Moderate Or Intense Low-oxygen Dilution Conditions Research Articles

Heat release characteristics of ammonia flames in MILD conditions

Ammonia (NH3) combustion is an attractive energy source with zero carbon dioxide (CO2) emissions and a potential hydrogen-combustion enabler. In addition, the availability of ammonia and its amenability to transportation make it appealing to future energy supply. To provide fundamental insights into the heat release characteristics of ammonia in moderate or intense low oxygen dilution (MILD) combustion, one-dimensional laminar and two-dimensional turbulent premixed flames are studied numerically. In particular, the flame structure and heat release distribution of ammonia and ammonia-hydrogen premixed flames are examined under conventional and MILD, and preheated conditions, respectively. The results show that stoichiometric ammonia MILD flames differ from normal ammonia flames producing NH2 earlier, which results in the formation of a radical pool that shifts the chemical pathway towards hydrogen-related chemistry. The NH2 radical serves as a precursor for the ammonia flame and characterizes the reaction zone, and plays a crucial role in bringing the explosive nature into the system. For fixed ratios of the root-mean-square turbulent velocity fluctuation to the laminar flame speed and integral length scale to the laminar flame thickness, the flame surface wrinkling is comparable between the stoichiometric ammonia flame at MILD and non-MILD conditions. Furthermore, the hydrogen enrichment of the lean ammonia flame results in a significant increase in heat release rate due to the preferential diffusion effect. The study also examines chemical markers to identify heat-releasing regions of the flames and proposes the use of NH2 × O (on a mass fraction basis) as a suitable indicator of regions with high chemical activity and correlates well with the heat release for the MILD and non-MILD ammonia flames as well as ammonia-hydrogen flames at both laminar and turbulent conditions.

On the effects of adding syngas to an ammonia-MILD combustion regime—A computational study of the reaction zone structure

This study examines the reaction zone structure of syngas/NH3/CH4 blends in moderate or intense low oxygen dilution (MILD) conditions to determine the effects of variations in NH3 and syngas mole fractions upon the stability of the reacting system and NOx emission. A series of axisymmetric turbulent combustion simulations are performed using OpenFOAM and selected reaction mechanisms and, a modified version of the eddy dissipation concept. An increasing amount of syngas is added to the reacting mixtures of NH3/CH4 in MILD conditions. It is observed that a reduction in the NH3 mole fraction increases the OH radical concentration, which implies a reduction in the ignition delay. Further, for a given NH3 mole fraction, an increase in the syngas content of the fuel blend along with a reduction in CH4 leads to a rise in the process maximum temperature near the entrance zone. This reduces the flow temperature gradient, decreases the CH2O mass fraction, and boosts the stability of MILD combustion. Additionally, adding syngas to ammonia for MILD combustion decreases the reaction zone weakening, which is a major problem in MILD combustion of ammonia. In particular, blending syngas with NH3 and reducing the amount of CH4 in the fuel blend increases flame speed and heat generation. When syngas is added to ammonia MILD combustion, NOx and NxO decrease, and more complete combustion is achieved.

Four-step global kinetics mechanism for diluted combustion fueled with kerosene

The aim of the present study was to develop a global kinetics mechanism capable of predicting characteristics of the combustion of kerosene diluted with combustion gases. A four-step model of the Aachen mechanism was developed via modification of a mechanism for lower molecular weight hydrocarbons. A simplified fuel was used to simulate kerosene, the thermodynamic properties of which were estimated based on the surrogate fuel in the Aachen mechanism. Combustion of this model fuel in a perfectly stirred reactor (PSR) operating under high-temperature air combustion (HiTAC) conditions was simulated using inflow mixtures preheated and diluted with nitrogen. In this new mechanism, the activation energy for the fuel consumption reaction and the pre-exponential factor for the hydrogen oxidation reaction were adjusted. The resulting model accurately predicted variations in temperature and major species over time. The four-step mechanism was also validated under conventional flame and moderate or intense low oxygen dilution (MILD) combustion conditions. The PSR simulations with the four-step mechanism accurately predicted variations in temperature and major species under MILD combustion conditions. In addition, this four-step mechanism was confirmed to be superior to other global mechanisms with regard to predicting the mole fractions of various species. Experimental trials were carried out with diluted kerosene flames formed above parallel jet burners composed of a fuel spray nozzle and an oxidizer nozzle, and it was found that NOx emission decreased with increasing distance between nozzles even for spray combustion. These diluted flames were numerically simulated using the eddy dissipation concept model combined with the four-step mechanism. The temperature distributions obtained from the simulations showed good agreement with the experimental data in the downstream region.

