Wall-Adapting Local Eddy-viscosity Research Articles

Simulation of propane-air premixed combustion process in randomly packed beds

Real geometric structures of randomly packed beds are modeled using the discrete element software LIGGGHTS. The wall-adapting local eddy-viscosity (WALE) model and the EBU-Arrhenius combustion model are used to simulate the propane-air premixed combustion process in the randomly packed beds, and the calculated results are compared with experimental data. The results reveal that the turbulence model and combustion model used in this paper are reasonable. Next, propagation velocity, area, mean vorticity and fractal dimension of a flame surface are calculated at various time points with different inlet velocities to investigate the changes in flame characteristics during the combustion process and the effect of increasing the inlet velocity. According to our results, the flame propagation velocity changes do not exhibit a clear trend over time. However, the variation trends of the two curves under the different inlet velocities are similar. In addition, the fractal dimension exhibits no obvious rule of increasing or decreasing during the combustion process. The area and mean vorticity of the flame surface increase with time. However, the rules of increase are not exactly the same. In addition, the flame regimes at various time points are identified. The results reveal that the turbulent premixed flames in a packed bed under two inlet velocities are concentrated in the thin reaction zone.

Numerical investigation of turbulent flow in an annular sector channel with staggered semi-circular ribs using large eddy simulation

Computational fluid dynamics (CFD) simulations are conducted to explore flow inside a channel with staggered semi-circular ribs attached to both the inner and outer annular walls. Large eddy simulation (LES) with the wall-adapting local eddy-viscosity (WALE) sub-grid scale (SGS) model is used to simulate turbulent flow in the computational domain. The flow channel is modeled on the test section of a simplified helical coil steam generator (HCSG) design. Owing to their compactness and higher heat transfer coefficient in comparison to straight tube steam generators, HCSGs are employed in recent designs of advanced light water reactors in the nuclear industry. A Reynolds number of 13,900 is considered based on the mean inlet velocity, channel height, and water properties. Results from LES are compared with both an unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation and experimental data acquired from the particle image velocimetry (PIV) method. Flow features observed inside the channel are presented by visualizing contours of velocities, vorticity, and Reynolds stresses on a monitoring plane. Time-averaged profiles of velocities and Reynolds stresses at five monitoring lines are also presented to validate the flow field resolved by LES with PIV. Additionally, vortical structures of the flow are captured using the Q-criterion. This study aims at providing flow characteristics of a specifically designed flow channel and acquiring the scalability of the numerical experiment.

Large-eddy simulations of the vortex-induced vibration of a low mass ratio two-degree-of-freedom circular cylinder at subcritical Reynolds numbers

The vortex induced vibration phenomenon of a low mass ratio (m*=2.6) two-degree-of-freedom circular cylinder at subcritical Reynolds numbers (Re=3900, 5300, 11,000) has been investigated by means of large-eddy simulations. A low-dissipative spatial and time discretisation finite element schemes have been implemented and combined with the Wall-Adapting Local-Eddy viscosity (WALE) subgrid-scale model to solve the filtered incompressible flow equations. Several values of the reduced velocity in the range 3 ⩽ U* ⩽ 12 have been considered. The numerical results are extensively compared with available experimental and numerical data. Particular interest has been placed in the region of maximum cross-flow amplitudes, the super-upper branch, where previous high-fidelity numerical simulations have underestimated the peak amplitudes compared with experimental results. The transition between the super-upper and lower branches is also shown and described. The numerical simulations successfully reproduce the three-branch response maximum oscillation amplitudes and associated vortex formation modes. The 2T vortex formation mode, i.e. two triplets of vortices per oscillation period, has been observed to occur in the super-upper branch, for the three different values of the Reynolds number investigated. These results contradict the claim made in previous works [27] that the vortex formation mode in the super-upper branch is Reynolds number dependent. Beats are observed to appear prior the transition from the super-upper to the lower branch. It is argued that they may be related with the coherence and strength of the third vortex shed at the shoulder of the cylinder each half-cycle, which is finally suppressed in the transition to the lower branch.

