ESA Ablative Radiation Project

FIRST RESULTS ON ABLATION RADIATION COUPLING THROUGH OPTICAL EMISSION SPECTROSCOPY FROM THE VACUUM ULTRAVIOLET TO THE VISIBLE Tobias Hermann1 , Stefan L¨ohle1 , P´en´elope Leyland2 , Lionel Marraffa3 , Jean-Marc Bouilly4 , and Stefanos Fasoulas5 1

High Enthalpy Flow Diagnostics Group, Institute of Space Systems (IRS), University of Stuttgart, Stuttgart, Germany 2 Ecole Polytechnique F´ed´erale de Lausanne, Interdisciplinary Aerodynamics Group 3 ESA/ESTEC, Aerothermodynamics Section 4 Astrium SAS 5 Institute of Space Systems (IRS), University of Stuttgart, Stuttgart, Germany

ABSTRACT

Plasma wind tunnel experiments simulating a Hayabusa re-entry trajectory point at 78.8 km with a local mass– specific enthalpy of 68.4 MJ/kg and a stagnation pressure of 24.4 hPa have been performed. Three materials, a carbon preform, a lightweight carbon phenolic ablator and cooled copper, have been tested in order to investigate radiation and ablation and its interaction. Optical emission spectroscopic measurements in the vacuum ultraviolet (VUV) regime (116–197 nm) have been conducted through a bore hole in the stagnation point of the material samples. Optical emission spectroscopic measurements in the UV/VIS spectral range (320–810 nm) have been conducted viewing the plasma from the side 5 mm in front of the sample surface. The radiation transport to the boundary layer has been analysed from these measurements combined with theoretical modeling. The resulting radiation heat flux to the surface has been compared: The stagnation point VUV radiation for the carbon sample is 1.81 times stronger compared to copper. For the carbon phenolic material the stagnation VUV radiation is over a magnitude weaker compared to copper. In the UV/VIS however both carbon based material samples exhibit stronger radiation than copper. On the stagnation streamline molecular rotational and vibrational temperatures are lower for both the carbon based materials compared to copper while the electronic excitation temperature increases slightly in front of a carbon based material. Atom number densities are largest for the carbon preform sample and lowest for the phenolic carbon sample. In conclusion there is a strong coupling of ablation and radiation surface heat flux. The measurements using the real ablation material, i.e. carbon phenolic, appear to perform best in this test campaign with respect to radiative heat flux mitigation.

Key words: Vacuum ultraviolet spectroscopy, plasma wind tunnel, re-entry, Boltzmann plot, ablation, radiation.

1.

INTRODUCTION

Re-entry vehicles entering the earth atmosphere on hyperbolic trajectories are exposed to very high thermal loads, a convective and a radiative component. As the reentry speed increases, the radiative part of the heat flux becomes more and more dominant [LHM12, GJC+ 11, Gno99]. In order to protect the vehicle from these thermal loads, ablative heat shields are used. Within the flow field, energy transfer is conducted by radiative transport in addition to convective and diffusive mechanisms [And06]. The introduction of carbonaceous and phenolic species originating from the ablative heat shield can have an effect on all energy transport mechanisms [Gup00, RRM72, HL67, JGM13].For high radiative heat fluxes the coupling of ablation and radiation needs to be investigated in order to set useful design margins. New radiating species are introduced, e. g. atomic carbon and cyanogen (CN) increasing the radiative heat flux. However, these species originating from the heat shield can act also as strong absorbers thus reducing the radiative heat flux. Absorbed radiation is either re-emitted or transported downstream becoming an additional convective heat flux to the wall [NPBM08]. Both increase and decrease in radiative heating through ablation have been reported in numerical analyses [Gup00, Par07, JGS09]. In terms of total radiative heat flux encountered by the vehicle the vacuum ultraviolet (VUV) regime, i. e. 100–200 nm, is considered the main source [NPBM08, JMG+ 11, Par04, LWM+ 09, PAI98, JGM13]. However, experimental investigations of this wavelength regime are sparse [CMGO09, McC72, SMZ+ 12, WHAW69, Sut84, PCWP97]. Considering the fact that ablation-radiation coupling is still not sufficiently understood, the need for experimental data on the subject is clear.

In this study the ablation and radiation processes have been investigated in a plasma wind tunnel since they provide steady-state high enthalpy flow environments representative of re-entry flows [LBHP12, AKKL96, AKW99, DHAK94, LAKW+ 98]. Several material sample tests

in the plasma wind tunnel PWK1 have been conducted at a local mass-specific enthalpy of 68.4 MJ/kg and a stagnation pressure of 24.4 hPa which corresponds to a Hayabusa re-entry trajectory point at 78.8 km [LSH+ 14].