Surface Density Function and Its Evolution in Homogeneous and Inhomogeneous Mixture n-Heptane MILD Combustion

ABSTRACT Moderate or intense low-oxygen dilution (MILD) combustion is a combustion technique with potential to simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the mean behavior of the reactive scalar gradient characterized by the surface density function (SDF = magnitude of the reaction progress variable gradient) and its evolution for exhaust gas recirculation (EGR) type, homogeneous and inhomogeneous mixture n-heptane combustion under MILD conditions using Direct Numerical Simulations (DNS) with reduced chemical mechanism. Two oxygen concentrations have been considered here for the homogeneous mixture case, namely, 3.0% and 4.5% by volume. The characteristics of the SDF and displacement speed were also analyzed in the reaction and propagating-flame-dominated regions of the domain to illustrate the effect of combustion mode on the mean variations of these quantities. The mean values of the SDF in turbulent MILD combustion cases are found to be smaller compared to the corresponding laminar flame cases. Moreover, the effect of mixture inhomogeneity on the SDF variation is found to be marginal for the parameters considered here. It is found that increasing the dilution factor reduces the percentage of flame thickening with respect to the corresponding laminar flame thickness under identical turbulence conditions. The effects of dilatation rate are found to be weak in the studied cases due to the expected low heat release under MILD conditions which leads to weak thermal expansion effects. The effective flame normal and tangential strain rates are found to be dominated by the additional flame normal and tangential strain rates due to flame propagation, respectively. The effective flame normal strain rate has positive values across the flame which promotes flame thickening and could explain the decrease in the SDF values, while the effective flame tangential strain rate is negative throughout the flame and indicates a fractional reduction in flame area that could be attributed to flame surface interactions. The reaction-dominated regions give rise to reduced level of the SDF in the homogeneous mixture cases and increased level of in all cases compared to those calculated in the propagating-flame region.

Characterisation of hydrogen jet flames under different pressures with varying coflow oxygen concentrations

The elevated temperature of hydrogen combustion increases the formation of thermal NOx. Moderate or intense low oxygen dilution (MILD) combustion is known to reduce NOx emissions and increase thermal efficiency. Pressure is often also used for increasing thermal efficiency. The impact that pressure has on fluid dynamics and chemical kinetics is especially relevant in MILD combustion conditions. Hydrogen jet flames issuing into a hot and vitiated coflow were imaged using OH∗ chemiluminescence at different pressures (1–7 bar) and oxygen levels (3–9% by vol.). Laminar flame simulations complemented the experiments. The observed mean radial OH∗ width increased with increased pressure, but only at O2 content less than 9%, suggesting that pressure has greater influence on kinetics when oxygen is reduced. The integrated OH∗ signal strength remained constant at 3% coflow O2, despite an apparent increase in flame width, suggesting a spatial broadening of the flame with pressure. Numerical results indicate that at 3–6% O2, conditions for MILD combustion of H2 are met across a wide range of strains and pressures, supporting the experimental observations for 3% O2.

Optimized TGA-based experimental method for studying intrinsic kinetics of coal char oxidation under moderate or intense low-oxygen dilution oxy-fuel conditions

Accurate assessment of coal char oxidation kinetics is essential for the design of efficient combustion systems; nonetheless currently, there is little research on this aspect under moderate or intense low-oxygen dilution (MILD) oxy-fuel (MO) conditions. In this study, experimental and modeling studies were conducted for analyzing the intrinsic kinetic properties of coal char oxidation under MILD air-combustion (MA) and MO conditions based on thermogravimetric analysis (TGA). The experimental conditions were improved to minimize various diffusion resistances in the crucible during the TGA experiments. Under the optimal conditions of TGA experiments, the reactivity differences of coal char under the MA and MO atmospheres were small. Both nonlinear model-fitting and linear model-free analysis methods were used to analyze kinetic parameters of coal char oxidation. The results showed that the random pore model correlated the best with the experimental results. The activation energies of coal char under the optimal conditions in MA and MO atmospheres (i.e., 104.4 kJ/mol and 98.2 kJ/mol, respectively) were much higher than those obtained by the conventional method. The reaction orders of coal char oxidation in MA and MO atmospheres were close to 1.0, and the specific values were affected by the diffusion effect and background atmosphere.