Thermal and emission characteristics of reverse air flow CAN combustor

Relative assessment of thermal and emission characteristics of conventional and reverse air flow CAN type gas turbine combustor is reported. Large Eddy Simulations (LES) are carried out with Wall Adaptive Local Eddy (WALE) viscosity model for sub grid scale stresses. Turbulence-chemistry interaction is modelled using presumed shape Probability Density Function (PDF) approach. Discrete Ordinates (DO) model is used for radiative heat transfer. Experimental measurement of temperature and NOx emission level at the exit of combustors are reported. Flow pattern and flame shape obtained from numerical analysis for conventional and reverse air flow combustor are correlated to combustor liner wall heating, exit temperature characteristics and NOx emission. The liner wall region of conventional combustor is dominated by hot combustion gases whereas the location of hot flame in reverse air flow combustor is in the vicinity of centreline. The exit temperature measurement shows that the wall side temperature in reverse air flow combustor is relatively low compared to conventional combustor. Reverse air flow arrangement in combustor eliminates the hot spots of high temperature gradient from primary region of combustor which results into significant reduction in NOx emission level in reverse air flow combustor as compared to conventional combustor. Thermal imaging of outer wall of combustors demonstrates that the radiative energy transfer from liner wall to outer wall is small in reverse air flow combustor compared to conventional combustor. This depicts that the wall-cooling requirement decreases in reverse air flow combustor.

Application of a parallel solver to the LES modelling of turbulent buoyant flows with heat transfer

An existing fully implicit, non-dissipative direct numerical simulation (DNS) algorithm is reformulated to utilise the sub-grid scale (SGS) models in large eddy simulation (LES). The Favre-filtered equations with low-Mach number scaling are derived. The wall-adapting local eddy-viscosity (WALE) is used as SGS model. A fully parallel, finite volume solver is developed based on the resulting LES algorithm using PETSc library and applied to buoyancy- and thermally-driven transitional/turbulent flows in Rayleigh-Taylor instability and turbulent Rayleigh-Benard convection. Results verify that the proposed low-Mach number LES approach, which is physically more accurate than pure incompressible methods for flows with variable properties, perfectly captures the evolution and complex physics of turbulent buoyant flows with or without heat transfer by taking the effects of density and viscosity changes into account without the Oberbeck-boussinesq (OB) assumption even at large temperature differences with uniform accuracy and efficiency.

Pore-Scale Simulation of Hydrogen–Air Premixed Combustion Process in Randomly Packed Beds

Real geometric structures of randomly packed beds are modeled using discrete element software LIGGGHTS. The pore-scale computational region is screened and chosen with nonextreme local distortion, porosity, and other structural parameters, and in this region the distribution of spherical walls of pellets are representative and typical. The wall-adapting local eddy-viscosity (WALE) model and EBU-Arrhenius combustion model are used to simulate the ignition process in a closed cavity, and the calculated results are compared with the direct simulation results. The results show that the turbulence model and the combustion model used in this work are basically reasonable. Then the propagation of the hydrogen–air premixed flame and the interaction of flame with turbulence in the porous and nonporous structures during the ignition process are simulated and analyzed. Results show that the temperature propagating velocity in the porous structure is higher than that in the nonporous one, and the porous structure can...

