Thoralf G. Reichel

Vortex Breakdown Types and Global Modes in Swirling Combustor Flows with Axial Injection

This paper presents the results of a combined experimental, numerical, and analytical study of the occurrence of different vortex breakdown types and helical instabilities in realistic swirling combustor flows with axial air injection through a centerbody. A parametric study of the isothermal flowfield inside the combustion chamber and in the mixing tube upstream of the combustor is carried out in a water-tunnel test facility. Selected configurations were further assessed under reacting conditions. Next, a large-eddy simulation was conducted and successfully validated with the experimental data. The isothermal and reacting results show a strong effect of the inflow parameters on the type of the vortex breakdown and the frequency, amplitude, and shape of the global mode. Linear local hydrodynamic stability analyses, carried out on the time-average measured and simulated velocity data, yield the absolutely unstable domain inside the flowfield. Axial injection is shown to impede a zone of absolute instabilit...

Increasing Flashback Resistance in Lean Premixed Swirl-Stabilized Hydrogen Combustion by Axial Air Injection

Since lean premixed combustion allows for fuel-efficiency and low emissions, it is nowadays state of the art in stationary gas turbines. In the long term, it is also a promising approach for aero engines, when safety issues like lean blowout (LBO) and flame flashback in the premixer can be overcome. While for the use of hydrogen the LBO limits are extended, the flashback propensity is increased. Thus, axial air injection is applied in order to eliminate flashback in a swirl-stabilized combustor burning premixed hydrogen. Axial injection constitutes a non-swirling jet on the central axis of the radial swirl generator which influences the vortex breakdown position. In the present work changes in the flow field and their impact on flashback limits of a model combustor are evaluated. First, a parametric study is conducted under isothermal test conditions in a water tunnel employing particle image velocimetry (PIV). The varied parameters are the amount of axially injected air and swirl number. Subsequently, flashback safety is evaluated in the presence of axial air injection in an atmospheric combustor test rig and a stability map is recorded. The flame structure is measured using high-speed OH* chemiluminescence imaging. Simultaneous high-speed PIV measurements of the reacting flow provide insight in the time-resolved reacting flow field and indicate the flame location by evaluating the Mie scattering of the raw PIV images by the means of the Qualitative Light Sheet (QLS) technique.The isothermal tests identify the potential of axial air injection to overcome the axial velocity deficits at the nozzle outlet, which is considered crucial in order to provide flashback safety. This effect of axial air injection is shown to prevail in the presence of a flame. Generally, flashback safety is shown to benefit from an elevated amount of axial air injection and a lower swirl number. Note, that the latter also leads to increased NOx emissions, while axial air injection does not. Additionally, fuel momentum is indicated to positively influence flashback resistance, although based on a different mechanism, an explanation of which is suggested. In summary, flashback-proof operation of the burner with a high amount of axial air injection is achieved on the whole operating range of the test rig at inlet temperatures of 620 K and up to stoichiometric conditions while maintaining single digit NOx emissions below a flame temperature of 2000 K.Copyright

Acoustic Response of a Helmholtz Resonator Exposed to Hot-Gas Penetration and High Amplitude Oscillations

Helmholtz resonators are often used in the gas turbine industry for the damping of thermoacoustic instabilities. To prevent thermal destruction, these devices are usually cooled by a purging flow. Since the acoustic velocity inside the neck of the resonator becomes very high already at moderate pressure oscillation levels, hot-gas penetration cannot always be fully avoided. This study extends a well-known nonlinear impedance model to include the influence of hot-gas intrusion into the Helmholtz resonator neck. A time-dependent but spatially averaged density function of the volume flow in the neck is developed. The steady component of this density function is implemented into the nonlinear impedance model to account for the effect of hot-gas intrusion. The proposed model predicts a significant shift in the resonance frequency of the damper towards higher frequencies, depending on the amplitude of the acoustic velocity in the neck and the temperature of the penetrating hot gas. Subsequently, the model is verified by the experimental investigation of two resonance frequencies (86 Hz and 128 Hz) for two hot gas temperatures (1470 K and 570 K) and various pressure oscillation amplitudes. The multimicrophone method, in combination with a microphone flush-mounted in the resonator volume, is used to determine the impedance of the Helmholtz damper. Additionally, a movable ultra-thin thermocouple was used to determine the degree of hot-gas penetration and the change of the mean temperature at various axial positions in the neck. A very good agreement between the model and the experimental data is obtained for all levels of pressure amplitudes and of hot-gas penetration depths. The mean air temperatures in the neck were accurately predicted too.

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.

Shockless Explosion Combustion: Experimental Investigation of a New Approximate Constant Volume Combustion Process

Approximate constant volume combustion (aCVC) is a promising way to achieve a step change in the efficiency of gas turbines. This work investigates a recently proposed approach to implement aCVC in a gas turbine combustion system: shockless explosion combustion (SEC). The new concept overcomes several disadvantages such as sharp pressure transitions, entropy generation due to shock waves, and exergy losses due to kinetic energy which are associated with other aCVC approaches such as pulsed detonation combustion. The combustion is controlled via the fuel/air mixture distribution which is adjusted such that the entire fuel/air volume undergoes a spatially quasi-homogeneous auto-ignition. Accordingly, no shock waves occur and the losses associated with a detonation wave are not present in the proposed system. Instead, a smooth pressure rise is created due to the heat release of the homogeneous combustion. An atmospheric combustion test rig is designed to investigate the auto-ignition behavior of relevant fuels under intermittent operation, currently up to a frequency of 2 Hz. Application of OH*– and dynamic pressure sensors allows for a spatially and time-resolved detection of ignition delay times and locations. Dimethyl ether (DME) is used as fuel since it exhibits reliable auto-ignition already at 920 K mixture temperature and ambient pressure. First, a model-based control algorithm is used to demonstrate that the fuel valve can produce arbitrary fuel profiles in the combustion tube. Next, the control algorithm is used to achieve the desired fuel stratification, resulting in a significant reduction in spatial variance of the auto-ignition delay times. This proves that the control approach is a useful tool for increasing the homogeneity of the auto-ignition.

