Insights into the dynamics of conical breakdown modes in coaxial swirling flow field

2018 ◽  
Vol 853 ◽  
pp. 72-110 ◽  
Author(s):  
Kuppuraj Rajamanickam ◽  
Saptarshi Basu
Keyword(s):  
Flow Field ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
High Energy ◽  
Structure Identification ◽  
Diagnostic Tools ◽  
Dynamic Mode ◽  
Time Resolved ◽  
Toroidal Bubble ◽  
Dominant Frequencies

The main idea of this paper is to understand the fundamental vortex breakdown mechanisms in the coaxial swirling flow field. In particular, the interaction dynamics of the flow field is meticulously addressed with the help of high fidelity laser diagnostic tools. Time-resolved particle image velocimetry (PIV) (${\sim}1500~\text{frames}~\text{s}^{-1}$) is employed in $y{-}r$ and multiple $r{-}\unicode[STIX]{x1D703}$ planes to precisely delineate the flow dynamics. Experiments are carried out for three sets of co-annular flow Reynolds number $Re_{a}=4896$, 10 545, 17 546. Furthermore, for each $Re_{a}$ condition, the swirl number ‘$S_{G}$’ is varied independently from $0\leqslant S_{G}\leqslant 3$. The global evolution of flow field across various swirl numbers is presented using the time-averaged PIV data. Three distinct forms of vortex breakdown namely, pre-vortex breakdown (PVB), central toroidal recirculation zone (CTRZ; axisymmetric toroidal bubble type breakdown) and sudden conical breakdown are witnessed. Among these, the conical form of vortex breakdown is less explored in the literature. In this paper, much attention is therefore focused on exploring the governing mechanism of conical breakdown. It is should be interesting to note that, unlike other vortex breakdown modes, conical breakdown persists only for a very short band of $S_{G}$. For any small increase/decrease in $S_{G}$ beyond a certain threshold, the flow spontaneously reverts back to the CTRZ state. Energy ranked and frequency-resolved/ranked robust structure identification methods – proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) respectively – are implemented over instantaneous time-resolved PIV data sets to extract the dynamics of the coherent structures associated with each vortex breakdown mode. The dominant structures obtained from POD analysis suggest the dominance of the Kelvin–Helmholtz (KH) instability (axial $+$ azimuthal; accounts for ${\sim}80\,\%$ of total turbulent kinetic energy, TKE) for both PVB and CTRZ while the remaining energy is contributed by shedding modes. On the other hand, shedding modes contribute the majority of the TKE in conical breakdown. The frequency signatures quantified from POD temporal modes and DMD analysis reveal the occurrence of multiple dominant frequencies in the range of ${\sim}10{-}400~\text{Hz}$ with conical breakdown. This phenomenon may be a manifestation of high energy contribution by shedding eddies in the shear layer. Contrarily, with PVB and CTRZ, the dominant frequencies are observed in the range of ${\sim}20{-}40~\text{Hz}$ only. We have provided a detailed exposition of the mechanism through which conical breakdown occurs. In addition, the current work explores the hysteresis (path dependence) phenomena of conical breakdown as functions of the Reynolds and Rossby numbers. It has been observed that the conical mode is not reversible and highly dependent on the initial conditions.

Investigation of the Dynamics of an Ultra Low NOx Injection System by POD Data Post-Processing

2015 ◽  
Author(s):  
M. Berrino ◽  
D. Lengani ◽  
F. Satta ◽  
M. Ubaldi ◽  
P. Zunino ◽  
...  
Keyword(s):  
Flow Field ◽  
Spatial Information ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Measurement Techniques ◽  
Injection System ◽  
Post Processing ◽  
Particle Image Velocimetry Technique ◽  
Annular Sector ◽  
Random Fluctuations

The present paper is focused on the investigation of the dynamics of the flow downstream of an Ultra Low NOx (ULN) injection system, designed to reduce NOx emissions and combustor axial length. Two rectangular flame tubes have been experimentally investigated: one aimed at simulating an unconfined exit flow, and another with the same transverse dimensions of the combustor annular sector, to simulate the confined flow field. The effects induced by the realistic flame tube presence are investigated comparing the flow field with that generated in the unconfined case. Particular attention is paid to the vortex breakdown phenomena associated with the flow generated by the two co-rotating swirlers constituting the injection system. Two different and complementary measurement techniques have been adopted to characterize the aerodynamics of the vortex breakdown. The hot-wire investigation results reveal the frequencies associated with the precession motion due to the vortex breakdown. The Particle Image Velocimetry technique has been coupled with Proper Orthogonal Decomposition (POD) for data post-processing in order to reconstruct the swirling motion generated by the injection system. The property of POD, which consists of splitting temporal from spatial information of the flow field in analysis, allows the distinction between deterministic and random fluctuations without the need of an external trigger signal. This feature is fundamental for the better understanding of an highly-swirling flow.

