Turbulent Non-premixed Flames Research Articles

Quantitative 2D thermometry in turbulent sooting non-premixed flames using filtered Rayleigh scattering.

This paper demonstrates the application of a thermometry method in turbulent sooting non-premixed flames using filtered Rayleigh scattering (FRS). Fuel tailoring is used to establish a specific C2H2-based fuel mixture such that temperature can be determined accurately by a single FRS measurement over the entirety of mixture fraction space, or equivalently, for all relevant thermo-chemical states. Evaluation is performed in a hierarchy of flows to establish measurement precision and accuracy. Initial assessments in a series of heated fuel mixtures; non-sooting, near-adiabatic flat flames; and laminar non-premixed sooting flames show accuracy of the approach over a full range of expected temperatures and high single-shot measurement precision (e.g., 65<SNR<80) for temperatures between 1900 and 2200 K. Subsequently, 2D temperature measurements are evaluated in a turbulent non-premixed sooting flame. For the current fuel mixture, the local mixture-averaged Rayleigh scattering cross section is nearly constant over all composition space, and thus traditional laser Rayleigh scattering (LRS) is used as a measurement standard in the absence of soot. The results show excellent agreement between the proposed FRS thermometry approach and LRS in non-sooting regions. In the presence of soot, the proposed FRS-based approach shows no signs of interference. In addition, 2D temperature imaging shows high SNR (60-74) over all temperature conditions. Thus, we believe the proposed methodology successfully provides a high-resolution 2D temperature method under turbulent sooting flame conditions.

Stability Characteristics of a Turbulent Nonpremixed Conical Bluff Body Flame

The thermal characteristics of turbulent non-premixed methane flames were investigated by four burner heads with the same exit diameter but different heights. The fuel flow rate was kept constant with an exit velocity of 15 m/s, while the co-flow air speed was increased from 0 to 7.6 m/s. The radial profiles of the temperature and flame visualizations were obtained to investigate the stability limits. The results evidenced that the air co-flow and the cone angle have essential roles in the stabilization of the flame: An increase in the cone angle and/or the co-flow speed deteriorated the stability of the flame, which eventually tended to blow off. As the cone angle was reduced, the flame was attached to the bluff body. However, when the cone angle is very small, it has no effect on stability. The mixing and entrainment processes were described by the statistical moments of the temperature fluctuations. It appears that the rise in temperature coincides with the intensified mixing, and it becomes constant in the entrainment region.

CFD Analysis and NOx Prediction in H2 and Ch4 Turbulent Non-premixed Flame Compared with Swirling Flame

This work is devoted to the comparative study for the formation and dissociation of nitrogen oxides by the numerical simulation of turbulent combustion without premix in a combustion chamber having a cylindrical shape with two coaxial jets, two flames using the ANSYS fluent software16.0. The study focuses on the influence of the type of fuel on the composition of discharges in content with NOx, that is to say two cases are treated and compared. Turbulence is modeled by the k-ε model and the chemical aspect of combustion is treated by the PDF model for each flame. The calculation results relate to the characteristics of dynamic fields, temperature, the mass fractions of different species involved in the combustion process and the NOx prediction. The effect of the swirl is also tested in this study with a CFD prediction of non premixed swirling g flame. These results are compared with measurements and confrontations is satisfactory.

A novel model for incorporation of differential diffusion effects in PDF simulations of non-premixed turbulent flames based on reaction-diffusion manifolds (REDIM)

In this work, reaction-diffusion manifold (REDIM) reduced chemistry is used in the simulation of turbulent non-premixed flames based on a transported-probability density function model. Differential molecular diffusion is applied in the generation of the manifolds. This is the first work to consider the gradients of the reduced variables as additional parameters in the REDIM model, and one-directional gradients are utilized to generate the REDIM reduced chemistry. Hereby, the influence of turbulence on differential molecular diffusion is automatically considered in terms of reduced variable gradients, and the physical transport properties (e.g., diffusion coefficients) are used in a detailed way, without any additional modeling (e.g., unity-Lewis number assumption). Although the scalar gradients appear as multi-directional in a general turbulent reacting flow, previous direct numerical simulation analysis reveals that REDIMs generated from one-directional gradients can accurately describe the system featuring multi-directional gradients, if this one-directional gradient has a major effect on the chemistry. Here, it is proposed to obtain such gradients under the hypothesis that the flame structure is locally one-dimensional at each spatial position. In order to retrieve the gradients of the reduced variables for the interpolation of the thermo-kinetic states from the REDIM table, the sub-grid gradient is evaluated here from the particle fields. The well-known Sandia series of flames is selected to validate the proposed algorithm. The results show that the new algorithm can reproduce the thermo-kinetic quantities with high accuracy for all investigated flames.

