Numerical study of the effect of turbulence on rate of reactions in the MILD combustion regime

In this paper, the importance of fluctuations in flow field parameters is studied under MILD combustion conditions. In this way, a turbulent non-premixed CH₄+H₂ jet flame issuing into a hot and deficient co-flow air is modeled using the RANS Axisymmetric equations. The modeling is carried out using the EDC model to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H₂/methane jet flame. Results illustrate that although the fluctuations in temperature field are small and the reaction zone volume are large in the MILD regime, the fluctuations in temperature and species concentrations are still effective on the flow field. Also, inappropriate dealing with the turbulence effect on chemistry leads to errors in prediction of temperature up to 15% in the present flame. By decreasing of O₂ concentration of hot co-flow air, the effect of fluctuations in flow field parameters on flame characteristics are still significant and its effect on species reaction rates does not decrease. On the other hand, although decreasing of jet inlet Reynolds number at constant inlet turbulence intensity addresses to smaller fluctuations in flow filed, it does not lead to lower the effect of turbulence on species distribution and temperature field under MILD combustion conditions.     

Simulations of laminar non-premixed flames of kerosene with hot combustion products as oxidiser

The effects of hot combustion product dilution in a pressurised kerosene-burning system at gas turbine conditions were investigated with laminar counterflow flame simulations. Hot combustion products from a lean (φ = 0.6) premixed flame were used as an oxidiser with kerosene surrogate as fuel in a non-premixed counterflow flame at 5, 7, 9 and 11 bar. Kerosene-hot product flames, referred to as ‘MILD’, exhibit a flame structure similar to that of kerosene–air flames, referred to as ‘conventional’, at low strain rates. The Heat Release Rate (HRR) of both conventional and MILD flames reflects the pyrolysis of the primary and intermediate fuels on the rich side of the reaction zone. Positive HRR and OH regions in mixture fraction space are of similar width to conventional kerosene flames, suggesting that MILD flames are thin fronts. MILD flames do not exhibit typical extinction behaviour, but gradually transition to a mixing solution at very high rates of strain (above A = 160, 000 s^?1 for all pressures). This is in agreement with literature that suggests heavily preheated and diluted flames have a monotonic S-shaped curve. Despite these differences in comparison with kerosene–air flames, MILD flames follow typical trends as a function of both strain and pressure. Further still, the peak locations of the overlap of OH and CH₂O mass fractions in comparison with the peak HRR indicate that the pixel-by-pixel product of OH- and CH₂O-PLIF signals is a valid experimental marker for non-premixed kerosene MILD and conventional flames.     

Transported PDF simulation of turbulent CH4/H2 flames under MILD conditions with particle-level sensitivity analysis

Transported probability density function (TPDF) simulation with sensitivity analysis has been conducted for turbulent non-premixed CH₄/H₂ flames of the jet-into-hot-coflow (JHC) burner, which is a typical model to emulate moderate or intense low oxygen dilution combustion (MILD). Specifically, two cases with different levels of oxygen in the coflow stream, namely HM1 and HM3, are simulated to reveal the differences between MILD and hot-temperature combustion. The TPDF simulation well predicts the temperature and species distributions including those of OH, CO and NO for both cases with a 25-species mechanism. The reduced reaction activity in HM1 as reflected in the peak OH concentration is well correlated to the reduced oxygen in the coflow stream. The particle-level local sensitivities with respect to mixing and chemical reaction further show dramatic differences in the flame characteristics. HM1 is less sensitive to mixing and reaction parameters than HM3 due to the suppressed combustion process. Specifically, for HM1 the sensitivities to mixing and chemical reactions have comparable magnitude, indicating that the combustion progress is controlled by both mixing and reaction in MILD combustion. For HM3, there is however a change in the combustion mode: during the flame initialization, the combustion progress is more sensitive to chemical reactions, indicating that finite-rate chemistry is the controlling process during the autoignition process for flame stabilization; at further downstream where the flame has established, the combustion progress is controlled by mixing, which is characteristic of nonpremixed flames. An examination of the particles with the largest sensitivities reveals the difference in the controlling mixtures for flame stabilization, namely, the stoichiometric mixtures are important for HM1, whereas, fuel-lean mixtures are controlling for HM3. The study demonstrates the potential of TPDF simulations with sensitivity analysis to investigate the effects of finite-rate chemistry on the flame characteristics and emissions, and reveal the controlling physio-chemical processes in MILD combustion.     