scholarly journals Unsteady Flow Field Characterization of Effusion Cooling Systems with Swirling Main Flow: Comparison Between Cylindrical and Shaped Holes

Energies ◽  
2020 ◽  
Vol 13 (19) ◽  
pp. 4993
Author(s):  
Tommaso Lenzi ◽  
Alessio Picchi ◽  
Tommaso Bacci ◽  
Antonio Andreini ◽  
Bruno Facchini
Keyword(s):  
Flow Field ◽  
Film Cooling ◽  
Gas Turbines ◽  
Swirling Flow ◽  
Cooling System ◽  
Complete Characterization ◽  
Time Resolved ◽  
Staggered Arrangement ◽  
Unsteady Characteristics

The presence of injectors with strongly swirled flows, used to promote flame stability in the combustion chambers of gas turbines, influences the behaviour of the effusion cooling jets and consequently of the liner’s cooling capabilities. For this reason, unsteady behaviour of the jets in the presence of swirling flow requires a characterization by means of experimental flow field analyses. The experimental setup of this work consists of a non-reactive single-sector linear combustor test rig, scaled up with respect to the real engine geometry to increase spatial resolution and to reduce the frequencies of the unsteadiness. It is equipped with a radial swirler and multi-perforated effusion plates to simulate the liner cooling system. Two effusion plates were tested and compared: with cylindrical and with laid-back fan-shaped 7-7-7 holes in staggered arrangement. Time resolved Particle Image Velocimetry has been carried out: the unsteady characteristics of the jets, promoted by the intermittent interactions with the turbulent mainstream, have been investigated as their vortex structures and turbulent decay. The results demonstrate how an unsteady analysis is necessary to provide a complete characterization of the coolant behaviour and of its turbulent mixing with mainflow, which affect, in turn, the film cooling capability and liner’s lifetime.

Geometry Effects on the Flow Field and the Spectral Characteristics of a Triple Annular Swirler

2003 ◽  
Author(s):  
Guoqiang Li ◽  
Ephraim J. Gutmark
Keyword(s):  
Flow Field ◽  
Boundary Conditions ◽  
High Frequency ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Low Frequency ◽  
Tube Length ◽  
Formation Mechanisms ◽  
Recirculation Bubble ◽  
Inlet Conditions

The dynamics of vortex breakdown are important to the performance of gas turbine combustors where swirling flows are extensively used to stabilize the flame and extend the lean flammability limit (LBO). Due to the strong interaction of vortical structures in the swirling flow with heat release and acoustical modes, vortex breakdown mechanism is essential to understanding the thermoacoustic behavior and to the development of combustion instability control strategy. This paper analyzes the vortex breakdown behavior downstream of a Triple Annular Research Swirler (TARS) based on velocity flow field data from stereoscopic PIV measurement and spectral data from hotwire/film measurements. The vortical structure is highly dependent on the different swirler combinations (swirler geometry) as well as on inlet conditions such as air flow-rate, mixing tube length and downstream conditions such as exhaust nozzle contraction ratio. The scale, location, strength, and formation mechanisms of the large-scale vortices vary for different geometries. The shape of the recirculation bubble changes with the outlet boundary conditions, suggesting that the swirling flow inside the combustion chamber remains subcritical downstream of the vortex breakdown. However, spectral analysis reveals that the dominant frequencies close to the exit of the TARS show only slight change for different outlet boundary conditions. Three ranges of frequencies characterize the spectral domain of TARS: high frequency close to the TARS exit, middle range frequency downstream of this region, and low frequency in most regions further downstream. The sources of instabilities in these three regions could be attributed to the strong shear layer, precessing vortex core and interaction between spanwise and azimuthal instabilities. The outlet boundary conditions affect the middle and low frequency range but have no effect on the high frequency. The inlet conditions have global effect on the entire flow region.