Effects of pressure on soot production in piloted turbulent non-premixed jet flames

Laser-induced incandescence (LII) was used to quantify the soot volume fraction in five piloted turbulent non-premixed C2H4-N2 jet flames at elevated pressures. In one series of flames, the bulk jet velocity was held constant as the pressure was increased from 1 bar (Re = 10,000) to 3 bar (Re = 30,000), and then 5 bar (Re = 50,000). In the other series, a Reynolds number of 10,000 was held constant at 1 bar, 3 bar, and 5 bar. LII measurements were calibrated with laminar diffusion flames at each pressure using 2D diffuse line-of-sight attenuation. Mean and RMS soot volume fractions along the flame centerline exhibit Gaussian profiles for all of the flames. The magnitude of soot volume fraction and the spatial soot distribution are enhanced by increases in pressure. In both series of flames, the peak mean soot volume fraction scales with the pressure as p2.2. In the constant velocity series, the scaling drops to p1.5 if the volume-integrated mean soot volume fraction on a per fuel mass basis is considered instead. At 3 bar, a threefold increase in Re leads to a decrease in the mean soot volume fraction despite the decrease in soot intermittency. At 5 bar, a fivefold increase in Re results in soot intermittency near zero at the flame’s center and a slight increase in the mean soot volume fraction.

Experimental and numerical study of NOx formation in a domestic H2/air coaxial burner at low Reynolds number

Thermal NOx formation in H2/air jet flames from a coaxial burner is studied experimentally and numerically. The aim is to study possible NOx reduction strategies for domestic gas boiler burners. Following a flame splitting method strategy, a single burner is studied at different inlet powers (from 0.2 to 1.0 kW). The effect of three different fuel-air ratios (or equivalence ratio φ) is considered by varying the coaxial air stream, with fuel-air ratios corresponding to values of φ<1, relevant for domestic boiler applications (here φ=0.77, φ=0.83 and φ=0.91). NOx concentrations increase with increasing inlet power between 0.2 and 0.6 kW and numerical results are in good correspondence with available experimental data. The opposite trend is observed above 0.6 kW and no numerical results are obtained, indicating a transition from laminar to turbulent flames. On the other hand, in contrast to the observations made in turbulent non-premixed flames, reducing the equivalence ratio implies higher NOx concentrations in the low Reynolds number flames considered. The numerical results in the laminar regime are used to highlight and quantify three competing main factors concerning NOx production in order to interpret the experimental observations: the volume of the region where NOx is produced, and within this region, the competition between residence time and NOx reaction rate. Based on this analysis, different design strategies for low NOx hydrogen diffusion burners are finally discussed.

Direct numerical simulations of turbulent non-premixed flames: Assessment of turbulence within swirling flows

Direct numerical simulations of non-premixed swirling fuel-rich/fuel-lean flames within a high-pressure model gas turbine combustor are conducted to investigate the flow and flame structures, as well as the transport mechanisms of both turbulent kinetic energy (TKE) and enstrophy. The effects of non-premixed flames upon these characteristics are also analyzed through comparison with the corresponding non-reacting swirling flows. We demonstrate that the turbulence state in the swirling flows behaves axisymmetrically overall in the current cylindrical laboratory-type combustor and is more likely to be cigar shaped in the presence of combustion. The analysis of TKE budgets within non-reacting swirling flows indicates that TKE is predominantly produced by mean shear in the shear layers and redistributed by transport effects from the inner shear layer (ISL) to the internal-recirculation zone; however, these transport effects are suppressed by combustion in fuel-lean non-premixed flames. Although the total pressure effects consume TKE with a similar magnitude in all cases, the essential cause is different. The influence of combustion upon TKE budgets is more significant for fuel-lean flames than for fuel-rich flames as a result of the stronger burning intensity in the ISL of the former. Analysis of enstrophy dynamics shows that dilatation and baroclinic torque play relatively noticeable roles in swirling non-premixed flames, unlike their negligible effects in high-intensity homogeneous isotropic turbulence. The augmentation of baroclinic torque caused by non-premixed swirling combustion mainly arises from the remarkable decrease in density and enhancement of preferential alignment between the vorticity and baroclinic torque vectors.

