Numerical Study Of Flow Characteristics Research Articles

Core-winding of Sodium Cold Fast Reactor Based on CFD Method Numerical Study of Flow Characteristics

Abstract Liquid metal cooled fast reactor often uses spiral winding wire as the positioning component of fuel rod bundle. Besides fixing, spiral winding has the functions of strengthening subchannel mixing and heat transfer. These characteristics have a very important impact on the internal flow temperature characteristics of the fuel assembly and the safety of the core system, so it is necessary to conduct in-depth research on the flow field distribution of the fuel rod bundle channel with wire wound, the transverse and axial flow characteristics of the edge channel and its internal channel, the flow characteristics of the circulation and eddy current, the local temperature field, and the outlet temperature distribution characteristics. In the actual research, there are some problems such as high cost, difficulty and high risk to carry out thermal hydraulic characteristic experiment directly. This paper uses CFD fine simulation technology to study 7 rod bundles with wire winding, uses Fluent meshing to partition high-quality polyhedral unstructured mesh, uses ANSYS Fluent to conduct numerical simulation, studies the influence of the presence of rod bundles with wire winding on the distribution of flow field and local temperature field, and forms a mature and reliable simulation experiment scheme. The temperature change curve and lateral mixing phenomenon obtained by CFD simulation were investigated. The outlet temperature differences of side channel, inner channel and corner channel were investigated, and the convective heat transfer characteristics between channels were investigated due to the existence of wire winding.

Combustion in a chamber furnace with a tangentially swirling vortex

Relevance. The need to analyze the furnace processes in boiler units with vertical vortex when arranging pulverized coal combustion with the enhancement of environmental parameters through the installation of tertiary blast nozzles. Despite the carbon-free policy in modern power engineering coal remains one of the main sources of energy, so the reconstruction of boiler units in order to reduce harmful emissions is very relevant. Aim. To study aerodynamics and combustion processes of pulverized coal fuel in the furnace chamber of a boiler unit with a tangential arrangement of burner devices using a combustion scheme with tertiary air blowing. Methods. The Eulerian–Lagrangian approach was applied to numerical study of flow characteristics, heat transfer and combustion in a furnace with a tangential burner arrangement. Also, k-ε model of gas turbulence, particle-turbulence interaction, diffusion model of particle dispersion, P-1 radiation modeling method and pulverized coal particle combustion model based on global particle kinetics and experimental data were used for numerical calculations. Results. Based on numerical calculations and analysis, the dependences of combustion product concentrations, temperature, hydrodynamics at combustion of Kuznetsky coal grade D in the furnace chamber with tangential arrangement of burners for brown coal, as well as at installation of tertiary blast nozzles have been obtained. Insufficient redistribution of air between burners and tertiary nozzles is revealed, as for maintenance of stable twist of vertical vortex and accordingly presence of high values of velocities at the outlet of burner devices it is not possible to increase the portion of tertiary air without reconstruction of burners. It is also established that the volume of the furnace chamber below the burners is poorly involved in heat exchange.

Numerical study of flow characteristics around a 30° yawed circular cylinder at Re=104

Unstable motions of bridge stay cables have been observed on site and in wind tunnel tests when a cable is yawed at certain orientations to wind. To uncover the underlying mechanisms, flow around a circular cylinder at a yaw angle of 30° has been numerically analyzed in the current study using delayed detached eddy simulation (DDES) at Re=104. A comparison with the reference normal flow case indicates the presence of a more coherent span-wise flow structure when the cylinder is yawed at 30°. The application of proper orthogonal decomposition further reveals that at this orientation, a synchronized flow structure exists, which is characterized by continuous anti-symmetric pressure blocks. In addition, a low-frequency flow fluctuation has been identified, the Strouhal number of which is roughly a quarter of that of the conventional Kármán vortex shedding. The pivotal role of axial flow in the intermittent amplification of cylinder sectional lift and the subsequent span-wise propagation of this enhanced local lift event has been revealed. The former is evident from the low-frequency sectional lift peaks occurred during vortex shedding, whereas the propagation speed associated with the latter is in good agreement with the span-wise component of the incoming flow speed. The temporal and spatial impact of axial flow on the surrounding flow structure of the cylinder may serve as a periodic excitation source, which could trigger an unstable response of a cylinder. This, in the context of bridge stay cables, could possibly contribute to the onset mechanism of dry cable galloping.

