Search

Abstract: A new line intensity distribution function is introduced in order to improve the accuracy of statistical narrow-band (SNB) models in the Doppler line broadening regime and in a wide temperature range. This distribution function generalizes the Malkmus distribution through a parameter enabling to adjust the contribution of small line intensities. This new model is shown to enhance significantly SNB accuracy for CO2 uniform column radiation, especially at low temperatures. For non-uniform columns, Lindquist–Simmons type approximations are derived for this new intensity distribution. Their results are in much closer agreement with line by line results than Curtis–Godson approximation ones when steep temperature gradients are considered. [ABSTRACT FROM AUTHOR]

Abstract: Band model parameters for high-temperature methane have been generated up to 2000K from an extended spectroscopic database. Part of the spectroscopic data are issued from calculations made in the framework of the effective Hamiltonian approach. These data have been completed by a statistical extrapolation. The calculations of global radiative properties such as band absorbtances and total emissivities are in good agreement with the available experimental data showing that the contribution of the hot bands is correctly taken into account. Finally, the degree of correlation between CH4, CO2 and H2O spectra in typical conditions of combustion applications is discussed. Band model parameters are available upon request. [Copyright &y& Elsevier]

We present in this paper numerical simulations of coupled radiative transfer and turbulent flows at high temperature and pressure, typical of multiphase flows encountered in aluminised solid propellant rocket engines. The radiating medium is constituted of gases and of liquid or solid particles of oxidised aluminum. The turbulent flow of the gaseous phase is treated by using a four equation, low Reynolds number, boundary-layer-type turbulence model. The distributions of concentrations, temperatures, and temperature fluctuation variances of particles are calculated from a Lagrangian approach and a turbulence dispersion model. Thermal and mechanical non-equilibrium between the gas and different classes of particles is allowed. A locally one dimensional, iteratively based, radiative transfer solver is developed to compute wall fluxes and radiative source terms. It is shown that the thermal boundary layer attenuates significantly the radiative fluxes coming from the outer regions. Particle radiation is found to be much more important than gas radiation. Turbulent dispersion of particles in the boundary layer induces a decrease of particle concentration in the region of maximum turbulent kinetic energy, and then, decreases the attenuation effect of wall fluxes due to the boundary layer. The effects of turbulent temperature fluctuations are found to be small in the problem under consideration. [Copyright &y& Elsevier]

An original approximate model, similar to Ludwig''s multigroup statistical narrow-band model, and to the fictitious gas CK model (CKFG) has been developed and validated to treat IR long-range sensing applications involving H2O as emitting and absorbing species. The main idea is to split the lines of a gas into N classes depending on the values of E″, the energies of the lower levels of the transitions. The statistical narrow-band (SNB) model and the Curtis–Godson approximation are applied separately to each class. Parameters of the model have been extracted from an adjustment of the curves of growth obtained from line-by-line calculations. Relative errors of the model in typical conditions of atmospheric long-range sensing are about a few per cent by reference to line-by-line calculations, much smaller than errors due to a classical SNB approach in the same conditions. [Copyright &y& Elsevier]

The IR emission spectra of combustion gases in a quasi-two-dimensional burner have been measured and processed for temperature and concentration determinations. A Fourier transform spectrometer with a spectral resolution up to 0.02 cm−1 has been used in the spectral range 1700–4300 cm−1. An original data reduction procedure based on the adjustment of the measured spectra and calculated ones at low spectral resolution has been developed and is shown to be efficient for temperature, CO2 and H2O measurements. The temperature was also deduced from the ratio of the intensities of a suitable pair of CO lines and CO concentration was determined from individual CO line intensities. The sensitivity of the different data reduction procedures is discussed and FTIR results are also compared to probe measurements. [Copyright &y& Elsevier]

Abstract: Based on the superposition principle, we propose in this study a Monte Carlo (MC) formulation for radiative transfer in quasi-isothermal media which consists in directly computing the difference between the actual radiative field and the equilibrium radiative field at the minimum temperature in the medium. This shift formulation is implemented for the prediction of radiative fluxes and volumetric powers in a combined free convection–radiation problem where a differentially heated cubical cavity is filled with air with a small amount of H2O and CO2. High resolution spectra are used to describe radiative properties of the gas in this 3D configuration. We show that, compared to the standard analog MC method, the shift approach leads to a huge saving of required computational times to reach a given convergence level. In addition, this approach is compared to reciprocal MC formulations and is shown to be more efficient for the prediction of wall fluxes but slightly less efficient for volumetric powers. [Copyright &y& Elsevier]