Investigation of Lean Premixed Swirl-Stabilized Hydrogen Burner With Axial Air Injection Using OH-PLIF Imaging

In the context of lean premixed combustion, the prevention of upstream flame propagation in the premixing zone, referred to as flashback, is a crucial challenge related to the application of hydrogen as a fuel for gas turbines. The location of flame anchoring and its impact on flashback tendencies in a technically premixed, swirl-stabilized hydrogen burner are investigated experimentally at atmospheric pressure conditions using planar laser-induced fluorescence of hydroxyl radicals (OH-PLIF). The inlet conditions are systematically varied with respect to equivalence ratio (ϕ = 0.2–1.0), bulk air velocity u0 = 30–90m/s and burner preheat temperature ranging from 300K to 700K. The burner is mounted in the atmospheric combustion test rig at the HFI, firing at a power of up to 220 kW into a 105 mm diameter quartz cylinder, which provides optical access to the flame region. The experiments were performed using an in-house burner design that previously proved to be highly resistant against flashback occurrence by applying the axial air injection strategy. Axial air injection constitutes a non-swirling air jet on the central axis of the radial swirl generator, thus, influencing the vortex breakdown position. High axial air injection yields excellent flashback resistance and is used to investigate the whole inlet parameter space. In order to trigger flashback, the amount of axially injected air is reduced, which allowed to investigate the near flashback flame behavior. Results show that both, fuel momentum of hydrogen and axial air injection alter the isothermal flow field and cause a downstream shift of the axial flame front location. Such a shift is proven beneficial for flashback resistance. This effect was quantified by applying an edge detection algorithm to the OH-PLIF images, in order to extract the location of maximum flame front likelihood xF. The temperature and equivalence ratio dependence of the parameters xF is identified to be governed by the momentum ratio between fuel and air flow J. These results contribute to the understanding of the superior flashback limits of configurations applying high amounts of axial air injection over medium or none air injection.Copyright

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

Design and Assessment of a Fuel-Flexible Low Emission Combustor for Dry and Steam-Diluted Conditions

In the present study a new combustor design for swirl-stabilized premixed combustion is presented and analyzed. By means of axial air injection into the mixing tube, the flow field inside the mixing tube and combustion chamber is altered for the purpose of increasing flame stability, resulting in low lean blow-out limits and flashback safety. For this reason, the combustor design is suited for fuel-flexible combustion over a wide range of reactivities including those of pure hydrogen and natural gas. Furthermore, the combustor design is suitable for high amounts of steam injection, significantly decreasing NOx emissions. The assessment of the flow field properties is conducted under isothermal and reacting conditions and the results are in good agreement. The fuel-air-mixing quality is examined by water tunnel experiments using laser induced fluorescence and show low levels of unmixedness. Flame properties are assessed by means of OH planar laser induced fluorescence and OH*-chemiluminescence. Emission probing of major combustion species was also conducted.Copyright

Vortex Breakdown and Global Modes in Swirling Combustor Flows with Axial Air Injection

Strongly swirling flows are used in the vast majority of gas turbine combustors. The complex flow field downstream of the vortex breakdown provides good flame stabilization but is also prone to self-excited large scale hydrodynamic instabilities. The role of these global modes, which usually manifest in a Precessing Vortex Core (PVC), for the combustion process is still an open question and influences on mixing processes and thermoacoustic oscillations have been proposed. In the current study the effect of axial air injection through a truncated center body on the type of the Vortex Breakdown (VB) and the global hydrodynamic mode is investigated using a combined experimental, numerical, and analytical approach. A parametric study of the isothermal flow field inside the combustion chamber and in the mixing tube upstream of the combustor is carried out in a water tunnel test facility. Selected configurations were further assessed under reacting conditions using methane fuel. Next, a Large Eddy Simulation (LES) was conducted and successfully validated with the experimental data. All results show a strong effect of the inflow parameters (axial injection rate and inlet swirl number) on the type of the vortexbreakdown and the frequency, amplitude, and shape of the global mode. The reacting cases show very similar results as the isothermal cases, proving the relevance of the isothermal investigation. Linear local hydrodynamic stability analyses, carried out on the time-average measured velocity data and the numerically obtained data, yield the absolutely unstable domain inside the flow field. Axial injection is shown to impede a zone of absolute instability near the combustor inlet while a a second zone further downstream remains. An excellent agreement of the measured to the calculated frequencies of the global modes is achieved over the whole range of investigated axial injection rates. The findings of this paper help to understand the mechanisms that are involved into the occurrence of global modes in swirling combustor flows and how they may be controlled by small flow field modifications. Furthermore, axial air injection is shown to provide a suitable flow field for flashback-proof combustor operation.