Numerical study of flow characteristics of tornado-like vortices considering both swirl ratio and aspect ratio

Flow characteristics of tornado-like vortices are primarily influenced by the swirl ratio and aspect ratio. This study systematically investigates effects of these two essential parameters on vortex visualization, mean flow fields, and fluctuations by large eddy simulations (LES). The geometric dimensions of a tornado-like vortex simulator are characterized by the inner cylinder radius and floor height. The former affects the core radii where maximum tangential velocities occur, as well as the vortex type which is used to separate flow structures while the latter influences the vortex type. Furthermore, the ratios of flow characteristics of tornado-like vortices, such as radial distances where the maximum tangential and radial velocities occur, to the inner cylinder radius (RV.max/r0; RU.max/r0, Rc/r0) have a high correlation with the swirl ratio. The ratios of heights where the maximum tangential and radial velocities occur to the floor heights (ZV.max/H0; ZU.max/H0) are mainly related to the aspect ratio. These parameters can be expressed by the swirl ratio and aspect ratio with high R-squared values of 0.941, 0.794, 0.984, 0.964, 0.958. Finally, vortex types can be modeled as a function of both swirl ratio and aspect ratio. This study reveals the relationship between the simulator geometry and generated vortices.

Numerical study of flow characteristics in compound meandering channels with vegetated floodplains

Large eddy simulations were conducted to simulate the flow in compound meandering channels whose main channel sinuosity was 1.381. Then, the floodplain vegetation was generalized using the momentum equation coupled with the drag force formula. The mean flow pattern, secondary flow, coherent structure, turbulence characteristics, and lateral mass and momentum transport with and without floodplain vegetation with relative depths (Dr) of 0.3–0.5 were studied. Results showed that the floodplain vegetation enabled the flow of the main channel to be more concentrated. The maximum average velocity in the cross section of the main channel increased by 100% and 30% when the relative depth was 0.3 and 0.5. Under the influence of floodplain vegetation, the secondary flow cell transformed greatly with the change in relative depth. When Dr < 0.3, the vegetation caused the vortex center of the secondary flow to move closer to the concave bank side, and the secondary flow distribution presents a flow pattern not flooding the floodplain. When Dr > 0.3, the spatial change in the secondary flow was not obvious. In addition, the floodplain vegetation did not change the large-scale vortex that was separated from the boundary layer of the convex bank side. Meanwhile, the floodplain vegetation increased the overall turbulence intensity, turbulent kinetic energy, and Reynolds stress of the main channel, and it increased the range of lateral mass exchange of the inbank flow and the mean and turbulent transport flux of each cross section.

Numerical study of flow characteristics and energy dissipation over the slotted roller bucket system

Numerical study of flow characteristics and energy dissipation over the slotted roller bucket system

Three dimensional flow over elliptic cylinders arrays in octagonal arrangement

A numerical study of flow characteristics around eight elliptic cross-section cylinders in an octagonal configuration is presented. The axis ratio of elliptic cylinder is taken as 1:2. Three spacings between the cylinders (0.07 m, 0.14 m and 0.2 m), two angles of attack (α=0° and α=15°) and two Reynolds numbers (Re=4060 and Re=45800) are considered to investigate the parametric influences. The flow is modelled using three-dimensional large eddy simulation. Simulations are performed by utilizing ANSYS Fluent software. The results comprise of flow patterns and force coefficients (drag and lift). It is seen that both the drag and lift forces acting on the cylinders vary with the spacings values, angle of attack and Reynolds numbers. The lift force peaks at α=15° and Reynolds number 45800. The drag force on the upstream cylinder reaches its peak at α=0° and Reynolds number 4060. It is observed that the drag on the upstream cylinders decreases as the value of the Reynolds number increases. Contours of velocity and subgrid turbulent viscosity are also presented. Moreover, it is concluded from the present study that the effect of change in the Reynolds number on flow characteristics is more significant as compared to the change in the spacing between the cylinders.

