Article

Numerical Investigation of Effusion Cooling Air Influence on the CO Emissions for a Single-Sector Aero-Engine Model Combustor

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Abstract

Stricter aviation emissions regulations have led to the desire for lean-premixed-vaporized combustors over rich-quench-lean burners. While this operation mode is beneficial for reducing NOx and particulate emissions, the interaction of the flame and hot exhaust gases with the cooling flow results in increased CO emissions. Predicting CO in computational fluid dynamics (CFD) simulations remains challenging. To assess current model performance under practically relevant conditions, Large-Eddy Simulation (LES) of a lab-scale effusion cooling test rig is performed. Flamelet-based manifolds, in combination with the Artificial Thickened Flame (ATF) approach, are utilized to model the Turbulence-Chemistry Interaction (TCI) in the test rig with detailed chemical kinetics at reduced computational costs. Heat losses are considered via exhaust gas recirculation (EGR). Local transport effects in CO emissions are included through an additional transport equation. Additionally, a Conjugate Heat Transfer (CHT) simulation is performed for good estimations of the thermal boundary conditions. Extensive validation of this comprehensive model is conducted using the available experimental dataset for the studied configuration. Subsequently, model sensitivities for predicting CO are assessed, including the progress variable definition and the formulation of the CO source term in the corresponding transport equation. To investigate the flame thickening influence in the calculated CO, an ATF-postprocessing correction is further developed. Integrating multiple sophisticated pollutant submodels and evaluating their sensitivity offers insights for future investigations into modeling CO emissions in aero-engines and stationary gas turbines.

