A reduced chemical kinetic model for Titan's atmosphere has been developed. This new model with 18 species and 28 reactions includes the mainfeatures of a more complete scheme, respecting the radiative fluxes. It has been verified against three key elements: a sensitivity analysis, the equilibrium chemical composition using shock tube simulations in CHEMKIN, and the results of computational fluid dynamics (CFDs) simulations. 1. Introduction The Saturn largest moon Titan, with its thick atmosphere rich in organic compounds and nitrogen, provides similar aspects to Earth. As a consequence, numerous scientists are interested in exploring it and are hoping for hints on how life began on Earth. Thus, it is likely that new missions will follow Cassini-Huygens and try to bring more information on this orange moon. The accurate prediction of the heat fluxes during the entry of the sounding probes is crucial to the integrity of the probe and to the quality of the protection of the scientific instruments. To investigate further the limits of these fluxes, obligatory for efficient thermal protection system design and sizing, CFDs tools are constantly developed and improved. One of the critical parameters for CFDs codes is the chemical kinetic model, as it describes the reactions schemes. However, the complexity of complete modelling is incompatible with design tools, which require fast response time simulation, and it is not necessary for such atmospheres, where the dominant species dictate the physics. The kinetic model depends primarily on the composition of the atmosphere. Despite the recent success of the Cassini-Huygens mission, uncertainties remain regarding the composition of Titan's atmosphere. The main components are N2, CH4, and Ar. One of the first kinetic models commonly used was proposed by Nelson et al. [1] in 1991. However, this first model does not take into account the formation of CN via HCN (as this species is not included). Also, as shown in many papers [2, 3], CN is a strong radiator and the radiative heat flux is predominant during Titan atmospheric entries. Moreover, the reaction rates used were not up to date. Consequently, a new chemical kinetic model was proposed by G？k？en [4], it is composed of 21 species (N2, CH4, CH3, CH2, CH, C2, H2, CN, NH, HCN, N, C, H, Ar, , CN+, N+, C+, H+, Ar+, and e？) and 35 reactions (rates for these reactions are given in Appendix A). Despite this new reduced model, CFDs simulations are still time and memory consuming (several hours depending on the mesh and the physics implemented). As a consequence, each
References
[1]
H. F. Nelson, C. Park, and E. E. Whiting, “Titan atmospheric composition by hypervelocity shock-layer analysis,” Journal of Thermophysics and Heat Transfer, vol. 5, no. 2, pp. 157–165, 1991.
[2]
H. Osawa, S. Matsuyama, N. Ohnishi, M. Furudate, and K. Sawada, “Numerical computation of radiative heating environment for huygens probe entry flight,” Journal of Thermophysics and Heat Transfer, vol. 22, no. 2, pp. 140–149, 2008.
[3]
M. J. Wright, D. Bose, and J. Olejniczak, “The impact of flowfield-radiation coupling on aeroheating for Titan aerocapture,” in Proceedings of the 42nd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nev, USA, 2004.
[4]
T. G？k？en, “N2-CH4-Ar chemical kinetic model for simulations of Titan atmospheric entry,” Journal of Thermophysics and Heat Transfer, vol. 21, no. 1, pp. 9–18, 2007.
[5]
P. Leyland, R. Sobbia, and J. B. Vos, “Chemical kinetic and radiating species studies of Titan aerocapture entry,” in Proceedings of the 16th Australian Fluid Mechanics Conference, Australia, 2007.
[6]
F. Mazoue and L. Marraffa, “Flow-field/radiation coupling analysis for huygens probe entry into Titan atmosphere,” in Proceedings of the 38th AIAA Thermophysics Conference, Toronto, Canada, June 2005.
[7]
D. L. Baulch, C. J. Cobos, R. A. Cox, et al., “Evaluated kinetic data for combustion modelling, supplement I,” Journal of Physical Chemistry Reference Data, vol. 26, no. 6, pp. 847–1033, 1994.
[8]
D. Bose, M. J. Wright, D. W. Bogdanoff, G. A. Raiche, and G. A. Allen Jr., “Modeling and experimental assessment of CN radiation behind a strong shock wave,” Journal of Thermophysics and Heat Transfer, vol. 20, no. 2, pp. 220–230, 2006.
[9]
D. Potter, S. Karl, and P. Leyland, “TN032-EPFL contribution to ESA-TRP AMOD,” Tech. Rep., ESA, 2009.
[10]
R. V. Yelle, D. F. Strobell, E. Lellouch, and D. Gautier, “Engineering models for Titan's atmosphere,” ESA SP-1177, pp. 243–256, 1997.
