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• W2315629488 abstract "The influence of the internal degrees of freedom on chemical reactions has to be well understood in order to predict the multi-temperature reaction rates with reasonable accuracy. In this paper the influence of the rotational excitation on chemical reactions is implemented into the CVCV-model (Coupled VibrationChemistry-Vibration-model) of Knab. The reaction and energy exchange rates computed with the extended and the original model are compared with each other. Rates predicted by the extended model are much closer to the equilibrium values. Comparisons with the three-temperature model proposed by Macheret et al. show good agreement if equilibrium is assumed between translational and rotational energy. Finally US-Shuttle STS-2 windward flowfields are calculated using the extended model. These calculations show that also overall quantities like wall heat flux and shock standoff distance are significantly influenced by the rotational energy inherent to the molecules. Introduction For an accurate computation of hypersonic flows around reentry vehicles the physical and chemical processes in the gas phase have to be well understood. Since the flow in the shock layer is in chemical and thermal nonequilibrium, reaction rates and relaxation processes have to be modelled quite accurately, in order to predict the heat load or the shock standoff distance reliably. Chemical reaction rates are strongly influenced by the excitation of the internal degrees of freedom. If 'Research Scientist, Member AIAA t Research Scientist, Member AIAA * Senior Scientist 5 Professor, Member AIAA a molecule is vibrationally excited its energy can contribute to overcome the activation barrier of a chemical process. This activation barrier can be changed by rotational energy. In the case of dissociation rotational excitation lowers the activation energy. Exchange reactions on the other hand can be, depending on the type of reaction, either enhanced or inhibited by rotational energy. Furthermore the distributions of vibrational and of rotational energy are not independent from each other, because the maximum amount of vibrational energy inherent to a molecule depends on its rotational quantum number and vice versa. On the other hand the excitation of the internal degrees of freedom is influenced by chemical kinetics due to the internal energy which is gained or removed in chemical reactions. This influence has to be modelled as well, consistently with the modelling of the reaction rates. In the past different models describing the influence of thermal nonequilibrium on reaction rates have been proposed. In 1963 Marrone and Treanor suggested a dissociation rate model for vibrational nonequilibrium, the so-called CVDV-model (Coupled VibrationDissociation-Vibration-model). The CVDV-model also includes an expression to allow for the influence of dissociation reactions on vibrational excitation, which has been derived consistently with the reaction rates. This model was extended by Knab to exchange and associative ionization reactions. To limit the maximum vibrational energy contributing to the reaction activation an additional parameter a was introduced into the CVCV-model (Coupled Vibration-ChemistryVibration-model). The dissociation energy of a molecule and hence the chemical reaction rates depend on the rotational quantum number of the molecule. Therefore JafFe 4 proposed to take into account this effect and derived mulCopyrtght © 1996 by the American Institute of Aeronautics and Astronautics, Inc AU rights reserved titemperature reaction rates by calculating the dissociation energy of the molecule depending on its rotational quantum number. This was realized using a Morsepotential plus an additional centrifugal term. Another three-temperature model to calculate nonequilibrium dissociation rates was proposed in 1994 by Macheret et al.. It is based on the assumptions of classical impulsive collisions and includes the effect of both the vibrational and the rotational energy inherent to the dissociating molecule. Furthermore in the case of diatom-diatom collisions also the rotational energy inherent to the collision partner is taken into account. Extended Model As shown by Jaffe and Macheret rotational energy has an influence on chemical kinetics which has to be taken into account in order to calculate the reaction rates accurately. In this paper a three-temperature reaction rate model, an extension of the CVCV-model, will be proposed, which accounts for this phenomenon not only in dissociation but also in exchange reactions. The CVCV-model is based on vibrational state selective reaction rates: k(T) = = Pe~ «„(») aA0(2) Here ev (v) is the molar vibrational energy of the vth level, AQ is the molar activation energy and DQ the molar dissociation energy of the molecule in the ground state. U and a are model parameters. For the determination of the global reaction rate these rates have to be summed up over all vibrational levels of a truncated harmonic oscillator with respect to their distribution function. The distribution of the energy levels is assumed to be Boltzmann like with the temperature Tv. P is a proportionality factor and is calculated in such way that in the limiting case Tv —* T the reaction rates correspond to the equilibrium reaction rates k (e.g. the reaction rates proposed by Park). For further detail see Ref.fl]. However this model neglects the effects of rotational energy. In order to allow for this influence an extension of the CVCV-model is suggested. The basic idea is to formulate the dissociation and activation energy for the state selective reaction rates dependent on the rotational quantum number. These extended state selective reaction rates k must then be summed up over all existing rotational-vibrational energy levels. For the global reaction rates one obtains: J=0w=0 f is the vibrational-rotational distribution function of the energy levels. The average internal energy removed (Gva,int) or gained (Gapp,mt) in a reaction is modelled in accordance with the CVCV-model:" @default.
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• W2315629488 date "1996-06-17" @default.
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• W2315629488 title "The influence of rotational excitation on vibration-chemistry-vibration-coupling" @default.
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• W2315629488 doi "https://doi.org/10.2514/6.1996-1802" @default.
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