Length Of Recirculation Zone Research Articles

Experimental and numerical study on flow field characteristics of a combustion chamber with double-stage counter-rotating swirlers

To discover a good predictive turbulent model and reveal the mechanism of turbulent mixing enhancement, a new combustion chamber equipped with a double-stage counter-rotating swirler with vane angles of both stages as 45° is proposed and the flow field characteristics are explored by combining 2D-PIV experiment and Reynolds averaged numerical simulation (RANS). Firstly, the performance of various turbulence models in predicting the flow field characteristics is evaluated by comparing them with experimental results. The study concludes that the RNG k-ε turbulence model exhibits superior performance in predicting velocity distribution, recirculation zone length, and vorticity distribution. Hence, it is selected to analyze the influence of the Reynolds number (Re) of swirlers on the flow field characteristics. In this study, seven Re numbers ranged from 5425 to 54,245 were selected for analysis. Through comparison, it appears that changes in Re number have a strengthening effect on the flow field at various locations, including the recirculation zone, wall surface, and shear layer. This suggests that Re number has an influence on turbulence or boundary layer behavior. Interestingly, there seems to be a critical value for Re number of 21,698, which may be related to a competition between Re number and wall effects.

Influence of secondary air blade angle and oxygen-rich combustion characteristics of an improved Babcock swirl burner

Innovations in research methods and stable combustion technologies were urgent to enhance flexibility in faulty coal-fired boiler. Focusing boiler start-stop processes was vital for effective peak shaving in power plants. The integration of enriched oxygen stable combustion with swirling techniques showed promise in improving low-load performance. In the cold single–phase experiment, the influence of secondary blade angle on the flow field of an improved Babcock burner was observed. Industrial experiments were conducted on a 700 MW boiler, exclusively implementing the improved burners in the lower layer of the boiler front wall. Stable annular recirculation zones were observed at the burner outlet with blade angles of 30°, 45°, 60°, and 75° for the secondary air. Increasing the blade angle expanded the recirculation zone length and decreased the gas net flow ratio. The angle, between 45° and 60°, facilitated air mixing and protected against outlet slagging and high-temperature corrosion. During cold start–up process of the boiler, supplying 0 kg·s−1 of pure oxygen to a single improved burner increased the flue gas temperature by about 50 °C within 0.4 m of the primary air duct outlet. Increasing the oxygen flow rate to 0.2456 kg·s−1 steadily raised the flue gas temperature at 405 MW. The gas temperature in the burner central region was generally lower than the secondary air zone, resulting in a longer ignition distance for the pulverized coal. Higher oxygen levels intensified the heating rate and reduced the ignition distance in the central region.

Experimental analysis of pulsatile flow effects on flow structure in transitional-type cavity

Studies combining flow separation and pulsatile flow phenomena are inherent in the field of biomedicine and have many engineering and biomedical applications. In this study, the pulsatile flow structure is investigated experimentally based on a transitional-type cavity with a length-to-depth ratio of 8 using a microparticle image velocimetry system. A pulsatile flow is generated by pulsating the inlet pressure in sinusoidal pulses using a pressure control unit. Stationary flow and four sets of pulsatile parameters are investigated: two pulsation amplitudes (A = 0.15 and 0.60) and two pulsation frequencies (f = 0.5 and 1 Hz) in the Reynolds range of 50–2000. The recirculation flow dynamics in a cavity are analyzed by investigating the effect of pulsations on the flow structure and statistical flow parameters such as vorticity, shear rate, and turbulence intensity. The analysis shows that the effect of the pulsation amplitude on the recirculation zone dynamics is more prominent than that of the pulsation frequency. The magnitude of the recirculation zone reduction achieved by the pulsatile flow is inversely proportional to the pulsation amplitude. A reduction in the recirculation zone length decreases the shear rate distribution along the cavity. Additionally, an analysis into the turbulence intensity shows that effect of pulsations is negligible when the flow approaches the turbulent flow regime.

