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

Characterization of Isogrid Structure in GFRP

Lightening parts, maintaining also a high strength, is a request of the transport industry. Isogrid structures represent one of the best answer to face these issues, especially if composite materials are considered for their production. However, the fabrication of these structures is very complex, as defects can arise that cause the part discarding or the part failure during service. The properties of the fabricated structure depend on some process characteristics, as the forming technology, the process parameters and the tools that have to be wisely designed. Isogrid structures are characterized by the ribs, so the mould shape must be carefully planned. In fact, a common defect that usually occurs is a scarce compaction of the ribs, which involves porosity and low mechanical strength. In this paper, the manufacturing process peculiarities for GFRP (Glass Fibre Reinforced Polymer) isogrid structures were defined, then both the mould and the parts were produced. Structural tests were carried out on these structures in order to validate the process design methodology, paying particular attention to the structural properties of the ribs, as the compaction degree and the interlaminar shear strength. Finally, some actions were undertaken to avoid the problems found in the first production run

Cutting Forces in Milling of Carbon Fibre Reinforced Plastics

The machining of fibre reinforced composites is an important activity for optimal application of these advanced materials into engineering fields. During machining any excessive cutting forces have to be avoided in order to prevent any waste product in the last stages of production cycle. Therefore, the ability to predict the cutting forces is essential to select process parameters necessary for an optimal machining. In this paper the effect of cutting conditions during milling machining on cutting force and surface roughness has been investigated. In particular the cutting force components have been analysed in function of the principal process parameters and of the contact angle. This work proposes experimental models for the determination of cutting force components for CFRP milling

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

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

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)

Uniformity of thickness of metal sheets by patchwork blanks: potential of adhesive bonding

The sheet metal forming operations generally involve the production of parts characterized by a non-uniform thickness distribution. However, in some cases, a product characterized by a distribution of thicknesses that is as uniform as possible may be desirable. This result can be obtained by using multiphase processes or by subtraction or addition of material from the blank. In this work, which deals with the method for adding material, an innovative methodology has been proposed as an alternative to the welding process. Specifically, the methodology is based on the bonding of a patch (before the deformation process), on the base plate with a constant thickness, in the area that most suffers from the thinning caused by the forming process. In this way, it was possible to influence the deformation of the patchwork blank and its thicknesses distribution. Through finite element analysis, it was possible to study the formability of a patchwork blank by varying the thickness and size of the patch, in order to produce an axially symmetric component by stretching through a hemispherical punch. Preliminary experimental tests demonstrated the reliability of the bonding and the potential of this method to uniform the final thickness of the sheet