Large Eddy Simulation and CDNS Investigation of T106C Low-Pressure Turbine

This study investigates the aerodynamic performance of a low-pressure turbine, namely the T106C, by large eddy simulation (LES) and coarse grid direct numerical simulation (CDNS) at a Reynolds number of 100,000. Existing experimental data were used to validate the computational fluid dynamics (CFD) tool. The effects of subgrid scale (SGS) models, mesh densities, computational domains and boundary conditions on the CFD predictions are studied. On the blade suction surface, a separation zone starts at a location of about 55% along the suction surface. The prediction of flow separation on the turbine blade is always found to be difficult and is one of the focuses of this work. The ability of Smagorinsky and wall-adapting local eddy viscosity (WALE) model in predicting the flow separation is compared. WALE model produces better predictions than the Smagorinsky model. CDNS produces very similar predictions to WALE model. With a finer mesh, the difference due to SGS models becomes smaller. The size of the computational domain is also important. At blade midspan, three-dimensional (3D) features of the separated flow have an effect on the downstream flows, especially for the area near the reattachment. By further considering the effects of endwall secondary flows, a better prediction of the flow separation near the blade midspan can be achieved. The effect of the endwall secondary flow on the blade suction surface separation at the midspan is explained with the analytical method based on the Biot–Savart Law.

A priori study of subgrid-scale features in turbulent Rayleigh-Bénard convection

At the crossroad between flow topology analysis and turbulence modeling, a priori studies are a reliable tool to understand the underlying physics of the subgrid-scale (SGS) motions in turbulent flows. In this paper, properties of the SGS features in the framework of a large-eddy simulation are studied for a turbulent Rayleigh-Bénard convection (RBC). To do so, data from direct numerical simulation (DNS) of a turbulent air-filled RBC in a rectangular cavity of aspect ratio unity and π spanwise open-ended distance are used at two Rayleigh numbers Ra∈{108,1010} [Dabbagh et al., “On the evolution of flow topology in turbulent Rayleigh-Bénard convection,” Phys. Fluids 28, 115105 (2016)]. First, DNS at Ra = 108 is used to assess the performance of eddy-viscosity models such as QR, Wall-Adapting Local Eddy-viscosity (WALE), and the recent S3PQR-models proposed by Trias et al. [“Building proper invariants for eddy-viscosity subgrid-scale models,” Phys. Fluids 27, 065103 (2015)]. The outcomes imply that the eddy-viscosity modeling smoothes the coarse-grained viscous straining and retrieves fairly well the effect of the kinetic unfiltered scales in order to reproduce the coherent large scales. However, these models fail to approach the exact evolution of the SGS heat flux and are incapable to reproduce well the further dominant rotational enstrophy pertaining to the buoyant production. Afterwards, the key ingredients of eddy-viscosity, νt, and eddy-diffusivity, κt, are calculated a priori and revealed positive prevalent values to maintain a turbulent wind essentially driven by the mean buoyant force at the sidewalls. The topological analysis suggests that the effective turbulent diffusion paradigm and the hypothesis of a constant turbulent Prandtl number are only applicable in the large-scale strain-dominated areas in the bulk. It is shown that the bulk-dominated rotational structures of vortex-stretching (and its synchronous viscous dissipative structures) hold the highest positive values of νt; however, the zones of backscatter energy and counter-gradient heat transport are related to the areas of compressed focal vorticity. More arguments have been attained through a priori investigation of the alignment trends imposed by existing parameterizations for the SGS heat flux, tested here inside RBC. It is shown that the parameterizations based linearly on the resolved thermal gradient are invalid in RBC. Alternatively, the tensor-diffusivity approach becomes a crucial choice of modeling the SGS heat flux, in particular, the tensorial diffusivity that includes the SGS stress tensor. This and other crucial scrutinies on a future modeling to the SGS heat flux in RBC are sought.

LES And URANS simulations of the swirling flow in a dynamic model of a uniflow-scavenged cylinder

The turbulent swirling flow in a uniflow-scavenged two-stroke engine cylinder is investigated using computational fluid dynamics. The investigation is based on the flow in a scale model with a moving piston. Two numerical approaches are tested; a large eddy simulation (LES) approach with the wall-adaptive local eddy-viscosity (WALE) model and a Reynolds-Averaged Navier-Stokes approach using the k−ω Shear-Stress Transport model. Combustion and compression are neglected. The simulations are verified by a sensitivity study and the performance of the turbulence models are evaluated by comparison with experimental results. Both turbulence models produce results in good agreement with experimental data. The agreement is particularly good for the LES, immediately after the piston passes the bottom dead center. Furthermore, in the piston standstill period, the LES predicts a tangential profile in agreement with the measurements, whereas the k−ω SST model predicts a solid body rotation. Several instabilities are identified during the scavenging process. The formation of a vortex breakdown with multiple helical vortex structures are observed after the scavenge port opening, along with the shedding of vortex rings with superimposed swirl. The turbulence models predict several flow reversals in the vortex breakdown region through the scavenge process. Flow separations in the scavenge ports lead to a secondary axial flow, in the separated region. The secondary flow exits in the top of the scavenge ports, resulting in large velocity gradients near the cylinder liner above the scavenge ports.