This paper presents the development of a Statistical Narrow Band (SNB) model for CO 2 in a nonequilibrium vibrational state. The population densities of the energy levels are modeled by a three-temperature approach (T , T 12 , T 3) associated resp. to translation–rotation modes, symmetric-stretching and bending modes, and antisymmetric stretching mode, to compute nonequilibrium line by line (LBL) spectra. The nonequilibrium between vibrational temperatures induces spectral correlation issues between η σ / κ σ and κ σ , where η σ and κ σ are the emission and absorption coefficients respectively. To overcome this issue, a vibrational splitting of the spectroscopic database is made to separate lines with completely different vibrational transitions. The SNB parameters are obtained by fitting the curves of growth from the LBL approach by a least squares error minimization using a Newton method for pure Lorentz and Doppler broadening regimes for each line class. The total SNB transmissivity is obtained by computing the product of each line class transmissivity, and the decorrelation between the spectra of the different classes is checked. Finally, the model is tested in Voigt broadening regime using a mixing rule by computing outlet radiative intensity of uniform and non-uniform columns and agrees well with the LBL approach. SNB parameters are available under request to the corresponding author. • A 3-temperature SNB model is developed for vibrational nonequilibrium CO 2 radiation. • CO 2 lines are grouped into classes with similar vibrational transitions. • Emission to absorption ratio and absorption spectra of each class are uncorrelated. • The model is implemented over a wide pressure range under Voigt broadening regime. • The model is applied to high altitude expanding plumes. [ABSTRACT FROM AUTHOR]

In this study, a Hybrid Statistical Narrow Band (HSNB) model is implemented to make fast and accurate predictions of radiative transfer effects on hypersonic entry flows. The HSNB model combines a Statistical Narrow Band (SNB) model for optically thick molecular systems, a box model for optically thin molecular systems and continua, and a Line-By-Line (LBL) description of atomic radiation. Radiative transfer calculations are coupled to a 1D stagnation-line flow model under thermal and chemical nonequilibrium. Earth entry conditions corresponding to the FIRE 2 experiment, as well as Titan entry conditions corresponding to the Huygens probe, are considered in this work. Thermal nonequilibrium is described by a two temperature model, although non-Boltzmann distributions of electronic levels provided by a Quasi-Steady State model are also considered for radiative transfer. For all the studied configurations, radiative transfer effects on the flow, the plasma chemistry and the total heat flux at the wall are analyzed in detail. The HSNB model is shown to reproduce LBL results with an accuracy better than 5% and a speed up of the computational time around two orders of magnitude. Concerning molecular radiation, the HSNB model provides a significant improvement in accuracy compared to the Smeared-Rotational-Band model, especially for Titan entries dominated by optically thick CN radiation. [ABSTRACT FROM AUTHOR]

Abstract: A band model is developed for the prediction of radiative transfer in air plasma applications under equilibrium and non-equilibrium conditions. For non-equilibrium applications, the medium is described by rotational–translational and vibrational temperatures but the populations of electronic states can be arbitrary. A specific formulation of the statistical narrow-band (SNB) model is developed for optically thick electronic systems of diatomic molecules when their populations are described by an electronic temperature. Model parameters, deduced from line by line calculations in the Voigt regime, are shown to be also convenient for arbitrary distribution of molecular electronic populations. This model is then complemented to include optically thin electronic systems and the continuum radiation through the simple box model, and line by line calculations for atomic lines. Several tests including equilibrium, non-equilibrium, uniform, and non-uniform conditions show the ability of this hybrid model to provide accurate and efficient solutions for radiative transfer problems in air plasmas. [Copyright &y& Elsevier]

Abstract: The spectroscopic data required for radiative transfer calculations in CO2–N2 plasmas are investigated. For diatomic electronic systems, we extend the systematic approach previously developed in Chauveau et al. [Contributions of diatomic molecular electronic systems to heated air radiation. JQSRT 2002;72:503–30], to the carbonaceous molecules C2, CN, CO, and . The calculated vibrational transition moments are systematically compared to experimental data or other calculations when available. The positions and intensities of the rotational lines have been determined in intermediate a/b Hund''s coupling case with up-to-date molecular constants. The other important mechanisms include triatomic (CO2) rovibrational transitions, atomic and ion lines, and continuum radiation. The analysis of the optically thin emission of pure CO2 and CO2(97%)–N2(3%) mixture plasmas at chemical and thermal equilibrium in the temperature range 2000–15000K shows a persistence of CO2 IR radiation predominance up to about 3800K in spite of its small molar fraction. The analysis confirmed the importance of the CO fourth positive system for temperatures between 5000 and 7000K and of atomic lines above. The contribution of continuum radiation to the optically thin emission is found to be weak. Vibrational Einstein coefficients and Franck–Condon factors of the investigated diatomic molecules are provided as supplementary material and the corresponding rotational line lists are available under request. [Copyright &y& Elsevier]

