van Ja Jeroen Oijen

MODELLING OF PREMIXED LAMINAR FLAMES USING FLAMELET-GENERATED MANIFOLDS

In order to reduce the computational cost of flame simulations, several methods have been developed during the last decades, which simplify the description of the reaction kinetics. Most of these methods are based on partial-equilibrium and steady-state assumptions, assuming that most chemical processes have a much smaller time scale than the flow time scale. These assumptions, however, give poor approximations in the ‘colder’ regions of a flame, where transport processes are also important. The method presented here, can be considered as a combination of two approaches to simplify flame calculations, i.e. a flamelet and a manifold approach. The method, to which we will refer as the Flamelet-Generated Manifold (FGM) method, shares the idea with flamelet approaches that a multi-dimensional flame may be considered as an ensemble of one-dimensional flames. The implementation, however, is typical for manifold methods: a low-dimensional manifold in composition space is constructed, and the thermo-chemical variables are stored in a database which can be used in subsequent flame simulations. In the FGM method a manifold is constructed using one-dimensional flamelets. Like in other manifold methods, the dimension of the manifold can be increased to satisfy a desired accuracy. Although the method can be applied to different kinds of flames, only laminar premixed flames are considered here. Since the major parts of convection and diffusion processes are present in one-dimensional flamelets, the FGM is more accurate in the ‘colder’ zones of premixed flames than methods based on local chemical equilibria. Therefore, less controlling variables are sufficient to represent the combustion process. Test results of one and two-dimensional premixed methane/air flames show that detailed computations are reproduced very well with a FGM consisting of only one progress variable apart from the enthalpy to account for energy losses.

Modeling of complex premixed burner systems by using flamelet-generated manifolds

Abstract The numerical modeling of realistic burner systems puts a very high demand on computational resources. The computational cost of combustion simulations can be reduced by techniques that simplify the chemical kinetics. In this paper, the recently introduced flamelet-generated manifold method for premixed combustion systems is applied to laminar flames. In this method, the reduced mechanism is created by using solutions of one-dimensional flamelet equations as steady-state relations. For a methane/air mixture a manifold is constructed with two controlling variables: one progress variable and the enthalpy to account for energy losses. This manifold is used for the computation of a two-dimensional burner-stabilized flame and the results are compared with results of detailed computations. The results show that these two controlling variables are sufficient to reproduce the results of detailed computations. The influence of flame stretch on the accuracy of the method is investigated by simulating strained flames in stagnation-point flows. The computation time can be reduced by a factor of 20 when a flamelet-generated manifold is applied. The reduction in computation time enables us to perform simulations of combustion in more complex combustion systems. To show that the method can be used to give accurate predictions, a semi-practical furnace is modeled and the results are compared with temperature measurements. The experimental setup consists of a cylindrical radiating furnace with a ceramic-foam surface burner in the top disc. Radial profiles of temperature have been measured at two different heights in the furnace. The measurements agree quite well with the results of the numerical simulation using a flamelet-generated manifold.

Modelling of premixed counterflow flames using the flamelet-generated manifold method

In the recently introduced flamelet-generated manifold (FGM) method the ideas of the manifold and the flamelet approach are combined: a manifold is constructed using one-dimensional (1D) flamelets. In this paper the effect of flame stretch on the accuracy of the FGM method is investigated. In order to isolate the effect of flame stretch, premixed methane/air counterflow flames are simulated. In the case of unit Lewis numbers, a 1D manifold is sufficient to model the main effects of flame stretch. A manifold with two progress variables reproduces the results computed using detailed kinetics almost exactly. When non-unit Lewis numbers are used, the enthalpy and element composition of the burnt mixture change, which may influence the mass burning rate significantly. If these composition changes are included in the manifold using one additional controlling variable, the results agree well with detailed computations.

