Precessing Vortex Core Research Articles

Identification of the precessing vortex core in a hydro turbine model using local stability analysis and stochastic modeling

Stochastic modeling and local linear stability analysis (LSA) is employed to predict the onset of the precessing vortex core (PVC) in the hydro turbine model. The method of the stochastic modeling based on the pressure fluctuation signals correctly predicts the instability of the azimuthal mode m = 1 at flow rates below 0.7Q c . This is in line with local LSA that shows that the azimuthal modes m = 1 and m = 2 are absolutely unstable below the flow rate of 0.7Q c . The absolute instability of mode m = 2 is a new observation in the part load regimes of hydro turbines and plays a significant role in the dynamics of the PVC. As demonstrated in this paper, local LSA and stochastic modelling are both methods to uncover the driver of the PVC using sparse experimental data stemming from either spatially resolved but non-timeresolved PIV snapshots or single-point time-resolved wall pressure recordings, respectively. This makes these methods suitable to be applied to configurations of industrial relevance.

Impact of runner crown shape modifications on the onset of the precessing vortex core

In this experimental study, a passive flow control concept is provided to extend the stable part load operation regime by shifting the rise of the precessing vortex core (PVC), also known as vortex rope, toward lower mass flows respectively deeper part loads. A parametrized runner crown design working as a passive flow control device is derived. This control device aims for shifting the bifurcation point of the PVC to lower flow rates. To determine the most influential design parameters and derive an optimized runner crown design, a design of experiments (DoE) approach is used. This DoE approach is based on data obtained from differential pressure sensors inside the draft tube wall of a generic hydro turbine test rig using air as working fluid. By means of stochastic modeling, the growth rate of the PVC mode is derived from the statistics of the measured pressure signals. The growth rate is used to estimate the bifurcation point of the PVC characterized by a certain normalized flow rate. It is shown that the stable part load operation regime is extended by up to 25% due to the passive impact of the modified runner crown. Moreover, the operational range featuring considerable PVC-induced pressure oscillations is diminished and the pressure recovery of the draft tube is improved compared to the baseline case.

Impact of precessing vortex core dynamics on the thermoacoustic instabilities in a swirl-stabilized combustor

Global instabilities in swirling flows can significantly alter the flame and flow dynamics of swirl-stabilized flames, such as those in modern gas turbine engines. In this study, we characterize the interaction between the precessing vortex core (PVC), which is the consequence of a global hydrodynamic instability, and thermoacoustic instabilities, which are the result of a coupling between combustor acoustics and the unsteady heat release rate. This study is performed using experimental data obtained from a model gas turbine combustor employing two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at five bar pressure. The flow split between the two streams is systematically varied to observe the impact of flow structure variation on the system dynamics at both non-reacting and reacting conditions. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence and acetone planar laser-induced fluorescence are used to obtain information about the velocity fields, flame and fuel flow behaviour, respectively. Spectral proper orthogonal decomposition and a complex network analysis are used to identify and characterize the dominant oscillation mechanisms driving the system. In the non-reacting data, a PVC is present in most cases and the amplitude of the oscillation increases with increasing flow through the centre nozzle. In the reacting data, three dominant modes are seen: two thermoacoustic modes and the PVC. Our results show that in the cases where the frequency of the PVC overlaps with either of the thermoacoustic modes, the thermoacoustic modes are suppressed. The complex network analysis coupled with a weakly nonlinear theoretical analysis suggests the mechanisms by which this coupling and suppression of the thermoacoustic mode occur.

Investigation of flame structure and precessing vortex core instability of a gas turbine model combustor with different swirler configurations

Numerical simulation of a dual-swirl gas turbine model combustor is performed under cold and reacting flow conditions using a three-dimensional unsteady Reynolds-averaged Navier–Stokes approach. A multi-species chemical mechanism is used in this study for the analysis of the numerous radicals participating in the ignition process and the flame structure. The other objective of this study is to investigate the flow field under different injector configurations, including both co-rotating and counter-rotating swirler arrangements, different swirl intensities, and vane areas. A comparison of the results with experimental data shows that the predicted velocity and temperature profiles follow the experimental data reasonably. In these studies, a precessing vortex core is found in the shear layer of the inner recirculation zone for all injector arrangements considered, and a co-rotating vortex exists in the outer shear layer for some of these arrangements. OH mass fraction field shows that the reactions take place mostly near the vortex core. Furthermore, it is shown that the build-up process of H2O2 and CH2O inside the cold jet has an important effect on combustion initiation. In addition, the formation and consumption of the H atoms in the recirculation zones and the balance between OH and H2O2 are shown to have important roles in the flame formation process. Finally, the precession frequency of the PVC is found to scale almost linearly with the spatial gradient of swirl velocity in the inner swirler and almost independent from the inclusion of the combustion reactions.