Visualization of flameholding mechanisms in a supersonic combustor using PLIF

Laser-induced fluorescence of OH and CH₂O was imaged to investigate the flame stabilization mechanism in a flameholder with a Mach 2.4 free stream. Ethylene was burned in a rectangular cavity with two points of injection: the aft wall and the cavity floor. When injected from the aft wall, the fuel came into immediate contact with hot combustion products from the reaction zone under the shear layer. Primary combustion occurred under the shear layer and in the aft region of the cavity volume. In contrast, when fuel was injected from the floor, a jet-driven recirculation zone of hot products near the upstream wall of the cavity served as a flameholder. The reaction then occurred on the underside of the shear layer. In conditions near lean blowout, significant changes in the flameholding mechanisms were observed. Improved CH₂O fluorescence signal was obtained by taking advantage of the long fluorescence lifetime at low pressures and delaying the camera gate to reduce the background signal.     

Generalisation of the eddy-dissipation concept for jet flames with low turbulence and low Damköhler number

Moderate or intense low oxygen dilution (MILD) combustion has been the focus of a range of fundamental experimental and numerical studies. Reasonable agreement between experimental and numerical investigations, however, requires finite-rate chemistry models and, often, ad hoc model adjustment. To remedy this, an adaptive eddy dissipation concept (EDC) combustion model has previously been developed to target conditions encountered in MILD combustion; however, this model relies on a simplified, pre-defined assumption about the combustion chemistry. The present paper reports a generalised version of the modified EDC model without the need for an assumed, single-step chemical reaction or ad hoc coefficient tuning. The results show good agreement with experimental measurements of two CH₄/H₂ flames in hot coflows, showing improvements over the standard EDC model as well as the previously published modified EDC model. The updated version of the EDC model also demonstrates the capacity to reproduce the downstream transition in flame structure of a MILD jet flame seen experimentally, but which has previously proven challenging to capture computationally. Analyses of the previously identified dominant heat-release reactions provide insight into the structural differences between a conventional autoignitive flame and a flame in the MILD combustion regime, whilst highlighting the requirement for a generalised EDC combustion model.     

Investigation of flame structure and burning intensity of partially premixed methane enrichment of syngas using OH-PLIF and kinetic simulation

Various experiments were conducted to study the combustion characteristics of partially premixed methane enrichment of syngas by using the OH-PLIF technique. Experiments were conducted on a co-flow burner, and the methane concentration (X_CH4 = CH₄/(H₂+CO+CH₄)) was varied from 0 to 20%, the overall equivalence ratio was varied from 0.4 to 1.2 and the inner equivalence ratio was varied from 1.5 to 3.5. Kinetic simulation was conducted by using OPPDIF module of CHEMKIN-Pro software. Results show that an increase in X_CH4 and ?_overall weakens the OH signal intensity. Adding methane into the fuel greatly increases the height of the inner flame front, and the increase of methane concentration has a negative effect on flame propagation speed. Meanwhile, simulation results remain consistent with the experiments. The main OH radical production reaction changes from R46: H+HO₂ = 2OH to R38: H+O₂ = O+OH when methane concentration contained in the fuel mixture increases. Sensitivity analysis also indicates that reaction which plays a dominant effect on temperature changes with the increase of methane concentration.     

CO2 addition and pressure effects on laminar and turbulent lean premixed CH4 air flames

An experimental study on CH₄–CO₂–air flames at various pressures is conducted by using both laminar and turbulent Bunsen flame configurations. The aim of this research is to contribute to the characterization of fuel lean methane/carbon dioxide/air premixed laminar and turbulent flames at different pressures, by studying laminar and turbulent flame propagation velocities, the flame surface density and the instantaneous flame front wrinkling parameters. PREMIX computations and experimental results indicate a decrease of the laminar flame propagation velocities with increasing CO₂ dilution rate. Instantaneous flame images are obtained by Mie scattering tomography. The image analysis shows that although the height of the turbulent flame increases with the CO₂ addition rate, the flame structure is quite similar. This implies that the flame wrinkling parameters and flame surface density are indifferent to the CO₂ addition. However, the pressure increase has a drastic effect on both parameters. This is also confirmed by a fractal analysis of instantaneous images. It is also observed that the combustion intensity S_T/S_L increases both with pressure and the CO₂ rate. Finally, the mean fuel consumption rate decreases with the CO₂ addition rate but increases with the pressure.     

