Finite-rate Chemistry Effects Research Articles

A consistent MMC-LES approach for turbulent premixed flames

A multiple mapping conditioning (MMC) approach is coupled with large eddy simulations (LES) for the prediction of the flame propagation, the flame structure and finite rate chemistry effects including NO formation in turbulent premixed flames. The mixing term in MMC relies on a reference field that characterizes the chemical composition within the flame and it is typically taken to be the reaction progress variable. In LES, this reference field needs to be artificially thickened to be resolved on the LES grid. A novel MMC mixing time scale model that had been developed with the aid of direct numerical simulation (DNS), has been adapted to account for the thickening of the reference field. Equally, the particle selection mechanism for particle mixing has been modified to ensure localness of mixing in composition space and to preserve the prediction of a realistic (thin) flame thickness by the stochastic particles. A set of freely propagating turbulent premixed flames with varying equivalence ratios is used to test the performance of the MMC model. Results of MMC-LES are compared with reference DNS that include full chemistry. Additional simulations were carried out where MMC is coupled with quasi-DNS where the flow field is fully resolved but the reaction progress variable field is thickened and its source term is closed with a flamelet approach. This allows to isolate the different modelling assumptions invoked in MMC-LES. Deviations from the exact solution can then be associated with the MMC approach and with thickening of the reference field separately and they are small. The MMC-LES predictions of the turbulent premixed flame propagation speed and flame structure are very good and finite rate chemistry effects can be captured well which is demonstrated by good agreement of predicted NO with DNS data.

Estimation and testing of single-step oxidation reactions for hydrogen and methane in low-oxygen, elevated pressure conditions

Advanced combustion concepts, such as moderate or intense low-oxygen dilution (MILD) combustion, offer a reduction in NOx emissions and increased thermal efficiency. MILD is characterised by low-oxygen, high-temperature conditions, where finite-rate chemistry effects are significant. Modelling this regime using reduced or single-step chemistry remains a challenge in part due to the finite-rate chemistry. This work proposes a generalised method for assigning Arrhenius coefficients of a single-step reaction using the outputs of a detailed mechanism. Assigning the typical chemical conservation equation to a progress variable, activation energy and pre-exponential factor for an Arrhenius kinetics global reaction are determined for hydrogen and methane across a number of conditions, with the proposed method extended to n-heptane as well. The temperature exponent in the modified Arrhenius equation is determined by minimising the ignition delay error between detailed and single-step simulations. Functional forms of each coefficient are calculated from a multivariate regression, dependent on initial temperature, pressure, and oxidant mole fraction. The predicted mechanisms are compared against the detailed kinetics in closed homogeneous batch reactors, with comparisons for hydrogen extended to both laminar opposed flow diffusion flames, and computational fluid dynamics (CFD) simulations. Ignition delay and equilibrium temperature are both well predicted for all three fuels in the batch reactors. Notably, the negative temperature coefficient behaviour of n-heptane is successfully recreated with the single-step mechanism. Temperature and heat release of hydrogen flames are well captured in both opposed flow laminar flames, and in turbulent CFD simulations. The computational time was also significantly reduced through the single-step mechanisms, resulting in ∼100 times reduction in compute time for CFD simulations. The function form of the Arrhenius coefficients shows promise for extension outside of the ranges and fuels analysed herein, and presents interesting phenomena for exploring how initial reactant temperature and pressure influence the effective activation energy of an oxidation process.

