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

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

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

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

A heat exchanger design for the separated window target of the EADF

The spallation target of the Energy Amplifier Demonstration Facility (EADF) [1] is cooled by a liquid lead-bismuth eutectic (LBE), while the secondary coolant is a diathermic oil. The reasons for these choices have been extensively discussed in [2] and [3]. Here we present the design and the optimisation of a heat exchanger using these fluids, whose additional requirements are the need of fitting into the top of the annular downcomer section of the target and the minimisation of the pressure losses on the LBE side, allowing the use of natural convection for the circulation of the primary fluid. Heat exchanger working temperatures are between 250 and 180°C in the LBE side, and between 150 and 190°C in the oil side (cold fluid), while the power to be removed is up to 3 MW. We selected a bayonet-type heat exchanger, as suggested in [4] for the primary loop of the EADF vessel, which seems to be the most appropriate choice to satisfy all the requirements

A thermal fluid-dynamic steady state analysis of the EADF downcomer channel

In this work a numerical simulation of the Energy Amplifier Demonstration Facility (EADF) [1, 2] downcomer channel is presented. The simulation is fully three-dimensional (3D) and is focused on a Steady-State Analysis. All relevant heat transfer phenomena are taken into account. The Intermediate Heat eXchangers (IHX) of the EADF reference configuration are immersed in the lead-bismuth eutectic of the downcomer and no physical barrier separates the hot and cold collectors. As expected, the simulation shows a thermal stratification outside the IHX, whose characteristics mainly depend on the IHX pressure loss. A parametric study of the effects of the IHX pressure loss coeficient on the thermal stratification pattern is presented

A beam window target design with independent cooling for the EADF

This report summarizes the study and the design of the window type target for the Energy Amplifier Demonstration Facility (EADF) [1]. The behaviour of the system in different condition has been analysed though extensive CFD simulations performed with the Star-CD commercial code [2]. The target represents one of the main technological problems related not only to the design of the EADF, but to all High Power Spallation Sources (HPSS) currently under study or in construction world-wide [4],[5]. Different configurations of the spallation EADF target are possible. Advantages and disadvantages of the different options are discussed elsewhere [6] and they are studied and analysed separately. The target device studied in this report is a window type target, cooled by diathermic oil in an independent loop. This target configuration is completely independent from the core operating conditions and gives advantages in terms of flexibility in the operation. The result of this report is a set of design data and constraints to take into account while engineering the spallation target