J.C. Roman Casado

Non-linear effects in a lean partially premixed combustor during limit cycle operation

In a gas turbine combustor limit cycles of pressure oscillations may occur due to a coupling of combustion dynamics to the acoustic field inside the system. In this case, the engine is subjected to high vibrations and the possibility of structural damage.Experimental research in this subject was carried out in a laboratory combustor operating in a lean, partially premixed methane/air flame, where the flame stabilizes on a triangular bluff body inside a rectangular combustor duct. Depending on the operating point, the flame shows a stable or unstable behavior. In this last case, amplitudes up to 155 dB (ref 20 μPa) have been recorded. The variation of behavior of the instability with operating conditions is well known. The stable combustion presents a low amplitude broadband noise. The unstable regime is more interesting. It has a main peak with high amplitude and fixed frequency and several secondary peaks at multiple times the frequency of the fundamental one. This peaks can be seen in the pressure and heat release spectrum.The secondary peaks of the pressure spectrum are due to non-linear effects. Odd numbered peaks came from a change in the acoustic boundary conditions in the burner. The even peaks are the result of frequency doubling of the odd frequencies. The frequency doubling comes from a second order source term of the Ligthill’s analogy.Copyright

Sensitivity of the Numerical Prediction of Turbulent Combustion Dynamics in the LIMOUSINE Combustor

The objective of this study is to investigate the sensitivity and accuracy of the reaction flow-field prediction for the LIMOUSINE combustor with regard to choices in computational mesh and turbulent combustion model. The LIMOUSINE combustor is a partially premixed, bluff body-stabilized natural gas combustor designed to operate at 40–80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermoacoustic instabilities is a very time-consuming process. For that reason, the meshing approach leading to accurate numerical prediction, known sensitivity, and minimized amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the computational domain. Typically, the structural mesh topology allows using much fewer grid elements compared to the unstructured grid; however, an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter, the studies are extended to combustible flows using standard available ansys-cfx combustion models. To validate the predicted variable fields of the combustors transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under nonreacting conditions. More significant differences are observed in the mixing behavior of air and fuel flows. Here, the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow, resolved with the use of the hexahedral mesh, presents better agreement with experimental data and demands less computational effort. Finally, in the paper, the performance of the combustion model for reacting flow is presented and the main issues of the applied combustion modeling are reviewed

Study of Unsteady Heat Transfer as a Key Parameter to Characterize Limit Cycle of High Amplitude Pressure Oscillations

The objective of the studies presented in this paper is the numerical prediction of unsteady heat flux and pressure fluctuations during the unstable regime of a combustor. The studied laboratory-scale lean partially premixed combustor was built in the LIMOUSINE project, to explore the mechanisms driving thermo-acoustic instabilities in conditions representative of gas turbine combustors.Due to the thermal interaction between hot gases and the colder liner wall, and also the correlation between gas temperature, density and speed of sound, prediction of the transient heat transfer rate is of high importance. In this paper analysis of transient heat transfer is conducted by coupling of fluid flow and solid body (liner) in one computational domain and thereby taking into account the thermal convection with the environment around the combustor and also the heat conduction transients within the liner. Conjugate heat transfer modeling can give access to the transient temperature distribution in the structure of the combustor which is important for the dynamic heat storage. Also this can be used to estimate the thermal stresses and creep strain as required to evaluate the lifetime assessment of the combustor. In this work the commercial CFD code ANSYS CFX is used to solve the problem, in which fluid and solid regions are solved simultaneously with a finite volume approach. In the fluid region, three dimensional compressible Reynolds Averaged Navier-Stokes equations are solved, while for the solid region only the enthalpy conservation equation is solved. To remove any interpolation errors, in all cases the skin (interface) mesh cells for both the fluid and solid are similar in resolution on either side of the interface. By comparing heat release and pressure data available from the measurements it follows that this simulation can give more accurate prediction of the amplitudes of thermoacoustic instabilities as compared to the solution with imposed thermal boundary conditions (such as isothermal). In the latter case the time history of heat accumulation in the solid is predicted incorrectly. Because the spatial scales of the solid temperature profiles are different in case of steady state or transient oscillatory heat transfer, care has to be taken in the meshing in these two situations. When meshing for a transient oscillatory heat transfer case, the solid mesh resolution needs to be adapted to the thermal penetration depth of the surface temperature oscillations. Hence for the transient heat transfer in limit cycle combustion oscillations, the meshing strategy and size of the grid in the solid part of the domain will play a very important role in determining the magnitude for the pressure fluctuations.Copyright