A super-grid approach for LES combustion closure using the Linear Eddy Model

LES–LEM is a simulation approach for turbulent combustion in which the stochastic Linear Eddy Model (LEM) is used for sub-grid mixing and combustion closure in Large-Eddy Simulation (LES). LEM resolves, along a one-dimensional line, all spatial and temporal scales, provides on-the-fly local turbulent flame statistics, captures finite rate chemistry effects and directly incorporates turbulence-chemistry interaction. However, the approach is computationally expensive as it requires advancing an LEM-line in each LES cell. This paper introduces a novel turbulent combustion closure model for LES using LEM to address this issue. It involves coarse-graining the LES mesh to generate a coarse- level ‘super-grid’ comprised of cell-clusters. Each cell-cluster, instead of each LES cell, then contains a single LEM domain. This domain advances the combined advection–reaction–diffusion solution and also provides suitably conditioned statistics for thermochemical scalars such as species mass fractions. Local LES-filtered thermochemical states are then obtained by probability-density-function (PDF) weighted integration of binned conditionally averaged scalars, akin to standard presumed PDF approaches for reactive LES but with physics-based determination of the full thermochemical state for particular values of the conditioning variables. The proposed method is termed ‘super-grid LEM’ or ‘SG-LEM’. The paper describes LEM reaction–diffusion advancement, the LEM representation of turbulent advection, a novel splicing algorithm (a key feature of LES–LEM) formulated for the super-grid approach, a wall treatment, and a thermochemical LES closure procedure. To validate the proposed model, a pressure-based solver was developed using the OpenFOAM library and tested on a premixed ethylene flame stabilised over a backward facing step, a setup for which some DNS data is available. SG-LEM provides high resolution flame structures, temperature and mass fractions suitable for LES thermochemical closure. Additionally, it provides reaction-rate data at the coarse level, a unique feature compared to other mapping-type closure methods. Quantitative comparisons are made between the proposed model and time-averaged DNS data, focussing on velocity, temperature and species mass fraction. Results show good agreement downstream of the step. Furthermore, comparison with an equivalent Partially-Stirred Reactor (PaSR) simulation demonstrates the superior predictive capability of SG-LEM. Additionally, the paper briefly examines the sensitivity of the model to coarse-graining parameters and finally, explores computational efficiency highlighting the substantial speedup achieved when compared to the standard LES–LEM approach with potentially significant speedup relative to PaSR closure for the intensely turbulent regimes of principal interest.

A Numerical Study of Flow Structures and Flame Shape Transition in Swirl-Stabilized Turbulent Premixed Flames Subject to Local Extinction

ABSTRACT Large Eddy Simulations (LES) of turbulent lean-premixed flames of V- and M-shape are presented. A simple algebraic closure with the ability to capture finite-rate chemistry effects is used for subgrid reaction rate modeling. The V-shaped flame is stabilized in the inner shear layer between a swirling annular jet and a central recirculating bubble in a sudden expansion duct. The M-shaped flame is stabilized in the inner and outer shear layer, adjoining the corner recirculation zone induced by the vertical step. The focus of the study is on the flow fields and shapes of the flames, which distinguish themselves through different heat load and sensitivity to local extinction. Good agreement with measurements is observed for the cold and the reacting flow cases. The numerical results suggest that the entrainment of hot gases into the outer recirculation zone occurs close to the impingement point of the swirling annular jet on the wall and this process is strongly dependent on intense vortical structures in the outer shear layer. The results further suggest that local extinction influences the position of the flame in the inner shear layer and, thereby, also the intensity of the local entrainment process.

Effects of pressure on the structure of partially premixed turbulent diffusion flames: Calculations using MMC-LES

This study numerically investigates the detailed structure of twelve partially premixed turbulent diffusion flames of methane and air across a pressure range between 1 and 20 bar. The flames are computed using the multiple mapping conditioning / large eddy simulations (MMC-LES) approach. The model is first validated against available experimental data at atmospheric pressure. This is followed by simulations of a series of flames with increasing pressure while holding either the bulk jet velocity or the jet Reynolds number constant. Results show that local extinction increases as pressure increases in cases with fixed bulk jet velocity. Meanwhile, an instability induced by body forces gradually arises as pressure increases at constant Reynolds number, but can be mitigated by reducing the gravitational acceleration, and hence the body forces, proportional to the pressure increase. Consequently, flames become more stable with less localised extinction when increasing pressure at constant Reynolds number. The study accurately reflects the effects of finite-rate chemistry with increasing pressure, as evidenced in representative plots of a Damköhler number ratio. Additionally, the approach to blow-off at different pressures is examined, and trends in temperature and species are analysed. The calculations presented in this study align well with existing data at atmospheric pressure and provide reliable blind test results that await detailed comparison with measurements not yet available at high pressures.