Abstract: It is well known that the Kirchhoff''s law, connecting emission to absorption through the Planck''s function, is no more valid when the medium is not at local thermal and chemical equilibrium. The aim of the present paper is to analyze the effects of this nonequilibrium on radiative transfer, and in particular, for atmospheric air re-entry applications. A general formulation is proposed in the framework of multi-temperature description of the thermodynamic state of the gas mixture. Application of this model to simulate an earth re-entry experiment first leads to a reasonable agreement and then outlines important effects of chemical nonequilibrium, especially through bound–free transitions. [Copyright &y& Elsevier]

Radiative transfer in local thermodynamic and chemical equilibrium N2–O2 plasmas is analyzed in this study using a line-by-line approach. The contributions of line absorption by atoms, ions and of continuous absorption by atoms, ions and molecules to the absorption coefficient of heated air are calculated. These data combined to our previous work on the contribution of molecular electronic systems to heated air radiation (J. Quant. Spectrosc. Radiat. Transfer 72 (2002) 503) lead to a reliable and exhaustive spectroscopic data base for radiative transfer in air plasmas and for temperatures up to 15,000 K. Line-by-line radiative transfer calculations are carried out for a simple planar geometry with prescribed temperature profiles. The spectral distribution of radiative fluxes and volumetric powers is analyzed and the relative contributions of continuum and line radiation are discussed. [Copyright &y& Elsevier]

• Lidar-relevant parameters (backscattering, extinction, lidar ratio, and depolarization ratio) are computed using MSTM for statistically representative ensembles of soot fractal aggregates. • The monomer radius strongly influences all of the studied lidar relevant radiative properties • The number of monomers per aggregate has also a strong effect on the lidar cross-sections, but has a limited impact on the lidar parameters such as the Lidar Ratio and the linear depolarization ratio. • The aggregate fractal dimension has a weak impact on all lidar relevant radiative properties. This study presents the averaged lidar-relevant radiative properties of numerically generated soot fractal aggregate ensembles. The radiative properties of these aggregates have been computed using the Superposition T-Matrix Method, with emphasis put on those that are most relevant to lidar application; the backscattering and extinction cross-sections, the Lidar Ratio and the Linear Depolarization Ratio. These lidar-relevant radiative properties have been computed over a broad spectrum, going from the ultraviolet to the near infrared, in order to address lidar instrument need for a priori knowledge in signal inversion procedures and measurement analysis. By averaging the computed radiative properties according to each set of morphological parameters, we obtain statistically representative results and we study the impacts of morphological changes on these lidar-relevant radiative properties. Our results show a strong impact of the primary particle radius on all considered radiative properties, while the number of primary particles induce significant variations on the cross-sections only. The fractal dimension, although being an essential morphological parameter, has a weak influences on the lidar-relevant radiative properties. [ABSTRACT FROM AUTHOR]

• A consistent model for CO 2 radiation under nonequilibrium conditions is developed. • Vibrational radiation source terms are derived from partial absorption and emission coefficients. • Vibrational temperature fields are obtained from Lagrangian post-processing of equilibrium calculations. • The model is applied to a nozzle expanding flow and to a high altitude rocket plume. • Emitted radiation is significantly increased due to the partial freeze of vibrational excitation. The aim of this paper is to develop a line by line model for CO 2 vibrational nonequilibrium radiation and to investigate nonequilibrium effects in the case of some applications involving high temperature expanding flows. A vibrational specific collisional relaxation model is developed and is incorporated in a multi-temperature thermodynamic description of the gas mixture, in order to compute vibrational level populations along the flow. The HITEMP-2010 spectroscopic database is employed with a model for level energy splitting to provide line by line absorption and emission total and per-vibrational mode specific spectra. The specific spectra allow us to derive the radiative source terms to be used in the multi-temperature model if a coupled approach is required. The model is applied first to a simple conical nozzle flow and then to the plume of a high altitude solid propellant rocket engine. It is shown that, for the considered applications, the partial freeze of vibrational excitation in the expanding flow increases significantly the radiative intensity escaping from the mixture. [ABSTRACT FROM AUTHOR]