Simulation of flows in shock-tube facilities by means of a detailed chemical mechanism for nitrogen excitation and dissociation
A database was recently developed at NASA Ames Research Center for the dissociation and excitation rates of the rovibrational energy levels of nitrogen. This work is a first step toward the development of a simplified mechanism for 3-D simulations of atmospheric entry flows. First, we have derived a Boltzmann average dissociation rate based on rotational and vibrational temperatures. At thermal equilibrium, a very good agreement is obtained with the rate proposed by Park (1990). Significant discrepancies are observed in thermal non-equilibrium conditions. Then, we have developed a 0-D vibrational state-to-state collisional model to simulate a typical shock in a nitrogen flow based on vibrational-specific excitation and dissociation reaction rates derived by assuming a Boltzmann distribution of the rotational energy levels at the translational temperature. We have shown that the vibrational energy relaxation for the monoquantum excitation processes deviates significantly from the conventional Landau-Teller law. The dissociation process, coupled with the energy level relaxation dynamics, strongly depends on multi-quantum jump processes. Finally, we have implemented the multi-temperature dissociation rate in a 1-D code for shock-tube flows and have compared our results with experimental results.
Analysis of the FIRE II Flight Experiment by Means of a Collisional Radiative Model
An accurate investigation of the behavior of electronically excited states of atoms and molecules in the post shock relaxation area is carried out by means of the Collisionalradiative model. The model is applied to a 1D shock tube code and the operating conditions are taken from three points in the trajectory of the FIRE II flight experiment. We account for thermal nonequilibrium between the translational and vibrational energy modes of individual molecular species and treat the electronic states of atoms and molecules as separate species. Relaxation of free-electrons is also accounted for by making use of a separate conservation equation for their energy. Non-Boltzmann distributions of the electronic state populations of atoms and molecules are allowed. Deviations from Boltzmann distributions are expected to occur in a rapidly ionizing regime behind a strong shock wave, due to the depletion of the high lying bound electronic states. In order to quantify the extent of departure from equilibrium of the electronic state populations, results are compared with those obtained assuming a Boltzmann distribution.
High temperature shock tube study of reactions of CH and C-atoms with N2
The reactions of CH and C-atoms with N2, which are believed to be initial steps in the prompt-NO mechanism, were studied at high temperature behind reflected shocks. CH was formed from the pyrolysis of highly dilute mixtures of CH4 or C2H6 ( C ( 3 P ) + N 2 → C N + N , (1) leading to k1=6.3×1013 exp(−23160 K/T) (±30%) cm3 mol−1 s−1 over the temperature range 2660 to 4660 K and pressure range 0.5 to 1 atm. In order to determine the rate coefficient of C H ( X 2 I I ) + N 2 → H C N + N , (2) a perturbation technique was employed. In this technique, the CH profile resulting from pyrolysis of CH4 or C2H6 dilute in argon was perturbed by the addition of N2. A detailed analysis of the CH profiles led to a rate coefficient, k2=4.4×1012 exp(−11060 K/T)(±50%) cm3 mol−1 s−1 over the temperature range 2500 to 3800 K and pressure range 0.6 to 1 atm. N-atom measurements provided an independent verification of k2 and the products of reaction 2.