Experimental investigation of the helical mode in a stratified swirling flame

In a centrally-staged combustor, the stratified swirling flame often manifests a helical mode, which is characterized by the emergence of helical vortices, including the precessing vortex core (PVC) and the outer helical vortex (OHV). To understand the flame-vortex interaction mechanism responsible for the coherent heat release rate (HRR) fluctuation of such a helical mode, this work employs 20 kHz simultaneous measurements of PIV/CH2O PLIF and PIV/CH* chemiluminescence to resolve the unsteady flow field and flame motion. Through a combination of time-resolved and phase-averaged analysis, together with the three-dimensional reconstruction of the coherent flow and flame structures, we arrive at the qualitative understanding that in the helical mode the pilot flame dominated by PVC is a major source of coherent HRR fluctuation, while the main flame only slightly modifies the fluctuation intensity. Quantitative analyses further demonstrate the different effects of PVC and OHV on the coherent HRR fluctuation. Specifically, the PVC affects the primary heat-release process by dictating the convective transport of the reactants upstream of the flame front. And the OHV contributes to additional heat release by inducing stretching and roll-up of the pilot flame tip; it happens during the OHV's second growth, which is likely caused by the thermal expansion resulting from the primary heat release. This explains our finding that the OHV-induced heat release occurs at a slightly delayed phase angle and a downstream location compared with the primary PVC-induced heat release.

Large eddy simulation of swirling flows in a non-reacting trapped-vortex combustor

Numerical investigation of swirling flows in a trapped-vortex combustor (TVC) is carried out using large eddy simulation (LES). A multi-block flow solver is employed to solve three-dimensional filtered compressible Navier-Stokes equations in generalized coordinate system through the use of high-order compact differencing schemes. The effect of swirl strength on the behavior of swirling flow features including vortex-breakdown, precessing-vortex core (PVC) and shear layer is practiced for a TVC with circumferential cavity and annular inlet mainstream. The axial curved-vane swirlers are designed to generate flows with swirl numbers of 0 and 0.34 (low-swirl), 0.6 and 0.75 (medium-swirl), and 1.03 and 1.34 (high-swirl). Evaluation of the mean flow field reveals that in the medium-swirl range, a central recirculation zone (CRZ) is established due to the vortex-breakdown mechanism apart from the bluff-body recirculation zone, while no CRZ is observed for low and high swirl ranges. The case with S = 0.6 in the medium-swirl range demonstrates a better performance by generating a larger central recirculation zone. The PVC is located in periphery of central recirculation zone and its precession frequency linearly increases with swirl number. The strain rate and overall pressure drop are invoked for further analysis of the flow field. The strain rate is sharp and intense in the low-swirl range and becomes diluted as the swirl number increases with its maximum point moving towards the cavity. Moreover, an overall pressure drop of under 5% is achieved for the low and medium-swirl ranges. Finally, turbulent kinetic energy (TKE) and local turbulence intensity as potential indicators of higher mixing in combustion systems are investigated. High levels of TKE are found to occur in all cases. For medium-swirl range, the high-TKE region is located at the proximity of shear layer between mainstream jet and cavity recirculation zone. Furthermore, for medium-swirl numbers the local turbulence intensity near the central recirculation zone boundary is between 50 and 100%.

