Daniel Lörstad

Evaluation of global mechanisms for les analysis of SGT-100 DLE combustion system

This paper presents the results of Computational Fluid Dynamics (CFD) analyses obtained for the experimental version of the SGT-100 Dry Low Emission (DLE) gas turbine burner provided by Siemens Industrial Turbomachinery Ltd (SIT). A testing and measurement campaign for this burner was previously carried out at the DLR Institute of Combustion Technology, Stuttgart, Germany, for various operating pressure conditions. The present work shows the successful validation of the CFD model in terms of time-averaged temperature and velocity data within measurement errors at an operating pressure of 3 bar. Several well known global mechanisms are tested in this work, namely the Westbrook Dryer 2-step (WD) scheme, the Jones and Lindstedt 4-step (JL4) scheme, the Meredith et al. 3-step (M3) scheme and a recently developed in-house 4-step scheme (M4) for methane-air mixtures. The M4 scheme is optimized by matching the detailed GRI-Mech 3.0 mechanism in terms of 1D laminar flame speed, using the CHEMKIN software for a wide range of pressures (1 to 6 bar), unburned gas temperatures (295 to 650 K) and equivalence ratios range (0.4 to 1.6). CFD simulations are performed using the Eddy Dissipation Model (EDM)/Finite Rate Chemistry (FRC) non-premixed turbulence chemistry interaction model. Both steady-state Reynolds Averaged Navier Stokes (RANS) and hybrid Unsteady Reynolds Averaged Navier Stokes /Large Eddy Simulation (URANS/LES) turbulence models are used. The LES Wall Adaptive Large Eddy-Viscosity (WALE) model with finite rate chemistry is also tested for validation. Velocity profiles, flame temperatures and major species are compared with experiments for different global reaction mechanisms used with different turbulence models. A reasonable agreement is found with the M4 global reaction mechanism in predicting mixing, temperatures and major species. RANS simulations are observed to underpredict the temperature profiles downstream and overpredict in the upstream region, while the velocity profiles are found to be in close agreement with experiments. The SAS-SST turbulence model predicts the velocity profiles in good agreement with experimental data and slightly better than the RANS model. Both the transient simulations slightly overpredict the temperature profiles. The LES-WALE model gives too high and unrealistic temperatures.

Investigation of Hydrogen Enriched Natural Gas Flames in a SGT-700/800 Burner Using OH PLIF and Chemiluminescence Imaging

The effect of hydrogen enrichment to natural gas flames was experimentally investigated at atmospheric pressure conditions using flame chemiluminescence imaging, planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and dynamic pressure monitoring. The experiments were performed using a 3rd generation dry low emission (DLE) burner used in both SGT-700 and SGT-800 industrial gas turbines from Siemens. The burner was mounted in an atmospheric combustion test rig at Siemens with optical access in the flame region. Four different hydrogen enriched natural gas flames were investigated; 0 vol.%, 30 vol.%, 60 vol.% and 80 vol.% of hydrogen. The results from flame chemiluminescence imaging and OH PLIF show that the size and shape of the flame was clearly affected by hydrogen addition. The flame becomes shorter and narrower when the amount of hydrogen is increased. For the 60 vol.% and 80 vol.% hydrogen flames the flame has moved upstream and the central recirculation zone that anchors the flame has moved upstream the burner exit. Furthermore, the position of the flame front fluctuated more for the full premixed flame with only natural gas as fuel than for the hydrogen enriched flames. Measurements of pressure drop over the burner show an increase with increased hydrogen in the natural gas despite same air flow thus confirming the observation that the flame front moves upstream towards the burner exit and thereby increasing the blockage of the exit. Dynamic pressure measurements in the combustion chamber wall confirms that small amounts of hydrogen in natural gas changes the amplitude of the dynamic pressure fluctuations and initially dampens the axial mode but at higher levels of hydrogen an enhancement of a transversal mode in the combustion chamber at higher frequencies could occur.

