Study of the Wall Thermal Condition Effect in a Lean-Premixed Downscaled Can Combustor Using Large-Eddy Simulation

The primary purpose of this study is to evaluate the ability of LES, with a turbulent combustion model based on steady flamelets, to predict the flame stabilization mechanisms in an industrial can combustor at full load conditions. The test case corresponds to the downscaled Siemens can combustor tested in the high pressure rig at the DLR. The effects of the wall temperature on the prediction capabilities of the codes is investigated by imposing several heat transfer conditions at the pilot and chamber walls. The codes used for this work are Alya and OpenFOAM, which are well established CFD codes in the fluid mechanics community. Prior to the simulation, results for 1-D laminar flames at the operating conditions of the combustor are compared with the detailed solutions. Subsequently, results from both codes at the mid-plane are compared against the experimental data available. Acceptable results are obtained for the axial velocity, while discrepancies are more evident for the mixture fraction and the temperature, particularly with Alya. However, both codes showed that the heat losses influence the size and length of the pilot and main flame.The research leading to these results has received funding through the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme (FP7, 2007-2013) under the grant agreement No. FP7-290042 for the project COPA-GT and the European Union’s Horizon 2020 Programme (2014-2020) and from Brazilian Ministry of Science, Technology and Innovation through Rede Nacional de Pesquisa (RNP) under the HPC4E Project, grant agreement No. 689772. The authors thankfully acknowledge the computer resources, technical expertise and assistance provided by the Red Española de Supercomputación (RES).Peer ReviewedPostprint (author's final draft

Modelling the effects of heat loss and fuel/air mixing on turbulent combustion in gas turbine combustion systems

The present study is concerned with the development and validation of a simulation framework for the accurate prediction of turbulent reacting flows at reduced computational costs. Therefore, a combustion model based on the tabulation of laminar premixed flamelets is employed. By compilation of several flamelets, the model is extended to account for the effects of heat loss and fuel/air mixing. While flamelets at different enthalpy levels are utilized to account for non-adiabatic chemical kinetics, premixed flamelets at varying equivalence ratios are combined to address non-premixed and partially premixed conditions. The tabulation is parametrized in terms of scalar controlling variables that are used to couple the chemistry with the fluid mechanics, namely the mixture fraction, reaction progress variable and a normalized enthalpy scalar. Closures are presented for Reynolds-Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES) based on presumed shape Probability Density Functions (PDF) to account for turbulence-chemistry interaction. The model is implemented in the High Performance Computing (HPC) multi-physics code Alya, which is based on the Finite Element Method (FEM) for spatial discretization. Transport equations are solved for the scalar variables that control the combustion chemistry along with a low-Mach number formulation of the Navier-Stokes equations. The prediction capabilities of the proposed approach for perfectly premixed conditions are assessed based on the reacting flow field of a confined turbulent jet flame. The effect of different heat transfer mechanisms and thermal conditions for the combustion chamber walls is investigated in detail using a Conjugate Heat Transfer (CHT) approach. The influence of radiation, convection and heat conduction over the solid walls is examined by comparing the gas temperature with reference experimental data. Finally, the effect of partial premixing and heat loss on the reacting flow field prediction is addressed based on a swirl stabilized gas turbine model combustor

Compressible Large Eddy Simulation of Thermoacoustic Instabilities in the PRECCINSTA Combustor using Flamelet Generated Manifolds with Dynamic Thickened Flame Model

The fully compressible, density-based CFD-solver TRACE has been extended for simulations of turbulent reacting flows in aero engine gas turbine combustors. The flamelet generated manifolds combustion model is utilized to account for detailed chemical kinetics and combined with the dynamically thickened flame model to resolve the flame front on the large eddy simulation (LES) mesh. The chemistry tabulation is coupled with the LES solver by inversion of the transported energy equation using tabulated mixture averaged NASA polynomial coefficients. LES of the PRECCINSTA test case, a lean, partially-premixed swirl combustor are performed and the two distinctive regimes are correctly predicted: a stable regime with a 'quite' stable flame and an unstable regime with an oscillating flame driven by self-excited thermoacoustic instabilities. Statistics collected from the simulations, mean and root-mean-square values, are in good agreement with the experimental reference data for both operating conditions. The dominant frequency of the unstable flame deviates from the measurement by about 100 Hz and requires further investigation. The results demonstrate the general suitability of the simulation framework for reacting flow simulations in gas turbine combustion systems and the prediction of self-excited thermoacoustic oscillations

Thermoacoustic characterization of a swirl premixed flame using Doak's Momentum Potential Theory

This study proposes a novel approach, based on Doak's Momentum Potential Theory, for the characterization of the thermoacoustic behavior of turbulent premixed flames. The novel approach has the following two advantages: firstly, an unambiguously separation of acoustic, thermal and turbulent dynamics is achieved by means of a Helmholtz decomposition of the momentum fluctuations density; secondly, the development as well as the interaction between the different dynamics are related to the fluxes of turbulent, acoustic, and thermal mean energies, which can be identified in the fluctuating stagnation enthalpy. The Momentum Potential Theory is applied here for the first time to describe the thermoacoustic behaviour of a confined turbulent flame, represented by large-eddy simulation data of a premixed CH4/Air model combustor. Interaction and energy exchange mechanisms between the fluctuations are analyzed in order to show the potential of the theory as a general framework for the characterization of thermoacoustic problems

