Lothar Rukes

Why nonuniform density suppresses the precessing vortex core

Linear stability analysis is applied to a swirl-stabilized combustor flow with the aim to understand how the flame shape and associated density field affects the manifestation of self-excited flow instabilities. In isothermal swirling jets, self-excited flow oscillations typically manifest in a precessing vortex core and synchronized growth of large-scale spiral-shaped vortical structures. Recent theoretical studies relate these dynamics to a hydrodynamic global instability. These global modes also emerge in reacting flows, thereby crucially affecting the mixing characteristics and the flame dynamics. It is, however, observed that these self-excited flow oscillations are often suppressed in the reacting flow, while they are clearly present at isothermal conditions. This study provides strong evidence that the suppression of the precessing vortex core is caused by density inhomogeneities created by the flame. This mechanism is revealed by considering two reacting flow configurations: The first configuration represents a perfectly premixed steam-diluted detached flame featuring a strong precessing vortex core. The second represents a perfectly premixed dry flame anchoring near the combustor inlet, which does not exhibit self-excited oscillations. Experiments are conducted in a generic combustor test rig and the flow dynamics are captured using PIV and LDA. The corresponding density fields are approximated from the seeding density using a quantitative light sheet technique. The experimental results are compared to the global instability properties derived from hydrodynamic linear stability theory. Excellent agreement between the theoretically derived global mode frequency and measured precession frequency provide sufficient evidence to conclude that the self-excited oscillations are, indeed, driven by a global hydrodynamic instability. The effect of the density field on the global instability is studied explicitly by performing the analysis with and without density stratification. It turns out that the significant change in instability is caused by the radial density gradients in the inner recirculation zone and not by the change of the mean velocity field. The present work provides a theoretical framework to analyze the global hydrodynamic instability of realistic combustion configurations. It allows for relating the flame position and the resulting density field to the emergence of a precessing vortex core.

Mean flow stability analysis of oscillating jet experiments

Linear stability analysis is applied to the mean flow of an oscillating round jet with the aim to investigate the robustness and accuracy of mean flow stability wave models. The jets axisymmetric mode is excited at the nozzle lip through a sinusoidal modulation of the flow rate at amplitudes ranging from 0.1 % to 100 %. The instantaneous flow field is measured via particle image velocimetry and decomposed into a mean and periodic part utilizing proper orthogonal decomposition. Local linear stability analysis is applied to the measured mean flow adopting a weakly nonparallel flow approach. The resulting global perturbation field is carefully compared to the measurements in terms of spatial growth rate, phase velocity, and phase and amplitude distribution. It is shown that the stability wave model accurately predicts the excited flow oscillations during their entire growth phase and during a large part of their decay phase. The stability wave model applies over a wide range of forcing amplitudes, showing no pronounced sensitivity to the strength of nonlinear saturation. The upstream displacement of the neutral point and the successive reduction of gain with increasing forcing amplitude is very well captured by the stability wave model. At very strong forcing (>40%), the flow becomes essentially stable to the axisymmetric mode. For these extreme cases, the prediction deteriorates from the measurements due to an interaction of the forced wave with the geometric confinement of the nozzle. Moreover, the model fails far downstream in a region where energy is transferred from the oscillation back to the mean flow. This study supports previously conducted mean flow stability analysis of self-excited flow oscillations in the cylinder wake and in the vortex breakdown bubble and extends the methodology to externally forced convectively unstable flows.

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...

The impact of heating the breakdown bubble on the global mode of a swirling jet: Experiments and linear stability analysis

This study investigates the dynamics of non-isothermal swirling jets undergoing vortex breakdown, with an emphasis on helical coherent structures. It is proposed that the dominant helical coherent structure can be suppressed by heating the recirculation bubble. This proposition is assessed with Stereo Particle Image Velocimetry (PIV) measurements of the breakdown region of isothermal and heated swirling jets. The coherent kinetic energy of the dominant helical structure was derived from PIV snapshots via Proper Orthogonal Decomposition. For one set of experimental parameters, mild heating is found to increase the energy content of the dominant helical mode. Strong heating leads to a reduction by 30\% of the coherent structures energy. For a second set of experimental parameters, no alteration of the dominant coherent structure is detectable. Local linear stability analysis of the time-averaged velocity fields shows that the key difference between the two configurations is the density ratio at the respective wavemaker location. A density ratio of approximately 0.8 is found to correlate to a suppression of the global mode in the experiments. A parametric study with model density and velocity profiles indicates the most important parameters that govern the local absolute growth rate: The density ratio and the relative position of the density profiles and the inner shear layer.

