Context Of Large Eddy Simulation Research Articles

Direct Numerical Simulation of Partial Fuel Stratification Assisted Lean Premixed Combustion for Assessment of Hybrid G-Equation/Well-Stirred Reactor Model

Abstract Partial fuel stratification (PFS) is a promising fuel injection strategy to stabilize lean premixed combustion in spark-ignition (SI) engines. PFS creates a locally stratified mixture by injecting a fraction of the fuel, just before spark timing, into the engine cylinder containing homogeneous lean fuel/air mixture. This locally stratified mixture, when ignited, results in complex flame structure and propagation modes similar to partially premixed flames and allows for faster and more stable flame propagation than a homogeneous lean mixture. This study focuses on understanding the detailed flame structures associated with PFS-assisted lean premixed combustion. First, a two-dimensional direct numerical simulation (DNS) is performed using detailed fuel chemistry, experimental pressure trace, and realistic initial conditions mapped from a prior engine large-eddy simulation (LES), replicating practical lean SI operating conditions. DNS results suggest that the conventional triple flame structure is prevalent during the initial stage of flame kernel growth. Both premixed and nonpremixed combustion modes are present with the premixed mode contributing dominantly to the total heat release. Detailed analysis further reveals the effects of flame stretch and fuel pyrolysis on flame displacement speed. Based on the DNS findings, the accuracy of a hybrid G-equation/well-stirred reactor (WSR) combustion model is assessed for the PFS-assisted lean operation in the LES context. It is found that the G-equation model qualitatively captures the premixed branches of the triple flame, while the WSR model predicts the nonpremixed branch of the triple flame. Finally, potential needs for improvements to the hybrid G-equation/WSR modeling approach are discussed.

Effect of flame–flame interaction on scalar PDF in turbulent premixed flames

Direct numerical simulation (DNS) results are analysed to investigate the effect of flame–flame interaction on the scalar distribution in a large eddy simulation context. The DNS data consist of turbulent premixed planar flame, V-flame and swirl flame cases with different turbulence and equivalence ratio, and relatively large Damköhler number conditions are considered for the three DNS cases. The volumetric fraction of non-flamelet type reaction zones Φ¯, which are caused by flame–flame interaction (FFI), is identified by the trained neural network. The quantification of scalar PDF mode is carried out by means of the bimodality coefficient. Various scalar PDFs are constructed with different sample volumes of the filter size. The PDFs with high bimodality coefficient samples show clear bimodal distributions, whereas low bimodality coefficient samples show unimodal or plateau distributions. The conditional PDF and conditional average of the bimodality coefficient conditioned based on Φ¯ clearly show negative correlation between these two quantities. These results suggest that the presence of FFI events leads to bimodality loss of the scalar field, even when the Damköhler number is large. Reaction zones with non-bimodal scalar distribution are also found to have very small scalar gradient. Thus, the reaction rate of such reaction zones could be underestimated in flamelet-type modelling. However, such deficit may be straightforwardly complemented additively, for example, by considering a zero-dimensional canonical reactor with the fraction Φ¯.

Implementation of a probabilistic surface density volume of fluid approach for spray atomisation

Eulerian Stochastic methods in Large Eddy Simulation context have been successfully used in reactive flows. They have been recently applied to spray atomisation, using surface and liquid volume as independent variables. These methods involve the solutions of Stochastic Partial Differential Equations, which are continuous in space and discontinuous in time. The implementation of these equations into a conventional finite volume solver can be challenging due to both the numerical diffusion of the scalar fields and the numerical integration of the stochastic source terms. This work presents the implementation and validation of the Eulerian Stochastic Fields surface/liquid volume atomisation method within the OpenFOAM framework. The paper discusses how algebraic compression methods can be used for the stochastic fields to minimise the numerical diffusion and the effects of the stochastic source term implementation. The model targets a-priori spray atomisation at large Weber numbers, although the implementation recovers the correct solution at low Weber model and captures correctly the capillary effects on liquid–gas interfaces. A turbulent liquid jet is used for validation and discussion of the accuracy and relative cost of the model implementation. The approach is then applied to a multi-hole gasoline direct injector where the agreement with experimental data showcases the capabilities of the solver for realistic injectors and atomisation regimes.

