Assessing the Predictive Capabilities of Combustion LES as Applied to the Sydney Flame Series

Large Eddy Simulation (LES) and flamelet-based combustion models were applied to four bluff-body stabilized nonpremixed and partially premixed flames selected from the Sydney flame series, based on Masri’s bluff-body test rig (University of Sydney). Three related non-reacting flow cases were also investigated to assess the performance of the LES solver. Both un-swirled and swirled cases were studied exhibiting different flow features, such as recirculation, jet precessing and vortex breakdown. Due to various fuel compositions, flow rates and swirl numbers, the combustion characteristics of the flames varied greatly. On six meshes with different blocking structure and mesh sizes, good prediction of flow and scalar fields using LES/flamelet approaches and known fuel and oxidizer mass fluxes was achieved. The accuracy of predictions was strongly influenced by the combustion model used. All flames were calculated using at least two modeling strategies. Starting with calculations of isothermal flow cases, simple single flamelet based calculations were carried out for the corresponding reacting cases. The combustion models were then adjusted to fit the requirements of each flame. For all flame calculations good agreement of the main flow features with the measured data was achieved. For purely nonpremixed flames burning attached to the bluff-body’s outer edge, flamelet modeling including strain rate effects provided good results for the flow field and for most scalars. The prediction of a partially premixed swirl flame could only be achieved by applying a flamelet-based progress variable approach.     

A numerical investigation of CO_2 dilution on the thermochemical characteristics of a swirl stabilized diffusion flame

The turbulent combustion flow modeling is performed to study the effects of CO₂ addition to the fuel and oxidizer streams on the thermochemical characteristics of a swirl stabilized diffusion flame. A flamelet approach along with three well-known turbulence models is utilized to model the turbulent combustion flow field. The k-ω shear stress transport(SST) model shows the best agreement with the experimental measurements compared with other models. Therefore, the k-ω SST model is used...     

Large Eddy Simulation of Turbulent Reacting Shear Layers Including Finite-Rate Chemistry and Detailed Diffusion Processes

Large eddy simulations (LES) of turbulent temporal shear layers with hydrogen chemistry are performed. In these simulations, approximate deconvolution is applied as an implicit subgrid-scale modeling approach to a reacting flow in combination with a steady flamelet model for the filtered heat release term. No additional heuristical or physical subgrid models are used. The formulation of the flamelet equations in physical space does not only allow to consider a detailed reaction scheme and the extinguished phase but also to take into account detailed diffusion mechanisms (Soret and Dufour effects, multicomponent diffusion coefficients). Two different levels of diffusion approximations are investigated in this work, the aim of which is twofold: Firstly, to verify approximate deconvolution as a tool for convective transport of mass, momentum and energy in gas flow, by comparing the LES results with those of a direct numerical simulation and secondly, to investigate the influence of detailed diffusion on the laminar flamelets and the LES results.     

Direct measurement of mixture fraction in reacting flow using Rayleigh scattering

The mixture fraction variable, , is very useful in describing reaction zone structure in nonpremixed flames. Extinction limits and turbulent mixing are often described as a function of this variable. Experimental evaluation of is critical for improving our understanding of the influence of turbulent mixing on the chemistry process. Heretofore, the evaluation of mixture fraction in combusting flow required multiple simultaneous concentration measurements. In this paper we present a fuel designed to permit measurements of mixture fraction by Rayleigh scattering technique only. A Rayleigh intensity/mixture fraction correspondence has been obtained experimentally in a laminar coflow flame. The influence of strain rate and differential diffusion effects have been investigated using laminar counterflow diffusion flame and shifting equilibrium chemistry models. The results obtained from comparisons between experiments and these models are very encouraging and suggest that the Rayleigh/mixture fraction correspondence established is valid under both the turbulent mixing and laminar strained flamelet combustion regimes.     

