Absorption Coefficients Of Gases Research Articles

Artificial neural network models for heat transfer in the freeboard of a bubbling fluidised bed combustion system

The heat transfer in the combustion chamber (freeboard) of a fluidised bed combustor is of critical importance, due to the need for thermal efficiency, the possible environmental impacts, and the increasing costs of fuel and electricity. In this study, an artificial neural network model was used to analyse the heat transfer in a bubbling fluidised bed combustion system. The heat transfer of the system was investigated using temperature measurements of the combustion chamber and side wall of a 0.3 MWe fluidised bed combustor, the fly ash contents (%) of 40 types of coal, the gas absorption coefficients (CO2 and H2O vapour), and the enthalpy of the flue gas. The whole system was analysed using a backpropagation algorithm (10 hidden layers with 10 neurons). It was found that the most important factor when calculating the heat transfer in the combustion chamber was the height of the chamber, and the influence of the flue gas enthalpy and the ash particle content on the heat transfer accounted for 4% and 8.6%, respectively. The accuracy of the results from the proposed multilayer perceptron algorithm is high, and analyses could be performed for different values of the input data.

Effect of moisture release on radiation behavior of burning coal bed between enclosed confined surfaces

In this study, a numerical analysis was performed to predict the radiation characteristics of burning coal layers heated on confined surfaces in an enclosed chamber. The one-dimensional (1D) unsteady energy balance equation and radiative transfer equation (RTE) were solved simultaneously using the semi-implicit Runge–Kutta method (SIRK) and discrete ordinate method (DOM). In addition to the 1D analysis, a commercial software was used to analyze the two-dimensional (2D) radiation behavior of the burning coal layers using the DOM.The heat generation rate (relevant to the combustion rate) and particle absorption coefficient were independently determined using combustion and thermal emission experiments. The gas absorption coefficient was calculated from gray gas models to account for the change of H2O vapor along coal and gas mixture with high moisture content. The dimensionless radiative heat flux and temperature distribution were determined for the three calculation cases. Two different predictions based on 1D and 2D geometries were validated against measured heat flux and temperature data. The effect of the moisture release inside the particle layer was particularly examined. The result indicates that the condensation of H2O is probably more responsible than the absorption of H2O vapor for a reduced heat flux observed in the combustion condition of high moisture coal burning at high pressure.

Detection of SF6 gas decomposition component H2S based on fiber-optic photoacoustic sensing

A high-sensitivity fiber-optic Fabry-Perot (F-P) photoacoustic gas detection system with properties of anti-electromagnetic interference and intrinsically safe was presented for quantitative analysis of SF6 decomposition component H2S. Erbium-doped fiber amplification was combined with acoustic resonance amplification to enhance the photoacoustic signal. A fiber-optic cantilever acoustic sensor (FOCAS) was used to sense weak photoacoustic signals. To enhance the sensitivity and stability of photoacoustic signals detection, the effect of SF6 gas on the responses of FOCAS and photoacoustic cell (PAC) was investigated. When SF6 is the background gas, the resonance frequency of the PAC will be greatly reduced and the sensitivity of the FOCAS will be improved. With the excitation light power of 1 W and an averaging time was 200 s, the minimum detection limit (MDL) of H2S gas in the SF6 background reached 14 ppb, and the normalized noise equivalence absorption (NNEA) was calculated to be 3.14 × 10−9 cm−1·W·Hz−1/2. The combination of excitation enhancement and detection enhancement of the photoacoustic signal solves the problem that the traditional sensor is difficult to improve the detection sensitivity due to the low absorption coefficient of H2S gas in the near-infrared region.

