CFD Code ANSYS Fluent Research Articles

Separation efficiency of a wastewater bar screen based on a 3D computational fluid dynamics modeling

The wastewater screening process is a crucial step in the majority of wastewater treatment plants, frequently using mechanical bar screens as the first filtration step to remove heavy debris from the wastewater flow. In the present study, two 3D geometries of bar screens with different screen openings were presented to investigate the effect of the variation in screen spacing of bars on the separation efficiency of debris. Based on the Ansys Fluent CFD code within the Eulerian-Lagrangian approach, the particle trajectories were tracked, and the retention efficiency of these two bar screens was evaluated. The results show that a higher particle separation efficiency is favored for small bar spacing and coarse particle sizes. Actually, despite the impact of bar spacing on the separation efficiency of particles, the adequate choice of this parameter is related to particle properties and the fluid flow inlet velocity. These parameters also have a great influence on the separation efficiency of bar screens. The study can help for more structural improvements in the shape of the screening media, which leads to enhanced performance of wastewater screening facilities.

Response to the design conditions of a tube-bundle thermal energy storage unit with paraffin-copper foam composite as a storage medium

Owing to its potential to limit the issues of intermittency and instability in solar energy by means of phase-change materials (PCMs), latent heat thermal energy storage (LHS) has met increasing attention and been widely employed in thermal-based systems. Most PCMs, however, inherently exhibit poor heat conductance leading to modest rate of charging. To master this drawback, highly conductive metal foams are utilized to upgrade the bulk heat conductance of a PCM resulting in an enhanced rate of heat transport; hence, accelerating the melting process. This enhancement way is employed to promote the response of a tube-bundle thermal energy storage unit configured of staggered tubes filled with high-porosity copper foam embedded in paraffin wax as a storage medium. To charge the thermal energy storage (TES) unit proposed, a relatively hot stream of water is allowed to flow across the bundled tubes through the voids in-between. To check how feasible the proposed design is, the tubes enclosing the TES medium and the water flow in the shell surrounding have been simulated by ANSYS Fluent CFD code. A variety of design factors has been examined to explore their influence on the charging performance attained including structural properties of the metal foam employed along with the configuration of the tube-bundle considered. The foam structural characteristics inspected are the porosity ∅∈(0.89∼0.97) and pore density ω∈(5∼80)PPI, while various tube-bundle configurations have been tested in terms of the packing density N∈(15∼65)TubesLines/meterwidth as well as the shape of PCM tubes used b/a∈(0.25∼1). Results indicate that for a certain tube-bundle configuration, the best response is acquired when denser foam with lower porosity is used, while the best performance can only be achieved with higher porosity due to the better TES capacity offered. For certain foam structural properties of (∅=0.93&ω=80PPI), it has been observed that increasing the packing density can save up to 56 % of the time taken for charging completion with up to 200 % increase in storage capacity leading to up to 48 % improvement in the system overall performance. It has also been found that using paraffin embedded with copper foam as a TES medium accelerates the melting process considerably and saves more than half the time taken to charge the corresponding TES unit based on pure paraffin. It is also worth mentioning that the currently suggested LHS unit is not only of simple configuration, but practically efficient as well, with outstanding gross performance OP∈(104∼105). Through a proper manipulation of design conditions, it has been found that the charging response can be notably improved leading to substantial promotion in the overall performance realized.

Numerical prediction of mean flow and acoustic field of a supersonic impinging jet

The three-dimensional turbulent mean flow and acoustic field of a supersonic jet impinging on a solid plate is studied computationally using the general purpose CFD code Ansys Fluent. A pressure-based coupled solver formulation with the second order weighted central-upwind spatial discretization is applied to compute transient solutions. Cold and hot jet thermal conditions are considered. Mean flow characteristics are investigated by a steady-state modeling approach. Acoustic radiation of impingement tones is simulated using a transient time-domain formulation. The effects of turbulence in steady-state are modeled by the SST k-ω turbulence model. The Wall-Modeled Large-Eddy Simulation (WMLES) model is applied to compute transient solutions. The near-wall mesh on the impingement plate is fine enough to resolve the viscosity-affected near-wall region all the way to the laminar sublayer. Nozzle-to-plate distance is parameterized in the model for automatic re-generation of the mesh and results. Steady-state predictions of hover lift loss and mean jet velocity distributions are compared with experimental data, and favorable agreement is reported. The transient solution reproduces the mechanism of impingement tone generation by the interaction of large scale vortical structures with the impingement plate. The acoustic near-field is directly resolved by Computational Aeroacoustics (CAA) to accurately propagate impingement tone waves to near-field microphone locations. Calculated impingement tone frequencies and sound pressure levels agree with experimental values.

