Central Toroidal Recirculation Zone Research Articles

Spray characteristics of different regions downstream of a swirl cup

The spray characteristics of different regions downstream of swirl cups play a critical role in cold start and re-ignition of gas turbines. The spray measurements were performed at the fuel pressures of 0.5, 0.8, 1.0, 1.5, and 2.0 MPa and the fuel temperatures of −23, −13, −3, 7, 17 and 27 ℃, respectively. The droplet size, droplet velocity, droplet number, and instantaneous spatial spray image of sprays from an aviation kerosene Jet-A were measured using a two-component phase Doppler particle analyzer and a digital off-axis holography system. As the fuel pressure and temperature increase, the Sauter Mean Diameter (SMD) and spray non-uniformity of the Spray Shear Layer (SSL) gradually decrease. As the fuel pressure increases, the SMD and spray non-uniformity of the Central Toroidal Recirculation Zone (CTRZ) gradually decrease, and the slopes of these curves both decrease. As the fuel pressure increases, the SMD and spray non-uniformity of the CTRZ rapidly decrease at the fuel temperature of −23 ℃, while slightly decrease at the fuel temperature of 27 ℃. The droplets in the CTRZ come from 3 different sources: simplex nozzle, venturi, and outside the CTRZ. As the fuel pressure increases, the proportion of droplets recirculated from outside the CTRZ decreases. This study proposed the concept of the “pressure critical point” for the swirl cups. As the fuel temperature decreases, the proportion of droplets recirculated from outside the CTRZ increases below the critical pressure, while decreases above the critical pressure. In addition, through the models of liquid film formation and breakup on the curved cylindrical wall, a semi-theoretical model was established to predict the SMD of SSL for swirl cups. The prediction uncertainty of this model is less than 6% for all 14 conditions in this paper.

Dual-camera off-axis holographic particle tracking velocimetry: Development and application to air-blast swirl spray measurement

Quantifying the droplet field near the atomizer exit poses a challenge due to its three-dimensional (3D), multiscale and turbulent characteristics. An optical measurement method named dual-camera high-resolution holographic particle tracking velocimetry (DCHR-HPTV) is proposed. This method enables the extraction of near-field spatial Sauter mean diameter (SMD) distribution and three-dimensional two-component (3D-2C) velocity field, from off-axis hologram pairs with a frame interval of 6 μs, recorded using two synchronized industrial cameras and pulsed lasers. The system achieves a resolution of 9344 × 7000 pixels with an individual pixel size (dpix) of 3.2 μm. The droplet size measurement range is from 10 μm to above 500 μm with an absolute error within 3 μm. The velocity measurement range is adjustable and is set to be 0 to 38.4 m/s in this study, with errors of 0.23 m/s and 0.15 m/s for horizontal and axial components, respectively. The method is applied to investigate a spray region of 29 mm × 20 mm with a depth of 35 mm (equivalent to 20.3 cm3) at the exit of an air-blast atomizer. The spatial distribution of circumferentially integrated SMD along radial and axial directions is obtained. Statistical analysis is performed on both spatial and quantity distributions of droplet 3D-2C velocities. Concurrently acquired droplet size and velocity in the central toroidal recirculation zone (CTRZ) indicate that both the increasing air pressure and enlarged size range lead to a weakened droplet backflow motion. This method, distinguished by its large field of view and high resolution for particle velocimetry, is suited for dynamic analysis of near-nozzle droplet fields and can be employed in various testing scenarios involving 3D motion of micron-sized particles within multiscale flow fields.

Confinement effect on recirculation structures in isothermal coaxial swirling jet

The present experimental work reports first observations of effect of confinement on recirculation structures in non-reacting coaxial swirling jet in geometric swirl number range of SG= 2.14–2.88. In particular, particle image velocimetry (PIV) has been employed to study the effect of two confinement ratios (CR) viz. 1.82 and 2.73 on two types of recirculation zones (RZ) i.e., pre-breakdown dual rings (PVB) and central toroidal recirculation zone (CTRZ) observed in coaxial swirling jet. It is observed thatCR = 1.82 produces a slight radial expansion in PVB whereas significant compaction and streamwise elongation is observed in CTRZ. The difference in the observation is explained on the basis of modified Rossby number (Rom). The PVB flow state which is characterised byRom > 1 presents significant resistance to wall effect to be felt till the centerline. Contrarily, the CTRZ flow mode characterised byRom < 1allows the wall effect to be felt till the centerline, in turn affecting the RZ entirely. TheCR = 2.73 is shown to offer larger area for the RZ to spread radially outwards. Resultantly, the PVB and CTRZ are shown to significantly widen. Other significant comparisons betweenCR = 1.82 and 2.73 for PVB and CTRZ are elaborated using contour plots of axial and radial strain rates.

