MHD pressure drop at bare welding positions in pipes of DCLL blankets (KIT Scientific Reports ; 7636)

A systematic parametric analysis has been performed using asymptotic numerical methods for determination of MHD flows near gaps of electrically insulating inserts in well conducting pipes. Such gaps could be present at several positions in fusion blankets, where cutting and rewelding by remotely controlled tools is foreseen. Gaps in the insulation provide additional current paths which leads to increased current density and braking electromagnetic Lorentz forces

Geometric Optimization of Electrically Coupled Liquid Metal Manifolds for WCLL Blankets

A number of previous theoretical and experimental studies for helium-cooled or water-cooled lead lithium (WCLL) blankets show that the major fraction of magnetohydrodynamic (MHD) pressure drop in the breeder flow originates from manifolds that distribute and collect the liquid metal into and from the breeder units (BUs). Moreover, those studies revealed that without a proper design of the manifolds, the flow partitioning among breeder units would be strongly nonuniform along the poloidal direction. In the present work, MHD flows in electrically coupled liquid metal manifolds are studied by using an efficient hybrid model that has been developed for prediction of MHD pressure drop in such geometries and for determining flow distribution in BUs. The tool combines global mass conservation and pressure drop correlations with detailed 3-D simulations. From the experience gained when applying the model to the geometry of a test blanket module (TBM), it is concluded that the design of the manifolds requires optimization for achieving a balanced flow partitioning among BUs. In the second step, the hybrid model is applied to determine the optimum position of the baffle plates that separate the feeding and collecting ducts in manifolds in order to guarantee comparable flow rates in all BUs

MHD Flow in Curved Pipes Under a Nonuniform Magnetic Field

In fusion reactors, a very hot deuterium–tritium plasma is confined in a toroidal volume by means of a strong magnetic field. In the blanket structure that surrounds the fusion plasma, high-energy neutrons, produced in the D-T fusion reaction, are absorbed by the lithium-containing liquid metal releasing their kinetic energy in the form of volumetric thermal load and breeding the fuel component tritium. The liquid metal flows from the blanket toward external ancillary systems for purification and tritium extraction. When the electrically conducting fluid moves in the strong plasma-confining magnetic field, induced electric currents generate electromagnetic Lorentz forces, which modify velocity distribution and increase pressure losses compared with hydrodynamic flows. These magnetohydrodynamic (MHD) effects have to be investigated to determine their impact on blanket performance. A number of studies on pressure -driven and buoyant MHD flows in geometries related to blanket modules are available, while only few works consider MHD flows in pipelines connecting blanket and ancillary systems. In the present study, we investigate numerically liquid metal MHD flows in the pipes, which cross the shield that protects the superconducting magnets from neutron radiation-induced damages. The geometry features two bends in series that turn the flow from the radial direction perpendicular to the magnetic field into a direction parallel to it and then back to a perpendicular orientation. The correct radial distribution of the magnetic field, as expected along the pipe axis, is taken into account. The flow experiences strong 3-D effects caused by Lorentz forces due to large-scale current loops driven by axial potential differences along the bend axis. In spite of very strong local MHD effects on velocity and pressure distribution, the overall pressure drop does not increase significantly compared with the one in a fully developed flow in a straight pipe of same length

MHD flow in a prototypical manifold of DCLL blankets (KIT Scientific Reports ; 7673)

Critical issues for the feasibility of dual coolant lead lithium blankets are large pressure drop and flow imbalance in parallel ducts due to 3D induced electric currents and 3D magnetohydrodynamic (MHD) phenomena that occur in liquid metal manifolds. In the present work we simulate MHD flows in a manifold where the liquid metal is distributed from a single duct into three parallel channels. The aim is identifying sources of flow imbalance, predicting velocity and pressure distribution

The effect of finite-conductvity Hartmann walls on the linear stability of Hunt's flow

We analyse numerically the linear stability of the fully developed liquid metal flow in a square duct with insulating side walls and thin electrically conducting horizontal walls with the wall conductance ratio $c=0.01\cdots 1$ subject to a vertical magnetic field with the Hartmann numbers up to $Ha=10^{4}.$ In a sufficiently strong magnetic field, the flow consists of two jets at the side walls walls and a near-stagnant core with the relative velocity $\sim(cHa)^{-1}.$ We find that for $Ha\gtrsim300,$ the effect of wall conductivity on the stability of the flow is mainly determined by the effective Hartmann wall conductance ratio $cHa.$ For $c\ll 1,$ the increase of the magnetic field or that of the wall conductivity has a destabilizing effect on the flow. Maximal destabilization of the flow occurs at $Ha\approx30/c.$ In a stronger magnetic field with $cHa\gtrsim 30,$ the destabilizing effect vanishes and the asymptotic results of Priede et al. [J. Fluid Mech. 649, 115, 2010] for the ideal Hunt's flow with perfectly conducting Hartmann walls are recovered.Comment: 11 pages, 6 figures, (minor revision, to appear in J Fluid Mech). arXiv admin note: text overlap with arXiv:1510.0922

Magnetoconvection in HCLL Blankets (KIT Scientific Reports ; 7672)

The present numerical study aims at clarifying the influence of electromagnetic and thermal coupling of neighboring fluid domains on magneto-convective flows in geometries relevant for the helium cooled fusion blanket concept

Numerical simulations of MHD flow transition in ducts with conducting Hartmann walls : Limtech Project A3 D4 (TUI) (KIT Scientific Reports ; 7713)

Pressure-driven magnetohydrodynamic duct flows in a transverse, wall-parallel and uniform field have been studied by direct numerical. The conducting Hartmann walls give rise to a laminar velocity distribution with strong jets at the side walls, which are susceptible to flow instability. The onset of time-dependent flow as well as fully developed turbulent flow have been explored in a wide range of parameters

Linear stability of magnetohydrodynamic flow in a square duct with thin conducting walls

This study is concerned with numerical linear stability analysis of liquid metal flow in a square duct with thin electrically conducting walls subject to a uniform transverse magnetic field. We derive an asymptotic solution for the base flow which is valid not only for high but also moderate magnetic fields. This solution shows that for low wall conductance ratios $c\ll1,$ an extremely strong magnetic field with the Hartmann number $Ha\sim c^{-4}$ is required to attain the asymptotic flow regime considered in the previous studies. We use a vector stream function/vorticity formulation and a Chebyshev collocation method to solve the eigenvalue problem for three-dimensional small-amplitude perturbations in ducts with realistic wall conductance ratios $c=1,0.1,0.01$ and Hartmann numbers up to $10^{4}.$ As for similar flows, instability in a sufficiently strong magnetic field is found to occur in the side-wall jets with the characteristic thickness $\delta\sim Ha^{-1/2}.$ This results in the critical Reynolds number and wavenumber increasing asymptotically with the magnetic field as $Re_{c}\sim110Ha^{1/2}$ and $k_{c}\sim0.5Ha^{1/2}.$ The respective critical Reynolds number based on the total volume flux in a square duct with $c\ll1$ is $\bar{Re}_{c}\approx520.$ Although this value is somewhat larger than$\bar{Re}_{c}\approx313$ found by Ting et al. (1991) for the asymptotic side-wall jet profile, it still appears significantly lower than the Reynolds numbers at which turbulence is observed in experiments as well as in direct numerical simulations of this type of flows.Comment: 18 pages, 9 figures, final versio