TEFLU computational benchmark

This document summarises the results obtained for the WP2 (TEFLU benchmark) of the ASCHLIM project. The main task of the benchmark was the assessment of the limitations of an eddy viscosity approach for HLM flows. The test case set-up and boundary conditions are described, highlighting some problems coming from the definition of inlet boundary conditions for turbulence quantities. A summary of the approaches followed and of the results obtained by each participant is then reported, making reference to the full reports annexed in the Appendix. Some general conclusions are drawn, as a synthesis of the conclusions reached by every participant. The main conclusion is that an approach based on the Reynolds analogy with Prt ∼ 1 is not able to simulate the correct temperature spreading rate of the TEFLU experiment, and, more in general, it is unsuitable for low Peclet number flows. Higher values of Prt, possibly as a function of local flow characteristics should be used, or higher order methods, not based on the Reynolds analogy. The tendency of two-equation models to overestimate the velocity spreading-rate is confirmed, even if definite conclusions about this point can not be drawn, due to the uncertainties in the inlet turbulence conditions

Assessment of turbulence models for heavy liquid metals in computational fluid dynamics

A summary and analysis of the results obtained from work-packages 1-6 of the ASCHLIM project is presented in this document, with the aim of defining the performances and shortcomings of CFD turbulence models currently adopted for the simulation of Heavy Liquid Metals flows in nuclear applications. Two classes of problems are analysed, one related to liquid metals physical characteristics (low Prandtl number) and one related to the flow morphology in typical ADS applications. Concerning the first class, some drawbacks were found in the use of wall-functions for HLM flows. In fact, thermal wall-functions currently implemented in commercial CFD codes are in general unsuitable for HLM flows, unless the first grid point lays in the thermal sublayer y+ < 70 ÷ 100. It was also proved that the Reynolds analogy is not applicable for very low Peclet number flows (~100). However, correct results were obtained for higher Pe (~1000), even using a constant value for the turbulent Prandtl number (0.9). Due to the lack of experimental measurements of turbulence quantities, it was not possible to draw strong conclusions concerning the second class of problems. However, results confirmed the capability of two-equation models to give a reasonably good prediction of the main flow characteristics in complex flow morphology typical of spallation-targets applications. Higher order models, both for momentum and heat turbulence transport, should be used in cases where turbulence anisotropy is important

CFD simulation of a free-surface isothermal water flow in the myrrha target geometry (MYRRHA benchmark)

The simulation of the MYRRHA free-surface water experiment using an Arbitrary Eulerian-Lagrangian Moving Mesh Algorithm implemented in the Star-CD commercial CFD code is presented. In spite of the complexity of the surface morphology, relatively to this type of approach for free surfaces modelling, results show that the algorithm is able to give quantitatively correct results in terms of velocity profiles. Although no measurements of the surface shape are available, it can be guessed that it is well predicted. Better results are obtained without taking into account surface tension, whose effects seem to be overestimated, probably due to the presence of surface discontinuities in the real free surface

Thermal fluid dynamic analysis of the MEGAPIE target heat exchanger in steady state and transient conditions

The MEGAwatt PIlot Experiment (MEGAPIE) project has the aim to demonstrate the feasibility of a liquid Lead-Bismuth target for spallation facilities at a beam power level of 1 MW. About 650 kW of thermal power has to be removed from the target through a bunch of 12 pin-coolers, using a diathermic oil as secondary coolant. In order to improve the heat exchange in the oil side, a spiral is introduced in the oil riser, which enforces a swirling flow thus increasing the Reynolds number and the heat transfer coefficient. A single-pin experimental rig was set up and tested in the ENEA-Brasimone facility. A detailed CFD simulation of the experiment was carried out by CRS4, showing the capability of the simulation to give a quantitatively correct prediction of the heat exchange mechanisms in the cooling pin. In order to analyse the performances of the actual Megapie Target Heat eXchanger (THX), the thermal-hydraulic simulation of a sector of the THX, including only one of the 12 pin coolers, has been carried out by CRS4 in steady state and transient conditions, with the assumption of periodical flow-conditions along the THX circumference. The main tasks of this work were the assessment of the global performances of the THX and the evaluation of the thermal field in the solid structure, to be used successively for the calculation of thermal stresses in the target structures