Characterisation of ethylene flames under a range of low-oxygen concentrations

New understanding of turbulent ethylene (C2H4) jet flames, issuing into a range of preheated coflowing oxidisers with reduced oxygen (O2) concentrations are reported. These conditions emulate moderate or intense low-oxygen dilution (MILD) combustion. To resolve previously reported non-monotonic trends, comparisons are made for coflow O2 concentrations of 3%, 4%, 5%, 6%, 9% and 11% and coflow temperatures of 1250K, 1315K and 1385K. Instantaneous and simultaneously planar imaging measurements of temperature, hydroxyl radicals (OH) and formaldehyde (CH2O) were taken at eight downstream locations ranging from 9mm to 75mm. The new data reveal non-monotonic lift-off trends in the OH and CH2O formation heights at all three temperatures. Compared with extant measurements, the higher resolution and greater control of coflow composition provide a detailed exploration of the transitional behaviour that occurs when MILD combustion conditions are achieved. The results experimentally demonstrate non-monotonic variation in lift-off height, and show that the same behaviour is present in both the OH and CH2O formation heights, and occurs at all three temperatures, which is due to the movement of the location of the stoichiometric and most reactive mixture fractions to the coflow side of the jet shear layer. The results also show that increasing the oxidiser temperature does not have a significant effect on the OH number density for a given temperature but does result in a significant decrease in lift-off height and CH2O concentration.

On the effects of NH3 addition to a reacting mixture of H2/CH4 under MILD combustion regime: Numerical modeling with a modified EDC combustion model

This paper examines the behavior of reacting NH3/H2/CH4 mixtures in moderate or intense low oxygen dilution (MILD) condition. A series of axisymmetric, turbulent reacting flow simulations are carried out incorporating a modified version of eddy dissipation concept and a few reaction mechanisms. The effects of adding a progressively increasing amount of NH3 to a reacting H2/CH4 mixture in moderate condition are investigated. It is observed that addition of NH3 to MILD combustion leads to markedly different behaviors compared to that in conventional combustion. Most notably, the inherently strong preheating of reactants in MILD combustion causes thermal cracking of NH3 prior to ignition. The resultant production of H2 profoundly affects the reacting flow as such increasing the NH3 mass fraction in the fuel blend decreases the flame lift-off. Further, unlike that in conventional combustion, adding NH3 to MILD combustion increases the process reactivity. In addition, the usual flame thickening typically seen in NH3 flames is not observed here, while in keeping with the thermodynamic predictions, NH3 addition lowers the temperature of combustion products. The results also show that in sharp contrast to that reported for conventional combustion, addition of NH3 in MILD condition does not increase the emission of NO, while the mass fraction of NO2 drops slightly. Overall, it is concluded that MILD combustion could be a promising route to NH3 combustion.

SGS Reaction rate modelling for MILD combustion based on machine-learning combustion mode classification: Development and a priori study

A neural network (NN) aided model is proposed for the filtered reaction rate in moderate or intense low-oxygen dilution (MILD) combustion. The framework of the present model is based on the partially stirred reactor (PaSR) approach, and the fraction of the reactive structure appearing in the PaSR is predicted using different NN’s, to consider both premixed and non-premixed conditions while allowing the use of imbalanced training data between premixed and non-premixed combustion direct numerical simulation (DNS) data. The key ingredient in the present model is the use of local combustion mode prediction performed by using another NN, which is developed in a previous study. The trained model was then assessed by using two unknown combustion DNS cases, which yields much higher dilution level (more intense MILD condition) and higher Karlovitz number than the DNS cases used as training data. The model performance assessment has been carried out by means of the Pearson’s correlation coefficient and mean squared error. For both the present model and zeroth-order approximated reaction rate, the correlation coefficient with the target values shows relatively high values, suggesting that the trend of predicted field, by the present model and zeroth-order approximation, is well correlated with the actual reaction rate field. This suggests that the use of PaSR equation is promising if the fraction of the reactive structure is appropriately predicted, which is the objective in the present study. On the other hand, substantially lower mean squared error is observed for a range of filter sizes for the present model than that for the zeroth-order approximation. This suggests that the present filtered reaction rate model can account for the SGS contribution reasonably well.

