EXPERIMENTAL-STUDY OF MAGNETOHYDRODYNAMIC FLOWS IN ELECTRICALLY COUPLED BENDS

An experimental study of a magnetohydrodynamic flow in a system of n ( n less than or equal to 5) U-bends is presented. The bends are electri cally coupled via common electrically conducting walls parallel to the external magnetic field. In the test section the fluid flows perpendi cular-parallel-perpendicular to the magnetic field. The Hartmann numbe r M varies in the range 6 x 10(2) less than or equal to M less than or equal to 2.4 x 10(3), and the interaction parameter N in the range 10 (2) less than or equal to N less than or equal to 4.3 x 10(4). The exp erimental data for the wall electric potentials and the pressure have been compared with the theoretical asymptotic values calculated for N much greater than M-3/2 much greater than 1. This assumption in theory ensures the inertialess nature of the flow. For n = 1 the agreement b etween the theory and the experiment is good. With increasing number o f bends quantitative (for n = 3) and then qualitative (for n = 5) disa greement appears. For the first time in strong-field magnetohydrodynam ics this disagreement has been observed on the Hartmann walls, i.e. wa lls perpendicular to the field. The experimental results for the wall potential indicate that for M = 5 in some of the ducts parallel to the field qualitatively different how 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 inve stigated there is only a slight tendency of the wall potential to appr oach theoretical values. This demonstrates the stability of the new fl ow pattern and that even such high values of N as 4.3 x 10(4) are insu fficient for the core flow to be inertialess. A strong dependence of t he pressure drop on N has been observed in all the flow configurations investigated. The dependence of the inertial part of the pressure dro p in each bend scales with N-1/3, as long as N-1/3 much less than 1. T his is characteristic of electromagnetic-inertia interaction in the bo undary 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 be nds have also been studied.