Soot formation in turbulent flames of ethylene/hydrogen/ammonia

This paper presents an experimental study of turbulent non-premixed jet flames of ethylene/nitrogen where the nitrogen is substituted with different proportions of hydrogen and/or ammonia. The focus is largely on the effects of hydrogen and ammonia on soot production in turbulent flames. A combination of pointwise, laser-induced fluorescence in the visible and UV bands (LIF-UV–visible), and laser-induced incandescence (LII) is used to measure soot precursors and soot along the flame centerline. All signals are collected at a repetition rate of 10 Hz with fast photomultiplier tubes to resolve the time decay. In a separate experiment, joint imaging of LIF-OH CH is also performed at a repetition rate of 10 kHz. Hydrogen substitution is found to increase the production of soot, whereas ammonia substitution inhibits soot formation. The peak mean soot volume fraction is almost a factor of 3 lower in the 25% ammonia case in comparison to the 25% nitrogen case. The mean signal decay time constant decreases with ammonia substitution, implying the formation of smaller soot nanoparticles. The mean signal decay time constant remains unaffected with hydrogen substitution. Measured peaks in LIF-CH and LIF-OH are reduced with ammonia substitution but only in regions upstream of where soot is formed. Further downstream in the sooting region, neither OH nor CH appear to be affected by the substitution of N 2 with H 2 or NH 3 .

A comprehensive assessment of fractal wrinkling/eddy dissipation based combustion model for simulating conventional turbulent premixed and non-premixed flames

This paper presents a step-forward extension of the well-known chemistry/flow interaction Eddy Dissipation Concept approach for modelling both, non-premixed and premixed, combustion regimes in a Reynolds-Averaged Navier Stokes framework. The Eddy Dissipation Concept approach and its extended versions (e.g. Partially Stirred reactors, PaSR) are widely used for CFD modelling of a variety of combustion systems. This is mainly due to their capability of incorporating chemical kinetic rates in turbulent flows. However, in defining the averaged chemical reaction rates, EDC describes the fine-scale of turbulent reacting flowfield based on the so-called mixing-rate calculated from turbulent velocity field, which consequently makes this model more suitable for diffusion flame/combustion regimes. In a recent study by the present authors, a hybrid Wrinkling-EDC approach is introduced to model fine scales of turbulent reacting flows in MILD combustion regimes based on flame surface density in premixed flame regimes and turbulent intermittency in diffusion flame regimes. In the present study, we study the performance of this newly developed approach for the simulation of conventional turbulent flames. The simulations are performed and validated using well-documented turbulent premixed and predominantly non-premixed flames (e.g. Flame F3 of Chen et al. and Sandia Flame D). In addition, all cases are simulated using a recently developed EDC model (referred to as extended EDC) along with the standard EDC version where both their results are used as a reference. The obtained results reveal better performance of the Wrinkling-EDC approach over the standard and extended versions of EDC model for the simulation of turbulent premixed flame, and a comparable performance between the extended EDC and wrinkling-EDC approach for the predictions of species concentration and temperature fields of turbulent non-premixed flames.

On the combined effect of internal and external intermittency in turbulent non-premixed jet flames

This paper analyzes the combined effect of internal intermittency and external intermittency on the dynamics of small-scale turbulent mixing in a turbulent non-premixed jet flame. The phenomenon of external intermittency in turbulent jet flames originates from a very thin layer, known as turbulent/non-turbulent interface, that separates the inner turbulent core from the outer irrotational surrounding fluid. The impact of external intermittency on turbulence is evaluated across the jet flame by the self-similarity scaling of higher-order structure functions of the mixture fraction. It is shown that the scaling of structure functions exhibits a growing departure from the prediction of classical scaling laws toward the edge of the flame. Empirical evidence is provided that this departure is primarily due to external intermittency and the associated break down of self-similarity. External intermittency creates local gradients that are significantly more intense compared to those created by internal intermittency alone. As chemical reactions usually take place in thin layers in the vicinity of the turbulent/non-turbulent interface, an accurate statistical description of these intense events is necessary to predict the turbulence-chemistry interaction. The study is based on data from a highly-resolved direct numerical simulation of a temporally evolving planar jet flame.

