Gray Gas Research Articles

Study on radiation heat transfer characteristics of aeroengine combustion chamber

The thermal radiation of gaseous combustion products in an aero-engine combustion chamber has obvious non-ash-body characteristics. In this paper, based on the OpenFOAM platform, a numerical study on multi-field coupled turbulence-combustion-radiation radiative heat transfer was carried out for a certain type of aero-engine combustion chamber using the mean weighted sum of grey gas (WSGG) gas radiation model, which can accurately describe the non-ash characteristics of the gas. The results show that the radiation heat transfer characteristics are influenced by several factors such as equivalent ratio, combustion pressure and wall emissivity, among which the equivalent ratio is dominant. Increasing the equivalence ratio from 0.11 to 0.4 resulted in a 189% increase in average incident radiation at the combustion chamber wall and a 233% increase in average incident radiation at the exit. Increasing the equivalent ratio, combustion pressure or wall emissivity will increase the average radiation intensity in the combustion chamber and the inhomogeneity of radiation distribution at the exit of the combustion chamber. The radial radiation distribution factor (RRDF) also increases with the increase of equivalent ratio, combustion pressure and wall emissivity.

LES of a swirl-stabilized 40 kWth biomass flame and comparison to a coal flame

Recent advancements have introduced high-fidelity simulations in pulverized coal combustion, yet the shift toward renewable fuels like biomass raises questions about suitable modeling approaches to understand the influence of the new fuel. The current study aims to examine the variations between biomass and pulverized coal flames by employing extended high-fidelity modeling approaches integrated into a Large Eddy Simulation (LES) framework. A fully coupled solid fuel-gas phase model is developed to investigate pulverized biomass and coal flames. Models for particles that are treated in a Lagrangian manner include state-of-the-art devolatilization and char models calibrated with advanced conversion models for the conditions under investigation. Using five-dimensional tabulated flamelet manifolds, gas phase reactions are modeled by two chemical mechanisms tailored to the respective fuel properties. Radiation is considered by solving the radiative transfer equation employing the weighted sum of gray gas model. This high-fidelity simulation framework is applied to a self-sustained, swirl-stabilized 40 kWth burner that is fired with walnut shells and Rhenish lignite, respectively. Both fuels are studied under the same flow conditions and thermal output to enable a thorough analysis of the fuel effects. The results are validated with experimental particle velocities. Using advanced post-processing for particle grouping, a close link between flame shape and differences in particle trajectories is established, indicating the high significance of particle size when transitioning between fuels.

Large eddy simulation investigation of the effect of radiative heat transfer on the ignition progress in a model combustor

Radiation significantly influences turbulent combustion. This paper investigates the impact of radiative heat transfer on the ignition process in a multi-staged model combustor. The ignition process is simulated both without and with consideration of radiation. Large Eddy Simulations based on the Euler-Lagrange description of the liquid spray and the Dynamic Thickened Flame model coupled with a skeletal chemical reaction mechanism are employed. The Weighted Sum of Gray Gases method are employed to solve the spectral properties and the Discrete Ordinates method is employed to solve the radiative transfer equation. The results indicate that radiative heat transfer influences the flame structure during the ignition process. In the flame propagation phase, both the total heat transfer and the exit average temperature are higher, with the radiative heat transfer during ignition being less than 15% of the total heat transfer. During the kernel generation phase, radiative heat transfer leads to a 7.24% increase in the maximum temperature of the flame kernel, a 7.74% expansion of the flame surface area, and outward movement of the position of the maximum heat release rate. Additionally, the ignition delay time extends by 14.91%. In the flame propagation phase, the flame transfers heat to the mixed gas and droplets near the flame surface through radiation, promoting flame spread.

