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Numerical simulation is applied to investigate the outflow of an incompressible three-dimensional turbulent swirled wall jet. The purpose of the study is to determine the jet flow structure and to compare the characteristics of swirled and nonswirled jets. The numerical solution of the equations of motion is obtained using large eddy simulation with wall resolution (WRLES). The results of the modeling are compared with the data of a unique published study devoted to experimental investigation of swirled wall jets. [ABSTRACT FROM AUTHOR]

Swirl has been employed as a mechanism for enhancing the spreading rate of jets, especially in the multiphase context of liquid droplets suspended in a gaseous jet. This work examines the effect of the swirl phenomenon on the downstream development of turbulent round jet sprays using Euler–Lagrange simulations. Two simulation sets are performed: The first set consists of single-phase simulations, and the second set consists of two-phase large eddy simulations where the droplets are stochastically added to the computational domain using an injection model. The simulations consider five different non-dimensional swirl strengths from 0.0 to 2.0 at a Reynolds number of 50,000. As simulation results, we obtain the total normalized number flux of droplets and for each droplet size bin along with the mean and standard deviation of the droplet velocity distributions. We quantify the influence of swirl on the higher-order statistics of the spray behavior. [ABSTRACT FROM AUTHOR]

The arteriovenous fistula (AVF) is commonly faced with stenosis at the juxta-anastomotic (JXA) region of the vein. Implantation of a flexible nitinol stent across the stenosed JXA has led to the retention of functioning AVFs leading to the resulting AVF geometry being distinctly altered, thereby affecting the haemodynamic environment within it. In this study, large eddy simulations of the flow field within a patient-specific AVF geometry before and after stent implantation were conducted to detail the change in flow features. Although the diseased AVF had much lower flow rates, adverse flow features, such as recirculation zones and swirling flow at the anastomosis, and jet flow at the stenosis site were present. Larger velocity fluctuations (leading to higher turbulent kinetic energy) stemming from these flow features were apparent in the diseased AVF compared to the stented AVF. The unsteadiness at the stenosis created large regions of wall shear stress (WSS) fluctuations downstream of the stenosis site that were not as apparent in the stented AVF geometry. The larger pressure drop across the diseased vein, compared to the stented vein, was primarily caused by the constriction at the stenosis, potentially causing the lower flow rate. Furthermore, the WSS fluctuations in the diseased AVF could lead to further disease progression downstream of the stenosis. The change in bulk flow unsteadiness, pressure drop, and WSS behaviour confirms that the haemodynamic environment of the diseased AVF has substantially improved following the flexible stent implantation. [ABSTRACT FROM AUTHOR]

The effect of the injection pressure on the dynamic flow characteristics of a lean direct injection gas turbine combustor under nonreactive conditions during liquid fuel atomization was investigated. The location of the internal recirculation core, size of the vortex breakdown bubble, and shape of the precessing vortex core and shear layer were confirmed by increasing the fuel injection pressure. It was found that as the fuel injection pressure increased, the momentum of the droplets increased. The tangential momentum of the constant main air swirl flow transferred to the droplets increased as the pressure increased, causing the swirl strength to decrease. It was confirmed that, as the swirl intensity decreased with increasing injection pressure, the shape of the precessing vortex core generated in front of the combustor became shorter, and the size of the central recirculation area also decreased. [ABSTRACT FROM AUTHOR]

In order to deeply understand the complex swirling flow characteristics of the typical staged and zoned combustor, particle image velocimetry measurement and large eddy simulation were used to study the steady and instantaneous flow field. The results show that an obviously central recirculation zone (CTRZ) cannot be formed by the single pilot stage swirler. When a main stage swirler with strong swirling flow is added, a large CTRZ appears in the concentric staged dual-swirl combustor. The CTRZ size in the co-rotating combustor is obviously larger than that in the counter-rotating combustor. The development process of the CTRZ is divided into three stages: P, P-M and M. The stages P and M are mainly dominated by the pilot stage and main stage swirling air, respectively. In addition, it can be seen from the instantaneous flow field that there is a single spiral precessing vortex core in the co-rotating and counter-rotating combustor. [ABSTRACT FROM AUTHOR]

