Large Eddy Simulation of Spark Ignition in a Gas Turbine Combustor

Ignition in an aircraft gas turbine combustion chamber is simulated using Large Eddy Simulation (LES) in conjunction with the filtered probability density function (pdf) equation approach, which is solved using the Eulerian stochastic field method. The LES-pdf methodology is used for both dispersed (liquid) and gas phases. The liquid phase is described using a Lagrangian formulation whilst an Eulerian approach is employed for the gas phase. The spark energy deposition was mimicked by a distributed energy source term added to the enthalpy equation. Unsuccessful and successful ignition sequences have been simulated and the results suggest that spark ‘size’ is an important parameter in the ignition of kerosene fuelled combustion chambers.     

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.     

Confinement-Induced Instabilities in a Jet-Stabilized Gas Turbine Model Combustor

Self-sustained jet flapping is observed in a confined, premixed and preheated methane-air turbulent flame, generated in a single-nozzle jet-stabilized gas turbine model combustor designed based on the FLOX ^? concept. The flapping frequency and its complex motion within the confinement of the combustor are characterized in detail using proper orthogonal decomposition (POD) of the flow fields measured by particle imaging velocimetry (PIV). The influence of jet flapping on combustion stability is examined using simultaneous PIV/OH chemiluminescence imaging and PIV/planar laser-induced fluorescence of OH radicals (OH PLIF) at 5 kHz repetition rate. By influencing the size and location of the recirculation zones, jet flapping modifies the flame shape and flame lift-off height. It also controls the amount of hot gas entrainment into the recirculation zones. In extreme cases, jet flapping is found to cause temporary local extinction of the flame, due to jet impingement on the combustor wall and partial blockage of burned gas entrainment. The flame is only able to recover after the jet detaches from the wall and reopens the back flow channel. The results suggest that jet flapping could play a key role in the stabilization mechanisms in similar jet-stabilized combustors.     

Large Eddy Simulations of a Small-Scale Flameless Combustor by Means of Diluted Homogeneous Reactors

A new model for Flameless Combustion (FC) based on the tabulation of diluted homogeneous reactors (DHR) is presented. This model is developed within the Large Eddy Simulations (LES) approach because LES has a good potential for correctly predicting the ternary mixing of FC. In DHR, a ternary mixture of fuel, air and burnt gases at equilibrium is considered as an initial condition for the reactor calculations. The auto-ignition of this mixture is then tabulated as a function of the input parameters which are mixture fraction, fresh gases temperature, dilution fraction, progress variable and enthalpy loss. The enthalpy loss is introduced by decreasing the temperature of the diluting burnt gases. The DHR model is first evaluated over the partially premixed Sandia Flame D. Correct results are obtained for this flame, although CO is overestimated by the model. This discrepancy is attributed to the usage of homogeneous reactors which impedes to account for the influence of scalar dissipation. Secondly, the flameless configuration of Verissimo et al., characterized by a strong enthalpy loss due to wall heat losses, is used to assess the performance of the model. Combustion results are found in correct agreement for temperature and major species. The largest discrepancies are found for CO again, although the axial shape for this species is correctly predicted.     

Modeling of Liquid Fuel Injection, Evaporation and Mixing in a Gas Turbine Burner Using Large Eddy Simulations

The interaction of turbulence, temperature fluctuation, liquid fuel transport, mixing and evaporation is studied by using Large Eddy Simulations (LES). To assess the accuracy of the different components of the methods we consider first isothermal, single phase flow in a straight duct. The results using different numerical methods incorporating dynamic Sub-Grid-Scale (SGS) models are compared with DNS and experimental data. The effects of the interactions among turbulence, temperature fluctuation, spray transport, evaporation and mixing of the gaseous fuel are studied by using different assumptions on the temperature field. It has been found that there are strong non-linear interactions among temperature-fluctuation, evaporation and turbulent mixing which require additional modeling if not full LES is used. The developed models and methods have been applied to a gas turbine burner into which liquid fuel is injected. The dispersion of the droplets in the burner is described. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of Axisymmetric Assumptions on Large Eddy Simulations of a Confined Jet and a Swirl Flow

Axisymmetric geometries can be found in many practical flow applications. In the attempt to predict these flows numerically, RANS flow solvers can decrease the computational efforts dramatically by taking this axisymmetry into account and by computing only a pie-segment of the flow. However, the extension of the concept of axisymmetric flows to LES computations is not straightforward, since the boundary conditions on the axis of symmetry are altering the instantaneous flow field. In this study, the influence of the introduction of an axis of symmetry to LES computations is assessed by computations of a flow with and without swirl over an axisymmetric expansion. The LES computations are performed on a full three-dimensional and a 90° segment of the geometry. The results are compared and the influence of the axis put into relation with the gain in computational costs.     