Computational Study of a Swirl Stabilized Flame in a Gas Turbine Combustor

Size of recirculation zone and temperature field is examined in a swirl stabilized aero gas turbine combustor using computational fluid dynamics. A sector of 22.5° of an annular combustor is modeled for the study. Unstructured tetrahedral meshes comprising 1.2x106 elements are employed in the model where the governing equations are solved using CFD flow solver CFX. Grid sensitivity study and code validation through limited test data has also been carried out. The effect of pressure and fuel-air ratio on the size of recirculation zone and apparent size of flame zone has been studied at steady state conditions. The heat release rate due to combustion is found to affect the length of recirculation zone as well as the velocity field inside the combustion liner.

Wind tunnel measurements on the effects of leading edge geometry on surface pressures and wake flow of finite blunt plate

In this paper, the surface wind pressure distribution and wake flow of finite blunt plates with different leading edge geometries are investigated. Particle image velocimetry and surface pressure measurement are performed on five typical leading-edge plates in a uniform wind. The results show that the wind pressure distribution on the blunt plate surface is largely affected by the leading-edge geometry. Specifically, as the leading edge angle increases from 60° to 180°, the peak values of the mean pressure coefficient and peak suction pressure coefficient decrease by 20.1 % and 11.6 % respectively. Meanwhile, the peak value of the r.m.s pressure coefficient increases from 0.041 to 0.126 and gradually approaches the mid-axis of blunt plate. In addition, larger separation angles lead to an increase in the reattachment length from 0.7d to 5.26d, but monotonically decrease the characteristic frequency of the plate. The analysis of the wake velocity field shows that an increase in leading-edge angle leads to an increase in the length of the low-velocity recirculation zone in the wake from 0.7d to 1.1d, and both the turbulent kinetic energy and Reynolds stress are larger in this region. Meanwhile, the flow streamlines and vorticity distribution become more unstable and complex, exhibiting a faster rate of evolution. Furthermore, the extraction of the wake structure using a proper orthogonal decomposition shows that the aerodynamic shape of leading edge has a limited effect on the large-scale vortex structure, but significantly affects small-scale vortex structure of high-order modes.

Analysis of flame instability in low-emission tower-type gas turbine combustor with multi-stage methane injection

A low-emission tower-type coaxial-staged combustor with a pilot stage, first main stage and second main stage is simulated in this study. The flow field, flame structure, vortex-mixing interaction, and flame instabilities are predicted with large eddy simulation and partially stirred reactor model. Heat radiation is not considered in the simulations. The results show that the maximum of the averaged length and width of the inner recirculation zone are approximately 249.8 and 71.3 mm, respectively. The distributions of the fuel from various stages in the combustor are analyzed based on the respective mixture fractions. The pressure and heat release rate (HRR) in the combustor fluctuate periodically with a frequency of about 373 Hz. Both HRR and pressure fluctuations increase and then decrease with time, and there is a phase difference of 46.1° between them. Due to the combustion instabilities, the flame structures change periodically, including attached flame and lifted V-shaped flame, in one HRR oscillation cycle. Moreover, the swirling flame length also varies periodically with time. As the pilot stage and first main stages are turned-off, the frequency of HRR fluctuations and pressure fluctuations, as well as the flame root dynamics, are significantly affected. It is also found that the effects of the fuel ratio of the first and second main stages have a significant influence on the flame root dynamics, e.g., different degrees of flashback observed in different cases.

Use of recirculation theory and reduced dimensionality for efficient calculation of jet flow in cylindrical combustors