Dynamics and bifurcations of laminar annular swirling and non-swirling jets

This paper presents bifurcation analyses characterising the nonlinear dynamics of fully developed laminar annular jets with respect to the centrebody diameter${ {d}}$, Reynolds number${ {Re}}$, and swirl ratio${ {S}}$. Similar flows appear in numerous applications and feature a vibrant range of topological and dynamical characteristics associated with phenomena including shear layer separation and vortex breakdown. Our results begin by describing the non-monotonic evolution of the axisymmetric jet's steady topology under varying${ {S}}$. In accord with earlier reports, the jet progresses through a sequence of wake, breakdown and wall jet regimes in a qualitatively similar manner across a wide span of${ {d}}$and${ {Re}}$values. In the wake regime, the non-swirling jet bifurcates to a plane-symmetric, but not axisymmetric, steady flow pattern beyond a${ {d}}$-dependent critical${ {Re}}$value. With further increase in${ {Re}}$, the steady non-swirling jet destabilises subsequently via multiple distinct Hopf bifurcations. Introducing${ {S}}>0$to the jet also induces unsteadiness by twisting the singly azimuthally periodic ($|m|=1$) asymmetric wake structure and causing it to precess periodically in time about the central axis. Intermediate swirl stabilises this unsteady dynamics and restores the jet's axisymmetry. This stabilising effect is then reversed in the breakdown regime at higher${ {S}}$, where a variety of different$|m|=1$and$|m|=2$instabilities bifurcate from the steady flow as${ {S}}$is increased. Several instances of hysteresis and subcritical behaviour are reported and discussed, including one that manifests precessing vortex core oscillations.

Investigation of the precessing vortex core reconnection phenomenon

The paper investigates the unstable behavior of a precessing vortex core in a diffuser behind a rotating swirler. The studies were carried out by experimental methods using a hydrodynamic stand, and by numerical simulation of turbulent flow in the stand in the various flow swirl modes. Both experimental and calculation studies show nonperiodic reconnection of the vortex core and the formation of vortex rings. This instability is observed within a narrow range of the swirl parameter between a weakly swirled flow without vortex breakdown and a strongly swirled flow with forming a stable precessing vortex core. The vortex core reconnection leads to forming a wave disturbance propagating along the vortex rope.

Modal Decomposition of the Precessing Vortex Core in a Hydro Turbine Model

We report on the experimental study of a precessing vortex core (PVC) in an air model of a Francis turbine. The focus is placed on the modal decomposition of the PVC that occurs in the draft tube of the model turbine for a range of operation conditions. The turbulent flow fluctuations in the draft tube are assessed using stereo particle image velocimetry (PIV) measurements. Proper orthogonal decomposition (POD) is applied to the antisymmetric and symmetric components of the velocity fields to distinguish the dynamics of the azimuthal instabilities. The pressure pulsations induced by the PVC are measured by four pressure sensors mounted on the wall of the hydro turbine draft tube. Spatial Fourier decomposition is applied to the signals of the pressure sensors to identify the contributions of azimuthal modes, m=1 and m=2, to the total pressure fluctuations. The analysis based on velocity and pressure data shows similar results regarding the identification of the PVC. The contribution of the m=2 mode to the overall turbulent kinetic energy is significant for the part load regimes, where the flow rates are twice as low as at the best efficiency point (BEP). It is also shown that this mode is not the higher harmonic of the PVC, suggesting that it is driven by a different instability. Finally, we show a linear fit of the saturation amplitudes of the m=1 and m=2 oscillations to determine the critical bifurcation points of these modes. This yields critical swirl numbers of Scr=0.47 and 0.61, respectively. The fact that the PVC dynamics in hydro turbines are driven by two individual instabilities is relevant for the development of tailored active flow control of the PVC.

Influence of key geometrical features on the non-reacting flow of a Lean Direct Injection (LDI) combustor through Large-Eddy Simulation and a Design of Experiments

