Numerical simulations on the TEFLU sodium jet experiment using the CFD code Karalis

The TEFLU experiments were performed at the Karlsruhe Forschungszentrum in order to investigate the behaviour of a low-Prandtl number jet under various flow condition, from forced flow to purely buoyant. Here, numerical simulations are presented and results compared with the experimental data. The computation are made within the Benchmark Working Group activities in order to test the capabilities of CFD codes to simulate Heavy Liquid Metal flows with heat transfer. The simulation are performed with the CFD code Karalis

Assessment of fission fragments enhancement for nuclear thermal propulsion

A novel concept of Nuclear Thermal Rocket (NTR) propulsion is presented. It is based onthe direct conversion of the kinetic energy of the Fission Fragments (FFs) into the propellantenthalpy. The FFs can escape from an extremely thin layer of fissionable material: a sufficiently large surface coated with few micrometers of Americium 242m, confined by a neutron diffuser, may become a critical reactor. The novel FF NTR propulsion concept may allow the propellant to achieve temperatures higher than the nuclear fuel, thus overcoming the limit in the specific enthalpy achievable by the propellant in the conventional solid-core NTR propulsion. Such a limit comes from the need to keep the temperature of the fuel material within a safe interval by using conventional convective heat transfer mechanism. A preliminary assessment of the FF NTR concept’s propulsion characteristics has been carried out using an in-house developed software system which integrates a Computational Fluid Dynamic code, a neutronic code and a Monte Carlo code. The assessment shows the potential to reach specific impulses of about 15,000 m/s and thrust levels in the range 4,000 to 6,000 N, with a trhust to weight ratio of a few percent of the acceleration of gravity. Such performances may make the FF propulsion a candidate for human missions to the planet Mars.645-652Pubblicat

Assessment of the boussinesq approximation for buoyancy-driven flows

The numerical simulation of buoyant flows often makes use of the Boussinesq approximation. This is particularly true when Direct Numerical Simulation is used for the analysis of heat transfer loops or other applications of natural convection flows. Boussinesq approximation consists of considering the density strictly constant, adding the buoyant force to the momentum balance and coupling an equation for the temperature to the incompressible Navier-Stokes system that has to be solved. In this paper, the validity of the Boussinesq approximation is investigated in some details via numerical simulations. The test cases chosen include a differentially heated cavity and two buoyant heated loops with internal heating. Results show clearly that the error on some performance parameters depends linearly on the compressibility parameter βΔΤ The error in some cases can be 20% or higher, but it is drastically reduced to a few percent by the use of a 2ⁿͩ order formulation of the Boussinesq source term. Nevertheless, some flow features are strictly tied to the compressible coupling and are not captured in a Boussinesq framework

A GENERAL COMPUTATIONAL APPROACH FOR MAGNETOHYDRODYNAMIC FLOWS USING THE CFX CODE: BUOYANT FLOW THROUGH A VERTICAL SQUARE CHANNEL

The buoyancy-driven magnetoconvection in the cross section of an infinitely long vertical square duct is investigated numerically using the CFX code package. The implementation of a magnetohydrodynamic (MHD) problem in CFX is discussed, with particular reference to the Lorentz forces and the electric potential boundary conditions for arbitrary electrical conductivity of the walls. The method proposed is general and applies to arbitrary geometries with an arbitrary orientation of the magnetic field. Results for fully developed flow under various thermal boundary conditions are compared with asymptotic analytical solutions. The comparison shows that the asymptotic analysis is confirmed for highly conducting walls as high velocity jets occur at the side walls. For weakly conducting walls, the side layers become more conducting than the side walls, and strong electric currents flow within these layers parallel to the magnetic field. As a consequence, the velocity jets are suppressed, and the core solution is only corrected by the viscous forces near the wall. The implementation of MHD in CFX is achieved