This work evaluates use of a dimensionally adaptive (DA) meshing technique to speed CFD modeling of non-reacting pressurized flow in a cylindrical geometry with a center jet and non-axisymmetric secondary jets. The DA-capable approach incorporates an a priori dimensional boundary location and extension of 3-D RANS flow solvers. Recirculation zone length, not flow asymmetry, was the limiting factor in placement of the dimensional boundary between 3-D and axisymmetric meshes. Boundary placement was based on confined turbulent jet theory, which suggests jet attachment is a linear function of cylinder radius, L, expressed as 5.85L. CFD results for a turbulent axial pipe flow showed that as the dimensional boundary was moved upstream, computational time dropped quadratically with the reduction in mesh cells. Further simulations of a 0.2032-m diameter, 1.5-m long reactor with turbulent center primary jet and outer radius secondary jets placed the dimensional boundary at 0.7 m. Results for pressures of 20, 10, and 5 bar and mass flow rates of 75% and 50% of baseline demonstrated two key results. First, flow similarity was preserved across all cases and the predicted jet attachment location was approximately 0.61 m, compared with the theoretical value of 0.594 m. Second, runtime reductions of 79% (pressure) and 82% (flow rate) were seen for the DA meshes compared with fully 3-D meshes. Differences in average exit axial velocities between DA and fully 3-D meshes were 2–4% for the pressure cases and 0.7–1.5% for the flow rate cases. Lastly, comparison with 3-D quarter-geometry calculations showed the DA approach to be ∼10% faster with exit axial velocities within 3–5% of 3-D velocities.

Numerical Investigation on Mechanism of Swirling Flow of the Prefilming Air-Blast Fuel Injector

Prefilming air-blast atomizers are widely used in modern gas turbine combustors. Due to insufficient awareness of the coupling mechanism of multi-stage swirling flow in gas turbines, there is a lack of effective methods for flow field optimization in combustor. In this study, the effect of some critical parameters on the flow field of a prefilming air-blast atomizer was analyzed with CFD. The parameters include the angle and number of the first swirler blades, the angle of the second swirler blades and the angle of sleeve. Furthermore, the coupling mechanism of two-stage swirling airflows of prefilming air-blast atomizer was discussed. Moreover, the influence of the interaction between two-stage counter swirling airflows on the characteristics of flow field was explained. The results show that with the increase in SNi, the axial length of the primary recirculation zone decreased, while the radial width increased. The starting position of primary recirculation zone (PRZ) moves forward with the increase in SNo. Reducing the sleeve angle β helps to form the primary recirculation zone. The results indicate that it is the transition of tangential velocity of airflow to radial velocity that promotes the formation of the PRZ. These results provide theoretical support for optimization of the flow field in swirl combustor.

Computational Investigation on the Impact of Coal Feed Size in a Tangentially Fired Gasifier

ABSTRACT Using a validated comprehensive model, the effect of coal particle size on two-phase flow inside a tangentially fired, Mitsubishi Heavy Industry entrained-flow coal gasifier is investigated. It is observed that the length of the central recirculation zone (CRZ) formed in the gasifier flow increases with an increase in the mean size of the injected coal particles. The CRZ lengths are 5.6, 4.5 & 2.1 m, respectively, for the cases with particle size distributions (PSDs) having mass mean diameters of 80, 40 & 20 µm. The generic swirl number, , captures this impact of particle size on the CRZ length, whereas the conventional swirl number, , is unable to do so. The value of does not vary significantly over most of the gasifier’s constant diameter zone in each case simulated in the present work, thereby underlining the utility of this swirl number to characterize similarity in swirl dynamics. Further, the analysis of the impact of coal particle size on char conversion yields that the PSD with finer particles leads to higher conversion due to the better combustibility of finer particles.

Analysis of Lewis number effects on dynamic response of laminar premixed flames

The dynamic response of premixed flames with different fuel Lewis numbers is crucial to understand for the design of safe combustion chambers with wide operating range. Due to preferential diffusion effects associated with fuel Lewis number for lean mixtures, a premixed flame responds differently to flame stretch. These responses could manifest in the flame’s response to small perturbations. In order to investigate this effect, in this study, lean premixed hydrogen, methane and propane flames at the same adiabatic unstretched flame speed are studied using numerical simulations. Steady unperturbed flames show significant differences in flame stand-off distances, flame height, flame stabilization limits, burner temperature and recirculation zone length. Flames are perturbed using a step function excitation and noticeable changes in the gain and phase of the flame transfer function are found even when the flame burning velocity and the inlet flow speed are kept the same. Maximum gain of the response is found to increase with the mixture Lewis number and is shown to be directly dependent on the flame tip burning strong or weak. Phase difference at low frequencies is found to depend on the average of the time scales of the recirculation vortex which in turn depends on the flame burning stronger or weaker under the influence of positive stretch at the flame base, axial convective wave and perturbation travelling along the flame front. Time for the flame to settle to a new position after the step excitation is found to correlate well with the sum of time scales of vortex, flame tangential speed and flame tip speed. Overall, it is found that hydrogen flame responds the fastest to the perturbations and exhibits smaller increase in gain compared to methane and propane.