Volatile Releasing Characteristics of Pulverized Coals under Moderate or Intense Low-Oxygen Dilution Oxy-Combustion Conditions in a Flat-Flame Assisted Entrained Flow Reactor

There has been little research on volatile releasing characteristics of pulverized coals under moderate or intense low-oxygen dilution (MILD) oxy-combustion (MO) conditions. For the first time, volatile releasing characteristics of bituminous coal and semi-anthracite under both MILD air-combustion (MA) and MO conditions were investigated using a flat-flame assisted entrained flow reactor. Both heating rate (~105 K/s) and residence time (65 ms) were carefully selected to mimic the conditions in typical industrial boilers. The combustion processes and properties of the volatiles were characterized through direct observation and char analysis. The results showed that the lower diffusion rate of the volatile in CO2 resulted in the decreasing of the volatile envelope flame size and a longer volatile burnout time (more than 20%). For bituminous coal (volatile content of 25%), the lower amount of apparent volatile yield under MO conditions reduced the heating value of the volatile. For semi-anthracite coal (volatile content of 7%), the short devolatilization time led to char-CO2 gasification reaction, which increased the apparent volatile yield and the heating value of the volatile by 47% and the volatile-N by 19%. This paper indeed provided new insight into the MILD oxy-combustion of solid fuels.

MILD Combustion and Biofuels: A Minireview

Even in the presence of the formidable growth of renewable energy sources, advanced combustion processes have and will continue to have an irreplaceable role as a driving force for energy market. This is indeed motivated by the same huge amount of energy that can be conveniently stored in the form of energy carriers, made available from the renewable sources. The coupling of the appropriate combustion conversion technology with locally available biofuels certainly is among the most effective solutions, now on the table, to face the decarbonization challenge. In this framework, moderate or intense low oxygen dilution (MILD) combustion is among the best candidates to support the transition toward the net zero emission target. Indeed, due to its inherent features, it is highly fuel flexible and efficient, allowing for the utilization of whatever energy carrier can be considered in the decarbonization strategy. It is also characterized by an inherent low or absent pollutant emission. The novelty of this minireview is to highlight the relevance of the kinetics involved in MILD combustion processes with particular focus on biofuels, identifying invariant temperatures, relevant for process stabilization for any energy carrier. A critical evaluation of advantages and drawbacks of the use of raw bioliquids in MILD combustion conditions is reported. Then, challenges, open questions, and future perspectives on the use of biofuel in MILD combustion are discussed.

Comparison of the Reactive Scalar Gradient Evolution between Homogeneous MILD Combustion and Premixed Turbulent Flames