This chosen flow condition has been investigated previously for ground test based interpretation of the Hayabusa re-entry in 2010, which was a superorbital re-entry that has been observed with instruments aboard an aircraft [LJ14, LBHP12, Jen10]. Experiments have been performed with a water cooled copper sample, a carbon fiber preform material sample (C ALCARB) and a phenolic carbon ablative material. A whole suite of optical diagnostic techniques have been employed in order to acquire the driving sources of radiation and ablation: Spectroscopy in the VUV measuring radiation through a hole in the stagnation point of the sample, spectroscopic measurements in the ultraviolet and visible spectral range (UV/VIS) viewing the plasma from the side, in-situ recession measurements with photogrammetry, pyrometry and thermography [LHZ+ 14]. The combination of the different instruments allows for the first time a detailed insight into the coupling of ablation and radiation including the VUV spectral range. Using the different materials, the influences can be investigated in detail. As the VUV spectral region is the essential source of the radiative heat flux, these spectroscopic measurements give insight into their influence on the ablation performance.

This paper provides an analysis of the spectroscopic data.

The influence of different materials on the local emission coefficient in the UV/VIS as well as on the measured VUV radiance is studied. Both spectroscopic datasets are used to evaluate self-absorption processes in the flow allowing the determination of absolute particle densities of various species [HZF+ 15]. Different excited states of atoms are used to measure electronic excitation temperatures [Fle99, Win07, vdSvdM90]. Molecular radiation is used to determine rotational and vibrational excitation temperatures [RAK97, LWH+ 10, LWM+ 09, WEMH13]. Using these techniques the influence of ablation on the thermochemical state of the boundary layer is studied. The paper will give a first analysis of the performed tests and the performance of the ground testing facility for future sample return mission testing.

2.

EXPERIMENTAL SETUP

Experiments have been conducted in the plasma wind tunnel PWK1. Details of the facility setup for this campaign can be found in [LHZ+ 14]. The different samples are mounted in the same probe holder on a moving platform inside the vacuum chamber. The high-enthalpy air flow is provided by a magnetoplasmadynamic arcjet generator (RD5) [AKKL96]. The probe is moved horizontally inside the chamber to adjust for heat load and total pressure. The sample diameter is 40 mm. A photography of the probe with an installed sample is shown in 1. The carbon preform C ALCARB is used as a reference material without phenolic resin in order to investigate surface effects, such as volumetric oxidation as is assumed to oc-

Figure 1: Probe with material sample installed.

cur with real lightweight ablators but without the influence of pyrolysis and blowing [HCMH12]. The carbon phenolic material is a lightweight ablator manufactured and used by A IRBUS and therefore the real material of interest within this study [BL05]. For comparison to the non-ablating case a cooled copper sample is mounted.

The VUV spectroscopy covering the 116–197 nm wavelength region has been designed to measure through the sample as it has been successfully applied by Palumbo et al. [PCWP97]. The measured radiation enters an evacuated light path through a MgF2 window directly behind a 2 mm diameter bore hole and is directed with mirrors outside the vessel to the spectrometer mounted on top of the vacuum chamber. An ACTON R ESEARCH C ORPORA TION VM-521-SG 1 m focal length spectrometer coupled with an A NDOR iStar 340T intensified charge-coupled device (CCD) camera is used. An absolute calibration for radiance of the VUV system is conducted using a M C P HERSON 632 deuterium lamp. See Ref. [HZF+ 14] .

The UV/VIS spectroscopy covering the 300–810 nm wavelength range has been setup on the side of the vacuum chamber. The boundary layer has been investigated using a classical experimental setup as it has been applied by several researchers at IRS [RAK97, Win07].

Measurements are taken with a P RINCETON I NSTRU MENTS SpectraPro 2758 spectrometer coupled with an A NDOR DU920N-0E camera. Using a f = 150 mm focusing mirror, a vertically orientated rectangular region 60 mm high and 1.6 mm wide at approximately 5 mm in front of the probe surface is imaged. Measurements are taken with a 300 l/mm grating. Separately acquired spectra covering 120 nm each are concatenated during postprocessing to a full spectrum. The UV/VIS system is calibrated using a G IGAHERTZ -O PTIK BN0102 integrating sphere for absolute radiance. This is done by positioning the integrating sphere at the measurement position and taking calibration measurements over the full spectral range. The calibrated measurements are Abeltransformed in order to obtain the local emission coefficient. See Ref. [HZF+ 15] .