Influence of gas composition and properties on the combustion and heat transfer characteristics of pulverized oxy-coal boiler

This paper presents a numerical study on the influences of gas composition and properties on the combustion and heat transfer characteristics of pulverized oxy-coal boiler. Previous studies were primarily focused on impacts of gas mass flow rate under different flue gas recirculation (FGR) rates while the important influences of gas composition and physical properties were overlooked. The results show that the changes in the gas composition and physical properties under different combustion modes, such as density, heat capacity and gas absorption coefficient, can significantly affect the flow, temperature, and heat transfer distributions in the furnace. Specifically, due to the differences in the heat capacities of CO2, H2O and N2, the gas heat capacity of oxy-coal combustion is considerably higher than that of air-coal combustion. As a result, oxy-coal combustion attains furnace temperature comparable to that of air-coal combustion at remarkably lower gas mass flow rate. Moreover, because of the differences in the radiation properties of CO2, H2O and N2, the gas absorption coefficient of oxy-coal combustion is higher than that of air-coal combustion, especially at the near-wall region, which hinders the radiation heat transfer between the high temperature flame and furnace wall. Consequently, oxy-coal combustion needs higher furnace temperature at further lower gas mass flow rate to achieve matched wall heat absorption with that of air-coal combustion. Therefore, the differences in the gas composition and physical properties under different combustion modes need to be comprehensively considered in the design of oxy-coal combustion systems or retrofit of existing air-coal combustion systems.

The choice of optimal absorption coefficient in the Rank-Correlated SLW model for prediction of radiative transfer in high temperature gases

The choice of gray gas absorption coefficient in the Rank Correlated Spectral Line Weighted-sum-of-gray-gases (RC-SLW) model is investigated. Several options are considered for calculating the gray gas absorption cross-section from the bounding supplemental absorption cross-sections, including that determined by inversion of the F-variable using Gauss Quadratures, the Geometric Mean of the bounding values, the Arithmetic Mean, and finally, a value determined from the weighting of the bounding supplemental absorption cross-sections using a variable interval fraction f. It is shown that an optimal value of the gray gas absorption coefficient can only be determined by accounting for the path length L. However, test cases and theoretical confirmation reveal that the Geometric Mean of the bounding supplemental absorption cross-sections for the gray gas absorption coefficient is the preferred method. The Geometric Mean approach is also nominally twice as fast computationally as the Gauss Quadrature approach used in the original RC-SLW model formulation. Therefore, this approach may be viewed as an enhancement to the model.

Emission and absorption coefficients of diatomic gases in air plasmas produced by Earth entry

Accurate radiative properties data are required for the further calculations of radiative transfer in thermodynamic nonequilibrium plasmas produced by Earth entry. However, most of the published radiative properties were determined using the outdated spectroscopic constants, transition probabilities, and partition functions, some of which are less accurate and may deviate from the experimental results. In this paper, the spectral emission and absorption coefficients of diatomic molecules in air plasmas are thoroughly calculated based upon the exact line by line approach. In this scheme, the accurate spectroscopic constants and Franck–Condon factors were selected from the recent publications to reconstruct the rotational structure of spectral lines. Moreover, the level populations were determined by means of our newly reported two-temperature partition functions. The required thermodynamic parameters of each species were obtained by simulating a hypersonic flow field of the RAMC II vehicle. The calculated radiative properties were verified by the experimental and numerical equilibrium radiative source strength and the results show good consistency with measured and theoretical values in the available literature. Based upon the obtained flow field parameters, we finally predicted the distributions of nonequilibrium source strength along the stagnation line. Estimates and analysis were also made for the contributions of each transition system of the diatomic molecule to the total source strength.