Effects of Swirl Number and Central Rod on Flow in a Lean Premixed Swirl Combustor

ABSTRACT In most modern-day lean gas turbine combustors, flame is stabilized by swirl flows. A central toroidal recirculation zone (CTRZ), developed by these swirl flows, helps to stabilize the flame. The current study is focused on the effects of swirl number and a central rod on the flow behavior in a lean premixed swirl-stabilized combustor. Turbulence is modeled using the two-equation, realizable k-ϵ model of Reynolds-Averaged Navier–Stokes (RANS), and flamelet generated manifold (FGM) technique is used to model combustion. Both swirl number and central rod have a very significant effect on the flow and the flame in the combustor. The CTRZ moves upstream along with the flame, creating a flashback, with an increase in the swirl number. Also, the presence of a central rod accelerates the flame flashback at high swirl numbers. Higher turbulence is observed in inner shear layers, especially at high swirl numbers. emissions are higher in the regions of high temperature, especially in the CTRZ. These emissions decrease with the increase in the swirl intensities. At low swirl numbers, the case with the central rod exhibits lower production, and at high swirl numbers, the case without the central rod yields better results in the mitigation.

Aerodynamic effect on atomization characteristics in a swirl cup airblast fuel injector

To investigate the influence of swirling air flow field on the spray characteristics and the droplet behaviors of the swirl cup airblast fuel injector, a single swirler cup and a double swirlers cup airblast injector were operated in a wide range of fuel mass flow rates (mf) and relative air pressure drops (ΔPa). Various laser based diagnostics, including planar Mie scattering, phase-Doppler particle analyzer, and high speed particle imaging velocimetry, were employed to provide the information of spray angle, droplet Sauter mean diameter (SMD), droplet velocity, and air flow velocity. The results show that the air flow field was characterized by central toroidal recirculation zone (CTRZ), the double swirlers cup airblast injector generated stronger CTRZ than single swirlers cup airblast injector. The spray characteristics and the droplet behaviors with different size classes were significantly affected by the air flow field. The larger droplets (25–100 μm) with higher initial momentum were likely to keep its velocity when passing through CTRZ, when the mf increased, more large fuel droplets could pass through the CTRZ and expanded to both sides, resulting an increase in a spray angle. On the other hand, smaller droplets (&amp;lt;25 μm) with lower initial momentum were trend to be enrolled by the CTRZ; therefore, the spray droplets at lower mf were confined in the CTRZ. These droplets trend to coalesce with each other resulting in an increase of SMD in this region.

Investigations of the effect of the primary hole on ignition performance of a three-dome model combustor with RP-3 liquid aviation fuel

The primary hole exerts a substantial influence on the ignition performance of gas turbine combustors. The current investigation delves into the effect of primary hole jets on the ignition performance of a three-dome model combustor utilizing RP – 3 liquid aviation fuel, providing a comprehensive understanding of the ignition performance. The number of primary holes in the identical dome is considered as two, four, and six, involved in creating different primary hole jets. The limitations in the effectiveness of ignition and propagation have been brought to light by the combustion experiment that was performed under ambient temperature and pressure at a reference velocity ranging from 3 – 8 m/s. Moreover, a validated numerical simulation methodology was employed to investigate the flow field characteristics in the model combustor at a reference velocity of 5.4 m/s. Both experiment and numerical simulation results indicate that the model combustor with four primary holes attains the weakest mutual support between adjacent domes, the highest average radial velocity, and the lowest restrictive effect on the center toroidal recirculation zone.