Mixing in Turbulent Flows: An Overview of Physics and Modelling

Turbulent flows featuring additional scalar fields, such as chemical species or temperature, are common in environmental and industrial applications. Their physics is complex because of a broad range of scales involved; hence, efficient computational approaches remain a challenge. In this paper, we present an overview of such flows (with no particular emphasis on combustion, however) and we recall the major types of micro-mixing models developed within the statistical approaches to turbulence (the probability density function approach) as well as in the large-eddy simulation context (the filtered density function). We also report on some trends in algorithm development with respect to the recent progress in computing technology.

A-Priori Assessment of Interfacial Sub-grid Scale Closures in the Two-Phase Flow LES Context

Due to the continuous increase in available computing power, the Large Eddy Simulation (LES) of two-phase flows started to receive more attention in recent years. Well-established models from single-phase flows are often used to close the sub-grid scale convective momentum transport and recently some modifications have been suggested to account for the jump of density and viscosity at the interface of multi-phase flows. However, additional unclosed terms in multi-phase flows, which are absent in single-phase flows, often remain ignored. This paper focuses on the crucial gaps in literature, namely the modeling of volume fraction advection and surface tension effects on sub-grid level. An a-priori analysis has been conducted for this purpose, i.e. the Direct Numerical Simulation of an academic two-phase flow configuration (single wobbling bubble in a turbulent background flow) has been explicitly filtered (corresponding to implicit filtering in actual LES) for varying filter width and the corresponding sub-grid terms have been compared to potentially suitable model expressions. Besides other approaches, adequately formulated models based on the scale similarity principle emerged to be promising candidates for both sub-grid volume fraction advection as well as sub-grid surface tension effects. In this context, special attention has to be paid to the secondary filter. Owing to the nature of the quasi-singular surface tension term, surface-weighted filtering may be more appropriate and robust than standard volume filtering.

Filtered Density Function Simulations of a Near-Limit Turbulent Lean Premixed Flame

The predictive numerical simulation of near-limit turbulent premixed combustion, in which the turbulent intensity is high and the fuel/air mixture is near the flammability limit, remains challenging. In this study, large-eddy simulation (LES)/filtered density function (FDF) simulations of a high-speed piloted premixed jet burner flame are performed to illustrate the importance of the resolved level of reactive scalars and micromixing modeling on predicting the overall combustion process. The simulations with increased resolved levels of reactive scalars yield notably improved predictions throughout the extinction–reignition region of this flame. The sources of grid sensitivity are thoroughly examined through the quantification of the resolved levels of the velocity and scalar fields, as well as the mixing-reaction budgets. Subgrid mixing is identified as the most prominent factor for grid sensitivity. The effects of mixing timescale modeling are investigated and quantified through a parametric study of the mixing rate parameter and the development of an augmented hybrid mixing timescale model in the LES context. As far as the mixing formulation is concerned, the Euclidean minimum spanning tree model shows a lower level of grid dependence than the interaction by exchange with the mean model due to the enforcement of localness in composition space.

Coarse large-eddy simulations in a transitional wake flow with flow models under location uncertainty

The focus of this paper is to perform coarse-grid large-eddy simulation (LES) using recently developed sub-grid scale (SGS) models of cylinder wake flow at Reynolds number (Re) of 3900. As we approach coarser resolutions, a drop in accuracy is noted for all LES models but more importantly, the numerical stability of classical models is called into question. The objective is to identify a statistically accurate, stable sub-grid scale (SGS) model for this transitional flow at a coarse resolution. The proposed new models under location uncertainty (MULU) are applied in a deterministic coarse LES context and the statistical results are compared with variants of the Smagorinsky model and various reference data-sets (both experimental and Direct Numerical Simulation (DNS)). MULU are shown to better estimate statistics for coarse resolution (at 0.46% the cost of a DNS) while being numerically stable. The performance of the MULU is studied through statistical comparisons, energy spectra, and sub-grid scale (SGS) contributions. The physics behind the MULU are characterised and explored using divergence and curl functions. The additional terms present (velocity bias) in the MULU are shown to improve model performance. The spanwise periodicity observed at low Reynolds is achieved at this moderate Reynolds number through the curl function, in coherence with the birth of streamwise vortices.