Computational assessment of soot models in ethylene/air laminar diffusion flames

To improve the accuracy of soot formation and evolution predictions, several physical and chemical models have been developed over the last decades. These models include (i) detailed chemical kinetic mechanisms describing both gas-phase chemistry related to combustion processes and reaction pathways leading to large-sized aromatic molecules, which are needed for modelling soot formation, and (ii) soot models providing a comprehensive description of soot particle dynamics and interactions with gas-phase chemical species. Accordingly, in this work, two detailed soot models, the method of moments (MOM) and the discrete sectional method (DSM), are evaluated in ethylene/air laminar diffusion flames, and their corresponding results are compared with experimental measurements. Furthermore, the NBP and KM2 chemical kinetic mechanisms are assessed and compared with each other by examining key chemical species related to soot formation and evolution. To compute gas mixture’s radiative properties, the weighted sum of grey gases model considering a grey medium is also utilised. Finally, the contributions of the soot precursors known as PAH (polycyclic aromatic hydrocarbon) to soot formation are also analysed. The main results show that the discrepancies in PAH concentrations obtained with different chemical kinetic mechanisms can be significant. In addition, compared to MOM ones, DSM results obtained here show a better agreement with experimental data. Finally, the analysis of PAH shows that those with two (A2) to four (A4) aromatic rings impact the most on soot modelling. Specifically, contributions of A4 were found to be more significant at lower heights above the burner, whereas A2 was found to be more impactful downstream as the flame develops. Maximum contributions of A2 and A4 to the soot inception rate were 66% and 85%, respectively, whereas the maximum summed contribution of PAH with five (A4R5) to seven (A7) aromatic rings accounted for only 13% of the inception rate.

Finite-rate chemistry Favre-Averaged Navier-Stokes based simulation of a non-premixed SynGas/Air flame

Abstract The present paper is devoted to the study of the influence of chemical mechanisms, turbulence models and gas radiative properties models on the characteristics of a turbulent diffusion CO/H2/N2 -air flame, i.e., the so-called syngas flame in a Favre-averaged Navier-Stokes (FANS) environment. For this purpose, a transient FANS solver for combustion is used. The simulations are carried out using three distinct turbulence models, i.e., the standard k-ε (SKE), the renormalization group (RNG) k-ε, and the Shear Stress Transport (SST) models. The turbulence-chemistry interaction is modeled using the Partially Stired Reaction (PaSR) model. The chemical mechanisms used in the present study are: (i) a compact skeletal C2 mechanism, (ii) a mechanism developped by Frassoldati-Faravelli-Ranzi (FFR) containing 14 species and 33 reactions and (iii) the optimised syngas mechanism by Varga. Radiation heat transfer is handled by the P-1 method. In addition, the performances of two gas radiative properties models, i.e., the grey mean gas and the weighted-sum-of-gray-gases (WSGG) models, are assessed in radiative heat transfer modeling of the syngas flame. The predicted results reveal that the combination of the RNG turbulence model and the C2 skeletal mechanism shows the best agreement with measurements. The WSGG model used predicts results with the same level accuracy as the grey gas model in modeling of the syngas flame.

Gray gas emissivities and absorptivities for H2O[sbnd]CO2[sbnd]CO[sbnd]N2 mixtures

Our previously published EXCEL program for the calculation of gray gas emissivities for H2OCO2CON2 mixtures is extended to the calculation of gray gas absorptivities (see Supplementary Material).

Numerical investigation of radiative transfer during oxyfuel combustion of hydrogen and hydrogen enriched natural gas in the container glass industry

The substitution of natural gas with hydrogen is one way to eliminate direct CO2 emissions. However, oxyfuel combustion of hydrogen or hydrogen enriched natural gas leads to different exhaust gas properties due to a changed composition compared to conventional combustion. In combustion simulation, the emissivity of a gas mixture is usually approximated using a Weighted Sum of Gray Gases (WSGG) model. Most of the existing WSGG models have been validated for natural gas combustion with air or oxyfuel and are therefore not applicable to hydrogen-oxyfuel combustion. CFD simulations showed, that none of the investigated WSGG models is able to predict the radiative heat transfer for all considered combustion scenarios with appropriate accuracy. In addition, in container glass manufacturing more than 95% of the heat flux to the glass surface is transferred by radiation because of the high process temperatures. Due to the changed gray gas emissivity, the high content of water vapor leads to a different emission spectrum of the exhaust gas. The influence of the changed emission spectrum on radiative heat transfer and the penetration depth of radiation in the glass melt is investigated using a simulation model of a pilot plant and non-gray modelling of the radiation transport. The CFD simulations show slightly enhanced radiative heat transfer to the glass and a slightly deeper penetration depth especially for wavelength below 2.2 μm for hydrogen-oxyfuel combustion.