Large Eddy Simulation of a Polydisperse Ethanol Spray Flame

Ethanol is identified as an interesting alternative fuel. In this regards, the predictive capability of combustion Large Eddy Simulation approach coupled to Lagrangian droplet dynamic model to retrieve the turbulent droplet dispersion, droplet size distribution, spray evolution and combustion properties is investigated in this paper for an ethanol spray flame. Following the Eulerian-Lagrangian approach with a fully two way coupling, the Favre-filtered low Mach number Navier-Stokes equations are solved on structured grids with dynamic sub-grid scale models to describe the turbulent carrier gas phase. Droplets are injected in polydisperse manner and generated in time dependent boundary conditions. They evaporate to form an air-fuel mixture that yields spray flame. Part of the ethanol droplets evaporates within the prevaporization area before reaching the combustion zone, making the flame to burn in a partially premixed regime. The chemistry is described by a tabulated detailed chemistry based on the flamelet generated manifold approach. The fuel, ethanol, is modeled by a detailed reaction mechanism consisting of 56 species and 351 reversible reactions. The simulation results including excess gas temperature, droplet velocities and corresponding fluctuations, droplet mean diameters and spray volume flux at different distances from the exit plane show good agreement with experimental data. Analysis of combustion spray features allows gaining a deep insight into the two-phase flow process ongoing.     

Large Eddy Simulation of a Methane–Air Diffusion Flame

Large Eddy Simulation has been applied to a piloted methane/air diffusion flame—the Sandia D flame—for which detailed experimental data are available. To evaluate the reacting density, temperature and species mass fractions a conserved scalar laminar flamelet formulation is employed, utilising a single virtually unstrained flamelet. The results of two simulations are discussed, comparing the use of the standard Smagorinsky model and a dynamic variant for closure of the unknown sub-grid stress. The chosen sub-grid scale model is shown to be extremely influential on the final solution. Whilst the use of the standard model results in a relatively poor simulation the dynamic closure offers an excellent velocity field prediction throughout the flame. Although the flame does show some strain rate influence on burning, particularly close to the inlet nozzle, the relatively simple ‘unstrained’ flamelet model applied is shown to provide an accurate representation of temperature and major species distribution.     

Subgrid Scale Modeling in Large-Eddy Simulation of Turbulent Combustion Using Premixed Flamelet Chemistry

Large-eddy simulation (LES) of turbulent combustion with premixed flamelets is investigated in this paper. The approach solves the filtered Navier–Stokes equations supplemented with two transport equations, one for the mixture fraction and another for a progress variable. The LES premixed flamelet approach is tested for two flows: a premixed preheated Bunsen flame and a partially premixed diffusion flame (Sandia Flame D). In the first case, we compare the LES with a direct numerical simulation (DNS). Four non-trivial models for the chemical source term are considered for the Bunsen flame: the standard presumed beta-pdf model, and three new propositions (simpler than the beta-pdf model): the filtered flamelet model, the shift-filter model and the shift-inversion model. A priori and a posteriori tests are performed for these subgrid reaction models. In the present preheated Bunsen flame, the filtered flamelet model gives the best results in a priori tests. The LES tests for the Bunsen flame are limited to a case in which the filter width is only slightly larger than the flame thickness. According to the a posteriori tests the three models (beta-pdf, filtered flamelet and shift-inversion) show more or less the same results as the trivial model, in which subgrid reaction effects are ignored, while the shift-filter model leads to worse results. Since LES needs to resolve the large turbulent eddies, the LES filter width is bounded by a maximum. For the present Bunsen flame this means that the filter width should be of the order of the flame thickness or smaller. In this regime, the effects of subgrid reaction and subgrid flame wrinkling turn out to be quite modest. The LES-results of the second case (Sandia Flame D) are compared to experimental data. Satisfactory agreement is obtained for the main species. Comparison is made between different eddy-viscosity models for the subgrid turbulence, and the Smagorinsky eddy-viscosity is found to give worse results than eddy-viscosities that are not dominated by the mean shear. Paper presented on the Eccomas Thematic Conference Computational Combustion 2007, submitted for a special issue of Flow, Turbulence and Combustion.     

Effect of Partial Premixing on Stabilization and Local Extinction of Turbulent Methane/Air Flames

The stabilization characteristics and local extinction structures of partially premixed methane/air flames were studied using simultaneous OH-PLIF/PIV techniques, and large eddy simulations employing a two-scalar flamelet model. Partial premixing was made in a mixing chamber comprised of two concentric tubes, where the degree of partial premixing of fuel and air was controlled by varying the mixing length of the chamber. At the exit of the mixing chamber a cone was mounted to stabilize the flames at high turbulence intensities. The stability regime of flames was determined for different degree of partial premixing and Reynolds numbers. It was found that in general partially premixed flames at low Reynolds numbers become more stable when the level of partial premixing of air to the fuel stream decreases. At high Reynolds numbers, for the presently studied burner configuration there is an optimal partial premixing level of air to the fuel stream at which the flame is most stable. OH-PLIF images revealed that for the stable flames not very close to the blowout regime, significant local extinction holes appear already. By increasing premixing air to fuel stream successively, local extinction holes grow in size leading to eventual flame blowout. Local flame extinction was found to frequently attain to locations where locally high velocity flows impinging to the flame. The local flame extinction poses a future challenge for model simulations and the present flames provide a possible test case for such study.     

