Turbulent reacting flows in a generic swirl gas turbine combustor are investigated numerically. Turbulence is modelled by a URANS formulation in combination with the SST turbulence model, as the basic modelling approach. For comparison, URANS is applied also in combination with the RSM turbulence model to one of the investigated cases. For this case, LES is also used for turbulence modelling. For modelling turbulence-chemistry interaction, a laminar flamelet model is used, which is based on the mixture fraction and the reaction progress variable. This model is implemented in the open source CFD code OpenFOAM, which has been used as the basis for the present investigation. For validation purposes, predictions are compared with the measurements for a natural gas flame with external flue gas recirculation. A good agreement with the experimental data is observed. Subsequently, the numerical study is extended to syngas, for comparing its combustion behavior with that of natural gas. Here, the analysis is carried out for cases without external flue gas recirculation. The computational model is observed to provide a fair prediction of the experimental data and predict the increased flashback propensity of syngas.
Modern gas turbines are to provide high efficiency, reliability, and stability, while meeting strict low emission requirements, with emerging additional requirements such as fuel flexibility. In that respect, the combustor is obviously a core component, and a detailed understanding of the complex flow, heat, and mass transfer processes in the flame is of great importance. Experimental investigation of gas turbine combustion is difficult and can provide only limited information due to practical limitations. Numerical simulations can provide detailed insight and reduce the number of costly experiments. Nevertheless, the highly complex processes in the combustor are difficult to model and the simulations are afflicted with inaccuracies. Thus, development of mathematical and numerical models for GTC and their experimental validation have been a continuous endeavor, to which the present work is aimed to provide a contribution.
Fureby [
In the simulation of GTC, one of the main challenges is turbulence modelling. This is caused by the highly nonisotropic turbulence structure, which is created by the high swirl levels applied to induce a flame-stabilizing vortex breakdown. In the previous work of present authors [
Modelling of the turbulence-chemistry interaction is the further main challenge, of course. A method, which is found to be adequate in modelling turbulence-chemistry interaction is the EDC [
The computational model is applied to predict turbulent combustion in a model gas turbine combustor firing natural gas (NG) and syngas (SG). First, a validation study is performed for the NG flame with flue gas recirculation (FGR). Numerical results are compared with the experimental data, where a reasonably fair agreement is observed. Then, the validated model is applied to predict the combustion behavior of SG, in comparison to NG, without FGR.
The motivation of the present work has been the assessment of the performance of the advocated laminar flamelet method in predicting gas turbine swirl flames considering the realistic fuel injection configuration of an industrial swirl burner. Moreover, the assessment of the predictive capability of the approach for the CO and NO emissions under flue gas recirculation and using syngas as fuel (instead of natural gas) has been a further motivation. The coherent consideration of the all abovementioned aspects on the same rig and within the same modelling framework is a novelty of the present investigation.
Experiments were performed at the Turbomachinery Laboratory of Helmut Schmidt University, Hamburg. The CAD drawing of the atmospheric model combustor, which is equipped by a single swirl burner with 12 channels, is illustrated in Figure
Combustor geometry.
Fuel is injected into the cross-flowing air by 12 injection holes, each located at the wall of each swirler channel. After passing through the swirler channels, each of which having an inclination of 45°, the fuel-air mixture enters the central converging-diverging burner nozzle and, subsequently, into the octagonal main combustion chamber. A converging exhaust gas nozzle is attached to the combustor exit to avoid reverse flow. Based on the local unburnt bulk axial velocity and the diameter at the exit of the converging-diverging burner nozzle, the Reynolds number turns out to be about 34,000. Assuming a perfect guidance by the swirler channels and conservation of angular momentum within the burner, and defining the swirl number as the ratio of the maximum swirl velocity to the bulk axial velocity, a swirl number of about 0.9 can be estimated at the burner nozzle outlet. A detailed description of the setup and measurement methods are provided in [
The combustor was designed for premixed/partially premixed operation. In the experiments [
Composition (vol, %) of fuel stream.
CH_{4} | C_{3}H_{8} | CO_{2} | CO | H_{2} | O_{2} | N_{2} | |
---|---|---|---|---|---|---|---|
NG | 92.5 | 5.2 | 1.3 | — | — | — | 1 |
SG | 10 | 22 | 4 | 22 | 40 | — | 2 |
The measurements [
Composition (vol, %) of oxidizer stream.