Pressure gradient tailoring effects on vorticity dynamics in the near-wake of bluff-body premixed flames

We investigate the role of mean streamwise pressure gradients in the development of a bluff-body-stabilized premixed flame in the near-wake of the bluff body. To this end, a triangular prism flame holder is situated in three different channel geometries: a nominal case with straight walls, a nozzle with a stronger mean pressure gradient, and a diffuser with a comparatively weaker mean pressure gradient. All geometries are implemented using embedded boundaries, and adaptive mesh refinement is used to locally resolve all relevant thermal (i.e., flame) and fluid-mechanical (i.e., vorticity) scales. A premixed propane flame, modeled using a 66-step skeletal mechanism, interacts with vorticity in the boundary layer of the triangular bluff body in the presence of each mean pressure gradient. Analysis of flame-related enstrophy budget terms reveals key differences in the behavior of baroclinic torque between cases, the specifics of which are tied to larger variations in the mean flow structure, recirculation zone structure, and confinement effects. Our results show that the baroclinic torque changes significantly among the configurations, with the nozzle exhibiting the largest baroclinic torque production. However, these differences are shown to be only a secondary consequence of the background pressure gradient, with the primary consequence being the change in the recirculation zone length resulting from the different channel configurations. These results are relevant for flame stabilization with bluff bodies, where clear understanding of the sensitivities to global mean pressure gradient is important to engineering design.

Numerical Study of Flow Field Over the Deck with Active Flow Control Method

When the destroyer is sailing on the sea, the turbulent ship airwake will be formed above the deck, due to the flow separation after the airflow passes through the edge of the hangar, and the most obvious vortex structure is the recirculation area, which will seriously affect the safety of take-off and landing of the ship-borne helicopter on the deck. Based on the means of flow control in fluid dynamics, this paper improves the quality of the flow field above the deck of the destroyer by adopting different blowing ways (3 blowing angles and 5 blowing speeds). The results show that for different blowing directions, the length of recirculation zone tends to decrease with the increase of blowing speed. The optimal blowing way is the blowing speed of 10m/s in the blowing direction of -45°, and the length of recirculation zone is reduced by 59.4%. This will obviously improve the landing safety of ship-borne helicopters and reduce the pilot's control load.

Modeling of Turbulent Heat-Transfer Augmentation in Gas-Droplet Non-Boiling Flow in Diverging and Converging Axisymmetric Ducts with Sudden Expansion

The effect of positive (adverse) and negative (favorable) longitudinal pressure gradients on the structure and heat transfer of gas-droplet (air and water) flow in axisymmetric duct with sudden expansion are examined. The superimposed pressure gradient has a large influence on the flow structure and heat transfer in a two-phase mist flow in both a confuser and a diffuser. A narrowing of the confuser angle leads to significant suppression of flow turbulence (more than four times that of the gas-drop flow after sudden pipe expansion without a pressure gradient at φ = 0°). Recirculation zone length decreases significantly compared to the gas-droplet flow without a longitudinal pressure gradient (by up to 30%), and the locus of the heat-transfer maximum shifts slightly downstream, and roughly aligns with the reattachment point of the two-phase flow. Growth of the diffuser opening angle leads to additional production of kinetic energy of gas flow turbulence (almost twice as much as gas-droplet flow after a sudden pipe expansion at φ = 0°). The length of the flow recirculating region in the diffuser increases significantly compared to the separated gas-droplet flow without a pressure gradient (φ = 0°), and the location of maximum heat transfer shifts downstream in the diffuser.

