Absorption Coefficients Of Gases Research Articles

A Computational Fluid Dynamics Study of Turbulence, Radiation, and Combustion Models for Natural Gas Combustion Burner

This paper presents a Computational Fluid Dynamics (CFD) study of a natural gas combustion burner focusing on the effect of combustion, thermal radiation and turbulence models on the temperature and chemical species concentration fields. The combustion was modelled using the finite rate/eddy dissipation (FR/EDM) and partially premixed flame models. Detailed chemistry kinetics CHEMKIN GRI-MECH 3.0 consisting of 325 reactions was employed to model the methane combustion. Discrete ordinates (DO) and spherical harmonics (P1) model were employed to predict the thermal radiation. The gas absorption coefficient dependence on the wavelength is resolved by the weighted-sum-of-gray-gases model (WSGGM). Turbulence flow was simulated using Reynolds-averaged Navier-Stokes (RANS) based models. The findings showed that a combination of partially premixed flame, P1 and standard k-ε (SKE) gave the most accurate prediction with an average deviation of around 7.8% of combustion temperature and 15.5% for reactant composition (methane and oxygen). The results show the multi-step chemistry in the partially premixed model is more accurate than the two-step FR/EDM. Meanwhile, inclusion of thermal radiation has a minor effect on the heat transfer and species concentration. SKE turbulence model yielded better prediction compared to the realizable k-ε (RKE) and renormalized k-ε (RNG). The CFD simulation presented in this work may serve as a useful tool to evaluate a performance of a natural gas combustor.

Assessment of gas radiative property models in the presence of nongray particles

ABSTRACTIn this study, a radiation code based on the method of lines solution of the discrete ordinates method for the prediction of radiative heat transfer in nongray gaseous media is developed by incorporation of two different spectral gas radiative property models, banded spectral line-based weighted sum of gray gases (banded SLW) and gray wide band (GWB) approximation in the presence of nongray absorbing–emitting–scattering particles. The aim is to introduce an accurate and CPU efficient spectral gas radiation model, which is compatible with spectral fuel/ash particle property models. Input data required for the radiation code and its validation are provided from two combustion tests previously performed in a 300 kWt atmospheric bubbling fluidized bed combustor test rig burning low calorific value Turkish lignite with high volatile matter/fixed carbon (VM/FC) ratio in its own ash. The agreement between wall heat fluxes and source term predictions obtained by global and banded SLW models reveal that global SLW model can be converted to an accurate wide band gas model (banded SLW) which can directly be coupled with spectral particle radiation. Furthermore, assessment of GWB approximation by benchmarking its predictions against banded SLW model shows that GWB gives reasonable agreement with a higher CPU efficiency when the particle absorption coefficient is at least in the same order of magnitude with the gas absorption coefficient.

Application of the WSGG Model to Solve the Radiative Transfer in Gaseous Systems With Nongray Boundaries

This paper presents a methodology for the application of the weighted-sum-of-gray-gases (WSGG) model to systems where the medium is bounded by nongray surfaces. The method relies on the assumption that each gray gas absorption coefficient is randomly spread across the entire wavenumber spectrum. It follows that, in the spectral integration of the radiative transfer equation (RTE), the local emission term can be computed by the joint probability of emission from the subsections of the spectrum related to each gray gas coefficient and from each wall emissivity band. One advantage of the proposed methodology is that it allows the use without any modification of WSGG correlations that are available in the literature. The study presents a few test cases considering a one-dimensional (1D), nonuniform medium slab composed of H2O and CO2, bounded by nongray surfaces. The accuracy of the methodology is assessed by direct comparison with line-by-line (LBL) calculations.

Functional data analysis applied to the multi-spectral correlated–k distribution model