On the radiation extinction of opposed flame spread over curved solid surface in low flow velocity conditions

The controlling mechanisms of the radiation extinction of the flame spreading over cylinders in low flow velocity conditions are investigated with an analytical approach. Attention is focused on the interaction among solid surface curvature, solid surface radiation, and gas-phase volumetric radiation heat loss in the flame spread process. The analysis uses a classical thermal model, which considers the heat balance on the unburned fuel surface near the flame front. A non-dimensional number that evaluates the effects of volumetric radiation heat loss and finite rate chemistry on the flame temperature is also proposed. It is found that solid surface radiation cannot be a dominant mechanism for the radiation extinction of a flame spreading over a thin cylinder. Furthermore, in the case of flame spread over a cylinder, flame spread rate significantly increases with decreasing opposed flow velocity due to curvature effect, thus the consideration of the relative gas flow velocity seen by the spreading flame becomes essential in low flow velocity conditions. However, when the volumetric radiation heat loss in the gas phase is taken into account in the thermal model, flame spread rate for a thin cylinder once increases with decreasing opposed flow velocity and then turns to decrease. As a result, the flame spread rate of a thin cylinder shows a peak value in low flow velocity conditions. This trend is in good agreement with previous microgravity experiments (Fujita et al., 2002 [1]). Moreover, the limit condition of flame spread appears as a bifurcation point where the stable and unstable solutions of flame spread rate coincide in the low flow velocity condition. The analytical results indicate that it is essential to consider the volumetric radiation heat loss in the gas phase to predict the flame spread over a curved surface in low flow velocity conditions.

Development of a turbulence dissipation based reaction rate model for progress variable in turbulent premixed flames

This study presents an algebraic combustion closure for Large eddy simulation (LES) exhibiting attributes of simplicity and simultaneous accuracy under realistic combustion conditions. The model makes use of the interlink between the reaction and dissipation rates in premixed turbulent combustion but relaxes the thin flame assumption by considering finite-rate chemistry effects in the small-scale turbulence structure. The core idea of the approach is to approximate the reaction progress in the unresolved spectrum of wave lengths and to use it within a filtered reaction rate expression. The model is implemented in OpenFOAM 4.0 and is tested on a turbulent, premixed flame behind a bluff-body, applying an LES approach for turbulence modelling. The cross comparison of velocity, temperature and composition data with experiments and a well-investigated combustion model in literature reveals competitive performance of the new model. Especially in the near-field of the bluff body flame, corresponding to thin and moderately thickened flame regions, its ability to capture the flame structure is highly promising. The chosen, partly explicit approach to recover the temperature from the transported sensible enthalpy, involving a strong coupling between filtered reaction and heat release rate, also shows advantages over obtaining the temperature from presumed probability density functions.

Flameless combustion of low calorific value gases, experiments, and simulations with advanced radiative heat transfer modeling

Thermal radiation is the dominant mode of heat transfer in many combustion systems, and in typical flameless furnaces, it can represent up to 80% of the total heat transfer. Accurate modeling of radiative heat transfer is, thus, crucial in the design of these large-scale combustion systems. Thermal radiation impacts the thermochemistry, thereby the energy efficiency and the temperature sensitive species prediction, such as NOx and soot. The requirement to accurately describe the spectral dependence of gaseous radiative properties of combustion products interacts with the modeling of finite rate chemistry effects and conjugates heat transfer and turbulence. Additionally, because of the multiple injection of fuels and/or oxidizers of various compositions, case-specific radiative properties' expressions are required. Along these lines, a comprehensive modeling to couple radiation and combustion in reacting flows is attempted and applied to the simulation of flameless combustion. Radiation is modeled using the spectral line-based weighted-sum-of-gray-gases approach to calculate gaseous radiative properties of combustion products using the correlation of the line-by-line spectra of H2O and CO2. The emissivity weights and absorption coefficients were optimized for a range of optical thicknesses and temperatures encountered in the considered furnace. Efforts were also made on the development of a reliable and detailed experimental dataset for validation. Measurements are performed in a low calorific value syngas furnace operating under flameless combustion. This test rig features a thermal charge which can extract about 60% of combustion heat release via 80% of radiative heat transfer, making it of special interest for modeling validation. The comparison between the simulation and the experiment demonstrated a fair prediction of heat transfer, energy balance, temperature, and chemical species fields.