Large eddy simulation of particle aggregation in turbulent jets

Aggregation is an inter-particle process that involves a multitude of different physical and chemical mechanisms. Aggregation processes often occur within turbulent flows; for example in spray drying, soot formation, or nanoparticle formation. When the concentration of particles is very large, a direct simulation of individual particles is not possible and alternative approaches are needed. The present work follows the stochastic aggregation modelling based on a Lagrangian framework by Pesmazoglou, Kempf, and Navarro-Martinez (2016) and implements it in the Large Eddy Simulation context. The new coupled model is used to investigate particle aggregation in turbulent jets. Two cases are considered: an existent Direct Numerical Simulation of nanoparticle agglomeration in a planar jet and an experimental configuration of SiO2 nanoparticles in a round jet. The results show a good agreement in both cases, demonstrating the advantages of the Lagrangian framework to model agglomeration and it capacity to describe the full particle size distribution.

Modelling of Landau–Darrieus and thermo-diffusive instability effects for CFD simulations of laminar and turbulent premixed combustion

An algebraic model is derived that accounts for the effects of non-resolved Landau–Darrieus and thermo-diffusive instabilities on the propagation speed of fully premixed laminar and turbulent flame fronts in the Large Eddy Simulation (LES) context provided that the laminar flame speed appears as a model parameter in the LES combustion model. The model is derived assuming fractal characteristics of flames which exhibit cellular structures due to instabilities. The smallest and largest unstable wavelengths are computed employing a dispersion relation for nominally planar flames. Values for the fractal dimension characterising the flame structures are taken from the literature. A phenomenological model accounts for the stabilising effect of strain. Based on experimental data, a correlation for a critical strain rate, which indicates the onset of instabilities, is formulated. To validate the new model which accounts for instabilities on the effective speed of laminar flame propagation, laminar expanding spherical methane–air flames at p = 5 bar and p = 10 bar are simulated in the LES context. Values for the fractal dimension, as proposed in the literature, are varied. The predicted flame propagation speed is in very good agreement with experimental data when applying a fractal dimension of about D = 2.06. The critical strain turns out to be a suitable parameter to indicate the onset of instabilities and to quantify the influence of instabilities. Simulations applying a second model proposed by Bradley and valid for spherically expanding flames show similar results. LES of turbulent Bunsen flames at 1, 5 and 10 bar, which are characterised by u′/s0L < 1, are performed to evaluate the derived instability model for turbulent flames. The simulated flames (from the Kobayashi database) have already been experimentally investigated in the context of Landau–Darrieus and thermo-diffusive instabilities. In agreement with conclusions from these investigations, for the considered cases the new model does not predict any onset of instabilities.

MODELS FOR THE LARGE EDDY SIMULATION EQUATIONS TO DESCRIBE MULTI-SPECIES MIXING OCCURING AT SUPERCRITICAL PRESSURE

An existing database of direct numerical simulations (DNS) representing mixing of species under supercritical pressure (supercritical-ρ) conditions has been investigated for the purpose of understanding the modeling of the gradient of the filtered pressure, the divergence of the filtered heat flux, and the divergence of the filtered species mass flux, all in the context of large eddy simulation (LES). The analysis consists of two separate parts. The activities of all terms appearing in the LES equations are first evaluated, and the dominant terms for each of the transport equations are identified. These data are used to check whether the standard LES assumptions−i.e., that the three above quantities are equal to the gradient of the pressure and the divergences of the fluxes computed from the filtered flow field, respectively−that are routinely used for atmospheric-ρ flows, continue to be valid also in the realm of supercritical-ρ conditions. Having found that these assumptions do not hold under supercritical-ρ conditions, alternative modeling strategies for these terms are proposed, and their accuracy with respect to the standard LES assumptions is assessed.

