scholarly journals Coherent structures in a swirl injector at Re  = 4800 by nonlinear simulations and linear global modes

2016 ◽  
Vol 792 ◽  
pp. 620-657 ◽  
Author(s):  
O. Tammisola ◽  
M. P. Juniper
Keyword(s):  
Shear Layer ◽  
Vortex Core ◽  
Eddy Viscosity ◽  
Vortex Breakdown ◽  
Turbulent Viscosity ◽  
Fuel Injector ◽  
Mean Flow ◽  
Turbulent Dissipation ◽  
Precessing Vortex Core ◽  
Global Modes

The large-scale coherent motions in a realistic swirl fuel-injector geometry are analysed by direct numerical simulations (DNS), proper orthogonal decomposition (POD), and linear global modes. The aim is to identify the origin of instability in this turbulent flow in a complex internal geometry. The flow field in the nonlinear simulation is highly turbulent, but with a distinguishable coherent structure: the precessing vortex core (a spiralling mode). The most energetic POD mode pair is identified as the precessing vortex core. By analysing the fast Fourier transform (FFT) of the time coefficients of the POD modes, we conclude that the first four POD modes contain the coherent fluctuations. The remaining POD modes (incoherent fluctuations) are used to form a turbulent viscosity field, using the Newtonian eddy model. The turbulence sets in from convective shear layer instabilities even before the nonlinear flow reaches the other end of the domain, indicating that equilibrium solutions of the Navier–Stokes are never observed. Linear global modes are computed around the mean flow from DNS, applying the turbulent viscosity extracted from POD modes. A slightly stable discrete $m=1$ eigenmode is found, well separated from the continuous spectrum, in very good agreement with the POD mode shape and frequency. The structural sensitivity of the precessing vortex core is located upstream of the central recirculation zone, identifying it as a spiral vortex breakdown instability in the nozzle. Furthermore, the structural sensitivity indicates that the dominant instability mechanism is the Kelvin–Helmholtz instability at the inflection point forming near vortex breakdown. Adjoint modes are strong in the shear layer along the whole extent of the nozzle, showing that the optimal initial condition for the global mode is localized in the shear layer. We analyse the qualitative influence of turbulent dissipation in the stability problem (eddy viscosity) on the eigenmodes by comparing them to eigenmodes computed without eddy viscosity. The results show that the eddy viscosity improves the complex frequency and shape of global modes around the fuel-injector mean flow, while a qualitative wavemaker position can be obtained with or without turbulent dissipation, in agreement with previous studies. This study shows how sensitivity analysis can identify which parts of the flow in a complex geometry need to be altered in order to change its hydrodynamic stability characteristics.

Impact of Precessing Vortex Core Dynamics on Shear Layer Response in a Swirling Jet

2018 ◽  
Vol 140 (6) ◽  
Author(s):  
Mark Frederick ◽  
Kiran Manoharan ◽  
Joshua Dudash ◽  
Brian Brubaker ◽  
Santosh Hemchandra ◽  
...  
Keyword(s):  
Stability Analysis ◽  
Shear Layer ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Vortex Core ◽  
Combustion Instability ◽  
Forced Response ◽  
Longitudinal Acoustic ◽  
Precessing Vortex Core ◽  
Acoustic Forcing

Combustion instability, the coupling between flame heat release rate oscillations and combustor acoustics, is a significant issue in the operation of gas turbine combustors. This coupling is often driven by oscillations in the flow field. Shear layer roll-up, in particular, has been shown to drive longitudinal combustion instability in a number of systems, including both laboratory and industrial combustors. One method for suppressing combustion instability would be to suppress the receptivity of the shear layer to acoustic oscillations, severing the coupling mechanism between the acoustics and the flame. Previous work suggested that the existence of a precessing vortex core (PVC) may suppress the receptivity of the shear layer, and the goal of this study is to first, confirm that this suppression is occurring, and second, understand the mechanism by which the PVC suppresses the shear layer receptivity. In this paper, we couple experiment with linear stability analysis to determine whether a PVC can suppress shear layer receptivity to longitudinal acoustic modes in a nonreacting swirling flow at a range of swirl numbers. The shear layer response to the longitudinal acoustic forcing manifests as an m = 0 mode since the acoustic field is axisymmetric. The PVC has been shown both in experiment and linear stability analysis to have m = 1 and m = −1 modal content. By comparing the relative magnitude of the m = 0 and m = −1,1 modes, we quantify the impact that the PVC has on the shear layer response. The mechanism for shear layer response is determined using companion forced response analysis, where the shear layer disturbance growth rates mirror the experimental results. Differences in shear layer thickness and azimuthal velocity profiles drive the suppression of the shear layer receptivity to acoustic forcing.

scholarly journals Stability and Sensitivity Analysis of Hydrodynamic Instabilities in Industrial Swirled Injection Systems

