Swirl Vortex Research Articles

Effects of inlet air holes on swirl flow characteristics and outlet temperature distribution in an axial swirl combustor

The air inlet holes on the swirl combustor are crucial for high-performance micro gas turbine (MGT), affecting internal airflow, fuel-air mixing, temperature distribution, and cooling. However, there are seldom studies that pay attention to the influence of the air inlet holes coupled with low volatility macromolecular fuels for fast and better fuel-air mixing and combustion processes in MGT. This study investigates the influence of primary and dilution hole positions on airflow, temperature, emissions, and combustion efficiency. Numerical simulations in ANSYS Fluent examine five inlet hole configurations in a two-stage axial swirl combustor. Simulation validation is conducted under three test conditions. The results showed that the cut-off effect of the primary hole affected the size of the swirl vortex core, and the backward movement of the primary hole resulted in a significant increase in the length of the recirculation zone and the coherence high-temperature zone, accelerated the droplet evaporation rate, and reduced CO and NO emissions. Backward movement of dilution hole alters flow field, affecting swirl vortex stability and reflow intensity. Case 3, with original dilution hole and backward extended primary hole, exhibits optimal outlet temperature distribution and combustion efficiency.

Analysis of vortex mixing in passive micromixers with misaligned inlet and rectangular winglets

In the present study, a three-dimensional design of a passive micromixer is proposed which induces the swirling flow pattern by employing the misaligned inlets and winglets. The concepts of fluid stretching and twisting are utilized to improve the mixing efficiency. Stretching is quantified in terms of the modified flow tortuosity parameter. The dependence of various geometrical parameters on flow tortuosity is examined to increase the diffusive mixing by increasing the lateral Peclet number. A detailed analysis is carried out to understand the impact of geometrical features on the vortex formation, propagation, and intensification. Lambda 2 and the helicity approach are used to identify the vortex core region and quantified using a modified swirl number & vortex index. For vortex T misalignment, the aspect ratio 1 along with the staggered common flow-down configuration of winglets is highly effective. For misaligned Y inlet, both interfacial diffusive mixing along with longitudinal vortices at winglets generate highly effective mixing even at a low Reynolds number. For Reynolds number 1 to 70 and channel length 3.5 mm, mixing efficiency above 82% is achieved, and the mixing energy cost is subsequently reduced. A detailed comparative study is done to establish the effectiveness of the present design.

AUGMENTATION HEAT TRANSFER IN A CIRCULAR TUBE USING TWISTED - TAPE INSERTS: A REVIEW

Heat transfer enhancement is the process of increasing the heat-transfer coefficient, which enhances the system's performance. Enhancing heat transfer is a major problem for saving energy and is also beneficial economically. Many passive devices are used inside tubes to improve heat transfer such as twisted tape inserts, rough parts, extended surfaces, additives for liquids wire plugs, etc. This research reviewed one of the most effective passive devices which are twisted tape inserts. Since it has many advantages such as simple fabrication, simple operation, and ease of maintenance. The twisted tape inserts generated swirl flow and vortex inside the tube. Therefore, the internal convective heat transfer process is significantly improved. The current research article provides an overview of different twisting tape inserts that can improve heat transfer rates. By reducing boundary layer thickness near tube walls. Which lead to reduce the size and cost of many industrial applications, including heat exchangers, refrigeration systems, air conditioners, reactors, thermal power plants, spacecraft, and automobiles. A summary of previous experimental and numerical studies is presented as well. The primary results indicated that the twisted tape inserts are demonstrated to be efficient in enhancing heat transfer inside the tube for laminar and turbulent flow. But during a turbulent flow, twisted tapes increased pressure loss more than laminar flow because of flow obstruction.