Structure of partially premixed n-heptane–air counterflow flames

To avoid the complexities associated with the droplet/vapor transport and nonuniform evaporation processes, a fundamental investigation of liquid fuel combustion in idealized configurations is very useful. An experimental–computational investigation of prevaporized n-heptane nonpremixed and partially premixed flames established in a counterflow burner is described. There is a general agreement between various facets of our nonpremixed flame measurements and the literature data. The partially premixed flames are characterized by a double flame structure. This becomes more distinct as the strain rate decreases and partial premixing increases, which also increases the separation distance between the two reaction zones. The peak partially premixed flame temperature increases with increasing premixing of the fuel stream. The peak CO₂ and H₂O concentrations are relatively insensitive to partial premixing. The CO and H₂ peak concentrations on the premixed side increase as the fuel-side equivalence ratio decreases. These species are transported to the nonpremixed reaction zone where they oxidize. The C₂ species have peaks in the premixed reaction zone. The concentrations of olefins are ten times larger than those of the corresponding paraffins. The oxidizer is present in partially premixed flames throughout the combustion system and there are no regions characterized by simultaneous high temperature and high fuel concentration. As a result, pyrolysis reactions leading to soot formation are greatly diminished.     

Spherical turbulent flame propagation of pulverized coal particle clouds in an O2/N2 atmosphere

The present study aims to clarify the effects of turbulence intensity and coal concentration on the spherical turbulent flame propagation of a pulverized coal particle cloud. A unique experimental apparatus was developed in which coal particles can be dispersed homogeneously in a turbulent flow field generated by two fans. Experiments on spherical turbulent flame propagation of pulverized coal particle clouds in a constant volume spherical chamber in various turbulence intensities and coal concentrations were conducted. A common bituminous coal was used in the present study. The flame propagation velocity was obtained from an analysis of flame propagation images taken using a high-speed camera. It was found that the flame propagation velocity increased with increasing flame radius. The flame propagation velocity increases as the turbulence intensity increases. Similar trends were observed in spherical flames using gaseous fuel. The coal concentration has a weak effect on the flame propagation velocity, which is unique to pulverized coal combustions in a turbulent field. These are the first reports of experimental results for the spherical turbulent flame propagation behavior of pulverized coal particle clouds. The results obtained in the present study are obviously different from those of previous pulverized coal combustion studies and any other results of gaseous fuel combustion research.     

燃料敏感性对内燃机部分预混燃烧(PPC)光谱特性的影响研究

在一台光学发动机上,利用火焰高速成像技术和自发光光谱分析法,研究了燃料敏感性(S)为0和6时对发动机缸内火焰发展和燃烧发光光谱的影响。试验过程中,通过改变喷油时刻 (SOI=-25,-15和-5°CA ATDC) 使燃烧模式从部分预混燃烧过渡到传统柴油燃烧模式。通过使用正庚烷、异辛烷、乙醇混合燃料来改变燃料敏感性。结果表明,在PPC模式下(-25°CA ATDC),火焰发展过程是从近壁面区域开始着火,而后向燃烧室中心发展,即存在类似火焰传播过程,同时在燃烧室下部未燃区域也形成新的着火自燃点。敏感性对燃烧相位影响较大,对缸内燃烧火焰发展历程影响较小;高敏感性燃料OH和CH带状光谱出现的时刻推迟,表明高敏感性燃料高温反应过程推迟,且光谱强度更低,表明碳烟辐射强度减弱。在PPC到CDC之间的过渡区域(-15°CA ATDC),燃烧火焰发光更亮,燃烧反应速率比-25°CA ATDC时刻的反应速率更快。高、低敏感性燃料对缸压放热率的影响规律与-25°CA ATDC相近,此时的燃烧反应更剧烈,放热率更高,碳烟出现时刻更早。该喷油时刻下的光谱强度高于PPC模式下的光谱强度,说明此时的CO氧化反应与碳烟辐射更强。在CDC模式下(-5°CA ATDC),由于使用的燃料活性较低,燃烧放热时刻过于推迟,放热量很小,缸内燃烧压力低,因此燃料敏感性对缸压和放热率的影响不明显,但从燃烧着火图像中可以看到高敏感性燃料的火焰出现时刻较低敏感性燃料推迟。低敏感性燃料的燃烧初期蓝色火焰首先出现在燃烧室中心,着火火焰出现时刻更早,之后蓝色火焰从中心向周围扩散,呈现火焰传播为主导的燃烧过程;燃烧后期,局部混合气过浓区导致亮黄色火焰面积逐渐增大并向周围扩散。高敏感性燃料的火焰发展趋势与低敏感性燃料类似,黄色火焰的亮度与面积更小。尽管高、低敏感性燃料的OH和CH带状光谱的出现时间相近,但高敏感性燃料的光谱强度仍更低。综合分析,火焰发展结构与自发光光谱特征主要受喷油时刻的影响,燃料的敏感性主要影响着火时刻和火焰自发光光谱强度,且高敏感性燃料的光谱强度更低。     