Experimental and numerical studies of the effects of the contraction ratios on the swirling flow characteristics of the model combustor outlet in lean gas turbines

Large Eddy Simulation (LES) is utilized in this work to study isothermal turbulent swirling flow characteristics for a lean partially premixed gas turbine model combustor. The goal is to investigate the influence of contraction ratio (CR) variation on the vortex breakdown phenomena features and the turbulence structure in these combustors. Particle image velocimetry (2D-PIV) measurements were carried out on a single-case model combustor using (air-air) phase to perform the mean flow behavior and the validation process under atmospheric conditions with CR equal to 0 (largest diameter, Dinlet = Doutlet), the equivalence ratio (φ) of 0.3, swirl number of (Sn) = 0.66 and Reynolds number of (Re) = 42,000. While five test cases with various CRs were applied in LES under the same conditions using (air-CH4) phase to predict the vortical structure and the precession vortex core (PVC). The CRs ranged as follows: 0, 50 %, 60 %, 70 % and 80 % (smallest diameter). Experimental and LES results show that there was a reasonable agreement between the PIV and LES results with maximum deviations of 2.8 % and 4.65 % for the normalized mean axial and radial velocities, respectively. As the CR was increased, the pressure drop and the mean axial velocity increased linearly at the contraction part. In addition, the downstream stagnation point and the size of the central toroidal recirculation zone (CTRZ) stabilized when CR = 60 %, 70 %, and 80 %. The swirl level was further enhanced by 40 % when CR = 60 % and 80 %. The presence of CR successfully suppressed the pressure and axial velocity fluctuations generated in the shear layer zones by 35 %. LES data was analyzed by Fast Fourier Transformation (FFT) and Proper Orthogonal Decomposition (POD) to investigate the PVC dynamics. The FFT showed four dominant frequencies as a result of the appearance of the PVC and vortex breakdown oscillation. The frequency of PVC (fpvc) decreased linearly with an increase in the CR where the fpvc was 20.8 Hz when CR = 0 and became 12.5 Hz at CR = 80 %. In contrast, the other frequencies were still constant even with increased CR at 187 Hz, 562 Hz, and 979 Hz. The POD analysis showed the first mode was more energetic and had 40 % of the total pressure fluctuations.

Experimental investigation into the effect of main stage swirl on flow and spray characteristics inside a stratified partially premixed combustor

Lean combustion has been applied in gas turbine combustors to meet increasingly stringent regulations for pollutant emissions. As a representative low-emission technology, stratified partially premixed (SPP) combustion is capable of effectively reducing NOx emissions while still providing robust flames. In this work, the flow and spray processes inside an SPP combustor were experimentally investigated to further reveal the role of main stage swirl in flow development and fuel placement. Particle image velocimetry (PIV) and Planar Mie scattering (PMie) techniques were applied to provide the velocity distributions and droplet distributions under isothermal conditions with a relative pressure drop ranging from 1% to 5%. A spray post-processing algorithm was developed to acquire the characteristic parameters of the sprays without clear boundaries. Results show that the strong swirl from the main stage leads to the radial flow of pilot air streams and dominates the formation of central toroidal recirculation zone (CTRZ). With the addition of the main stage swirl, a proper flow structure including jet zones, recirculation zones and shear layers is formed, which promotes the widening of droplet radial distribution and the increase of the spray cone angle. The spray distribution therefore transforms from a bubble shape into a cone shape. The findings and conclusions could promote the understanding of interactions between pilot and main stage swirls for stage combustors and contribute to the optimization of such SPP injectors.

Numerical simulation through Fluent of a cold, confined and swirling airflow in a combustion chamber

The study focuses on modelling a cold, confined and swirling airflow in a magnesium burner. Direct observations of the magnesium combustion in the combustion chamber proved the existence of a significant recirculation zone in the burner. Before modelling the high flame temperature and the turbulent mixture of solid fuel and products in this burner, the correct simulation of a high swirl cold flow and the creation of a recirculation zone is a crucial first step. Experimental measurements were made placing a hot-wire anemometry at different positions in a PMMA replica of the burner, to validate the simulation results returned by the Fluent software. In a low swirl case (S=0.13), simulations performed using different eddy viscosity models were compared and the standard k-ɛ model was proved to more accurately predict the velocity profiles and the central recirculation zone. In a high swirl case (S=2.94), the turbulence and its anisotropy were too strong and the RSM model was proved to more accurately predict the position, size and shape of the central toroidal recirculation zone. Important errors were still observed for the velocity values. Higher discretization orders (second- or higher-order) could not be used in this case, due to induced numerical instabilities.