Large eddy simulation of a supersonic lifted hydrogen flame with sparse-Lagrangian multiple mapping conditioning approach

The Multiple Mapping Conditioning / Large Eddy Simulation (MMC-LES) approach is used to simulate a supersonic lifted hydrogen jet flame, which features shock-induced autoignition, shock-flame interaction, lifted flame stabilization, and finite-rate chemistry effects. The shocks and expansion waves, shock-reaction interactions and overall flame characteristics are accurately reproduced by the model. Predictions are compared with the detailed experimental data for the mean axial velocity, mean and root-mean-square temperature, species mole fractions, and mixture fraction at various locations. The predicted and experimentally observed flame structures are compared through scatter plots of species mole fractions and temperature against mixture fraction. Unlike most past MMC-LES which has been applied to low-Mach flames, in this supersonic flame case pressure work and viscous heating are included in the stochastic FDF equations. Analysis indicates that the pressure work plays an important role in autoignition induction and flame stabilization, whereas viscous heating is only significant in shear layers (but still negligibly small compared to the pressure work). The evolutions of particle information subject to local gas dynamics are extracted through trajectory analysis on representative fuel and oxidizer particles. The particles intermittently enter the extinction region and may be deviated from the full burning or mixing lines under the effects of shocks, expansion waves and viscous heating. The chemical explosive mode analysis performed on the Lagrangian particles shows that temperature, the H and OH radicals contribute dominantly to CEM respectively in the central fuel jet, fuel-rich and fuel-lean sides. The pronounced particle Damköhler numbers first occur in the fuel jet / coflow shear layer, enhanced at the first shock intersection point and peak around the flame stabilization point.

Analysis of high-speed combustion regimes of hydrogen jet in supersonic vitiated airstream

Highly-resolved reactive simulations of a hydrogen jet injected into a transverse supersonic flow of vitiated air at Mach 2 are conducted. The operating conditions are chosen to be relevant of scramjet operative conditions and are representative of experiments conducted at ONERA Palaiseau Research Center. Hydrogen injection in the supersonic vitiated airstream leads to the formation of a bow shock, which interacts with the boundary layer and gives rise to separation zones. In the resulting recirculation zones, only small amounts of OH radicals are produced. Combustion stabilization and development take place further downstream. Heat release is significant in the vicinity of the wall within a sonic region where the equivalence ratio is around two. Farther from walls, combustion takes place at supersonic speeds but features lower levels of heat release rates. Supersonic turbulent combustion regimes are analyzed in detail: first in a standard set of coordinates (u′/SL0,Lt/δL0) using data collected in regions featuring a sufficiently large degree of premixing. Then, they are analyzed in the turbulent Reynolds and Damköhler numbers sub-space (Ret,Da). In this representation, three distinct Damköhler number definitions based on (i) production rate of H2O, (ii) flame propagation characteristic time, and (iii) ignition delay time are considered. These various representations put into evidence turbulent combustion regimes featuring significant finite-rate chemical kinetics effects. The whole set of computational results confirms that (i) the use of models based on the fast chemistry hypothesis is questionable for such conditions, (ii) taking into account finite-rate chemistry effects is essential, and (iii) ignition processes play a key role in combustion stabilization. The manuscript ends with some perspectives and challenging issues for future works.

Finite-rate chemistry effects in turbulent hypersonic boundary layers: A direct numerical simulation study

The accurate prediction of hypersonic flows is a key challenge for the design of reentry bodies or hypersonic aircraft. Knowledge concerning the wall-bounded turbulence is especially needed. The present work is a fundamental study of a turbulent spatially developing flat-plate boundary layer subjected to high-enthalpy effects. The influence of air chemical reactions, typical of high-speed regimes, is investigated by means of direct numerical simulations. First- and second-order statistics are accurately examined, as well as spectral content and classical correlations.

