Speaker
Description
The VKI Plasmatron facility is the world’s largest Inductively Coupled Plasma (ICP) torch, providing a chemically pure plasma flow for material response studies in atmospheric entry conditions.
This paper presents the experimental investigation of the freestream plasma flow by means of optical emission spectroscopy in the UV to NIR wavelength region. We used a mixture of synthetic air and hydrogen (77.9% N2, 20.1% O2 and 2% H2), with the aim of providing sufficient line strength of the Balmer beta line of the hydrogen atom for diagnostic purposes. The gas phase radiative signature of dominant atomic and molecular species is observed using a high resolution spectrograph, combined with a two-dimensional intensified CCD array. From the rebuilding of local emission intensities, obtained through Abel inversion, radial temperature profiles are determined under the assumption of Local Thermodynamic Equilibrium (LTE). The procedure is based either on fitting the experimental spectra with a line-by-line code (NEQAIR) or by analytical results assuming a Boltzmann distribution.
Experimental results
LTE temperature profiles, measured from the absolute line intensities of H, O and N, agreed on a consistent trend. Boltzmann temperature profiles from oxygen lines also agreed with the LTE temperatures within the uncertainty bounds. Electron number density was measured from the Stark broadening of the Balmer beta line of hydrogen at 486.1nm, showing consistent overlap to the profile computed from the LTE temperature of the same line, under the assumption of chemical equilibrium. Synthetic spectra were computed with NEQAIR and compared to the measured ones over the whole range. Although the peaks were well represented, a residual baseline prevented good matching of the weaker parts of the spectra. A two parameters spectral fitting, including an LTE temperature and a uniform baseline, provided temperature profiles which agreed with the LTE temperatures within the uncertainty bounds.
Conclusions and future work
Overall, a local thermodynamic equilibrium model seems to provide an accurate description of the thermodynamic state of the plasma jet for the condition studied in this work. The model assumptions are corroborated by the consistent temperature profiles from different atomic line intensities, agreement with Boltzmann plot temperatures and electron number density measurements. LTE synthetic spectra also provide a good representation of the local emission intensities at different radial positions. Further investigation should be devoted to the understanding of the residual baseline effect. Possible explanations could be related to the uncertainties in the higher quantum number vibrational states, continuum radiation effects or to the presence of stray light. More precise baseline subtraction will also be performed on the Hβ line, based on measurements of synthetic air spectra.
Summary
Free-stream characterization is a tedious task in supersonic plasma flows. On top of traditional intrusive measurements of cold-wall heat flux and total pressure, we perform locally-resolved emission spectroscopy by means of a spectrograph and an intensified camera. The CFD simulation of the flow-field is post-processed with a radiative transfer code, allowing to compare the simulated radiance to the measured one. The technique aims at providing additional validation of the simulations tools, thus improving the characterization capabilities.