Numerical analysis of gray gas radiation effects on heat and mass transfer in an annular cavity

This study deals with a numerical investigation of coupled double diffusive natural-convection with thermal radiation in an annular cavity containing a gray gas mixture. The black vertical cylindrical walls are maintained at different temperatures and concentrations to create cooperating flows. The finite volume method (using the SIMPLER algorithm) is used to solve the governing equations and the discrete ordinate method (with S8 quadrature) to treat the radiative aspect of the problem. A parametric study illustrating the influence of the optical thickness and the ratio of buoyancy forces, on the flow field and heat and mass transfer for Reyleigh number equal to 5?106 and aspect ratio equal to 1, is performed. The numerical results show that gas radiation modifies the flow structure and the distribution of temperature and concentration in the cavity. The effect of permutation of boundary conditions, between the vertical walls, on heat and mass transfer is also considered. The thermal radiation reduces the total heat transfer in the annular space regardless of the configuration of the boundary conditions.

To simple waves and small-amplitude perturbations in radiation gasodynamics

The paper analyzes one-dimensional simple waves and small-amplitude perturbations in radiating and scattering gray gas. The governing equation of radiation acoustics describing the dynamics of simple waves is derived. The conditions of radiation-thermal dissipation and radiation resistance force are introduced into this equation to describe the propagation with dissipation and attenuation of various radiation perturbation waves. The phenomenological approximate Whitham method is used to investigate non-equilibrium wave phenomena in radiative medium. This method is an effective way to analyze fundamental modes when more than one velocity appears in the governing equation. The use of this method is demonstrated in this paper by considering the evolution of one-dimensional harmonic waves caused by a short-wave initial perturbation of the equilibrium state of the radiating and scattering medium. Analytical solutions are obtained for all wave modes, which allow us to interpret their physical meaning. These solutions can be, in particular, an additional test for radiative hydrodynamic codes operating in the radiative acoustics regime. The presented approach may be useful in detailing higher-order numerical Godunov schemes for radiation acoustics problems.

New gas radiation model of high accuracy based on the principle of weighted sum of gray gases

The standard WSGG (Weighted Sum of Gray Gases) model is very fast and simple but can lead to relatively high discrepancies in the computation of the radiative heat transfer. A new model of high accuracy relying on the principle of the WSGG model is proposed. Contrary to the standard WSGG model, the pressure absorption coefficient depends here weakly on the temperature while the weighting factor is not predefined by a mathematical function. Mathematical properties on the model parameters are obtained and we show that all the model parameters can be determined from only one of them. This last is reconstructed with an efficient inverse algorithm from the total radiative heat source data computed with the LBL method using HITEMP 2010. We propose an efficient method to find good initial guesses (of the model parameters) leading to the best accuracy of the model. It is shown, on 17 selected 1D cases representative of the combustion of CO2H2O mixtures, that the maximum relative errors on the radiative heat source for the new model (based on 6 gray gases) does not exceed 3.5% (for these 17 cases) whereas for the standard WSGG model, these errors vary up to 14.5% (five cases have errors higher than 10.0% and nine cases have errors between 5.0% and 10.0%). The accuracy of the total radiative heat flux is also greatly enhanced with the new model. We also show that the present model is robust and can be used in 3D geometry.