Strongly Coupled Fluid–Structure Interaction in a Three-Dimensional Model Combustor During Limit Cycle Oscillations

Due to the high temperature of the flue gas flowing at high velocity and pressure, the wall cooling is extremely important for the liner of a gas turbine engine combustor. The liner material is heat-resistant steel with relatively low heat conductivity. To accommodate outside wall forced air cooling, the liner is designed to be thin, which unfortunately facilitates the possibility of high-amplitude wall vibrations (and failure due to fatigue) in case of pressure fluctuations in the combustor. The latter may occur due to a possible occurrence of a feedback loop between the aerodynamics, the combustion, the acoustics, and the structural vibrations. The structural vibrations act as a source of acoustic emitting the acoustic waves to the confined fluid. This leads to amplification in the acoustic filed and hence the magnitude of instability in the system. The aim of this paper is to explore the mechanism of fluid–structure interaction (FSI) on the LIMOUSINE setup which leads to limit cycle of pressure oscillations (LCO). Computational fluid dynamics (CFD) analysis using a RANS approach is performed to obtain the thermal and mechanical loading of the combustor liner, and finite element model (FEM) renders the temperature, stress distribution, and deformation in the liner. Results are compared to other numerical approaches like zero-way interaction and conjugated heat transfer model (CHT). To recognize the advantage/disadvantage of each method, validation is made with the available measured data for the pressure and vibration signals, showing that the thermoacoustic instabilities are well predicted using the CHT and two-way coupled approaches, while the zero-way interaction model prediction gives the largest discrepancy from experimental results.

Sensitivity of the Numerical Prediction of Flow in the LIMOUSINE Combustor on the Chosen Mesh and Turbulent Combustion Model

The objective of this study is to investigate the sensitivity and accuracy of the combustible flow field prediction for the LIMOUSINE combustor with regards to choices in computational mesh and turbulent combustion model. The LIMOUSINE combustor is a partially premixed bluff body stabilized natural gas combustor designed to operate at 40–80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermo-acoustic instabilities is a very time consuming process. For that reason the meshing approach leading to accurate numerical prediction, known sensitivity, and reduced amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the numerical model. Typically, the structural mesh topology allows using much less grid elements compared to the unstructured grid, however an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter the studies are extended to combustible flows using standard available ANSYS-CFX combustion models. To validate the predicted variable fields of the combustor’s transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under non-reacting conditions. More significant differences are observed in the mixing behavior of air and fuel flows. Here the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow resolved with the use of the hexahedral mesh presents better agreement with experimental data and demands less computational effort. Finally in the paper the performance of the combustion model for reacting flow as a function of mesh configuration is presented, and the main issues of the applied combustion modeling are reviewed.Copyright

Modelling Flame-Generated Noise in a Partially Premixed, Bluff Body Stabilized Model Combustor

Numerical simulation using Computational Fluid Dynamics (CFD) has become increasingly important as a tool to predict the potential occurrence of combustion instabilities in gas turbine combustors operating in lean premixed mode. Within the EU-funded Marie Curie project, LIMOUSINE (Limit cycles of thermo-acoustic oscillations in gas turbine combustors), a model test burner has been built in order to have reproducible experimental results for model validation. The burner consists of a Rijke tube of rectangular section having a flame-stabilizing wedge at about 1/4 of its length. Fuel and air supplies were carefully designed to give closed end acoustic inlet boundary conditions while the atmospheric outlet representing an acoustically open end. A transient CFD simulation of the turbulent, partially premixed, bluff body stabilized combusting flow has been carried out for the LIMOUSINE burner using ANSYS CFX commercial software. A 2-D section has been modelled by means of the scale resolving turbulence model, Scale-Adaptive Simulation (SAS), and a two-step Eddy Dissipation combustion model.Experiments were performed on the LIMOUSINE model burner to measure the dynamic variation of pressure and temperature. Results were obtained for several cases with power input ranging from 40 to 60 kW and air factors between 1.2 and 1.8.The CFD results are found to be in good agreement with experiments: the flame is predicted to stabilise on the bluff body in the fluid recirculation zone; resonance frequencies are found to change depending on power and air excess ratio and have a good agreement with experimental results and analytical values; pressure oscillations are consistent with pipe acoustic modes.Copyright