CFD simulation of an isothermal water flow in the EADF-target geometry in dynamical similarity (COULI experiment)

The results of the numerical simulation of an isothermal water-flow in a geometry typical of the spallation target of the Energy Amplifier Demonstration Facility (EADF) are presented, and compared with the experimental results obtained at CEA-Cadarache (COULI experiment). All the calculations were performed with the Star-CD finite-volume commercial code. Basically, a 2D axisymmetric model was adopted, although a full 3D simulation was carried out as well, as explained below. The Chen k-ε high-Reynolds model was used, joined with a Norris & Reynolds Two-Layer model for the simulation of the near-wall turbulence. The grid-independence of the solution has been verified, and the results obtained with two different convection schemes (QUICK and MARS) has been compared, in order to minimise numerical uncertainties. No appreciable differences were found in the results. Because of the fact that experimental measurements revealed a high non-axisymmetric and non stationary flow behaviour, the possible presence of instabilities intrinsic in the flow topology has been analysed through a full 3D simulation, obtained with the circumferential extrusion of the 2D model. The simulation yielded a steady solution, with results in perfect agreement with the 2D case. It is worth to notice that it was possible to obtain a converged solution in the 3D case only using the MARS scheme, while the QUICK scheme had numerical problems. In spite of the above-mentioned lack of axial symmetry in the experimental set-up, the comparison with computational results showed the capability of Star-CD to correctly simulate the main flow characteristics

Optimisation of the pin cooler design for the megapie target using full 3D numerical simulations

The MEGAwatt PIlot Experiment (MEGAPIE) project has been recently proposed to demonstrate the feasibility of a liquid lead bismuth target for spallation facilities at a beam power level of 1 MW. The target will be put into operation at the Paul Scherrer Institut (PSI, Switzerland) in 2004 and will be used in the existing target block of SINQ. About 650 kW of thermal power has to be removed through a bunch of 12 pin-coolers. In order to improve the heat exchange, it was decided to investigate the possibility of accelerating the oil coolant by introducing a spiral in the oil cylindrical channel. This forces the flow to rotate while rising, thus increasing the Reynolds number and the heat transfer coefficient. We show some numerical simulations, which have supported the dimensioning of the pins as well as the choice of the secondary coolant, that is Diphyl THT. The spiral option has been confirmed. The spiral diameter must be a little smaller than the channel width, to allow the effective mechanical assemblage of the pin. The existence of a gap between the spiral and the external wall adds complexity to the numerical simulation, being fully 3D with several orders of magnitude of length scales involved. A single pin has been tested by Enea-Brasimone and entirely simulated by CRS4 for a matrix of various operational settings. Results are shown and compare

A windowless design for the target of the EADF

In this note, we review the main features of the windowless target requirement. Then, we derive some necessary characteristics of the flow. We also make some comments hopefully useful for an eventual design optimisation process. From a first analysis, it seems that the requirements imposed on the maximum temperature and the pressure losses can be met but care must be taken to avoid a buoyancy induced flow critical instability

Benchmark calculation of the MEGAPIE target (M1)