Effect of inlet flow turbulence on flame–vortex dynamics during thermo-acoustically induced flame flashback in a premixed dump combustor

The present article reports the investigation on the effect of inlet flow turbulence on flame–vortex dynamics during the thermo-acoustically induced flame flashback in a premixed dump combustor. The inlet turbulence intensity can be varied independently of the Reynolds number by using plates of different slot widths and is placed significantly upstream of the nominal flame-stabilization zone. The study aims to experimentally address the flame flashback behavior and the conventional combustion dynamics mechanisms involving flame–vortex interaction. The reacting flow-field is characterized using high-speed simultaneous measurement of dynamic pressure fluctuation with the time-resolved/phase-resolved visualization of flame and flow-field utilizing CH*/OH* chemiluminescence and particle image velocimetry. We note that higher turbulence intensity delays the onset of combustion dynamics and significantly influence the flame flashback behavior. In the present work, we observed two types of flame flashback behavior with large amplitude oscillations — (a) intermittent flame flashback, which relates to upstream flame displacement during certain phases of the acoustic pressure cycle and (b) permanent flame flashback, which results in flame quenching. The results reveals that the recirculation zone length plays a key role in determining the flame flashback. The role of enhanced turbulence is seen in two effects- (1) Dictating the length of the recirculation zone and (2) The modulation of the flame by the wake vortex, which correlates with the tendency of flame to flashback directly.

Effect of inclined angle on a wall-mounted finite-height square cylinder with a passive vertical suction

Drilling a vertical cavity from the bottom to the free-end surface has been a passive method of controlling wake flow around a wall-mounted square cylinder. The mean flow field around a wall-mounted finite-height square cylinder with a vertical rectangular cavity was studied experimentally in a water tunnel to explore the effects of inclined angle α. Measurements were conducted using an acoustic Doppler velocimeter at the Reynolds number of Reh = 17,525, in which the incidence angles of the cylinder were α = 0, 10°, 30°, and 45°. Results depict the presence of a recirculation zone in the cavity and a near-wake zone. As α increases from 0 to 10°, the mean wake behind the cylinder shifts in the opposite position, and a single peak is observed. When α changes from 10° to 45°, the mean wake regains its symmetry, and two peaks are observed at the wake. Self-similarity in terms of the streamwise velocities and Reynolds shear stress can be achieved by properly selecting the velocity and length scales irrespective of the angle of incidence. For the wake flow with a higher value of α, the length of the mean recirculation zone, momentum exchange, and turbulence kinetic energy increase, whereas the mean and variance values of the vortex shedding frequency reach a smaller one. In this study, the cavity flow and wake flow are also observed in terms of velocity distribution, turbulent kinetic energy budget, and turbulent events.

Numerical Study of Laminar Bingham Fluid in Axisymmetric Sudden Expansion

A numerical study was carried out to investigate the laminar flow of Bingham fluid through an axisymmetric sudden expansion of four aspect ratios and various values of Reynolds number between [50~200] and Bingham number [0~2]. By using the commercial code Ansys-Fluent, this paper focuses on presenting Bingham's flow through an axisymmetric sudden expansion to determine the length and intensity of recirculation zones and shed light on the local loss coefficient. The results show an increase in the reattachment length and the eddy intensity of the recirculation zones by increasing the Reynolds number and the aspect ratio and decreasing with increasing the Bingham number and vice versa, the local loss coefficient increases as the aspect ratio increases for the Newtonian fluid this effect is reflected in the Bingham fluid, the increase of the Bingham number also increases the local loss coefficient, dimensionless equations has built to predict all the reattachment lengths, the eddy intensity and the local loss coefficient.

