R.J.M. Bastiaans

LES and RANS of Premixed Combustion in a Gas-Turbine Like Combustor Using the Flamelet Generated Manifold Approach

Dry-low NOx gas turbine technology relies on lean premixed combustion of fuel. Additionally the accurate prediction of turbulent premixed combustion is still very difficult. In the present paper the calculation of reduced chemistry is assessed efficiently through the use of the flamelet generated manifold (FGM), which is used in conjunction with a CFD code in a RANS as well as in an LES context. In order to predict the combustion phenomena in a high swirl and high Reynolds number flow (the SimVal setup, at atmospheric pressure with elevated temperature), the present model is used concomitantly with a pre-assumed PDF for which fluctuations are completely determined in terms of an algebraic model. The mixing model for the variance has an arbitrary model constant, and the results show that the flame stabilization is not very sensitive to the model parameter present in the model. Stabilization of the combustion occurs at a location comparable to that found in experiments. In order to investigate the effects of this parameter on the numerical solutions, first RANS simulations were addressed considering arbitrary values for this parameter, defined within a certain range, and in a next step the grid resolution was changed. LES calculations were also performed showing similar features predicted in RANS. It is found that with the use of FGM combustion features in gas turbine conditions can be reproduced in a robust way.Copyright

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.

Steady large-scale modulation of a moderately turbulent co-flow jet

The effects of a spatial modulation acting at the inflow of a moderately turbulent planar jet surrounded by a faster co-flow are investigated using direct numerical simulation of the Navier–Stokes equations. We adopt a superposition of spatially filtered small-scale random perturbations and a structured large-scale flow modulation. The large-scale modulation is characterised in terms of a Beltrami flow, specified by a wavenumber K. These large-scale modulations are steady and spatially periodic, while the random small-scale perturbations fluctuate in time and in space. The flow configuration studied in this paper is agitated by this combined large- and small-scale agitation at the inflow plane of a rectangular domain of size L × L × 2L in the x-, y- and streamwise z-directions. The inflow perturbation is focused on a strip of size L × D in the x- and y-directions. A parametric variation is carried out considering different choices for the wavenumber of the large-scale modulation. We focus on effects that the inflow modulation has on global characteristics of the flow, e.g. the width of the mixing region formed between the two streams and the dissipation rate, ϵ. Results show that the width of the mixing region increases faster compared to the case without the large-scale perturbation, when the flow is agitated by structures of size comparable to the integral scales of the flow. For the dissipation rate, results show the presence of a maximum response at a certain wavenumber K in case we apply a large-scale modulation. This maximum is attained at modulation scales that vary locally with respect to the distance from the inflow plane. Close to the inflow, the maximum response occurs at small modulation scales, while further into the domain a maximum response is present at comparably large modulation scales.

Accuracy of Large-Eddy Simulation of Premixed Turbulent Combustion

The accuracy of large-eddy simulation (LES) of a turbulent premixed Bunsen flame is investigated in this paper. To distinguish between discretization and modeling errors, multiple large-eddy simulations, using different grid size h but the same filterwidth Δ, are compared with the direct numerical simulation (DNS). In addition, large-eddy simulations using multiple Δ but the same ratio Δ/h are compared. The chemistry in the LES and DNS is parametrized with the standard steady premixed flamelet for stochiometric methane-air combustion. The subgrid terms are closed with an eddy-viscosity or eddy-diffusivity approach, with an exception of the dominant subgrid term, which is the subgrid part of the chemical source term. The latter subgrid contribution is modeled by a similarity model based upon Δ, which is found to be superior to such a model based upon Δ. Using the 2Δ similarity model for the subgrid chemistry the LES produces good results, certainly in view of the fact that the LES is completely wrong if the subgrid chemistry model is omitted. The grid refinements of the LES show that the results for Δ = h do depend on the numerical scheme, much more than for h = Δ/2 and h = Δ/4. Nevertheless, modeling errors and discretization error may partially cancel each other; occasionally the Δ = h results were more accurate than the h ≤ Δ results.