The Ck (Correlated-k) approach is among the most used method for the approximate modelling of the radiative properties of gases both in uniform and non-uniform media. One of its main defects is that the treatment of non-uniform gas paths is founded on the assumption of correlation - in fact co-monotonicity - of gas absorption coefficients in distinct states which is not rigorously verified for actual spectra. This correlation assumption fails as soon as large temperature gradients are encountered along the radiative path lengths. In order to circumvent this problem, a method based on functional data analysis (FDA) - referred to as the MSCk model in this work - was proposed in Refs. [1,2]. The principle of the method is to group together wavenumbers with respect to the spectral scaling functions - defined as the ratio between spectral absorption coefficients in distinct states - so that the correlation/co-monotonicity assumption can be considered as exact over the corresponding intervals of wavenumbers. Very few details were provided up to now about the application of FDA within the frame of the MSCk model. Indeed, most of our previous works were dedicated to the derivation of the methods itself. Accordingly, in the present paper, we mostly focus our attention on the mathematical definition of clusters of scaling function, quantities which are used to build spectral intervals over which gas spectra in distinct states are assumed to be scaled. The comparison of different clustering methods together with the criterion to select an appropriate number of clusters are described and discussed and the application of this approach for several test cases, including 3D geometries, are presented.

WITHDRAWN: Effects of angle of attack on IR radiation for a hypersonic boost-glide vehicle

Hypersonic vehicles emit strong infrared radiation signatures that can be treated as a detecting source for object identification and routine diagnosis. This paper is aimed at examining the intrinsic radiation characteristics of a Boost-Glide Vehicle (BGV) under the condition of various angles of attack (AOA). A two-temperature model considering the thermal and chemical non-equilibrium effects is coupled with Navier–Stokes equations solved by the finite volume technique. A gas-solid conjunction heat transfer model is also added into the fluid solver to simulate the surface temperature of the vehicle. Gas absorption coefficients are evaluated through Line-by-Line (LBL) method and the radiative transfer equation is solved with line of sight (LOS) algorithm. Each part of the numerical modules is validated separately. The computational results for a Hypersonic Technology Vehicle 2 (HTV-2) type vehicle show that radiances emitted from gas are at least three orders of magnitude lower than that from the surface. The presence of AOA results in different distributions in surface temperature and therefore changes infrared radiation characteristics in intensity and spectral band. Two group simulations are performed in the steady and transient states, these results indicate that the variation of AOA does have a great effect on the infrared radiance and closely relates to observation angle, spectral band, angle size, angular velocity and time history. A low AOA rate of change is beneficial to reduce the radiance in the side view, but increase the radiance in the upward view. AOA rate of change has little effect on radiation intensity-time variations in the top view.

Step-scan differential Fourier transform infrared photoacoustic spectroscopy (DFTIR-PAS): a spectral deconvolution method for weak absorber detection in the presence of strongly overlapping background absorptions.

The determination of small absorption coefficients of trace gases in the atmosphere constitutes a challenge for analytical air contaminant measurements, especially in the presence of strongly absorbing backgrounds. A step-scan differential Fourier transform infrared photoacoustic spectroscopy (DFTIR-PAS) method was developed to suppress the coherent external noise and spurious photoacoustic (PA) signals caused by strongly absorbing backgrounds. The infrared absorption spectra of acetylene (C2H2) and local air were used to verify the performance of the step-scan DFTIR-PAS method. A linear amplitude response to C2H2 concentrations from 100 to 5000ppmv was observed, leading to a theoretical detection limit of 5ppmv. The differential mode was capable of eliminating the coherent noise and dominant background gas signals, thereby revealing the presence of the otherwise hidden C2H2 weak absorption. Thus, the step-scan DFTIR-PAS modality was demonstrated to be an effective approach for monitoring weakly absorbing gases with absorption bands overlapped by strongly absorbing background species.

Mathematical Model of the Radiative Heat Exchange in the Selective Gases of a Diffusion Flame

Possibilities of improvement of the differential model of radiative transfer, used in engineering investigations of the heat exchange in the products of combustion of a gas fuel, were analyzed. The equations, boundary conditions, and algorithms of this model were refined. A method of calculating the local absorption coefficients of selective gases, involved in differential equations of radiative transfer, has been determined.

Theoretical study of an evanescent optical integrated sensor for multipurpose detection of gases and liquids in the Mid-Infrared