An assessment of turbulence models for linear hydrodynamic stability analysis of strongly swirling jets

Abstract Linear stability analysis has proven to be a useful tool in the analysis of dominant coherent structures, such as the von Karman vortex street and the global spiral mode associated with the vortex breakdown of swirling jets. In recent years, linear stability analysis has been applied successfully to turbulent time-mean flows, instead of laminar base-flows, which requires turbulent models that account for the interaction of the turbulent field with the coherent structures. To retain the stability equations of laminar flows, the Boussinesq approximation with a spatially nonuniform but isotropic eddy viscosity is typically employed. In this work we assess the applicability of this concept to turbulent strongly swirling jets, a class of flows that is particularly unsuited for isotropic eddy viscosity models. Indeed we find that unsteady RANS simulations only match with experiments with a Reynolds stress model that accounts for an anisotropic eddy viscosity. However, linear stability analysis of the mean flow is shown to accurately predict the global mode growth rate and frequency if the employed isotropic eddy viscosity represents a least-squares approximation of the anisotropic eddy viscosity. Viscosities derived from the k − ϵ model did not achieve a good prediction of the mean flow nor did they allow for accurate stability calculations. We conclude from this study that linear stability analysis can be accurate for flows with strongly anisotropic turbulent viscosity and the capability of the Boussinesq approximation in terms of URANS-based mean flow prediction is not a prerequisite.

Why Non-Uniform Density Suppresses the Precessing Vortex Core

Isothermal swirling jets undergoing vortex breakdown are known to be susceptible to self-excited flow oscillations. They manifest in a precessing vortex core and synchronized growth of large-scale vortical structures. Recent theoretical studies associate these dynamics with the onset of a global hydrodynamic instability mode. These global modes also emerge in reacting flows, thereby crucially affecting the mixing characteristics and the flame dynamics. It is, however, observed that these self-excited flow oscillations are often suppressed in the reacting flow, while they are clearly present at isothermal conditions. This study provides strong evidence that the suppression of the precessing vortex core is caused by density stratification created by the flame. This mechanism is revealed by considering two reacting flow configurations: The first configuration represents a detached steam-diluted natural gas swirl-stabilized flame featuring a strong precessing vortex core. The second represents a natural gas swirl-stabilized flame anchoring near the combustor inlet, which does not exhibit self-excited oscillations. Experiments are conducted in a generic combustor test rig and the flow dynamics are captured using PIV and LDA. The corresponding density fields are approximated from the seeding density using a quantitative light sheet technique. The experimental results are compared to the global instability properties derived from hydrodynamic linear stability theory. Excellent agreement between the theoretically derived global mode frequency and measured precession frequency provide sufficient evidence to conclude that the self-excited oscillations are, indeed, driven by a global hydrodynamic instability. The effect of the density field on the global instability is studied explicitly by performing the analysis with and without density stratification. It turns out that the significant change on instability is caused by the radial density gradients in the inner recirculation zone and not by the change of the mean velocity field. The present work provides a theoretical framework to analyze the global hydrodynamic instability of realistic combustion configurations. It allows relating the flame position and the resulting density field to the emergence of a precessing vortex core.Copyright

Methods for the Extraction and Analysis of the Global Mode in Swirling Jets Undergoing Vortex Breakdown