Scaling of Lean Aeronautical Gas Turbine Combustors

A numerical procedure is presented for the scaling of lean aeronautical gas turbine combustors to different thrust classes. The procedure considers multiple operating points and aims for a self-similar flow field with respect to a reference configuration. The developed scaling approach relies on an optimization-based workflow which involves automated geometry and numerical grid generation, unsteady Reynolds-averaged Navier-Stokes (URANS) simulations and post-processing of the reacting flow field. Kriging is applied as a meta model to identify new sets of parameters for combustor geometry generation. A generic lean-burn high pressure aeronautical combustor has been designed to serve as a first verification test case with reactive flow characteristics comparable to real combustion chambers. The burner geometry is parameterized by 23 free parameters which are altered within the scaling process. The definition of a suitable scaling function is essential for the success of the scaling approach. A scaling function based on pressure loss, axial location of heat release, pilot air split and the temperature profile at the combustor exit is proposed. The developed procedure is tested and applied for the scaling of an internally-staged lean combustor to a lower thrust class considering multiple operating points simultaneously. In total, 65 different combustor variants have been evaluated by the scaling procedure. Simulations were performed for each of these configurations at take-off, approach and idle operating conditions. The final combustor configuration, scaled to a lower thrust class, shows good agreement to the reference configuration in terms of the scaling targets and reasonably resembles the emission indices. Integrating the scaling procedure into the design process of future combustion systems could reduce the required design iterations and thereby contribute to significantly reduced development times and costs

Thermoacoustic characterization of a swirl premixed flame using Doak's Momentum Potential Theory

This study proposes a novel approach, based on Doak's Momentum Potential Theory, for the characterization of the thermoacoustic behavior of turbulent premixed flames. The novel approach has the following two advantages: firstly, an unambiguously separation of acoustic, thermal and turbulent dynamics is achieved by means of a Helmholtz decomposition of the momentum fluctuations density; secondly, the development as well as the interaction between the different dynamics are related to the fluxes of turbulent, acoustic, and thermal mean energies, which can be identified in the fluctuating stagnation enthalpy. The Momentum Potential Theory is applied here for the first time to describe the thermoacoustic behaviour of a confined turbulent flame, represented by large-eddy simulation data of a premixed CH4/Air model combustor. Interaction and energy exchange mechanisms between the fluctuations are analyzed in order to show the potential of the theory as a general framework for the characterization of thermoacoustic problems

Compressible Large Eddy Simulation of Thermoacoustic Instabilities in the PRECCINSTA Combustor Using Flamelet Generated Manifolds With Dynamic Thickened Flame Model

The fully compressible, density-based CFD-solver TRACE has been extended for simulations of turbulent reacting flows in aero engine gas turbine combustors. The flamelet generated manifolds combustion model is utilized to account for detailed chemical kinetics and combined with the dynamically thickened flame model to resolve the flame front on the large eddy simulation (LES) mesh. The chemistry tabulation is coupled with the LES solver by inversion of the transported energy equation using tabulated mixture averaged NASA polynomial coefficients. LES of the PRECCINSTA test case, a lean, partially-premixed swirl combustor are performed and the two distinctive regimes are correctly predicted: a stable regime with a 'quite' stable flame and an unstable regime with an oscillating flame driven by self-excited thermoacoustic instabilities. Statistics collected from the simulations, mean and root-mean-square values, are in good agreement with the experimental reference data for both operating conditions. The dominant frequency of the unstable flame deviates from the measurement by about 100 Hz and requires further investigation. The results demonstrate the general suitability of the simulation framework for reacting flow simulations in gas turbine combustion systems and the prediction of self-excited thermoacoustic oscillations

Thermoacoustic characterization of the PRECCINSTA combustor using Doak's Momentum Potential Theory

The dynamics of a confined, partially premixed flame is analyzed in this study using a novel approach based on an extension to multi-components and reactive flows of Doak's Momentum Potential Theory. The main advantage of this novel approach lies in the unambiguous identification of turbulent, acoustic, thermal, and mixture fluctuations. Starting with an Helmholtz decomposition of the momentum fluctuations vector, a clear separation in turbulent, acoustic, thermal, and mixture components can be achieved for all model quantities described by the theory. These quantities can be used to characterize the thermoacoustic behavior of the combustor system. The Momentum Potential Theory is applied to LES data of the flow inside the PRECCINSTA combustor in stable operating conditions. The most of the contents presented in this paper are based on a more comprehensive study presented by the authors at the ASME Turbo Expo 2023. The following main results are demonstrated here: (i) turbulent, acoustic, thermal, and mixture dynamics can be effectively separated from each others; (ii) in particular, the acoustics of the combustor can be better highlighted using the concept of the Generalized Acoustic Field, defined as the acoustic component of the Total Fluctuating Enthalpy; (iii) turbulent, acoustic, and thermal Total Fluctuating Enthalpy components can be used to characterize the thermoacoustic behavior of the combustor. All of these findings intend to demonstrate the potential of the proposed approach as a more general and consistent framework for the thermoacoustic characterization of combustors

A general acoustic framework for the assessment of noise emitted by combustors: a first case study

This study proposes an alternative and general framework for the assessment of the sound generated within a combustion chamber. The novel approach is based on Doak's Momentum Potential Theory and combines the advantage of using the single assumption of a time-stationary fluctuating flow with a clear separation of turbulent, thermal and acoustic dynamics. The latter can furthermore be quantified by defining a generalised acoustic variable, which can be identified in the fluctuations of the stagnation enthalpy. The proposed model provides a new and more comprehensive explanation for the generation of combustion noise and is used here for the first time to analyse the acoustic properties of the turbulent and non-isentropic flow in a simplified combustion chamber, represented by LES data of a non-reacting combustor simulator. The application highlights the ability of the model to identify and clearly separate the main dynamical effects characteristic for the flow. Finally, the acoustic production can be quantified using the concept of the generalised acoustic field and related to the turbulent dynamics, which seems to represent the major acoustic generation mechanism for the considered setup