Pedro J. Coelho

Numerical simulation of a mild combustion burner

Abstract A combustor with high air preheating and strong internal exhaust gas recirculation is numerically simulated. The combustor operates in the mild combustion mode, characterized by relatively low flame temperatures, low NO x emissions, no visible flame and no sound. The Eulerian Particle Flamelet model by Barths et al. [1] is employed to calculate NO in a postprocessing stage. Predictions of the mean and fluctuating velocity components, as well as local residence times, are compared with experimental data. It is found that the predicted profiles reproduce the data reasonably well, but some discrepancies were found. The NO emission calculated using this unsteady flamelet model is relatively close to measurements available from a previous study, while steady flamelets overpredict the NO emission by an order of magnitude.

Spectral radiative effects and turbulence/radiation interaction in a non-luminous turbulent jet diffusion flame

A non-luminous turbulent jet diffusion flame is numerically simulated using a Reynolds stress second-order closure, the steady laminar flamelet model, and different approaches for radiative transfer. The commonly used optically thin approximation is compared with the discrete ordinates method. Calculations using the Planck mean absorption coefficient are compared with computations performed using the spectral line-based weighted-sum-of-gray-gases model. The interaction between turbulence and radiation is simulated, and its influence on the predicted results is investigated. It is shown that the discrete ordinates method and the optically thin approximation yield relatively close results for the present flame if the medium is modelled as gray using the Planck mean absorption coefficient. In both cases, the predicted fraction of radiative heat loss is significantly overestimated. However, if the spectral nature of gaseous radiation is accounted for, the computed radiation loss is closer to the experimental data. The fluctuations of the species have a minor role in the interaction between turbulence and radiation, which is mainly due to the temperature fluctuations.

The role of ray effects and false scattering on the accuracy of the standard and modified discrete ordinates methods

Ray effects and false scattering are two major sources of inaccuracy of the discrete ordinates method. High order schemes may reduce false scattering, and the modified discrete ordinates method may mitigate ray effects. Although the origin of the two errors is different, there is an interaction between them, since they tend to compensate each other. It is shown that decreasing one of the errors while keeping the other unchanged may decrease the solution accuracy because the compensation effect disappears. It is also shown that the modified discrete ordinates method does not decrease ray effects caused by sharp gradients of the temperature of the medium. A new version is proposed that successfully mitigates ray effects in that case.

Numerical simulation of radiative heat transfer from non-gray gases in three-dimensional enclosures

Abstract The discrete ordinates and the discrete transfer methods are applied to the numerical simulation of radiative heat transfer from non-gray gases in three-dimensional enclosures. Several gas radiative property models are used, namely the correlated k-distribution (CK), the spectral line-based weighted-sum-of-gray-gases (SLW) and the weighted-sum-of-gray-gases (WSGG) methods. The results are compared with recently published accurate calculations based on the statistical narrow band model. The WSGG model is computationally efficient, but often yields relatively large errors. It should be used only if moderate accuracy is sufficient. The SLW model is the best alternative regarding the compromise between accuracy and numerical efficiency. However, an optimization of the coefficients of the model is essential to reduce the computational requirements, especially in the case of gas mixtures. The CK model is the most accurate of the methods evaluated here, but too time consuming for engineering applications, although recent developments may partly overcome this shortcoming.

Unsteady modelling of a piloted methane/air jet flame based on the Eulerian particle flamelet model

A piloted methane/air jet flame is numerically simulated by using both steady and unsteady flamelets. Unsteady calculations are performed in a postprocessing stage using Eulerian transport equations for passive scalars to describe the temporal evolution of the scalar dissipation rate, conditional on the stoichiometric mixture fraction. The influence of the number of particles, and the initial conditions of the fluid particle transport equations and unsteady flamelet equations on the predicted Favre-average species mass fractions are investigated. It is shown that the predictions obtained using unsteady flamelets are in closer agreement with the experimental data than the steady flamelet predictions, both for the minor and for the major species. Initialization of the probability of finding fluid particles over the fuel rich region and initialization of the unsteady flamelet profiles based on a piecewise-linear interpolation of the boundary conditions at the fuel, pilot flame and air inlets yield good agreement between the predicted and the experimental data. However, the OH and the NO mass fractions are overestimated.