Numerical parametric study of turbulent counterflowing jets

In order to study the sensitivity to flow and geometrical conditions, a turbulent round jet in counterflow is numerically investigated for different Reynolds numbers and slight inclination angles at various jet-to-counterflow velocity ratios. Numerical simulations are executed with the second order Reynolds Stress Model (RSM) using the CFD code Ansys Fluent. It is found that the linearity between the penetration length and the velocity ratio remains valid as long as the jet-to-counterflow momentum flux ratio is up to 0.06. It is shown that no influence of the Reynolds number of either the main flow (Re0) or the jet flow (Rej) is found as long as Re0 > 10,000 and Rej > 3000, respectively. Also, even a small inclination angle seems to result in an asymmetric flow field with appreciable reduction in jet penetration length.

Numerical Analysis of Aerodynamic and Shock Wave Characteristics of Biconvex and Double-Wedge Shape Airfoils for Supersonic Flow

This present study describes the aerodynamic characteristics of supersonic flow over biconvex and double wedge airfoils using a finite volume method-based commercial CFD code Ansys Fluent. A steady-state RANS approach is used with SST k-ω viscous modeling. A series of simulations are conducted to analyze the characteristics of shock and expansion waves formed around the airfoils for Mach numbers ranging from 1.4 to 3.4 with varying angles of attack (α) from 0° to 20°. It is observed that the lift and drag coefficients both increase with the angle of attack for a fixed Mach number and decrease with the Mach number for a fixed angle of attack. Double wedge airfoil generates about 5% more lift at a low Mach number and 1% more lift at a higher Mach number compared to the biconvex airfoil. However, the biconvex airfoil generates lesser drag than the double wedge airfoil. The maximum value of the pressure coefficient (Cp ) is found to be 1.7 for biconvex airfoil and 1.4 for double wedge airfoil. The maximum value of the lift-to-drag ratio for biconvex airfoil is 7.63, occurs at 1.4 Mach number and 3.46° angle of attack, whereas the value for double wedge airfoil is 5.19 at the same Mach number with 4.47° angle of attack, which suggest that biconvex airfoil has a higher lift-to-drag ratio and gives a better aerodynamics performance. The shock waves start to detach after an angle of attack of 5° and the shock wave is fully detached at a 15° angle of attack for biconvex airfoil for Mach number of 1.4. For the same Mach number, the double wedge airfoil, the shock wave starts to form the same as the biconvex airfoil but the waves are fully detached at a lower angle of attack of 10°. With the increasing Mach number, the shock waves remain attached to the airfoil.

Study on aerodynamics characteristics of the effect of leading-edge radius of FDM printed (ABSi material) supersonic aerofoil

Currently, supersonic vehicles are studied as future civil transports. The selection and design of aerofoils plays an important step in the aircraft design process. NACA 64-008A aerofoil is chosen for the investigation which is produced using ABSi (Acrylonitrile butadiene styrene) by Fused Deposition Modelling (FDM). The focus of this work is to investigate numerically the aerodynamic characteristics of supersonic aerofoil, NACA 64-008A, for the effect of varying the leading-edge radii along with variations in angle of attacks. The simulation is carried out using commercial CFD code ANSYS Fluent. The closure for the Reynolds Averaged Navier Stokes equations is achieved by using the k-ω SST turbulence model. The coefficients of lift and drag for three distinct leading-edge radii, namely 4.8 mm, 5 mm, and 5.2 mm at Mach 2.25 with a varied angle of attack (-5°to 5°) are investigated. k-ω SST turbulence model is selected to deal with the flow turbulence and achieve the closure for Reynolds averaged Navier Stokes equations. The CL/CD ratio increases as the leading radius increases. The maximum value of CL/CD ratio is observed for 5.2 mm leading edge radius at an angle of attack, α = 5°. Moreover, the drag decreases with increase in lift values. It is clearly seen that increase in the leading-edge radii of the supersonic profile has a considerable effect in the increase in coefficient of lift. Thus, the variations in the leading-edge radii of the supersonic aerofoil enhances the aerodynamical characteristics.