Measurements and LES of a SGT-800 Burner in a Combustion Rig

The Siemens gas turbine SGT-800 is the largest industrial gas turbine produced by Siemens Industrial Turbomachinery (SIT) offering a dry low emission (DLE) capability below 15 ppm NOx. It has a very high reliability using an annular combustor system with passive damping and 30 DLE burners. To obtain a greater understanding of the mixing process and the flame dynamics and in order to further reduce the emission levels, single burner rig tests have been performed. The laboratory measurements are complemented by Large Eddy Simulation (LES) and Reynolds Averaged Navier-Stokes (RANS) simulations to further investigate the transient fuel distribution and subsequent flame behavior. The measurements were performed jointly by SIT and Lund University using the SIT single burner combustion rig, where the square chamber allows great optical access in the flame region. The experimental data includes wall temperature, pressure fluctuations, light intensity variation and simultaneous Planar Laser Induced Fluorescence of OH and acetone. This investigation is complemented using fuel concentration field laser measurements of the fuel distribution upstream of the flame region in SIT water rig, using a burner partly made of Plexiglas to allow for optical access. The LES model was developed jointly by SIT and FOI. The LES computations were performed using a combustion code developed from the OpenFOAM library utilizing the mixed subgrid flow model, complemented with a subgrid wall model. The reacting flow was simulated using a Finite Rate Chemistry (FRC) combustion model based on the Partially Stirred Reactor (PaSR) model. For this study, a two-step global/reduced methane-air reaction mechanism was employed to describe the combustion chemistry. The RANS simulations were performed with ANSYS Fluent, using the k-e Realizable eddy viscosity turbulence model in combination with the Fluent partially premixed combustion model. This model is a combination of the Zimont flamelet progress variable model and a Probability Density Function based non-premixed combustion model. The investigation includes a detailed evaluation of the numerical results compared to the measurement data. The numerical model includes the upstream air supply and fuel line systems up to well-defined constrictions to ensure appropriate acoustic inlet conditions. The measurements reveal large fluctuations in the flame region, which has been investigated using LES. (Less)

Experimental and LES investigations of a SGT-800 burner in a combustion rig

The Siemens gas turbine SGT-800 has an annular combustor and 30 dry low emission burners. In order to further reduce the emission levels and to obtain improved understanding of the flow and associated flame dynamics, single burner rig tests have been performed. The laboratory measurements are complemented by Large Eddy Simulation (LES) to analyze the effect on the flame dynamics due to the transient fuel distribution and mixing process in the burner. The study includes both atmospheric and high pressure conditions. The computational model was developed jointly by Siemens Industrial Turbomachinery AB (SIT) and FOI. It is based on a finite rate chemistry LES model using a Partially Stirred Reactor (PaSR) turbulence chemistry interaction model and a two-step CH4 /air mechanism developed by FOI. The results are compared to measurements performed jointly by SIT and Lund Institute of Technology. The experimental data includes wall temperature, pressure fluctuations, light intensity variation and simultaneous Planar Laser Induced Fluorescence of OH and acetone. The study is further complemented by Reynolds Averaged Navier-Stokes (RANS) calculations of the fuel concentration field evaluated to laser measurements in a water rig using the same burner configuration. Different burner fuel distributions are examined and the corresponding influence on the downstream mixing, fuel distribution and flame dynamics are studied. The results indicate that the fuel distribution upstream the flame, the detailed modeling of the fuel supply manifold, including the specification of numerical boundary conditions, and the flow in the fuel and air supply pipes, have significant influence on the flame dynamics. This is proven by the successful combustion LES of an unstable flame that experiences high flame dynamics and that a modification of the boundary conditions alters the dynamics resulting in a more stable flame. This is well in accordance with the experimental data and previous experience at SIT. The modal structures caused by the interaction between the flow, acoustics and flame dynamics are analyzed using the Proper Orthogonal Decomposition (POD) technique. The dominating modes in general originate from the burner mixing tube air-fuel-mass flow-interaction and flame-combustion chamber interaction. (Less)