Moderate or intense low-oxygen dilution (MILD) combustion is a novel combustion technique that can simultaneously improve thermal efficiency and reduce emissions. This paper focuses on the differences in statistical behaviours of the surface density function (SDF = magnitude of the reaction progress variable gradient) between conventional premixed flames and exhaust gas recirculation (EGR) type homogeneous-mixture combustion under MILD conditions using direct numerical simulations (DNS) data. The mean values of the SDF in the MILD combustion cases were found to be significantly smaller than those in the corresponding premixed flame cases. Moreover, the mean behaviour of the SDF in response to the variations of turbulence intensity were compared between MILD and premixed flame cases, and the differences are explained in terms of the strain rates induced by fluid motion and the ones arising from flame displacement speed. It was found that the effects of dilatation rate were much weaker in the MILD combustion cases than in the premixed flame cases, and the reactive scalar gradient in MILD combustion cases preferentially aligns with the most compressive principal strain-rate eigendirection. By contrast, the reactive scalar gradient preferentially aligned with the most extensive principal strain-rate eigendirection within the flame in the premixed flame cases considered here, but the extent of this alignment weakened with increasing turbulence intensity. This gave rise to a predominantly positive mean value of normal strain rate in the premixed flames, whereas the mean normal strain rate remained negative, and its magnitude increased with increasing turbulence intensity in the MILD combustion cases. The mean value of the reaction component of displacement speed assumed non-negligible values in the MILD combustion cases for a broader range of reaction progress variable, compared with the conventional premixed flames. Moreover, the mean displacement speed increased from the unburned gas side to the burned gas side in the conventional premixed flames, whereas the mean displacement speed in MILD combustion cases decreased from the unburned gas side to the middle of the flame before increasing mildly towards the burned gas side. These differences in the mean displacement speed gave rise to significant differences in the mean behaviour of the normal strain rate induced by the flame propagation and effective strain rate, which explains the differences in the SDF evolution and its response to the variation of turbulence intensity between the conventional premixed flames and MILD combustion cases. The tangential fluid-dynamic strain rate assumed positive mean values, but it was overcome by negative mean values of curvature stretch rate to yield negative mean values of stretch rate for both the premixed flames and MILD combustion cases. This behaviour is explained in terms of the curvature dependence of displacement speed. These findings suggest that the curvature dependence of displacement speed and the scalar gradient alignment with local principal strain rate eigendirections need to be addressed for modelling EGR-type homogeneous-mixture MILD combustion.

Reactive Structures of Ammonia MILD Combustion in Diffusion Ignition Processes

Reactive structures have been analyzed, when ammonia is used as a fuel, in a steady 1D counterflow diffusion flame layer, mimicking diffusion ignition processes. The characterization has been carried out in a wide range of feeding parameters under Moderate or Intense Low-oxygen Dilution (MILD) combustion conditions. Both the Hot-Fuel-Diluted-Fuel (HFDF) and Hot-Oxidant-Diluted-Fuel (HODF) configurations were studied to analyze the main effects of the inlet feeding conditions on the oxidative structures. The reaction zone has been analyzed in terms of temperature and heat release profiles in the mixture fraction space for various ranges of inlet parameters, using a standard code and a validated chemical kinetic scheme. Several features of the reaction zone have been recognized as reported also in previous works for hydrocarbon flames. They were used as discriminative for the achievement of various combustion regimes. In particular, the flame thickening process and the absence of correlation between the maximum heat release and the stoichiometric mixture fraction were analyzed to build maps of behaviors. The latter were reported on an inlet preheating level-temperature increase plane for fixed values of the bulk strain rate and system pressures. Another relevant feature previously reported with hydrocarbons in the literature, in Hot Diluted Diffusion Ignition (HDDI) processes under MILD conditions, was the pyrolysis depression. The latter characteristic has been not observed when ammonia is used as a fuel, for the operative conditions here investigated. Indeed, the heat release profiles do not show the presence of negative heat release regions. The results obtained for the HFDF configuration are strongly dependent on the system pressure level. Finally, the HODF condition has been also analyzed for ammonia at the atmospheric pressure. Boundaries of the combustion regimes and reactive structure features showed several differences between HFDF and HODF cases with respect to the inlet parameters.

Data driven analysis and prediction of MILD combustion mode

Abstract Direct numerical simulation (DNS) data of moderate or intense low-oxygen dilution (MILD) combustion and a planar flame are analysed to identify quantities influencing the unique features of co-existing combustion modes and develop a model to identify them in MILD combustion. The results show the existence of direct relationship between the scalar dissipation and reaction rates in MILD combustion, whereas the correlation is weaker than the planar flame. Also, rotational turbulent motions identified by the enstrophy-strain balance also show a substantial influence on the MILD combustion field, via the principal component analysis, suggesting the mechanism by which the non-flamelet part of reaction zones is controlled. A neural network (NN)-based model was proposed to identify the filtered local combustion mode for MILD combustion fields in LES context. The NN is trained based on DNS data of MILD combustion at two different conditions for different filter sizes. The model assessment was performed using the third MILD condition having a higher Ka and dilution level than those of the two training data sets. Several filter sizes ranging from 0.5 to 2.0 times of a corresponding laminar flame thickness were considered, and also prediction performance of a “zeroth-order model” is compared with that of the NN-based prediction. The assessment shows very high correlation between the NN-based prediction and the mode directly obtained from DNS. The predicted local combustion mode could be used to exploit advantages of both flamelet and non-flamelet-type combustion models to predict co-existing MILD reaction zones.

