Oliver Krüger

Influence of Pressure and Steam Dilution on NOx and CO Emissions in a Premixed Natural Gas Flame

In the current study, the influence of pressure and steam on the emission formation in a premixed natural gas flame is investigated at pressures between 1.5 bar and 9 bar. A premixed, swirl-stabilized combustor is developed that provides a stable flame up to very high steam contents. Combustion tests are conducted at different pressure levels for equivalence ratios from lean blowout to near-stoichiometric conditions and steam-to-air mass ratios from 0% to 25%. A reactor network is developed to model the combustion process. The simulation results match the measured NOx and CO concentrations very well for all operating conditions. The reactor network is used for a detailed investigation of the influence of steam and pressure on the NOx formation pathways. In the experiments, adding 20% steam reduces NOx and CO emissions to below 10 ppm at all tested pressures up to near-stoichiometric conditions. Pressure scaling laws are derived: CO changes with a pressure exponent of approximately −0.5 that is not noticeably affected by the steam. For the NOx emissions, the exponent increases with equivalence ratio from 0.1 to 0.65 at dry conditions. At a steam-to-air mass ratio of 20%, the NOx pressure exponent is reduced to −0.1 to +0.25. The numerical analysis reveals that steam has a strong effect on the combustion chemistry. The reduction in NOx emissions is mainly caused by lower concentrations of atomic oxygen at steam-diluted conditions, constraining the thermal pathway.

Large Eddy Simulations of Hydrogen Oxidation at Ultra-Wet Conditions in a Model Gas Turbine Combustor Applying Detailed Chemistry

Humidified gas turbines (HGT) offer the attractive possibility of increasing plant efficiency without the cost of an additional steam turbine as is the case for a combined gas-steam cycle. In addition to efficiency gains, adding steam into the combustion process reduces NOx emissions. It increases the specific heat capacity (hence, lowering possible temperature peaks) and reduces the oxygen concentration. Despite the thermophysical effects, steam alters the kinetics and, thus, reduces NOx formation significantly. In addition, it allows operation using a variety of fuels, including hydrogen and hydrogen-rich fuels. Therefore, ultra-wet gas turbine operation is an attractive solution for industrial applications. The major modification compared to current gas turbines lies in the design of the combustion chamber, which should accommodate a large amount of steam without losing in stability. In the current study, the premixed combustion of pure hydrogen diluted with different steam levels is investigated. The effect of steam on the combustion process is addressed using detailed chemistry. In order to identify an adequate oxidation mechanism, several candidates are identified and compared. The respective performances are assessed based on laminar premixed flame calculations under dry and wet conditions, for which experimentally determined flame speeds are available. Further insight is gained by observing the effect of steam on the flame structure, in particular HO2 and OH* profiles. Moreover, the mechanism is used for the simulation of a turbulent flame in a generic swirl burner fed with hydrogen and humidified air. Large eddy simulations (LES) are employed. It is shown that by adding steam, the heat release peak spreads. At high steam content, the flame front is thicker and the flame extends further downstream. The dynamics of the oxidation layer under dry and wet conditions is captured; thus, an accurate prediction of the velocity field, flame shape, and position is achieved. The latter is compared with experimental data (PIV and OH* chemiluminescence). The reacting simulations were conducted under atmospheric conditions. The steam-air ratio was varied from 0% to 50%. [DOI: 10.1115/1.4007718] (Less)

Numerical Modeling and Validation of the Flow in a Fluidic Oscillator

A fluidic actuator is a device, which only needs one fluid supply to generate a self-induced and self-sustaining oscillating jet at its outlets. The present study investigates numerically the flow dynamics of a fluidic oscillator operated with water. Simulation results are validated with experimental data obtained with PIV and time-resolved pressure measurements. The numerical simulations are based on unsteady Reynolds-averaged Navier-Stokes equations (URANS) considering a turbulent, incompressible, and isothermal flow. Beforehand, a sensitivity analysis regarding the turbulence closure, the spatial grid solution, and the outlet geometry was conducted. In addition, to gain a deeper understanding of the flow dynamics a modal analysis is provided. It was found that the two-dimensional simulation employing the SST was sufficient to describe the flow field and dynamics qualitatively as well as quantitatively. However, nonlinear effects could only be observed in the threedimensional computations.

