J.A. van Oijen

A 5-D implementation of FGM for the large eddy simulation of a stratified swirled flame with heat loss in a gas turbine combustor

Numerical simulations are foreseen to provide a tremendous increase in gas-turbine burners efficiency in the near future. Modern developments in numerical schemes, turbulence models and the consistent increase of computing power allow Large Eddy Simulation (LES) to be applied to real cold flow industrial applications. However, the detailed simulation of the gas-turbine combustion process remains still prohibited because of its enormous computational cost. Several numerical models have been developed in order to reduce the costs of flame simulations for engineering applications. In this paper, the Flamelet-Generated Manifold (FGM) chemistry reduction technique is implemented and progressively extended for the inclusion of all the combustion features that are typically observed in stationary gas-turbine combustion. These consist of stratification effects, heat loss and turbulence. Three control variables are included for the chemistry representation: the reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the stratification effect is expressed by the mixture fraction. The interaction between chemistry and turbulence is considered through a presumed beta-shaped probability density function (PDF) approach, which is considered for progress variable and mixture fraction, finally attaining a 5-D manifold. The application of FGM in combination with heat loss, fuel stratification and turbulence has never been studied in literature. To this aim, a highly turbulent and swirling flame in a gas turbine combustor is computed by means of the present 5-D FGM implementation coupled to an LES turbulence model, and the results are compared with experimental data. In general, the model gives a rather good agreement with experimental data. It is shown that the inclusion of heat loss strongly enhances the temperature predictions in the whole burner and leads to greatly improved NO predictions. The use of FGM as a combustion model shows that combustion features at gas turbine conditions can be satisfactorily reproduced with a reasonable computational effort. The implemented combustion model retains most of the physical accuracy of a detailed simulation while drastically reducing its computational time, paving the way for new developments of alternative fuel usage in a cleaner and more efficient combustion.

LES of pre-vaporized Kerosene Combustion at high Pressures in a single Sector Combustor taking Advantage of the Flamelet generated Manifolds Method

This paper summarizes the development of an LES based model for reliable description of combustion in a gas turbine combustion chamber. Combustion is described by means of the flamelet generated manifolds (FGM) method. A Smagorinskymodel with dynamic procedure is applied to determine the subgrid scale stresses. A gradient ansatz model is used to represent the sub-grid scale scalar flux in the mixture fraction and in the reaction progress variable equations. Soot formation and radiation are not considered. In order to evaluate the capability of the model for predicting combustion processes induced by complex real fuels a high pressure single sector combustor (SSC) is investigated. This combustion chamber is fuelled with pre-vaporized kerosene fuel and features very complex unsteady swirling flow and partially premixed combustion properties. The validation of the designed tool along with the prediction analysis is carried out in terms of comparison between experimental data (achieved with a nozzle fired at 0.6 Mpa) and numerical results. This reveals that the proposed LES model is able to capture satisfactorily the flow and combustion properties involving. In particular the flame is predicted to be not always attached to the nozzle. It fluctuates between a lifted and an attached regime. This agrees with experimental findings.

Analysis of impinging wall effects on hydrogen non-premixed flame

Investigations of the flame–vortex and flame–wall interactions have been performed for hydrogen impinging non-premixed flame at a Reynolds number of 2000 and a nozzle-to-plate distance of 4 jet diameters by direct numerical simulation (DNS) and flamelet generated manifold (FGM) based on detailed chemical kinetics. The results presented in this study were obtained from simulations using a uniform Cartesian grid with 200 × 600 × 600 points. The spatial discretization was carried out using a sixth-order accurate compact finite difference scheme, and the discretized equations were advanced using a third-order accurate fully explicit compact-storage Runge–Kutta scheme. The results show that the inner vortical structures dominate the mixing of the primary jet for the nonbuoyant case, while outer vortical structures dominate over the inner vortical structures in the flow fields of the buoyant cases. The formation of vortical structures due to buoyancy has a direct impact on the flow patterns in both the primary and wall jet streams, which in turn affects the flame temperature and the near-wall heat transfer. It has been found that the buoyancy instability plays a key role in the formation of the much wider and higher value wall heat flux compared with the nonbuoyant case, while external perturbation does not play a significant role. The computational results show an increased wall heat flux with the presence of buoyancy.

A Priori Assessment of the Potential of Flamelet Generated Manifolds to Model Lean Turbulent Premixed Hydrogen Combustion

The numerical modeling of combustion systems is a very challenging task. The interaction of turbulence, chemical reactions and thermodynamics in reacting flows is of exceptional complexity. Computing power is too limited to solve practical problems in detail. This problem asks for special treatments in the modeling of flames.