Vortex Control Using Plate Type Floor Splitter in Pump Intake

Pump sump design is very crucial in ensuring a minimum requirement for its operation. Improper pump sump design will influence the development of vortices and swirls that can adversely affect the pump efficiency which subsequently could damage the pump. This can be mitigated by installing an anti-vortex device (AVD) beneath the intake pipe in the pump sump. The AVD works by suppressing the development of vortices and swirls in the flow of water into the intake pipe. ANSI/HI 9.8-2012 standard has outlined various AVD shapes and concepts that can be used. This experimental study was conducted to determine the appropriate parameters and configurations of twin plate floor splitter (TPFS) to suppress the development of vortex in the intake pipe. Geometries and other outlines in experiments were based on the guidelines by ANSI/HI 9.8-2012. The AVD model variations used were models with no floor splitter, single plate floor splitter (SPFS), and TPFS with 25 mm, 45 mm and 65 mm gap variations. The results showed that the distance between the two sides of twin plate affects the number of vortex generation and the swirl angle reduction. The number of vortices increased with the distance between the plates but with lower localized vortex intensity. However, the TPFS models that were used produced a swirl angle of less than 5° with the 25 mm variation producing the lowest swirl angle of 2.41°. The use of TPFS shows significantly improved performance in swirl angle reduction and vortex suppression as compared to no AVD case.

Numerical investigation of thermal-hydraulic performance of a heat exchanger tube with helical dimples

This study proposes a novel type of tube with helical dimples. Turbulent heat transfer and flow resistance in a heat exchanger tube with helical dimples were numerically investigated. Furthermore, the effects of the geometric parameters, including spiral pitch, transverse length, number of starts, depth, and radius on the performance were investigated. The results demonstrated that the longitudinal swirling flow and vortex generated behind the helical dimples result in adequate fluid mixing between the mainstream and near-wall regions. Therefore, the thermal performance of the tube with helical dimples was enhanced by 120–270% in comparison with the plain tube. Both Nu/Nu0 and f/f0 increased with increasing dimple starts, depth, and radius and decreasing spiral pitch and transverse length. Additionally, a maximum PEC = 1.93 was obtained by employing the geometric model with P = 30 mm, L = 6.9 mm, N = 4, D = 2 mm, and R = 2 mm in all cases.

Occurrence of a Swirling Vortex without an Intake

In this study, mechanisms that cause the occurrence of a swirling vortex without an intake are explained. There are more than one different reasons which may cause the occurrence of an air-core vortex or a swirling-vortex flow filament within an ambient fluid. An air-core vortex or a swirling vortex flow filament can also occur in an ambient fluid media without an intake provided that the coaxial subsurface depressions and thereby the imaginary subsurface spherical sink surface sectors are hydraulically developed by any means (such as rotation of the fluid mass of spherical sector-form, impinging or penetrating jet flow, continuously entering solid body or string etc.). Although studies related to occurrence of a swirling vortex in case of an intake can be found in available literature, there are currently no studies which explains the occurrence mechanisms of a swirling vortex without an intake in detail. In this study, it is showed that the causes for the occurrence of the air-core vortex or the swirling vortex filament can be different but the main mechanism of development of the air-core vortex or the swirling vortex filament is identical for all cases.

The influence of inlet swirl intensity and hot-streak on aerodynamics and thermal characteristics of a high pressure turbine vane

In modern gas turbines, the High Pressure Turbine (HPT) is exposed to an extreme thermal environment due to the burned gases leaving the combustor. The burned gases are characterized by flow and temperature distortions that effect the aerodynamics and heat transfer of the turbine. The purpose of this paper is to investigate numerically the effect of the intensity of the swirling flow combined with the temperature non-uniformity “Hot-Streak” (H-S) on the aerothermal performances of a HPT Nozzle Guide Vane (NGV). The investigations are conducted on the solid untwisted NGV annular cascade developed in NASA Lewis Research Center. Four swirl intensities (|Sn| = 0, 0.1, 0.25 and 0.5), two swirl orientations (positive and negative) and two hot-streaks (rounded and radial) at the NGV inlet are considered. The simulations are done by solving the Reynolds Averaged Navier-Stokes (RANS) equations using ANSYS-CFX software. The results show that the H-S with swirl undergoes twisting following the orientation of the swirl. The H-S twist is aggressive under positive swirl compared to the negative swirl case. The inlet swirl generates a new secondary flow structure, so called Swirl Vortex (SV), which induces more aerodynamic losses. The aerodynamic efficiency under negative swirl found to be higher than that under positive swirl. The maximum temperature on the vane surface is controlled by the radial transport of the SV towards the endwalls.

