Experimental Assessment of PTFE Post-Arc Ablation

The study addresses the post arc ablation (PAA) of PTFE (Polytetrafluoroethylene) material after being stressed by high current arcs. Arcs were generated in ambient air applying AC or DC current profiles to reach energy input in the range 7-22 kJ. The characterization has been performed essentially based on standard optical measurement techniques. The shadowgraph technique enabled us to show that the PAA flow is composed by a significant amount of carbonaceous soots lasting for several dozens of milliseconds after current interruption. The pyrometry technique allowed to estimate the soots temperature in the range 1400-2200 K

Plasma radiation for atmospheric entry at Titan: Emission spectroscopy measurements and numerical rebuilding

Emission spectroscopy measurements on a plasma representative of Titan atmosphere composition were obtained in the Inductively Coupled Plasma wind tunnel facility (VKI-Minitorch) at the von Karman Institute in Belgium. Temperatures ranged from 3600 to 5000 K, pressure was fixed at 300 mbar, and the molar composition was 1.9% CH4 and 98.1% N2. The high-pressure plasma was produced to obtain conditions close to equilibrium. In conjunction, line-by-line calculations have been carried out to assess the reliability of two distinct sets of molecular electronic transition moments, recently released, by predicting the radiative signature of high-temperature N2-CH4 plasma. The radiative transfer problem was solved by considering the plasma plume at local thermodynamic equilibrium conditions in an axisymmetric configuration. Comparisons between the synthetic and experimental spectra demonstrated good agreement for the CN Violet and high-wavelength CN Red bands, while some discrepancies were observed for the C2 Swan bands and low-wavelength CN Red band

INVESTIGATION OF SUPERSONIC AIR PLASMA JET PRODUCED IN THE VKI PLASMATRON FACILITY

Thermodynamic characterization of a supersonic air plasma jet produced in the VKI Plasmatron facility is presented. Plasma flow regimes corresponding to underexpanded and highly underexpanded situations are investigated by means of Optical Emission Spectroscopy (OES) technique. Several diagnostics are presented using both molecular and atomic radiative contributions. Electron density, excitational energy distribution and molecular spectra fitting are presented

Investigation of the Gas-surface Interaction of Innovative Carbon Composite Ablators in the VKI Plasmatron

For a new class of low density carbon/resin composite ablators, which has been introduced and successfully applied to flight by the Stardust mission, the process of ablation is not only restricted to the surface but can also occur in-depth of the material if oxygen is able to diffuse into the porous material. The occurrence of such new porous carbon/resin composites requires an important effort in theoretical and experimental investigation for an adequate understanding of the ablation process to enable development and validation of material response models. At the von Karman Institute for Fluid Dynamics (VKI) research activities were developed to establish a methodology for experimental characterization of innovative low-density ablators using the inductively coupled 1.2MW Plasmatron facility. A comprehensive setup of measurement techniques was applied to the facility in order to determine and characterize the in-situ material response of ablative samples in different test conditions. Optical emission spectroscopy was utilized to address the thermo-chemistry of the plasma free-stream and its interaction with the ablating sample. In addition microscopic analysis tools for sample examination, at the carbon fibre length scale (~10μm), are used to investigate the material physics. The degradation behaviour of the material is then being analyzed by scanning electron microscopy to be able to evaluate the depth of degradation and the thinning of the carbon-fibres. In particular, to provide information about the diffusion/reaction competition of oxygen, which controls the oxidation of carbonized resin and exposed fibres in-depth. Material surface properties, as emissivity, are also determined in-situ using an IR-radiometer combined with two-colour pyrometer measurements. Preliminary results showed that nitridation, leading to CN (CN violet & CN red), is highly apparent in pure nitrogen plasma flows but significantly drops when oxygen is involved, speaking for dominant oxidation reactions (CO, CO2, NO). Additionally, different chemical mechanisms were found to occur rather in nitrogen than in air plasma. In such a way, diatomic carbon (C2 Swan) transitions were highly radiating after injection of the sample into N2 plasma but truncated after a few seconds. This was not observed with air as test gas. As expected, oxygen is the driving force to provoke reactions as the system undergoes the ablation process, but its uncertain state of diffusion into the porous material and on the contrary, reactions undergone in the absence of oxygen, necessitate the usage of appropriate micro- and spectroscopic tools

Vision-based tomographic reconstruction of emissivity distribution in asymmetric thermal plasma

We report an original on-line optical diagnostics technique for the vision-based tomographic reconstruction of emissivity distribution in asymmetric thermal plasma using CCD cameras. The least-square QR decomposition (LSQR) method has been employed to solve the inverse ill-posed problem of the reconstruction efficiently. A numerical case is used for validating the diagnostics technique, and effects of different CCD camera combinations and measurement errors on the reconstruction performance are examined. Results show that the emissivity distribution of thermal plasma can be reconstructed well with a short reconstruction time for different noisy input data, proving the feasibility and the on-line ability of this technique to characterize the asymmetric plasma medium