A detailed one-dimensional model of combustion of a woody biomass particle

A detailed one-dimensional model for combustion of a single biomass particle is presented. It accounts for particle heating up, pyrolysis, char gasification and oxidation and gas phase reactions within and in the vicinity of the particle. The biomass pyrolysis is assumed to take place through three competing reactions yielding char, light gas and tar. The model is validated using different sets of experiments reported in the literature. Special emphasis is placed on examination of the effects of pyrolysis kinetic constants and gas phase reactions on the combustion process which have not been thoroughly discussed in previous works. It is shown that depending on the process condition and reactor temperature, correct selection of the pyrolysis kinetic data is a necessary step for simulation of biomass particle conversion. The computer program developed for the purpose of this study enables one to get a deeper insight into the biomass particle combustion process.

Intrinsic Low-Dimensional Manifold Method Extended with Diffusion

It is necessary to use chemical reduction techniques for the modeling of complex burner systems. The intrinsic low-dimensional manifold (ILDM) method has proved to be a useful method to simplify detailed reaction mechanisms. However, diffusion is neglected in the construction of the manifold, which results in less accurate manifolds in regions where both chemistry and diffusion are significant, which is generally the case in a large part of the reaction zone. This paper presents a new reduction method, which uses the basic principles of intrinsic low-dimensional manifolds. A manifold is constructed in the composition phase space instead of the composition space, so that the gradients of species concentrations and enthalpy can be used to include diffusion within the manifold. The composition phase space is spanned by the species mass fractions, the enthalpy, and their corresponding diffusive fluxes. The second-order conservation equations are transformed into first-order equations by introducing flux terms. The resulting set of first-order equations are used to construct the manifold. The procedure used to define the manifold is equivalent to the ILDM method. The new method, called phase space ILDM (PS-ILDM), is applied to a simple but illustrative example, and finally a manifold for a CO/H2 mechanism is presented. The results show that the new method gives more accurate manifolds than the ILDM method for parts of the flame where both reaction and diffusion are important. In addition, fluctuations of enthalpy and element fractions due to preferential diffusion are included in the manifold. This may result in a database with a lower dimension and therefore a more effective reduction.

A numerical study of confined triple flames using a flamelet-generated manifold

Laminar triple flames are investigated numerically using detailed and reduced chemical reaction mechanisms. Triple flames are believed to play an essential role in flame propagation in partially premixed systems. In order to reduce the computational cost of the simulations, the flamelet-generated manifold (FGM) method is used to simplify the chemistry model. The FGM method is based on a library of premixed laminar flamelets. Mixture fraction variations in partially premixed flames are taken into account by using the mixture fraction as an extra degree of freedom. A comparison of the results computed with FGM and with detailed chemistry shows that FGM predicts the structure of triple flames very accurately. The gradient of the mixture fraction in the unburnt mixture is varied, and its influence on the structure and propagation of triple flames is studied. For decreasing gradients, the curvature of the premixed flame branch decreases and the propagation velocity increases. Due to the diffusive nature of the...

Measurements of absolute concentrations of CH in a premixed atmospheric flat flame by cavity ring-down spectroscopy

Abstract Absolute concentrations of CH in a premixed, atmospheric flat flame of CH 4 and air have been determined with cavity ring-down spectroscopy (CRDS). CH is excited from the X 2 Π to the A 2 Δ state at 430 nm. Since at atmospheric pressure the CH radical is present only in a very narrow layer at the flame front, specific problems due to the finite size of the laser beam and thermal deflection are encountered. An intensified CCD camera was used as an aid to be able to take these effects into account. Distributions of [CH] were obtained for two different stoichiometries ( φ = 1.2 and 1.1) in a burner-stabilized flame. Signal-to-noise ratios indicate that [CH] number densities down to 4 × 10 11 cm −3 , corresponding to 1.5 ppb (S/N = 2) can be detected easily at 1 atm. Flame uniformity was verified with an Abel inversion technique. The rotational flame temperature was derived from Boltzmann distributions. The results were compared to modeling calculations using GRI-Mech 2.11 and 3.0. The predictions for both models show higher maximum [CH] located further away from the burner. The computed maximum [CH] is predicted in both cases at a higher temperature. Analyses of the effect of errors in the experimental settings and direct absorption measurements of [OH] have been used to verify positional differences. The results indicate that the differences may be attributed to reaction mechanisms.