CFD analysis of a SGT-800 burner in a combustion RIG

This work focuses on 3D turbulent reacting flow modeling of a SGT-800 3rd generation dry low emission (DLE) burner at both atmospheric and engine-like conditions. At atmospheric pressure the burner is fitted in a test rig with high pre-heating of the incoming air. To reduce the computational cost, the M4 mechanism previously developed by Abou-Taouk et al. (2013) is used for operating pressure of 1 bar. A new novel optimized 4-step reaction mechanism for methane-air mixture is developed in the present work at an operating pressure of 20 bar. The mechanism is based on a large sample of detailed chemistry solutions that are processed by an iterative optimization procedure. This leads to a reduced 4-step mechanism, reproducing the targeted detailed chemistry solutions in terms of laminar flame speeds, species profiles and temperatures. The CFD simulations are performed using the combined eddy dissipation model / finite rate chemistry (EDM/FRC) turbulence chemistry interaction model. The turbulence is modeled using both the k-ω SST and the scale adaptive simulation (SAS) turbulence models. A comprehensive testing and measurement campaign carried out at atmospheric pressure for this burner was previously performed in a combustion test rig. The CFD results are compared to measurement data which includes for example flame position and pressure drop.

Numerical and Experimental Investigations of the Siemens SGT-800 Burner Fitted to a Water Rig

DLE (Dry Low Emission) technology is widely used in land based gas turbines due to the increasing demands on low NOx levels. One of the key aspects in DLE combustion is achieving a good fuel and air mixing where the desired flame temperature is achieved without too high levels of combustion instabilities. To experimentally study fuel and air mixing it is convenient to use water along with a tracer instead of air and fuel. In this study fuel and air mixing and flow field inside an industrial gas turbine burner fitted to a water rig has been studied experimentally and numerically. The Reynolds number is approximately 75000 and the amount of fuel tracer is scaled to represent real engine conditions. The fuel concentration in the rig is experimentally visualized using a fluorescing dye in the water passing through the fuel system of the burner and recorded using a laser along with a CCD (Charge Couple Device) camera. The flow and concentration field in the burner is numerically studied using both the scale resolving SAS (Scale Adaptive Simulation) method and the LES (Large Eddy Simulation) method as well as using a traditional two equation URANS (Unsteady Reynolds Average Navier Stokes) approach. The aim of this study is to explore the differences and similarities between the URANS, SAS and LES models when applied to industrial geometries as well as their capabilities to accurately predict relevant features of an industrial burner such as concentration and velocity profiles. Both steady and unsteady RANS along with a standard two equation turbulence model fail to accurately predict the concentration field within the burner, instead they predict a concentration field with too sharp gradients, regions with almost no fuel tracer as well as regions with far too high concentration of the fuel tracer. The SAS and LES approach both predict a more smooth time averaged concentration field with the main difference that the tracer profile predicted by the LES has smoother gradients as compared to the tracer profile predicted by the SAS. The concentration predictions by the SAS model is in reasonable agreement with the measured concentration fields while the agreement for the LES model is excellent. The LES shows stronger fluctuations in velocity over time as compared to both URANS and SAS which is due to the reduced amounts of eddy viscosity in the LES model as compared to both URANS and SAS. This study shows that numerical methods are capable of predicting both velocity and concentration in a gas turbine burner. It is clear that both time and scale resolved methods are required to accurately capture the flow features of this and probably most industrial DLE gas turbine burners. (Less)

