Jean Taine

Gas IR radiative properties : From spectroscopic data to approximate models

Abstract Different IR gas radiative property models suitable for a wide range of engineering applications (statistical narrowband, correlated- k :, correlated- k -fictitious gases, and models based on global absorption coefficient distribution functions, such as WSGG, SLW, ADF, and ADFFG) are presented. High-temperature properties, especially with CO 2 and H 2 O as active species, are emphasized. The accuracy of the different models is discussed by comparing their results with those of a line-by-line (LBL) approach, considered as a reference. This last model is introduced step by step, and the effects of the properties of the molecular states of the absorbing and emitting species on the properties of the corresponding spectra are discussed. The main physical features of complex IR spectra of CO 2 and H 2 O are presented in order to explain important phenomena occuring in different applications. The key point in the use of an LBL approach is the development of a complete and accurate spectroscopic database suitable for high-temperature applications. The parameters of approximate models are generated from the LBL approach in order to provide accurate predictions of radiative properties of uniform media. The intrinsic accuracy of these models applied to radiative transfer in nonuniform media is then studied. The implementation of the models is discussed on the basis of their formulations in terms of absorption coefficients or in terms of averaged transmissivities, and on the basis of their classification into band models and global models. Different methods for the treatment of mixtures of absorbing species are discussed.

Direct identification of absorption and scattering coefficients and phase function of a porous medium by a Monte Carlo technique

Abstract A general method of direct identification of absorption and scattering coefficients and phase function of porous media, assumed statistically homogeneous and isotropic, has been developed for wavelengths small in regard of the typical structure length (i.e. by neglecting diffraction). This method is here limited to a transparent fluid phase and an opaque solid phase. Diffuse and specular reflection laws and combination of these laws are considered. First results have been obtained for sets of Dispersed radius Overlapping Opaque Spheres (DOOS) in a transparent fluid phase, or sets of Dispersed radius Overlapping Transparent Spheres (DOTS) in an opaque solid phase. In the case of DOOS models, the absorption and scattering coefficients have the same analytical expressions as those characterizing the optically thin limit for any porous medium. For DOTS models, these coefficients have been, for porosity higher than 0.65, identified from Monte Carlo calculations versus the porosity or the specific area per unit volume of the fluid phase. For DOOS models, the phase functions have been expressed by a simple analytical expression in the case of a specular reflection law, and derived from Monte Carlo calculations in the case of the diffuse reflection law. For DOTS models, phase function have been numerically calculated from Monte Carlo calculations. The case of the combination of the two reflection laws is finally discussed.

Accuracy of narrow-band and global models for radiative transfer in H2O, CO2, and H2OCO2 mixtures at high temperature

The accuracy of several narrow-band (SNB, CK, CKFG) and global (WSGG, SLW, ADF, ADFFG) gas infrared radiative property models applied to radiative transfer in a planar geometry with different types of temperature profiles is studied. The considered gaseous mixtures are H2ON2, CO2N2 and H2OCO2N2. Reference solutions are provided by line-by-line (LBL) calculations. All model parameters are based on the same spectroscopic data bases so that only the intrinsic accuracy of each model is tested. All narrow-band models lead in most cases to accurate results, but errors induced by the transmissivity-based models SNB and CKFG increase with wall reflectivity if the reflected radiation is assumed spectrally uncorrelated with gaseous transmissivity. Global models are less time consuming than narrow-band models but are generally less accurate and limited to media with gray boundaries and/or participating particles. The WSGG model leads in many cases to very important errors. The relative accuracy of the SLW and ADF models is typically about 10–20% but care must be taken in the choice of the reference temperature. The ADFFG model is the most accurate global model but requires greater computing times than the ADF and SLW models. For long range sensing of hot gases, only the fictitious-gas based models CKFG and ADFFG lead to accurate results. In the case of mixtures containing H2O and CO2, the spectral uncorrelation assumption is accurate for narrow-band models and its implementation results only in greater computing times for the CK model. On the contrary, this assumption is not generally accurate for the whole spectra and specific parameters must be generated from the joint distribution function of the absorption coefficients in the case of global models.