Swirling flow of viscoelastic fluids. Part 1. Interaction between inertia and elasticity

2001 ◽  
Vol 429 ◽  
pp. 67-115 ◽  
Author(s):  
JASON R. STOKES ◽  
LACHLAN J. W. GRAHAM ◽  
NICK J. LAWSON ◽  
DAVID V. BOGER
Keyword(s):  
Flow Field ◽  
Axial Velocity ◽  
Xanthan Gum ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Central Axis ◽  
Driven Cavity ◽  
Low Viscosity ◽  
Existence Domain ◽  
Boger Fluids

A torsionally driven cavity, consisting of a fully enclosed cylinder with rotating bottom lid, is used to examine the confined swirling flow of low-viscosity Boger fluids for situations where inertia dominates the flow field. Flow visualization and the optical technique of particle image velocimetry (PIV) are used to examine the effect of small amounts of fluid elasticity on the phenomenon of vortex breakdown. Low-viscosity Boger fluids are used which consist of dilute concentrations of high molecular weight polyacrylamide or semi-dilute concentrations of xanthan gum in a Newtonian solvent. The introduction of elasticity results in a 20% and 40% increase in the minimum critical aspect ratio required for vortex breakdown to occur using polyacrylamide and xanthan gum, respectively, at concentrations of 45 p.p.m. When the concentrations of either polyacrylamide or xanthan gum are raised to 75 p.p.m., vortex breakdown is entirely suppressed for the cylinder aspect ratios examined. Radial and axial velocity measurements along the axial centreline show that the alteration in existence domain is linked to a decrease in the magnitude of the peak in axial velocity along the central axis. The minimum peak axial velocities along the central axis for the 75 p.p.m. polyacrylamide and 75 p.p.m. xanthan gum Boger fluids are 67% and 86% lower in magnitude, respectively, than for the Newtonian fluid at Reynolds number of Re ≈ 1500–1600. This decrease in axial velocity is associated with the interaction of elasticity in the governing boundary on the rotating base lid and/or the interaction of extensional viscosity in areas with high velocity gradients. The low-viscosity Boger fluids used in this study are rheologically characterized and the steady complex flow field has well-defined boundary conditions. Therefore, the results will allow validation of non-Newtonian constitutive models in a numerical model of a torsionally driven cavity flow.

Dynamic Mode Decomposition for Extracting Cycle-to-Cycle Variation of SIDI Engine In-Cylinder Flow Under Motoring Condition

2020 ◽  
Author(s):  
Mengqi Liu ◽  
Fengnian Zhao ◽  
Xuesong Li ◽  
Min Xu ◽  
David L. S. Hung
Keyword(s):  
Flow Field ◽  
Direct Injection ◽  
Dynamic Mode Decomposition ◽  
Dynamic Mode ◽  
Field Variation ◽  
Dynamic Features ◽  
Time Resolved ◽  
Cycle Variation ◽  
Mode Decomposition ◽  
Cylinder Flow

Abstract Cycle-to-cycle variation (CCV) of in-cylinder flow strongly affects the performance and efficiency of spark ignition direct injection (SIDI) engines. In order to achieve a precise flow control inside the engine, the underlying dynamic features of flow field CCV must be thoroughly investigated. In this work, large-eddy simulations (LES) with 50 consecutive cycles are employed for high fidelity numerical realizations of engine flow under motoring condition. To supplement the numerical analysis, time-resolved particle image velocimetry (PIV) measurements are also conducted in several cutting planes. Although the velocity root mean square (RMS) is calculated to quantify the cyclic variation intensity of simulation and experiment results, some important dynamic characteristics cannot be observed directly from velocity data. Therefore, dynamic mode decompositions (DMD), which is a widely used modal decomposition algorithm on fluid study, is used to decompose flow fields into modes with specific frequencies and provide growth rates of corresponding flow structures. This spectral information of in-cylinder flow field is ponderable for uncovering dynamic features of engine CCV. In this study, DMD algorithm is applied on both LES and PIV datasets. The frequency and growth rate differences are employed to elucidate the CCV feature deviations captured by LES and PIV. This research provides a guideline for extracting engine flow field cyclic variability feature using DMD algorithm. Based on the discussion for spectral features and potential sources of flow field variation, the capability of LES to capture CCV features is evaluated. The DMD spectrum differences between PIV and LES can guide the boundary condition perturbations used for simulation fidelity improvements.