Flow Field and Flame Dynamics of Swirling Methane and Hydrogen Flames at Dry and Steam Diluted Conditions

The majority of recent stationary gas turbine combustors employ swirling flows for flame stabilization. The swirling flow undergoes vortex breakdown and exhibits a complex flow field including zones of recirculating fluid and regions of high shear. Often, self-excited helical flow instabilities are found in these flows that may influence the combustion process in beneficial and adverse ways. In the present study we investigate the occurrence and shape of self-excited hydrodynamic instabilities and the related heat-release fluctuations over a wide range of operating conditions. We employ high-speed stereoscopic particle image velocimetry and simultaneous OH*-chemiluminescence imaging to resolve the flow velocities and heat release distribution, respectively. The results reveal four different flame shapes: A detached annular flame, a long trumpet shaped flame, a typical V-flame, and a very short flame anchored near the combustor inlet. The flame shapes were found to closely correlate with the reactivity of the mixture. Highly steam-diluted or very lean flames cause a detachment, whereas hydrogen fuel leads to very short flames. The detached flames feature a helical instability, which in terms of frequency and shape is similar to the isothermal case. A complete suppression of the helical structure is found for the V-flame. Both, the trumpet shaped flame and the very short flame feature helical instabilities of different frequencies and appearances. The phase-averaged OH*-chemiluminescence images show that the helical instabilities cause large scale-heat release fluctuations. The helical structure of the fluctuations is verified using a tomographic reconstruction technique.Copyright

Laminar Burning Velocities and Emissions of Hydrogen-Methane-Air-Steam Mixtures

Humidified gas turbines using steam generated from excess heat feature increased cycle efficiencies. Injecting the steam into the combustor reduces NOx emissions, flame temperatures and burning velocities, promising a clean and stable combustion of highly reactive fuels, such as hydrogen or hydrogen-methane blends. This study presents laminar burning velocities for methane, and hydrogen-enriched methane (10 mol% and 50 mol%) at steam contents up to 30% of the air mass flow. Experiments were conducted on prismatic Bunsen flames stabilized on a slot-burner employing OH planar laser-induced fluorescence for determining the flame front areas. The experimental burning velocities agree well with results from one dimensional simulations using the GRI 3.0 mechanism. Burning velocities are increased with hydrogen enrichment, and reduce non-linearly with ascending steam mole fractions, showing the potential of steam dilution for a stable combustion of these fuels over a wide flammability range. Additionally measured NOx and CO emissions reveal a strong reduction in NOx emissions for an increasing dilution with steam, whereas CO curves are shifted towards higher equivalence ratios.Copyright

Laminar Burning Velocities and Emissions of Hydrogen–Methane–Air–Steam Mixtures

Humidified gas turbines using steam generated from excess heat feature increased cycle efficiencies. Injecting the steam into the combustor reduces NOx emissions, flame temperatures and burning velocities, promising a clean and stable combustion of highly reactive fuels, such as hydrogen or hydrogen-methane blends. This study presents laminar burning velocities for methane, and hydrogen-enriched methane (10 mol% and 50 mol%) at steam contents up to 30% of the air mass flow. Experiments were conducted on prismatic Bunsen flames stabilized on a slot-burner employing OH planar laser-induced fluorescence for determining the flame front areas. The experimental burning velocities agree well with results from one dimensional simulations using the GRI 3.0 mechanism. Burning velocities are increased with hydrogen enrichment, and reduce non-linearly with ascending steam mole fractions, showing the potential of steam dilution for a stable combustion of these fuels over a wide flammability range. Additionally measured NOx and CO emissions reveal a strong reduction in NOx emissions for an increasing dilution with steam, whereas CO curves are shifted towards higher equivalence ratios.Copyright