A priori filtering and LES modeling of turbulent two-phase flows application to phase separation

The Large Eddy Simulation (LES) of two-phase flows with resolved scale interfaces is investigated through the a priori filtering of Direct Numerical Simulations (DNS) of one-fluid and multifield models. A phase inversion benchmark [1–4] is considered highlighting many coalescence and interface rupture events in a kind of atomization process. The order of magnitude of specific two-phase subgrid LES terms is first considered with the two modeling approaches. Then, different existing models such as Smagorinsky [5], Wall-Adapting Local Eddy-viscosity (WALE) model [6], Bardina [7], Mixed [8] and Approximate Deconvolution Model (ADM) [9] are used to account for two-phase subgrid effects. These models are compared to filtered DNS results. The main conclusion concerning a priori LES filtering is that the inertia term is not predominant in two-phase flows with fragmentation and rupture of interface. This conclusion is different from that of the studies of [3, 10–13]. Concerning LES models, functional modeling do not correlate to filtered DNS results whereas structural approaches do. Bardina and ADM are clearly the good LES framework to consider for two-phase flows with resolved scale interfaces. ADM is clearly better than Bardina in our study.

Jet noise computation based on enhanced DES formulations accelerating the RANS-to-LES transition in free shear layers

The paper presents jet noise computations performed using the original and some recently proposed modified formulations of detached-eddy simulation, based on alternative definitions of the subgrid length-scale and/or a modified version of the subgrid Spalart-Allmaras model, which involves the Wall-Adapting Local Eddy-viscosity model. The modifications are aimed at the elimination or, at least, a significant weakening of a known flaw of detached-eddy simulation, namely, the severe delay of the Kelvin–Helmholtz instability and transition from fully modeled to mostly resolved turbulence in the free and separated shear layers. This flaw makes the original detached-eddy simulation formulation, in fact, non-applicable to the prediction of the noise of high Reynolds number jet flows when typical grids are used, and has led to the use of Implicit Large-Eddy Simulation instead. Based on examples of application of these modifications to such flows, already available in the literature, they were found capable of resolving the issue and providing quite accurate predictions of the aerodynamic characteristics and turbulent statistics of the high-Re jets. The study performed in the present work allows us to draw a similar conclusion regarding the accuracy of the enhanced detached-eddy simulation formulations in terms of jet-noise prediction. The new formulation gives noise results very close to those of ILES except at the highest frequencies, while being more general and less sensitive to grid resolution. In addition, it removes a (1–3) dB underestimation of the noise spectral maximums for the peak radiation direction, which is typical of ILES on fine grids.

Assessment of subgrid-scale modeling for large-eddy simulation of a spatially-evolving compressible turbulent boundary layer

Abstract The performance of three standard subgrid-scale (SGS) models, namely the wall-adapting local eddy-viscosity (WALE) model, the Dynamic Smagorinsky model (DSM) and the Coherent Structures model (CSM), are investigated in the case of a spatially-evolving supersonic turbulent boundary layer (STBL) over a flat plate at M ∞ = 2 and Reθ ≈ 2600. A high-order split-centered scheme is used to discretize the convective fluxes of the Navier–Stokes equations, and is found to be highly effective to overcome the dissipative character of the standard shock-capturing WENO scheme. The consistency and the accuracy of the simulations are evaluated using direct numerical simulations taken from the literature. It is demonstrated that all SGS models require a comparable minimum grid refinement in order to capture accurately the near-wall turbulence. Overall, the models exhibit correct behavior when predicting the dynamic properties, but show different performances for the temperature distribution in the near-wall region even for cases with satisfactory energy resolution of more than 80%. For a well-resolved LES, the SGS dissipation due to the fluctuating velocity gradients is found to dominate the total SGS dissipation.

