The flamelet generated manifold (FGM) model is suitable for moderate or intense low oxygen dilution (MILD) combustion provided the flamelets underlying the manifold include the effects of strong dilution by products of the fuel/oxidizer mixture. Here we propose such an extended model based on the use of non-premixed flamelets diluted at the airside and develop its application to non-adiabatic combustion in a lab-scale furnace. The extended model is referred to as diluted air FGM (DA-FGM) model. In the DA-FGM model in addition to mixture fraction, progress variable and scaled enthalpy loss, one additional controlling parameter named air dilution level, is introduced leading to a four-dimensional lookup table for laminar flames. For turbulent flames also variances of mixture fraction and progress variable are taken into account as independent variables leading to a six-dimensional table. Using a RANS approach implemented in OpenFOAM-2.3.1, the DA-FGM model has been applied to MILD combustion of Dutch natural gas in a lab-scale furnace operated at a thermal power 9 kW and at equivalence ratio 0.8. Radiation is described using a weighted-sum-of-gray-gases (WSGG) model. The validation study is mainly done using a grey WSGG model with TRI taken into account. The relative importance of including turbulence radiation interaction (TRI) and spectral treatment of radiative transfer is also studied. The predicted velocity and temperature statistics are in good agreement with the experimental LDA and CARS data provided not only the mixture fraction fluctuations but also the progress variable fluctuations are taken into account.

Flameless combustion, also called MILD combustion (Moderate or Intense Low Oxygen Dilution), is a technology that reduces NO_x emissions and improves combustion efficiency. Appropriate turbulence-chemistry interaction models are needed to address this combustion regime via computational modelling. Following a similar analysis to that used in the Extended EDC model (E-EDC), the purpose of the present work is to develop and test a Novel Extended Eddy Dissipation Concept model (NE-EDC) to be better able to predict flameless combustion. In the E-EDC and NE-EDC models, in order to consider the influence of the dilution on the reaction rate and temperature, the coefficients are considered to be space dependent as a function of the local Reynolds and Damköhler numbers. A comparative study of four models is carried out: the E-EDC and NE-EDC models, the EDC model with specific, fixed values of the model coefficients optimized for the current application, and the Flamelet Generated Manifold (FGM) model with pure fuel and air as boundary conditions for flamelet generation. The models are validated using experimental data of the Delft Lab Scale furnace (9 kW) burning Natural Gas (T = 446 K) and preheated air (T = 886 K) injected via separate jets, at an overall equivalence ratio of 0.8. among the considered models, the NE-EDC results show the best agreement with experimental data, with a slight improvement over the E-EDC model and a significant improvement over the EDC model with tuned constant coefficients and the FGM model.

Mild combustion in a lab-scale furnace has been experimentally and numerically studied. The furnace was operated with Dutch natural gas (DNG) at 10 kW and at an equivalence ratio of 0.8. OH∗chemiluminescence images were taken to characterize the reaction zone. The chemiluminescence intensity is relatively low compared to conventional flames and relatively uniformly distributed in the reaction zone due to the dilution effects of recirculated burnt gases. Visible flames were not observed. To characterize the dilution effects of burnt gases on reactions, flamelets generated with diluted fuel and diluted air, instead of flamelets based on pure fuel and air, were applied in an extended Flamelet Generated Manifold (FGM) approach. Burnt gases at stoichiometric mixture fraction rather than those at global equivalence ratio were considered as diluent, which is more appropriate for furnaces operating at lean condition. The numerical simulations were performed using the open source CFD package-OpenFOAM.

The Delft Jet-in Hot Coflow (DJHC) burner is used to investigate flameless combustion by imitating the recirculation flow characteristics appearing in a real complex furnace via a hot diluted coflow[1]. A welldefined stream of high temperature, low oxygen concentration combustion products is injected around the fuel jet as oxidizer in order to obtain ‘Moderate and Intense Low-oxygen Dilution (MILD)’ combustion conditions. For a range of jet and coflow conditions detailed experiments were made [2] and also several numerical validation studies, see e.g. [4,5]. The Eddy Dissipation Concept (EDC) model for turbulence chemistry interaction modeling has been widely used for modeling MILD combustion. EDC is providing a closure for the mean chemical source term based on a proposed microstructure of the reacting flow following from energy cascade concepts. It assumes that chemical reactions can only happen in the smallest eddies, whose size are of the same order of magnitude as the Kolmogorov scales, the so-called fine structures. Thus, the fraction of fine structure 훾훾∗ and mean residence time 휏휏∗ (the reciprocal of it denotes the mass exchange between reactants inside fine structure and the surrounding) are necessary for EDC simulation. They are related to turbulent kinetic energy 푘푘 and eddy dissipation rate 휀휀 (which are calculated from turbulent models) via two constants 퐶퐶퐷퐷1 and 퐶퐶퐷퐷2 . It has been confirmed that 휀휀 = 2퐶퐶퐷퐷1푢푢∗3/퐿퐿∗ = 4퐶퐶퐷퐷2푢푢∗2/3퐿퐿∗2.