MHD-velocity distributions in U-bends partly parallel to the magnetic field

J. Reimann¹ - I. Bucenieks² - S. Dementev² - A. Flerov² - S. Molokov¹ - I. Platnieks²

¹ Association KfK-EURATOM, Kernforschungszentrum Karlsruhe GmbH, IATF, Postfach 3640. 76021 Karlsruhe, Germany
² Institute of Physics, Latvian University, Salaspils-1, LV-2169, Latvia

Abstract
A U-hend where the magnetic field is perpendicular to the inlet and outlet "radial" ducts and parallel to the "toroidal" duct in between is part of a recently proposed fusion reactor blanket. For heat transfer from the "first wall" of the toroidal duct the velocity distributions are of prime importance. Experiments in Hg were performed in a sharp-edged U-bend where both the size of the radial inlet duct and the wall conductance ratios C in the U-bend could be varied (C = 0, 0.06, 0.12). Maximum Hartmann numbers Ha and interaction parameters N were Hamax =- 460 and Nmax = 150. Measurements of the mean velocity and velocity fluctuations in different cross sections were performed using hot wires and electric potential probes. Theoretically, the flow distribution was predicted by the Core Flow Approximation (CFA), valid for Ha and N -> oo. With increasing Ha and N the fluid in the toroidal duct is increasingly pushed towards the side walls. (The CFA predicts that the liquid transport in direction of the duct axis occurs only in thin side layers.) In the core of the cross section a pair of strong vortices exist with axis parallel to the magnetic field. These vortices have transversal velocity components significantly larger than the mean duct velocity and exist over the total length of the toroidal duct.