A supercritical CO2-diluted oxy-fuel combustor was analyzed focusing on the flame structure and stability. A parametric study on the background pressure (20–115 bar) was performed for swirl-stabilized combustion using large eddy simulation (LES) with a detailed finite-rate chemistry model. The employed physical conditions match those of an experiment. The differences between the subcritical and supercritical states were analyzed in terms of the mixing and reaction characteristics as well as the flame stability. Also, the 1-D simulations were performed to examine the basic reaction characteristics and evaluate different kinetic mechanisms in the supercritical conditions. Different turbulent flame structures between the subcritical and supercritical states could be explained using the swirl number and Reynolds number affected by a viscosity change. The flame stability was analyzed using the instantaneous and RMS flow fields, Rayleigh criterion, flame brush thickness, mean shear rate, and heat release. Relatively improved flame stability was observed in the supercritical state. The flame brush thickness and turbulent intensity are proportional and decrease as the background pressure increases. [ABSTRACT FROM AUTHOR]

The effects of flow structure on the separation efficiency and pressure drop in a cyclone-type oil separator were investigated via Reynolds-averaged Navier-Stokes (RANS) simulations and large eddy simulations (LES), and the results were compared with experiments. Compared with the RANS simulations, the LES results were more similar to the experimental data as they simulated the complex flow structure more realistically. Swirling flow with strong turbulent kinetic energy (TKE) at the top section hindered the flow of particles toward the separator wall. In addition, a decrease in tangential velocity along the wall at the bottom reduced the centrifugal force, resulting in a decreased separation efficiency as particles were able to flow directly toward the outlet. The LES also predicted the pressure drop slightly better than the RANS simulation did, due to increased pressure drop caused by collision of the flow with the helix and outlet tube, which led to the formation of vortical flow structures with strong TKE. [ABSTRACT FROM AUTHOR]

In this paper, the so-called REDIM–DTF sub-grid scale combustion model is proposed to improve the well-known dynamically thickened flame (DTF) combustion model by combining the DTF modeling strategy with the reaction-diffusion manifold (REDIM) chemistry table, which is generated from a detailed reaction mechanism. The new model is then used to calculate two lean premixed swirling flames in the PRECCINSTA combustor via large eddy simulation. The radial profiles of velocity, temperature, and species concentration, as well as the flame surface area and vortex structure are analyzed. The results are in good overall agreement with the corresponding experiment, and the performance of the REDIM–DTF model is very similar to that of the DTF model. The predicted CO mass fraction profiles, however, show a relatively larger discrepancy between the original DTF model and the proposed REDIM–DTF model, which is thought to be due to different reaction mechanisms used. As a smaller number of species transport equations are solved in the REDIM–DTF model, its computational efficiency is about 15% higher than that of the original DTF model. [ABSTRACT FROM AUTHOR]

In the present study, the effect of bend angle on pressure drop and flow behavior in a small-diameter corrugated duct is numerically investigated and experimentally validated under a fully developed flow condition. The large eddy simulation, together with the proper orthogonal decomposition (POD) method, is employed to study the pressure drop, mean flow pattern, and unsteady flow evolution for a corrugated duct with various bend angles. The results show that the pressure drop exhibits a monotonic increase with increasing bend angle. Specifically, as the bend angle increases from 0 ∘ to 90 ∘ , the pressure drop of the corrugated duct experiences a striking increase of about 43%. Accordingly, a larger bend angle is found to induce the occurrence of stronger Dean cells or larger swirl intensity downstream the duct bend. Meanwhile, as for larger bend angles, the main turbulent properties of the Dean cells could be, to some extent, governed by the first few POD modes, which appear to be featured with one or a few large-scale vortices. Generally, the larger bend angle causes stronger swirl intensity and wave-like structures, thus rendering severer pressure drop or larger pressure loss coefficient in the corrugated duct. [ABSTRACT FROM AUTHOR]