Assessing the impact of wet and dry flue gas recycling on heat transfer in oxygen-fired circulating fluidized bed furnaces

Oxygen-driven circulating fluidized bed combustion (oxy-CFB) has been suggested as a potential technique for facilitating carbon capture in large-scale power generation. Precise modeling of radiative heat transfer is vital for forecasting the thermal performance of such processes. This research examines the impact of wet and dry flue gas recycling on the thermal behavior of a large-scale oxy-CFB furnace, utilizing a combined modeling strategy that merges a three-dimensional furnace process model with a thermal radiation solver. The semi-empirical furnace model operates in steady-state conditions, accounting for fluid dynamics, reactions, and heat transfer phenomena. The radiation solver, based on the zone method, employs novel correlations for radiative heat exchange between optically thick zones. The radiation solver employs a weighted sum of the gray gases model, developed specifically for oxy-combustion, and geometric optics to determine the radiative characteristics of combustion gases and particles, respectively.This research covers various process conditions and boiler loads to evaluate the influence of wet versus dry flue gas recycling on heat transfer and temperature profiles. Model results were successfully compared against small-scale oxygen-fired CFB tests and large-scale measurements in a supercritical air-fired CFB. Utilizing an external radiation solver allows for a more precise representation of radiative heat transfer and the influence of varying process conditions. The insights from this research can aid in the design of large-scale oxy-CFB facilities, and the demonstrated modeling approach can be employed for additional model development, such as integrating the external radiation solver with other modeling tools.

Numerical study on oxy-fuel combustion characteristics of industrial furnace firing coking dry gas

One of the most promising methods for clean utilization of hydrocarbon fuels and CO2 capture is oxy-fuel combustion. However, report on oxy-fuel burning of industrial gas is still limited. In the study, numerical simulation was conducted to explore the oxy-fuel combustion performance of a coking dry gas fired industrial furnace. A modified weighted sum gray gas (WSGG) model with a four-gray body and fourth-degree temperature polynomial was proposed for both air and oxy-fuel combustion with the introduction of the fourth graybody. The impacts of oxygen content, excess oxygen ratio, oxygen purity, and dehydration efficiency on combustion and heat transfer were mainly investigated. The maximum furnace temperature increases significantly, and the high temperature region inside the furnace shrinks and moves downward as the oxygen content rises. The H2O content increases from ∼20% to ∼37%, and the CO2 content decreases from ∼77% to ∼60%. The furnace temperature is up to 1835 K under air conditions, while the furnace temperature is reduced by 195 K under oxy-fuel condition of 21% O2 content. The composition of flue gas barely changes as the excess oxygen ratio increases from 1.02 to 1.20. Although the CO2 enrichment in flue gas and the improvement of oxy-fuel combustion performance benefit from an increase in oxygen purity, the combustion performance further advances little when it is raised to over 98%. Additionally, when dehydration efficiency grows, radiant heat transfer decreases and the flue gas temperature at the furnace outlet rises. The content of O2 and N2 has little relationship with dehydration efficiency. This study aids in understanding the actual oxy-fuel combustion process of an industrial gas-fired boiler and offers a possible theoretical basis for clean burning of petrochemical combustible gas and carbon capture.

CFD analyses of large-scale tests highlighting the effects of thermal radiation in steam-containing atmospheres

In simulations of heat transfer in large gaseous volumes under moderate temperatures, thermal radiation is traditionally neglected to simplify the computations. In recent years however, some small as well as large-scale experiments have demonstrated that thermal radiation may have a strong effect on the temperature field if the atmosphere contains an infrared absorbing gas such as vapor, even in small concentrations. This has in particular been shown in the H2P2 series of tests performed in the large-scale PANDA facility at the Paul Scherrer Institute. For these tests an air-helium-steam mixture was compressed with a helium injection, which caused the formation of a hot bubble. The initial vapor volume fraction varied between 0.1 % and 60%. In this paper, we report the results of CFD simulations conducted for the five H2P2 tests. The turbulence model k-ω SST was used, while thermal radiation was considered using the simple P1 model as a default, in conjunction with the Weighted Sum of Gray Gases Model (WSGGM) to specify steam absorptivity. For comparison purposes we also made use of the more advanced Monte-Carlo model for selected experiments. CFD Best Practice Guidelines were employed, in particular through preliminary grid and time-step sensitivity studies.The temperature and helium concentration fields matched the experimental data quite accurately, except for the test with very low vapor content (H2P2_1_2, 0.1% vol. steam), where the temperature was somewhat over-estimated. This may be due to the inadequacy of the radiation models at these extreme conditions. Two major conclusions can be drawn from this study: 1) ignoring thermal radiation can lead to large over-predictions of temperature, even when the steam content is low (order of 1%) and 2) the simple P1 model provides sufficient accuracy. Use of the more sophisticated Monte-Carlo model yielded similar and slightly more accurate results, albeit at a considerably larger CPU expenditure (5–7 times).