Numerical study of a turbulent co-axial non-premixed flame for methanol hydrothermal combustion: Comparison of the EDC and FGM models

Eddy dissipation concept (EDC) model and flamelet generated manifolds (FGM) model are developed separately to study the temperature profiles and extinction limits of non-premixed hydrothermal flames. Predictions by the two models are evaluated comparatively by experimental data in literatures. FGM model shows relatively better prediction of temperature than EDC model in the near nozzle field. Extinction temperatures can be predicted by EDC model with deviations of 10–33 K. The extinction flow rates predicted by the FGM model are higher than those by the EDC model. Flow fields and reaction source terms are analysed to identify the inherent mechanism leading different results by the two models. It is illustrated that the positive effect of turbulence on reaction rate near the nozzle by the FGM model is the essential reason causing different flame characteristics from the EDC model by which the turbulence only has negative effect on reaction rate.

Study on the Effects of Cone Height on the Turbulent Nonpremixed Flames Downstream of a Conical Bluff Body

Abstract Flow, thermal, and emission characteristics of turbulent nonpremixed CH4 flames were investigated for three burner heads of different cone heights. The fuel velocity was kept constant at 15 m/s, while the coflow air speed was varied between 0 and 7.4 m/s. Detailed radial profiles of the velocity and temperature were obtained in the bluff body wake at three vertical locations of 0.5D, 1D, and 1.5D. Emissions of CO2, CO, NOx, and O2 were also measured at the tail end of every flame. Flames were digitally photographed to support the point measurements with the visual observations. Fifteen different stability points were examined, which were the results of three bluff body variants and five coflow velocities. The results show that a blue-colored ring flame is formed, especially at high coflow velocities. The results also illustrate that depending on the mixing at the bluff-body wake, the flames exhibit two modes of combustion regimes, namely fuel jet- and coflow-dominated flames. In the jet-dominated regime, the flames become longer when compared with the flames of the coflow-dominated regime. In the latter regime, emissions were largely reduced due to the dilution by the excess air, which also surpasses their production.

A-Priori Validation of Scalar Dissipation Rate Models for Turbulent Non-Premixed Flames

The modelling of scalar dissipation rate in conditional methods for large-eddy simulations is investigated based on a priori direct numerical simulation analysis using a dataset representing an igniting non-premixed planar jet flame. The main objective is to provide a comprehensive assessment of models typically used for large-eddy simulations of non-premixed turbulent flames with the Conditional Moment Closure combustion model. The linear relaxation model gives a good estimate of the Favre-filtered scalar dissipation rate throughout the ignition with a value of the related constant close to the one deduced from theoretical arguments. Such value of the constant is one order of magnitude higher than typical values used in Reynolds-averaged approaches. The amplitude mapping closure model provides a satisfactory estimate of the conditionally filtered scalar dissipation rate even in flows characterised by shear driven turbulence and strong density variation.

Modeling curvature effects in turbulent autoigniting non-premixed flames using tabulated chemistry

Turbulent flames are intrinsically curved. In the presence of preferential diffusion, curvature effects either enhance or suppress molecular diffusion, depending on the diffusivity of the species and the direction of the flame curvature. When a tabulated chemistry type of modeling is employed, curvature-preferential diffusion interactions have to be taken into consideration in the construction of manifolds. In this study, we employ multistage stage flamelet generated manifolds (MuSt-FGM) method to model autoigniting non-premixed turbulent flames with preferential diffusion effects included. The conditions for the modeled flame are in MILD combustion regime. To model the above-mentioned curvature-preferential diffusion interactions, a new mixture fraction which has a non-unity Lewis number is defined and used as a new control variable in the manifold generation. 1D curved flames are simulated to create the necessary flamelets. The resulting MuSt-FGM tables are used in the simulation of 1D laminar flames, and then also applied to turbulent flames using 2D direct numerical simulations (DNS). It was observed that when the curvature effects are included in the manifold, the MuSt-FGM results agree well with the detailed chemistry results; whereas the results become unsatisfactory when the curvature effects are ignored.