The Flamelet-Generated Manifold Method applied to steady planar partially premixed counterflow flames

ABSTRACT The recently introduced Flamelet Generated Manifold (FGM) method has proved to be an accurate and efficient reduction method for the modelling of premixed flames. The FGM method uses a chemical library based on one-dimensional unstrained premixed flames to model the chemistry of a multi-dimensional flame. Recently, the method has also been applied successfully to a so-called triple flame configuration, which is partially premixed. In this configuration, the gradient of the mixture fraction was relatively small compared to the flame thickness. In this paper the applicability of FGM in partially premixed combustion systems is investigated further. The method is tested in a planar counterflow configuration, which enables the control of the mixture fraction gradient. The gradient of the mixture fraction is changed by modifying the applied strain in one case and the inlet mixture fraction in the other case. The results show that, even though the mixing and flow time scales are of the same order as the flame time scales from the database. FGM is still relatively accurate. This can be explained by the fact that in a large part of the flame, the chemistry is not far from equilibrium, which means that the chemistry is still much faster than the flow and mixing processes. It has already been shown that this holds for strain, but this paper shows that it is also true for the dissipation rate. For very high strain and dissipation rates (up to 2000 s−1), errors up to 10% are obtained.

Numerical Simulations of a Premixed Turbulent Confined Jet Flame Using the Flamelet Generated Manifold Approach With Heat Loss Inclusion

In the present paper a computational analysis of a confined premixed turbulent methane/air jet flame is presented. In this scope, chemistry is reduced by the use of the Flamelet Generated Manifold (FGM) method [1, 2], and the fluid flow is modeled in a RANS context. In the FGM technique the reaction progress of the flame is generally described by a few control variables, for which a transport equation is solved during runtime. 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. In the present implementation 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. The turbulence-chemistry interaction is considered through the use of a presumed pdf approach.A generic lab scale burner for high-velocity preheated jets is used for validation [3, 4]. It consists of a rectangular confinement, and an off-center positioning of the jet nozzle enables flame stabilization by recirculation of hot combustion products. The inlet speed is appropriately high, in order to be close to the blow out limit. Flame structures were visualized by OH* chemiluminescence imaging and planar laser-induced fluorescence of the OH radical. Laser Raman scattering was used to determine concentrations of the major species and the temperature. Velocity fields were measured with particle image velocimetry.The important effect of conductive heat loss to the walls is included in the FGM chemistry reduction method in a RANS context, in order to predict the evolution and description of a turbulent jet flame in high Reynolds number flow conditions. Comparisons of various mean fields (velocities, temperatures) with RANS results are shown. 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.Copyright

Measurements of the absolute concentrations of HCO and 1CH2 in a premixed atmospheric flat flame by cavity ring-down spectroscopy

Abstract Singlet methylene ( 1 CH 2 ) and the formyl radical (HCO) have been studied in a premixed flat flame of CH 4 and air by cavity ring-down spectroscopy at 1 atm. The absorption lines lie in the same spectral region for both species. The 1 CH 2 radicals were probed via the b 1 B 1 (0,13,0) ←a 1 A 1 (0,0,0) band at 622 nm and the HCO radicals via the A 2 A′ (0,9,0) ←X 2 A″ (0,0,0) band at 615 nm. Absolute concentrations of 1 CH 2 and HCO have been obtained at various heights above the burner and compared to numerical simulations using both the GRI-Mech 2.11 and 3.0 mechanisms, showing relatively good agreement.