Designing a Radial Swirler Vortex Breakdown Burner

2006 ◽  
Author(s):  
Stephan Burmberger ◽  
Christoph Hirsch ◽  
Thomas Sattelmayer
Keyword(s):  
Flow Field ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Azimuthal Velocity ◽  
Design Guidelines ◽  
Flame Stabilization ◽  
Fuel Flexibility ◽  
Primary Zone ◽  
Large Eddy ◽  
Flame Flashback

Most gas turbine premix burners without centrebody employ the breakdown of a swirling flow at the transition between the mixing section and the combustor for aerodynamic flame stabilization [1]. As the formation of the desired vortex breakdown pattern depends very sensibly on the distribution of axial and azimuthal velocity in the mixing section, the design of suitable swirlers is usually a cumbersome iterative process. The presented burner design was found through the implementation of design guidelines derived from CFD-calculations and on the basis of analytical considerations [5]. The swirling flow is generated by a radial swirler with tangential inlets. In order to stabilize the flow pattern, the swirling flow confines a slow non-swirling flow on the centreline. The centre flow being set into azimuthal motion creates increasing azimuthal velocity in streamwise direction in the vortex core. This process is reinforced by a conical nozzle and leads to the production of positive azimuthal vorticity inside the nozzle which stabilizes the flow field. First atmospheric test runs and Large Eddy Simulations of the isothermal as well as reactive flow field prove that the design goals have been reached: The burner creates stable vortex breakdown in the primary zone of the combustion chamber without flame flashback or backflow on the centreline over the entire operating range and even for difficult fuels like hydrogen containing gases. This finding indicates that reliable vortex breakdown burners with remarkable fuel flexibility can be designed using the guidelines presented in [5].

Flashback Propensity in Swirl Stabilized Burner With Syngas Fuels

2010 ◽  
Author(s):  
Bidhan Dam ◽  
Gilberto Corona ◽  
Ahsan Choudhuri
Keyword(s):  
Flow Field ◽  
Recirculation Zone ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Fuel Mixture ◽  
Stoichiometric Ratio ◽  
Swirl Number ◽  
Significant Phenomenon ◽  
Transient Velocity ◽  
Fuel Mixtures

In swirl stabilized burner, combustion induced vortex breakdown (CIVB) flashback is a significant phenomenon. This paper presents experimental measurements of CIVB flashback propensity for hydrogen (H2)-carbon monoxide (CO) flames. The effects of H2 concentration, and swirl number on the flashback propensity of H2-CO flames are discussed. For a given air mass flow rate, the stoichiometric ratio (%F) at which the CIVB flashback occurs decreases with the increase in H2 concentration in fuel mixtures. However it appears that near the CIVB flashback limit, the swirl strength plays a more dominating role over the H2 concentration in the fuel mixture. The flashback propensity decreases with the increase in swirl number. An analysis of the nonreacting flow field (Air 6 g/s) as well as reacting (CH4-Air and H2-CO-Air) flame near the CIVB transient velocity field was conducted. The analysis revealed that a complex vortex-chemistry interaction leading to vortex breakdown and flashback occurred. The vector flow field showed that the high swirling flow generates a more stabilized and wider recirculation zone. It also showed that the presence of H2 dictates the intensity of the flashback process.

Analysis of synthetic jet PIV experiment based on dynamic mode decomposition

2020 ◽  
Vol 34 (14n16) ◽  
pp. 2040103
Author(s):  
Zhi-Xian Ye ◽  
Yi-Yang Jiang ◽  
Ze-Nan Tian ◽  
Shao-Chang Mo ◽  
Yuan-Qi Fang ◽  
...  
Keyword(s):  
Flow Field ◽  
Field Research ◽  
Input Voltage ◽  
Synthetic Jet ◽  
Dynamic Mode Decomposition ◽  
Dynamic Mode ◽  
Vibration Displacement ◽  
Time Resolved ◽  
Mode Decomposition ◽  
Flow Field Characteristics

Synthetic Jet Actuator (SJA) works cyclically with the directionally transportation of fluid near the exit, and the paper presents the performance of a loudspeaker driven SJA in static flow field. Time-Resolved Particle Image Velocimetry (TR-PIV) system is used to measure the flow field characteristics near the SJA slot exit with input voltage changing, and the flow field snapshots obtained by TR-PIV are modality analyzed by Dynamic Mode Decomposition (DMD) method. The PIV experiments show that by varying input voltage at fixed oscillating frequency, the loudspeaker diaphragm vibration displacement is the parameter that affects the jet velocity and the performance of the SJA. The traveling vortex vanishes at high voltage due to the interaction between vortex structures and the synthetic main jet. In DMD method, the first three-order modes can characterize the main information of the original flow field with reverting the flow field snapshot sequence. It indicates that the DMD method is applicable in the SJA flow field research and the reduced order model can effectively simplify the analysis of flow field.