Acceleration of premixed H[formula omitted]-Air-Steam flames when accounting for thermal radiation

Diluting hydrogen flames with steam can increase flame stability and reduce nitrogen oxide emissions. While the effects of dilution on the flame characteristics due to variations in chemistry or transport properties have been well investigated, this is not the case for those related to thermal radiation. Thermal radiation is indeed often neglected in free flame simulations. However, it can be critical in diluted flames in which participating species are present in the fresh gases. This study explores thermal radiation’s impact on H2-Air flames diluted with steam through 1D laminar flame computations. To this end, a reactive fluid solver is coupled with a semi-analytical thermal radiation code. Both the grey gas approximation, for a preliminary understanding, and the more accurate CK narrow-band model, for detailed simulations that account for spectral-dependent gas properties, are used. The combustion numerical setup is validated against literature results. Three main thermal radiation effects are highlighted: a preheating of the fresh gases, an increased laminar flame speed, and a decreased temperature in the burnt gases. It is found that domain length considerably affects this preheating and flame speed, which can then affects laminar burning velocity measurements. It is shown that thermal radiation and combustion can often be decoupled and that a preheated adiabatic flame can recover the main characteristics of the corresponding radiative flame. The increase of the laminar flame speed by thermal radiation is reported for a wide range of equivalence ratios, dilution levels, and pressures. The observed acceleration is especially significant for lean and very lean flames which can be up to 500% faster when considering thermal radiation.

Applications of weighted-sum-of-gray-gas model with soot to an experimental aero-engine combustion chamber

Due to the importance of radiation during combustion in the aero-engine, the weighted-sum-of-gray-gas (WSGG) model considering both gas and soot is applied to the combustion modelling for an experimental aero-engine in this work. Five gray gases are chosen to resolve the absorption coefficients of CO2-H2O mixture, with which the Planck-mean absorption coefficients of soot are used for the spectral consideration of gas-soot mixture. Together with the discrete ordinate method, the radiation effects using WSGG model on the combustion are presented. Comparisons are made between full-scale combustion simulations without radiation and those with radiation from gas only or gas-soot mixture. Results show that radiation from either gas or gas-soot mixture leads to no more than 3% differences in temperature and 5% differences in species concentrations. However, the temperature of combustor liner walls in the primary zone (i.e., the main combustion region) is significantly altered with an average of 100 K rise when considering radiation from gas only and a maximum of 500 K rise when considering radiation from gas-soot mixture. Results also show that the temperature of combustor liner walls with radiation from gas-soot mixture becomes more well-distributed than that either without radiation or with radiation from gas only.

Effects of Radiation Model on Soot Modeling in Laminar Coflow Diffusion Flames at Elevated Pressure

ABSTRACT Detailed numerical simulations of C2H4/Air coflow laminar sooting flames at atmospheric and elevated pressures are performed with fully coupled flow, gas-phase reactions, soot dynamics, and nongray radiative heat transfer. The soot dynamics is modeled using a hybrid method of moments combined with a polycyclic aromatic hydrocarbon (PAH) mechanism. Nongray radiative heat transfer by CO2, H2O, and soot is calculated using different methods to study the effect of radiation model on the predictions. The discrete ordinates radiation model (DOM) and P1 radiation model are compared. Three radiative property models with different accuracy and efficiency are included: a) full-spectrum correlated- distribution (FSCK) model, b) weighted sum of gray gas model with original parameters (WSGG-Smith), and c) WSGG model with optimized parameters for variable mole ratios and elevated pressure (WSGG-SK). The optically thin approximation (OTA) model is also compared to a simple baseline case. The results show that the temperature and soot volume fraction predicted by the DOM combined with the WSGG-SK model are consistent with the FSCK model, with errors less than 2%. When the pressure increases to 8 bar, errors of P1 and OTA models increase significantly, the maximum temperatures predicted by the P1 and OTA models have errors of 36 K and 63 K, and the relative errors of the peak soot volume fraction are 18% and 24%, respectively.

