Lithium Jet Hydraulics
By Valeriy Kolesnik, Alexander Mikheyev, and Nikolay Loginov, Institute for Physics and Power Engineering, Obninsk, Russia
Target assembly of the IFMIF: the deuteron beam strikes liquid lithium and produces |
For more than fifty years, scientists have been striving to achieve the controlled fusion of light elements (hydrogen isotopes) into heavier products(mostly helium, with the additional production of energetic neutrons) as a means of producing energy. Several concepts have been under study at laboratories around the world, and have met with varying degrees of success. One key ingredient of a probable fusion reactor of the future is a blanket of lithium lining the interior walls. The lithium will absorb the energetic neutrons, fueling a heat exchanger for the production of electricity, and will also produce tritium, a hydrogen isotope that can be used in subsequent fusion reactions. In addition to lithium, other materials will be present inside the reactor, and their response to continued bombardment by neutrons is the subject of ongoing research.
The path of the liquid lithium through the target assembly |
Outline of the CFD domain, and detail of the mesh in the nozzle region |
The International Fusion Materials Irradiation Facility (IFMIF) is currently being planned by Japan, Russia, the EU, and the United States to test materials for their potential use in fusion reactors of the future. At the IFMIF,a beam of energetic neutrons will strike samples of various materials to test their performance under likely reactor conditions. The neutrons will be produced in a target assembly by an accelerated beam of deuterons (deuterium nuclei) that strikes a liquid lithium jet. Since the lithium jet will be exposed to unusual conditions, such as vacuum and imposed curvature, numerical simulations are being carried out to better understand its behavior.
The lithium jet will be emitted from a special nozzle and will flow at high velocity along a concave surface inside the target assembly area. While the outside surface of the jet will be bounded by a rigid wall, the inside (free) surface will be exposed to a vacuum. At this stage of the research, 2D isothermal simulations of the jet flow have been carried out using FIDAP. The studies have examined the flow inside the nozzle and along the free surface of the jet, as well as the influence of the nozzle edge angle on the shape and behavior at the jet free surface.
The target assembly through which the lithium flows consists of:
- A flow straightener;
- A two-stage convergent nozzle [1,2];
- A curvilinear section
The initial calculation corresponds to an experimental model where the nozzle edge is oriented at an angle of 62°30' from the test chamber wall. A uniform velocity profile is set at the inlet to the flow straightener. Inside the flow passages, the walls are assumed to be ideally smooth and completely wetted by the lithium. At the jet free surface, the vacuum condition is represented by a pressure of0.001 Pa. The material in the vacuum region is assumed to consist of non-condensable gases that are chemically passive to lithium. The turbulent jet flow through the assembly is captured using the standard k-ε model. The results show that the liquid is squeezed and accelerated where the channel in the nozzle first narrows, and again at the second stage of the nozzle. The mean jet velocity at the nozzle outlet is 20m/s, and the pressure drop across it is about 100 kPa. Close inspection of the liquid velocity in the nozzle edge zone shows tiny drops of liquid leaving the free surface, even though the calculations did not estimate the size and frequency of these drops.
Contours of velocity magnitude for the lithium jet after 0.013 seconds for a nozzle edge angle of 62°30' (left), Turbulent kinetic energy for a nozzle edge angle of 62°30'(center), Deformation of the jet velocity field at the vacuum interface, where the channel curvature changes (right) |
A calculation of that part of the jet volume occupied by the liquid is also of great interest to researchers. Described by the function F, F=1 corresponds to the case when all the volume is filled with liquid, and F=0to the case when the given volume is empty. The results show that there is no vacant space in the bulk of the jet volume or adjacent to the wall. At the jet free surface, however, a significant drop in F occurs in a 1-2 mm thick layer, which is associated with wave generation. The perturbations at the jet surface in this region develop as a result of the curvilinear path that the jet is forced to take. The curvature in the wall geometry behind the jet leads to a radial acceleration of the lithium and a subsequent centrifugal force of the liquid on the wall. The liquid near the wall is compressed quickly, but to conserve the jet volume,the jet surface deforms. Following the change in curvature, the thinned jet profile increases gradually along the remainder of the wall.
References
- Ida, M. et al.: Thermal-Hydraulic Characteristics of IFMIF Liquid Lithium Target. Fusion Eng. Des., 63-64,2002.
- Shima, A.: Theory of Direct and Inverse Methods to Obtain Nozzle Shape. Mem. Inst. High Sp. Mech.,Japan., Vol.17, No. 164, p.61-86, 1961/1962.
- Ida, M; Nakamura, H; Ezato, K; Takeuchi, H.:IFMIF_WalkThrough03_0317, JAERI, Japan, 2004.
- www.jaeri.go.jp/english/index.cgi
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