Numerical Simulations of a Turbulent High-Pressure Premixed Cooled Jet Flame With the Flamelet Generated Manifolds Technique

In the present paper, a computational analysis of a high pressure confined premixed turbulent methane/air jet flames with heat loss to the walls is presented. In this scope, chemistry is reduced by the use of the flamelet generated manifold (FGM) method and the fluid flow is modeled in an large eddy simulation (LES) and Reynolds-averaged Navier–Stokes (RANS) context. The reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the turbulence effect on the reaction is represented by the progress variable variance. A generic lab scale burner for methane high-pressure (5 bar) high-velocity (40?m/s at the inlet) preheated jet is adopted for the simulations, because of its gas-turbine relevant conditions. 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. Furthermore, the present analysis indicates that the physical and chemical processes controlling carbon monoxide (CO) emissions can be captured only by means of unsteady simulations.

The Application of Flamelet-Generated Manifold in the Modeling of Stratified Premixed Cooled Flames

CFD predictions of flame position, stability and emissions are essential in order to obtain optimized combustor designs in a cost efficient way. However, the numerical modeling of practical combustion systems is a very challenging task. As a matter of fact, the use of detailed reaction mechanisms is necessary for such reliable predictions. Unfortunately, the modeling of the full detail of practical combustion equipment is currently prohibited by the limitations in computing power, given the large number of species and reactions involved. The Flamelet-Generated Manifold (FGM) method reduces these computational costs by several orders of magnitude without loosing too much accuracy. Hereby FGM enables the application of reliable chemistry mechanisms in CFD simulations of combustion processes. In the present paper a computational analysis of partially premixed non-adiabatic flames is presented. In this scope, chemistry is reduced by the use of the FGM method. In the FGM technique the progress of the flame is generally described by a few control variables. For each control variable a transport equation is solved during run-time. The flamelet system is computed in a pre-processing stage, and a manifold with all the information about combustion is stored in a tabulated form. This research applies the FGM chemistry reduction method to describe partially premixed flames in combination with heat loss, which is a relevant condition for stationary gas turbine combustors. In order to take this into account, in the present implementation the reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the local equivalence ratio effect on the reaction is represented by the mixture fraction. A series of test simulations is performed for a two dimensional geometry, characterized by a distinctive stratified methane/air inlet, and compared with detailed chemistry simulations. The results indicate that detailed simulations are reproduced in an excellent way with FGM.

The Implementation of Five-Dimensional FGM Combustion Model for the Simulation of a Gas Turbine Model Combustor

Gas turbines are one of the most important energy conversion methods in the world today. This is because using gas turbines, large scale, high efficiency, low cost and low emission energy production is possible. For this type of engines, low pollutants emissions can be achieved by very lean premixed combustion systems. Numerical simulation is foreseen to provide a tremendous increase in gas turbine combustors design efficiency and quality over the next future. However, the numerical simulation of modern stationary gas-turbine combustion systems represents a very challenging task. Several numerical models have been developed in order to reduce the costs of flame simulations for engineering applications. In the present paper the Flamelet-Generated Manifold (FGM) chemistry reduction method is implemented and extended for the inclusion of all the features that are typically observed in stationary gas-turbine combustion. These consist of stratification effects, heat loss and turbulence. The latter is included by coupling FGM with the Reynolds Averaged Navier Stokes (RANS) model. 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 probability density function (PDF) approach, which is considered for progress variable and mixture fraction. This results in two extra control variables: progress variable variance and mixture fraction variance. The resulting manifold is therefore five-dimensional, in which the dimensions are progress variable, enthalpy, mixture fraction, progress variable variance and mixture fraction variance. A highly turbulent and swirling flame in a gas turbine model combustor is computed in order to test the 5-D FGM implementation. 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.© 2015 ASME

Numerical Analysis of a Swirl Stabilized Premixed Combustor with the Flamelet Generated Manifold approach

In this paper the effectiveness of LES for modeling premixed methane combustion will be investigated in the context of gas turbine modeling. The required reduction of the chemistry is provided by the flamelet generated manifold (FGM) approach of van Oijen (2002). For turbulence-chemistry interactions an algebraic model is used to calculate variations which are used to invoke a pre-assumed pdf, Vreman et al. (2009). The algebraic model has a tunable parameter.

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.

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.