Turbulent dispersion of gas bubbles in quasi-two-dimensional MHD turbulence

The transport properties of the liquid metal two-phase flow are strongly modified due to the influence of an external magnetic field.
Experimental investigations of the gas dispersion in a turbulent channel flow have been performed at the sodium loop of the Forschungszentrum Rossendorf applying a transverse magnetic field and the mercury facility located at the Institute of Physics Riga/Salaspils (Latvia) producing a magnetic field along the mean flow direction by means of a solenoid.
Small Argon bubbles have been injected into the liquid metal flow by a rectangular test section. A single orifice positioned in the centre of the channel cross section just at the entrance of the flow into the magnetic field has been used to inject the gas. The local void fraction has been measured in a distance of 300 mm (sodium flow) and 500 mm (mercury flow) downstream from the gas injector by means of resistive probes.

Transverse magnetic field

The figures show isoplots of the cross sectional void fraction distributions obtained for growing values of the magnetic field strength at Reynolds numbers of 9300 and 18600, respectively.

                  Re = 9300                                  Re = 18600

In the ordinary hydrodynamic case (Ha = 0) the void fraction shows a distinct tendency to a uniform distribution over the cross sectional area. The application of a magnetic field causes a concentration of the gas bubbles in the channel centre. This concentration process advances further with increasing field strength indicating a significant damping of the turbulent motion.
The fact that the damping of the turbulent bubble dispersion parallel to the field lines is much more pronounced than in the perpendicular direction reveals the anisotopic effect of the electromagnetic force. The mass transfer properties are determined by the quasi-two-dimensional turbulent structure of the flow.
For sufficiently large values of the magnetic interaction parameter (N ~ 800) a relaminarization of the flow is observed. The gas phase is again isotropic and confined to a narrow channel region above the position of gas injection.

Longitudinal magnetic field

In the longitudinal magnetic field configuration no breaking of the symmetry occurs by the applied field. We also find a significant concentration of the gas phase with growing field strength due to the electromagnetic damping of the turbulent fluctuations. In contrast to the transverse case the void fraction distributions remain isotropic.

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