模型燃烧室内瞬态紊流的大涡模拟

本文采用三种不同亚网格尺度模型对带有V型稳定器的模型燃烧室二维瞬态紊流流动进行了大涡模拟。并在交错网格系下用SIMPLE算法和混合差分格式求解离散方程。数值研究拟不同型式入口速度分布和不同亚网格尺度模型下模型燃烧室二维瞬态紊流流场。计算结果表明不同入口速度分布和不同亚网格尺度模型对瞬态流场和出口速度分布有一定的影响。本文通过数值模拟,揭示了V型稳定器后旋涡的产生和脱落过程。通过计算结果及实验数据的比较可知,本文采用的亚网格尺度模型可以用来模拟模型燃烧室紊流流场及稳定器后面回流区的流动情况。     

Large Eddy Simulations and Experiments of Flow Around Finite-Height Cylinders

The paper presents experimental and numerical results for the flow around a surface-mounted circular cylinder at the two height-to-diameter ratios of 2.5 and 5. The Reynolds number based on approach flow velocity and cylinder diameter is 43,000 and 22,000 for these two cases and the boundary layer of the approach flow has a thickness of about 10% of the cylinder height. The experiments comprise both flow visualizations with dye and laser Doppler velocimeter measurements of all mean velocity and fluctuation components. The numerical study is performed by an elaborate large eddy simulation on a staggered Cartesian grid using the immersed boundary method. The instantaneous flow behaviour including the shedding is analysed with information based on animations. For the long cylinder alternating shedding is found to occur over most of the height while for the shorter cylinder the shedding is observed mainly near the ground where it is also mostly alternating but intermittently also symmetrical. The mean-flow behaviour is analysed with the aid of streamlines and contour plots of mean-velocity and fluctuation components in various planes and a detailed comparison of LES and LDV results is provided, showing generally good agreement. The LES with very fine resolution near the free end allow a detailed study of the complex flow in this region with owl-face topology on the end wall previously observed in experiments. Behind the cylinder, the longitudinal recirculation region, the downstream development of tip vortices and the emergence of trailing vortices further downstream are analysed. The sum of the results, together with those from previous studies that were reviewed extensively, provides a comprehensive picture of the very complex flow behaviour.     

A 5-D Implementation of FGM for the Large Eddy Simulation of a Stratified Swirled Flame with Heat Loss in a Gas Turbine Combustor

Numerical simulations are foreseen to provide a tremendous increase in gas-turbine burners efficiency in the near future. Modern developments in numerical schemes, turbulence models and the consistent increase of computing power allow Large Eddy Simulation (LES) to be applied to real cold flow industrial applications. However, the detailed simulation of the gas-turbine combustion process remains still prohibited because of its enormous computational cost. Several numerical models have been developed in order to reduce the costs of flame simulations for engineering applications. In this paper, the Flamelet-Generated Manifold (FGM) chemistry reduction technique is implemented and progressively extended for the inclusion of all the combustion features that are typically observed in stationary gas-turbine combustion. These consist of stratification effects, heat loss and turbulence. Three control variables are included for the chemistry representation: the reaction evolution is described by the reaction progress variable, the heat loss is described by the enthalpy and the stratification effect is expressed by the mixture fraction. The interaction between chemistry and turbulence is considered through a presumed beta-shaped probability density function (PDF) approach, which is considered for progress variable and mixture fraction, finally attaining a 5-D manifold. The application of FGM in combination with heat loss, fuel stratification and turbulence has never been studied in literature. To this aim, a highly turbulent and swirling flame in a gas turbine combustor is computed by means of the present 5-D FGM implementation coupled to an LES turbulence model, and the results are compared with experimental data. In general, the model gives a rather good agreement with experimental data. It is shown that the inclusion of heat loss strongly enhances the temperature predictions in the whole burner and leads to greatly improved NO predictions. The use of FGM as a combustion model shows that combustion features at gas turbine conditions can be satisfactorily reproduced with a reasonable computational effort. The implemented combustion model retains most of the physical accuracy of a detailed simulation while drastically reducing its computational time, paving the way for new developments of alternative fuel usage in a cleaner and more efficient combustion.     