Figure 2: Stagnation point VUV spectrum for C AL CARB and copper (126 – 180 nm excluding 148 – 150 nm because of saturation).

3.

RESULTS

The results are divided in spectra close to the stagnation point which includes the VUV spectra and radially distributed emission coefficients analysed mainly from the spectroscopy in the UV/VIS.

3.1.

Stagnation point spectra

In this section the resulting calibrated stagnation point spectra obtained for the three different materials are presented.

The two materials C ALCARB and a lightweight carbon phenolic are related to the cooled copper material as a non-ablating reference for better visibility in the graphs (see Figs. 2 and 3). Figure 2 shows the stagnation point spectrum (spectral and cumulative radiance) in front of C ALCARB and copper in the spectral range 126 – 180 nm.

The 148 – 150 nm range is excluded as this wavelength region was saturated during measurement. The carbon lines at 165 nm and 156 nm are not calibrated, but since they are black body limited by the same limiting curve as the carbon multiplet at 127 nm the lines have been scaled to match the respective limiting curve. Both spectra are corrected for absorption by molecular oxygen in the bore hole between the stagnation point and the window [HZF+ 15]. In front of C ALCARB the absorption is negligible except for the 141 nm nitrogen multiplet.

Atomic nitrogen and oxygen lines dominate the VUV regime for both material cases. When comparing the materials, respective lines of oxygen and nitrogen are of equal intensity except for the 174 nm multiplet which is stronger in front of C ALCARB. Particularly the carbon (neutral and singly ionized) lines in the spectrum in front of the C ALCARB sample add a significant amount to the total radiance which is 1.81 times stronger when compared to the values in front of copper.

Figure 3 presents the comparison of the stagnation point

Figure 3: Stagnation point VUV spectrum for the carbon phenolic and copper (140 – 197 nm).

spectrum (spectral and cumulative radiance) in front of the carbon phenolic and copper in the spectral range 140 – 197 nm. Both spectra exhibit the same atomic nitrogen lines. The spectrum in front of the carbon phenolic also consists of three relatively strong carbon lines. The copper spectrum is again corrected for the absorption by molecular oxygen in the bore hole. As the atomic lines in front of the carbon phenolic are unlikely to be black body limited and other absorbing species, e. g. C3 , are expected to be present, a correction for the absorption in the bore hole of the carbon phenolic is impossible.

Therefore, this uncertainty prevents reliable quantitative results. However, a qualitative trend can be observed.

The radiance of nitrogen lines in front of the carbon phenolic is much weaker than for the copper case. The total radiance in front of the carbon phenolic in the measured wavelength interval is over one magnitude weaker than the respective copper value.

Figure 4 shows the local emission coefficient in front of C ALCARB, the carbon phenolic, and copper in the spectral range 320 – 810 nm for the stagnation streamline 5 mm in front of the probe surface 25 s after the probe has been first exposed to the plasma. The spectral region is dominated by molecular emission of N+ 2 for all materials.

Only in front of the copper and the C ALCARB sample, N2 is observed. In front of the C ALCARB and the carbon phenolic sample, there appears furthermore a strong radiation of CN. Atomic lines of nitrogen and oxygen do not significantly change the total radiance and are similar in magnitude for copper and the carbon phenolic samples.

The atomic lines in front of C ALCARB are significantly stronger. In the respective wavelength intervals, the integrated emission coefficient in front of C ALCARB is 4.2 times stronger and in front of the carbon phenolic 1.23 times stronger than the respective copper value.

In conclusion, in the VUV range, the emission spectra differ significantly in terms of total radiance as well as species distribution. In the UV/VIS range, the spectra are much less different. The carbon phenolic ablator apparently reduces the N2 emission that arises in case of a copper and carbon preform. The atomic oxygen and nitrogen

Figure 4: UV/VIS spectrum on the stagnation streamline 5 mm in front of the probe surface for C ALCARB, the carbon phenolic and copper (320 – 810 nm).

Figure 5: Radial distribution of the molecular emission 5 mm in front of the probe surface.

emissions are highest in front of the C ALCARB sample.

3.2.

Radial distribution of excitation temperatures and excited species in the stagnation point region

This section presents results obtained from an analysis of the local emission spectra in the UV/VIS spectral region.

