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SHOCK LAYER KINETICS OF CO AND CO2-BASED ATMOSPHERES
Brett A. Cruden(1), Aaron M. Brandis
(1)AMA Inc, NASA Ames Research Center, Moffett Field, CA 94035, USA, E-mail:Brett.A.Cruden@nasa.gov
ABSTRACT
The present paper reports the analysis of shocks in pure CO at velocities from 3-9 km/s in NASA Ames’ Electric Arc Shock Tube (EAST) facility, with the intent of simplifying the analysis of CO2 shockwaves produced during Martian and Venus entries. Tunable diode laser absorption spectroscopy measurements were performed concurrently with emission spectroscopy over a wide spectral range (VUV through Mid-infrared). The temperature trends extracted from the data show CO dissociation rates much faster than suggested by previous models. The trend of C2 radiation suggests rates for C2 exchange and dissociation that are 1-2 orders of magnitude higher than those employed in the Park or Johnston models. Radiation in the mid-infrared from molecular CO is at odds with absorption measurements.
1. INTRODUCTION
In previous years, spectrally and spatially resolved measurements of shock layer radiation in shocks composed of CO2 and mixtures (with N2/Ar) relevant to Martian and Venus entries have been reported [1]. Among other things, these measurements have been used to revise shock layer radiation models that are used for aeroheating predictions [2]. These updated models are now being employed by NASA flight projects. While these models were found to better predict the EAST data than the legacy models of Park [3], and are not inconsistent with available flight and ground test data in CO2, some question remains as to their correctness.
A more recent set of experiments was conducted to separate the impact of CO2 dissociation by starting the shock from CO molecules and using tunable diode laser absorption spectroscopy (TDLAS) to probe the molecular ground state and obtain translational temperature information [4]. This work focuses on the analysis of this shock tube data.
2. EXPERIMENTAL
Experiments were conducted in the NASA Ames Electric Arc Shock Tube (EAST) and have been reported elsewhere [4]. The EAST facility is a 10 cm inner diameter shock tube which is driven by an electric arc source. The shock waves travel 7.9 m downstream to the test section where measurements are performed. The TDLAS measurement performs a scan of CO absorption as a function of time at a fixed position in the tube. The properties of the laser allow for the measurement of a 0.07 nm spectral range in 0.5 s. The line measured corresponds to the fundamental vibrational transition of CO at a rotational quantum of J=51. A Gaussian or Voigt fit to the lineshape allows the extraction of an average translational temperature over the 0.5 s scan period. The intensity of the absorption may also be used to obtain the number density of CO.
Fig. 1. Spectrally and spatially resolved radiance data obtained from an incident shockwave in CO
Concurrently, four spectrometers obtain an image of the shock at one moment of time (accumulated over 0.1-1.0 s) at a fixed position, covering an axial length in the tube of 12.5 cm. The data is spectrally resolved and calibrated to units of absolute radiance. An example of such data is given in Fig. 1.
3. ANALYSIS
Temperature analysis of the shock is obtained from three different methods. The translational temperature is obtained from the Doppler width in the TDLAS data [4]. Rotational and vibrational temperatures are obtained by fitting the shape of C2 spectra. Electronic temperature is obtained by taking the Planck-limited data in the vacuum ultraviolet [1]. All four of these temperatures are co-plotted in Fig. 2 for a representative test. These are compared to predicted temperatures using three different CO dissociation rates. For this condition, the rate of Hanson [5] best represents the data. This rate is at odds with that of Schwenke, which is based on ab initio calculations from the electronic ground state of CO [6]. It is suggested that the difference may be attributed to the dissociation through metastable states of CO which would cut the activation energy for dissociation nearly in half.
Fig. 2. Measured temperature profiles and predicted temperatures using different CO dissociation rates
Fig. 3. Comparison of radiance attributed to C2 Swan bands on the basis of different reaction kinetic choices for CO dissociation and C2 dissociation/exchange
There are two reactions that influence the C2 density behind the shock. The rates proposed by Park [3] are 1-2 orders of magnitude larger than other measurements in the literature. An example of altering these rates is shown in Fig. 3 where the rates of Fairbarn [7, 8], along with the dissociation rate from Hanson, changes the shape of the C2 radiance curve versus that obtained from the Johnston model, which is based on Park’s rates, bringing it into better agreement with the measured data. The overprediction at the peak radiance is attributed to the lack of a non-Boltzmann model for C2. Measurements at much higher velocity, however, are better matched with the Johnston model. Thus, it is suggested that a new rate be constructed that matches the Johnston rate at high temperature and the Hanson rate at low temperature.
4. CONCLUSIONS
The full paper will discuss the above analysis in further detail along with aspects of CO radiation in the vacuum ultraviolet and near infrared.
5. REFERENCES
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Cruden, B. A., Prabhu, D., et al., Absolute Radiation Measurement in Venus and Mars Entry Conditions, Journal of Spacecraft and Rockets, Vol. 49, 1069-1079, 2012.
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Johnston, C. O., and Brandis, A. M., Modeling of nonequilibrium CO Fourth-Positive and CN Violet emission in CO2–N2 gases, Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 149, 303-317, 2014.
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Park, C., Howe, J. T., et al., Review of chemical-kinetic problems of future NASA missions, II: Mars entries, Journal of Thermophysics and Heat transfer, Vol. 8, 9-22, 1994.
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MacDonald, M. E., Brandis, A. M., et al., Post-Shock Temperature and CO Number Density Measurements in CO and CO2, Vol. 2017-4342, 2017.
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Hanson, R. K., Shock-tube study of carbon monoxide dissociation kinetics, The Journal of Chemical Physics, Vol. 60, 4970, 1974.
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Schwenke, D. W., Jaffe, R. L., et al., Collisional Dissociation of CO: ab initio Potential Energy Surfaces and Quasiclassical Trajectory Rate Coefficients, Journal of Physical Chemistry, submitted.
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Fairbairn, A. R., The dissociation of carbon monoxide, Proc. R. Soc. London A, Vol. 312, 207-227, 1969.
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Fairbairn, A. R., Shock-tube study of the dissociation rate of CN, Journal of Chemical Physics, Vol. 51, 972-975, 1969.
Summary
Shock tests in pure CO have been examined to update modeling parameters in CO/CO2 atmospheres