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, η_mix?d_p, 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.     

Influence of pressure on near nozzle flow field and soot formation in laminar co-flow diffusion flames

Soot formation from combustion devices, which tend to operate at high pressure, is a health and environmental concern, thus investigating the effect of pressure on soot formation is important. While most fundamental studies have utilised the co-flow laminar diffusion flame configuration to study the effect of pressure on soot, there is a lack of investigations into the effect of pressure on the flow field of diffusion flames and the resultant influence on soot formation. A recent work has displayed that recirculation zones can form along the centreline of atmospheric pressure diffusion flames. This present work seeks to investigate whether these zones can form due to higher pressure as well, which has never been explored experimentally or numerically. The CoFlame code, which models co-flow laminar, sooting, diffusion flames, is validated for the prediction of recirculation zones using experimental flow field data for a set of atmospheric pressure flames. The code is subsequently utilised to model ethane-air diffusion flames from 2 to 33 atm. Above 10 atm, recirculation zones are predicted to form. The reason for the formation of the zones is determined to be due to increasing shear between the air and fuel steams, with the air stream having higher velocities in the vicinity of the fuel tube tip than the fuel stream. This increase in shear is shown to be the cause of the recirculation zones formed in previously investigated atmospheric flames as well. Finally, the recirculation zone is determined as a probable cause of the experimentally observed formation of a large mass of soot covering the entire fuel tube exit for an ethane diffusion flame at 36.5 atm. Previously, no adequate explanation for the formation of the large mass of soot existed.     

Soot formation and temperature field structure in co-flow laminar methane-air diffusion flames at pressures from 10 to 60 atm

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane-air laminar diffusion flames over a wide pressure range, from 10 to 60 atm in a high-pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 10 to 100 atm. Peak soot concentrations showed a strong dependence on pressure at relatively lower pressures; but this dependence got weaker as the pressure is increased. The maximum conversion of the fuel’s carbon to soot, 12.6%, was observed at 60 atm at approximately the mid-height of the flame. Radial temperature gradients within the flame increased with pressure and decreased with flame height above the burner rim. Higher radial temperature gradients near the burner exit at higher pressures mean that the thermal diffusion from the hot regions of the flame towards the flame centerline is enhanced. This leads to higher fuel pyrolysis rates causing accelerated soot nucleation and growth as the pressure increases.     

On the role of mixing models in the simulation of MILD combustion using finite-rate chemistry combustion models

The present work shows an in-depth analysis about the role of mixing models on the simulation of MILD combustion using a finite-rate combustion model, the Partially Stirred Reactor approach (PaSR). Different approaches of increasing complexity are compared: a simple model based on a fraction of the integral time scale, a fractal-based mixing model and a dynamic mixing model based on the resolution of transport equations for scalar variance and dissipation rate. The approach is validated using detailed experimental data from flames stabilized on the Adelaide Jet-in-Hot Co-flow (JHC) burner at different fuel-jet Reynolds numbers (5k, 10k and 20k) and different co-flow oxygen dilution levels (3%, 6% and 9%). The results indicate the major role of mixing models to correctly handle turbulence/chemistry interactions and clearly indicate the superior performances of the dynamic mixing model over the other tested approaches.     

