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.

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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.

The majority of recent stationary gas turbine combustors employ swirling flows for flame stabilization. The swirling flow undergoes vortex breakdown (VB) and exhibits a complex flow field including zones of recirculating fluid and regions of high shear intensities. Of ten, self-excited helical flow instabilities, which manifest in a precession of the vortex core, are found in these flows and 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 their impact on heat release fluctuations and mixing characteristics over a wide range of operating conditions. We employ high-speed stereoscopic particle image velocimetry (S-PIV) 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 V flame, and a very short flame anchored near the combus tor inlet. The flame shapes were found to closely correlate with the reactivity of the mix ture. 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 exploited to use a tomographic reconstruction technique. Furthermore, it is shown that the helical instability significantly enhances the mixing between the emanating jet and the central recirculation zone.

C h ris tia n O liv e r P a s c h e r e it1In flu en ce of Pressure and S team D ilu tio n on NOx and CO Em issions in a P rem ixed N atu ral Gas F lam eIn the current study, the influence o f 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 pre mixed, swirl-stabilized combustor is developed that provides a stable flame up to very high steam contents. Combustion tests are conducted at different pressure levels fo r 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 fo r all operating conditions. The reactor network is used fo r a detailed investigation o f the influence o f steam and pressure on the NOx formation pathways. In the experi ments, 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 o f 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 o f 20%, the NOx pressure expo nent 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 o f atomic oxygen at steam-diluted conditions, constraining the ther mal pathway. plants [1,2] but with lower installation costs and emission levels [3]. In contrast to the complex combined-cycle plants, ultra wet gas turbines have a substantially smaller footprint. Depending on the cycle configuration, short start-up times and excellent load control capabilities can be achieved. Furthermore, the high steam content allows for low-NOx, near-stoichiometric combustor oper ation and, thus, enables postcombustion CO2 capture at low cost, since the concentration of CO2 reaches the highest possible value for air breathing gas turbines after condensation of the steam. It was recently shown for a lean premixed and for a rich-quenchlean combustor, that steam dilution significantly reduces NOx

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-ε 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.

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