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Flame-wall-interaction (FWI) is investigated numerically using a premixed stoichiometric Side-Wall-Quenching configuration. Within the 2D fully resolving laminar simulation, detailed chemistry is used to study the stationary quenching of a methane–air (CH4) flame at an isothermal inert wall of 300 K. The investigation is related to a recent experimental study that revealed that the carbon-monoxide distribution substantially differs in the near-wall region when compared to an undisturbed flame. Simulations are carried out using different reaction mechanisms (GRI and Smooke) as well as diffusion treatments (unity Lewis and mixture averaged transport) and the results are compared to the measured temperature and CO concentrations. Specifically regarding the latter, being an important pollutant, recent attempts based on tabulated chemistry failed in predicting its near-wall accumulation. Accordingly, within this work the detailed chemistry simulations are used to investigate the origin of CO near the wall. Therefore, a Lagrangian analysis is applied to quantify the contribution of chemical production and consumption as well as diffusion to understand the root mechanism of the high CO concentrations measured. The analysis revealed that the high CO concentrations near the wall results from a transport originating from CO produced at larger wall distances. In that region being not submitted to large heat losses, a high chemical activity and corresponding CO production is found. Accordingly, a diffusion process is initiated towards the wall where the chemical sources itself were actually found to be negative.
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In this paper transient fluid-structure thermal analyses of the Limousine test rig have been conducted while the combustor was exposed to saturated amplitude limit cycle combustion oscillations (LCO). The heat transfer between hot combustion gases and the liner wall cooled by convection will affect thermoacoustic instabilities, and therefore the relevance of prediction of the transient heat transfer rate in gas turbine combustors is explored. The commercial CFD code ANSYS CFX is used to analyze the problem. Fluid and solid regions are resolved simultaneously in a monolithical approach with application of a finite volume approach. Since the spatial scales of the solid temperature profiles are different in case of steady state and transient oscillatory heat transfer, special care has to be taken in the meshing strategy. It is shown that for the transient oscillatory heat transfer in to the solid in LCO operation, the mesh distribution and size of the grid in the solid part of the domain will play a very important role in determining the magnitude for the heat flow in the solid and the gas pressure fluctuations, and the grid resolution needs to be adapted to the thermal penetration depth. Moreover, compared to the calculations of only the fluid domain with adiabatic/isothermal boundary wall conditions, the results demonstrated that application of the Conjugated Heat Transfer (CHT) model leads to significant accuracy improvements in the prediction of the characteristics of the combustion instability.
Article
Tabulated chemistry models like the Flamelet Generated Manifolds method are a good approach to include detailed information on the reaction kinetics in a turbulent flame at reasonable computational costs. However, so far, not all information on e.g. heat losses are contained in these models. As those often appear in typical technical applications with enclosed flames in combustion chambers, extensions to the standard FGM approach will be presented in this paper, allowing for the representation of non-adiabatic boundaries. The enthalpy as additional control variable for the table access is introduced, such that the chemistry database becomes three-dimensional with mixture fraction, reaction progress variable and enthalpy describing the thermo-chemical state. The model presented here is first validated with a two-dimensional enclosed Bunsen flame and then applied within the Large Eddy Simulations of a turbulent premixed swirl flame with a water-cooled bluff body and a turbulent stratified flame, where additional modeling for the flame structure using artificially thickened flames was included. The results are encouraging, as the temperature decrease towards the bluff body in the swirl flame and the cooling of the pilot flame exhaust gases in the stratified configuration can be observed in both experiments and simulation.
Article
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.
Article
In this work, an integrated Large Eddy Simulation (LES) model is developed for sooting turbulent nonpremixed flames and validated in a laboratory scale flame. The integrated approach leverages state-of-the-art developments in both soot modeling and turbulent combustion modeling and gives special consideration to the small-scale interactions between turbulence, soot, and chemistry. The oxidation of the fuel and the formation of gas-phase soot precursors is described by the Flamelet/Progress Variable model, which has been previously extended to account for radiation losses. However, previous DNS studies have shown that Polycyclic Aromatic Hydrocarbons (PAH), the immediate precursors of soot particles, exhibit significant unsteady effects due to relatively slow chemistry. To model these unsteady effects, a transport equation is solved for a lumped PAH species. In addition, due to the removal of PAH from the gas-phase, alternative definitions of the mixture fraction, progress variable, and enthalpy are proposed. The evolution of the soot population is modeled with the Hybrid Method of Moments (HMOM), an efficient statistical model requiring the solution of only a few transport equations describing statistics of the soot population. The filtered source terms in these equations that describe the various formation, growth, and destruction processes are closed with a recently developed presumed subfilter PDF approach that accounts for the high spatial intermittency of soot. The integrated LES model is validated in a piloted natural gas turbulent jet diffusion flame and is shown to predict the magnitude of the maximum soot volume fraction in the flame relatively accurately, although the maximum soot volume fraction is shown to be rather sensitive to the subfilter scalar dissipation rate model.
Article
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.
Article
Large-Eddy Simulations (LES) and Direct Numerical Simulation (DNS) are applied to the analysis of a swirl burner operated with a lean methane–air mixture and experimentally studied by Meier et al. [19]. LES is performed for various mesh refinements, to study unsteady and coherent large-scale behavior and to validate the simulation tool from measurements, while DNS enables to gain insight into the flame structure and dynamics. The DNS features a 2.6 billion cells unstructured-mesh and a resolution of less than 100 microns, which is sufficient to capture all the turbulent scales and the major species of the flame brush; the unresolved species are taken into account thanks to a tabulated chemistry approach. In a second part of the paper, the DNS is filtered at several filter widths to estimate the prediction capabilities of modeling based on premixed flamelet and presumed probability density functions. The similarities and differences between spatially-filtered laminar and turbulent flames are discussed and a new sub-grid scale closure for premixed turbulent combustion is proposed, which preserves spectral properties of sub-filter flame length scales. All these simulations are performed with a solver specifically tailored for large-scale computations on massively parallel machines.
Article
Many models are now available to describe chemistry at a low CPU cost, but only a few of them can be used to describe correctly premixed, partially premixed and diffusion combustion. One of them is the FPI model that uses two coordinates: the mixture fraction Z and the progress variable c. In this paper, we introduce a new evolution of the FPI method that can now handle heat losses. After a short review of kinetic models used in turbulent combustion, the main features of the new three-dimensional FPI method, in which we introduce a third coordinate for enthalpy h, are presented. First, a one-dimensional radiative premixed flame validation case is presented for a large range of radiative heat losses. Second, we present the results of simulations of two laminar burners. Both the fully and the partially premixed burner simulations give a good estimation of all the flame features such as the flame stabilization (driven by heat losses), the flame structure and the profile of major and minor species.
Article
The prediction of combustion processes using Large Eddy Simulation (LES) combined with tabulated chemistry and presumed probability density modeling has proven to be very successful and become very popular, especially in academia, during the last years. A variety of time and length scales occur within combustion systems which need to be resolved. The comparably slow unsteady flow is well described by the LES, whereas Flamelet Generated Manifolds (FGM) provide a good means to represent the fast chemical reactions. However, the slow production of minor species such as nitrogen oxide is not well captured by the manifold defined by fast evolving major species. To overcome this deficiency, an additional transport equation for nitrogen oxide (NO) is solved here. The source term of NO is taken from the chemistry database depending on mixture fraction and progress variable. Two different modeling assumptions for this source term are presented in this paper. The models are applied to a standard test case from the Sydney bluff body flame series and compared to experimental data and the classic FGM approach. Both models show a large improvement over the results obtained by the standard FGM model.
Article
In this paper a new test rig for the characterization of advanced combustor cooling concepts for gas turbine combustors is introduced. The test rig is designed to allow investigations at realistic operating conditions of future lean combustors at elevated pressures and temperatures. The features and capabilities of the test rig in comparison to existing rigs are described. The properties of the hot gas flow are measured in order to provide the necessary data for an detailed analysis of the measured cooling efficiency of combustor wall test samples. Results of the characterization of the velocity and temperature distribution in the hot gas flow at the leading edge of the test sample at pressures up to p=10bar and global flame temperatures up to T=2000K are presented.
GRI 3.0 Reaction Mechanism
  • G P Smith
  • D M Golden
  • M Frenklach
  • N W Moriarty
  • B Eiteneer
  • M Goldenberg
  • C T Bowman
Laseroptische Untersuchung der Flamme-Kühlluft-Interaktion in einer effusionsgekühlten Brennkammer
  • J Hermann
Experimental Investigations of Flame-Cooling Air Interaction in an Effusion Cooled Pressurized Single Sector Model Gas Turbine Combustor
  • M Greifenstein