Effect of inlet flow turbulence on the combustion instability in a premixed backward-facing step combustor

This paper reports the effect of inlet flow turbulence intensity on the combustion instability characteristics in a backward facing step combustor. The inlet turbulence intensity is varied by a turbulence generator. Unsteady pressure measurements and OH* chemiluminescence images are recorded over a wide range of operating conditions at different inlet turbulence intensities. The study shows an early onset of instability at low turbulence level, i.e., higher turbulence postpones the onset of instability to higher Reynolds number Re and/or higher equivalence ratio Φ. The early onset of instability in the Re and Φ parameter spaces is due to the change in system parameters such as flame speed and size of the recirculation zone downstream of the step at different turbulence levels. Further, the onset is characterized as subcritical bifurcation. At low Re, the hysteresis zone width is small for low turbulence levels and it is large at higher turbulence levels; and at higher Re, the hysteresis width remains constant at all turbulence levels. Investigation of instability characteristics reveals that there are momentary slippages from limit cycle orbit into brief silent regimes in an intermittent manner. The frequency of occurrence of the momentary silent regimes increases with reduction in turbulence, indicating that higher turbulence helps in maintaining the system in a stable limit cycle orbit. High-speed chemiluminescence imaging reveals the necessity of the vortex rollup in the recirculation zone to grow up to the top wall by dilatation from the heat release for the onset of instability. Considerations of the effect of turbulence on both the flame speed and the recirculation zone size together explain all the observed bifurcation trends. These results suggest that inlet flow turbulence should not just be considered as background noise. The turbulence effects on both the flame and flow should be considered in predicting the instability characteristics.     

Stability and emissions control using air injection and H2 addition in premixed combustion

We experimentally study lean premixed combustion stabilized behind a backward-facing step. For a propane–air mixture, the lean blowout limit is associated with strong pressure fluctuation arising simultaneously with strong flame–vortex interactions, which have been shown to constitute the mechanism of heat release dynamics in this flow. A high-speed air jet, issuing from a small slot and injected perpendicular to the main flow near the step, is used to disrupt this mechanism. For momentum ratio of jet to main flow below unity, the jet dilutes the mixture, further destabilizing the flame or leading to complete blowout. Above unity, the flame becomes more stable, and the pressure oscillations are suppressed. Flow visualization and OH*/CH* chemiluminescence measurements show that a strong jet produces a more compact flame that is less driven by the wake vortex, anchored closer to the step, and deflected upwards away from the lower wall of the channel. This renders the flame less vulnerable to heat loss and strong strains, which improves its stability and extends the flammability limit. Adding hydrogen to the main fuel improves the flame stability over the entire range of the air jet mass flow, with better results for momentum ratio larger than 1; H₂ pulls the flame further upstream, away from the shear zone and the unsteady vortex. NO_x emission benefits from the air jet, while, with H₂ addition, NO_x concentration is higher in the products as the overall burning temperature rises. However, hydrogen addition enables extending the flammability limit further by increasing air supply in the primary stream, hence achieving lower NO_x. The study suggests a simpler, almost passive, multi-objective combustion control technique and indicates that hydrogen addition can be a successful in situ approach for NO_x reduction.     

The impact of N2 micro-jets on the V-to-M flame shape transition in a premixed swirl burner

Injection of N₂ through micro-jets located on the dump plane of a lean premixed swirl stabilized combustor is investigated as a new method for mitigating combustion instabilities. This study focuses on the chemical and fluid dynamic processes by which the N₂ micro-jets impact the flame dynamics. An experimental and numerical investigation is performed to characterize the combustion instability during the V-to-M flame shape transition in a swirl burner fueled with premixed CH₄/air, at an equivalence ratio of 0.62. Reasonable agreements have been found between the experimental measurements and simulation results. Both of them present that the flame changes from V-shape to M-shape periodically, and a low-frequency instability around 10 Hz is observed accordingly. It is confirmed that intermittent flame extinction in the outer recirculation zone (ORZ) is the source of the combustion instability. Furthermore, injection of N₂ through micro-jets located on the combustor dump plane, into the outer recirculation zone, results in a stable V shape flame. It is clearly seen that the ORZ dilution can eliminate the combustion instability without inhibiting the combustion efficiency. A special focus is placed on the impact of the diluent injection on the local flame-flow interaction. The nitrogen micro-jets increase the local nitrogen concentration by 7% on average, lowering the flame speed and extinction strain rates by 27% and 17% respectively. Moreover, the micro-jets increase the turbulence intensity in the ORZ, leading to a significant increase in the Karlovitz number and transferring the local combustion regime from the thin reaction zone regime to the broken reaction zone regime. Hence, the nitrogen micro-jets impact on both the turbulence and the chemical reaction rates prevents flame propagation into the ORZ and results in a stable flame.     