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.

Experimental investigation of performance of high-shear atomizer with discrete radial-jet fuel nozzle: mean and dynamic characteristics

The present study focuses on the performance of a novel high-shear atomizer with a discrete radial-jet fuel nozzle to overcome the constraints associated with the simplex-pressure-swirl and duplex-fuel nozzles at the high-end power demand of a gas turbine combustor. The high-shear atomizer consists of multiple inner and outer radial swirlers with interchangeable flare and fuel nozzle. The performance of the atomizer with discrete radial-jet fuel nozzle is elucidated at ALR (mass ratio of air to liquid) 14.1 through variations in geometrical design parameters of the swirl cup. The parameters of interest are the split ratio (γ), relative swirl direction of inner and outer swirler (co- and counter-rotation), flare angle (θ) and flare mixing length (η). Spray characteristics at ALR 4.72, 7.08 and 9.44 are also presented for an atomizer by freezing the geometrical design. The particle image velocimetry diagnostic technique is employed to capture the spray flow field. The non-dimensional radial (W/Df; W, radial width of CTRZ (in mm) and Df, exit diameter of flare (mm)) and axial (L/Df) sizes of the central toroidal recirculation zone and near field swirl number (SN5) of the flow are explored. Further, variations in the droplet size distribution of the atomizer across all the ALR are discussed in detail. The Sauter mean diameter across all the test cases is found to be in the range of 9–30 μm, 15–37 μm, 15–50 μm and 23–75 μm at ALR 14.1, 9.44, 7.08 and 4.72 respectively, which shows good atomization capability of the atomizer with discrete jets. The spatial distribution of the spray volume/mass in an azimuthal plane is examined in the circumferential and radial directions, which shows consistent and excellent azimuthal symmetry of the spray even with a decrease in ALR value. The overall mean and dynamic spray characteristics of the atomizer suggest that high-shear atomizer in combination with a discrete radial-jet fuel nozzle would be a better candidate than an atomizer with a simplex pressure-swirl fuel nozzle in rich-quench-lean concept-based gas turbine combustors.

Investigations of the effect of dome spacing on ignition performance of a model combustor with RP-3 liquid aviation fuel

Dome spacing is one of the most critical factors to affect the ignition performance of the aerospace engine. In this paper, a three-dome model combustor is designed to investigate the effect of dome spacing on ignition performance with RP-3 liquid aviation fuel. The dome spacing considered are 48 mm, 60 mm, 72 mm, respectively, and the effect on ignition performance including ignition and propagation limits are revealed by a combustion experiments under ambient temperature and pressure, and the reference velocity is from 3 m/s to 8 m/s. The ignition performance is evaluated in terms of the fuel/air ratio, at which ignition occurs. The results show that the best ignition performance is achieved when the dome spacing is 60 mm. Furthermore, numerical simulations with validated methodology of non reacting flows inside the combustor have been conducted. The results show that when the dome spacing is 60 mm, the best mutual support between adjacent domes is attained according to the maximum value of mergin rate and dimensionless width of central toroidal recirculation zone. Besides, the restrictive effect of the primary hole jet on the central toroidal recirculation zone is found to be increased with the decrease of dome spacing.

Isothermal swirling flow characteristics and pressure drop analysis of a novel double swirl burner

Nonreacting swirling flow behavior through a novel double swirl burner has been investigated in this paper using Detached Eddy Simulation (DES) calculations and experimental measurements. The effect of mixing both the swirling flow of the inner jet (primary air or inner swirler) and the swirling flow of the annular jet (secondary air or outer swirler) on the flow structure has been analyzed. Moreover, pressure losses and flow field characteristics have been investigated at different inlet air mass flow rates and combustor diameters. Due to the drawbacks of general pressure loss to predict the pressure drop through high turbulence swirl flow as well as the complex geometry, the time-averaged viscous dissipation field function is used to predict the pressure drop through the burner. This function is based on the relationship between viscous dissipation and pressure losses. DES flow fields are compared with high-speed particle image velocimetry measurements at different inlet air mass flow rates. It is found that there is good agreement between numerical and experimental results, and both the central toroidal recirculation zone and the corner recirculation zone are well captured. In the case of a single inner swirler, the general pressure drop is very high compared to single outer and double swirl generators, and 84% of the inlet air pass through the outer swirler and just 16% of the air pass through the inner swirler in the case of a double burner (partially premixed). The time-averaged viscous dissipation field function is very important to identify the locations that lead to pressure drop inside the burner, which induces to select the appropriate design for the best performance. In addition, it is concluded that the performance of the double swirl burner is more efficient compared with the other two single swirlers in terms of pressure losses and flow structure.