Numerical Investigations and Modal Analysis of the Coherent Structures in a Generic Swirl Burner

The dynamic near-field characteristics of an isothermal turbulent swirling jet in a generic swirl burner is numerically studied and validated. The results are compared to experimentally obtained PIV data. Large Eddy Simulations are employed and their respective sensitivity is assessed regarding the grid resolution and the subgrid-scale modeling. The flow dynamics are investigated for varying swirl intensities. It was found that the low-swirling case was considered in an intermediate state in the critical region of the onset of vortex breakdown. The high-swirling case revealed a dominant frequency in the turbulent energy spectra, which could be identified as a convective helical instability by applying a modal analysis. This helical instability is assumed to be triggered by a precessing vortex core (PVC). Additionally, it was found that the azimuthal momentum transfer is increased by the PVC, leading to a better mixing. Furthermore, it is shown that the dynamics of a swirling flow can be excellently captured by using LES.

Numerical Investigation of the Flow Field and Mixing in a Swirl-Stabilized Burner With a Non-Swirling Axial Jet

In the present study numerical results of simulations, using RANS and LES, of the non-reacting flow in a swirl-stabilized burner are presented. The burner was developed for lean premixed combustion with high fuel flexibility at low emissions. An important challenge for a fuel-flexible, low emission combustor is the prevention of flashback for fuels of high reactivity, such as hydrogen, without compromising on lean blow out safety and mixing quality. Flashback safety can be increased by a sufficiently high and uniform axial velocity at the end of the mixing tube. In the investigated combustor the velocity deficit in the center of the mixing tube, which results from the swirl, is prevented by a non-swirling axial jet. In a parametric study the effect of different amounts of axial injection on the flow field is investigated. The results are validated with experimental data, gained from PIV measurements in a vertical water tunnel. It is shown that the mean flow field can be well captured by steady-state RANS simulations using a realizable k-e turbulence model. The most suitable geometry is identified and, subsequently, transient LES simulations are conducted. The dynamic flow field characteristics are investigated. It was found that in spite of the high swirl, the flow field is quite stable and no dominating frequency is detected. The flow field of the swirling flow in the combustion chamber can be captured well using LES. Furthermore, the mixing quality is compared to the experiments, which are performed in a water tunnel. In contrast to the RANS simulation, the LES can qualitatively capture the spatial unmixedness observed from experimental data. All simulations were conducted using water as fluid.Copyright

Flow Field and Flame Dynamics of Swirling Methane and Hydrogen Flames at Dry and Steam-Diluted Conditions

The majority of recent stationary gas turbine combustors employ swirling flows for flame stabilization. The swirling flow undergoes vortex breakdown and exhibits a complex flow field including zones of recirculating fluid and regions of high shear. Often, self-excited helical flow instabilities are found in these flows that may influence the combustion process in beneficial and adverse ways. In the present study we investigate the occurrence and shape of self-excited hydrodynamic instabilities and the related heat-release fluctuations over a wide range of operating conditions. We employ high-speed stereoscopic particle image velocimetry and simultaneous OH*-chemiluminescence imaging to resolve the flow velocities and heat release distribution, respectively. The results reveal four different flame shapes: A detached annular flame, a long trumpet shaped flame, a typical V-flame, and a very short flame anchored near the combustor inlet. The flame shapes were found to closely correlate with the reactivity of the mixture. Highly steam-diluted or very lean flames cause a detachment, whereas hydrogen fuel leads to very short flames. The detached flames feature a helical instability, which in terms of frequency and shape is similar to the isothermal case. A complete suppression of the helical structure is found for the V-flame. Both, the trumpet shaped flame and the very short flame feature helical instabilities of different frequencies and appearances. The phase-averaged OH*-chemiluminescence images show that the helical instabilities cause large scale-heat release fluctuations. The helical structure of the fluctuations is verified using a tomographic reconstruction technique.Copyright