Numerical Study of Turbulent Swirling Diffusion Flame Under Lean and Rich Conditions Using Turbulence Realizable k-epsilon Model

ABSTRACT This work investigates numerically a non-premixed swirling flame in a coaxial burner with a radial fuel injection under lean and rich conditions of mixtures. The swirler is placed in the annular part of the burner. The fuel is injected radially through eight holes symmetrically distributed on the periphery of the central tube. All simulations are carried out using the ANSYS-Fluent CFD code. The turbulence is captured using the Reynolds Averaged Navier-Stokes approach. Chemistry/turbulence interaction is resolved using the Eddy Dissipation Model. Simulations are performed with a global equivalence ratio ranging from 0.5 to 1.3. Model validation is achieved by comparing computed results to our experimental data of Stereoscopic Particle Image Velocimetry obtained in the case of the stoichiometric regime. Good agreement between numerical results and experimental measurements is assigned. The central recirculation zone and the swirling jet region due to the presence of the swirl are well predicted by the simulations. The effect of the global equivalence ratio on the profile of axial velocity, temperature distributions and pollutant emissions (CO and NOx) is numerically studied. From dynamic point of view, the equivalence ratio modifies the mean axial velocity of the swirling diffusion flame. The increase of equivalence ratio destabilizes the flame by increasing the liftoff height. NO production decreases by the increasing of the equivalence ratio.

Vortex-induced vibration of two tandem square cylinders with different restraint conditions

The present work investigates the vortex-induced vibration of twin square cylinders at Re = 150 with three restraint conditions. (Ⅰ) the upstream cylinder (UC) is free to oscillate while the downstream cylinder (DC) is fixed; (Ⅱ) both cylinders are free to oscillate, and (Ⅲ) the UC is fixed while the DC is free to oscillate. The centre-to-centre distance between the cylinders is fixed as 4.0B, and the mass ratio of the cylinder is m* = 3. The computations are performed for the reduced velocity ranging from 1 to 15 under each restraint condition. The flow dynamics are described by the two-dimensional incompressible Navier-Stokes equations and solved numerically using the Ansys Fluent CFD code (based on the finite volume method and SIMPLEC algorithm. It is found that whether the DC oscillates, the UC will exhibit the soft-lock-in phenomenon within the co-shedding regime. At high reduced velocities, the UC re-shows the soft-lock-in phenomenon when the DC is fixed. Whether the UC oscillates, the DC will exhibit the soft-lock-in phenomenon within the reattachment regime. For Case-Ⅱ, the strong vortex falling off the leeward side of the UC leads to a sharp decrease in distance between the two cylinders, and the DC exhibits the soft-lock-in phenomenon again.

Resistance Evaluation for the Submerged Glider System using CFD Modelling

The underwater gliders are type of autonomous underwater vehicles, which are in their way to be used instead of traditional propellers or thrusters. They are used as submerged gliders or as a part connected to floating hull which is propelled by it. The underwater system depends on different number of hydrofoils (underwater wings) that allow it to glide forward while descending through the water. This paper concentrates on three critical design details: profile of the wings, multiple flapping wings and their angle of attack (AoA), and a method is focused on the mesh generation to predict calm water resistance for the submerged glider system while considering the profile and the maximum rotating angle of the wings and rudder. Flow around submerged wing system model has been calculated at different Froude numbers ranging from 0.1 to 0.6. The grid generator is established for meshing the computational domain with structured hexahedral and unstructured tetrahedral grid. Simulations are carried out using commercial CFD code ANSYS Fluent 19 to calculate calm water resistance of the submerged glider system with different number of hydrofoils. The results conclude that the cambered plate was chosen to design the rudder at 19o AOA and the hydrofoil NACA0012 can be practically applied in the design of the wings at 15o AOA from the resistance point of view. In addition, this investigation introduces a new application for CFD calculations in estimating the resistance of the submerged glider system.