The effect of fuel composition and Reynolds number on soot formation processes in turbulent non-premixed toluene jet flames

The soot formation processes in three different turbulent prevaporized non-premixed toluene jet flames stabilized on a jet-in-hot-coflow (JHC) burner were investigated in this study. The jet Reynolds number and the stoichiometric mixture fraction were varied in order to manipulate the flow time scales and the chemistry, respectively. Time-resolved laser-induced incandescence (TiRe-LII), non-linear two-line atomic fluorescence of indium (nTLAF), and OH planar laser induced fluorescence (PLIF) were simultaneously applied to yield spatially resolved and instantaneous fields of soot volume fraction, primary particle size, temperature, and OH. The mean distributions of the detected quantities are used to identify major differences among the flames. The highest soot loading is observed for the low Reynolds number and low stoichiometric mixture fraction flame. However, this flame features also the lowest temperature and primary particle size. Based on these observations, the simultaneously detected data sets and flamelet computations are employed to elucidate differences in the soot formation pathways in the flames. The analyses reveal that the high soot loading causes greater heat losses in the low Reynolds number and low stoichiometric mixture fraction flame. This has a significant impact on the soot formation pathways and causes a reduction in the particle size.

Physical models of flame height and air entrainment of two adjacent buoyant turbulent jet non-premixed flames with different heat release rates

This paper investigated the flame height and air entrainment of two adjacent buoyant turbulent jet non-premixed flames with the same base dimension whilst having different heat release rates (HRRs). Propane was used as fuel. The HRR of one of the burners was fixed at 8, 16, 24, 32 and 40 kW, respectively. By increasing the HRR of the other burner, the interaction behaviors of two jet flames were studied under various burner spacings. A total of 15 categories of HRRs combinations of two jet flames were conducted in this work. Two flames heights are different to lead the heat pressure on both sides of the flame to be non-axisymmetric. And the interaction law, air entrainment mechanism etc. become more complicated than two same HRRs flames. Results showed that with increasing burner spacing, the flame merging behaviors can be divided into three stages. The flame height firstly deceases to a minimum value, then slightly rises again and finally maintains stable due to the weak interaction of non-merging flames. Two effective weighting factors are supposed to quantize the maximum and minimum flame heights to derive a flame height model which then is validated using the experimental data in literatures. The merging point height increases with increasing HRR and burner spacing. Based on the air entrainment mechanism of two interaction flames, a general method for calculating the global volume air entrainment is presented to build a physical entrainment model. Above flame height and physical entrainment models are suitable for the calculation with two flames combinations with the same and different HRRs, which greatly extends the application scope of interaction fires. In addition, two creative methods of calculating flame height and global air entrainment volume have been provided a reference for the research on the combustion behaviors of multiple flames with different HRRs combination.

Turbulence/flame/wall interactions in non-premixed inclined slot-jet flames impinging at a wall using direct numerical simulation

In the present work, three-dimensional turbulent non-premixed oblique slot-jet flames impinging at a wall were investigated using direct numerical simulation (DNS). Two cases are considered with the Damköhler number (Da) of case A being twice that of case B. A 17 species and 73-step mechanism for methane combustion was employed in the simulations. It was found that flame extinction in case B is more prominent compared to case A. Reignition in the lower branch of combustion for case A occurs when the scalar dissipation rate relaxes, while no reignition occurs in the lower branch for case B due to excessive scalar dissipation rate. A method was proposed to identify the flame quenching edges of turbulent non-premixed flames in wall-bounded flows based on the intersections of mixture fraction and OH mass fraction iso-surfaces. The flame/wall interactions were examined in terms of the quenching distance and the wall heat flux along the quenching edges. There is essentially no flame/wall interaction in case B due to the extinction caused by excessive turbulent mixing. In contrast, significant interactions between flames and the wall are observed in case A. The quenching distance is found to be negatively correlated with wall heat flux as previously reported in turbulent premixed flames. The influence of chemical reactions and wall on flow topologies was identified. The FS/U and FC/U topologies are found near flame edges, and the NNN/U topology appears when reignition occurs. The vortex-dominant topologies, FC/U and FS/S, play an increasingly important role as the jet turbulence develops.