The double backward-facing step: interaction of multiple separated flow regions

The backward-facing step is perhaps the quintessential geometry used to study separated flow. Extensive previous research has quantified its detailed flow characteristics. However, often regions of separated flow do not exist in isolation; rather, interaction occurs between multiple regions. This motivated an experimental investigation into the time-averaged and dynamic flow features of a double backward-facing step, covering separations of zero to eight step heights between equal-height steps. Three flow regimes are identified. A single reattachment regime occurs for separations of less than four step heights, perhaps remarkable for the lack of variation in key flow characteristics from a single backward-facing step response. Next, an intermediate regime is identified for a separation of four step heights. In this case, the flow does not yet reattach on the first step, although significant differences in reattachment length, surface pressure on the vertical step faces and turbulence statistics occur. Finally, for greater step separations, a double reattachment regime, with reattachment on both steps, is identified. Downwash, induced by the first recirculation zone, reduces the reattachment length and turbulent fluctuations of the second recirculation zone. The surface pressure on the first-step vertical face is reduced, seemingly a result of an upstream influence due to the low pressure in the second-step recirculation zone. Detailed characterisation of the regimes offers insight into the fundamental interaction of regions of separated flow, revealing aspects of complex dynamics relevant to a broad range of practical scenarios.

Analysis of Storage Effects in the Recirculation Zone Based on the Junction Angle of Channel Confluence

In this study, numerical simulations using the Environmental Fluid Dynamics Code model were conducted to elucidate the effects of flow structures in the recirculation zone on solute storage based on the junction angle. Numerical simulations were performed at a junction angle of 30° to 90° with a momentum flux ratio of 1.62. The simulation results revealed that an increase in the junction angle caused the recirculation zone length and width to increase and strengthened the development of helical motion. The helical motion increased the vertical gradient of the mixing layer and the mixing metric of the dosage curves. The recirculation zone accumulated the solute as a storage zone, which formed a long tail in the concentration curves. The interaction between the helical motion and recirculation zone affected the transverse mixing, such that the transverse dispersion had a positive relationship with the helical motion intensity and a negative relationship with the recirculation zone size. Transverse mixing exhibited an inverse relationship with the mass exchange rate of the recirculation zone. These results indicate that the transverse dispersion is replaced by mixing due to strongly developed storage zones.

Investigation of pulsatile flow structure in closed-type cavity

Pulsatile flow in a channel with sudden expansion and contraction, referred to as a closed-type cavity, is experimentally and numerically investigated in the range of Re = 50–1650, covering laminar and transitional flow regimes. Investigations are performed in the range of pulsation frequencies corresponding to Wo = 0.28–0.62 and at a constant pulsation amplitude. Pulsation frequency influence to time-averaged recirculation zone length and the development of recirculation zone as well as upper and corner eddies during the pulse cycle at different pulsation frequencies are investigated. A fixed amplitude from zero to maximum velocity is chosen to investigate flow behaviour throughout a whole pulsation cycle. The results show that the pulsation effect on the recirculation zone length is insignificant in the laminar flow regime at investigated frequencies. However, in the transitional flow regime, recirculation zone length was shortened, regardless of the Wo. The analysis of recirculation zone and upper eddy dynamics during the pulse cycle revealed that their growth rate depends on Wo. The development lag effect is observed at certain velocity phase angles. The analysis of shear rate and turbulence intensity profiles revealed that increased instabilities are determined by the interaction of recirculation zone, upper eddy and the forward-facing step during the pulse cycle.

The Effect of the Ratio of the Secondary and Tertiary Air on the Outlet Velocity Field of the New Swirling Pulverized Coal Burner

ABSTRACT The outlet velocity field of the pulverized coal burner is one of the important factors affecting its operating characteristics. In this article, cold tests were carried out for a new swirl burner with a pre-combustion chamber, and the influence of the air ratio on the outlet velocity field of the burner was studied. Through the cold single-phase test on the 1:3 model test bench, the burner outlet cold air velocities were obtained. The results shown that the formation of the central recirculation zone was mainly affected by the secondary air, a stable central recirculation zone could not be formed when the velocity of secondary air was too low. When the total air of the burner was fixed, the secondary air increased, the length of the central recirculation zone increased, and the mixing speed between different flows increased. When the ratio of the mass flow of the secondary air to tertiary air increased from 26:74 to 85:15, the reflux rate increases 5.5 times, which helped to keep the combustion stable.