Numerical study of flow characteristics of the blade with different structures

In order to study the effect of blade structures on the induced excitation characteristics, two different structures are investigated, which are the blade with special trailing edge and the staggered blade. Firstly, based on the steady numerical simulation, the time-averaged velocity and pressure distribution are obtained at different Re numbers (5×103 and 1×104). Secondly, the unsteady numerical simulation method LES is applied to acquire the transient and complex flow field around the blade. It is found that both two blade structures significantly change the flow characteristics around the blade, especially in the wake flow region. The evident frequency is observed from the frequency spectrums. It is clear that the dominant frequency is originated from the vortex shedding, which is similar to the Karmen vortex street. Finally, the results will be useful for the design of blade with low noise purpose.

Numerical study of flow characteristics around rectangular cylinders in tandem

The phenomena of mean pressure distribution around two square cylinders in tandem has been carried out. Also the mean velocity distribution in the wake of the two cylinder are studies. The Navier-Stokes equation for conservation of momentum and the continuity equation for conservation of mass are used to study the pressure and velocity distribution for various longitudinal spacing of the cylinder. The turbulent k-ω stationary flow at a high Reynolds number of Re= 6.5e4 is used to simulate single-phase flow. A critical longitudinal spacing equal to four times the side dimension of the square cylinder (L/D= 4) is investigated. Beyond that critical spacing both the cylinders are subjected to drag force. When L/D < 4 only the downstream cylinder is subjected to negative drag. Also, the form of the streamline profile behind upstream cylinder is different from that of single cylinder. However, beyond L/D= 4 the nature of the pressure behind both of the cylinder becomes kind of like that of the single cylinder.

Comparative numerical study of flow characteristics in shell & helical coil heat exchangers with hybrid models

The present paper dispenses with effectiveness and energy actions of Shell and Coil heat exchanger used to reject heat from working fluid in industries. In particular, shell and coil heat exchanger had examined for 3 vivid coil configurations, so to examine the flow attributes for these models for common boundary conditions and under varying working operations. Important inference reflected out are rate of heat transfer from heat exchanger and effectiveness for all the three systems. Computations had been formulated using the software Ansys Fluent and it had covered the simulations for all the different working operations separately for all the heat exchangers. The major attribute for rate of heat transfer enhancement of numerical models pointed is the overall heat transfer coefficient which depends on coil thermal conductivity. The value of volume flow rate in tubes were found another essential element. The shell-helical coil heat exchanger arrangement resulted as preferred one over other two heat exchanger due to its consistency while slinky coil can be used for enhanced heat transfer conditions if total pressure drop is not a performance measuring parameter.

A three-dimensional numerical study of flow characteristics in strongly curved channel bends with different side slopes

The influence of channel side slope on flow in strongly curved channel bends is studied numerically. The performances of five different turbulence models are investigated. Comparison to experimental measurements demonstrates that the fully 3D numerical model can reliably simulate a channel bend flow field. Among the tested turbulence models, the realizable k − e model performed best. The present study also demonstrates that the realizable k − e model can satisfactorily predict smaller flow features in bend flow, such as the outer-bank circulation cell. The validated model is employed to carry out additional computations for channel bends with different side slopes. It is found that the number, position, and strength of secondary flow cells varies with the channel side slope, with corresponding influence on flow distribution and flow vorticity.

Three-dimensional numerical study of flow characteristics and membrane fouling evolution in an enzymatic membrane reactor

In order to enhance the understanding of membrane fouling mechanism, the hydrodynamics of granular flow in a stirred enzymatic membrane reactor was numerically investigated in the present study. A three-dimensional Euler-Euler model, coupled with k-e mixture turbulence model and drag function for interphase momentum exchange, was applied to simulate the two-phase (fluid-solid) turbulent flow. Numerical simulations of single- or two-phase turbulent flow under various stirring speed were implemented. The numerical results coincide very well with some published experimental data. Results for the distributions of velocity, shear stress and turbulent kinetic energy were provided. Our results show that the increase of stirring speed could not only enlarge the circulation loops in the reactor, but it can also increase the shear stress on the membrane surface and accelerate the mixing process of granular materials. The time evolution of volumetric function of granular materials on the membrane surface has qualitatively explained the evolution of membrane fouling.