A Thickened-Hole Model for Large Eddy Simulations over Multiperforated Liners

Flow instabilities such as Rotating Stall and Surge limit the operating range of centrifugal compressors at low mass-flow rates. Employing compressible Large Eddy Simulations (LES), their generation mechanisms are exposed. Toward low mass-flow rate operating conditions, flow reversal over the blade tips (generated by the back pressure) causes an inflection point of the inlet flow profile. There, a shear-layer induces vortical structures circulating at the compressor inlet. Traces of these flow structures are observed until far downstream in the radial diffuser. The tip leakage flow exhibits angular momentum imparted by the impeller, which deteriorates the incidence angles at the blade tips through an over imposed swirling component to the incoming flow. We show that the impeller is incapable to maintain constant efficiency at surge operating conditions due to the extreme alteration of the incidence angle. This induces unsteady flow momentum transfer downstream, which is reflected as compression wave at the compressor outlet traveling toward the impeller. There, the pressure oscillations govern the tip leakage flow and hence, the incidence angles at the impeller. When these individual self-exited processes occurs in-phase, a surge limit-cycle establishes.     

LES of a Multi-burner Annular Gas Turbine Combustor

In this study, Large Eddy Simulation (LES) has been used to predict the flow, mixing and combustion in both a single burner laboratory gas turbine combustor and in an 18 burner annular combustor, having identical cross sections. The LES results for the single burner laboratory combustor are compared with experimental data for a laboratory model of this combustor, and with other LES predictions, with good agreement. An explicit finite volume based LES model, using the mixed subgrid model together with a partially stirred reactor model for the turbulence chemistry interactions, is used. For the annular combustor, with the swirlers parameterized by jet inflow boundary conditions, we have investigated the influence of the a-priori unknown combustor exit impedance, the influence of the swirler characteristics and the fuel type. The combustion chemistry of methane–air and n-decane–air combustion is modeled by a two-step reaction mechanism, whereas NOx is separately modeled with a one-step mechanism. No experimental data exists for the annular combustor, but these results are compared with the single burner LES and experimental results available. The combustor exit impedance, the swirler- and fuel characteristics all seem to influence the combusting flow through the acoustics of the annular combustor. To examine this in greater detail time-series and eigenmodes of the combustor flow fields are analyzed and comparisons are made also with results from conventional thermoacoustic eigenmode analysis, with reasonable agreement. The flow and pressure distributions in the annular combustor are described in some detail and the mechanisms by which the burners interact are outlined.     

Implementation Issues of the Conditional Moment Closure Model in Large Eddy Simulations

Different ways of transferring information regarding the mixture fraction, its sub-grid scale variance and the scalar dissipation rate are examined in terms of a Large Eddy Simulation (LES)/Conditional Moment Closure (CMC) calculation. In such a simulation, information must be transferred from a fine LES grid to a usually coarser CMC grid. Different options of calculating conditional and unconditional quantities in the CMC resolution are assessed by filtering experimental mixture fraction and scalar dissipation rate data at various resolutions. It was found that when a presumed shape for the Filtered Density Function at the CMC resolution is used, special care must be given to the mixture fraction variance. It was also found that the Amplitude Mapping Closure model can be used for the conditional scalar dissipation rate. LES/CMC with detailed chemistry of a bluff-body stabilised burner was performed using two different ways of calculating the turbulent diffusivity. The structure of the flame is realistic, with little difference noticed when using the two diffusivities.     

Large Eddy Simulations in Turbines: Influence of Roughness and Free-Stream Turbulence

The individual and coupled effects of the incoming free-stream turbulence (FST) and surface roughness on the transition of a separated shear layer over a flat plate is numerically investigated using Large eddy simulation (LES). The upper wall of the test section is inviscid and specifically contoured to impose a streamwise pressure distribution over the flat plate to simulate the suction surface of a low pressure turbine (LPT) blade. The interaction of the streamwise streaks due to FST and roughness with the separated shear layer is captured. The streaks induced by FST are intermittent in nature and the streaks due to roughness are steady for a given topology of the rough surface. Both FST and roughness promoted near-wall mixing. The ‘net effect’ of mixing in the pre-separated region is manifested by a shift the inflection point of the velocity profile towards the wall. This resulted in the upstream shift of the transition point and a significant reduction in the size of the separation bubble. The combined effect of FST and roughness further reduced the size of separation bubble. The streamwise evolution of the boundary layer parameters has been compared for different cases. The potential ‘roughness benefit’ obtained in the case of highly loaded turbine blades in terms of its considerable reduction of profile loss is also shown.     