Capturing the Swirling Vortex and the Impact of Ventilation Conditions on Small-Scale Fire Whirls

The fundamental flow structure and temperature distribution of small-scale fire whirls, including tangential and axial velocities, temperature variation, and air entrainment in the lower boundary layer, were successfully captured using a generic fire field model with large eddy simulation (LES) turbulence closure. Numerical predictions were validated thoroughly against two small-scale experimental measurements, where detailed temperature and velocity distributions were recorded. Good agreement between numerical and experimental results was achieved. Normalization was also performed to compare the numerical predictions with the empirical correlations by Lei et al. (2015) developed from medium-scale fire whirl measurements. The transient development stages of small-scale fire whirls and the impact of air entrainment on the stability of the fire whirls were also investigated based on the validated numerical results. The numerical validations showed the potential of the current LES fire field model in capturing the dynamic behaviour of the fire whirl plume and performing a quantitative analysis on its onset criteria and combustion dynamics in future.

Dynamics of compact vortex rings generated by axial swirlers at early stage

This work concentrates on the study of flow dynamics of swirl vortex rings at the Reynolds number Re = 20 000 using a combination of the planar- and stereo-particle image velocimetry (PIV) measurements and dynamic delayed detached-eddy simulation. Particular attention is paid to the identification of the large-scale azimuthal modes in the vortex ring propagation process. In the experiments, vortex rings are issued from piston-driven axial swirlers with the swirl number ranges from S = 0 to 1.10. The stroke ratio L/D = 1.5 is used to produce a compact vortex ring without a trailing jet. PIV measurements are conducted in a water tank, while the in-plane component flow velocities on the longitudinal center plane and the three-component flow velocities on the cross section plane at several downstream locations according to the ring trajectories are obtained. In the simulation, the axial swirlers are also included, while the piston motion is realized by imposing a time-dependent inflow condition. Two types of dynamic effects in the vortex ring propagation process are captured by the planar-PIV measurement: the arriving time effect and the azimuthal effect, which induce parallel shift of the vortex ring core and the radial tilting of the vortex sheet, respectively. These modes are identified using the stereo-PIV results by applying the fast Fourier transform in the azimuthal direction, followed by the proper orthogonal decomposition on the radial and temporal directions. It shows that both m = 0 and 1 modes (m is the azimuthal wave number) coexist in the weakly swirled vortex rings, while the m = 2 mode arises and the m = 0 mode decays at high swirl numbers. The simulation also identifies the m = 1 and 2 modes, while the m = 2 mode has a large pitch with respect to the formation time.

Mass and momentum transport in the near field of swirling turbulent jets. Effect of swirl rate

Local transport of the flow momentum and scalar admixture in the near-field of turbulent swirling jets (Re = 5,000) has been investigated by using a combination of the particle image velocimetry and planar laser-induced fluorescence methods. Advection and turbulent and molecular diffusions are evaluated based on the measured distributions of the mean velocity and concentration and the Reynolds stresses and fluxes. As has been quantified from the data, the flow swirl intensifies the entrainment of the surrounding fluid and promotes mass and momentum exchange in the outer mixing layer. A superimposed swirl results in the appearance of a wake/recirculation region at the jet axis and, consequently, the formation of an inner shear layer. In contrast to the scalar admixture, the momentum exchange in the inner shear layer is found to be strongly intensified by the swirl. For the jet with the highest considered swirl rate, a substantial portion of the surrounding fluid is found to enter the unsteady central recirculation zone, where it mixes with the jet that is issued from the nozzle. The contribution of the coherent velocity fluctuations, which are induced by large-scale vortex structures, to the turbulent transport has been evaluated based on triple decomposition, which was based on proper orthogonal decomposition analysis of the velocity data sets. For the considered domain of the jet with the highest swirl rate and vortex breakdown, the contributions of detected helical vortex structures, inducing pressing vortex core, to the radial fluxes of the flow momentum and the scalar admixture are found to locally exceed 65% and 80%, respectively.