Lean Direct Injection (LDI) emerged as an interesting concept to limit NOx emissions in aero engines at the cost of operating close to the flame lean blow-off limit. In this technology, fuel is injected into a swirled airstream that generates recirculating flow structures that stabilize the flame. It is then of paramount importance at the design stage to understand the effect of various features on these structures. The present investigation makes use of Eulerian-Lagrangian Large-Eddy Simulations (LES) previously validated against existing experimental data for a reference condition to study the liquid non-reacting flow inside the CORIA Spray LDI burner with the help of Adaptive Mesh Refinement (AMR). A Design of Experiments (DoE) is proposed to analyze the significance of several geometrical features on the flow field, namely the combustor width, the air swirler vane angle, the number of swirler vanes and the axial location of the fuel injector tip. The study covers the qualitative appearance of the flow and the quantitative characterization of the spray dispersion and fuel-air mixing process. In this way, the chosen response variables include the size of the relevant coherent flow structures (Central Toroidal Recirculation Zone induced by the Vortex Breakdown Bubble, Corner Recirculation Zone and Swirled Jet) and their associated velocities, spray features (global drop sizes and spray penetration), pressure drop across the swirler and induced swirl number. Besides, the Precessing Vortex Core (PVC) relevance and frequency content is studied through Proper Orthogonal Decomposition (POD). Results from the statistical analysis show that the number of swirler vanes and their angle are the geometrical parameters that most importantly influence the flow features: stronger recirculation zones leading to an improved atomization and mixing have been found both when decreasing the number of swirler blades and increasing the blade angle. However, both solutions also increase the pressure losses across the swirler. As far as the spectral analysis is concerned, the number of swirler vanes is the most influencing factor on both the frequency and intensity of the PVC modes, being crucial for the possible activation and the energetic content of a double-helix PVC mode.

Investigation on flame dynamic characteristics to air-inlet excitation in a gas turbine model combustor

The combustion characteristics for a gas turbine (GT) model combustor in air-inlet excitation are studied by Large Eddy Simulation (LES). Time series of heat release rate (HRR) fluctuation is captured during simulation. The precessing vortex core (PVC) structure is shown by pressure iso-surface, and the single/double helix branches of PVC alternately evolve in one forcing cycle. With the rotation of PVC, the high-speed ring zone (HSRZ) is periodically squeezed, and large-scale vortex roll-up results from that. The flame root structure is also significantly affected by PVC, which is one of the reasons for the HRR fluctuation. With the increase of the forcing amplitude, the flame response is divided into three parts. The linear growth zone and secondary rising zone are separated by saturation plateau. The flame dynamics analysis shows that the change of the flame structures and flame front oscillation modes are the apparent mechanism of the nonlinear flame response appearance.

Effect of the conical diffuser angle on the confined swirling flow induced Precessing Vortex Core

This paper presents the effect of the conical diffuser angle (θ=0°,2°,4°) on the Precessing Vortex Core (PVC) induced in confined swirling airflows generated by axial swirlers with two swirl number (Sw=1.1,0.7). Planar Particle Image Velocimetry (2D-PIV) synchronized with unsteady wall pressure fluctuation measurements are performed to identify the Strouhal number of the precession frequency and pressure fluctuations amplitude induced by the PVC, as well as the PVC parameters related to its phase-averaged vortex structure. Furthermore, the distribution of pressure recovery rate based on the mass flow averaged pressure, and the relationship between the local axial pressure gradient and the local PVC structure is also investigated to understand the relationship between the PVC parameters and pipe geometries. These relationships are derived from transient simulations with the Shear Stress Transport Curvature Correction model, since the pressure field data cannot be obtained from the experiments conducted in this paper. Experimental and numerical results in pipe configurations 1 (diffuser followed by a straight pipe) and 2 (diffuser followed by an open exit) clarify the diffuser angle effect. As the diffuser angle increases, the Strouhal number decreases, whereas the pressure fluctuations amplitude increases. Moreover, the PVC parameters are also affected by increased diffuser angle: the vortex trajectory radius is enlarged, and the helical vortex pitch is shortened. In addition, the geometry change (from diffuser to straight pipe) leads to a sudden change in the vortex pitch and a modification of the pressure recovery rate evolution. The vortex structure in the conical diffuser is affected by a difference in the diffuser angle, but the latter does not affect the vortex structure in the downstream straight pipe. This highlights that the local vortex pitch depends only on the local pipe geometry. Finally, a theoretical model of the relationship between the pressure fluctuation amplitude and PVC parameters is proposed, and the relationship between the local pressure gradient and the local vortex trajectory radius gradient is clarified. The amplitude factor Cp∗ is a function of the dimensionless vortex trajectory radius r∗. Positive or negative local pressure gradient leads to the mitigation or promotion of the PVC development.

