MHD turbulence measurements in a sodium channel flow exposed to a transverse magnetic field

The knowledge about properties of the anisotropic turbulence in liquid metal flows exposed to an external magnetic field is essential for several technological applications such as the concept of a self-cooled liquid metal blanket for thermonuclear fusion reactors or the electromagnetic brake in metallurgical processes, but also of basic interest for turbulence research. Flow arameters like pressure drop and heat transfer rate are essentially determined by the interaction between the external magnetic field and the liquid metal flow. Several experimental studies [1-3] revealed that the application of a magnetic field leads not exclusively to a suppression of the turbulent perturbations. Velocity fluctuations remain and demonstrate a distinct anisotropy of the MHD turbulence showing a tendency to become two-dimensional. The properties of the local mass transfer are strongly
governed by the anisotropic character of the flow [4,5]. In this context a number of questions arises regarding the origin, the decay time or the size of the turbulent elements. A typical feature of the two-dimensional turbulence is that the energy becomes concentrated in organised large scale fluctuations. This fact ensures an intensive heat transfer on longer distances. Because of the anisotropy of the electromagnetic dissipation term vortices will be scarcely damped over long distances if their axes are aligned with the magnetic field lines [6]. For practical applications it is important to identify possibilities to promote the formation of such quasi-two-dimensional vortices in order to control the heat or mass transfer rate of the flow. It is well-known that the mentioned perturbations can be generated by rather specific means, for example, mechanical inserts or electrical currents between definitely arranged electrodes, etc..
In our experiments we force the turbulence intensity by mechanical means employing a grid of cylindrical bars or flat stripes. Due to the favourable material properties of the used liquid sodium we are able to extend the measurements into the region of high interaction parameters. In this way the obtained results complement the already existing knowledge about the two-dimensional MHD turbulence. The results concerning the measured turbulence intensity as a function of the interaction parameter N and the corresponding power spectral densities will be presented. Electrical potential probes were used to determine the velocity fluctuations. The measurements took place in two different test sections allowing either the variation of the axial distance between the turbulence promoter and the probe or the installation of special channel inserts in order to suppress the generation of the M-profile of the local velocity and to decrease the wall conductance ratio. The results obtained in both test sections will be compared and the influence of the altered boundary conditions onto the turbulent structure of the flow will be discussed.