2017 ◽  
Author(s):  
Thomas L. Kaiser ◽  
Thierry Poinsot ◽  
Kilian Oberleithner
Keyword(s):  
Stability Analysis ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Vortex Core ◽  
Large Eddy Simulations ◽  
Mode Shapes ◽  
Mean Flow ◽  
Precessing Vortex Core ◽  
Large Eddy ◽  
Good Agreement

The hydrodynamic instability in an industrial, two-staged, counter-rotative, swirled injector of highly complex geometry is under investigation. Large eddy simulations show that the complicated and strongly nonparallel flow field in the injector is superimposed by a strong precessing vortex core. Mean flow fields of large eddy simulations, validated by experimental particle image velocimetry measurements are used as input for both local and global linear stability analysis. It is shown that the origin of the instability is located at the exit plane of the primary injector. Mode shapes of both global and local linear stability analysis are compared to a dynamic mode decomposition based on large eddy simulation snapshots, showing good agreement. The estimated frequencies for the instability are in good agreement with both the experiment and the simulation. Furthermore, the adjoint mode shapes retrieved by the global approach are used to find the best location for periodic forcing in order to control the precessing vortex core.

scholarly journals Numerical study of the vortex breakdown and vortex reconnection in the flow path of high-pressure water turbine

2021 ◽  
Vol 2088 (1) ◽  
pp. 012040
Author(s):  
A V Sentyabov ◽  
D V Platonov ◽  
A V Minakov ◽  
A S Lobasov
Keyword(s):  
Vortex Core ◽  
Vortex Breakdown ◽  
Numerical Study ◽  
Pressure Fluctuations ◽  
Hydraulic Turbine ◽  
Sliding Mesh ◽  
Precessing Vortex Core ◽  
Vortex Reconnection ◽  
Water Turbine ◽  
Pressure Water

Abstract The paper presents a study of the instability of the precessing vortex core in the model of the draft tube of a hydraulic turbine. The study was carried out using numerical modeling using various approaches: URANS, RSM, LES. The best agreement with the experimental data was shown by the RSM and LES methods with the modelling of the runner rotation by the sliding mesh method. In the regime under consideration, the precessing vortex rope is subject to instability, which leads to reconnection of its turns and the formation of an isolated vortex ring. Reconnection of the vortex core leads to aperiodic and intense pressure fluctuations recorded on the diffuser wall.

The Effects of Exit Boundary Condition on Precessing Vortex Core Dynamics

2019 ◽  
Author(s):  
Danielle Mason ◽  
Sean Clees ◽  
Mark Frederick ◽  
Jacqueline O’Connor
Keyword(s):  
Boundary Condition ◽  
Vortex Core ◽  
Gas Turbines ◽  
Vortex Breakdown ◽  
Base Flow ◽  
Eigenvalue Decomposition ◽  
Precessing Vortex Core ◽  
Swirling Jet ◽  
Exit Boundary ◽  
The Impact

Abstract Many industrial combustion systems, especially power generation gas turbines, use fuel-lean combustion to reduce NOx emissions. However, these systems are highly susceptible to combustion instability, the coupling between combustor acoustics and heat release rate oscillations of the flame. It has been shown in previous work by the authors that a precessing vortex core (PVC) can suppress shear layer receptivity to external perturbations, reducing the potential for thermoacoustic coupling. The goal of this study is to understand the effect of combustor exit boundary condition on the flow structure of a swirling jet to increase fundamental understanding of how combustor design impacts PVC dynamics. The swirling jet is generated with a radial-entry, variable-angle swirler, and a quartz cylinder is fixed on the dump plane for confinement. Combustor exit constriction plates of different diameters are used to determine the impact of exit boundary condition on the flow field. Particle image velocimetry (PIV) is used to capture the velocity field inside the combustor. Spectral proper orthogonal decomposition, a frequency-resolved eigenvalue decomposition that can identify energetic structures in the flow, is implemented to identify the PVC at each condition in both energy and frequency space. We find that exit boundary diameter affects both the structure of the flow and the dynamics of the PVC. Higher levels of constriction (smaller diameters) force the downstream stagnation point of the vortex breakdown bubble upstream, resulting in greater divergence of the swirling jet. Further, as the exit diameter decreases, the PVC becomes less energetic and less spatially defined. Despite these changes in the base flow and PVC coherence, the PVC frequency is not altered by the exit boundary constriction. These trends will help inform our understanding of the impact of boundary conditions on both static and dynamic flame stability.