Numerical study of flow characteristics and pollutant dispersion using three RANS turbulence closure models

The flow characteristics and pollutant dispersion around building are investigated numerically using three RANS turbulence closure models, i.e. Standard k–ɛ model, Realizable k–ɛ and RNG k–ɛ model. Two physical configurations are considered in this study: a single isolated obstacle and two obstacles forming an idealized street canyon. Numerical simulations are performed with OpenFOAM® source code which utilise the finite volume method to solve the transport equations of mass, momentum, turbulent kinetic energy and turbulent dissipation rate. When numerical results were compared to wind tunnel experiments, it was found that the RNG k–ɛ turbulence model yielded the best agreement with experimental data. Moreover, the coefficients of the RNG k–ɛ turbulence model were adapted to better predict the flow characteristics in the urban boundary layer. The results obtained using the appropriate model show good agreement with experimental measurements for velocity flow profile and turbulent kinetic intensity. The prediction of pollutant dispersion reveals that in addition to the Reynolds number, the emission location plays an important role in the pollutant concentration distribution.

Numerical study of flow characteristics on wing airfoil eppler 562 with whitcomb winglet variations

Winglet is a tool used to improve the efficiency of aircraft and UAV performance by preventing fluid flow jump from lower surface to upper surface at wingtip. The addition of this winglet resulted in improved lift and reduction of drag force from the aircraft wing or UAV. From Whitcomb's research, it was found that the use of winglet on a full size airplane can increase fuel efficiency by 7%. The research led to the idea of conducting research on fluid flow characteristics on the UAV wing with the Eppler 562 airfoil combined with the whitcomb winglet. This numerical study was conducted using the Computational Fluid Dynamics (CFD) method based on the advantages of using this simulation that can review the fluid flow in macroscopic way. This study is provide accurate fluid flow visualization results and can improve the performance of the wings when compared with wings without winglet (plain wing). Wing with the Eppler 562 airfoil combined with the whitcomb winglet results reduction in rotating motion that makes velocity components as opposed to lift.

A numerical study of flow characteristics in a helical pipe

Flow characteristics and loss mechanism inside the helical pipe with large-caliber and large-scale Dean number were analyzed in this study. Numerical simulation was carried out for exploring velocity distribution, pressure field, and secondary flow by varying coil parameters such as Dean number, curvature radius, and coil pitch. The velocity gradient in the cross-section increases along the pipe and causes unsteady flow in the pipe. Large pressure differences in the 180° and 315° cross-section generate centrifugal forces on the pipe. The secondary flow is the major factor resulting in flow loss, presented obviously by the streamlines to analyze the effects of pipe parameters on the vortices. The vortex center shifts toward the upper wall with the increase in Dean number and takes a slight deflection with the increase in coil pitch. Meanwhile, a correlation of the flow loss extent inside the pipe as a function of friction factor was presented. The increases in curvature radius and coil pitch can diminish the friction factor to reduce flow losses. The accuracy of the numerical methodology was also validated by conducting corresponding experiments and empirical mathematical analysis. The maximum deviation between the experimental values and the simulated results of the pressure drop is just 2.9%.

Experimental and numerical study of flow characteristics of flat-walled diffuser/nozzles for valveless piezoelectric micropumps

The flat-walled diffuser/nozzles are classified into two types, i.e. Tube I and Tube II, based on their different flow resistance characteristics. Tube I has less pressure loss coefficients in the diffuser direction than the nozzle direction, and Tube II has larger pressure loss coefficients in the diffuser direction than the nozzle direction. This work focuses on the characterization of the diffuser efficiency, and flow rectification of these two types of diffuser/nozzles. The characterization is performed with diverging angles in the range of 5°–60° and length–width ratios in the range of 1–20, and the pressure drop ranging from 1 to 10 kPa. The results show that with the increase in pressure drop and the decrease in length–width ratio, Tube I type of diffuser/nozzles can change to Tube II. For Tube I type of diffuser/nozzles, the smaller the diverging angle and the longer the length, the better the performance. For Tube II type of diffuser/nozzles, the larger the diverging angle and shorter the length, the better the flow rectification performance. Simulation results match well with the experiment data. Of particular interest, simulation of the diffuser flow fields suggests that the flow separation has a significant impact on pressure loss coefficients in the diffuser direction.