A two-zone subgrid flame model for predicting radiant emission from fires

A simple two-zone subgrid temperature distribution is developed to model turbulence-radiation interaction in the Fire Dynamics Simulator. The new approach enforces consistency between the subgrid flame distribution and the heat release rate from the eddy dissipation model. To investigate the performance of the new model, we simulate the University of Maryland turbulent line burner and compare global radiative fraction as a function of coflow oxygen dilution. The results suggest that the model can improve grid resolution dependence in prediction of radiative emissions. Sensitivity of the model predictions to the assumed fuel carbon to CO fraction (a parameter of the two-step fast chemistry scheme) was found to be significant, which is not surprising since this parameter essentially controls the in-flame soot concentration and generally has a first-order effect on resolved emission. Four different model variations were tested, corresponding to varying degrees of complexity in modeling the subgrid correlations for the absorption coefficient. Differences in results between model implementations were on par with the variation in results obtained with different path lengths applied to the gray gas model for the absorption coefficient.

New gas radiation model based on the principle of weighted sum of gray gases. Application to CO2–H2O mixtures at high temperature

A new gas radiation model based on the principle of weighted-sum-of-gray-gases (WSGG) is proposed. It is shown that expressing the weighting factors as linear functions of the temperature is more suitable than the fourth-order polynomial dependence to the temperature that is commonly adopted in the WSGG model. Using an efficient inverse algorithm, it was estimated a set of twelve spatially nonconstant parameters of the new model based on four gray gases, namely the coefficients of the linear functions and the pressure absorption coefficients. The inverse algorithm is based on the adjoint state of the radiative transfer equation (RTE) using as data the total radiative heat source instead of the total gas emissivity data. The parameters are fitted using data generated with line-by-line (LBL) simulations based on HITEMP2010. In the present work, the proposed gas radiation model is applied to determine the parameters of H2O–CO2 mixtures at atmospheric pressure with partial pressure ratio equal to 2, within the range of temperature between 400 K and 1800 K and for thicknesses of the gas layer up to 3.0 m. By applying the inverse algorithm to a given case, the present gas radiation model has the level of accuracy of the LBL method. The application of the same parameters to other test cases does not lead to the same level of accuracy, since the parameters obtained are spatially dependent. However, averaging these parameters in the inverse algorithm results in a gas radiation model with spatially constant parameters. When applied to several (1-D and 3-D) test cases in the conditions for which the parameters were obtained, this led to a significant reduction in the normalized errors on the radiative heat source and, for many of these cases, on the radiative heat flux compared to that obtained with the standard WSGG model.

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.

Portfolio performance implications of investment in renewable energy equities: Green versus gray

AbstractThe Sustainable Development Goals of the United Nation and interest by investors in Environmental, Social and Governance (ESG) investment strategies have caused a rapid shift to the green or renewable energy sector, from traditional or gray (oil, gas, and coal) energy companies. In this study, we examine whether and to what extent, financially speaking, there is a price to pay for investing in renewable energy sector equity. Moreover, we seek to determine whether green investments can be considered a hedge during times of financial stress. We find that alphas from investments in a portfolio of gray (overall energy sector) stocks and versus a portfolio of renewable energy equities during an exogenous, non‐financial shock—the COVID‐19 pandemic—and during non‐crisis periods did not differ statistically. However, the renewable energy index showed higher idiosyncratic volatility than the energy index, as expected. The results are robust to alternative model specifications. From a practical perspective, our results are informative in that they provide insights into the tradeoffs associated with renewable energy investments. In particular, risk‐adjusted returns to a renewable energy portfolio may be affected by greater idiosyncratic risk.