A theoretical study of evanescent optical sensor for multipurpose detection in the Mid-Infrared of gases and pollutants in water is presented in this paper. The opto-geometrical parameters of the transducers – ridge waveguides – have been optimized in order to obtain the highest evanescent power factor for monomodal propagation in the Mid-Infrared. The highest sensitivity has been obtained for a configuration with an optimal length of waveguide Lopt=4.3cm for intrinsic propagation loss equal to 1dB/cm. Then a spiral waveguide configuration is suggested to obtain this optical length path in a monolithic structure. A numerical example is also included using a ridge waveguide based on chalcogenide glasses (GeSbSe). In case of gas detection, a generic calculation of the minima concentrations to be detected as a function of the molar absorption for any working wavelength is presented. Extremely low limits of detection can be achieved due to the strong absorption coefficients of gases and chemical species in the Mid-Infrared spectral range, 268ppb in case of carbon dioxide at λ=4.3μm, 1.848ppm and 781ppb for methane at λ=3.31μm and at λ=7.66μm respectively. For the pollutants detection in water, an improvement of the integrated structure has been proposed to avoid water absorption in this spectral region by deposing a polymer (PIB) as waveguide superstrate, thus the limit of detection for toluene is 26ppb at λ=6.68μm. These concentration minima that could be detected by the Mid-IR sensor are lower than the threshold limit values determined in the international environmental and health standards. Hence this integrated optical sensor may be considered as an attractive support tool in monitoring environmental and health fields.

Prediction of shock-layer ultraviolet radiation for hypersonic vehicles in near space

A systemic and validated model was developed to predict ultraviolet spectra features from the shock layer of near-space hypersonic vehicles in the “solar blind” band region. Computational procedures were performed with 7-species thermal non-equilibrium fluid mechanics, finite rate chemistry, and radiation calculations. The thermal non-equilibrium flow field was calculated with a two-temperature model by the finite volume technique and verified against the bow-shock ultra-violet (BSUV) flight experiments. The absorption coefficient of the mixture gases was evaluated with a line-by-line method and validated through laboratory shock tube measurements. Using the line of sight (LOS) method, radiation was calculated from three BSUV flights at altitudes of 38, 53.5 and 71km. The investigation focused on the level and structure of ultraviolet spectra radiated from a NO band system in wavelengths of 200–400nm. Results predicted by the current model show qualitative spatial agreement with the measured data. At a velocity of 3.5km/s (about Mach 11), the peak absolute intensity at an altitude of 38km is two orders of magnitude higher than that at 53.5km. Under the same flight conditions, the spectra structures have quite a similar distribution at different viewing angles. The present computational model performs well in the prediction of the ultraviolet spectra emitted from the shock layer and will contribute to the investigation and analysis of radiative features of hypersonic vehicles in near space.

Impact of relative humidity on visibility degradation during a haze event: A case study

Light scattering of aerosols depends on ambient relative humidity (RH) since hygroscopic particles absorb significant water at high RH, and this results in low visibility. This paper used custom-made parallel nephelometers (PNEPs) to measure aerosol light scattering enhancement factor ƒ(RH), and utilized data including visibility, PM2.5, black carbon, water-soluble ions mass concentrations and surface meteorological parameters, in conjunction with background weather conditions, to analyze a haze event in Guangzhou during 8th–15th Dec. 2013. Unfavorable weather conditions, such as high RH and low wind speed, were observed during the haze event. The hourly average mass concentration of PM2.5 was 127μg/m3, with concentration of 192.4μg/m3 on 9th and 196μg/m3 on 13th. The ƒ(RH) did not exhibit significant changes during this haze process, with value of ƒ(80%)=1.58±0.07. Although the mass fraction of water-soluble ions to PM2.5 decreased after 12th Dec., the aerosol hygroscopicity might not have changed significantly since the mass fraction of nitrate became more dominant, which has stronger ability to take up water. The best-fitted parameterized function for ƒ(RH) is ƒ(RH)=0.731+0.1375∗(1–RH/100)−1+0.00719∗(1–RH/100)−2. Combining the fixed parameterization of ƒ(RH) above, the visibility was calculated with the measured light scattering and absorption coefficient of particles and gas under dry condition, as well as ambient RH. The predicted visibility range agrees well with the measurements without precipitation. Using ISORROPIA II model, the calculated aerosol liquid water content (ALWC) at ambient RH varied consistently with the PM2.5 under lower RH, while it was more influenced by high RH. This work also show that high RH accompanied with precipitation will enhance aerosol hygroscopic growth effect, leading to further visibility degradation, even if PM2.5 mass decreased due to precipitation.

Integrating cavity based gas cells: a multibeam compensation scheme for pathlength variation.

