Ph. Rivière

Correlated-k and fictitious gas methods for H2O near 2.7 μm

Abstract The correlated-k distribution (CK) method is extended with a fictitious gas approach in order to account for strong temperature-gradient effects. The correlated-k fictitious gas (CKFG) method involves consideration of the absorbing gas as a mixture of fictitious gases, each characterized by real gas lines with a lower level energy belonging to a fixed range. CK and CKFG parameters have been obtained from a line-by-line (LBL) approach for H2O between 2900 and 4200 cm-1 in the 300–1500 K temperature and 0.2–3 atm pressure ranges. Results obtained from CK, CKFG and narrow-band statistical (NBS) models are compared under various conditions with LBL results. The CK approach yields accurate results for isothermal and slightly nonisothermal media. But for large temperature gradients, only CKFG results agree well with LBL calculations. It is shown that the CK approach leads to an underestimation of more than 50% under some conditions of the radiation emitted by a hot medium and transmitted through a cold medium. For practical use, the variations of the CK and CKFG model parameters with temperature and pressure are approximated by analytical functions with few coefficients fitted from LBL calculations. It is shown that this fitting procedure does not degrade the accuracy of either model. Finally, the utility of each of the models is discussed for use under various radiative transfer situations.

AIR MIXTURE RADIATIVE PROPERTY MODELLING IN THE TEMPERATURE RANGE 10,000-40,000 K

Abstract A model based on the distribution functions of the absorption coefficients is used to describe the radiative properties of an air plasma at atmospheric pressure between 10,000 and 40,000 K. In order to account for spectral fine structure effects on radiative transfer in nonuniform media, fictitious species whose spectra reasonably satisfy the scaling approximation are introduced. The absorption spectra of these fictitious species are assumed to be spectrally uncorrelated. Radiative properties are described in terms of absorption coefficients and the model can be used with arbitrary radiative transfer equation solvers. Model parameters are generated from spectral high resolution calculations. The different approximations required to implement the model are discussed and their validity is checked by comparisons with line-by-line benchmarks. It is shown that the model can predict radiative transfer along strongly nonisothermal columns within a few percent.

Contributions of diatomic molecular electronic systems to heated air radiation

Abstract A spectroscopic database has been constructed for all the contributing electronic systems of air diatomic molecules for elevated temperatures (up to 15,000 K ). The electronic systems which have been investigated are N2 first- and second-positive, Birge–Hopfield 1 and 2, Carroll–Yoshino, Worley and Worley–Jenkins, N2+ Meinel, first- and second-negative, O2 Schumann–Runge and NO γ,β,δ,e,γ′,β′,11,000 A and infrared. The RKR potentials have been reconstructed in order to calculate the vibrational wavefunctions, and the electronic-vibrational part of line intensities has been computed with up-to-date ab-initio electronic transition moment functions. The results of our calculated vibrational band strengths are in good agreement with the available measurements. The positions and intensities of the rotational lines have been determined in intermediate a/b Hunds coupling case with up-to-date molecular constants, and the lambda doubling has been resolved when related parameters were available in the literature. Absorption spectra and the optically thin radiation source strengths of the studied electronic systems are presented and discussed in terms of their comparative contribution to emission and absorption.

Correlated-k fictitious gas model for H2O infrared radiation in the Voigt regime

The ability of the correlated-k fictitious gas (ckfg) approach to accurately predict radiative transfer in strongly nonhomogeneous media is studied for the case of water vapor absorption lines with Voigt profiles. In the ckfg approach, the real gas is considered as a mixture of fictitious gases, each one characterized by absorption lines with similar transition lower level energies. The correlated-k approach is then applied to each gas and the spectra of different gases are assumed to be uncorrelated. The model is studied for a range of temperatures (180–2500 K) and pressures (10-2 atm). In addition, the model takes into account the nonlorentzian behavior of the far wings. The model parameters are obtained from line-by-line calculations based on approximate spectroscopic data suitable for high temperature and low spectral resolution applications. The choice of the quadrature for spectral integrations and of the lower level energy E″ ranges is discussed. ckfg results are compared to the reference line-by-line calculations and to the results from the correlated-k (ck) approach, and from available statistical narrow-band (SNB) models in the Voigt regime. It is shown that a 10 point quadrature may be sufficiently accurate, even for very small line widths. ckfg results agree well with line-by-line calculations while ck and SNB models may strongly underestimate the intensity of radiation emitted by a hot gaseous column and transmitted through a long cold column.

An approximate data base of H2O infrared lines for high temperature applications at low resolution. Statistical narrow-band model parameters

Abstract An approximate spectroscopic H2O data base suitable for high temperature and a few tens cm-1 spectral resolution applications has been developed as an extrapolation of HITRAN92 and Flaud and Camy-Peyret bases for the spectral and temperature ranges 150–9000 cm-1 and 300–2500 K, respectively. The energies of high rovibrational levels have been extrapolated by using analytical expressions derived from Tyuterevs Hamiltonian. The intensities of lines issued from the fundamental vibrational level and from high rotational levels have been extrapolated empirically. The intensities of lines issued from other vibrational levels are calculated from the intensities of the corresponding lines issued from the fundamental vibrational level by using the independent rigid harmonic oscillator approach. Satisfactory agreement between line-by-line calculations based on the extended data base and experimental data is obtained. Statistical narrow-band model parameters have been adjusted using line-by-line calculations; they are available under request.