Effects of confinement length of the central toroidal recirculation zone partly confined by the small pilot stage chamber on ignition characteristics

Distributed Lean Direct Injection (DLDI) combustor is a new promising low NOx emission combustor. The ignition process independently occurs in the pilot stage chamber, which partly confines the swirled spray at the small confinement ratio. This study investigates ignition characteristics of the partly confined swirled spray experimentally, where effects of confinement length on flame propagation path and ignition probability are mainly considered. Particle image velocimetry, Mie scattering, phase Doppler anemometry and high-speed visualization were used to reveal the mechanisms of flame propagation and stabilization. The ignition probability was firstly counted in three chambers with the length varying from 18 mm to 27 mm, when pressure drop, ΔP, was 3000 Pa, and total equivalence ratio, ϕoverall, varied from 0.67 to 0.73. Results show that reduction of the chamber length increases the ignition probability. To interpret this trend, aerodynamics, droplets characteristics and ignition process in three chambers were analyzed at ΔP=3000 Pa and ϕoverall=0.73. The time-average flow field shows that the modification of chamber length results in changes of the confinement length of the central toroidal recirculation zone (CTRZ). Furthermore, the structure of CTRZ is directly impacted by changes of the confinement length. The reduction of the confinement length makes more air flow back, which means the greater recirculation capability of the CTRZ. More recirculating air can transport more droplets back into the CTRZ before sparking, which can also transport more high-temperature mixture there to help hold flame during ignition process. The high-speed visualization shows that the failed mode of ignition process is that the swirling flame can grow up at the beginning of Phase 3, but will blow out gradually after the extinction of the flame in corner recirculation zone (CRZ). According to characteristics of aerodynamics, droplets characteristics and ignition process, it can be concluded that since significant time is taken for flame to reach the fully burning state, the reduction of the confinement length improves the recirculation capability of the CTRZ which helps the partly confined flame get better stability. In addition, in Phase 2 of ignition process, greater recirculation capability makes the anchored position of the edge flame more upstream, which can make the propagation path shorter.

Parametric study for reducing emission characteristics using different computational schemes

The present study numerically predicts emissions (NOx & CO) characteristics issued from swirl burner using different computational schemes adopted in ANSYS 17.2. Various turbulence models were tested and compared in a structured grid having an average skewness of 0.04. RSM model shows the best results in predicting the central toroidal recirculation zone for such severe swirling flow (S>0.6). Three different schemes of combustion modeling were also tested and compared. Flamelet PDF model was preferred among other combustion models for its potential to consider non-equilibrium chemistry, where its choice was recommended form many studies in the literature. A radiation heat transfer analysis was carried out using P1 radiation model to account radiation effects in estimating NOx and CO concentrations. The effect of varying swirl ratio, excess air, diluting fuel with N2 and CO2, oxidizer preheating, and oxidizer composition on emissions (NOx and CO) reduction strategy concluded that increasing the swirl number, excess air factor, and (N2 and CO2) mixing ratios have a considerable influence on NOx and CO emissions reduction. While increasing the preheating temperature, and the oxidizer Oxygen concentration leads to an increase the NOx and CO emissions. Hence, a considerable reduction in NOx and CO formation was apparently observed by lowering the peak flame temperature less than 1800 K. These predicted outcomes are in agreement with measurements data published in the literature.