O_{2} | H_{2}O | CO_{2} | Ar | N_{2} | |
---|---|---|---|---|---|
FGR0 | 20.6 | 1 | — | 0.9 | 77.5 |
FGR20 | 17.9 | 2.3 | 2.2 | 0.9 | 76.7 |
In the present analysis, totally three cases are analyzed: NG with FGR (NG-FG20), NG without FGR (NG-FGR0), and SG without FGR (SG-FGR0). For the NG-FG20 case, the mixture composition was adjusted to have an adiabatic flame temperature of about 1525 K (corresponding to an equivalence ratio of about 0.5, fuel and oxidizer mass flow rates being 0.0009217 kg/s and 0.03575 kg/s, and combustor inlet temperature 573 K). For SG-FGR0 case, an operation point was chosen corresponding to a slightly lower adiabatic temperature of 1450 K.
A block structured mesh, consisting of 1.2 million cells, is used. A detailed view of the surface mesh is illustrated in Figure
The mesh (detail view of surface mesh).
For the computational investigation, the finite volume method based open source CFD code OpenFOAM [
A second-order upwind scheme was applied to discretize the convective terms in the transport equations for all the variables. A first-order Euler scheme was used for time stepping, since stability problems were quite often encountered with a second-order time discretization. The time step size is chosen in such a way that cell Courant numbers do not exceed unity. Starting from an initial field, the numerical simulations were performed for a time period, which is long enough to allow the development of a quasi-periodic flow field that is no more dependent on the initial conditions. After this state, the time-averaging of the results was started, which was continued until the time-averaged fields did not show any substantial change in time.
Along with the three momentum equations, the pressure-correction equation, and two equations for the turbulence model, three additional differential transport equations (four equations, if NO is included) are solved for combustion modelling which are discussed in the following section.
The turbulence-chemistry interactions necessitate the use of a combustion model, if flow turbulence is not directly simulated but modelled. In the present work, the laminar flamelet method (LFM) is used. According to the usual assumptions of the LFM [
Therefore, in the present study, a flamelet model based on the mixture fraction and the reaction progress variable (
S-shaped curve for the NG-FGR20 flame.
The reaction progress variable can be defined in different ways. In the present work, a temperature-based definition is preferred (the alternative species-based definitions are rather difficult to handle in the present case with FGR due to the existence of combustion products at the oxidizer inlet)
Since NO reactions are very slow compared to the main combustion reactions, the extraction of NO mass fractions out of the flamelet data (see (
The flamelet libraries (see (
A formal grid independence study was not performed. The grid is constructed based on our previous experience on similar flames [
Distribution of GI in planes through (a) swirler and (b) combustor.
One can see that there are large regions fulfilling GI < 25. In the remaining regions, the values remain mostly within the band 25 < GI < 50. Thus, the calculated GI values (Figure
The results will be presented in two parts; in the first part, the results for the NG flame with FGR (NG-FGR20) will be discussed. In the second part, the results for the NG and SG flames without FGR (NG-FGR0, SG-FGR0) will be presented.
The present combustor is designed to operate in premixed/partially premixed mode [
Although it was obvious that the LFM based on the mixture fraction and the scalar dissipation rate (see (
Predicted temperature field at a time step in a plane through combustor, calculated using mixture fraction-scalar dissipation rate based LFM (see (
In the present work, the LFM based on the mixture fraction and the reaction progress variable (see (
Distributions of the axial velocity predicted by the mixture fraction-reaction progress variable LFM for the NG-FGR20 flame, at two different time steps, in a plane through the combustor are presented in Figure
Predicted fields of axial velocity at an arbitrary time step in a plane through combustor by the mixture fraction-reaction progress variable based LFM for NG-FGR20 flame: (a) URANS-SST, (b) URANS-RSM, and (c) LES.
Time-averaged predictions of the axial velocity component and the velocity magnitude, as predicted by URANS-SST, URANS-RSM, and LES, for the NG-FGR20 flame, are shown in a plane through the combustor in Figure
Predicted time-averaged velocity fields in a plane through combustor for NG-FGR20 flame: (a) axial velocity URANS-SST, (b) velocity magnitude URANS-SST, (c) axial velocity URANS-RSM, and (d) axial velocity LES.
As an indication of the flow turbulence, the distribution of the representative RMS value of the velocity fluctuations (
Predicted
The predicted time-averaged fields of temperature and CO mass fraction for the NG-FGR20 flame resulting from URANS-SST, URANS-RSM, and LES calculations are displayed in Figure
Predicted time-averaged fields of (a) temperature URANS-SST, (b) CO mass fraction URANS-SST, (c) temperature URANS-RSM, and (d) temperature LES, in a plane through combustor.