Control of the Precessing Vortex Core by Open and Closed-Loop Forcing in the Jet Core

2016 ◽  
Author(s):  
Phoebe Kuhn ◽  
Jonas P. Moeck ◽  
Christian Oliver Paschereit ◽  
Kilian Oberleithner
Keyword(s):  
Vortex Core ◽  
Oscillation Frequency ◽  
Closed Loop ◽  
Hydrodynamic Instability ◽  
Axial Position ◽  
Hot Wire ◽  
Mean Flow ◽  
Precessing Vortex Core ◽  
The Mean ◽  
Lock In

The precessing vortex core (PVC) is the dominant coherent structure of swirling jets, which are commonly applied in gas turbine combustion. It stems from a global hydrodynamic instability that is caused by internal feedback mechanisms in the jet core. In this work, we apply open and closed-loop forcing in a generic non-reacting jet to control this mechanism and the PVC. Control is exerted by two oppositely facing, counter-phased zero-net mass flux jets, which are introduced radially into the flow through a thin lance positioned on the jet center axis. By using this type of forcing, the instability mode m = 1, corresponding to the PVC, can either be excited or damped. This markedly affects the PVC oscillation frequency and amplitude. The passive influence of the actuation lance on the mean flow field properties and the coherent flow dynamics is studied first without forcing. PIV and hot-wire measurements reveal an effect on the mean flow, but no qualitative changes of the PVC dynamics. Lock-in experiments are conducted, in which the synchronization behavior of the PVC with the forcing is determined. Here, two different cases are considered. First, actuation is applied at different streamwise positions in order to identify the region of highest receptivity towards external forcing. This region of lowest lock-in amplitude is shown to coincide with the location of the wavemaker, shortly upstream of the vortex breakdown bubble. Second, the lock-in behavior at a fixed axial position and various forcing frequencies ff is studied. A linear correlation between the lock-in amplitude and the deviation of the forcing frequency from the natural oscillation frequency |ff – fn| is observed. Closed-loop control is then applied with the aim to suppress the PVC. The actuator lance is positioned in the wavemaker region, where the flow is most receptive. Magnitude and phase of the natural flow oscillation associated with the PVC are estimated from four hot-wire signals using an extended Kalman filter. The estimated PVC signal is phase-shifted and fed back to the actuator. PIV measurements reveal that feedback control achieves a reduction of the PVC oscillation energy of about 40%.

Characterization of Different Actuator Designs for the Control of the Precessing Vortex Core in a Swirl-Stabilized Combustor

2017 ◽  
Vol 140 (4) ◽  
Author(s):  
Finn Lückoff ◽  
Moritz Sieber ◽  
Christian Oliver Paschereit ◽  
Kilian Oberleithner
Keyword(s):  
Feedback Control ◽  
Vortex Core ◽  
Pressure Sensors ◽  
Pollutant Emissions ◽  
Mean Flow ◽  
Thermoacoustic Instabilities ◽  
Precessing Vortex Core ◽  
System A ◽  
Excellent Control ◽  
Lock In

The precessing vortex core (PVC) represents a helical-shaped coherent flow structure typically occurring in both reacting and nonreacting swirling flows. Until now, the fundamental impact of the PVC on flame dynamics, thermoacoustic instabilities, and pollutant emissions is still unclear. In order to identify and investigate these mechanisms, the PVC needs to be controlled effectively with a feedback control system. A previous study successfully applied feedback control in a generic swirling jet setup. The next step is to transfer this approach into a swirl-stabilized combustor, which poses big challenges on the actuator and sensor design and placement. In this paper, different actuator designs are investigated with the goal of controlling the PVC dynamics. The actuation strategy aims to force the flow near the origin of the instability—the so-called wavemaker. To monitor the PVC dynamics, arrays of pressure sensors are flush-mounted at the combustor inlet and the combustion chamber walls. The best sensor placement is evaluated with respect to the prediction of the PVC dynamics. Particle image velocimetry (PIV) is used to evaluate the passive impact of the actuator shape on the mean flow field. The performance of each actuator design is evaluated from lock-in experiments showing excellent control authority for two out of seven actuators. All measurements are conducted at isothermal conditions in a prototype of a swirl-stabilized combustor.

Guiding Actuator Designs for Active Flow Control of the Precessing Vortex Core by Adjoint Linear Stability Analysis

2018 ◽  
Author(s):  
Jens S. Müller ◽  
Finn Lückoff ◽  
Kilian Oberleithner
Keyword(s):  
Stability Analysis ◽  
Combustion Chamber ◽  
Linear Stability ◽  
Linear Stability Analysis ◽  
Vortex Core ◽  
Stereoscopic Particle Image Velocimetry ◽  
Axial Position ◽  
Mean Flow ◽  
Precessing Vortex Core ◽  
Lock In