The Formation and Evolution of Turbulent Swirling Vortex Rings Generated by Axial Swirlers

The present work investigates the formation process and early stage evolution of turbulent swirling vortex rings, by using planar Particle Image Velocimetry (PIV) and Large Eddy Simulation (LES). Vortex rings are produced in a piston-nozzle arrangement with swirl generated by 3D-printed axial swirlers in experiments. Idealised solid-body rotation is applied in LES to evaluate the effect of nozzle exit velocity profile in experiments. The Reynolds number (Re) based on the nozzle diameter D and the slug velocity U0 in the nozzle is 20,000. The swirl number S generated ranges from 0 (zero-swirl vortex ring) and 1.1, covering the two critical swirl numbers previously identified in a swirling jet. Both PIV and LES results show that the formation number F decreases linearly as S increases, with the maximum F ≈ 2.6 at S = 0 (produced by the swirler with straight vanes) and minimum F = 1.9 at S = 1.1. The corresponding maximum attainable circulation in the nozzle axis parallel plane also diminishes with increasing S. Evolution of compact rings produced by a stroke ratio L/D = 1.5 reveals that circulation decay rate is largely proportional to S. The trajectory of the vortex core in the axial direction, hence the ring axial propagation velocity, decreases as S, while that in the radial direction and the radial propagation velocity, increase with S. An empirical scaling function is proposed to scale these variables.

Intensification of uniflow cyclone performance at low loading

This paper presents an extensive uniflow cyclone dedusting study and analysis of respective performance data based on separation efficiency, pressure drop and cut size. A laboratory-scale uniflow cyclone with axial swirl vanes is used to test a range of setups employed by the industry. The most important swirl vane and vortex finder parameters, as well as concentration levels are examined in detail with reference to geometric changes. Comprehensive separation data is provided, showing the impact of angle of attack, core size ratio, blade overlap, vortex finder length and vortex finder diameter on performance levels. Grade efficiency curves and cut sizes for each setup are analyzed. A cut size below 4 μm is achieved for marble dust with a particle density of 2700 kg m−3. Results show that the cut size can be reduced by over 50% with an increase of pressure drop by approximately 30% in relation to the base configuration. For a steep angle of attack of 15° and a core size ratio of 0.125, the cut size can be reduced by 25%, with a 35% reduction of pressure drop.

Wandering corrections from PIV measurements of tornado-like vortices

Wandering of tornado-like vortices consists in random oscillations of the vortex core from its time-averaged position, which complicates efforts to characterize vortex characteristics. A procedure is then necessary to retrieve characteristics of tornado-like vortices not affected by wandering smoothing effects. This study explores two procedures to correct wandering effects on Particle Image Velocimetry data obtained from a down-scaled model of the WindEEE Dome simulator. The first procedure re-centers the velocity data as a function of the instantaneous location of the vortex center. The second procedure treats the time-averaged vortex velocity field as the convolution of a bi-variate normal probability density function, which represents the distribution of vortex center locations over horizontal planes orthogonal to the vortex axis and the instantaneous tornado velocity field. Depending on swirl ratio and vortex height, wandering amplitude was generally from 5% to 9% of the updraft radius. The re-centering procedure was found to be more accurate than the deconvolution procedure. When applied to the turbulence statistics of the velocity field, the correction revealed that a higher level of turbulence at the center of low swirl tornadoes is a result of wandering. Also, the corrected shear stresses revealed a spiral pattern for cases of higher swirl. Wandering effects increases with reducing swirl ratio. For this experiment, swirl ratio is reduced from 0.96 down to 0.22 and errors on the vortex core radius as high as 50% and reduction of the maximum tangential velocity of 13% were observed.