This data is reduced by an Abel transformation of the line of sight integrated measurements under the assumption of an axisymmetric plasma plume. The molecular spectra in the wavelength range 320 – 440 nm are used to deduce radially resolved rotational and vibrational excitation temperatures. Furthermore, the contribution of each molecular species’ radiation to the total radiation in this wavelength range is determined. The measured local emission coefficient is compared with simulated spectra varying the input parameters (rotational temperature, vibrational temperature, and relative number density) to best fit the measured data. The radiation database PARADE is used for the simulations [LHM12, SWD+ 09].

Due to the overlapping bands of N2 , N+ 2 and CN, a separate analysis for each molecule is impossible. Therefore, a common vibrational and a common rotational temperature are assumed [HZF+ 15] .

Figure 5 shows the radial distribution of the integrated emission coefficient for the wavelength range 320 – 440 nm, divided into the contributions from each molecular species. The material sample holder design is a flush mounted sample of 40 mm in diameter inside an 80 mm water-cooled copper housing (see Fig. 1). The end of the sample and the probe is indicated in the figures. The + biggest contributors are N+ 2 and CN. The N2 distribution in front of C ALCARB declines more steeply towards the probe edge than in front of copper and is furthermore stronger on the center line. The qualitative N+ 2 distribution in front of the carbon phenolic is similar as in front of copper, but at significantly lower quantitative values. The N2 distribution of C ALCARB follows the trend of N+ 2 and is significantly larger in magnitude than in front of cop-

Figure 6: Radial distribution of excited atoms 5 mm in front of the probe surface.

per. In front of both the carbon phenolic and C ALCARB, the CN distribution peaks near the edge of the material and declines towards the center line as well as towards the probe edge.

Figure 6 shows the radial distribution of the normalized population density of excited states of atomic nitrogen (Ei = 112909.9 cm−1 , λ0 = 574.71 nm), oxygen (Ei = 106751.5 cm−1 , λ0 = 496.96 nm) and carbon (Ei = 88568.0 cm−1 , λ0 = 581.63 nm). The integrated emission coefficient of atomic multiplets with the respective upper energy states and indicated wavelengths are used to obtain these distributions. The plot is normalized for each species. In front of C ALCARB, the distribution of all three atomic species is virtually identical.

Similar to the distribution of N+ 2 , the population density declines more steeply towards the probe edge compared to the distribution in front of the copper case. The magnitude of the atomic population density in front of C AL CARB is far larger as in front of copper. The atomic distribution in front of the carbon phenolic behaves similarly to the values in front of C ALCARB as all three species are similarly distributed. However, the magnitude is slightly lower compared to the values in front of the copper sample.

Figure 7: Radial distribution of the nitrogen atomic electronic excitation temperature 5 mm in front of the probe surface (carbon phenolic, only highly excited energy states considered).

Figure 7 shows the radial distribution of the electronic excitation temperature of atomic nitrogen in front of the carbon phenolic obtained from the UV/VIS spectroscopic data. The temperature is obtained by applying the Boltzmann plot method for atomic lines originating from highly excited states [Fle99, Win07, vdSvdM90, HZF+ 15]. A clear separation between the sample material and the probe area can be identified. The temperature reaches a value of about 8800 K in the center and decreases towards the material edge, where it reaches a plateau value of 8400 K in front of the probe housing. At the probe edge the temperature drops to about 8000 K.

3.3.

Excitation temperatures and number densities on the stagnation streamline

This section presents results obtained utilizing both the VUV spectra as well as the UV/VIS local emission coefficient. The branching ratio of optically thick multiplets in the measured VUV spectra is used to obtain number densities of various low energy states including ground states of the three atomic species [HZF+ 15]. The method requires a radiative transport calculation along the line of sight of the VUV spectroscopic system through the flowfield. The required flowfield data (distribution of highly excited atoms and rotational temperature) is obtained by the analysis of the UV/VIS spectroscopy from the side.

As a first analysis step, the copper flowfield is used for the radiative transport calculation for all three material cases. The result is presented in a Boltzmann plot on the stagnation streamline 5 mm in front of the sample.

It comprises high energy states obtained from UV/VIS spectroscopy from the side as well as low energy states obtained by analysing the branching ratio of the multiplets in the VUV regime.

Figure 8 presents the Boltzmann plot on the stagnation streamline 5 mm in front of the probe surface. The plot consists of discrete energy states of atomic nitrogen, oxygen and carbon (note that the oxygen axis is offset for

Figure 8: Atomic nitrogen, oxygen and carbon Boltzmann plot on the stagnation streamline 5 mm in front of the probe surface (oxygen number density is offset for clarity).

clarity) in front of all three materials. The ground states of oxygen, carbon and singly ionized carbon have been measured, however, the ground state of nitrogen was inaccessible in front of C ALCARB and the carbon phenolic.

