LES of a premixed jet flame DNS using a strained flamelet model

Turbulent premixed flames in the thin and broken reaction zones regimes are difficult to model with Large Eddy Simulation (LES) because turbulence strongly perturbs subfilter scale flame structures. This study addresses the difficulty by proposing a strained flamelet model for LES of high Karlovitz number flames. The proposed model extends a previously developed premixed flamelet approach to account for turbulence’s perturbation of subfilter premixed flame structures. The model describes combustion processes by solving strained premixed flamelets, tabulating the results in terms of a progress variable and a hydrogen radical, and invoking a presumed PDF framework to account for subfilter physics. The model is validated using two dimensional laminar flame studies, and is then tested by performing an LES of a premixed slot-jet direct numerical simulation (DNS). In the premixed regime diagram this slot-jet is found at the edge of the broken reaction zones regime. Comparisons of the DNS, the strained flamelet model LES, and an unstrained flamelet model LES confirm that turbulence perturbs flame structure to leading order effect, and that the use of an unstrained flamelet LES model under-predicts flame height. It is shown that the strained flamelet model captures the physics characterizing interactions of mixing and chemistry in highly turbulent regimes.     

Modelling of hydrogen-blended dual-fuel combustion using flamelet-generated manifold and preferential diffusion effects

In the present study, Reynolds-Averaged Navier-Stokes simulations together with a novel flamelet generated manifold (FGM) hybrid combustion model incorporating preferential diffusion effects is utilised for the investigation of a hydrogen-blended diesel-hydrogen dual-fuel engine combustion process with high hydrogen energy share. The FGM hybrid combustion model was developed by coupling laminar flamelet databases obtained from diffusion flamelets and premixed flamelets. The model employed three control variables, namely, mixture fraction, reaction progress variable and enthalpy. The preferential diffusion effects were included in the laminar flamelet calculations and in the diffusion terms in the transport equations of the control variables. The resulting model is then validated against an experimental diesel-hydrogen dual-fuel combustion engine. The results show that the FGM hybrid combustion model incorporating preferential diffusion effects in the flame chemistry and transport equations yields better predictions with good accuracy for the in-cylinder characteristics. The inclusion of preferential diffusion effects in the flame chemistry and transport equations was found to predict well several characteristics of the diesel-hydrogen dual-fuel combustion process: 1) ignition delay, 2) start and end of combustion, 3) faster flame propagation and quicker burning rate of hydrogen, 4) high temperature combustion due to highly reactive nature of hydrogen radicals, 5) peak values of the heat release rate due to high temperature combustion of the partially premixed pilot fuel spray with entrained hydrogen/air and then background hydrogen-air premixed mixture. The comparison between diesel-hydrogen dual-fuel combustion and diesel only combustion shows early start of combustion, longer ignition delay time, higher flame temperature and NOx emissions for dual-fuel combustion compared to diesel only combustion.     

Approximating the chemical structure of partially premixed and diffusion counterflow flames using FPI flamelet tabulation

Various strategies have been proposed to tabulate complex chemistry for subsequent introduction into fluid mechanics computations. Some of them are grounded on laminar flame calculations, which are useful to seek out key relations linking a few control parameters with relevant species responses. The objective of this paper is to estimate whether approaches based on premixed flamelets (FPI or FGM) can be extended to partially premixed and diffusion flames. Prototypes of nonpremixed laminar and strained counterflow flames are simulated using fully detailed chemistry. The configuration studied is a jet of methane/air mixture opposed to an air stream. A set of reference flames is then obtained, to which FPI results are compared. By varying the equivalence ratio of the free stream of methane/air mixture, from stoichiometry up to pure methane, premixed, partially premixed, and diffusion flames are analyzed. When the fresh fuel/oxidizer mixture equivalence ratio takes values within the flammability limits, excellent results are obtained with FPI. When this equivalence ratio is outside the flammability limits, diffusive fluxes across isomixture fraction surfaces lead to a departure between the FPI tabulation and the reference detailed chemistry flames. This is associated mainly with the appearance of a double-flame structure, progressively evolving into a single diffusion flame when the fuel side equivalence ratio is further increased. Using an improved flame index to distinguish between premixed and diffusion flame burning, hybrid partially premixed combustion is reproduced from a combination of FPI and diffusion flamelets.     