A new modeling approach for mixture fraction statistics based on dissipation elements

The probabilty density function (PDF) of the mixture fraction is of integral importance to a large number of combustion models. Here, a novel modelling approach for the PDF of the mixture fraction is proposed which employs dissipation elements. While being restricted to the commonly used mean and variance of the mixture fraction, this model approach individually considers contributions of the laminar regions as well as the turbulent core and the turbulent/non-turbulent interface region. The later region poses a highly intermittent part of the flow which is of high relevance to the non-premixed combustion of pure hydrocarbon fuels. The model assumptions are justified by means of the gradient trajectory based analysis of high fidelity direct numerical simulation (DNS) datasets of two turbulent inert configurations and a turbulent non-premixed jet flame. The new dissipation element based model is validated against the DNS datasets and a comparison with the beta PDF is presented.

Soot particle size distribution measurements in a turbulent ethylene swirl flame

There is a need to better understand particle size distributions (PSDs) from turbulent flames from a theoretical, practical and even regulatory perspective. Experiments were conducted on a sooting turbulent non-premixed swirled ethylene flame with secondary (dilution) air injection to investigate exhaust and in-burner PSDs measured with a Scanning Mobility Particle Sizer (SMPS) and soot volume fractions (fv) using extinction measurements. The focus was to understand the effect of systematically changing the amount and location of dilution air injection on the PSDs and fv inside the burner and at the exhaust. The PSDs were also compared with planar Laser Induced Incandescence (LII) calibrated against the average fv. LII provides some supplemental information on the relative soot amounts and spatial distribution among the various flow conditions that helps interpret the results. For the flame with no air dilution, fv drops gradually along the centreline of the burner towards the exhaust and the PSD shows a shift from larger particles to smaller. However, with dilution air fv reduces sharply where the dilution jets meet the burner axis. Downstream of the dilution jets fv reduces gradually and the PSDs remain unchanged until the exhaust. At the exhaust, the flame with no air dilution shows significantly more particles with an fv one to two orders of magnitude greater compared to the Cases with dilution. This dataset provides insights into soot spatial and particle size distributions within turbulent flames of relevance to gas turbine combustion with differing dilution parameters and the effect dilution has on the particle size. Additionally, this work measures fv using both ex situ and in situ techniques, and highlights the difficulties associated with comparing results across the two. The results are useful for validating advanced models for turbulent combustion.

In-Situ Adaptive Manifolds: Enabling computationally efficient simulations of complex turbulent reacting flows

Reduced-order manifold approaches to turbulent combustion modeling traditionally involve precomputation of manifold solutions and pretabulation of the thermochemical database versus a small number of manifold variables. However, additional manifold variables are required as the complexity of turbulent combustion processes increases through consideration of, for example, multi-modal, non-adiabatic, or non-isobaric combustion, or combustion featuring multiple and/or inhomogeneous inlets. This increase in the number of manifold variables comes with an increase in the computational cost of precomputing a greater number of manifold solutions, most of which are never actually utilized in a CFD calculation. The memory required to store the pretabulated high-dimensional thermochemical database also increases, practically limiting the complexity of manifold-based combustion models. In this work, a new In-Situ Adaptive Manifolds (ISAM) approach is developed that overcomes this limitation by combining ‘on-the-fly’ calculation of manifold solutions with In-Situ Adaptive Tabulation (ISAT), enabling the use of more complex manifold-based turbulent combustion models. The performance of ISAM is evaluated via LES of turbulent nonpremixed jet flames with both hydrogen and hydrocarbon fuels. A performance assessment indicates that the computational overhead associated with ISAM compared to pretabulation ranges from negligible up to a factor of two, with most of this overhead associated with convolution of the thermochemical state against a presumed subfilter PDF. In addition, the memory requirements of ISAM are more than two orders of magnitude less than conventional tabulation. These results demonstrate the potential for ISAM to accommodate significantly more complex manifold-based combustion models.