The benchmark calculations performed by CRS4 with Star-CD on a reference geometry of the MEGAPIE target are presented in this report (benchmark M1). Scope of the benchmark is a comparison of the results obtained by the various partners involved in the MEGAPIE project using different codes and turbulence modelling approaches. The considered target geometry is the one with the final part of the guide tube slanted at an angle of about 9 degrees. The Pb-Bi flow in the last 2150 mm of the target have been simulated, including the calculation of the thermal field in all the solid structures (window, hull and flow guide). Due to geometrical symmetry, only half of the real domain was considered. Turbulence was simulated using a Chen k-ε model, combined with a Two-layer model in the most critical near-wall regions (window and flow guide in the spallation region) and with Wall Functions along the riser and the down-comer. Modified wall functions for low Prandtl number fluids were implemented. Results are presented for both cases with the beam footprint major axis parallel (benchmark M1.0) and normal (benchmark M1.1) to the guide-tube slant. In order to estimate the effect of the variation of the turbulent Prandtl number on the heat exchange, two calculation have been performed, one with Prt = 0.9 and one using a relationship Prt = f(Ret, Pr), yielding a locally variable turbulent Prandtl number. Results show a very complex flow pattern in the spallation region, with 3D vortex structures being generated in the reversing region and dragged along the rising duct. In case M1.0 with Prt = 0.9, results show maximum window temperatures of 521 °C and 487 °C in the external and internal side respectively, with a maximum Pb-Bi temperature of 486 °C located nearby the window centre. The maximum flow velocity is 1.35 m/s. A significant heat exchange takes place across the 1.5 mm thick flow guide, causing a mean temperature increase along the down-comer of about 34 °C. Due to the high Reynolds number of the flow, the effect of using a variable Prt is limited to near wall regions, where the heat exchange is slightly reduced. The combination of a lower heat exchange across the flow guide (resulting in a lower temperature increase of the Pb-Bi along the down-comer) and a worse window cooling yielded a maximum window temperature of 524°C, namely 3 °C more than in the case with Prt = 0.9. In case M1.1, maximum window temperatures of 447 °C and 414 oC were found using Prt = 0.9 with a maximum Pb-Bi temperature of 423 °C located in the central part of the spallation region. Using a variable Prt, window temperatures increased of about 2 °C while a 1 °C lower maximum Pb-Bi temperature was found

Thermal analysis of the TOF lead target at CERN

The lead target at the Time Of Flight (TOF) facility at CERN, currently under commissioning, undergoes relevant temperature transients due to the intensity of the four 20 GeV/c pulses of 7 x 1012 protons, carrying an energy of 21.4 kJ delivered in 7 ns each. A 3D thermal analysis of the target system in both steady-state and transient conditions has been performed using the finite volume commercial code StarCD coupled with the results from Fluka simulations. Results show that the maximum temperature inside the lead target using the parameters of the TOF commissioning phase (4 pulses every 1.2 s in a 14.4 s super-cycle) is 127°C at steady-state operations, which is an acceptable value, compatible with safe and durable target operations. A significant improvement could be obtained by doubling the beam size (108°C maximum temperature in the bulk of the central block). The transients coming from the pulsed operation are not such as to create structural problems related to thermal fatigue. It is interesting to notice that the thermal oscillation in the hottest point in the bulk of the central block is much lower in the case where the 4 pulses are spaced of 3.6 s during the PS super-cycle (about 20°C), than in the case where they are spaced of only 1.2 s (about 40° C)

Analysis and optimisation of a gas-cooled pipe for solar thermal energy production using parabolic collectors

In the framework of the design of a solar thermal power plant proposed by ENEA, the activity carried out by CRS4 on the thermal-fluid-dynamic simulation of a gas-cooled pipe irradiated by a parabolic solar collector is described in this paper. Two methods have been adopted in parallel: a simplified one-dimensional approach and a Computational-Fluid-Dynamics (CFD) three-dimensional approach. The first method was used to build a tool able to give quick answers to parameters changes with an acceptable degree of accuracy. The CFD analysis is used both to validate 1D-model results and to study in details all 3D physical phenomena. The multi-zone one-dimensional model developed at CRS4 is described first. The pipe is split along its axis into a discrete number (typically 100) of sub-domains. Each sub-domain is split further on into five different zones, corresponding to the various components of the pipe. All the main energy-exchange mechanisms between the various parts of the pipe have been implemented, resulting in a system of five equations for each sub-domain, solved iteratively within an EXCEL framework. The 3D-CFD model is then described. The model is fully parametric, allowing a quick variation of the geometrical parameters of the system. Convection, conduction and thermal radiation heat exchanges are solved in a coupled way. The model can give any information about field variables of fluids and solid structures as well as all energy balances. The two models have been used for a parametric study of the effect of the pipe diameter variation on the system efficiency. Although not negligible differences between the two models can be noticed concerning local energy balances, the error on the evaluation of the system efficiency is order 1%. The result of the optimization was practically the same for the two models