Water mist fire fighting has achieved a well established position in fire protection for industrial and civil applications. The performances of water mist nozzles, largely used in water mist production, are deeply influenced by the internal nozzle flow characteristics and CFD modelling is a powerful tool to analyse them in details. The paper focuses on the numerical investigation of the internal flow in pressure swirl injectors for water mist applications. 3D large eddy simulations based on the Volume-Of-Fluid methodology have been implemented. The flow is assumed to be incompressible and under isothermal non-reacting conditions. Validation of the model against available experimental data is performed with satisfactory results. The effect of internal nozzle geometry on the injector behaviour is investigated by modifying the inclined swirling channels (five configurations) and the injection pressure in the range 5 to 320 bar. The effect of the size of cylindrical and conical swirl chambers are also independently investigated. Three different flow regimes can be distinguished as a function of imposed swirl and injection pressure and a map is reported to clarify the effect of these parameters. When a stable hollow cone spray is formed the mass flow rate fluctuations are < 5%, corresponding to a discharge coefficient between 0.34 and 0.50. When no air core is present in the discharge hole, mass flow rate fluctuations as high as 16% are observed, which correspond to discharge coefficient as high as 0.7. A detailed quantification of in-nozzle characteristics, like swirl number and momentum flux distribution along the nozzle and lamella thickness in the discharge hole, is reported and discussed, with particular emphasis on stability and transient behaviour of the atomiser internal flow, which represents the main novelty of the present study. [ABSTRACT FROM AUTHOR]

Large-eddy simulation (LES) of CH4-air low-swirl flame was carried out in a multi-nozzle combustor with two burner configurations by using a premixed flamelet model. The multi-nozzle burner includes a co-swirling array where all five nozzles act in the same direction and a counter-swirling array where the center nozzle is in the opposite swirling direction to the outer nozzles. LES results are in good agreement with OH-planar laser-induced florescence data in terms of OH concentrations and combustion progress variables. Numerical results show that the flow of each nozzle is constant before merging. The neighboring flows interact with each other and generate a highvelocity zone with intensive turbulence. The kinetic energy in the interacting region for the co-swirling array is larger than that for the counter-swirling array. After neighboring flow combining, the flow develops into a unified swirling motion similar to a single swirling flow for the co-swirling array, whereas the flow maintains the individual swirling structures for the counter-swirling arrangement. However, the swirling array exerts minimal effect on multi-nozzle combustion in terms of the temperature distributions and combustion progress of premixed low-swirl multi-nozzle flames. [ABSTRACT FROM AUTHOR]

A 2D PIV experiment was carried out on the confined swirling flow and compared with simulation results. Simulations were performed using Star CCM+ commercial software. The LES WALE method of air and water was used to investigate the effect of working fluid on the swirling flow field. A second comparison was conducted to study the gravitational effects on the swirling flow. Velocity analyses were performed by extracting the mean velocity data, and FFT analysis was conducted on the instantaneous flow field of axial and pressure distribution. In general, good agreements were obtained using the experimental data; interestingly, the air simulation showed relatively higher performance in terms of predicting the asymmetric behavior of the flow. Energy cascade was studied at two different points, which indicated a similar tendency between air and water, with a slight shift of 0.1 Strouhal number in terms of dominant frequencies. The discrepancy of the CRZ and CTRZ formation was found to be of minor degree due to changes in the working fluid or the gravitational effect. Thus, a conclusion was made regarding whether applying water instead of air or altering the vertical test rigs affects the structure of the flow. [ABSTRACT FROM AUTHOR]

The effects of flow structure and flame dynamics on combustion instabilities in a lean premixed swirl combustor were numerically investigated using Large eddy simulation (LES) by varying the inlet geometry of combustor. The dynamic k -equation and G-equation flamelet models were respectively employed as the LES subgrid models of turbulence and combustion. The divergent half angle (α) in the combustor inlet was varied systematically from 30° to 90° to quantify the effect of inlet geometry on the combustion instabilities. This variation caused considerable deformation in recirculation zones in terms of their size and location, leading to significant changes in flame dynamics. Analysis of unsteady pressure distributions in the combustor showed that the largest damping caused by combustion instabilities takes place at α = 45°, and the amplitude of acoustic pressure oscillation is largest at α = 30°. Examination of local Rayleigh parameters indicated that controlling flame-vortex interactions by modifying inlet geometry can change the local characteristics of combustion instabilities in terms of their amplification and suppression, and thus serve as a useful approach to reduce the instabilities in a lean premixed swirl combustor. These phenomena were studied in detail through unsteady analysis associated with flow and flame dynamics. [ABSTRACT FROM AUTHOR]