Large-eddy simulation and challenges for projection-based reduced-order modeling of a gas turbine model combustor

Computationally efficient modeling of gas turbine combustion is challenging due to the chaotic multi-scale physics and the complex non-linear interactions between acoustic, hydrodynamic, and chemical processes. A large-eddy simulation, referred to as the full order model (FOM), is performed for a gas turbine model combustor with turbulent combustion effects modeled using a flamelet-based method. Modal analysis reveals a high degree of correlation with averaged and instantaneous high-frequency particle image velocimetry fields. The dynamics of the precessing vortex core is quantitatively characterized using dynamic mode decomposition. The governing equations of the FOM are projected onto a low-dimensional linear manifold to construct a reduced-order model (ROM). A discretely-consistent least squares projection is used to guarantee global stability. The ROM provides an accurate reconstruction of the combustion dynamics within the training region, but faces a significant challenge in future state predictions. This limitation is mainly due to the increased projection error, which in turn is a direct consequence of the highly chaotic nature of the flow field, involving a wide range of dispersed coherent structures. This shortcoming is overcome using an adaptive basis method which yields accurate predictions of dynamics beyond the training region consistent with the FOM. Formal projection-based ROMs have not been applied to a problem of this scale and complexity, and achieving accurate and efficient ROMs is a grand challenge problem. A production-ready ROM method will significantly decrease the computational cost of the flame dynamics as well as the portability of this prediction to smaller-scale computers.

Experimental investigation of the flow-spray field in a realistic concentric staged high-temperature-rise combustor

The present work investigated the flow and spray fields in a realistic concentric staged high-temperature-rise (HTR) combustor in isothermal conditions using high-speed (10 kHz) particle image velocimetry (PIV) and 10 Hz kerosene planar laser-induced fluorescence (kerosene-PLIF). The two stages of the combustor, including a pilot stage and a main stage, were operated both individually and jointly to investigate their corresponding flow fields and the interactions between the flow from the two stages. From the time-averaged flow field results, strong coupling between the pilot and main swirling flows has been identified, leading to the formation of a primary recirculation zone (PRZ) and a lip recirculation zone (LRZ). From the instantaneous flow fields, the presence of a precessing vortex core (PVC) was observed when the two stages were flowing jointly, and the dominant PVC unsteady motions located in the inner shear layer of the pilot swirling was identified using snapshot proper orthogonal decomposition (snapshot POD) with dominant frequencies at 1400 and 1515.4 Hz. Comparing the flow field with the spray field indicates that the unsteady flow motion, in addition to the mean bulk flow, plays a key role in the understanding of the spray distribution. Noticeably, it is shown in present study that the dominant unsteady flow motions properly captured from the POD analyses show good agreement with some primary features of the spray distribution, which can be valuable to unraveling the complex flow-spray coupling in the HTR combustor operated under realistic conditions.

Influence of a Central Jet on Isothermal and Reacting Swirling Flow in a Model Combustion Chamber

We studied flow dynamics in a model combustion chamber using Large-eddy simulations (LES) and Particle image velocimetry (PIV) at the Reynolds number Re of 15,000. The swirl is produced using a Turbomeca swirler and air flow, while combustion is supported by a central methane/air jet. We compared four flow regimes, assessing the effect of the central jet for isothermal and lean reacting conditions. A detailed comparison for isothermal and reactive cases without the central jet is described, validating the LES results against PIV. We observe that unsteady dynamics are governed by global instability in the form of a well-known precessing vortex core (PVC). The central jet slightly changes the dynamics of PVC in the isothermal case where a strong recirculation zone is still formed. However, for the reacting case, the bubble is completely destroyed with no signs of strong vortical structures in the inner shear layer. These observations are confirmed using spectral analysis and proper orthogonal decomposition, describing the contribution of different flow modes in terms of azimuthal harmonics.