We present a four beam ratiometric setup for an integrating sphere based gas cell, which can correct for changes in pathlength due to sphere wall contamination. This allows for the gas absorption coefficient to be determined continuously without needing to recalibrate the setup. We demonstrate the technique experimentally, measuring methane gas at 1651nm. For example, contamination covering 1.2% of the sphere wall resulted in an uncompensated error in gas absorption coefficient of ≈41%. With the ratiometric scheme, this error was reduced to ≈2%. Potential limitations of the technique, due to subsequent deviations from mathematical assumptions are discussed, including severe sphere window contamination.

Geometric optimization of radiative heat transfer on surfaces of corrugated furnaces

Corrugated furnace technology is used in fire tube steam generators to improve mechanical strength and heat transfer in the furnace. In this paper, geometric optimization of radiative heat transfer of corrugated furnaces using finite volume method with two optimization methods (the method of simulated annealing and the simplex method) is proposed. The methodologies were applied to two benchmark cases: one is a theoretical furnace with a constant temperature profile, gray gas and black cold walls and the other is a combustion chamber with non-gray gas of a given temperature profile and constant temperature gray furnace surfaces. For the first case study, the optimization results indicate that when the gray gas absorption coefficient increases, the corrugation amplitude also increases. However, it has no influence on the number of corrugation points. In the second case study, the problem is ill-posed, thus requiring the non-deterministic characteristic of the simulated annealing method to obtain the optimum condition. In terms of optimization methods, the simulated annealing method obtained better optimization results, especially in the furnace with combustion, at the expense of a greater number of determinations of the objective function.

Fiber ring resonator based slow-light and high sensitivity gas sensing technology

A high sensitivity gas sensing technology based on fiber ring resonator slow-light is proposed and demonstrated in this paper. Slow-light produced by coupled resonator induced transparency in a fiber ring resonator has been studied theoretically and experimentally. Influences of parameters, such as cycle numbers, coupling ratio of couplers, ring loss and coupler additional loss, on slow-light characteristics of the fiber ring resonator were analyzed theoretically and numerically in detail. The delay time of slow-light in the fiber ring resonators has been measured using time domain measuring method, and the maximum delay time of 34.18ns at wavelength of 1531.63nm has been obtained, which is in accord with theoretical analysis. The gas absorption coefficient at the wavelength of 1531.63nm where slow-light generated reached 2.41, which increased by 6.73 times the figure for the traditional method. It is the first time to measure gas concentration by using slow-light in fiber ring resonators, and the method is simple in structure and easy to implement.

Calculations of narrow-band transimissities and the Planck mean absorption coefficients of real gases using line-by-line and statistical narrow-band models

Narrow-band transmissivities in the spectral range of 150 to 9300 cm−1 and at a uniform resolution of 25 cm−1 were calculated using the statistical narrow-band (SNB) model with the band parameters of Soufiani and Taine, the more recent parameters of Andre and Vaillon, and the line-by-line (LBL) method along with the HITEMP-2010 spectroscopic database. Calculations of narrow-band transmissivity were conducted for gas columns of different lengths and containing different isothermal and non-isothermal CO2-H2O-N2 mixtures at 1 atm. Narrow-band transmissivities calculated by the SNB model are in large relative error at many bands. The more recent SNB model parameters of Andre and Vaillon are more accurate than the earlier parameters of Soufiani and Taine. The Planck mean absorption coefficients of CO2, H2O, CO, and CH4 in the temperature range of 300 to 2500 K were calculated using the LBL method and different versions of the high resolution transmission (HITRAN) and high-temperature spectroscopic absorption parameters (HITEMP) spectroscopic databases. The SNB model was also used to calculate the Planck mean absorption coefficients of these four radiating gases. The LBL results of the Planck mean absorption coefficient were compared with the classical results of Tien and those from the SNB model.

Numerical simulation of radiation intensity of oxy-coal combustion with flue gas recirculation

Oxy-fuel combustion is one of potential technologies for carbon dioxide (CO2) capture in fossil fuel fired power plants. Characterization of flue gas composition in the oxy-fuel combustion differs from that of conventional air-coal combustion, which results in the change of radiative heat transfer in combustion processes. This paper presents a numerical study of radiation intensity on lateral walls based on the experimental results of a 0.5MW combustion test facility (CTF). Differences in the oxy-coal combustion are analyzed, such as flue gas recycle, absorption coefficient and radiation intensity. The simulation results show that an effective O2 concentration ([O2]effective) between 29 and 33vol% (equivalent to the flue gas recycle ratio of 72–69%) constitutes a reasonable range, within this range the behavior of oxy-coal combustion is similar to air-coal combustion. Compared with the air-coal combustion, the lower limit (29vol%) of this range results in a similar radiative heat flux at the region closed to the burner, but a lower radiative heat flux in the downstream region of the CTF; the upper limit (33vol%) of this range results in a higher radiative heat flux at the region closed to the burner, while a similar radiative heat flux in the downstream region of the CTF.