GEN-IV LFR development: Status & perspectives

Since Lead-cooled Fast Reactors (LFR) have been conceptualized in the frame of Generation IV International Forum (GIF), great interest has focused on the development and testing of new technologies related to Heavy Liquid Metal (HLM) nuclear reactors. In this frame, ENEA developed one of the larger European experimental fleet of experimental facilities aiming at investigating HLM thermal-hydraulics, coolant chemistry control, corrosion behavior for structural materials, and at developing components, instrumentations and innovative systems, supported by experiments and numerical tools. The present work aims at highlighting the capabilities and competencies developed by ENEA so far in the frame of the liquid metal technologies for GEN-IV LFR. In particular, an overview on the ongoing R&D experimental program will be depicted considering the actual fleet of facilities: CIRCE, NACIE-UP, LIFUS5, LECOR and HELENA. CIRCE (CIRColazione Eutettico) is the largest HLM pool facility presently in operation worldwide. Full scale component tests, thermal stratification studies, operational and accidental transients and integral tests for the nuclear safety and SGTR (Steam Generator Tube Rupture) events in a large pool system can be studied. NACIE-UP (NAtural CIrculation Experiment-UPgraded) is a loop with a HLM primary and pressurized water secondary side and a 250 kW power Fuel Pin Simulator working in natural and mixed convection. LIFUS5 (lithium for fusion) is a separated effect facility devoted to the HLM/Water interaction. HELENA (HEavy Liquid metal Experimental loop for advanced Nuclear applications) is a pure lead loop with a mechanical pump for high flow rates experiments. LECOR (LEad CORrosion) is a corrosion loop facility with oxygen control system installed. All the experiment actually ongoing on these facilities are described in the paper, depicting their role in the context of GEN-IV LFR development

Coupled simulations of the NACIE facility using RELAP5 and ANSYS FLUENT codes

This work deals with the development and preliminarily assessment of a coupling methodology between a modified version of RELAP5/Mod3.3 STH code and FLUENT commercial CFD code, applied to the NACIE (natural circulation experiment) LBE (lead bismuth eutectic) experimental loop (built and located at the ENEA Brasimone research centre). The coupling tool is used to simulate experiments representative of both natural circulation conditions and isothermal gas enhanced (assisted) circulation. Furthermore, an accidental test reproducing an Unprotected Loss of Flow (ULOF) scenario is also simulated and the outcomes are presented. A preliminary sensitivity analysis has shown that, to guarantee a suitable numerical convergence, the assisted circulation tests require a time step one order of magnitude lower compared to natural circulation ones. The comparison between the RELAP5 stand-alone simulations and RELAP5/FLUENT coupled simulations proved the capability to simulate the thermal-hydraulic behaviour of a loop experimental facility for all the examined conditions

The CFD code karalis

Karalis is a paralle MPI, Finite-Volume, multiblock CFD code which solves the fully compressible Euler and Navier-Stokes equations where all couplings between dynamics and thermodynamics are allowed. This the most general mathematical model for all fluid flows. The code solves the coupled system of continuity, momentum and full energy equation for the velocity components, pressure and temperature. Once, u, v, w, p are and T are updated, arbitrary thermodynamics is supplied. The second order Roe’s upwind TVD scheme is used to compute convective fluxes through the Finite-Volume cell interfaces. A V-cycle Coarse Grid Correction Multi-Grid algorithm is used, together with a 5-stage Runge-Kutta explicit time-marching method, to accelerate convergence to a steady state. This formulation, typical of aerodynamic flows, shows an eccellent efficiency even for incompressible flows as well as for flows of incompressible fluids (typically buoyancy flows), once equipped with a preconditioner. Merkel’s preconditioner has been chosen because it can be easily formulated for arbitrary equations of state given as a functional relation of two independent thermodynamic variables (typically the pressure p and the temperature T), or even in tabular form, read in as an input file and used with bilinear interpolation. Karalis implement two among the most popular turbulence models, namely the one-equation model by Spalart and Allmaras and the two-equations model by Wilcox, the k-ω model, which allow a good compromise between accuracy, robustness and stability of turbulent calculations. Code validation is presented for some typical benchmark test cases of incompressible fluid dynamics. Comparison with solutions obtained with a few popular commercial CFD codes is also presented

An analytical model of heat transfer and fluid dynamic performances of an unconventional NTR engine for manned interplanetary missions

An analytical model of fluid flow and heat transfer of a Nuclear Thermal Rocket (NTR) engine concept is presented. The engine is based on the direct conversion of the kinetic energy of the fission fragments (FFs) into the propellant enthalpy. The FFs can escape from an extremely thin layer of fissionable material: a sufficiently large surface coated with few micrometers of Americium 242m, confined by a neutron moderator–reflector, may become a critical reactor. Three dimensional coupled CFD-Monte Carlo simulations have already been presented in Di Piazza and Mulas (2006). In this paper, an analytical integral 1-D model of fluid dynamics and heat transfer is built in order to foresee the performances on the basis of simple, physically founded correlations. The Peclet number has been identified as the main governing parameter of the system, and theoretically based correlations have been found for the thermodynamic efficiency of the engine and for the specific impulse. The correlations show a good agreement with numerical results presented in Di Piazza and Mulas (2006) from fully coupled 3D CFD-Monte Carlo calculations