A general flamelet transformation useful for distinguishing between premixed and non-premixed modes of combustion

The flame index was originally proposed by Yamashita et al. as a method of locally distinguishing between premixed and non-premixed combustion. Although this index has been applied both passively in the analysis of direct numerical simulation data, and actively using single step combustion models, certain limitations restrict its use in more detailed combustion models. In this work a general flamelet transformation that holds in the limits of both premixed and non-premixed combustion is developed. This transformation makes use of two statistically independent variables: a mixture fraction and a reaction progress parameter. The transformation is used to produce a model for distinguishing between premixed and non-premixed combustion regimes. The new model locally examines the term budget of the general flamelet transformation. The magnitudes of each of the terms in the budget are calculated and compared to the chemical source term. Determining whether a flame burns in a premixed or a non-premixed regime then amounts to determining which sets of these terms most significantly contribute to balancing the source term. The model is tested in a numerical simulation of a laminar triple flame, and is compared to a recent manifestation of the flame index approach. Additionally, the model is applied in a presumed probability density function (PDF) large eddy simulation (LES) of a lean premixed swirl burner. The model is used to locally select whether tabulated premixed or tabulated non-premixed chemistry should be referenced in the LES. Results from the LES are compared to experiments.     

Experimental and numerical analysis of a lean premixed stratified burner using 1D Raman/Rayleigh scattering and large eddy simulation

A joint experimental and numerical approach is conducted to investigate a turbulent lean premixed stratified flame (flame TSF-A of the Darmstadt stratified burner). First, the distribution of the temperature and main species is obtained experimentally by 1D Raman/Rayleigh scattering. These measurements are used to provide insight into the physics of stratified combustion and to serve as validation data for numerical models. As a second step Large Eddy Simulations (LES) are carried out using tabulated chemistry combined with a thickened flame approach. The chemistry table uses the progress variable and additionally the mixture fraction as a second controlling variable to account for the variation in equivalence ratio. To test the applicability of the model, the influence of artificial thickening on the simulation of stratified flames is investigated by means of a one-dimensional test case. Furthermore, two different grids are used in the three-dimensional simulations to assess the modeling impact. The data obtained from the measurements and simulations are presented and compared along radial profiles at several axial positions. Further information about the interaction of the reaction zone with the mixing layer has been extracted from the LES which is currently not accessible by experiments.     

甲烷预混多喷嘴阵列燃烧器热声振荡模态实验研究

为研究燃气轮机模型燃烧室的非预混燃烧流场,采用大涡模拟方法分别结合火焰面生成流形模型（FGM）和部分预混稳态火焰面模型（PSFM）对甲烷/空气同轴射流非预混燃烧室开展了数值模拟研究,并与试验结果进行对比。结果表明：FGM所预测的速度分布、混合分数分布、燃烧产物及CO分布与试验结果更符合;两种模型均能捕捉到燃烧室中的火焰抬举现象;燃烧过程中的火焰结构较为复杂,同时存在预混燃烧区域和扩散燃烧区域,扩散燃烧主要分布在化学恰当比等值线附近,预混燃烧区域主要分布在贫油区。     

Assessing LES models based on tabulated chemistry for the simulation of Diesel spray combustion

In the context of large-eddy simulation (LES) of Diesel engine combustion, two LES combustion models are proposed. Their ability to predict autoignition delays and heat release of an autoigniting liquid α-methylnaphthalene/n-decane jet injected into a constant-volume chamber under Diesel-like conditions is assessed. These models retain the tabulation of a complex chemistry scheme using autoigniting homogeneous reactors (HR) at constant pressure. This allows accounting for the chemical complexity of heavy hydrocarbon fuels over the wide range of conditions representative for Diesel engines, at comparatively low CPU time overhead. The tabulated homogeneous reactor (THR) approach assumes the local structure of the reaction zone to be that of an HR, while the approximated diffusion flame (ADF) approach is based on autoigniting strained diffusion flames. Two variants of each approach are considered, either neglecting sub-grid-scale mixture fraction variance (THR and ADF models), or accounting for it via a presumed β-PDF (THR-pdf and ADF–PCM models). LES results indicate that the ADF model assuming diffusion flame structures tends to predict faster propagation of the combustion toward less reactive mixture fractions then the THR model. Moreover, neglecting the mixture fraction fluctuations strongly overestimates initial experimental heat release rates after autoignition. Comparison between models shows that this assumption yields higher reaction rates and temperature levels close to the stoichiometric mixture fraction zones. Predictions in terms of autoignition are remarkably close with all models, and exhibit very few variations from one realization to the other. Variations in global heat release rate become more apparent for different realizations at later instants, in relation to the interaction of large flow scales with combustion.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

LES of a turbulent premixed swirl burner using the Eulerian stochastic field method

A turbulent premixed swirl burner is simulated using the sgs-pdf evolution equation approach in conjunction with the Eulerian stochastic field solution method in the context of Large Eddy Simulation. Simple gradient diffusion models are adopted for the sub-grid stresses and eight stochastic fields were utilised to characterise the influence of the sub-grid fluctuations. The chemistry was represented by an augmented reduced mechanism derived from GRI 3.0 with 15 reaction steps and 19 species. Statistical means and instantaneous quantities show overall good agreement with the experimental data and demonstrate the capability of the pdf method in LES to simulate premixed combustion in complex flame configurations.     

