de Lph Philip Goey

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.

Measurement of flame temperature and adiabatic burning velocity of methane/air mixtures

A simple and accurate method is presented to determine the flame temperature and the adiabatic burning velocity of laminar premixed flat flames, using a specially constructed flat flame burner. The heat loss of the flat flame is determined from measurement of the temperature profile of the burner plate. The adiabatic burning velocity is found when the burner plate temperature profile is uniform, implying that the net heat loss of the flame is zero. Several methods are presented to determine the thermal conductivity of the burner plate. The results for methane/air mixtures are compared with experimental data found in the literature and one-dimensional flame calculations, and close agreement is found. The method is particularly useful in providing an accurate reference for other temperature measurement techniques (e.g. spectroscopic laser diagnostics), and to determine adiabatic burning velocities over the entire range of flamm ability

Detailed analysis of the heat flux method for measuring burning velocities

The heat flux method for determining the adiabatic burning velocity of gaseous mixtures of fuel and oxidizer and also producing well-defined reference flames is described in detail. Practical aspects of the heat flux method are discussed, especially the construction of the burner and attachment of thermocouples. An analysis is given of possible uncertainties and ways of correcting shortcomings. Conclusions from this analysis are applied to a typical measurement. The results are compared to other published results, including those from other often used methods, such as those with counter flow and closed vessels. For methane and air, a peak value of SL = 37.2 ± 0.5 cm/s was found.

Stabilization of Adiabatic Premixed Laminar Flames on a Flat Flame Burner

Abstract A simple analysis and measurements are presented, which show that adiabatic premixed laminar flames can be stabilized on a flat flame burner, especially designed for this purpose. The physical properties of these flames are identical to those of flat freely propagating flames. The adiabatic state can be accomplished in practice when the burner plate temperature is well above the temperature of the unburnt mixture. The net heat loss of the flame to the burner is zero (i.e. the flame is adiabatic) when the measured radial temperature profile of the burner plate is uniform. These flames are particularly suitable for comparison with theoretical or numerical flat flame studies.

Premixed combustion on ceramic foam burners

Abstract Combustion of a lean premixed methane–air mixture stabilized on a ceramic foam burner has been studied. The stabilization of the flame in the radiant mode has been simulated using a one-dimensional numerical model for a burner stabilized flat-flame, taking into account the heat transfer between the gas and the burner and the radiative properties of the ceramic material. The combustion has been modeled with the skeletal mechanism and the nitrogen chemistry using an accurate postprocessing technique based on the reaction mechanism of Glarborg et al. [16] . It is shown that the flue gas temperature is decreased significantly in the radiant mode. The emissions of CO and NO are therefore considerably lower compared to combustion in the blue-flame mode. The numerical results are validated with experiments. The temperature of the flue gases and the surface are measured in combination with the concentrations of the pollutants CO and NO. The temperatures have been obtained with nonintrusive techniques. Coherent anti-Stokes Raman scattering (CARS) has been used for the gas temperature and infrared pyrometry for the surface temperature measurements. Gas samples have been obtained with a suction probe and analyzed further by an infrared absorption technique (CO) and by a chemiluminescence analyzer (NO). From a comparison of the experimental and computational results it is concluded that the ceramic burner is chemically inert, since the results are similar to those for water cooled flat-flame burners. It is shown that modeling of the gas radiation is essential for an accurate prediction of CO in the postflame zone. Furthermore, it is shown that prompt NO, as well as the thermal NO, mechanisms are important for an accurate prediction of the total NO emission for combustion in the radiant mode.

Measurement of adiabatic burning velocity in methane-oxygen-nitrogen mixtures

Experimental measurements of the adiabatic burning velocity in methane-oxygen-nitrogen mixtures are presented. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The oxygen content in the artificial air was varied from 16 percent to 21 percent. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Major attention in this work has been paid to the identification of possible uncertainties and errors of the measurements. The overall error of the burning velocities is estimated to be smaller than ± 0.8 cm/s. Experimental results are in very good agreement with recent literature data for methane-air mixtures. They also agree well with detailed chemical model predictions.

A flamelet description of premixed laminar flames and the relation with flame stretch

A laminar flamelet description is derived for premixed laminar flames. The full set of 3D instationary combustion equations is decomposed in three parts: (1) a flow and mixing system without chemical reactions, described by the momentum, enthalpy, and element conservation equations, (2) the G-equation for the flame motion, and (3) a flamelet system describing the inner flame structure and the local mass burning rate. Local fields for the flame curvature and the flame stretch couple the flamelet system with the flow and flame motion. To derive an efficient model, the flamelet equations are analyzed in depth, using the Integral Analysis, first introduced by Chung and Law [1]. It appears that the flamelet response is governed by algebraic equations describing the influence of flame stretch on the local mass burning rate, the enthalpy variation, and element composition. Known expressions for the mass burning rate, found by Joulin, Clavin, and Williams are recovered in some special cases. Furthermore, the validity of the expressions has been shown for weak and strong stretch by comparing the results with numerical results of lean stretched premixed methane/air flames, computed with skeletal chemistry. Finally, the theory is illustrated for the tip of a 2D stationary Bunsen flame.

On the determination of the laminar burning velocity from closed vessel gas explosions

A methodology to determine the laminar burning velocity from closed vessel gas explosions is explored. Unlike other methods which have been used to measure burning velocities from closed vessel explosions, this approach belongs to the category which does not involve observation of a rapidly moving flame front. Only the pressure–time curve is required as experimental input. To verify the methodology, initially quiescent methane–air mixtures were ignited in a 20-l explosion sphere and the equivalence ratio was varied from 0.67 to 1.36. The behavior of the pressure in the vessel was measured as a function of time and two integral balance models, namely, the thin-flame and the three-zone model, were fitted to determine the laminar burning velocity. Data on the laminar burning velocity as a function of equivalence ratio, pressure and temperature, measured by a variety of other methods have been collected from the literature to enable a comparison. Empirical correlations for the effect of pressure and temperature on the laminar burning velocity have been reviewed and two were selected to be used in conjunction with the thin-flame model. For the three-zone model, a set of coupled correlations has been derived to describe the effect of pressure and temperature on the laminar burning velocity and the laminar flame thickness. Our laminar burning velocities are seen to fall within the band of data from the period 1953–2003. A comparison with recent data from the period 1994–2003 shows that our results are 5–10% higher than the laminar burning velocities which are currently believed to be the correct ones for methane–air mixtures. Based on this observation it is concluded that the methodology described in this work should only be used under circumstances where more accurate methods can not be applied.  2003 Elsevier Ltd. All rights reserved.

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.