The predicted and measured profiles of time-averaged temperature along the “combustor axis” and along the “evaluation line” (Figure
Predicted and measured profiles of time-averaged temperature along (a) combustor axis and (b) evaluation line for NG-FGR20 flame.
Both experiments and predictions show (Figure
The predicted and measured profiles of time-averaged CO mass fraction along the combustor axis and along the “evaluation line” (Figure
Predicted and measured profiles of time-averaged CO mass fraction along (a) combustor axis and (b) evaluation line for NG-FGR20 flame.
The predicted and measured profiles of time-averaged NO mass fraction along the combustor axis and along the “evaluation line” (Figure
Predicted and measured profiles of time-averaged NO mass fraction along (a) combustor axis and (b) evaluation line for NG-FGR20 flame.
All results presented in this section are obtained by URANS-SST. The predicted time-averaged temperature fields for the NG-FGR0 and SG-FGR0 flames are displayed in Figure
Predicted time-averaged temperature fields in a plane through combustor for (a) NG-FGR0 flame and (b) SG-FGR0 flame.
As seen in Figure
Figure
Predicted time-averaged isolines of the mixture fraction with respect to the time-averaged flame front (
For NG-FGR0 and SG-FGR0 cases, measurements were not available within the combustor but at the combustor outlet. Tables
Combustor exit CO and NO mass fractions for NG-FGR0.
Measured | Predicted | |
---|---|---|
CO ppm @ 15% O_{2} | 0.12 | 3.42 |
NO ppm @ 15% O_{2} | 3.02 | 7.95 |
Combustor exit CO and NO mass fractions for SG-FGR0.
Measured | Predicted | |
---|---|---|
CO ppm @ 15% O_{2} | 1.04 | 1.74 |
NO ppm @ 15% O_{2} | 0.41 | 2.61 |
For CO, the prediction for the SG-FGR0 flame is quite close to the measurement (Table
Turbulent flames in a generic swirl gas turbine combustor designed to operate in premixed/partially premixed mode are investigated numerically. Turbulence is modelled by a URANS formulation, using the SST turbulence model, as the basic modelling approach. For comparison, URANS is applied also in combination with the RSM to one of the investigated cases. For this case, LES is also used for turbulence modelling. For modelling the turbulence-chemistry interaction, a laminar flamelet model based on the mixture fraction and reaction progress variable is used, coupled with a presumed probability density function approach. Natural gas and syngas flames with and without external flue gas recirculation are investigated. Comparing the predictive performances of different turbulence models for one of the cases (NG-FGR20), it was observed that LES predicts a slightly thicker flame brush compared to URANS-SST, while URANS-RSM predicts a slightly sharper and more inclined flame front compared to URANS-SST, extending deeper into the burner on the axis. The numerical results obtained by different turbulence models are observed to show a comparable overall performance and a fair overall agreement with the experimental data. The slight overprediction of the combustor exit temperature is assumed to be affected by the assumption of adiabatic walls in the mathematical model. The model will be improved in the future to include nonadiabatic effects, which is also expected to lead to a more accurate prediction of NO emissions. For syngas, an increased flashback propensity could be predicted, which qualitatively agrees with the experimental observations.
Reaction progress variable (—)
Mean specific heat capacity at constant pressure (J
Turbulence kinetic energy (
Kolmogorov length scale (m)
Static pressure (Pa)
Favre or Reynolds probability density function (—)
Volumetric heat release rate (J
Source term of transport equation for species
Time (s)
Static temperature (K)
Space coordinates (m)
Velocity vector (
Axial velocity (
Velocity magnitude (
Mass fraction of species
Dimensionless wall distance (—)
Mixture fraction (—).
Beta probability density function (Favre or Reynolds) (—)
Local finite volume cell size (m)
Dissipation rate of
Source term for progress variable equation (kg
Viscosity (kg
Density (kg
Schmidt number (—)
Thermochemical variable to be extracted from flamelet libraries (—)
Scalar dissipation rate (
Turbulence eddy frequency (
Favre-averaged value
Reynolds-averaged value
Favre fluctuational value.
Burnt
Stoichiometric
Turbulent
Unburnt.
Eddy dissipation concept
Flue gas recirculation
Grid Index
Gas turbine
Gas turbine combustion/combustor
Large Eddy Simulation
Laminar flamelet method
Natural gas
Probability density function
Reynolds Averaged Numerical Simulation
Root Mean Square
Reynolds Stress Model
Syngas
Unsteady RANS
Shear Stress Transport.
The authors declare that they have no competing interests.