Local measurements in a liquid-metal two-phase flow under the influence of a magnetic field. Analytical study of end effects in liquid-metal magnetohydrodynamic generators

The first part of the present work is an experimental investigation of the influence of an imposed magnetic field on the local flow pattern in (a) a mercury-nitrogen pool experiment and (b) in a vertical mercury-nitrogen pipe flow. The emphasis was put on demonstrating the feasibility of using the hot-film technique for simultaneous measurements, in a liquid metal-gas mixture, of both the liquid phase and gas phase parameters. The two-phase pool facility was used for investigating the effect of the injection of gas bubbles on the turbulent intensity in the pool. Preliminary measurements in water-nitrogen have shown the turbulent intensity to increase as a function of the square-root of the local void fraction. In the measurements with mercury-nitrogen, the turbulent intensity was found to be about twice higher than in water at the same void fraction, and to increase with the void fraction at a lesser rate than the 1/2 power. The spectral distribution of the turbulent fluctuations exhibited, in two-phase flow, a region with a negative slope of about (-2.5) as opposed to the (-5/3) slope known from the single-phase turbulence spectra. The magnetic field was found to be less effective than in single-phase flow in suppression the turbulent fluctuations; this effect is believed to be due to the buoyant energy supplied continuously to the flow by the gas bubbles. In the mercury-nitrogen pipe flow, the magnetic field was found to cause dramatic changes in the void distribution of a bubbly flow in which asymmetric conditions existed at the inlet (mixing) section. This effect was attributed to the suppression of the bubble dispersion across the test section. In the second part of the work, a mathematical model of a MHD generator channel was used to study the reduction of end effects by changing the geometry of the channel, the magnetic field distribution or by inserting electrically insulating vanes in the magnetic end regions. (author)