Analysis of the Effect of the Swirl Flow Intensity on Combustion Characteristics in Liquid Fuel Powered Confined Swirling Flames

This article examines the implementation of CFD technology in the design of the industrial liquid fuel powered swirl flame burner. The coupling between the flow field and the combustion model is based on the eddy dissipation model. The choice of the LES (Large Eddy Simulation) turbulence model over standard RANS (Reynolds Averaged Navier-Stokes) offers a possibility to improve the quality of the combustion-flow field interaction. The Wall Adapting Local Eddy-Viscosity (WALE) sub-grid model was used. The reaction chemistry is a simple infinitely fast one step global irreversible reaction. The computational model was setup with the Ansys-CFX software. Through the detailed measurements of industrial size burner, it was possible to determine the natural operational state of the burner according to the type of fuel used. For the inlet conditions, axial and radial velocity components were calculated from known physical characteristics of both the fuel and air input, with the initial tangential velocity of the fuel assumed as 18% of the initial axial fuel velocity. Different swirl number (S) values were studied. Addition of a surplus (in comparison to conventional flame stabilization) of tangential air velocity component (W), the rotational component increases itself with a considerably high magnitude, contributing to the overall flame stabilization. The level of S especially influences the turbulent energy, its dissipation rate and turbulent (Reynolds) stresses. In the case of high swirl number values (S > 0,65) it is possible to divide the flow field in three principle areas: mixing area (fuel-air), where exothermal reactions are taking place, central recirculation area and outer recirculation area, which primarily contains the flow of burnt flue gases. The described model was used to determine the flow and chemical behavior, whereas the liquid atomization was accounted for by LISA (Linear Instability Sheet Atomization) model incorporating also the cavitation within injection boundary condition. The boundary conditions were determined based on the data from the experimental hot water system. Depending on system requirements, especially with continuous physical processes as well as the results of experimental measurements, the paper reports on determination of the mixing field and its intensity in the turbulent flow, the description of heat release and interaction of turbulent flow field and chemical kinetics in the case of confined swirling flames.

Flow characteristics prediction in pump mode of a pump turbine using large eddy simulation

To obtain more accurate flow characteristics of pump turbines, the method of large eddy simulation with wall-adapting local eddy viscosity model is applied in simulating several operating points in the pump mode. Firstly, based on the experimental validation, the method of large eddy simulation could better predict the external performance and internal flow characteristics in a pump turbine in the pump mode compared with the method of Reynolds-averaged Navier–Stokes with two-equation turbulence model shear stress transport k–ω. Then, flow characteristics under 1.00 QBEP (best efficiency point), 0.91 QBEP, 0.88 QBEP, and 0.85 QBEP operating points are investigated to find out the causes of the head drop in the energy-discharge curve through large eddy simulation. The detailed analysis reveals that the head drop at the point 0.85 QBEP is related to the recirculation flow at the runner inlet. Finally, unsteady studies confirm that vortex movement at the runner inlet lead to the variation of the amplitudes and directions of the velocity, which generates the rotation of the separation vortices in the runner and stay vane channels.

