C. Otani, H.S. Maciel, A.Marotta, D.S. da Silva, I. Spassovska. See these
Proceedings

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