Growth of Soot Volume Fraction and Aggregate Size in 1D Premixed C2H4/Air Flames Studied by Laser-Induced Incandescence and Angle-Dependent Light Scattering

The growth of soot volume fraction and aggregate size was studied in burner-stabilized premixed C2H4/air flames with equivalence ratios between 2.0 and 2.35 as function of height above the burner using laser-induced incandescence (LII) to measure soot volume fractions and angle-dependent light scattering (ADLS) to measure corresponding aggregate sizes. Flame temperatures were varied at fixed equivalence ratio by changing the exit velocity of the unburned gas mixture. Temperatures were measured using spontaneous Raman scattering in flames with equivalence ratios up to = 2.1, with results showing good correspondence (within 50 K) with temperatures calculated using the San Diego mechanism. Both the soot volume fraction and radius of gyration strongly increase in richer flames. Furthermore, both show a nonmonotonic dependence on flame temperature, with a maximum occurring at ~1675 K for the volume fraction and ~1700 K for the radius of gyration. The measurement results were compared with calculations using two different semiempirical two-equation models of soot formation. Numerical calculations using both mechanisms substantially overpredict the measured soot volume fractions, although the models do better in richer flames. The model accounting for particle coagulation overpredicts the measured radii of gyration substantially for all equivalence ratios, although the calculated values improve at = 2.35.

The implementation of 5-D FGM for LES of a gas turbine model combustor with heat loss

The interest in numerical simulation of combusting flows for industrial applications has gained a wide growth in the past decade.

A simple model for prediction of preheating and pyrolysis time of a thermally thin charring particle

The aim of this paper is to present a simple model, based on a time and space integral method, for prediction of preheating and conversion time of a charring solid particle exposed to a non-oxidative hot environment. The main assumptions are 1) thermo-physical properties remain constant throughout the process; 2) temperature profile within the particle is assumed to obey a quadratic function with respect to the space coordinate; 3) pyrolysis initiates when the surface temperature reaches a characteristic pyrolysis temperature; 4) decomposition of virgin material occurs at an infinitesimal thin layer dividing the particle into char and virgin material regions; 5) the volume of the particle remains unaltered; 6) volatiles escape through the pores immediately after formation.Employing assumption (2) allows one to convert the energy conservation equation of the particle, which is basically described in the form of a partial differential equation (PDE), into an ordinary differential equation (ODE) by performing space integration. Next, by applying approximate time integration the ODE is transformed into an algebraic equation. Applying this approach to the preheating and pyrolysis stages of a thermally thin charring solid particle leads to a set of algebraic equations which provides reactor designers with a convenient means for computation of the heating up time, mass loss history and total conversion of particle. The accuracy of the simple model is assessed by comparing its prediction with that of a one-dimensional detailed pyrolysis model. Overall, good agreement is achieved indicating that this new model can be used for engineering and design purposes.Copyright

Mathematical Modeling of Heat and Mass Transfer Processes During Pyrolysis and Combustion of a Single Biomass Particle

A detailed mathematical model is developed for simulation of heat and mass transfer processes during the pyrolysis and combustion of a single biomass particle. The kinetic scheme of Shafizadeh and Chin is employed to describe the pyrolysis process. The light gases formed during the biomass pyrolysis is assumed to consist of methane, carbon dioxide, carbon monoxide, hydrogen and water vapor with given mass fractions relevant to those found in the experiments of high heating conditions. The combustion model takes into account the reactions of oxygen with methane, hydrogen, carbon monoxide, tar and char as well as gasification of char with water vapor and carbon dioxide. Appropriate correlations taken from past studies are used for computation of the rate of these reactions.The model allows calculation of time and space evolution of various parameters including biomass and char densities, gaseous species and temperature. Different experimental data reported in the literature are employed to validate the pyrolysis and combustion models. The reasonable agreement obtained between the predictions and measured data reveals that the presented model is capable of successfully capturing various experiments of wood particle undergoing a pyrolysis or combustion process. In particular, the role of gas phase reactions within and adjacent to particle on the combustion process is examined. The results indicate that for the case of small particles in the order of millimeter size and less, one may neglect any effects of gas phase reactions. However, for larger particles, a combustion model may need to include hydrogen oxidation and even carbon monoxide combustion reactions.Copyright

Grate Furnace Combustion: A Submodel for the Solid Fuel Layer

The reduction of NOx-formation in biomass fired grate furnaces requires the development of numerical models. To represent the variety in scales and physical processes playing a role in the conversion, newly developed submodels are required. Here, a submodel for the reverse combustion process in the solid fuel layer on the grate is described. The submodel is shown to give good predictions for the velocity of the combustion front as well as for the spatial profiles of porosity, oxygen mass fraction and temperature. These predictions are essential input for NOx-calculations.