Swirling jets undergoing vortex breakdown are widely used in combustion applications, due to their ability to provide aerodynamic flame stabilization. It is well known that vortex breakdown is accompanied by a dominant coherent structure, the so called precessing vortex core (PVC). Reports on the impact of the PVC on the combustion process range from beneficial to detrimental. In any event, efficient methods for the analysis of the PVC help to increase the benefit or reduce the penalty resulting from it. This study uses Particle Image Velocimetry (PIV) measurements of a generic non-isothermal swirling jet to demonstrate the use of advanced data analysis techniques. In particular, the Finite Time Lyapunov Exponent (FTLE) and local linear stability analysis (LSA) are shown to reveal deep insight into the physical mechanisms that drive the PVC. Particularly, it is demonstrated that the PVC amplitude is strongly reduced, if heating is applied at the wavemaker of the flow. These techniques are complemented by the traditionally used Proper Orthogonal Decomposition (POD) and spatial correlation techniques. It is demonstrated how these methods complement each other and lead to a comprehensive understanding of the PVC that lays out the path to efficient control strategies.Copyright

The Impact of an Inhomogeneous Temperature Field on the Precessing Vortex Core in Swirling Jets Undergoing Vortex Breakdown

The vast majority of modern gas turbines employ swirling jets undergoing vortex breakdown in their burners. Usually, the vortex breakdown is accompanied by a large scale coherent structure, the so called precessing vortex core (PVC). It is known that under isothermal conditions the PVC always occurs in the natural flow, if the swirl number is sufficiently large. Nevertheless, it has been observed that the PVC may be suppressed in the reacting flows of gas turbine combustors, even though, the swirl number would allow for the occurrence of a PVC under isothermal conditions. Therefore, this study investigates the influence of a non-homogeneous temperature distribution in a non-reacting flow on the PVC. In order to isolate the influence of the temperature, a heated generic swirling jet is considered. This eliminates the need to consider the much more complex flow field of gas turbine combustors, where the PVC may be influenced by the confinement, the chemical reaction and thermoacoustics. The flow field of the generic swirling jet is investigated at a fixed Reynolds number and for different swirl numbers by means of Stereo Particle-ImageVelocimetry (SPIV). The temperature distribution is imposed by a heating element in the flow. Proper-Orthogonal-Decomposition (POD) is applied to the measured data to gain insight into the energy content and spatial structure of the PVC. Linear hydrodynamic stability analysis is used as an analytical tool to investigate the growth or dampening of the PVC. It is shown that the simplifications described above are justified. A heated generic swirling jet is a suitable model to investigate the basic phenomena that lead to the suppression of the PVC in non-isothermal swirling jets. The decisive factor for the suppression of the PVC in non-isothermal, non-reacting flows is the temperature gradient in the vicinity of the upstream stagnation point.

The influence of the inner shear layer on the suppression of the global mode in heated swirling jets

The vast majority of modern gas turbines employ swirling jets undergoing vortex breakdown in their burners. Usually, the vortex breakdown is accompanied by a large scale coherent structure, the so called global mode. It is known that under isothermal conditions the global mode always occurs in the natural flow, if the swirl number is sufficiently large. Nevertheless, it has been observed that the global mode may be suppressed in the reacting flows of gas turbine combustors, even though, the swirl number would allow for the occurrence of a global mode under isothermal conditions. Therefore, this study investigates the influence of a non-homogeneous temperature distribution in a non-reacting flow on the global mode. In order to isolate the influence of the temperature, a heated generic swirling jet is considered. This eliminates the need to consider the much more complex flow field of gas turbine combustors, where the global mode may be influenced by the confinement, the chemical reaction and thermoacoustics. The flow field of the generic swirling jet is investigated by means of Stereo Particle-Image-Velocimetry (SPIV) at various levels of heating. The temperature distribution is imposed by a heating element in the flow. Analysis of the experimental data suggests that a sufficient amount of heating hinders the synchronization of the inner and outer shear layer, a feature otherwise typical of the global mode. The authors propose that the applied heating leads to an altered growth of the inner shear layer, thus altering the stability characteristics of the flow in the vicinity of the wave-maker. Linear stability analysis is used to conduct a model study, in order to investigate the parametric influence of the shear layer thickness on the stability characteristics of a heated swirling jet.

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.