A High-Fidelity Modeling Tool to Support the Design of Oxy-Combustors for Direct-Fired Supercritical CO2 Cycles

Abstract The challenge in the design of oxy-combustors for direct-fired supercritical CO2 (sCO2) cycles is in addressing disparate performance metrics and objectives. Key design parameters to consider include, among others, injector design for mixing and flame stability, split of recycled CO2 diluent between injectors and cooling films, target flame temperatures to control noncondensable products, and strategies to inject the diluent CO2 for film cooling and thermal control. In order to support novel oxy-combustor designs, a high-fidelity yet numerically efficient modeling framework based on the CRUNCH CFD® flow solver is presented, featuring key physics-based submodels relevant in this regime. For computational efficiency in modeling large kinetic sets, a flamelet/progress variable (FPV) based tabulated-chemistry approach is utilized featuring a three-stream extension to allow for the simulation of the CO2 film cooling stream in addition to the fuel and oxidizer streams. Finite rate chemistry effects are modeled in terms of multiple progress variables for the primary flame as well as for slower-evolving chemical species such as NOx and SOx contaminants. Real fluid effects are modeled using advanced equations of states. The predictive capabilities of this computationally tractable design support tool are demonstrated on a conceptual injector design for an oxy-combustor operating near 30 MPa. Simulations results provide quantitative feedback on the effectiveness of the film cooling as well as the level of contaminants (CO, NO, and N) in the exhaust due to impurities entering from the injectors. These results indicate that this framework would be a useful tool for refining and optimizing the oxy-combustor designs as well as risk mitigation analyses.

Improved MMC-LES to compute the structure of a mixed-mode turbulent flame series

An improved version of the Lagrangian multiple mapping conditioning model coupled with large eddy simulation (MMC-LES) is applied to compute the structure of a range of turbulent mixed-mode flames stabilised on the Sydney piloted burner with compositionally inhomogeneous inlet. The calculations are compared with detailed measurements. While premixed MMC-LES models are yet to be developed, the most commonly deployed version and code for MMC-LES uses a mixture-fraction-based approach and a sparse-Lagrangian notional particle method that is most suitable for non-premixed combustion. The improvements reported here are due to (i) a more rigorous density coupling approach, (ii) a more refined mixing time scale, and (iii) selective particle intensification to improve localness in physical space as well as mixture fraction space. This intensification is selectively applied in upstream regions where mixed-mode combustion dominates and is relaxed further downstream as the jet flames transition to a non-premixed flame structure. The computed results show good agreement with experimental data hence confirming the feasibility of the improved MMC-LES approach in being able to account for mixed-mode combustion as well as for finite-rate chemistry effects. These promising results warrant future extension of the selective particle intensification method to become automatically adaptive.

Coupled Simulations of Finite-Rate Ablation with Pyrolysis in Rocket Nozzles

An ablation model based on finite-rate heterogeneous reactions is incorporated into a conjugate flowfield/ablation analysis framework, relaxing the assumption of equilibrium ablation made in previous studies using this conjugate analysis methodology. Fully coupled, conjugate, two-dimensional simulations modeling the ablation of pyrolyzing carbon–phenolic material in the HIPPO nozzle test case are presented; comparisons are made between equilibrium and finite-rate surface chemistry assumptions. The finite-rate surface chemistry method provides substantially improved agreement with experimental measurements. The difference in the surface recession predictions obtained with the equilibrium and finite-rate models is larger than expected. This suggests that finite-rate surface chemistry effects may be important for temperatures above the 2000–2500 K threshold that is typically assumed for equilibrium surface chemistry. Additional studies investigate the effects of finite-rate gas-phase reactions and radiative heat transfer on the predicted ablation response.