Large eddy simulation of turbulent vertical wall fires supplied with gaseous fuel through porous burners

This paper presents a large eddy simulation (LES) study of vertical turbulent wall fires, which aims at bringing fundamental insight into the near-wall flame structure and heat transfer characteristics. The LES simulations are wall-resolved, i.e., the simulations are performed with sufficient grid resolution to capture the wall gradients and do not require a wall heat transfer model. The wall fires considered herein correspond to a simplified configuration in which gaseous fuel is supplied from an array of vertical porous burners, featuring a series of meter-scale wall flames with different fuel mass flow rates and different fuel types. Four fuels (methane, ethane, ethylene and propylene) are studied with prescribed fuel flow rates. Simulations are performed using an LES solver called FireFOAM. The Wall-Adapting Local Eddy-viscosity (WALE) model is used for turbulence modeling. The Eddy Dissipation Concept (EDC) model is used for combustion modeling. The thermal radiation model uses the discrete ordinate method with the simplifying assumption of an optically-thin medium characterized by a fixed radiant fraction. In the fuel blowing region, grid convergence is achieved using a 3 mm computational grid; while in the downstream flame preheating region, a 1.5 mm near-wall grid spacing is required to fully resolve the wall convective heat transfer. The change in grid requirement is due to the presence or absence of fuel blowing which affects the thickness of the wall viscous sub-layer. The simulations are in good qualitative and quantitative agreement with experimental data in the turbulent flame region; in particular: the radiative heat flux increases with elevation and with the fuel mass loss rate, while the convective heat flux remains approximately constant with elevation and decreases with the fuel mass loss rate. The present wall-resolved simulations are considered a first step on the route to developing accurate wall models needed for engineering simulations of wall fires.

Study of three LES subgrid-scale turbulence models for predictions of heat and mass transfer in large-scale compartment fires

ABSTRACTNumerical assessment was performed to investigate the wall-adaptive features offered by two subgrid-scale (SGS) turbulence models: Wall-Adapting Local Eddy Viscosity (WALE) and Vreman against the Smagorinsky model. The gas temperature and velocity field predictions were enhanced using WALE over Smagorinsky, especially at the flaming and near-wall regions since WALE considers both strain and rotation rates of the turbulent structure and the turbulent viscosity approaches zero at the wall. Conversely, the simulation results by Vreman were under-predicted against the experimental data. The WALE model could notably enhance the simulation accuracy for large-scale compartment fires due to significant improvements of the flow diffusivity modeling.

Numerical simulations of combustion process in a gas turbine with a single and multi-point fuel injection system

The paper presents a numerical study of a medium size model of industrial gas turbine combustor. The research was conducted using the RANS (Reynolds Averaged Navier–Stokes) approach with k–∊ model and LES (Large Eddy Simulation) with WALE (Wall Adapting Local Eddy viscosity) subgrid model. The simulations were performed in cold and reacting flow conditions. In the latter case, the combustion process was modelled using a steady flamelet model with chemical mechanisms of Smooke with 16 species and 25 elementary reactions, and the GRI-2.11 with 49 species and 277 elementary reactions including NO chemistry. In the first part of the paper, the numerical results were validated against experimental data including velocity field, temperature and species concentrations. The velocity components predicted for the cold flow agree very well with measurements. In the case of the simulations of the reacting flow, some discrepancies were observed in both the temperature field and species concentrations. However, the main flame characteristics were captured correctly. It turned out that the chemical kinetics had a larger impact on the results than the turbulence model. In the second part of the paper, we modified the fuel and air injection method and analysed how the changes introduced affect the flame dynamics. It was shown that: (i) depending on the distribution of air, the velocity, temperature and species composition in the upper part of the combustion chamber can be significantly altered; (ii) more substantial changes can be achieved by shifting the fuel injection points; their location outside the main recirculation zone leads to a dangerous situation resulting in overheating of the walls; (iii) it turns out that substantial differences in the flame characteristics in the upper part of the combustion chamber vanish approaching the outlet plane and the resulting mixture compositions are very similar.

