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.

@en

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.

@en

The technique called "flameless combustion", also denoted as "MILD" combustion, was developed to reduce the nitrogen oxides (NOx) emission in the combustion process. The term "flameless" refers to the low visibility of the flame. The technique is particularly of interest when hot exhaust gas is used to preheat inlet air to high temperature. The combination of flameless combustion and exhaust gas heat recycling techniques simultaneously reduces the emission and increases the energy efficiency. Over the past few decades, flameless combustion has been successfully applied to industrial furnaces or tested at pilot scale setups in other applications. Nevertheless, despite the successful industrial application, many fundamental issues of flameless combustion are still unresolved. Detailed measurements of flameless combustion have been performed in jet in- hot-coflow (JHC) flames, but it is unclear whether the findings can be related to the flameless combustion in a furnace because only part of the features of flameless combustion are mimicked in JHC flames. Concerning modelling, it is found that the existing combustion models are not suitable for numerical modelling of flameless combustion and new model development is needed. The objective of this research is to characterize the flameless combustion in a labscale furnace that is simple enough to allow detailed measurements while keeping most relevant characteristics found in large scale furnaces. This thesis is divided into two parts, experimental measurements and model development and validation. The goal of experiments is to observe the flame behaviour and obtain detailed velocity and temperature data of flameless combustion in the furnace by means of high speed imaging and laser diagnostic techniques. The goal of the model development is to extend the Flamelet Generated Manifolds (FGM) method to take into account the effects of dilution by recirculated burnt gases. One of the databases of the Delft jet-in-hot-coflow (DJHC) flames and a new database obtained in a new lab-scale furnace are used for the model validation. @en

Flameless combustion, named as Moderate or Intense Low-oxygen Dilution (MILD) combustion or high-temperature air combustion (HiTAC), is a promising technology to improve the thermal efficiency while suppressing NOx formation in combustion systems. Flameless combustion can occur when fresh air (and/or fuel) streams are sufficiently diluted by entrained combustion products before reactions take place. It has recently been experimentally studied on laboratory-scale setups because of scientific challenges, environmental concerns and its potential industrial applications. Some burning features in flameless combustion have been observed in jet-in-hot-coflow burners which use hot coflows generated by a secondary burner or diluting air with N2 or/and CO2 to mimic the diluted air which is actually diluted by burnt gases entrainment in furnaces. With the help of highspeed cameras, the time-resolved studies on such burners have been done experimentally. E. Oldenhof et al. [1] reported that the jet-in-hot-coflow flame is stabilized by autoignition kernels and the entrainment of hot oxidizer plays an important role in the formation of autoignition kernels[2]. As O2 level in coflow is reduced, reaction zone becomes less intense leading to a greater degree of partial premixing in these flames[3]. P. R. Medwell et al.[4] also concluded that large-scale vortices can lead to a weakening of the flame front or even local extinction leading to a form of partial premixing, and may contribute to the stabilization of the flameless combustion reaction zone. With low level (5% by volume) hydrogen addition in the fuel, the flame also exhibits autoignition kernels, but this was not observed at higher level (10% and 25%) hydrogen addition cases[5]. However, how can these findings be related to the flames in a furnace is still unclear because of the lack of similar experimental observations in furnace. @en

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.

@en

We report results of a computational study of oxy-fuel spray jet flames. An experimental database on flames of ethanol burning in a coflow of a O2–CO2 mixture, created at CORIA (Rouen, France), is used for model validation (Cléon et al., 2015). Depending on the coflow composition and velocity the flames in these experiments start at nozzle (type A), just above the tip of the liquid sheet (type B) or are lifted (type C) and the challenge is to predict their structure and the transitions between them. The two-phase flow field is solved with an Eulerian–Lagrangian approach, with gas phase turbulence solved by Large Eddy Simulation (LES). The turbulence-chemistry interaction is accounted for using the Flamelet Generated Manifolds (FGM) method. The primary breakup process of the liquid fuel is neglected in the current study; instead droplets are directly injected at the location of the atomizer exit at the boundary of the simulation domain. It is found that for the type C flame, which is stabilized far downstream the dense region, some major features are successfully captured, e.g. the gas phase velocity field and flame structure. The flame lift-off height of type B flame is over-predicted. The type A flame, where the flame stabilizes inside the liquid sheet, cannot be described well by the current simulation model. A detailed analysis of the droplet properties along Lagrangian tracks has been carried out in order to explain the predicted flame structure and discuss the agreement with experiment. This analysis shows that differences in predicted flame structure are well-explained by the combined effects of droplet heating, dispersion and evaporation as function of droplet size. It is concluded that a possible reason for the difficulty to predict the type A and B flames is that strong atomization-combustion interaction exists in these flames, modifying the droplet formation process. This suggests that atomization-combustion interaction should be taken into account in future study of these flame types.@en

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. @en

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.@en

We report on a comparative study of model predictions of jet-in-hot-coflow flames. TheDelft Jet-in-Hot- Coflow (DJHC) burner was built to mimic the important characteristics of flameless combustion without the complications of a real furnace [1,2]. The DJHC burner has been used to create a turbulent diffusion flame of Dutch Natural Gas in a coflowing oxidizer stream of high temperature with low oxygen concentration. The experimental database contains the results of high speed chemiluminescence imaging, velocity statistics from LDA measurementsand temperature statistics from CARS measurements. In recent years several computational studies have been made using the DJHC burner as validation database [3-9]. It has been shown before that predictions are sensitive to the coflow radial profiles of temperature and oxygen concentration, to there presentation of effects of entrained air, and to turbulence-chemistry interaction and this is also the focus of the present study @en