Enhanced mid-infrared gas absorption spectroscopic detection using chalcogenide or porous germanium waveguides

Different integrated photonic sensors are investigated for the detection in the mid-infrared region of the two gases namely CO2 and CH4. The three studied structures are ridge waveguides, based both on chalcogenide films (ChG) or porous germanium (PGe) and slot waveguides based on ChG. Waveguide dimensions are optimized to obtain the highest power factor between guided light and gas while maintaining a single mode propagation in the mid-infrared wavelength range. The achievable power factor is 1% in case of ChG ridge-waveguide, 45% for PGe-ridge, and 58% in case of ChG-slot. Extremely low limits of detection (LOD), 0.1 ppm for CO2 at λ = 4.3 μm and 1.66 ppm for CH4 at λ = 7.7 μm are obtained for a ChG slot waveguide, due to the large gas absorption coefficients in the mid-infrared spectral range. For PGe waveguides, low LOD values are also computed: 0.12 ppm for CO2 at λ = 4.3 μm and 1.89 ppm for CH4 at λ = 7.7 μm. These results show that the proposed structures could achieve competitive performance required for generic spectroscopic detection on a chip for environment and health sensing.

Terahertz Time-Domain Spectroscopy of Low-Concentration N2O Using Long-Range Multipass Gas Cell

We developed a long-range multipass gas cell that extends the beam path to 18.61 m, for which a THz beam shows multiple reflections within a diameter of only 32 cm and a height of 54 cm. Due to the very long propagation THz beam path in the multipass gas cell, the absorbance and absorption coefficients of nitrous oxide can be measured very sensitively from 0.2 to 1.2 THz bandwidth. The gas concentration can be determined from the measured THz pulses for the absorption. As the gas concentration increased sequentially from 1.0% to 8.3% at atmospheric pressure, the absorbance lines also increased sequentially. The resulting absorptions and frequencies of transitions showed good agreement between experiment and calculations performed with the HITRAN database.

Assessments on SGG and WSGG spectral models in high-ash coal combustion

In radiation process, a variety of models can be used to represent the spectral radiative properties of combustion gases. In coal combustion, the high level of soot concentration brings higher complexity to the formulation. Several pieces of research are found, comparing different radiation models usually applied to idealized laboratory situations. Only a few works explore the behavior of such models in real scale applications, such as coal-fired furnaces. Comparisons among different arrangements of the SGG and WSGG models were carried out by means of CFD simulations, applied to a boiler of a real 160 MW coal-fired power plant. Five different cases were simulated. The most simple case was set with a fixed absorption coefficient SGG model, while the most detailed case was achieved by employing a WSGG model with gas and soot absorption coefficients overlay. Temperature fields were well represented by all cases, with maximum deviations of 6%. Predictions on radiative heat flux for the SGG models were significantly lower than the WSGG simulations, with relative deviations up to 16.46%. Even though deviations on radiative heat flux are significant, results suggest the suitability of simple gray gas models in fast assessments of industrial applications such as coal-fired furnaces.

Multi-component Gas Photoacoustic Detection

Based on the basic principles of photoacoustic spectrometry, this paper uses methane and acetylene as examples to conduct a research for the multi-component gas photoacoustic detection. The photoacoustic spectrometer designed in this paper has the advantages of good selectivity and large detection range, and the proposed multi-component gas photoacoustic quantification method based on least squares regression does not require cell constants and gas absorption coefficient parameters, thus avoiding errors introduced by these parameters. The method proposed in this paper provides a method reference for the engineering application of photoacoustic spectrometry.

ADVANCED PHOTOCHEMICAL BOX AND QUANTUM-KINETIC MODELS FOR SENSING ENERGY, RADIATION EXCHANGE IN ATMOSPHERIC GASES MIXTURES AND LASER- MOLECULES INTERACTION

The aim of the work is to develop a set of optimal photochemical models with the inclusion of a submodel of the boundary layer using complex plane field methods and spectral algorithms and optimized blocks describing nonlinear radiation transfer and chemical conversion mechanisms, quantum-kinetic and photoelectronic models for describing nonlinear optical effects due to the interaction of infrared laser radiation with the gas atmosphere of an industrial city. An obvious consequence of the resonant interaction (in particular, absorption) of electromagnetic radiation by atmospheric molecular gases is a quantitative redistribution of molecules by energy levels of internal degrees of freedom, which quantitatively changes the so-called gas absorption coefficient. A change in the population levels of the gas mixture causes a violation of the thermodynamic equilibrium between the vibrations of the molecules and their translational motion and causes a new nonlinear effect of the photokinetic cooling of the atmospheric environment.