LES of Low to High Turbulent Combustion in an Elevated Pressure Environment

A subgrid scale flame surface density combustion model for the Large Eddy Simulation (LES) of premixed combustion is derived and validated. The model is based on fractal characteristics of the flame surface, assuming a self similar wrinkling of the flame between smallest and largest wrinkling length scales. Experimental and direct numerical simulation databases as well as theoretical models are used to derive a model for the fractal parameters, namely the cut-off lengths and the fractal dimension suitable in the LES context. The combustion model is designed with the intent to simulate low as well as high Reynolds number premixed turbulent flame propagation and with a focus on correct scaling with pressure. The combustion model is validated by simulations of turbulent Bunsen flames with methane and propane fuel at pressure levels between 0.1 MPa and 2 MPa and at turbulence levels of $0 < u^{\prime }/s_{L}^{0} < 11$ , conditions typical for spark ignition engines. The predicted turbulent flame speed is in a very good agreement with the experimental data and a smooth transition from resolved flame wrinkling to fully modelled, nearly subgrid-only wrinkling is realized. Evaluating the influence of mesh resolution shows a predicted mean flame surface and turbulent flame speed independent of mesh resolution for cases with 9–86 % resolved flame surface. Additional simulations of a highly turbulent jet flame at 0.1 MPa and 0.5 MPa and the comparison with experimental data in terms of flame shape, velocity field and turbulent fluctuations validates the model also at conditions typical for gas turbines.

Large Eddy Simulation of Autoignition in a Turbulent Hydrogen Jet Flame Using a Progress Variable Approach

The potential of a progress variable formulation for predicting autoignition and subsequent kernel development in a nonpremixed jet flame is explored in the LES (Large Eddy Simulation) context. The chemistry is tabulated as a function of mixture fraction and a composite progress variable, which is defined as a combination of an intermediate and a product species. Transport equations are solved for mixture fraction and progress variable. The filtered mean source term for the progress variable is closed using a probability density function of presumed shape for the mixture fraction. Subgrid fluctuations of the progress variable conditioned on the mixture fraction are neglected. A diluted hydrogen jet issuing into a turbulent coflow of preheated air is chosen as a test case. The model predicts ignition lengths and subsequent kernel growth in good agreement with experiment without any adjustment of model parameters. The autoignition length predicted by the model depends noticeably on the chemical mechanism which the tabulated chemistry is based on. Compared to models using detailed chemistry, significant reduction in computational costs can be realized with the progress variable formulation.

A new LES model coupling flame surface density and tabulated kinetics approaches to investigate knock and pre-ignition in piston engines

Downsizing currently appears as a promising technique for reducing the CO 2 emissions of spark-ignition engines. Nevertheless boost levels are limited by the occurrence of uncontrolled auto-ignitions of fresh gases. These latter are sporadic and can have localized origins. The large-eddy simulation (LES) theoretically seems to be a promising numerical approach to study these abnormal combustion processes as a part of the turbulence spectrum is resolved and the effects of cycle-to-cycle variations are taken into account. In this context, a dedicated model integrating the auto-ignition model TKI (Tabulated Kinetics for Ignition) and ECFM-LES (Extended Coherent Flame for LES) is first detailed. It is then applied to the modeling of simple test cases representative of phenomena encountered during abnormal combustions. Finally, a qualitative study is performed to verify that well-known experimental trends related to knock can be retrieved numerically in a LES context. To this purpose, several operating points for a spark-ignition engine are computed. Signals of in-cylinder pressures that have the same appearance as those found in the literature are obtained. Retarding spark timing enables numerically to limit the occurrence of knock, as commonly used in engine control. A pre-ignition case is also regarded and shows the possibility to use the proposed model for further quantitative studies of this phenomenon.