A filtered-laminar-flame PDF sub-grid scale closure for LES of premixed turbulent flames. Part I: Formalism and application to a bluff-body burner with differential diffusion

A sub-grid scale closure for Large Eddy Simulation (LES) of turbulent combustion based on physical-space filtering of laminar flames is discussed. Applied to an unstructured grid, the combustion LES filter size is not fixed in this novel approach devoted to LES with refined meshes, but calibrated depending on the local level of unresolved scalar fluctuations. The context is premixed or stratified flames, the derived model relies on four balance equations for mixture fraction and its variance, and a progress variable and its variance. The proposed formalism is based on a presumed probability density function (PDF) derived from the filtered flames. Closures for the terms of the equations that are unresolved over LES grids are achieved through the PDF. The method uses flamelet tabulated detailed chemistry and is first applied to the simulation of laminar flames (1D and 2D) over various grids for validation, before simulating a turbulent burner studied experimentally by Sweeney et al. (2012). Since this burner also features differential diffusion effects, the numerical model is modified to account for accumulation of carbon in the recirculation zone behind the bluff-body. A differential diffusion number based on the gradient of residence times is proposed, in an attempt to globally quantify differential diffusion effects in burners.     

Generalized flame surface density transport conditional on flow topologies for turbulent H2-air premixed flames in different regimes of combustion

Abstract

The generalized flame surface density (FSD) transport conditional on local flow topologies in premixed turbulent flames has been analyzed based on a detailed chemistry direct numerical simulation database of statistically planar turbulent hydrogen-air premixed flames with an equivalence ratio of 0.7 representing the corrugated flamelets, thin reaction zones and broken reaction zones regimes of combustion. The local flow topologies have been categorized by the values of the three invariants of the velocity gradient tensor and the statistical behaviors of the generalized FSD and different terms of its transport equation conditional on these flow topologies have been analyzed in detail for different choices of the reaction progress variable. The qualitative behavior of the different terms of the generalized FSD transport equation has been found to be similar for different choices of reaction progress variable but the statistical behaviors of the tangential strain rate term and its components have been found to be affected by the regime of combustion. The topologies, which exist for all values of dilatation rate, contribute significantly to the generalized FSD transport in premixed turbulent flames for all regimes of combustion. An unstable nodal flow topology, which is representative of a counter-flow configuration, has been found to be a dominant contributor to the FSD transport for all regimes of combustion irrespective of the choice of reaction progress variable. Moreover, a focal topology which is obtained only for positive values of dilatation rate, has been found to contribute significantly, especially to the curvature and propagation terms of the FSD transport equation for all regimes of combustion including the broken reaction zones regime. However, the contributions of the flow topologies to the turbulent transport and tangential strain rate term, which are obtained only for positive dilatation rates, have been found to weaken from the corrugated flamelets to the broken reaction zones regime.     

部分预混旋流燃烧的大涡模拟

采用耦合涡耗散概念模型的大涡模拟方法,探究了Re和组分变化对部分预混旋流火焰动力学特性的影响。通过与实验结果定性和定量的比较,验证了大涡程序模拟燃烧过程的可靠性。计算结果显示Re的增加,会明显提高空-燃混合效率,从而导致部分预混火焰中预混燃烧模式的比例有所增加,且预混燃烧区域向上游移动,Re的增加也会使得火焰下游产生更多更快的涡破碎结构。N_2含量的增加,会减小流向回流区尺寸,降低空-燃混合效率,但对减小火焰温度具有明显效果,从而对降低NO_x排放产生积极作用。结论为进一步研究部分预混旋流燃烧室的不稳定性及燃烧效率提供了理论和方法上的指导。     

A consistent level set formulation for large-eddy simulation of premixed turbulent combustion