A Maximum Entropy-Inspired Interpolative Closure for the Prediction of Radiative Heat Transfer in Laminar Co-Flow Diffusion Flames

ABSTRACT An interpolative-based first-order maximum entropy (M1) moment closure for providing approximate solutions to the equation of radiative transfer in non-gray participating media is proposed and described. This newly developed non-gray moment closure technique results in significant computational savings compared to an approach that makes use of the direct numerical solution of the optimization problem for entropy maximization. Its predictive capabilities are also assessed by comparing its solutions to those of the more commonly adopted first-order spherical harmonics (P1) moment closure technique, as well as the popular discrete ordinates method (DOM), which is used as a benchmark for the model comparisons. The evaluations are performed for sooting co-flow laminar diffusion flames for blends of ethanol and methane fuels at atmospheric as well as at elevated pressures and include comparisons to available experimental data for soot volume fraction and flame temperature. The strong spectral dependence exhibited by the absorption coefficient of real gases is treated herein using the statistical narrow-band correlated-k method, and the chemical kinetics of the underlying species are modeled using a reduced mechanism for methane and ethanol fuels. Theoretical details of the proposed interpolative M1, along with a description of the proposed Godunov-type finite-volume scheme developed for the numerical solution of the resulting system of hyperbolic moment equations are briefly discussed. The finite-volume method makes use of limited second-order solution reconstruction, multi-block body-fitted quadrilateral meshes with anisotropic adaptive mesh refinement (AMR), and an efficient Newton–Krylov–Schwarz (NKS) iterative method for solution of the resulting nonlinear algebraic equations arising from the finite-volume discretization procedure. The numerical results for laminar co-flow flames show that the flame solutions and predictions of soot formation of the non-gray M1 maximum-entropy moment closure are very promising and in very good agreement with those of both the DOM and P1 spherical harmonics model, while offering a substantial reduction in the number of dependent solution variables, compared to the DOM.

Sequential Slope and Intercept Method for Estimation of Gas Absorption and Diffusion Coefficients in Binary Gas–Liquid Systems

A novel geometric method based on a sequential slope–intercept approach is derived for estimation of concentration-dependent diffusion, gas absorption, and gas dissolution concentration in gas–liquid binary systems. The gas absorption and diffusion are modeled using an inverse free boundary problem governed by Fick’s second law of diffusion and Henry’s absorption law. An unknown gas–liquid interface is governed by the Stefan’s type boundary condition. Implementation of the estimation method involves piecewise linear approximation of the transformed concentration data and sequential application of the estimated slopes and intercepts from this approximation. Application and validation of the developed estimation method is given for synthetic concentration profiles and for real concentration measurements for a dimethyl ether–bitumen binary system obtained using computer tomography. The developed estimation method overcomes the existing estimation methods derived from polynomial approximations that significantly underestimate or overestimate the diffusion coefficients at very low dissolved gas concentrations.

Analysis and Detection of a Target Gas System Based on TDLAS & LabVIEW

This paper introduces detection techniques of target gas concentration based on infrared absorption spectrum method and the simulation of the gas detection system using LabVIEW. In this work, the harmonic detection is performed by wavelength modulation technique with a related frequency signal to cover the measured signal and get the gas absorption coefficient that can be analysed to give a gas concentration. A series of simulations are carried out to scan weak absorption lines and extract the second harmonic curve of the absorption spectrum of the target gas. The absorption and second harmonic curves are acquired by changing the parameters of the target gas concentration and line width. This work claims to be a contribution to the study and the comprehension of gas detection system based on tunable diode laser absorption spectroscopy and can serve as a basis for practical data acquisition.