A new general circulation model for Mars based on the NCAR Community Atmosphere Model

We introduce and present results from a new general circulation model for Mars adapted from the National Center for Atmospheric Research (NCAR) Community Atmosphere Model (CAM) version 3.1 terrestrial model. The radiative transfer has been replaced with a two-stream correlated-k scheme with carbon dioxide gas absorption coefficients suited for Mars. A time-invariant dust field is assumed with a Conrath (Conrath, B.J. [1975]. Icarus 24, 34–46) vertical distribution. Carbon dioxide is allowed to sublimate and condense at the surface, and the mass is removed from the atmosphere. The topography is averaged from MOLA data. The surface albedos and thermal inertias are derived from TES observations. The model is freely distributed to interested users.Comparisons between model temperatures, and spacecraft and Lander observations show agreement within ±10K, depending on dust concentration. The annual pressure cycle is typically within 20Pa of Viking Lander observations, however the model underestimates the surface pressure during southern summer, possibly due to increased dust activity that is not reflected in the model. Predicted model boundary layer depths are typically within a few hundred meters of observations, and tend to depend inversely on surface pressure, agreeing with observations.

Modeling the cumulative distribution of absorption coefficients of gases using the generalized k-moment method

The generalized k-moment method is formulated in terms of Cutteridge–Devyatov polynomials (CDP). In this novel approach, the moments involved are spectral averages of integer powers of the logarithm of the absorption coefficient. The technique to obtain k-distributions from those generalized moments is detailed both theoretically and from a practical point of view. Its outputs are afterward assessed against reference data in several test cases of increasing complexity. Indeed, the first ones involve single lines in the Lorentz, Doppler and Voigt regimes. The most sophisticated situations investigated in this work concern applications of the method to high resolution LBL data for pure CO2 at temperatures between 300K and 2300K and at atmospheric pressure. In any case, the CDP solution to the generalized k-moment problem is found to provide very accurate results. The present technique outperforms our previous approach to k-moment modeling of the cumulative distribution of absorption coefficients of gases that were based on first, second, first inverse and logarithmic moments, in all the situations investigated. Equations required to apply the model are provided in the paper, both over narrow bands and the full spectrum.

Laser gas analysis of multicomponent gas mixtures in case of rough known matrix of gas absorption coefficients

Laser gas analysis of multicomponent gas mixtures in case of rough known matrix of gas absorption coefficients

Determination of spectral width of laser lines in the IR range using the absorption spectroscopy method

An engineering technique for determining the spectral width of laser lines is proposed, based on the attenuation of the radiation in a cuvette with known gas composition under fixed thermodynamicconditions using absorption spectroscopy. Preliminarily, based on the theoretical analysis and computational procedures, the dependence of the gas transmission function on the laser linewidthis specified for a particular laser transition, and then, using the nomographic chart and the experimental value of the function, the linewidth of the laser line is found. In the remote gas analysis using the differential absorption method the correction of the measured value of the probed gas absorption coefficient is carried out, taking the spectral width of the laser radiation linewidth into account. The technique requires provision with appropriate spectroscopic information. Particular consideration is carried out for the radiation of a carbon monoxide laser, absorbed by water vapour

Upgrading the sensitivity of spectroscopy gas analysis with application of supersonic molecular beams

We propose an approach to increase a sensitivity of microwave and THz spectroscopy, that involves application of supersonic molecular beams. The key advantage offered by such an approach is that a gas temperature can be decreased along with an increase in the gas density, which results in a much greater number of molecules interacting with radiation and, hence, in a higher absorption coefficient. This effect has been demonstrated experimentally on supersonic CO and NO beams, using a phase manipulation microwave spectrometer. The absorption coefficient was found to be three orders of magnitude higher than the value of gas absorption coefficient in a standard 1-m long cell at room temperature.