A Conservative Formulation of the Discrete Transfer Method

The discrete transfer method, often employed to calculate radiative heat transfer in combustion chambers, is not conservative. The reason for this behavior is examined and a conservative formulation is proposed and evaluated. A simple treatment of isotropic scattering media is also presented. The original and the conservative formulation of the method are applied to two-dimensional and three-dimensional enclosures containing a participating medium.

Flue gas recirculation in a gas-fired laboratory furnace: Measurements and modelling

This paper presents an experimental and numerical study of the effect of flue gas recirculation (FGR) on flame characteristics and pollutant emissions. The experimental study was performed in a small-scale laboratory furnace fired by a gas swirl burner of industrial type. The data reported include simultaneous flue gas concentrations of O 2, CO, CO2, unburnt hydrocarbons (UHC) and NOx. In addition, detailed in-flame data for major gas-phase species concentrations and gas temperatures were obtained in the near-burner region for two representative operating conditions. For these conditions, a mathematical model based on the numerical solution of the equations governing conservation of mass, momentum and energy and the transport equations for scalar quantities was used. The flue gas data show a marked decrease of NOx emissions with FGR without significant effects on flame stability, overall combustion efficiency and CO and UHC emissions. The transition between yellow and blue flame occurs at higher FGR rates as the excess air increases. The detailed in-flame data suggest that prompt NOx is an important mechanism of NOx formation for the present flow configuration without FGR and that FGR is an effective method for reducing it. These trends are correctly predicted by the mathematical model. However, discrepancies between the predicted and measured temperature and species concentrations, including NOx, were found, especially close to the burner. These may be due to the shortcomings of the turbulence model in the prediction of swirling flows.

Modelling of radiative heat transfer in enclosures with obstacles

Radiation models suitable for incorporation in reactive fluid flow codes are extended to calculate radiation in enclosures containing obstacles of very small thickness. The discrete transfer, the discrete ordinates and the finite volume method are employed to predict the heat transfer in two-dimensional enclosures and the results are compared with zone method calculations, with the total exchange areas determined by the Monte-Carlo method. All the methods predict similar heat fluxes, but the computational requirements are different. The discrete ordinates and the finite volume method are the most economical oneis. An application to a utility boiler is also presented. 0 1997 Elsevier Science Ltd.

On the application of the exponential wide band model to the calculation of radiative heat transfer in one- and two-dimensional enclosures

Abstract Various implementations of the exponential wide band model (EWBM) are used to model radiative heat transfer in one- and two-dimensional enclosures containing CO 2 and H 2 O. These are, first, the original banded approach using the four-region approximation for the total band absorption, second, a numerical integration of the spectral transmittance, and third, the wide band correlated k -distribution method (CKM). A correlated and a non-correlated formulation are used to solve the radiative transfer equation. In two-dimensional enclosures, these formulations are implemented using a ray tracing method (RTM) and the discrete ordinates method (DOM), respectively. The wide band CKM is found to be the best choice concerning accuracy and computational effort.

Parallelization of the discrete ordinates method

Abstract Two different paraUetization strategies of the discrete ordinates method (DOM) are described and implemented in a distributed-memory computer. In the angular decomposition (ADP), each processor performs the calculations for the whole domain but deals only with a few directions, while in the spatial domain decomposition (DDP), each processor treats all the directions but only for a subdomain. In the ADP the number of iterations is independent of the number of processors, contrary to the DDP, and higher efficiencies are obtained. The influence of the order of quadrature, grid size, and absorption coefficient of the medium is also investigated.