[11]
B. J. McBride and S. Gordon, “Computer program for calculation of complex chemical equilibrium compositions and applications. Part 2: users manual and program description. Reference publication 1311, NASA,” Tech. Rep., NASA, 1996.
[12]
C. R. Wilke, “A viscosity equation for gas mixtures,” The Journal of Chemical Physics, vol. 18, no. 4, pp. 517–519, 1950.
[13]
S. Gordon and B. J. McBride, “Computer program for calculation of complex chemical equilibrium compositions and applications. Part 1: analysis. Reference publication 1311, NASA,” Tech. Rep., NASA, 1994.
[14]
C. Park, R. L. Jaffe, and H. Partridge, “Chemical-kinetic parameters of hyperbolic earth entry,” Journal of Thermophysics and Heat Transfer, vol. 15, no. 1, pp. 76–90, 2001.
[15]
A. J. Dean and R. K. Hanson, “CH and C-atom time histories in dilute hydrocarbon pyrolysis: measurements and kinetics calculations,” International Journal of Chemical Kinetics, vol. 24, no. 6, pp. 517–532, 1992.
[16]
T. Kruse and P. Roth, “Kinetics of C2 reactions during high-temperature pyrolysis of acetylene,” Journal of Physical Chemistry A, vol. 101, no. 11, pp. 2138–2146, 1997.
[17]
D. L. Baulch, C. J. Cobos, R. A. Cox, et al., “Evaluated kinetic data for combustion modelling,” Journal of Physical Chemistry Reference Data, vol. 21, no. 3, pp. 411–429, 1992.
[18]
C. Park, J. T. Howe, R. L. Jaffe, and G. V. Candler, “Review of chemical-kinetic problems of future NASA missions, II: Mars entries,” Journal of Thermophysics and Heat Transfer, vol. 8, no. 1, pp. 9–23, 1994.
[19]
W. Tsang, “Chemical kinetic data base for propellant combustion 2: reactions involving CN, NCO, and HNCO,” Journal of Physical Chemistry Reference Data, vol. 21, no. 4, pp. 753–791, 1992.
[20]
NIST Chemical Kinetics Database, ver. 7.0, 2003.
[21]
W. Tsang and J. T. Herron, “Chemical kinetic data base for propellant combustion 1: reactions involving NO, NO2, HNO, HNO2, HCN and N2O,” Journal of Physical Chemistry Reference Data, vol. 20, no. 4, pp. 609–663, 1991.
[22]
A. J. Dean, R. K. Hanson, and C. T. Bowman, “High temperature shock tube study of reactions of CH and C-atoms with N2,” in Proceedings of the 23rd Symposium (International) on Combustion, pp. 259–265, 1990.
[23]
T. Sommer, T. Kruse, and P. Roth, “Perturbation study on the reaction of C2 with N2 in high-temperature C60/Ar + N2 mixtures,” Journal of Physical Chemistry A, vol. 101, no. 20, pp. 3720–3725, 1997.
[24]
S. T. Wooldridge, R. K. Hanson, and C. T. Bowman, “A shock tube study of reactions of CN with HCN, OH, and H2 using CN and OH laser absorption,” International Journal of Chemical Kinetics, vol. 28, no. 4, pp. 245–258, 1996.
[25]
D. F. Davidson and R. K. Hanson, “High temperature reaction rate coefficients derived from N-atom ARAS measurements and excimer photolysis of NO,” International Journal of Chemical Kinetics, vol. 22, no. 8, pp. 843–861, 1990.
[26]
A. J. Dean, D. F. Davidson, and R. K. Hanson, “A shock tube study of reactions of C atoms with H2 and O2 using excimer photolysis of C3O2 and C atom atomic resonance absorption spectroscopy,” The Journal of Physical Chemistry, vol. 95, no. 1, pp. 183–191, 1991.
[27]
T. R. Roose, R. K. Hanson, and C. H. Kruger, “Decomposition of NO in the presence of NH3,” in Proceedings of the International Symposium on Shock Tubes and Waves, vol. 11, pp. 245–253, 1978.
[28]
C. Park, “Review of chemical-kinetic problems of future NASA missions, I: Earth entries,” Journal of Thermophysics and Heat Transfer, vol. 7, no. 3, pp. 385–398, 1993.