Flow Physics of a Bluff-Body Swirl Stabilized Flame and their Prediction by Means of a Joint Eulerian Stochastic Field and Tabulated Chemistry Approach

In the frame of this work a transported joint scalar probability density function (PDF) method is combined with the flamelet generated manifolds (FGM) tabulated chemistry approach for large eddy simulation (LES) modeling of a three-dimensional turbulent premixed swirl burner. This strategy accounts for the turbulence-chemistry interaction at reasonable computational costs. At the same time, it allows the usage of detailed chemistry mechanisms for the creation of the chemical database. The simulation results obtained are comparatively assessed along with complementary measurements. Furthermore, transient and time-averaged data are used to provide insight into the flow physics of the bluff-body swirl stabilized flame considered. The sensitivity of the results to different modeling approaches regarding the predicted flame shape and its dynamics is also investigated, where the implemented approach is compared with the well-established artificially thickened flame (ATF) combustion model. Consequently, the investigation conducted in this work aims to provide a complete picture on the ability of the proposed combustion model to reproduce the flow conditions within complex bluff-body swirl stabilized flames.     

钝体射流火焰及其点火过程的大涡模拟

钝体燃烧器广泛应用于航空发动机、燃气轮机、锅炉等设备的燃烧室中.对其点火过程的了解和控制直接关系到设备的安全运行和污染物排放等重要问题.我们采用基于火焰面/过程变量燃烧模型的大涡模拟方法对湍流非预混钝体射流火焰及其点火过程进行了详细的数值模拟.以Sydney钝体燃烧器的无反应射流和有反应甲烷/氢气(CH_4/H_2)火焰为研究对象,首先通过统计平均的数值结果与实验测量及文献数据的对比,全面检验了所用数值方法和燃烧模型;随后,详细展示了钝体燃烧器点火和火焰发展的瞬态过程;最后,对钝体射流的点火过程进行了细致的分析和表征.根据温度峰值、羟基(OH)质量分数和甲醛(CH_2O)质量分数峰值随时间的变化表征了强制点火过程的4个阶段:点火源衰减、点火触发、点火核生成和点火成功.其中,点火核驻留的空间位置位于钝体燃烧器冷态流场外侧涡的尾部回流区域附近.     

Large Eddy Simulations of Unconfined Non-reacting and Reacting Turbulent Low Swirl Jets

The low swirl flow is a novel method for stabilizing lean premixed combustion to achieve low emissions of nitrogen oxides. Understanding the characteristics of low swirl flows is of both practical and fundamental interest. In this paper, in order to gain better insight into low swirl stabilized combustion, large eddy simulation and dynamically thickened flame combustion modeling are used to characterize various features of non-reacting and reacting low swirl flows including vortex breakdown, shear layers’ instability, and coherent structures. Furthermore, four test cases with different equivalence ratios are studied to evaluate the effects of equivalence ratio on the flame and flow characteristics. A finite volume scheme on a Cartesian grid with a dynamic one equation eddy viscosity subgrid model is used for large eddy simulations. The obtained results show that the combustion heat release and increase in equivalence ratio toward the stoichiometric value decrease the local swirl number of the flow field, while increasing the flow spreading at the burner outlet. Results show that the flame becomes W shaped as the equivalence ratio increases. Moreover, the combination of the swirling motion and combustion heat release temporally imposes a vortex breakdown in the post-flame region, which leads to occurrence of a transient recirculation zone. The temporal recirculation zone disappears downstream of the burner outlet due to merging of the inner shear layer from all sides at the centerline. Also, various analyses of shear layers’ wavy and vortical structures show that combustion heat release has the effect of decreasing the instability amplitude and vortex shedding frequency.     