Three-dimensional organization and dynamics of vortices in multichannel swirling jets

2018 ◽  
Vol 843 ◽  
pp. 180-210 ◽  
Author(s):  
Andrea Ianiro ◽  
Kyle P. Lynch ◽  
Daniele Violato ◽  
Gennaro Cardone ◽  
Fulvio Scarano
Keyword(s):  
Flow Field ◽  
Shear Layer ◽  
Vortex Core ◽  
Vortex Breakdown ◽  
Three Dimensional ◽  
Dynamic Mode Decomposition ◽  
Dynamic Mode ◽  
Flow Topology ◽  
Swirl Number ◽  
Initial Development

The unsteady three-dimensional flow organization of jets issued from a duct with swirl vanes at Reynolds number equal to 1000 and swirl number $S$ ranging between 0 and 0.8 is investigated. Time-resolved tomographic particle image velocimetry returns the instantaneous flow structure and its evolution by visualization of velocity and vortical features. The most relevant coherent motions are identified and characterized with the aid of dynamic mode decomposition. The time-averaged flow topology indicates that the vanes used to impart the swirling motion have a significant impact on the azimuthal modulation of momentum, with the jet exhibiting four sectors separated by a thin cross-like wake resulting from the boundary layer developed along the vane walls. The flow field is thus characterized by inner and outer shear regions. An increase in swirl, up to moderate levels ($S=0.4$), causes larger jet spreading angles. Further increase of the swirl number is accompanied by the appearance of a central recirculation zone due to vortex breakdown at $S=0.6$ which increases in size and is triggered upstream for increasing $S$. Although no shear layer instability development is observed at $S=0$, already at $S=0.2$ the swirling motion promotes the growth of helical vortices appearing as Kelvin–Helmholtz waves that deform the outer axial shear layer. The downstream evolution features successive pairing, which is observed for all the considered swirl numbers. The initial development of the instability is independent for each vane, whereas a mutual interaction between the vanes occurs after the vortex pairing. The reconnection from the four sectors vortices induces a significant increase of azimuthal vorticity, which affects the dynamical behaviour of the precessing vortex core. The latter is visualized by a low-order spatio-temporal reconstruction based on few dynamical modes. At a higher swirl number ($S\geqslant 0.6$), the axial vorticity component dominates the flow field; it interacts with the azimuthal vorticity, which penetrates inward through the meanders of the vane wakes and forces the vortex core precession and breakdown.

The promises and perfidy of cryomicroscopy and analysis

1994 ◽  
Vol 52 ◽  
pp. 382-383
Author(s):  
Patrick Echlin
Keyword(s):  
Low Temperature ◽  
High Energy ◽  
Short Paper ◽  
List Type ◽  
Time Resolved ◽  
Energy Beam ◽  
Cellular Analysis ◽  
Dissolved Ions ◽  
Embedding Medium ◽  
Low Temperature Microscopy

The unusual title of this short paper and its accompanying tutorial is deliberate, because the intent is to investigate the effectiveness of low temperature microscopy and analysis as one of the more significant elements of the less interventionist procedures we can use to prepare, examine and analyse hydrated and organic materials in high energy beam instruments. The promises offered by all these procedures are well rehearsed and the litany of petitions and responses may be enunciated in the following mantra.Vitrified water can form the perfect embedding medium for bio-organic samples.Frozen samples provide an important, but not exclusive, milieu for the in situ sub-cellular analysis of the dissolved ions and electrolytes whose activities are central to living processes.The rapid conversion of liquids to solids provides a means of arresting dynamic processes and permits resolution of the time resolved interactions between water and suspended and dissolved materials.The low temperature environment necessary for cryomicroscopy and analysis, diminish, but alas do not prevent, the deleterious side effects of ionizing radiation.Sample contamination is virtually eliminated.

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