Identification of combustion mode under MILD conditions using Chemical Explosive Mode Analysis

Direct Numerical Simulations (DNS) data of Moderate or Intense Low-oxygen Dilution (MILD) combustion are analysed to identify the contributions of the autoignition and flame modes. This is performed using an extended Chemical Explosive Mode Analysis (CEMA) which accounts for diffusion effects allowing it to discriminate between deflagration and autoignition. This analysis indicates that in premixed MILD combustion conditions, the main combustion mode is ignition for all dilution and turbulence levels and for the two reactant temperature conditions considered. In non-premixed conditions, the preponderance of the ignition mode was observed to depend on the axial location and mixture fraction stratification. With a large mixture fraction lengthscale, ignition is more preponderant in the early part of the domain while the deflagrative mode increases further downstream. On the other hand, when the mixture fraction lengthscale is small, sequential autoignition is observed. Finally, the various combustion modes are observed to correlate strongly with mixture fraction where lean mixtures are more likely to autoignite while stoichiometric and rich mixtures are more likely to react as deflagrative structures.

An a priori assessment of the Partially Stirred Reactor (PaSR) model for MILD combustion

Moderate or Intense Low-oxygen Dilution (MILD) combustion has drawn increasing attention as it allows to avoid the thermo-chemical conditions prone to the formation of pollutant species while ensuring high energy efficiency and fuel flexibility. MILD combustion is characterized by a strong competition between turbulent mixing and chemical kinetics so that turbulence-chemistry interactions are naturally strengthened and unsteady phenomena such as local extinction and re-ignition may occur. The underlying physical mechanisms are not fully understood yet and the validation of combustion models featuring enhanced predictive capabilities is required. Within this context, high-fidelity data from Direct Numerical Simulation (DNS) represent a great opportunity for the assessment and the validation of combustion closure formulations. In this study, the performance of the Partially Stirred Reactor (PaSR) combustion model in MILD conditions is a priori assessed on Direct Numerical Simulations (DNS) of turbulent combustion of MILD mixtures in a cubical domain. Modeled quantities of interest, such as heat release rate and reaction rates of major and minor species, are compared to the corresponding filtered quantities extracted from the DNS. Different submodels for the key model parameters, i.e., the chemical time scale τc and the mixing time scale τmix, are considered and their influence on the results is evaluated. The results show that the mixing time scale is the leading scale in the investigated cases. The best agreement with the DNS data regarding the prediction of heat release rate and chemical source terms is achieved by the PaSR model that employs a local dynamic approach for the estimation of the mixing time scale. An overestimation of the OH species source terms occurs in limited zones of the computational domain, characterized by low heat release rates.

Additional criteria for MILD coal combustion

We proposed a theoretical basis for Moderate or Intense Low-oxygen Dilution (MILD) coal combustion based on the turbulent scalar energy spectra. This is motivated by the hypothesis that smallest scalar mixing length scales should be on the order of the particle size or smaller to ensure that mixing can occur to prevent formation of diffusion flames. Our proposed criterion is evaluated using several experimental datasets from the literature for coal combustion in both MILD and traditional combustion regimes. The experimental results confirm that the smallest mixing length scales should be of the order of or smaller than the particle diameter, ηmix⪅dp, to breakup the heat and mass transfer boundary layers around particles in MILD coal combustion. Results indicate that poor mixing of species with small Schmidt numbers around small particles leads to the high luminous intensity in the reactor. The effects of inlet velocity and jet diameter on the mixing length scales are analyzed. Higher inlet velocity and smaller jet diameter are expected to reach MILD regime. The proposed criterion can be used to guide experimental design to achieve MILD conditions for coal combustion.