Simultaneous stereo‐PIV and OH×CH2O PLIF measurements in turbulent ultra lean CH4/H2 swirling wall‐impinging flames

We present experimental results from turbulent low-swirl lean H₂/CH₄ flames impinging on an inclined, cooled iso-thermal wall, based on simultaneous stereo-PIV and OH×CH₂O PLIF measurements. By increasing the H₂ fraction in the fuel while keeping Karlovitz number (Ka) fixed in a first series of flames, a fuel dependent near-wall flame structure is identified. Although Ka is constant, flames with high H₂ fraction exhibit significantly more broken reaction zones. In addition, these high H₂ fraction flames interact significantly more with the wall, stabilizing through the inner shear layer and well inside the near-wall swirling flow due to a higher resistance to mean strain rate. This flame-wall interaction is argued to increase the effective local Ka due to heat loss to the wall, as similar flames with a (near adiabatic) ceramic wall instead of a cooled wall exhibit significantly less flame brokenness. A second series of leaner flames were investigated near blow-off limit and showed complete quenching in the inner shear layer, where the mean strain rate matches the extinction strain rate extracted from 1D flames. For pure CH₄ flames (Ka ≈ 30), the reaction zone remains thin up to the quenching point, while conversely for the 70% H₂ flames (Ka ≈ 1100), the reaction zone is highly fragmented. Remarkably, in all near blow-off cases with CH₄ in the fuel, a large cloud of CH₂O persists downstream the quenching point, suggesting incomplete combustion. Finally, ultra lean pure hydrogen flames were also studied for equivalence ratios as low as 0.22, and through OH imaging, exhibit a clear transition from a cellular flame structure to a highly fragmented flame structure near blow-off.     

Validation of flow simulation and gas combustion sub-models for the CFD-based prediction of NOx formation in biomass grate furnaces

While reasonably accurate in simulating gas phase combustion in biomass grate furnaces, CFD tools based on simple turbulence–chemistry interaction models and global reaction mechanisms have been shown to lack in reliability regarding the prediction of NO_x formation. Coupling detailed NO_x reaction kinetics with advanced turbulence–chemistry interaction models is a promising alternative, yet computationally inefficient for engineering purposes. In the present work, a model is proposed to overcome these difficulties. The model is based on the Realizable k–? model for turbulence, Eddy Dissipation Concept for turbulence–chemistry interaction and the HK97 reaction mechanism. The assessment of the sub-models in terms of accuracy and computational effort was carried out on three laboratory-scale turbulent jet flames in comparison with the experimental data. Without taking NO_x formation into account, the accuracy of turbulence modelling and turbulence–chemistry interaction modelling was systematically examined on Sandia Flame D and Sandia CO/H₂/N₂ Flame B to support the choice of the associated models. As revealed by the Large Eddy Simulations of the former flame, the shortcomings of turbulence modelling by the Reynolds averaged Navier–Stokes (RANS) approach considerably influence the prediction of the mixing-dominated combustion process. This reduced the sensitivity of the RANS results to the variations of turbulence–chemistry interaction models and combustion kinetics. Issues related to the NO_x formation with a focus on fuel bound nitrogen sources were investigated on a NH₃-doped syngas flame. The experimentally observed trend in NO_x yield from NH₃ was correctly reproduced by HK97, whereas the replacement of its combustion subset by that of a detailed reaction scheme led to a more accurate agreement, but at increased computational costs. Moreover, based on results of simulations with HK97, the main features of the local course of the NO_x formation processes were identified by a detailed analysis of the interactions between the nitrogen chemistry and the underlying flow field.