Hermann Stuttgart Leyland EPFL

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible 8th European Symposium on Aerothermodynamics for Space Vehicles 03.03.2015 Tobias Hermann, Stefan Löhle, Pénélope Leyland, Jean-Marc Bouilly, Stefanos Fasoulas High Enthalpy Flow Diagnostics Group First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

Motivation • Future return missions with superorbital re-entry velocities - Radiative heat flux is significant - Essential source is the VUV (100 – 200 nm) - Significant coupling between ablation and radiation

• Ablation radiation coupling in plasma wind tunnels - Steady state testing - High enthalpy flow - Material samples: cooled copper, carbon preform (Calcarb), carbon phenolic ablator • Experimental strategy: - Multiple diagnostic techniques for a complete dataset: - VUV-spectroscopy, UV/VIS-spectroscopy, photogrammetry, thermography, pyrometry

[Carpenter, NASA]

[Zander, IRS]

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Plasma wind tunnel condition • Plasma wind tunnel facility PWK1 - Magnetoplasmadynamic arcjet generator (RD5) hlocal

68.4 MJ/kg

đ?‘ž

4.1 MW/m2

pamb

16.6 hPa

pstag

24.4 hPa

I

1220 A

U

133 V

đ?‘šair

18 g/s

• Subsonic condition - Based on local heat transfer simulation concept [1] and - Local mass-specific enthalpy from the model of Zoby [2] - Simulation of a Hayabusa re-entry trajectory point at 78.8 km

[1] Kolesnikov, VKI, 1999 [2] Zoby, NASA, 1968

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Experimental setup - VUV-spectroscopy • 2 mm stagnation point bore hole • Light enters under an angle of 28° • MgF2 window behind sample • 116-197 nm wavelength range (six single acquisitions) • Evacuated light path • Calibrated for absolute radiance with deuterium lamp First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Experimental setup - UV/VIS-spectroscopy • Measurement from side • 320 - 810 nm wavelength range • Imaged area: 60 mm x 1.5 mm

• 5 mm in front of sample surface • Calibrated for absolute radiance with integrating sphere

• Abel-transformation to obtain local emission coefficient [3] [3] Fulge et al., AIAA, 2011 First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Results – VUV stagnation point radiance • Corrected for O2 absorption in the bore hole (copper 34%, Calcarb 6%) [Hermann et al., AIAA, 2014] • Dominated by atomic radiation (N, O, C, C+) • Integral radiance in front of carbon phenolic an order of magnitude weaker than copper (140 – 197 nm) • Integral radiance in front of Calcarb 1.8 times stronger than copper (126 -180 nm, excluding 148 -150 nm)

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Results – UV/VIS centerline emission coefficient • Strong molecular radiation (N2, N2+, CN) • Atomic radiation of Calcarb stronger than carbon phenolic • Integral emission in front of carbon phenolic 1.2 times stronger than copper (320 - 810 nm, excluding 580 - 690 nm) • Integral emission in front of Calcarb 3 times stronger than carbon phenolic (320 - 580 nm)

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Spectroscopic analysis methods - Molecules • Spectral simulation of multiple species using PARADE [5] • Spectral shape - Assumption: all molecules have the same temperatures - Trot, Tvib • Relative intensity - excited state number density - emission of each species

Calcarb sample, Centerline, Trot= 8950 K, Tvib= 10190 K [5] Liebhart et al., AIAA, 2012 First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Spectroscopic analysis methods - Atoms

VUV multiplets (optically thick)

UV/VIS multiplets (optically thin)

đ?‘‘đ??żÎť = Îľ − Îşđ??żÎť đ?‘‘đ?‘Ľ

4đ?œ‹Îľđ?œ†0 = đ?‘›đ?‘˘ đ??´đ?‘˘đ?‘™â„Žđ?‘?

Ground and low energy states

High energy states

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Spectroscopic analysis results – Molecular radial distribution • Significant decrease of molecular excitation temperatures through material samples • CN emission peaks off center

• N2, N2+ emission peaks in the center

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Spectroscopic analysis results – Centerline atomic Boltzmann plot 5 mm in front of sample • No deviation from Boltzmann distribution observed • Electronic excitation temperature extracted from Boltzmann relation • Absolute number densities of carbon and oxygen (ground state measured) • Extrapolation of nitrogen ground state: qualitative comparison of materials

Carbon

Nitrogen

Oxygen

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Spectroscopic analysis results – Centerline atomic Boltzmann plot 5 mm in front of sample • Electronic excitation temperature slightly increased in front of Calcarb • Highest atom density for Calcarb, lowest for carbon phenolic

Copper Tel,exc (N, O) 8330

Calcarb

Carbon phenolic

9070

8350

nN (extr.)

3.5 x 1022

4.9 x 1022

2.1 x 1022

nO

5.1 x 1021

6.8 x 1021

-

nC

-

1.7 x 1021

5.5 x 1020

nC+

-

2.0 x 1020

-

(extr.) for extrapolated values

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Summary • Experiments investigating ablation radiation coupling conducted in PWK1 • VUV radiation strongest in front of Calcarb and significantly weaker in front of carbon phenolic (>1 OM)

• Molecular radiation in front of Calcarb three times stronger than in front of carbon phenolic • Material samples decrease vibrational temperature significantly • Carbon phenolic reduces atom number density in stagnation point region • Ablation radiation coupling is significant • Phenolic matrix reduces VUV radiative heat flux significantly

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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Big thanks to: the collegues from the high enthalpy flow diagnostic group, our workshop, the project partners, ESA (research grant No. 2011/ITT-6632/PL)

Thank you for your attention!

Questions?

First Results on Ablation Radiation Coupling Through Optical Emission Spectroscopy from the Vacuum Ultraviolet to the Visible

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