Aerodynamic Interaction Between an Incoming Vortex and Tip Leakage Flow in a Turbine Cascade

In turbines, secondary vortices and tip leakage vortices form in the blade passage and interact with each other. In order to understand the flow physics of this vortices interaction, the effects of incoming vortex on the downstream tip leakage flow are investigated by experimental, numerical, and analytical methods. In the experiment, a swirl generator was used upstream of a linear turbine cascade to generate the incoming vortex, which could interact with the downstream tip leakage vortex (TLV). The swirl generator was located at ten different pitchwise locations to simulate the quasi-steady effects. In the numerical study, a Rankine-like vortex was defined at the inlet of the computational domain to simulate the incoming swirling vortex (SV). The effects of the directions of the incoming vortices were investigated. In the case of a positive incoming SV, which has a large vorticity vector in the same direction as that of the TLV, the vortex mixes with the TLV to form one major vortex near the casing as it transports downstream. This vortices interaction reduces the loss by increasing the streamwise momentum within the TLV core. However, the negative incoming SV has little effects on the TLV and the loss. As the negative incoming SV transports downstream, it travels away from the TLV and two vortices can be identified near the casing. A triple-vortices-interaction kinetic model is used to explain the flow physics of vortex interaction, and a one-dimensional mixing analytical model are proposed to explain the loss mechanism.

Heat transfer analyses of film-cooled HP turbine vane considering effects of swirl and hot streak

Conjugate heat transfer (CHT) analyses were conducted on the film-cooled first stage vane of GE-E3 engine to reveal the influences of inlet swirl and hot streak (HS) on vane film cooling. Two cases with only HS and four cases considering with combined HS and swirl were studied, including the effects of HS/swirl to vane clocking positions (HS/swirl aligned to passage or vane) and swirl orientations (positive/negative). The results indicate that the variations of incidence angle combined with effects of radial transportation of the swirl vortex worsen the film coolant attachment, decrease the film cooling efficiency and increase the heat load onto vane surface. However, for film holes with radial angles toward the outer endwall, negative incidence near hub endwall is beneficial to the leading edge film coolant attachment. Therefore, a negative swirl aligned to the vane leads to lowering the temperature at the leading edge and the pressure side than the other three cases with combined swirl and HS. Heat transfer coefficient (HTC) on the suction surface (SS) is increased due to upwash and downwash of boundary layer fluids caused by swirl. The temperature on SS is therefore increased because the heat energy transferred from fluid to solid is increased.

IN-CYLINDER FLOW CHARACTERIZATION USING VORTICITY BASED PARAMETERS


 
 
 
 The paper deals with intake swirling flow characterization in the cylinder of IC engine. The commonly used method based on the swirl resp. tumble number from angular momentum flux evaluation does not need to give the appropriate values in some cases. Typically, in the case of two intake ports, a counter-rotating swirl vortex pair is presented. However their summary angular momentum flux is zero a thus corresponding swirl number is zero too. In order to correctly quantify these cases, it is proposed to evaluate so called vorticity numbers, i.e. dimensionless numbers based on vorticity evaluation. Concrete results are evaluated from data obtained using 3-D CFD simulation in AVL FIRE code. Comparison of variously defined vorticity numbers with each other and with the vortex numbers is performed. A practical way of calculating the vorticity numbers was also suggested with regard to possible adverse effects of the velocity gradients at the cylinder wall.
 
 
 

Piston geometry effects in a light-duty, swirl-supported diesel engine: Flow structure characterization

This work studied how in-cylinder flow structure is affected in a light-duty, swirl-supported diesel engine when equipped with three different piston geometries: the first two featuring a conventional re-entrant bowl, either with or without valve cut-outs on the piston surface and the third featuring a stepped-lip bowl. Particle image velocimetry experiments were conducted inside an optical engine to measure swirl vortex intensity and structure during the intake and compression strokes. A full computational model of the optical diesel engine was built using the FRESCO code, a recently developed object-oriented parallel computational fluid dynamics platform for engine simulations. The model was first validated against the measured swirl-plane velocity fields, and the simulation convergence for multiple cycles was assessed. Flow topology was studied by addressing bulk flow and turbulence quantities, including swirl structure, squish flux, plus geometric and operating parameters, such as the presence of valve cut-outs on the piston surface, compression ratio and engine speed. The results demonstrated that conventional re-entrant bowls have stronger flow separation at intake, hampering bowl swirl, but higher global swirl than for stepped-lip bowls thanks to a stronger and more axisymmetric squish mechanism and less tilted swirl. Stepped-lip bowls have larger inhomogeneities (tilt and axisymmetry) and higher turbulence levels, but also faster turbulence dissipation toward top dead center. They have weaker squish flux but larger squish inversion momentum as a result of the smaller inertia.