The fundamental impact of the precessing vortex core (PVC) as a dominant coherent flow structure in the flow field of swirl-stabilized gas turbine combustors has still not been investigated in depth. In order to do so, the PVC needs to be actively controlled to be able to set its parameters independently to any other of the combustion system. In this work, open-loop actuation is applied in the mixing section between the swirler and the generic combustion chamber of a non-reacting swirling jet setup to investigate the receptivity of the PVC with regard to its lock-in behavior at different streamwise positions. The mean flow in the mixing section as well as in the combustion chamber is measured by stereoscopic particle image velocimetry and the PVC is extracted from the snapshots using proper orthogonal decomposition. The lock-in experiments reveal the axial position in the mixing section that is most suitable for actuation. Furthermore, a global linear stability analysis is conducted to determine the adjoint mode of the PVC which reveals the regions of highest receptivity to periodic actuation based on mean flow input only. This theoretical receptivity model is compared with the experimentally obtained receptivity data and the applicability of the adjoint-based model for the prediction of optimal actuator designs is discussed.

The Role of the Centerbody Wake on the Precessing Vortex Core Dynamics of a Swirl Nozzle

2020 ◽  
Author(s):  
Arnab Mukherjee ◽  
Nishanth Muthichur ◽  
Chaitali More ◽  
Saarthak Gupta ◽  
Santosh Hemchandra
Keyword(s):  
Stability Analysis ◽  
Nozzle Exit ◽  
Vortex Core ◽  
Vortex Breakdown ◽  
Spatial Separation ◽  
Reynolds Numbers ◽  
Swirling Jets ◽  
Precessing Vortex Core ◽  
Swirl Flows ◽  
Structural Sensitivity Analysis

Abstract The precessing vortex core (PVC) phenomenon in swirling jets is a helical instability in the flow driven by the coherent precession of the vortex breakdown bubble (VBB) around the flow axis, resulting in the helical rollup of the shear layer. This instabilitty is driven mainly by flow processes in the region upstream of the VBB. Centerbodies, commonly employed in combustor nozzles create a central wake recirculation zone (CWRZ) that can interfere with VBB precession and hence suppress the PVC. We study this phenomenon in a swirl nozzle with a centerbody whose end face is flush with the nozzle exit plane, using large eddy simulations (LES) and linear hydrodynamic stability analysis for flow Reynolds numbers Re = 48,767 and 82,751, based on nozzle exit diameter and bulk flow velocity. For one of the Re = 82,751 cases the centerbody end face diameter is halved resulting in the onset of coherent VBB precession. Linear stability analysis reveals a marginally unstable mode in this case. The same mode is found to be stable in the nominal cases. Structural sensitivity analysis for these two cases, shows that the VBB precession eigenmode is sensitive to changes in the time averaged flow in the VBB-CWRZ merger region. This suggests that the reduction in CWRZ length due to halving the centerbody end face diameter is the reason for the onset of VBB precession. These results suggest that in general, spatial separation between the CWRZ and VBB can result in the onset of VBB precession and the emergence of PVC oscillations in swirl flows.

The Role of the Centerbody Wake on the Precessing Vortex Core Dynamics of a Swirl Nozzle

2021 ◽  
Vol 143 (5) ◽  
Author(s):  
Arnab Mukherjee ◽  
Nishanth Muthichur ◽  
Chaitali More ◽  
Saarthak Gupta ◽  
Santosh Hemchandra
Keyword(s):  
Stability Analysis ◽  
Nozzle Exit ◽  
Vortex Core ◽  
Vortex Breakdown ◽  
Spatial Separation ◽  
Reynolds Numbers ◽  
Swirling Jets ◽  
Precessing Vortex Core ◽  
Large Eddy ◽  
Structural Sensitivity Analysis

Abstract The precessing vortex core (PVC) phenomenon in swirling jets is a helical instability in the flow driven by the coherent precession of the vortex breakdown bubble (VBB) around the flow axis, resulting in the helical rollup of the shear layer. This instability is driven by flow processes in the region upstream of the VBB. Centerbodies, commonly employed in combustor nozzles, create a centerbody wake recirculation zone (CWRZ) that can interfere with VBB precession and hence suppress the PVC. We study this phenomenon in a swirl nozzle with a centerbody whose end face is flush with the nozzle exit plane, using large eddy simulations (LES) and linear hydrodynamic stability analysis for flow Reynolds numbers Re = 48,767 and 82,751, based on nozzle exit diameter and bulk flow velocity. For one of the Re = 82,751 cases, the centerbody end face diameter is halved, resulting in the onset of coherent VBB precession. Linear stability analysis reveals a marginally unstable mode in this case. The same mode is found to be stable in the nominal cases. Structural sensitivity analysis shows that the VBB precession eigenmode is sensitive to changes in the time-averaged flow in the VBB-CWRZ merger region. This suggests that the reduction in CWRZ length due to halving the centerbody end face diameter is the reason for the onset of VBB precession. These results suggest that in general, spatial separation between the CWRZ and VBB can result in the onset of VBB precession and the emergence of PVC oscillations in flows with swirl.

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