Large Eddy Simulations of the Flow in the Near-Field Region of a Turbulent Buoyant Helium Plume

Large eddy simulations are conducted in the near-field region of a large turbulent buoyant helium plume. The CFD package FireFOAM is applied to that purpose. The transient and mean flow dynamics are discussed as a function of grid resolution, with and without the use of the standard Smagorinsky subgrid scale (SGS) model. Small scale structures, formed at the edge of the plume inlet due to baroclinic and gravitational mechanisms and subject to flow instabilities, interact with large scale features of the flow, resulting in a puffing cycle. In general, the LES calculations reproduce the main features of the turbulent plume, with better agreement when the Smagorinsky type SGS model is applied. In particular, the puffing cycle is recovered in the simulations with correct frequency. The mean and rms values of the velocity components are well predicted with use of the SGS model, even on relatively coarse meshes. Agreement for the species mass fraction (mean and rms values) is less satisfactory, but in line with results found in the literature.     

Conditional Moment Closure for Large Eddy Simulations

A conditional moment closure (CMC) based combustion model for large-eddy simulations (LES) of turbulent reacting flow is proposed and evaluated. Transport equations for the conditionally filtered species are derived that are consistent with the LES formulation and closures are suggested for the modelling of the conditional velocity, conditional scalar dissipation and the fluctuations around the conditional mean. A conventional β-probability density distribution of the scalar is used together with dynamic modelling for the sub-grid fluxes. The model is validated by comparison of simulations with measurements of a piloted, turbulent methane-air jet diffusion flame.     

Large Eddy and Reynolds-Averaged Navier - Stokes Simulations of Turbulent Flow Over an Airfoil

A Large Eddy Simulation (LES) of turbulent flow over an airfoil near stall is performed. Results of the LES are compared with those of Reynolds-Averaged Navier-Stokes (RANS) simulations using two well-known turbulence models, namely the Baldwin-Lomax model and the Spalart-Allmaras model. The subgrid scale model used for the LES is the filtered structure function model. All simulations are performed using the same structured multi-block code. In order to reduce the CPU time, an implicit time stepping method is used for the LES. The purpose of this study is to show the possibilities and limitations of LES of complex flows associated with aeronautical applications using state of the art simulation techniques. Typical flow features are captured by the LES such as the adverse-pressure gradient and flow retardation. Visualization of instantaneous flow fields shows the typical streaky structures in the near-wall region.     

Laser Diagnostic Study of the Mechanism of a Periodic Combustion Instability in a Gas Turbine Model Combustor

To investigate the mechanisms leading to sustained thermoacoustic oscillations in swirl flames, a gas turbine model combustor was equipped with an optically accessible combustion chamber allowing the application of various laser techniques. The flame investigated was a swirled CH₄/air diffusion flame (thermal power 10 kW, global equivalence ratio φ = 0.75) at atmospheric pressure which exhibited self-excited thermoacoustic oscillations at a frequency of 290 Hz. In separate experiments, the flow velocities were measured by laser Doppler velocimetry, the flame structures and heat release rates by planar laser-induced fluorescence of CH and by OH chemiluminescence, and the joint probability density functions of the major species concentrations, mixture fraction, and temperature by laser Raman scattering. All measurements were performed in a phase-locked mode, i.e., triggered with respect to the oscillating pressure level measured by a microphone. The results revealed large periodic variations of all measured quantities and showed that the heat release rate was correlated with the degree of mixing of hot products with unburned fuel/air mixtures before ignition. The thermal expansion of the reacting gases had, in turn, a strong influence on the flow field and induced a periodic motion of the inner and outer recirculation zones. The combination of all results yielded a deeper understanding of the events sustaining the oscillations in the flame under investigation. The results also represent a data base that can be used for the validation and improvement of CFD codes.     

High-order Large Eddy Simulations of Confined Rotor-Stator Flows

In many engineering and industrial applications, the investigation of rotating turbulent flow is of great interest. In rotor-stator cavities, the centrifugal and Coriolis forces have a strong influence on the turbulence by producing a secondary flow in the meridian plane composed of two thin boundary layers along the disks separated by a non-viscous geostrophic core. Most numerical simulations have been performed using RANS and URANS modelling, and very few investigations have been performed using LES. This paper reports on quantitative comparisons of two high-order LES methods to predict a turbulent rotor-stator flow at the rotational Reynolds number Re(=?Ωb ²/ν)?=4 × 10⁵. The classical dynamic Smagorinsky model for the subgrid-scale stress (Germano et al., Phys Fluids A 3(7):1760–1765, 1991) is compared to a spectral vanishing viscosity technique (Séverac & Serre, J Comp Phys 226(2):1234–1255, 2007). Numerical results include both instantaneous data and post-processed statistics. The results show that both LES methods are able to accurately describe the unsteady flow structures and to satisfactorily predict mean velocities as well as Reynolds stress tensor components. A slight advantage is given to the spectral SVV approach in terms of accuracy and CPU cost. The strong improvements obtained in the present results with respect to RANS results confirm that LES is the appropriate level of modelling for flows in which fully turbulent and transition regimes are involved.