A seamless hybrid RANS-LES model based on transport equations for the subgrid stresses and elliptic blending

The aim of the present work is to develop a seamless hybrid Reynolds-averaged Navier–Stokes (RANS) large-eddy simulation (LES) model based on transport equations for the subgrid stresses, using the elliptic-blending method to account for the nonlocal kinematic blocking effect of the wall. It is shown that the elliptic relaxation strategy of Durbin is valid in a RANS (steady) as well as a LES context (unsteady). In order to reproduce the complex production and redistribution mechanisms when the cutoff wavenumber is located in the productive zone of the turbulent energy spectrum, the model is based on transport equations for the subgrid-stress tensor. The partially integrated transport model (PITM) methodology offers a consistent theoretical framework for such a model, enabling to control the cutoff wavenumber κc, and thus the transition from RANS to LES, by making the Cε2 coefficient in the dissipation equation of a RANS model a function of κc. The equivalence between the PITM and the Smagorinsky model is shown when κc is in the inertial range of the energy spectrum. The extension of the underlying RANS model used in the present work, the elliptic-blending Reynolds-stress model, to the hybrid RANS-LES context, brings out some modeling issues. The different modeling possibilities are compared in a channel flow at Reτ=395. Finally, a dynamic procedure is proposed in order to adjust during the computation the dissipation rate necessary to drive the model toward the expected amount of resolved energy. The final model gives very encouraging results in comparison to the direct numerical simulation data. In particular, the turbulence anisotropy in the near-wall region is satisfactorily reproduced. The contribution of the resolved and modeled fields to the Reynolds stresses behaves as expected: the modeled part is dominant in the near-wall zones (RANS mode) and decreases toward the center of the channel, where the relative contribution of the resolved part increases. Moreover, when the mesh is modified, the amount of resolved energy changes but the total Reynolds stresses remain nearly constant.

Large-Eddy Simulation of Lean Premixed Turbulent Flames of Three Different Combustion Configurations using a Novel Reaction Closure

This large eddy simulation (LES) study is applied to three different premixed turbulent flames under lean conditions at atmospheric pressure. The hierarchy of complexity of these flames in ascending order are a simple Bunsen-like burner, a sudden-expansion dump combustor, and a typical swirl-stabilized gas turbine burner–combustor. The purpose of this paper is to examine numerically whether the chosen combination of the Smagorinsky turbulence model for sgs fluxes and a novel turbulent premixed reaction closure is applicable over all the three combustion configurations with varied degree of flow and turbulence. A quality assessment method for the LES calculations is applied. The cold flow data obtained with the Smagorinsky closure on the dump combustor are in close proximity with the experiments. It moderately predicts the vortex breakdown and bubble shape, which control the flame position on the double-cone burner. Here, the jet break-up at the root of the burner is premature and differs with the experiments by as much as half the burner exit diameter, attributing the discrepancy to poor grid resolution. With the first two combustion configurations, the applied subgrid reaction model is in good correspondence with the experiments. For the third case, a complex swirl-stabilized burner–combustor configuration, although the flow field inside the burner is only modestly numerically explored, the level of flame stabilization at the junction of the burner–combustor has been rather well captured. Furthermore, the critical flame drift from the combustor into the burner was possible to capture in the LES context (which was not possible with the RANS plus k–ɛ model), however, requiring tuning of a prefactor in the reaction closure.

Validation of large-eddy simulation of strongly curved stationary and rotating U-duct flows

Numerical predictions of the developing turbulent flow through stationary and rotating U-ducts with strong curvature are presented using large-eddy simulation (LES). The primary aim is to validate LES in a strongly curved U-duct for three different cases: stationary (non-rotating), positive and negative rotational cases. Selected flow models have provided a challenging test for the methodology adopted in terms of boundary conditions, wall functions and subgrid-scale (SGS) models, which are still regarded as crucial research topics within the context of LES. The agreement between the predictions and experimental data is quite satisfactory considering the coarse grid adopted. Discrepancies are observed to diminish near the lateral walls due to the relatively fine grid resolution. It is seen that LES performs better than the two-component-limit (TCL) RANS model in predicting the quantitative features of the turbulent flow through a U-duct with a non-conforming mesh adaptation.