Characterization of wall filming and atomization inside a gas-turbine swirl injector

The internal atomization characteristics of a practical gas turbine fuel injector are investigated comprehensively using various laser flow diagnostic techniques. The injector consists of a pilot spray surrounded by a primary swirl air, with the spray impinging on a venturi prefilmer to form a liquid film that accumulates into a rim at the venturi tip, which in turn is sheared by a counter-rotating secondary swirl air. The atomizer geometry is modularized into different configurations to access the internal flow field. The quantitative measurements made are with a phase Doppler interferometer at different stages of the injector and time-resolved volume or planar laser induced fluorescence (TR-V/PLIF) imaging. The different internal atomization characteristics such as the liquid film thickness variation, size distribution of craters formed by primary spray droplets impinging on the liquid film, rim thickness at the venturi tip, and droplet size and velocity distribution at different stages are measured. It is found that the film dynamics is influenced by the droplet splashing and the shearing action of the primary air. The liquid rim at the venturi tip is the major source of droplets at the injector exit rather than the liquid film. The unsteady evolution of the multiphase flow inside the injector is dictated by the precessing vortex core of the primary swirl. However, the atomization process is mainly influenced by the central toroidal recirculation zone of the primary swirl flow and the counter-rotating shear layer acting on the accumulated liquid rim at the venturi tip.

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

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.

Impact of confinement on flowfield of swirl flow burners

Swirlers are commonly used in gas turbine combustors as they provide recirculation zones and reduce axial velocity for enhanced flame stability. Swirl provides hot gas recirculation zone at front end of the combustor for enhanced mixing between hot reactive species and the freshly introduced mixture. In this paper, the impact of confinement on a swirl assisted combustion was investigated with focus on the flowfield under unconfined and confined conditions. The features of the flowfield were characterized under both isothermal and reacting conditions. Experimental results showed that for the unconfined cases, the flowfield exhibited the traditional central toroidal recirculation zone. Upon confinement, this zone shortened and also widened with increased velocity fluctuations across the combustor. Increase in the Reynolds number further enhanced the recirculation zone and increased the velocity magnitudes and turbulence. For reacting conditions, minimal recirculation was noticed for the unconfined flame. The recirculation zone was significantly enlarged upon confinement (compared to the non-reacting case) and with increase in Reynolds number. In general, the fluctuating velocity was found to be higher in the confined case compared to the unconfined case, and even higher at increased Reynolds number.

Effect of rotational direction of triple-swirler on cold flow characteristics of a model combustor

Triple-swirler plays an important role for aero-engine combustors to achieve high temperature rise. In this paper, experimental investigations were carried out to explore the effect of triple-swirler rotational direction on swirling flow field in atmospheric condition. Two-dimensional-planar particle image velocimetry measurements show that the central toroidal recirculation zone (CTRZ) formation is significantly affected by the swirler rotational direction combinations: an obvious CTRZ can be formed by the triple-swirler with co-rotating intermediate swirler and outer swirler, while a much smaller CTRZ was obtained by the triple-swirler with a counter-rotating intermediate and outer swirler. Furthermore, the swirl level of the mixed flow is significantly affected by the rotational direction combination, and the integrated swirl numbers were calculated to help evaluating the swirl level generated by triple-swirlers. The rotational direction combination plays a key role on the tangential velocity distribution. The tangential velocity distribution is not only closely related to rotational direction, but also the swirl number combination and mass proportion of each swirler in a triple-swirler.

동축 반전 스월러의 플레어 각도변화가 스월러 유동에 미치는 영향 연구

ABSTRACTSwirler generates the overall swirling flow in the combustion chamber and this swirling flow governs the flame stability and enhances fuel atomization. This paper deals with the flare angle effects on flow streamlines, recirculation zone, Central Toroidal Recirculation Zone(CTRZ) and Corner Recirculation Zone(CRZ) in the model combustion chamber using counter-rotating swirler. 2D PIV system was employed to obtain the velocity components and test condition was obtained using Reynolds Analogy equivalent to air test. We observed transitional flow patterns of flare angle increased. The obtained results show that the flare angle controls the behavior of Recirculation zone, Central Toroidal Recirculation Zone and Corner Recirculation Zone. Key Words : Swirling flow, Flare, Counter-Rotating, PIV system, Recirculation zone 기 호 설 명 Alphabets CTRZ : Central Toroidal Recirculation ZoneCRZ : Corner Recirculation ZoneFA : Flare AngleS n : Swirl NumberRe : Reynolds Numberρ : DensityV : Velocityµ : Dynamic ViscosityD : Diameter sw : Swirlerhub : Hub θ