Improved spectral absorption-coefficient grouping strategies in radiation heat transfer calculations for combustion gases with pressure and temperature inhomogeneity

The grouping strategy for the spectral absorption coefficient of gases used in multigroup full spectrum k distribution and multiscale multigroup full spectrum k distribution models was improved to enable radiation-heat transfer calculations of combustion gas with temperature inhomogeneity under various pressures or large pressure gradients. The improvements in calculation accuracy resulting from different grouping strategies were evaluated using a series of semi-1D problems in which strong temperature and pressure inhomogeneity were present. Finally, the evaluations are given for the calculation of radiation heat transfer characteristics and thermal emission imaging of the exhaust system of a parallel turbine-based combined cycle (TBCC) engine.

Estimation of Gas Absorption and Diffusion Coefficients for Dissolved Gases in Liquids

The multifaceted process of gas absorption and diffusion of dissolved gases has several useful applications in engineering, medicine, pharmaceutics, and science. A new systematic study for modeling of this process using an inverse problem for concentration dependent diffusion with free boundary at the gas–liquid interface is presented in this work. The study includes the model derivation based on the Fick’s second law of diffusion, the Henry’s absorption law, the Stefan’s principles of free boundary problems, and the development of new techniques for estimation of concentration dependent diffusion coefficient and the gas absorption coefficient governing propagation of gas–liquid interface due to absorption. The developed estimation technique utilized the dissolved gas concentration profiles measured and different time moments and is applied to real X-ray CT measurement data for a binary gas liquid mixture containing dimethyl ether and bitumen.

Line by line based band identification for non-gray gas modeling with a banded approach

The banded approach or box model is a method to include the non-grayness of combustion gases in radiation heat transfer calculations. However, the determination of the correct limits for the bands and the effective band absorption coefficients is still something of a black art. In this study, the line by line (LBL) spectral absorption coefficient profile has been implemented to obtain the effective number of bands, and bands’ limits for pure H2O, pure CO2 and a H2O-CO2 gas mixture. A mathematical technique has been used to smooth the LBL profiles of pure gases in atmospheric pressure in order to be used for identifying the gray bands. The optimization for selecting the bands is done by analyzing the radiative heat transfer in several one-dimensional benchmarks. After obtaining the optimal band dividing scheme, a set of correlations for the pressure based gray band absorption coefficient of pure gases is found by integrating the line by line spectral absorption coefficient weighted by the corresponding black body intensity along the bands. In contrary to the previous similar works, by using the LBL data for pressure based gray absorption coefficient of the bands, the current correlations are independent of gas concentration and path length. The present approach could successfully support the non-gray walls. The method has been validated using several benchmarks and exhibited comparable accuracy with other available models.

Assessment of combustion models for numerical simulations of a turbulent non-premixed natural gas flame inside a cylindrical chamber

ABSTRACTThis work presents a Computational Fluid Dynamics (CFD) study of the non-premixed combustion of natural gas with air in an axisymmetric cylindrical chamber, focusing on the contribution of the chemical reaction modeling on the temperature and the chemical species concentration fields. Simulations are based on the solution of mass, momentum, energy and chemical species conservation equations. Thermal radiation heat transfer in the combustion chamber is computed through the Discrete Transfer Radiation Method, and the Weighted-Sum-of-Gray-Gases model solves the dependence of gas absorption coefficient on the wavelength. Turbulence is modeled by the standard k-ε model. Regarding the combustion modeling, it is performed a comparison of solutions obtained with the combined Eddy Break-Up/Arrhenius (EBU/Arrhenius) and the Steady Laminar Diffusion Flamelet (SLDF) models. The finite volume method is employed to treat the differential equations. Among other results, the solution of the governing equations allows for the determination of the region where combustion takes place, the distribution of the chemical species and the velocity fields. The numerical results are compared to experimental measurements, showing varied agreements. Results indicate that, in this case, the EBU/Arrhenius model can predict the flame temperature and the concentration of the most important species with better accuracy than the more sophisticated SLDF model.

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.