Detached Eddy Simulation of syngas combustion in a reverse-flow configuration

The combustion of low calorific value syngas in a 3.3-kW reverse-flow chamber under moderate or intense low-oxygen dilution (MILD) (global equivalence ratio Φglobal = 0.89) and ultra-lean (Φglobal = 0.32) conditions have been numerically investigated with detailed chemistry. Reynolds averaged Navier Stokes (RANS)/Detached eddy simulation (DES) models for flow and Eddy dissipation concept (EDC)/Flamelet generated manifold (FGM) models for combustion have been used. Quantitative measurements of temperature, OH radical, and CO emissions performed in the same combustor are used to validate and compare the models. The prediction of the minor species OH is a special focus of the present study. The fields of velocity, mixture fraction, and kinetic parameters have been investigated in detail. The DES-EDC model performs the best in predicting the scalar distributions, although the computational cost is highest. The RANS model predicts insufficient mixing and higher velocities leading to a delayed reaction zone. The FGM model consistently predicts early ignition that has been attributed to the lack of a suitable progress variable and the use of the diffusion flamelet to represent the thermochemistry. However, the FGM model is promising with suitable modifications due to the low computational cost. The present study also provides a comprehensive dataset in a reverse-flow chamber operating under MILD and ultra-lean conditions for validation of numerical models.

Effect of fuel type on the MILD combustion of syngas

The MILD (Moderate or Intense Low-oxygen Dilution) combustion of four typical syngas fuels was examined in CRN (chemical reaction network) model and in parallel jet forward flow combustor. The results were presented on the thermodynamics, ignition kinetics and NO chemistry using CRN calculation. The flow field mixing was obtained from numerical simulation, the global flame signatures and exhaust emissions from experiments. The syngas fuel with higher content of inert gas (N2 and CO2) and lower concentration of active components (H2 and CO) exhibited higher mixture ignition temperature under MILD condition, as a result, greater gas recirculation ratio was required to establish the thermodynamic condition of MILD scheme. H2-enriched syngas and CO-enriched syngas showed different chemical kinetics in the ignition process of MILD mixture. In the NO chemistry under MILD condition, it was revealed that the H2 containing syngas fuels produce the NO emission mainly through the N2O-intermediate and NO-reburning mechanisms, whereas for pure H2 MILD combustion, the NNH route plays a more important role. The N2 component in the syngas fuel had physical effect in delaying the interaction between the air and fuel stream and in favoring the turbulent mixing as well as the gas recirculation. The CO2 component in the syngas fuel had chemical effect in lowering the reaction rate. The critical equivalence ratio above which MILD combustion occurs (with the reaction region covering the entire combustor) was identified for the four syngas fuels. It was observed that the syngas fuel with higher N2 and CO2 content can achieve MILD combustion at leaner condition and with lower NOx emissions. Reduced CO emission and single-digit NOx emissions were achieved for the four syngas fuels.

Generalised Eddy Dissipation Concept for MILD combustion regime at low local Reynolds and Damköhler numbers. Part 1: Model framework development

Moderate or Intense Low Oxygen Dilution (MILD) combustion is a fuel-flexible combustion technology featuring high efficiency and low pollutant emissions. Fundamental studies reveal that turbulence-chemistry interactions are extremely complex in MILD conditions and reactor-type approaches seem to be the adequate modelling choice. In this work we develop a generalised Eddy Dissipation Concept (EDC) adapted to MILD combustion regime accounting for finite rate chemistry. We examine two recent modifications of the standard EDC and present a generalised model. It is based on functional expressions where the model parameters are adjusted to the local conditions in terms of Reynolds and Damköhler numbers, contrary to the usually proposed ad hoc tuning of the global EDC constants. Numerical results reveal that previously presented corrections are indeed suitable for specific conditions; their appropriate combination, guided by physical premises and a scrutiny of computation results, leads to a reformulation of the EDC framework. The study consists of two parts: the model development is described here (Part 1); in a companion paper (Part 2), we present a thorough validation process performed against twelve flames issuing from the jet-in-hot-coflow burners from Delft and Adelaide, representing a wide range of operating conditions. The new, generalised model can serve as a plug and play engineering tool without complex pre- or post-processing treatment.