Numerical study of flow characteristics behind a stationary circular cylinder with a flapping plate

The laminar flow over a stationary circular cylinder with a flapping plate is simulated in this study to investigate the flow characteristics by using our recently developed boundary condition-enforced immersed boundary-lattice Boltzmann method [Wu and Shu, J. Comput. Phys. 228, 1963 (2009); Wu et al., Int. J. Numer. Methods Fluids 62, 327 (2010); Wu and Shu, Comm. Comp. Phys. 7, 793 (2010)]. The purpose of this work is to study the flow control behind a bluff body by an alternative way different from the rotationally oscillating motion. The idea is in fact from the tadpole locomotion, where the bluff head-body is modeled by a circular cylinder, and the thin tail is simplified by a rigid plate with flapping motion. In this work, only the laminar flow is considered and thus the Reynolds number is chosen as 100. Similar to the case of rotationally oscillating cylinder, the flow wake behind the cylinder and flapping plate is strongly affected by the flapping amplitude and frequency of plate. On the other hand, because of the existence of flapping plate, the length of plate can also modify the flow structures. Due to flapping motion of plate, some typical flow patterns and drag reduction are found, and two different vortex interaction modes, known as constructive interaction and destructive interaction, are observed.

Three-dimensional numerical study of flow structure in channel confluences

Channel confluence is one of the important features of each river system and some hydraulic structures. The features that can dominantly control flow characteristics in a confluence are confluence angle, discharge, width ratios, and Froude number of flow. Several research studies have been conducted however a comprehensive three-dimensional (3-D) numerical study of flow characteristics in a confluence has not yet been reported. In the present study, SSIIM2.0, a 3-D numerical model, is validated and applied to investigate secondary currents, velocity distribution, flow separation, and water surface elevation in different conditions. The results of the present study illustrate that flow structure and water surface variations in a confluence are highly influenced by confluence angle, discharge, and width ratios as well as Froude number because of their effect on flow deflection, separation, and secondary currents. The graphs from the present study can be used to analyze water surface variation, velocity field, and flow separation dimensions in difference conditions for engineering designs.

Numerical Study of Flow Characteristics due to Interaction Between a Pair of Vortices in a Turbulent Boundary Layer

This paper represents a numerical study of the flow field due to the interactions between a pair of vortices produced by vortex generators in a rectangular channel flow. In order to analyze longitudinal vortices induced by the vortex generators, the pseudo-compressibility method is introduced into the Reynolds-averaged Navier-Strokes equations of a 3-dimensional unsteady, incompressible viscous flow. A two-layer k-e turbulence model is applied to a flat plate 3 dimensional turbulence boundary to predict the flow structure and turbulence characteristics of the vortices. The computational results predict accurately the vortex characteristics related to the flow field, the Reynolds shear stresses and turbulent kinetic energy. Also, in the prediction of skin friction characteristics the computational results are reasonably close to those of the experiment obtained from other researchers.

Numerical study of flow characteristics of the high speed train entering into a tunnel

When a high speed train enters into a tunnel, the aerodynamic forces severely change and, consequently, the stability and performance significantly deviate from the value of design point which is usually set at cruising speed on the plain ground. The compression wave is also generated ahead of the train due to the piston-like action of tunnel entry motion. The present work is to understand the flow field such as variation of aerodynamic forces and generation of compression wave during tunnel entry motion by applying three-dimensional unsteady Navier–Stokes equation solver. To account for the relative motion of stationary tunnel and moving train, the sliding multi-block method has been implemented.