Plasma Wind Tunnel: Diagnostics of Subsonic Plasma Jet

A.M. Essiptchouk, G. Petraconi*, C. Otani*, H.S. Maciel*, A.Marotta, D.S.
da Silva, 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 linear calorimetric probe is developed for the arc jet heat flux measurements.
Method de Abel’s inversion matrix is engaged for obtain the heat flux density distribution. Heat
flux density from plasma jet to catalytically calorimetric probe is investigated for efficient study
of the efficiency of thermal protection system (TPS) material used for protection of reusable
space transportation system. The heat flux density distribution permits determine the regions of
the plasma jet where heat conditions is more uniform and corresponds to those encountered
during atmosphere reentry.
Keywords: Re-entry, plasma jet, calorimetric probe, Abel inversion.
PACS: 52.70.-m
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.
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. This
allows to simulate heat transfers from supersonic flows by subsonic flow experiments,
provided these simulation variables match.
A linear plasma torch with “hot” cathode and stepped anode was used as the
reactive plasma source. DC arc plasma system was designed for continuous working at
power up to 50 kW. In order to carried out the plasma jet diagnostic the working
parameters were adjusted at current 135 A and arc voltage 300 V. Thus at power about
41 kW the net power in plasma jet was about 30 kW, giving a plasma enthalpy of
about 5.5 MJ/kg. The flow rate of plasma-forming gas (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 d=3 mm, inner diameter 2 mm. This probe is assumed to be fully catalytical
that permits to determine the free stream enthalpy. Since the radiation of the cold
surface is negligible, the measurement represents the total convective and chemical
heat flux reaching the surface.
The water flow rate Gw was adjusted for each experiment in order to obtain
maximal sensitivity of calorimeter. The maximum temperature of the outlet water was
settled below 60 °C in order to avoid the beginning of boiling on the calorimeter inner
wall. The probe was installed perpendicular to plasma jet and can be moved in axial
and radial direction (see Figure 1). The temperature increase ΔTw of the cooling water
was measured using chromel-alumel thermocouples (diameter 0.1 mm) installed at
inlet and outlet of cross-flow tube probe.
It was assumed that heat flux from plasma jet is symmetrical with respect to the zaxis
(plasma jet axis) and at any point is function of distance r and z in cylindrical
coordinate. Thus, by using Abel inversion it is possible to obtain the heat flux density
distribution knowing the heat flow from the plasma to the calorimetric probe. In this
work we will use the method of inverse matrix [1]. The method is similar to the
applied for rotate calorimetric probe [2].
The model accepted for calculations is shown in Figure 2. All region of the plasma

where ql and Q are column vectors of heat fluxes per unit length and total heat fluxes,
respectively. The total heat flux Qi entering to the probe in ith-position is calculated as
Qi=cpGwΔTw, where cp=4186 J/kg °C is specific heat of water. Thus the radial
distribution of heat flux density is
-1 ⋅
ql = L Q (5)
The heat flux in stagnation point is more important for the material test. An
example of computed heat flux distribution on the cylinder at temperature 400 K
which exposed to hot gas at 6000 K moving at velocity M=0.53 (dados from [3]) is
shown on the Figure 3. A strong variation of q along the cylinder surface is clearly
observed. In order to obtain q in stagnation point we can assume uniform heat flux
density distribution q=qmax=const and use only part of calorimetric probe surface. As it
shown on the Figure 3, for this particular case the working surface must be limited by
angle of 100 degrees. For other flux conditions this angle is varied in range from 80 to
110 degrees. Thus for our calculations we use Θ=90°.

FIGURE 3. Heat flux density distribution on
the cylinder surface and equivalent heat flux
for q=const equal heat flux in stagnation point.
FIGURE 4. Radial distribution of the
total heat flux Q (measured values)
Figure 4 shows radial distribution of the total heat flux Q(r) (in W)obtained on
distance of 5, 30 and 50 mm from the nozzle of plasma torch. After the use of inverse
matrix transformation, see eq. (5), the radial distribution of the heat flux density q (in
W/m2) was obtained and shown in the Figure 5. We can see that on small distances
from nozzle there is a “plate” with approximately constant heat flux density that
occupied most part of plasma jet. This plate (or core of flux) exist up to distance of 30
mm, when q(r=0) starts drastically diminish and the distribution become more narrow
as it can be see at z=50 mm. Obtained results permits calculate the value of the total
hest flux entering to the target material by simples integration.
The use of water cooled Pitot tube allows obtain the velocity distribution (as it
shown on the Figure 6). This measurements permit to calculate the enthalpy
distribution, in case of need, by using a methodology described in [4].
Moreover, a measurement of the plasma jet velocity distribution (see Figure 6)
permits to obtain the plasma jet enthalpy distribution.
The obtained results are important for the heating tests of material to protect the
reusable transportation system during the re-entry phase.
The authors thank to the Brazilian Space Agency, National Counsel of
Technological and Scientific Development and the FAEP (UNICAMP) for useful
1. W. Lochte-Holtgreven, Plasma diagnostics, North-Holland Publishing Company, Amsterdam, 1968.
2. N. Matsumo, T. Mieno, Vacuum 69, 557-562 (2003).
4. S.V. Dresvin, Physics and techniques of low temperature plasma, Moscow, Atomizdat, 1972, in Russian.

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