Radiative properties and radiative transfer in high pressure thermal air plasmas

The aim of this paper is to investigate radiative properties of thermal air plasmas in wide ranges of pressure and temperature, and to analyse the accuracy of some spectral and geometrical approximations in high-pressure radiative transfer applications. Comprehensive calculations of absorption spectra, including molecular, atomic and ionic line and continuum radiation, are presented and the dependence of these spectra on the pressure level is analysed. The high resolution spectra, in association with a rigorous ray-tracing method, are then used to study the accuracy of the P1 and the simplified SP3 geometrical approximations in 1D axisymmetric geometries. Cylindrical plasma columns at uniform pressure and with a non-uniform pressure distribution are considered. The P1 approximation provides acceptable results but the SP3 approximation is found to be more accurate. Concerning the spectral approximations, the use of band averaged Rosseland mean absorption coefficients yields volumetric radiative powers in fairly good agreement with line-by-line calculations.

Systematic semi-classical calculations of Stark broadening parameters of NI, OI, NII, OII multiplets for modelling the radiative transfer in atmospheric air mixture plasmas

Abstract Electron and ion Stark broadening parameters (widths and shifts) of NI, OI, NII, and OII for all the electric dipolar LS allowed transitions from the NIST data bases have been calculated following the semi-classical approach by Sahal–Brechot (Astron. & Astrophys. 1 (1969) 91). The input data, positions and oscillator strengths of electric dipolar transitions, have been taken from the TOPBASE data bases (The Opacity Project, Vol. 1. Bristol and Philadelphia: Institute of Physics Publishing, 1995). When required, ion contributions have been estimated using the quasistatic approximation. The computational procedures have been validated by comparisons with available experimental and theoretical data. Discrepancies between measured line widths and our calculated ones are typically about 20%, while the magnitude order of the neutral line shifts is generally well predicted. Calculations have been carried out systematically for air mixture plasmas in the temperature range 6000– 25,000 K under the assumption of chemical equilibrium at atmospheric pressure.

Subgrid-scale model for radiative transfer in turbulent participating media

The simulation of turbulent flows of radiating gases, taking into account all turbulence length scales with an accurate radiation transport solver, is computationally prohibitive for high Reynolds or Rayleigh numbers. This is particularly the case when the small structures are not optically thin. We develop in this paper a radiative transfer subgrid model suitable for the coupling with direct numerical simulations of turbulent radiating fluid flows. Owing to the linearity of the Radiative Transfer Equation (RTE), the emission source term is spatially filtered to define large-scale and subgrid-scale radiation intensities. The large-scale or filtered intensity is computed with a standard ray tracing method on a coarse grid, and the subgrid intensity is obtained analytically (in Fourier space) from the Fourier transform of the subgrid emission source term. A huge saving of computational time is obtained in comparison with direct ray tracing applied on the fine mesh. Model accuracy is checked for three 3D fluctuating temperature fields. The first field is stochastically generated and allows us to discuss the effects of the filtering level and of the optical thicknesses of the whole medium, of the integral length scale, and of the cutoff wave length. The second and third cases correspond respectively to turbulent natural convection of humid air in a cubical box, and to the flow of hot combustion products inside a channel. In all cases, the achieved accuracy on radiative powers and wall fluxes is about a few percents.

Radiation Models and Radiation Transfer in Hypersonics

The paper presents radiation models developed to investigate radiation in entry in Earth, Mars and Jupiter atmospheres. The capacity of ASTEROID computing code to simulate elementary radiative processes, calculate spectral and groups optical properties, and also solve simple radiative heat transfer problems is presented for Earth entry. The large number of radiative processes involved in the radiative flux is put forward. The contributions of the different radiative processes encountered in Mars entry are studied using the HTGR spectroscopic database. The validity of this database with respect to diatomic molecules systems and CO2 infrared radiation is illustrated through experimental validations. The accuracy of statistical narrow-band model to predict radiative flux is illustrated for an afterbody. Finally recent improvements of the model developed for the calculation of radiative properties of high-temperature H2/He mixtures representative of Jupiter atmosphere is presented. The model takes into account the most important radiative processes.

Modelling radiative properties of participating species in a microwave plasma reactor for diamond deposition

The paper details the modelling of radiation in a microwave assisted plasma reactor used to deposit synthetic diamond over a substrate. The main radiatively active constituents in the reactor are atomic and molecular hydrogen, acetylene, methane and soot (if produced). Radiation from hydrogen occurs in ultraviolet (UV) whereas the hydrocarbons are active in the infrared region. Soot absorb and scatter in the UV but only absorption is important in the infrared-visible (IR-V) region. Hence, the two spectral regions have been treated independently. A two temperature model has been adopted for hydrogen thermodynamic state where Tg represents rotational, vibrational and translational temperature and Te represents electronic excitation temperature. As scattering is significant in UV, the radiative transfer equation is solved using Discrete Ordinate Method (DOM) with cumulative-k narrow-band model for molecular hydrogen. Radiation from atomic hydrogen has been found to be negligibly small compared to molecular hydrogen. In the IR-V, radiative transfer equation is solved using ray tracing method with gas properties represented by statistical narrow-band models. Preliminary simulations for reactor conditions indicate that soot significantly increase the radiative transfer in the reactor and presence of soot can disrupt the operation of the plasma reactor.