Numerical investigations of transient nozzle flow separation

Large-eddy simulations (LES) are carried out to investigate the shock induced transient flow through a planar nozzle mimicking a shock tube experimental setup at shock Mach number Ms=1.86. A fifth-order Weighted Essentially Non-oscillatory (WENO) scheme based 3D numerical flow solver equipped with an immersed boundary method and Wall-Adapting Local Eddy-viscosity (WALE) model is used for this purpose. A comparative study is presented to show the effect of different flow initializations namely, i) without flow fluctuation, ii) with white random noise and iii) with homogeneous isotropic turbulence superimposed on the flow-field. Results are in good agreement with the experimentally measured speeds of the primary shock wave and the following secondary shock wave. It is found that an improper initialization of the flow-field may lead to erroneous predictions of the flow characteristics, particularly the location of the separation point and the unsteady shock/boundary layer interaction. Substantial improvement in the prediction of the early-stage Mach reflection, complex shock/boundary layer interaction is observed with superimposed turbulent fluctuations as an initial flow-field. This is when a homogeneous incompressible isotropic turbulence superimposed on the shocked section is assigned as initial fluctuating field. An additional test case with the latter method, having an approximately four times higher mesh resolution, is used for detailed investigation of the unsteady flow fields, turbulent statistics and boundary layer separation. Results show considerable improvements in the prediction of the secondary shock and the flow separation location compared to the previous findings which dealt with lower mesh resolution. The turbulent flow structures are depicted using the mean flow-field which is spatially averaged over the span-wise homogeneous direction. Time-averaging on the fly turns out to be inadequate, since it leads to a spatial shift of the separation bubble. The reason for this is the use of past flow information only, due to the lack of future information. This in-turn invokes the need of phase-averaging to extract suitable physically statistics of the turbulent flow-field for further deeper analysis.

Application of a Parallel Solver to the LES Modeling of Turbulent Buoyant Flows with Heat Transfer

An existing fully implicit, non-dissipative direct numerical simulation (DNS) algorithm is reformulated to utilise the sub-grid scale (SGS) models in large eddy simulation (LES). The Favre-filtered equations with low-Mach number scaling are derived. The wall-adapting local eddy-viscosity (WALE) is used as SGS model. A fully parallel, finite volume solver is developed based on the resulting LES algorithm using PETSc library and applied to buoyancy- and thermally-driven transitional/turbulent flows in Rayleigh-Taylor instability and turbulent Rayleigh-Bénard convection. Results verify that the proposed low-Mach number LES approach, which is physically more accurate than pure incompressible methods for flows with variable properties, perfectly captures the evolution and complex physics of turbulent buoyant flows with or without heat transfer by taking the effects of density and viscosity changes into account without the Oberbeck-Boussinesq (OB) assumption even at large temperature differences with uniform accuracy and efficiency.

A mathematical modeling study of the influence of small amounts of KCl solution tracers on mixing in water and residence time distribution of tracers in a continuous flow reactor-metallurgical tundish

In an earlier research (Chen et al., 2015a) a mathematical model was established to simulate tracer mixing (a KCl solution). The predicted Residence Time Distribution (RTD) curves showed good agreements with experimental RTD curves for larger amounts of tracer additions. However, for smaller additions (50mL) of a KCl solution into water, the predicted RTD curves tended to deviate from the experimental RTD curves for a tundish (a continuous flow reactor). The current paper focuses on the possibilities that the predictability for smaller additions could be resolved by using a suitable turbulence model. The performance of five different turbulence models representing different modeling techniques and levels of complexity were tested in combination with using a density-coupled mixed composition fluid model to simulate the mixing, i.e. the following models: LVEL, Chen–Kim k–ε, MMK k–ε, Explicit Algebraic Reynolds Stress Model (EARSM), and Large Eddy Simulation (LES): Wall-Adapting Local Eddy-viscosity (WALE). The results indicate that models that tend to resolve turbulence structures renders better predictions of the mixing process of smaller tracer amounts. In addition, the influence of different tracer amounts on the flow in tundish was assessed. The simulation results for 75mL, 100mL, 150mL, and 250mL KCl tracer additions were compared. The results showed that in an upward flow the tracer will, sooner or later (dependent on the tracer amount), sink to the bottom. This is due to the higher density of the tracer compared to the density of water. From a physical modeling perspective, this issue is like the “butterfly effect”. It is showed that for a slight increase of the amount of tracer, the flow field might be disturbed. This, in turn, will result in a shifted RTD curve.