INVESTIGATION OF SIEMENS SGT-800 INDUSTRIAL GAS TURBINE COMBUSTOR USING DIFFERENT COMBUSTION AND TURBULENCE MODELS

Siemens SGT-800 gas turbine is the largest industrial gas turbine within Siemens medium gas turbine size range. The power rating is 53MW at 39% electrical efficiency in open cycle (ISO) and, for its power range, world class combined cycle performance of >56%. The SGT-800 convectively cooled annular combustor with 30 Dry Low Emissions (DLE) burners has proven, for 50-100% load range, NOx emissions below 15/25ppm for gas/liquids fuels and CO emissions below 5ppm for all fuels, as well as extensive gas fuel flexible DLE capability. In this work the focus is on the combustion modelling of one burner sector of the SGT-800 annular combustor, which includes several challenges since various different physical phenomena interacts in the process. One of the most important aspects of the combustion in a gas turbine combustor is the turbulence chemistry interaction, which is dependent on both the turbulence model and the combustion model. Some turbulence-combustion model combinations that have shown reasonable results for academic generic cases and/or industrial applications at low pressure, might fail when applied to complex geometries at industrial gas turbine conditions since the combustion regime may be different. Therefore is here evaluated the performance of Reynolds Averaged Navier-Stokes (RANS) and Scale Adaptive Simulation (SAS) turbulence models combined with different combustion models, which includes the Eddy Dissipation Model (EDM) combined with Finite Rate Chemistry (FRC) using an optimized reduced 4-step scheme and two flamelet based models; Zimonts Burning Velocity model and Lindstedt & Vaos Fractal model. The results are compared to obtained engine data and field experience, which includes for example flame position in order to evaluate the advantages and drawbacks of each model. All models could predict the flame shape and position in reasonable agreement with available data; however, for the flamelet based methods adjusted calibration constants were required to avoid a flame too far upstream or non-sufficient burn out which is not in agreement with engine data. In addition both the flamelet based models suffer from spurious results when fresh air is mixed into fully reacted gases and BVM also from spurious results inside the fuel system. The combined EDM-FRC with a properly optimized reduced chemical kinetic scheme seems to minimize these issues without the need of any calibration, with only a slight increase in computational cost.

Numerical Investigation of Hydrogen Enriched Natural Gas in the SGT-800 Burner

The effects of hydrogen enrichment in the SGT-800 3rd generation DLE burner fitted in an atmospheric combustion rig have been numerically investigated. Three different mixtures with 0%, 60% and 80% hydrogen enrichment to methane have been studied. In this study a URANS (Unsteady Reynolds Averaged Navier-Stokes) approach is applied. The chemistry is included through the use of laminar flamelet libraries in combination with a presumed PDF (Probability Density Function). The mean reaction rate is acting as a source term to a reaction progress variable, and is modelled using a fractal combustion model. In the methane simulations two turbulence models, k–ω SST and k–ω SST-SAS, were evaluated. The latter model was found to predict results in good agreement with measurement data. The dynamic behaviour of the flame is captured by the SST-SAS model but not by the standard SST model. For the hydrogen enriched methane simulations the validated SST-SAS model with a calibrated model constant for the mean reaction rate from the methane simulations was used. The overall results such as flame position and global pressure drop are in good agreement with experimental data. The time averaged flame stabilization point is moving upstream towards the burner exit nozzle when the hydrogen enrichment is increasing. The total pressure drop over the burner is increasing with the increasing hydrogen level.Copyright

Investigation of Combustion in a Dump Combustor Using Different Combustion and Turbulence Models

Modelling combustion in gas turbine combustors remains to be a challenge since several different physical phenomena interact in the process. One of the most important aspects of the combustion in a gas turbine combustor is the chemistry-turbulence interaction. In order to study the effect of the combustion and turbulence models, a dump combustor geometry is selected. Two combustion models namely, finite rate chemistry and flamelet based models, together with different turbulent models including LES 1eq k-model, RANS k-epsilon and k-omega models are implemented using both CFX and OpenFoam codes. The predicted temperature and velocity fields are compared to the existing experimental results. It is shown that different turbulence models behave very differently and there are large discrepancies between the experimental and predicted results. Some part of the discrepancies may be due to unknown heat losses through the combustor wall in the experiment.Copyright