Experimental investigation of characteristics of a diffusion flame established over liquid ethanol surface under opposed air flow

A preliminary study of the shape and the extinction characteristics of a diffusion flame established over a circular liquid fuel surface under the influence of an opposed air flow, is presented. Renewable liquid fuel such as ethanol is employed. A simple heterogeneous combustion setup, which consists of a cylindrical tube containing ethanol located at the bottom, is exposed to an opposed air flow from a coaxial circular pipe of same size located at the top at a fixed separation distance. Axial and radial extents of flame for different air flow rates are qualitatively analyzed. Burning rates of ethanol for different separation distances and air flow rates are recorded. For a fixed separation distance, at a particular air flow rate the flame extinction takes place. Extinction air flow rates and corresponding strain rates for different separation distances are presented.     

Modelling of a Bluff-Body Nonpremixed Flame using a Coupled Radiation/Flamelet Combustion Model

A coupled radiation/flamelet combustion modelling technique is applied to the simulation of a bluff-body flame. Radiation heat transfer is incorporated into the laminar flamelet model for turbulent combustion through the enthalpy defect. A new method is developed for generating flamelet library with enthalpy defect. The radiation within the flame is modelled using a raytracing approach based on the discrete transfer method. The predicted results are compared with the reported experimental data. Comparison shows that the effects of radiative heat transferr on the temperature and major species are small for the flame considered. However, a significant improvement in the prediction of OH is achieved when radiation heat transfer is included. This revised version was published online in July 2006 with corrections to the Cover Date.     

PDF of distance function for level‐set flamelet library modelling

The difference between a presumed distribution of flamelet position and a numerically simulated distribution of distance function (a signed distance to flamelet) is investigated. It is shown that even if the distribution of flamelet position is symmetrical and close to Gaussian, the distribution of distance function away from the mean flame position is skewed towards the mean position and the mean of the distance function is also different from the distance to the mean position. The difference depends on the distance to the mean flame and the flame wrinkling amplitude. An extension method for the variance of the distance function and an upwind scheme for solving the re‐initialization equation are presented. A numerical simulation of a premixed turbulent flame is compared to experimental data. Copyright © 2003 John Wiley & Sons, Ltd.     

Effect on flow field characteristics in methane–air non-premixed flame with steam addition

The present paper reports on an experimental study to determine the effect of humidity on the flow field and the flame stability limit in turbulent non-premixed flame, and examines the dynamical behavior of the unsteady aerodynamic flow structures observed on a bluff-body burner at both humid and non-humid air combustion states. Particle image velocimetry (PIV) is used to capture the instantaneous appearance of vortex structures and obtain the quantitative velocity field. Streamlines and velocity contours analysis are used to identify specific flame structures and reveal the effect of steam added on the vortex structure. The results show both central fuel penetration limit and partially quenching limit in the humid air case reduce. The decrease in the critical penetration limit is primarily attributed to a reduction in momentum of the humid air. The flamelet concepts are applied to discuss the partially quenching limit in the blue neck region. The analysis reveals that the large decrease in the partially quenching limit is due to the increase in chemical reaction time of the humid air combustion.     

Investigation over the recirculation influence on the combustion of micro organic dust particles

This paper investigates the role of recircnlation and non-unity Lewis number on the combustion of organic dust particles. Since recirculation effect is more noticeable in micro-combustors, it is necessary to propose a modeling approach of this phenomenon to better simulate the performance of micro-combustors. In this research, in order to model the combustion of organic dust particles, it is assumed that the dust particles va- porize first to yield a known chemical structure which is oxidized in the gas phase, and the chemical structure of this gaseous fuel is assumed methane. To study the flame structure and solve the governing equations, it is considered that the flame structure consists of three zones titled the preheat-vaporization zone, the narrow reaction zone and finally the post flame zone. The recirculation phenomenon is evaluated by entering the exhausted heat from the post flame zone into the preheat zone. The solution is based on the follow- ing approach. First, the governing equations in each zone are nondimensionalized. Then the needed boundary and matching conditions are applied in each zone. After that, these equations and the required boundary and matching conditions are simultaneously solved with the analytical model. Consequently, the remarkable effects of recirculation and non- unity Lewis number on the combustion characteristics of the organic dust particles such as burning velocity and temperature profiles for different particle radii are obtained. The results show reasonable agreement with published experimental data.     