Fire smoke dispersion inside and outside of a warehouse building in moderate and strong wind conditions

A moderate-sized (8 MW) fire was modelled in a warehouse located in an urban area with diverse architecture, in wind conditions. The investigation includes the effects of neighbouring architecture on the wind field, and in consequence on the smoke control performance and the smoke dispersion in the near-field of the building. Overall, 25 CFD simulations were performed with ANSYS Fluent CFD code, for wind velocities of 5 m/s (moderate), 10 m/s (strong) at 12 wind angles each (0°–330°, 30° increment), and at 0 m/s for reference. The building smoke venting system's performance was affected by the wind, reaching 74%–114% and 78%–158% of the reference mass flow at the moderate and strong wind velocities, respectively. Surprisingly, in multiple cases the exhaust flow rate of ventilators was increased, rather than hindered by the wind. We attribute this to the arrangement of inlets and outlets on façades and the resulting pressure difference between the wall and roof openings. The smoke plume was highly dependent on the wind angle, and the type of architecture up- and downwind of the fire. Significant urban canyon effects and large vortices forming behind tall buildings were observed, leading to smoke accumulation in a large distance from the fire.

Analysis of magneto-natural-convection flow in a semi-annulus enclosure filled with a micropolar-nanofluid; a computational framework using CVFEM and FVM

Micropolar fluids are the media in which their behavior is affected by the microrotation of the substructure particles. In the present work, the natural-convection flow of a micropolar-nanofluid (Al2O3/water) in the presence of an inclined magnetic field in a semi-annulus enclosure is investigated. The influence of active parameters on magneto-natural convection is investigated. The governing equations (mass, momentum, microrotation, and energy) are numerically solved by CVFEM. Moreover, for some conservative comparisons, the equations are solved by Ansys Fluent CFD code which is conjugated by an innovative nondimensionlization scheme. The effects of important parameters are investigated on the streamlines, isotherms, microrotation contours, the maximum absolute value of stream function (Ψmax), the average Nusselt number (Nuave) and the maximum absolute value of microrotation Nmax∗. The results discovered that the values of Nuave descend with ascending the Hartmann number (Ha) and material parameter (K) while it ascends with ascending Rayleigh number (Ra) and particle volume fraction (ϕ). The value of Nuave descends 10.8% while Nmax∗ increases 19.8% when K increases from 1 to 2. The value of Nuave descends 44.83% when Ha increases from 25 to 75 at λ=45°. The minimum value of Nuave occurs at λ=45° for Ha=25 and at λ=30° for Ha=50 and Ha=75.

Numerical Analysis of Heating Technique in Corium Melt Pool Convection Flow Field & Thermal Interaction in a Volumetrically Heated Molten Pool

In-Vessel Retention (IVR) is one of the existing strategies of severe accident management of LWR, which intends to stabilize and isolate corium & fission products inside the reactor pressure vessel (RPV) and primary containment structure. Since it has become an important safety objective for nuclear reactors, it is therefore needed to model and evaluate relevant phenomena of IVR strategy in assessing safety of nuclear power reactors. One of the relevant phenomena during accident progression in the oxidic pool is non-uniform high heat generation occurring at large scale. Consequently, direct experimental studies at these scales are not possible. The role computer codes and models are therefore important in order to transpose experimental results to reactor safety applications. In this paper, the state-of-the-art ANSYS FLUENT CFD code is used to simulate Non-uniform heat generation in the lower plenum by the application of Cartridge heating under severe accident conditions to derive the basic accident scenario. However, very few studies have been performed to simulate non-uniform decay heat generation by Cartridge heaters in a pool corresponding lower plenum of power reactor. The current investigation focuses on non-uniform heating in the fluid domain by Cartridge heaters, which has been done using ANSYS FLUENT CFD code by K-epsilon model. The computed results are based on qualitative assessment in the form of temperature and velocity contour and quantitative assessment in terms of temperature and heat flux distribution to assess the impact of heating method on natural convective fluid flow and heat transfer.