Towards large eddy simulation of combustion in spark ignition engines

Internal combustion engine simulations are commonly performed using the RANS (Reynolds averaged Navier–Stokes) approach. It gives a correct estimates of global quantities but is by nature not adapted to describe phenomena strongly linked to cyclic variations. On the other hand, large eddy simulation (LES) is a promising technique to determine successive engine cycles. This work demonstrates the feasibility of LES engine cycles simulation by using a flame surface density (FSD) approach. This approach, presented in a first section, combines an Eulerian spark ignition model derived from the RANS AKTIM model [J.M. Duclos, O. Colin, Arc and Kernel Tracking Ignition Model for 3D SI Engines Calculations, Comodia, Nagoya, Japan, 2001, pp. 343–350] and a Coherent Flame Model (CFM) [S. Candel, T. Poinsot, Combust. Sci. Tech. 70 (1990) 1–15; O. Colin, A. Benkenida, C. Angelberger, Oil & Gas Sci. Techn.— Rev. IFP 58 (1) (2003) 47–32] describing the flame propagation. The CFM model, commonly used in RANS simulations, is here formulated in a LES context. In a second part, the whole ignition-combustion model is validated against an experiment relative to the turbulent ignition and flame propagation of a stoichiometric propane-air mixture [B. Renou, A. Boukhalfa, Combust. Sci. Tech. 162 (2001) 347–371]. Finally, LES engine cycles simulations are performed on a real engine configuration. First, the sensitivity of the model to the LES combustion filter size Δ ˆ is examined, showing a weak dependence of the modelling approach to Δ ˆ . Then results are compared to those obtained with the algebraic model for the FSD proposed by Boger et al. [M. Boger, D. Veynante, H. Boughanem, A. Trouvé, Proc. Combust. Inst. 27 (1998) 917–925] and the need for non-equilibrium combustion models is demonstrated.

Turbulent mixing in a tubular reactor: Assessment of an FDF/LES approach

AbstractThe aim of the present research was to investigate the feasibility of a probability density function (PDF) methodology combined with a large eddy simulation (LES) for turbulent reactive mixing in industrial geometries. In order to incorporate the PDF transport equation into a LES context, a filtered version was employed, denoted as filtered density function (FDF) equation. Using FDF/LES, the yield of mixing‐sensitive, parallel competitive reactants was numerically studied in a tubular reactor with perpendicular inlet operated at Re = 4,000 for varying Damköhler numbers and feed‐stream concentrations. The “interaction by exchange with the mean” (IEM) model was used to close the filtered conditional scalar energy dissipation rate (SED) appearing in the FDF equation. The modeling assumptions were assessed by means of laser induced fluorescence (LIF) experimental data, providing the mean conserved concentration field in a horizontal and vertical center plane downstream the injector, and concentration PDF's at several downstream positions. In the vicinity of the injector, the FDF/LES model slightly overpredicts both the spreading of jet and the dissipation of small scale fluctuations, which was attributed to the overprediction of the turbulent diffusivity in the transition region of the laminar jet to the the turbulent wake of the injector. Further downstream, however, the global macro transport of the reactants was qualitatively well predicted, and the sensitivity of the yield to the Damköhler number and feedstream concentration showed consistent behavior. © 2005 American Institute of Chemical Engineers AIChE J, 51: 725–739, 2005

On the Use of Shock-Capturing Schemes for Large-Eddy Simulation

Numerical simulations of freely decaying isotropic fluid turbulence were performed at various Mach numbers (from 0.2 to 1.0) using known shock-capturing Euler schemes (Jameson, TVD-MUSCL, ENO) often employed for aeronautical applications. The objective of these calculations was to evaluate the relevance of the use of such schemes in the large-eddy simulation (LES) context. The potential of the monotone integrated large-eddy simulation (MILES) approach was investigated by carrying out computations without viscous diffusion terms. Although some known physical trends were respected, it is found that the small scales of the simulated flow suffer from high numerical damping. In a quasi-incompressible case, this numerical dissipation is tentatively interpreted in terms of turbulent dissipation, yielding the evaluation of equivalent Taylor micro-scales. The Reynolds numbers based on these are found between 30 and 40, depending on the scheme and resolution (up to 1283). The numerical dissipation is also interpreted in terms of subgrid-scale dissipation in a LES context, yielding equivalent Smagorinsky “constants” which do not level off with time and which remain larger than the commonly accepted values of the classical Smagorinsky constant. On the grounds of tests with either the Smagorinsky or a dynamic model, the addition of explicit subgrid-scale (SGS) models to shock-capturing Euler codes is not recommended.