LES of a swirl-stabilized kerosene spray flame with a multi-component vaporization model and detailed chemistry

Due to the introduction of alternative aviation fuels, new methods and models are necessary which have the capability to predict the performance of combustors dependent on the fuel composition. Towards this target, a multi-component vaporization model is coupled to a direct, detailed chemistry solver in the context of Eulerian–Lagrangian LES. By means of the computational platform, a lab-scale, swirl-stabilized spray flame is computed. The burner exhibits some of the key features of current aero-engine combustors. Global features like the measured spray distribution and the position of the reaction zone are well reproduced by the LES. The comparison of droplet size, droplet velocity and liquid volume flux profiles with experimental data also show a good agreement. However, discrepancies in the temperature profiles in the central mixing zone exist. The computational results show that evaporation and mixing are the rate-controlling steps in the flame zone. In this zone, chemistry can be assumed to be infinitely fast. However, other zones exist where finite rate chemistry effects prevail. For these states, the direct computation of the elementary reactions by means of Arrhenius equations and the transport of all individual species are beneficial. Furthermore, the finite rate chemistry approach demonstrates a great potential with respect to pollutant formation, as precursors can be directly computed. Additionally, the example of benzene forming from one specific chemical class in the fuel suggests that a multi-component description of the liquid phase and the evaporation process is required to correctly predict soot emissions.

A One-Dimensional Turbulence-Based Closure Model for Combustion LES

ABSTRACTA large-eddy simulation (LES) closure method for the filtered density function (FDF) and reactive scalars’ conditional means in a turbulent non-premixed flame is developed and validated. The closure is based on data from one-dimensional turbulence (ODT) simulations. A novel FDF tabulation-based kernel density estimation from ODT simulations is used to model the statistical distributions of the conditioning variables. These distributions are combined with ODT conditional statistics to construct closure tables for filtered reactive scalars and density. The ODT-LES closure model is validated using the Sandia flames D and F of piloted jet diffusion flames, which are excellent test cases to demonstrate finite-rate chemistry effects in non-premixed combustion. The closure model exhibits good agreement with experimental results while also reproducing salient features associated with these flames, including the occurrence of extinction and reignition.

Flow Inhomogeneities in a Realistic Aeronautical Gas-Turbine Combustor: Formation, Evolution, and Indirect Noise

Indirect noise generated by the acceleration of combustion inhomogeneities is an important aspect in the design of aero-engines because of its impact on the overall noise emitted by an aircraft and the possible contribution to combustion instabilities. In this study, a realistic rich-quench-lean (RQL) combustor is numerically investigated, with the objective of quantitatively analyzing the formation and evolution of flow inhomogeneities and determining the level of indirect combustion noise in the nozzle guide vane (NGV). Both entropy and compositional noise are calculated in this work. A high-fidelity numerical simulation of the combustion chamber, based on the large-eddy simulation (LES) approach with the conditional moment closure (CMC) combustion model, is performed. The contributions of the different air streams to the formation of flow inhomogeneities are pinned down and separated with seven dedicated passive scalars. LES-CMC results are then used to determine the acoustic sources to feed an NGV aeroacoustic model, which outputs the noise generated by entropy and compositional inhomogeneities. Results show that non-negligible fluctuations of temperature and composition reach the combustor's exit. Combustion inhomogeneities originate both from finite-rate chemistry effects and incomplete mixing. In particular, the role of mixing with dilution and liner air flows on the level of combustion inhomogeneities at the combustor's exit is highlighted. The species that most contribute to indirect noise are identified and the transfer functions of a realistic NGV are computed. The noise level indicates that indirect noise generated by temperature fluctuations is larger than the indirect noise generated by compositional inhomogeneities, although the latter is not negligible and is expected to become louder in supersonic nozzles. It is also shown that relatively small fluctuations of the local flame structure can lead to significant variations of the nozzle transfer function, whose gain increases with the Mach number. This highlights the necessity of an on-line solution of the local flame structure, which is performed in this paper by CMC, for an accurate prediction of the level of compositional noise. This study opens new possibilities for the identification, separation, and calculation of the sources of indirect combustion noise in realistic aeronautical gas turbines.