Large-Eddy Simulations of Spray a Flames Using Explicit Coupling of the Energy Equation with the FGM Database

This paper provides a numerical study on n-dodecane flames using Large-Eddy Simulations (LES) along with the Flamelet Generated Manifold (FGM) method for combustion modeling. The computational setup follows the Engine Combustion Network Spray A operating condition, which consists of a single-hole spray injection into a constant volume vessel. Herein we propose a novel approach for the coupling of the energy equation with the FGM database for spray combustion simulations. Namely, the energy equation is solved in terms of the sensible enthalpy, while the heat of combustion is calculated from the FGM database. This approach decreases the computational cost of the simulation because it does not require a precise computation of the entire composition of the mixture. The flamelet database is generated by simulating a series of counterflow diffusion flames with two popular chemical kinetics mechanisms for n-dodecane. Further, the secondary breakup of the droplet is taken into account by a recently developed modified version of the Taylor Analogy Breakup model. The numerical results show that the proposed methodology captures accurately the main characteristics of the reacting spray, such as mixture formation, ignition delay time, and flame lift-off. Additionally, it captures the “cool flame" between the flame lift-off and the injection nozzle. Overall, the simulations show differences between the two kinetics mechanisms regarding the ignition characteristics, while similar flame structures are observed once the flame is stabilised at the lift-off distance.

     

Large eddy simulation of turbulent premixed piloted flame using artificial thickened flame model coupled with tabulated chemistry

A sub-grid scale (SGS) combustion model, which combines the artificial thickened flame (ATF) model with the flamelet generated manifold (FGM) tabulation method, is proposed. Based on the analysis of laminar flame structures, two self-contained flame sensors are used to track the diffusion and reaction processes with different spatial scales in the flame front, respectively. The dynamic formulation for the proposed SGS combustion model is also performed. Large eddy simulations (LESs) of Bunsen flame F3 are used to evaluate the different SGS combustion models. The results show that the proposed SGS model has the ability in predicting the distributions of temperature and velocity reasonably, while the predictions for the distributions of some species need further improvement. The snapshots of instantaneous normalized progress variables reveal that the flame is more remarkably and severely wrinkled at the flame tip for flame F3. More satisfactory results obtained by the dynamic model indicate that it can preserve the premixed flame propagation characteristics better.     

Leading-Edge Reaction Zones in Lifted-Jet Gas and Spray Flames

An investigation of the leading edge characteristics in lifted turbulent methane-air (gaseous) and ethanol-air (spray) diffusion flames is presented. Both combustion systems consist of a central nonpremixed fuel jet surrounded by low-speed air co-flow. Non-intrusive laser-based diagnostic techniques have been applied to each system to provide information regarding the behavior of the combustion structures and turbulent flow field in the regions of flame stabilization. Simultaneous sequential CH-PLIF/particle image velocimetry and CH-PLIF/Rayleigh scattering measurements are presented for the lifted gaseous flame. The CH-PLIF data for the lifted gas flame reveals the role that ``leading-edge' combustion plays as the stabilization mechanism in gaseous diffusion flames. This phenomenon, characterized by a fuel-lean premixed flame branch protruding radially outward at the flame base, permits partially premixed flame propagation against the incoming flow field. In contrast, the leading edge of the ethanol spray flame, examined using single-shot OH-PLIF imaging and smoke-based flow visualization, does not exhibit the same variety of leading-edge combustion structure, but instead develops a dual reaction zone structure as the liftoff height increases. This dual structure is a result of the partial evaporation (hence partial premixing) of the polydisperse spray and the enhanced rate of air entrainment with increased liftoff height (due to co-flow). The flame stabilizes in a region of the spray, near the edge, occupied by small fuel droplets and characterized by intense mixing due to the presence of turbulent structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Soot surface temperature measurements in pure and diluted flames at atmospheric and elevated pressures

Soot surface temperature was measured in laminar jet diffusion flames at atmospheric and elevated pressures. The soot surface temperature was measured in flames at one, two, four, and eight atmospheres with both pure and diluted (using helium, argon, nitrogen, or carbon dioxide individually) ethylene fuels with a calibrated two-color soot pyrometry technique. These two dimensional temperature profiles of the soot aid in the analysis and understanding of soot production, leading to possible methods for reducing soot emission. Each flame investigated was at its smoke point, i.e., at the fuel flow rate where the overall soot production and oxidation rates are equal. The smoke point was chosen because it was desirable to have similar soot loadings for each flame. A second set of measurements were also taken where the fuel flow rate was held constant to compare with earlier work. These measurements show that overall flame temperature decreases with increasing pressure, with increasing pressure the position of peak temperature shifts to the tip of the flame, and the temperatures measured were approximately 10% lower than those calculated assuming equilibrium and neglecting radiation.