A consistent formulation of the G-equation approach for LES is developed. The unfiltered G equation is valid only at the instantaneous flame front location. Hence, in a filtering procedure applied to derive the appropriate LES equation, only the instantaneous unfiltered flame surface can be considered. A new filter kernel is provided, which averages along the flame surface. The filter kernel is used to derive the G equation for the filtered flame front location. This equation has two unclosed terms, involving a flame front conditional averaged flow velocity, and a filtered propagation term. A model for the conditional velocity is derived, expressing this quantity in terms of the Favre-filtered flow velocity, which is typically known from a flow solver. This model leads to the appearance of a density ratio in the propagation term of the G equation. LES of combustion in the thin reaction zones regime is discussed in the LES regime diagram. A new line is identified separating the thin reaction zones regime into two parts, where the broadened flame thickness is larger and smaller than the filter size, respectively. A model for the propagation term is provided. This leads to a term including the subfilter turbulent burning velocity and an additional term proportional to the resolved flame front curvature. For the former, an algebraic model is provided from an equation for the subfilter flame front wrinkling. The latter term depends on the inverse of the subfilter Damköhler number and disappears in the corrugated flamelets regime.     

Numerical analysis of the Cambridge stratified flame series using artificial thickened flame LES with tabulated premixed flame chemistry

Detailed comparisons of LES results against measurement data are presented for the turbulent lean and rich stratified Cambridge flame series. The co-annular methane/air burner with a central bluff body for flame stabilization has been investigated experimentally by Sweeney et al.  and . Three cases with varying levels of stratification in the lean and rich combustion regime are taken into account. Turbulent combustion is modeled by using the artificial thickened flame (ATF) approach in combination with flamelet generated manifolds (FGM) lookup tables. The model is adapted for stratified combustion and an alternative formulation for the flame sensor is presented. Three different grids are used to investigate the influence of the filter width and the sub-filter modeling on the overall results. Velocities, temperatures, equivalence ratios, and major species mass fractions predictions are compared with measurements for three different stratification rates and an overall good overall agreement was found between simulation and experiment. Some deviations occur near the bluff body, which are analyzed further by evaluation of atomic and species mass fractions. The stratified combustion process was further investigated and characterized by probability density functions extracted from the simulation results.     

Numerical computations and optical diagnostics of unsteady partially premixed methane/air flames

The structures and dynamics of unsteady laminar partially premixed methane/air Bunsen flames are studied by means of numerical simulations, OH and CH PLIF imaging, and high speed chemiluminescence imaging employing a high framing speed intensified charge coupled device camera. The Bunsen burner has a diameter of 22 mm. Rich methane/air mixtures with an equivalence ratio of 1.5 are injected from the burner into atmosphere at different flow speeds ranging from 0.77 to 1.7 m/s, with Reynolds numbers based on the nozzle flow ranging from 1100 to 2500. The numerical simulations are based on a two-scalar flamelet manifold tabulation approach. Detailed chemistry is used to generate the flamelet manifold tabulation which relates the species concentrations, reaction rates, temperature and density to a distance function G and mixture fraction Z. Two distinct reaction zones are identified using CH and OH PLIF imaging and numerical simulations; one inner reaction zone corresponds to premixed flames on the rich side of the mixture and one outer reaction zone corresponds to mixing controlled diffusion flames on the lean side of the mixture. Under normal gravity conditions both the inner premixed flames and the outer diffusion flames are unsteady. The outer diffusion flames oscillate with a flickering frequency of about 15 Hz, which slightly increases with the burner exit velocity. The inner premixed flames are more random with much more small-scale wrinkling structures. Under zero gravity conditions the outer diffusion flames are stable whereas the inner premixed flames are unstable and highly wrinkled. It appears that the outer diffusion flames are governed by the Rayleigh-Taylor instability whereas the inner premixed flames are dictated by Landau-Darrieus instability. The two-scalar flamelet approach is shown to capture the basic structures and dynamics of the investigated unsteady partially premixed flames.     

Strained flamelets for turbulent premixed flames II: Laboratory flame results

The predictive ability of strained flamelets model for turbulent premixed flames is assessed using Reynolds Averaged Navier Stokes (RANS) calculations of laboratory flames covering a wide range of conditions. Reactant-to-product (RtP) opposed flow laminar flames parametrised using the scalar dissipation rate of reaction progress variable are used as strained flamelets. Two turbulent flames: a rod stabilised V-flame studied by Robin et al. [Combust. Flame 153 (2008) 288-315] and a set of pilot stabilised Bunsen flames studied by Chen et al. [Combust. Flame 107 (1996) 223-244] are calculated using a single set of model parameters. The V-flame corresponds to the corrugated flamelets regime. The strained flamelet model and an unstrained flamelet model yield similar predictions which are in good agreement with experimental measurements for this flame. On the other hand, for the Bunsen flames which are in the thin reaction zones regime, the unstrained flamelet model predicts a smaller flame brush compared to experiment. The predictions of the strained flamelets model allowing for fluid-dynamics stretch induced attenuation of the chemical reaction are in good agreement with the experimental data. This model predictions of major and minor species are also in good agreement with experimental data. The results demonstrate that the strained flamelets model using the scalar dissipation rate can be used across the combustion regimes.     