Heat transfer intensification of NEPCM-water suspension filled heat sink cavity with notches cooling tubes by applying the electric field

The effects of an electric field on heat transfer, free convection, and entropy generation of nano-encapsulated phase change material (NEPCM) dissolved in water in a complex inclined chamber are studied. The enclosure consists of a rectangular cavity heated from below and the remaining walls are considered adiabatic. The fluid is cooled by eight cooling tubes, each with four notches. The effects of the electric field, inclination angle (ζ = 45° and ζ = 90°), and Rayleigh number (Ra = 103, 104, and 105) in equal concentration of NEPCM (ϕ = 2 %) are studied using ANSYS Fluent CFD code. The dimensionless form of fluid governing equations, electric potential equation, electric charge density equation, and entropy generation equation are used to set the primitive parameters. Grid verification and validation tests are performed to ensure the reliability of the solution. Calculations show that with increasing Ra, the average Nusselt number (Nuave) decreases from ζ = 45° to 90°, but when an electric field is applied, the results are reversed. As the Rayleigh number increases in conjunction with the electric field, the buoyancy effect becomes more pronounced. Furthermore, owing to its high heat transfer potential and lowest irreversibility, the straight cavity (ζ = 90°) is desirable at Ra = 105.

CFD Modeling for the Influence of Fly Ash Particle Size on the Different Properties of High Concentrated Slurry Transportation in Horizontal Pipe

The disposal of fly ash from thermal power plants is becoming a serious concern for environmental engineers. This article attempts to study the influence of particle size of the fly ash on the various properties of high concentrated slurry (50 - 70 % by weight) flow through a horizontal pipe with the help of a commercial CFD code ANSYS FLUENT. Modified Herschel-Bulkley model and SST k-ω turbulence model is used for computational analysis, and the computational outcomes are validated with the available literature. In the present study, the fly ash samples were procured from different coal-based thermal power plants. Various properties: physical and rheological are obtained experimentally. The specific gravity of particles and the static settling concentration (SSC) of the slurry increases with the decrease in the particle size. The rheological properties increase as the concentration increases, and the slurry behaves like a non-Newtonian fluid at a high concentration. In addition, the particle size and concentration intensively impact on the skin friction coefficient and velocity distribution. However, no significant impact on the velocity profile at 70 % concentration (by weight).
 HIGHLIGHTS
 
 The rheological properties increase as the concentration increases, and the slurry behaves like a non-Newtonian fluid at a high concentration
 CFD modelling of the slurry flow at high concentration with modified Herschel-Bulkley model and SST k-ω turbulence model
 The particle size and concentration intensively impact on the skin friction coefficient
 
 GRAPHICAL ABSTRACT

CFD Analysis of a Small-Scale Solar Chimney Exposed to Ambient Crosswind

Solar chimneys are devices that use solar energy to generate a hot airflow that can be used for power production, the drying of agricultural products, and/or water desalination. The performance of a small-scale solar chimney is studied numerically. The computational domain includes the solar chimney, the ground, and the atmosphere. The turbulent airflow is simulated using the commercial CFD code Ansys Fluent. The only boundary conditions required for the simulation are the wind speed, the ambient temperature, and the absorbed energy from the ground, determined by an energy balance in the system. The system was simulated for one day in the summer in the city of Belo Horizonte, Brazil. The ambient crosswind plays an important role in the velocity and temperature. The velocity inside the solar chimney increased with the wind speed, increasing the heat transfer and decreasing the airflow temperature. When the wind speed increased from 0 to 10 m/s, the outlet velocity increased from 1 to 4 m/s, and the outlet temperature decreased from 313 to 304 K.

Computational investigation of erosion wear in the eco-friendly disposal of the fly ash through 90° horizontal bend of different radius ratios

Abstract In the present study, slurry erosion wear was evaluated in 90° horizontal pipe bends of various radius ratios (R/r = 2–10) through a commercial CFD code ANSYS FLUENT. For the suspension of fly ash-water, Euler–Lagrange and two way-coupling methods were employed to predict the slurry erosion wear. The flow through the horizontal bend pipe was simulated using a Standard k–ε turbulence modelling. The computational results were validated with the experimental result of the available literature. Fly ash was taken as the dispersed phase of the solid-liquid combination however water was used as the liquid phase. The fly ash particles size was taken as 150 µm. Various affecting factors, such as velocity (4–10 m/s) and solid concentration (2.5 and 7.5% by volume) of the fly ash, were also studied in this investigation. The erosion rate was maximum in the case of R/r = 4 and minimum for R/r = 10 at all velocities and concentrations. It was also found that the erosion rate increases with the increase in solid concentration and velocity.