The turbulent air natural-convection in a differentially heated square cavity is certainly a benchmark problem in thermo-hydrodynamics. Using large eddy simulation (LES), the natural convection at a Rayleigh number of 5.33 × 10 is numerically explored, in which a sub-grid scale model constructed by non-dimensional analysis on the basis of the swirling-strength and the harmonically- averaged grid interval is utilized. A factor of swirling strength intermittency (FSI), which is defined by the ratio of the swirling strength to the magnitude of the complex eigenvalue of the filtered velocity gradient tensor, is used instead of the near-wall length scale decaying factor in the description of the near-wall sub-grid viscosity. The cavity with the height-width-depth ratio of 1:1:2 is partitioned by a non-uniform staggered grid, whose resolution is obtained by the grid number of 100 ×100 × 120 . It was found that to some extent the LES results agree well with the existing experimental data, suggesting that the swirling-strength based sub-grid scale model used in the LES is of potential. Subjected the influences of the loop wall jet and the assigned temperature in the horizontal walls, as can be seen from the t-z averaged FSI distribution in the square section, the heterogeneity of the averaged FSI in the near wall regions is more evident, but in the core region of the square cavity, this heterogeneity decreases and the natural convection tends to become relatively homogeneous. The turbulent natural convection at Rayleigh number 5.33 × 10 certainly has its own geometry-dependent intermittency of turbulence. [ABSTRACT FROM AUTHOR]

A finite reaction rate model is presented as a closure of large eddy simulation (LES) to numerically study an open premixed methane/air swirling flame. The resultant model is firstly validated by comparing with reported data and then employed to investigate the effect of swirling intensity on flow field, flame characteristics and combustion instability of the swirling flame. Three different swirl numbers are considered. The LES results show that as swirling intensity increases, the vortex entrainment and micro-mixing are enhanced, leading to more lean equivalent ratios at flame front; consequently, higher swirling number causes lower flame temperatures and slower CO oxidization; for all simulated swirl numbers, flame fronts are completely located out of the recirculation zones and anchored at the inner surface of the annular swirling steams; swirl number has a crucial effect on swirling flame extension toward radial and tangential dimensions and then significantly affects streamwise flame length, which is a great influencing factor on combustion instability; vortex-induced disturbance on flame in streamwise plays a critical role in combustion instability. [ABSTRACT FROM AUTHOR]

In view of the fact that large scale vortices play the substantial role of momentum transport in turbulent flows, large eddy simulation (LES) is considered as a better simulation model. However, the sub-grid scale (SGS) models reported so far have not ascertained under what flow conditions the LES can lapse into the direct numerical simulation. To overcome this discrepancy, this paper develops a swirling strength based the SGS model to properly model the turbulence intermittency, with the primary characteristics that when the local swirling strength is zero, the local sub-grid viscosity will be vanished. In this paper, the model is used to investigate the flow characteristics of zero-incident incompressible turbulent flows around a single square cylinder (SC) at a low Reynolds number range Re ∈ [10, 10]. The flow characteristics investigated include the Reynolds number dependence of lift and drag coefficients, the distributions of time-spanwise averaged variables such as the sub-grid viscosity and the logarithm of Kolmogorov micro-scale to the base of 10 at Re = 2 500 and 10, the contours of spanwise and streamwise vorticity components at t = 170. It is revealed that the peak value of sub-grid viscosity ratio and its root mean square (RMS) values grow with the Reynolds number. The dissipation rate of turbulent kinetic energy is larger near the SC solid walls. The instantaneous factor of swirling strength intermittency (FSI) exhibits some laminated structure involved with vortex shedding. [ABSTRACT FROM AUTHOR]