Experimental study of magnetohydrodynamic flows in electrically coupled bends

Abstract

An experimental study of a magnetohydrodynamic flow in a system of n (n[les ]5) U-bends is presented. The bends are electrically coupled via common electrically conducting walls parallel to the external magnetic field. In the test section the fluid flows perpendicular–parallel–perpendicular to the magnetic field. The Hartmann number M varies in the range 6×102[les ]M[les ]2.4×103, and the interaction parameter N in the range 102[les ]N[les ]4.3×104. The experimental data for the wall electric potentials and the pressure have been compared with the theoretical asymptotic values calculated for N[Gt ]M3/2[Gt ]1. This assumption in theory ensures the inertialess nature of the flow. For n=1 the agreement between the theory and the experiment is good. With increasing number of bends quantitative (for n=3) and then qualitative (for n=5) disagreement appears. For the first time in strong-field magnetohydrodynamics this disagreement has been observed on the Hartmann walls, i.e. walls perpendicular to the field. The experimental results for the wall potential indicate that for n=5 in some of the ducts parallel to the field qualitatively different flow patterns are established than those predicted by the asymptotic inertialess theory. The flow in the core depends on N, i.e. is of inertial nature. In the whole range of N investigated there is only a slight tendency of the wall potential to approach theoretical values. This demonstrates the stability of the new flow pattern and that even such high values of N as 4.3×104 are insufficient for the core flow to be inertialess. A strong dependence of the pressure drop on N has been observed in all the flow configurations investigated. The dependence of the inertial part of the pressure drop in each bend scales with N−1/3, as long as N−1/3[Lt ]1. This is characteristic of electromagnetic–inertia interaction in the boundary and internal layers parallel to the field. A linear increase of the pressure drop with the number of coupled bends has been observed, confirming qualitatively previous theoretical results. The effects of magnetic field inclination and different flow distribution between bends have also been studied.