Cold flow characteristics of a novel bluff body hydrogen burner

The cold flow characteristics of a novel partial premixed bluff body (PPBB) low NO_x burner, capable of operating with hydrogen as well as methane-hydrogen blends, were investigated numerically. The PPBB burner features a frustum shaped conical bluff body generating a flame stabilizing recirculation zone. Fuel is partially premixed via jets in an accelerating cross-flow. Steady-state and transient non-reacting simulations using five different turbulence models, i.e. standard k-ε, realizable k-ε, shear stress transport (SST) k-ω, stress-blended eddy simulation (SBES) and large eddy simulation (LES), were conducted. The simulations were validated against particle image velocimetry (PIV) measurements of an unconfined non-reacting flow. All turbulent models were able to predict the recirculation zone length in good agreement with the experimental data. However, only scale resolving simulations could reproduce velocity magnitudes with sufficient accuracy. Time averaged and instantaneous results from the scale resolving simulation were analysed in order to investigate flow characteristics that are special about the PPBB burner design and of relevance for the combustion process. Two different burner configurations were studied and their effects on the flow field were examined. The recirculation zone volume as well as the entrainment into the wall jet around the bluff body were found to correlate with the elevation of the bluff body relative to the burner throat. Both of these parameters are expected to have a strong impact on the overall NO_x emission, since the near burner region is typically one of the main contributors to the NO_x formation.     

Free radical imaging techniques applied to hydrocarbon flames diagnosis

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Premixed and nonpremixed generated manifolds in large-eddy simulation of Sandia flame D and F

Premixed and nonpremixed flamelet-generated manifolds have been constructed and applied to large-eddy simulation of the piloted partially premixed turbulent flames Sandia Flame D and F. In both manifolds the chemistry is parameterized as a function of the mixture fraction and a progress variable. Compared to standard nonpremixed flamelets, premixed flamelets cover a much larger part of the reaction domain. Comparison of the results for the two manifolds with experimental data of flame D show that both manifolds yield predictions of comparable accuracy for the mean temperature, mixture fraction, and a number of chemical species, such as CO₂. However, the nonpremixed manifold outperforms the premixed manifold for other chemical species, the most notable being CO and H₂. If the mixture is rich, CO and H₂ in a premixed flamelet are larger than in a nonpremixed flamelet, for a given value of the progress variable. Simulations have been performed for two different grids to address the effect of the large-eddy filter width. The inclusion of modeled subgrid variances of mixture fraction and progress variable as additional entries to the manifold have only small effects on the simulation of either flame. An exception is the prediction of NO, which (through an extra transport equation) was found to be much closer to experimental results when modeled subgrid variances were included. The results obtained for flame D are satisfactory, but despite the unsteadiness of the LES, the extinction measured in flame F is not properly captured. The latter finding suggests that the extinction in flame F mainly occurs on scales smaller than those resolved by the simulation. With the presumed β-pdf approach, significant extinction does not occur, unless the scalar subgrid variances are overestimated. A thickened flame model, which maps unresolved small-scale dynamics upon resolved scales, is able to predict the experimentally observed extinction to some extent.     

LES modeling of premixed combustion using a thickened flame approach coupled with FGM tabulated chemistry

Flamelet Generated Manifolds (FGM) tabulated chemistry is used in combination with a thickened flame approach to perform Large Eddy Simulation (LES) of premixed combustion. Two-dimensional manifolds are used to describe the chemistry by the mixture fraction and progress variable. Simulations of one-dimensional flames have been used to verify the coupling of the tabulated chemistry and the LES solver where important features like the grid dependence of flame propagation are carefully addressed. Finally, the method is applied to the turbulent flame of a premixed swirl burner including the complex geometry of the swirl nozzle. Results of the velocity, species and temperature are compared with experimental data. Thereby different efficiency functions are used to show the sensitivity related to this model parameter. Some aspects regarding dynamic thickening, numerical accuracy and computational efficiency are also addressed.