Measurement and analysis of turbulent liquid metal flow in a high-power spallation neutron source-EURISOL

The European Isotope Separation On-Line (EURISOL) design study completed in 2009 examined means of producing exotic nuclei for fundamental research. One of the critical components identified in the study was a high-power neutron spallation source in which a target material is impacted by a proton beam producing neutrons by a process known as spallation. Due to the high heat power deposition, liquid metal, in this case mercury, is the only viable choice as target material. Complex issues arise from the use of liquid metal. It is characterised by an unusually low Prandtl number and a higher thermal expansivity than conventional fluids. The turbulence structure in LM is thereby affected and still an object of intense research, hampered in part by measurement difficulties. The use of Computational Fluid Dynamics (CFD) allowed a satisfactory design for the neutron source to be found rapidly with little iteration. However it was feared that the development of the boundary layer and associated turbulence would not be correctly represented by the CFD since the codes were developed for standard fluids. Therefore, an experiment and associated measurements were carried out to assess the reliability of the computations and to validate the design of the target. System level measurements such as cavitation noise, target structural vibrations and cover gas pressure were recorded as local pressure fluctuations in the fluid. In this manner, both the macroscopic effect of the turbulence on the target structure, and the liquid metal turbulence itself could be measured and serve as a benchmark for assessing the reliability of the computations. The current work shows how the Shear Stress Transfer (SST) and Large Eddy Simulation (LES) turbulence models were used to simulate the liquid metal flow. LES was proved more accurate than SST in predicting large and small turbulent structures in the liquid. The prediction of the turbulence intensity generated by large eddy structures in the fluid proved to be in agreement with experimental data at frequencies lower than 40 Hz and at speeds up to 6 m/s in the liquid metal. (c) 2011 Elsevier B.V. All rights reserved.