Description of the char conversion process in coal combustion based on premixed FGM chemistry

This work presents and assesses a tabulation approach to represent the complex coal combustion chemistry at low computational costs. It should enable to include the process of volatiles conversion and char burnout in scale resolving simulations (such as large eddy simulations) of realistic coal combustion chambers. It is based on premixed flamelets and gets investigated with respect to its capability to accurately describe char conversion. In this approach the gas phase chemistry is mapped onto a four-dimensional manifold controlled by two mixture fractions, the enthalpy and a reaction progress variable. Accordingly, mixing between oxidizer, volatiles and gaseous char conversion products is accounted for besides non-adiabatic physics and finite rate chemistry effects. In a previous work (Knappstein et al., 2017), several modeling assumptions with regard to the description of the volatiles reaction using tabulated chemistry were already assessed. For the full description of the coal combustion process the model functionality was extended to account also for char conversion in the present work. Here, special emphasis is therefore put on aspects related with the conversion of char. The consistency of the model formulation and its implementation is demonstrated by the examination of mass, species and energy balances in generic test cases for which analytical solutions are derivable. Furthermore, results of a simulation adopting the presented model are compared against a detailed chemistry reference solution accounting for the full reaction mechanism. This situation approximates a particle essentially consisting of char that is exposed to hot oxygen-containing product gases. Thereby, capabilities of the presented modeling approach with respect to prediction accuracies for the model’s intended application in LES can be estimated.

Prediction of Pollutant Emissions from Bluff-Body Stabilised Nonpremixed Flames

Construction of a stable flame is one of the critical design requirements in developing practical combustion systems. Flames stabilised by a bluff-body are extensively used in certain types of combustors. The design promotes mixing of cold reactants and hot products on the flame surface to improve the flame stability. In this study, bluff-body stabilised methane-hydrogen flames are computed using the steady laminar flamelet combustion method in conjunction with the Reynolds-averaged Navier-Stokes (RANS) approach. These flames are known as Sandia jet flames and have different jet mean velocities. The turbulence is modelled using the standard k-ϵ model and the chemical kinetics are modelled using the GRI-mechanism with 325 chemical reactions and 53 species. The computed mean reactive scalars of interest are compared with the experimental measurements at different axial locations in the flame. The computed values are in reasonably good agreement with the experimental data. Although some underpredictions are observed mainly for NO and CO at downstream locations in the flame, these results are consistent with earlier reported studies using more complex combustion models. The reason for these discrepancies is that the flamelet model is not adequate to capture the finite-rate chemistry effects and shear turbulence specifically, for species with a slow time scale such as nitrogen oxides.

Large eddy simulation/dynamic thickened flame modeling of a high Karlovitz number turbulent premixed jet flame

Due to the complex multiscale interaction between intense turbulence and relatively weak flames, turbulent premixed flames in the thin and broken reaction zones regimes exhibit strong finite-rate chemistry and strain effects and are hence challenging to model. In this work, a laboratory premixed jet flame in the broken reaction zone, which has recently been studied using direct numerical simulation (DNS), is modeled using a large eddy simulation (LES)/dynamic thickened flame (DTF) approach with detailed chemistry. The presence of substantial flame thickening due to strong turbulence-chemistry interactions, which can be characterized by a high Karlovitz number (Ka), requires the DTF model to thicken the flame in an adaptive way based on the local resolution of flame scales. Here, an appropriate flame sensor and strain-sensitive flame thickness are used to automatically determine the thickening location and thickening factor, respectively. To account for finite-rate chemistry and strain effects, the chemistry is described in two different ways: (1) detailed chemistry denoted as full transport and chemistry (FTC), and (2) tabulated chemistry based on a strained premixed flamelet (SPF) model. The performance of the augmented LES/DTF approach for modeling the high Ka premixed flame is assessed through detailed a posteriori comparisons with DNS of the same flame. It is found that the LES/DTF/FTC model is capable of reproducing most features of the high Ka turbulent premixed flame including accurate CO and NO prediction. The LES/DTF/SPF model has the potential to capture the impact of strong turbulence on the flame structure and provides reasonable prediction of pollutant emissions at a reasonable computational cost. In order to identify the impact of aerodynamic strain, the turbulent flame structure is analyzed and compared with unstrained and strained premixed flamelet solutions. The results indicate that detailed strain effects should be considered when using tabulated methods to model high Ka premixed flames.