Experimental and Numerical Heat Transfer Study of Swirling Air Jet Impingement

Experimental and numerical study has been performed to study effect of swirl on heat transfer. A twisted swirl tape with twist ratio (T. R)=2 is used to generate swirl and effect of parameters like z/d=2 to 8 and Re varying between 3000 to 9000 are studied experimentally. Experiments show heat transfer dependency on Reynolds number. This study is further extended numerically using commercial CFD code ANSYS Fluent for much higher Reynolds number flow up to 20000. It is seen that CFD centerline peak Nu profile matches pretty well with experiments conducted. A relation between peak Nu and Re is established numerically for the conditions mentioned above. Further, effect of multiple swirl jet impingement (two jets separated by distance 1.5D, 2D and 3D) on Peak Nu is explained through CFD studies. CFD results show that jet distance of 1.5D is more effective in achieving higher heat transfer compared to 2D and 3D distances.

Effects of pipe turns on vertical helically coiled tube heat exchangers for water heating in a household refrigerator

A new coupled numerical model of the overall vapor-compression cycle and water heater was developed in order to analyze the influence of condenser coil designs on the heat transfer process of a helical coil heat exchanger immersed in a storage tank. The model was developed using the CFD code ANSYS Fluent. In the numerical simulation, the helical condenser coil was treated as an internal heat source and the initial heat flux was obtained from experimental results and delivered to each coil element in the CFD model as boundary condition.The coupled model was validated with experimental results and showed that the numerical results of the proposed model are in good agreement with the experimental data with the deviation of ± 1.6%. Then, the impact of the main representative geometrical parameter on the overall heat transfer coefficient and water temperature distribution was investigated. The results indicated that the temperature distribution and overall heat transfer coefficient increase with the increase of the pipe turns of a helically coiled tube heat exchanger.

Three-Dimensional Simulation of Decaying Turbulent Swirling Flow Using Different Turbulence Models

This paper presents a numerical study of the turbulent swirling flow in a horizontal tangential inlet tube. The commercial CFD code ANSYS FLUENT 15 was used for solving the set of governing equations using different turbulence models. Eight turbulence models are tested which are, standard k–ε, realizable k–ε, RNG k–ε, SST k–ω, Non-Linear k–ε, v2-f, RSM (Quadratic Pressure-Strain Model), and RSM (Stress-Omega Model). All these turbulence models are available directly in the ANSYS FLUENT except the non-linear k–ε which was implemented in the solver using User Defined Functions (UDF). The numerical predictions are compared with experimental measurements from literature for tangential, axial velocity profiles and Reynolds stresses profiles within the tested tube. The results indicated that the axial velocity is predicted fairly well by the standard k–ε model while the tangential velocity is well predicted by RSM. On the other hand, v2-f model predicts the Reynolds stresses better than the other tested models. The statistical analysis of turbulence model performance showed that, the RSM (Quadratic Pressure-Strain) model gives the best agreement with all data of experiments followed by non-linear k–ε and standard k–ε turbulence models.

Numerical evaluation of geometric variations in internally heated melt pool

In Vessel Retention (IVR) is considered as one of the best safety strategy for a hypothetical severe accident in light water nuclear reactors (LWR). Since the strategy involves heat transfer between the corium melt pool and surrounding, it is highly impacted by the natural convection behaviour of such internally heated melt pool. Extensive experiments and numerical study has been performed worldwide to understand the behaviour of corium melt pool. COPRA (COrium Pool Research Apparatus) is one such experiment which uses molten salts of non-eutectic binary mixture (20 mol% NaNO3–80 mol% KNO3) and eutectic binary mixture (50 mol% NaNO3–50 mol% KNO3) as simulant material. Geometric variation in COPRA experiment is investigated to get a better understanding of the natural convection behaviour of the melt pool. Although the influence of geometry is reported in literature, the increase in geometry beyond the conventional size is yet to be explored. Eutectic salt (50 mol% NaNO3–50 mol% KNO3) is used as a simulant material for this study. A commercial CFD code ANSYS FLUENT 20.2 is employed for the numerical modelling of the corium pool. K-epsilon (k-ε) turbulence model and solidification-melting model are used in this simulation. Experimental data is used for validating this model. The geometric effect is investigated by assuming the same height and radius for the pool. Also, the ratio of heating power and cooling length is maintained for all the variations of geometry. The temperature field, liquid fraction contour, and velocity distribution of the melt pool are calculated. This paper provides an extension to the existing data thus further enhances the understanding of the thermal–hydraulic characteristics of the corium melt pool.