Validity of band-model calculations for CO2 and H2O applied to radiative properties and conductive-radiative transfer

Abstract A previously presented line-by-line (LBL) calculation is used to test the validity of various approximate models. Narrow-band model parameters are generated from the lines used by the LBL approach in the 150–8000 cm -1 and 300–1500 K ranges. The accuracy of narrow-band models and of approximations for nonuniform paths applied to transmissivities and intensities of columns is studied. Random-statistical models give good results for the transmissivities but inaccurate results for the emitted intensities. Applications to combined conductive-radiative transfer in one-dimensional media are then presented. The temperature and flux distributions predicted by wide-band models are accurate when radiative transfer is preponderant; narrow-band models must be used in the case of optically thin media (e.g. in boundary layers).

Line-by-line and narrow-band statistical model calculations for H2O

Abstract In line-by-line calculations we use hot-band lines generated from the AFGL or GEISA data. The parameters of a random statistical model are calculated from these lines for 25 cm -1 wide intervals. Comparisons are made between experimental spectra and results of the two theoretical models for the 1595 and 3755 cm -1 bands between 900 and 1500 K. The validities of the statistical model and of the Curtis-Godson and Lindquist-Simmons approximations for nonisothermal and inhomogeneous media are tested.

Accurate calculated tabulations of IR and Raman CO 2 line broadening by CO 2 , H 2 O, N 2 , O 2 in the 300–2400-K temperature range

Pressure-broadening coefficients for (12)C(16)O(2) lines have been calculated with a recent model derived from the Robert and Bonamy approach which leads to more accurate results than the previously used Anderson-Tsao- Curnutte model. Systematic calculations of CO(2)-CO(2), CO(2)-H(2)O, CO(2)-N(2), and CO(2)-O(2) broadening coefficients in the 300-2400-K temperature range are presented. The results are suitable for both IR and Raman lines and should be useful for spectra calculations. Tabulations of the broadening coefficients are given together with simple analytical expressions for their rotational quantum number and temperature dependences.

A line-by-line calculation of low-resolution radiative properties of CO2-CO-transparent nonisothermal gases mixtures up to 3000 K

Abstract Low-resolution radiative properties (absorptivities, transmissivities, intensities) of nonisothermal or inhomogeneous gas mixtures at high temperatures, suitable for radiative heat transfer applications (e.g., in combustion) are obtained from a line-by-line calculation, using recently published high-resolution spectroscopic data at 296 K given by AFGL. It is shown that absorptivities obtained for isothermal mixtures of CO2, CO, N2 (and sometimes H2O) agree with the corresponding measured spectra.

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.

Calculated tabulations of H 2 O line broadening by H 2 O, N 2 , O 2 , and CO 2 at high temperature

We present calculations of the temperature dependence of H(2)O pressure broadening parameters by H(2)O, CO(2), N(2), and O(2). They were made for Q-lines with a theoretical model which provides a correct treatment of close collisions and has been widely tested. The results should be useful for Raman spectra calculations. A simple law is proposed to deduce halfwidths of P- and R-lines from the Q-line results. The accuracy of this law at high temperature is demonstrated. A simple analytical representation of a constant halfwidth approximation is also given.

Accurate calculated tabulations of CO line broadening by H 2 O, N 2 , O 2 , and CO 2 in the 200–3000-K temperature range

We present accurate calculations of CO line-broadening coefficients. They have been calculated with a modelwhich has been tested with success on the broadening of CO,-; CO2 and H2O (Refs. 2,9,10) lines. A similar data base for broadening coefficients has already been made for CO2. The following results are presented as in Ref. 11. The broadening coefficients γ‌ m‌ of CO RJ=m-1 lines by H2O, CO2, N2, and O2 have been calculated in the 300-2400K range. The data of Refs. 7,6,4, and 12 have been used for CO-H2O, -CO2, -N2, and -O2, respectively. For a given value of the rotational quantum number m, calculations have only been made for temperatures >Tmin(m), where