Transient numerical simulations of a cold-flow bidirectional vortex chamber

Bidirectional chambers are well-studied in terms of the flow structure and influence of their input parameters. However, most of the available studies are based on steady-state or time-averaged research methods and do not allow to obtain data on bidirectional flow dynamics over time. The present paper reports on detailed numerical studies based on detached eddy simulations (DESs) and unsteady Reynolds-averaged Navier–Stokes methods applied for two vortex chambers with different aspect ratios. A comparison of the numerical results with the available experimental data shows that the DES method gives the most accurate results on bidirectional flow structure, turbulent fluctuations, and precessing vortex core (PVC) motion. A notable feature of the studied bidirectional flow is the central recirculation zone (CRZ) formation, which is correctly predicted by the DES method only. The presence of a CRZ in a bidirectional flow has a significant effect on turbulent velocity fluctuations and PVC behavior. It is found that CRZ formation leads to a significant decrease in radial and circumferential velocity fluctuations whereas the axial velocity fluctuations are slightly increased. Additionally, the paper reports new findings on CRZ and PVC interaction in bidirectional flows. PVC motion is almost completely nullified by the presence of a CRZ. This can prove useful in many industrial applications of bidirectional chambers, e.g., vortex thrusters and gas turbine combustors. The bidirectional swirling flow transient properties studied in this paper could assist in determining the most efficient operational modes and geometric configuration of industrial chambers as well as enabling control of turbulent fluctuations, which would allow for reliable ignition and stable combustion.

Comparative analysis of air and water flows in simplified hydraulic turbine models

This article presents a comparative analysis of flow characteristics behind a hydraulic turbine runner in air and water. Swirling flow with a precessing vortex core (PVC) was investigated using a laser Doppler anemometer and pressure pulsation sensors. The experiments were conducted on aerodynamic and hydrodynamic test rigs over a wide range of hydraulic turbine operating conditions. Part-load modes of hydraulic turbine operation were investigated using the Fourier transform of pressure pulsations obtained from acoustic sensors. The features of the swirling flow were shown for the range of operating conditions from deep partl-load to overload.

Evolution characteristics of 3D vortex structures in stratified swirling flames studied by dual-plane stereoscopic PIV

Stratified swirling flames are typically formed in centrally-staged combustors of modern aero engines. The pilot flame plays an indispensable role in stabilizing the stratified swirling flame. Evolutions of the primary recirculation zone (PRZ) and vortices in the central shear layer (CSL) notably affect dynamics of the pilot flame. This work employed a dual-plane stereoscopic particle image velocimetry (DP-s-PIV) to investigate the evolution characteristics of three-dimensional vortex structures in stratified swirling flows/flames at different Re numbers and working conditions. Time-resolved measurements obtained temporal evolutions of vortex structures and the scanning process resolved time-averaged volumetric flow fields under different working conditions. Results show that the flame-induced flow acceleration near the burner outlet enhances the vorticity and the strain rate of the central shear layer (CSL), where a precessing vortex core (PVC) resides and its dynamics is strongly coupled with the primary recirculation zone (PRZ). The lengths of the PRZ in swirling flames were found to be smaller than that of the swirling flow, owing to the reduced swirl intensity and the enhanced turbulent-induced momentum transport. Furthermore, statistical analyses based on the instantaneous flow fields of the reacting cases demonstrated that the averaged vorticity magnitude and averaged turbulent kinetic energy (TKE) for vortices in the CSL essentially obey the linear dependency and square dependency with the Re number, respectively. And, there exists a linear correlation between the average turbulent kinetic energy and averaged vorticity magnitude for vortices in the CSL. This implies that the enhanced vortices due to the flame-induced effect downstream of the pilot stage is the main source for local turbulence generation in stratified swirling flames.

Large-eddy simulations of a swirling flow in a model Francis turbine

We performed Large-eddy simulations of the flow in a model air Francis turbine in a range of low-load regimes with a swirler rotating at fixed frequency. All investigated regimes revealed the presence of coherent helical vortex structure in the draft tube: the precessing vortex core. We identified the frequency of this instability and obtained mean flow velocity fields to be utilized in further works.

Large-eddy simulation of a reacting swirling flow in a model combustion chamber

We perform Large-eddy simulations of a non-premixed swirling flame in a model of a combustion chamber with a swirling air bulk flow at Re = 15000 and a central pilot low-velocity jet with methane using the Flamelet-generated manifold model. The unsteady behaviour of this regime is well reproduced based on the flame dynamics. The distribution of turbulent kinetic energy suggests the presence of intensive vortical structures typical of high-swirl flows similar to the precessing vortex core.