M. Lino da Silva

State-Resolved Dissociation Rates for Extremely Nonequilibrium Atmospheric Entries

This paper presents the application of the forced harmonic oscillator method to the simulation of state-resolved dissociation processes behind high-temperature shock waves typical of atmospheric reentries. Improvements have been brought to the model, considering a more precise method for the calculation of the different vibrational level energies,thereforeincreasingtheaccuracyofthepredictedtransitionprobabilitiesbetweenhighervibrationallevels close and above the dissociation limit. The model has been validated against data issued from recent experiments, as well as data issued from semiclassical trajectory calculations for collisions between different species. A good overall agreementisachievedagainstsuchotherdata.Adatabaseofreactionrateshasbeenconstructed withthepurposeof simulating shock-heated nitrogen flows. Dissociation processes behind a shock wave have been simulated for different postshock translational temperatures. At lower temperatures, the well-known ladder-climbing phenomenon is the main dissociation channel behind a shock wave, with dissociation occurring for transitions from the vibrational levels close to the dissociation limit. At higher temperatures, transitions between the different vibrational levels of nitrogen become roughly equiprobable, and the overall range of bound vibrational levels contributes to the dissociation.

Nonequilibrium Dissociation Processes in Hyperbolic Atmospheric Entries

Physical-chemical processes encountered behind atmospheric entry shock waves are known to occur in extremely nonequilibrium conditions. Also, translational temperatures up to 100,000 K may be reached immediately behind the shock wave. Such challenging conditions require the development of adequate state-to-state models, which prevents using widespread first-order theories. A complete database for the simulation of state-resolved dissociation processes is presented in this paper. Rate coefficients valid up to very high temperatures have been obtained for diatom-diatom collisions, using the forced harmonic oscillator theory. The rate coefficients for atom-diatom collisions have been selected after a critical review of the existing data sets, as the forced harmonic oscillator theory proved inadequate for the simulation of such processes. Such a consistent state-to-state model has then been used for simulating nitrogen dissociation processes behind very high-temperature shock waves, and the obtained results have been compared with those obtained using popular one- and two-temperature models.

A review of non-equilibrium dissociation rates and models for atmospheric entry studies

The appropriate simulation of vibrational exchange and dissociation processes in high-temperature (above 1000 K) plasmas mandates the application of more detailed state-to-state models, when compared with those based on first-order theories, utilized for the simulation of low-pressure plasmas. Such can be achieved through the application of approaches such as the forced harmonic oscillator model, or the quasi-classical trajectory model. This allows obtaining multiquantum state-resolved rates significantly more accurate at high temperatures, as compared with rates issued from first-order perturbation theories. This work reviews the more recent high-temperature datasets proposed by several groups, including our own. Such datasets have then been applied to the simulation of dissociation processes in high-temperature shock-heated nitrogen flows, and a comparison against traditional multi-temperature models has been carried out, for post-shock temperatures ranging from 20 000-100 000 K. Differences between the results predicted by the state-resolved and macroscopic models are significant, except in the high-temperature limit.

Non-equilibrium kinetics in N2 discharges and post-discharges: a full picture by modelling and impact on the applications

The main concerns associated with the establishment of a self-consistent model for N2 discharges and post-discharges at low pressures (typically p ~ 1 Torr), as well as in mixtures of this gas with O2 and CH4 are analysed and discussed. The focus is given on the coupling of the various kinetics involved: electrons, vibrational molecules N2 , dissociated atoms N(4S), ionic species, and various atomic and molecular electronic states. The impact of N2–O2 and N2–CH4 systems on the applications is briefly summarized by reviewing the essential kinetics. The difficulty in incorporating a self-consistent model for the surface kinetics is also discussed and a state-of-the-art approach for wall reactions is presented.

Radiation from an equilibrium CO2–N2 plasma in the [250–850 nm] spectral region: II. Spectral modelling

In the first part of this work, described in a previous paper, the thermodynamic conditions in an atmospheric pressure inductively coupled CO2–N2 plasma have been determined, and the radiation emission spectrum has been measured and calibrated in the [250–850 nm] spectral region. In the second part of this work, a synthetic radiation spectrum is obtained taking into account (a) the geometry of the plasma torch and (b) the local thermodynamic conditions of the plasma. This synthetic spectrum has then been compared against the measured spectrum. The good agreement between the two spectra allows validating the spectral database of the line-by-line code SPARTAN for the simulation of the radiative emission of CO2–N2 plasmas from the near-UV to the near-IR spectral region.

GPRD, a database for the spectral properties of diatomic molecules of atmospheric interest

A short note describing the development of a database providing factual and numerical data on the spectral properties of diatomic molecules. This database is available online for the overall scientific community at the following adress: this http URL

Coupled Hydrodynamic/State-Specific High-Temperature Modeling of Nitrogen Vibrational Excitation and Dissociation

Vibrational state-specific kinetics are applied to the simulation of high-temperature gas behind strong shocks. A hydrodynamic numerical tool was developed based on recent excitation and dissociation state-resolved models from the FHO theory, and on accurate vibrational energy levels for the N2 electronic ground-state X 1 � + . The master equation is solved in timedependent, post-shock relaxation and multi-dimensional CFD simulations. The obtained results are compared to classical multi-temperature models often used in the aerospace community for non-equilibrium flow simulations. There is considerable interest in understanding non-equilibrium processes in shock-heated flows during high-speed atmospheric entries. For very high-temperature conditions, internal level populations significantly depart from the Boltzmann distribution, making the classical multi-temperature models totally in-appropriated. Accurate determination of internal levels populations requires the resolution of the so-called master equation, a system of coupled statespecific mass conservation equations associated to each internal level. Rotational levels are assumed to be populated by a Boltzamnn distribution at the gas temperature, this assumption being justified by the fast rotational relaxation times. Atomic nitrogen excitation and gas ionisation processes are discarded. Only the electronic ground-state X 1 � + of molecular nitrogen is considered. Its vibrational energy levels are computed through potential-curve reconstruction

Thermodynamics equilibrium and non equilibrium of plasma flows

A plasma flow is described using the properties of the entropy function. The plasma flow is a multi-temperature mixture with reactive processes (translation: T for heavy particles, Te for electrons and Tvj for the vibrational temperature of the species j), diffusion and non equilibrium chemical reactions. The entropy rate production is determined for a plasma at rest and for a plasma flow. Equilibrium conditions are deduced and phenomenological equations are presented for non equilibrium plasma flow conditions.