STUDY OF AXISYMMETRIC NATURE IN 3-D SWIRLING FLOW IN A CYLINDRICAL ANNULUS WITH A TOP ROTATING LID UNDER THE INFLUENCE OF AXIAL TEMPERATURE GRADIENT OR AXIAL MAGNETIC FIELD

The three dimensional swirling flow has been obtained by solving Navier Stokes equations, expressed in cylindrical coordinate system, using finite difference technique on a staggered grid. An explicit finite difference method using pressure correction technique, for the solution of Navier-Stokes has been implemented to solve three dimensional flows. Present study explores the 3-D axisymmetric nature of stratified swirling flow and vortex breakdown in a cylindrical annulus cavity with top rotating lid. The annulus is obtained by inserting a thin coaxial rod in cylindrical cavity. This rod may be stationary or rotating depending on the particular study. Three dimensional swirling flows in annuli have also been studied subjected to axial temperature gradient or under the influence of axial magnetic field. Influence of governing parameters Re, Ri and Ha on the overall heat transfer has been investigated through variation of the average Nusselt number with these parameters. Further, the present numerical results are shown to be in good agreement with the available benchmark solutions under the limiting conditions.

A multi-objective CFD optimization of liquid fuel spray injection in dry-low-emission gas-turbine combustors

The main goal of this research is to investigate the effects of fuel injection strategy on the performance of the premixing chamber of modern Dry-Low-Emission (DLE) Gas-Turbine (GT) combustors. Here, an Eulerian-Lagrangian model for multi-phase multi-component flows is evaluated and used to investigate the effects of different fuel spray design parameters, including the injection location, direction, mass-flow-rate partitioning, and flow Swirl number, on the performance of the premixing chamber. The analysis is enriched by multi-objective optimizations accounting for several goals, including the evaporation efficiency, mixture stratification, entropy generation, and flow recirculation. It is observed that the droplet breakup, spray-vortex interactions, and wall-wetting have significant influences on the performance objectives while the droplet residence time effect is minor. Among the design parameters, the injection location and direction have a profound impact on the droplet breakup which predominately controls the evaporation efficiency. In addition, the interactions between the spray and the two swirling vertices inside the chamber strongly affect the mixture stratification (uniformity), e.g. the location and direction of the injection should not be chosen such that a large proportion of fuel droplets are trapped in the shear layer between the two vortices (otherwise the evaporation efficiency drops significantly) or trapped in the strong outer swirling vortex (if large mixture non-uniformity should be avoided). Finally, the best designs meeting a series of targets, e.g. simultaneous perfect mixing and uniformity, etc., are introduced.

Numerical research of cavitation on Francis turbine runners

Francis turbines, as other hydraulic turbines, are custom-designed for nominal operating conditions specific to each power plant. However, they may need to be operated at off-design flow conditions because of variable reservoir levels and flowrates. Operating the turbine at off-design points can cause cavitation. Four cavitation types can be observed on Francis turbine runners. These are leading edge, travelling bubble, draft tube swirl and inter-blade vortex cavitation. They may cause erosion, reduction in efficiency, vibration, instability of operation and noise. Runner blades must be designed taking the flow characteristics into account for design conditions to prevent cavitation. Cavitation limits for the off-design points must also be determined. In this study, the runner geometry of an actual hydroelectric power plant that was designed and implemented in 1960s, is redesigned with the help of the state of the art computational fluid dynamics techniques for cavitation free operation. The cavitation limits for the off-design points are also determined.