All measured energy states can be described by a Boltzmann distribution which is indicated by a linear correlation function. In front of the carbon phenolic it is not possible to obtain the oxygen ground state and the high energy data is based on very weak multiplets which are overlapped by molecular bands adding too high uncertainty to the absolute population density of those states.

The data is not further analysed. The Boltzmann plot is further used to obtain electronic excitation temperatures for the different species. In the cases where the ground state was measured it is possible to sum over all energy states and thus obtain an absolute atom number density. Energy states which have not been measured directly have been interpolated using the Boltzmann relation which is assumed to be reasonable as all measured states obey a Boltzmann distribution. The respective data is presented in Table 1.

The nitrogen ground state has been extrapolated from the respective Boltzmann distribution. This procedure only allows for a qualitative comparison of the different materials. Additionally, the molecular radiation is used to obtain extrapolated number densities. The known rotational and vibrational temperatures and the known absolute emission coefficient obtained from the molecular spectra are used. Furthermore, assuming a Boltzmann distribution of the electronic states with the electronic excitation temperature taken from the atomic Boltzmann plot (Figure 8), absolute number densities are obtained by matching the absolute emission coefficient of the measurement with a PARADE simulation [LLE+ 09]. As in the case of atomic nitrogen, this procedure is an extrapolation to low energy and ground states and can therefore only provide qualitative information.

The electronic excitation temperatures of nitrogen and oxygen are both approximately 700 K larger in front of

Table 1: Excitation temperatures and number densities on the stagnation streamline 5 mm in front of the probe (extr. for extrapolated values).

Copper

C ALCARB

Trot Tvib Texc,N Texc,O Texc,C

12780 12930 8230 8430 -

8950 10190 8910 9230

nN (extr.) nO nC nC+ nN2 (extr.) nN+ (extr.)

2 nCN (extr.)

3.5 × 1022 5.1 × 1021 1.5 × 1021 1.9 × 1018 -

4.9 × 1022 6.8 × 1021 1.7 × 1021 2.0 × 1020 1.6 × 1021 2.3 × 1018 1.6 × 1018

C ALCARB compared to copper. The atom densities of all species are largest in front of C ALCARB. This is in accordance to Figs. 2 and 3 where the absolute atomic radiance also follows this trend. The molecular number densities of N2 and N+ 2 in front of copper and C ALCARB are similar. The dissociation degree of nitrogen in front of C ALCARB is ∆N2 , Calcarb = 94%, in front of copper it yields ∆N2 , copper = 92%. In front of C ALCARB ∆C, ion, Calcarb = 11% of the carbon atoms are ionized.

4.

CONCLUSION

Experiments aiming to investigate ablation-radiation coupling have been successfully conducted in the plasma wind tunnel PWK1. Optical emission spectroscopic measurement systems for different wavelength intervals have been setup to investigate the radiation of the hot flow in front of cooled copper, the carbon preform (C ALCARB) and the low density industrial carbon phenolic ablator.

Vacuum ultraviolet radiation is measured through a bore hole in the stagnation point while radiation in the UV/VIS regime is measured viewing the plasma from the side.

The VUV radiation incident on the stagnation point is strongest in front of C ALCARB and over a magnitude lower in front of the carbon phenolic. However, the stagnation point VUV radiation incident on an the carbon phenolic sample is assumed to be larger than the measured value as the radiation is expected to be partially absorbed in the bore hole.

Molecular radiation is weakest in front of copper and radiation in front of C ALCARB is about 3 times stronger than in front of the carbon phenolic. The electronic excitation temperature shows a slight increase in front of the surface of a carbon based material sample.

From the measurements presented in this study, a clear

influence of ablation on the measured radiation is stated.

In front of a purely carbonaceous material without phenolic resin the radiative heating to the surface is increased.

The introduction of a phenolic matrix reduces the VUV radiation to the surface. Apparently, the decreased number density of atoms produces less radiation and species strongly absorbing VUV radiation are injected into the boundary layer.

Analysing further measurement data (surface temperature and recession rate) will allow further understanding of the radiation effects to the actual ablator performance.

ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support by ESA through the research grant No. 2011/ITT-6632/PL (funding under ESA research grant 4000106422/12/NL/AF). The dedicated support of the institute’s workshop is indispensable and gratefully acknowledged. The authors would like to thank the project partners for sharing their hardware and knowledge and the colleagues from the High Enthalpy Flow Diagnostics Group for the continuous and prompt help.

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