Oleg Zikanov University of Michigan ? Dearborn
An applied constant magnetic field can completely change the structure and behavior of a flow of an electrically conducting fluid, such as a liquid metal, and the way in which the flow transports heat and reacts to temperature non-uniformities. The transformation is especially dramatic in the case of strong magnetic fields, as characterized by the values of the Hartmann number much larger than one. The starting point of the project is the finding of the recent experimental and computational studies of mixed convection in the pipe flow with transverse magnetic field. It has been demonstrated that the commonly accepted view that the anisotropic Joule dissipation of the induced electric currents leads to suppression of turbulence and establishing of a laminar flow with steady-state velocity and temperature fields is not always correct. There is a possibility that, while turbulence is suppressed, the buoyancy-induced instability to the perturbations uniform along the magnetic field lines survives and leads to development of large-scale unsteady coherent structures that cause anomalous temperature fluctuations characterized by high amplitudes and low dominant frequencies. The phenomenon has potentially disruptive and, at the moment, ignored implications for the Li-based cooling and breeding blankets for the nuclear fusion reactors. Should the anomalous fluctuations develop in the blanket, their amplitude can, in the presence of high thermal load, reach several tens of degrees. The resulting unsteady thermal stresses will cause rapid deterioration of the wall material. The objective of the proposal is to investigate the mechanisms leading to the anomalous fluctuations from the fundamental science perspective and to assess their likelihood in the configurations corresponding to the principal components of the blankets currently developed in the U.S. and Germany. The specific cases to be considered include the mixed convection in horizontal and vertical ducts with transverse temperature gradient and natural convection in rectangular enclosures. The research combines the high-resolution three-dimensional numerical simulations and experiments, both based on the recently developed methods that, for the first time, make the convection with strong magnetic field amenable to rigorous scientific analysis. The project is conducted in collaboration with the Ilmenau University of Technology and Karlsruhe Institute of Technology in Germany.
The project will explore the nature of the paradoxical strong fluctuations of temperature recently detected in flows of liquid metals subjected to very strong (several Tesla) magnetic fields. In addition to advancing the basic knowledge, particular attention will be paid to the danger of such fluctuations for the operation and structural integrity of the lithium blankets developed for nuclear fusion reactors and planned for testing at the currently constructed ITER (International Thermonuclear Experimental Reactor) facility. Using the massively parallel computations and novel numerical and experimental techniques, the project will clarify the mechanisms, by which the fluctuations develop, and identify the configurations of the blanket components that have to be avoided. The project will be undertaken as international collaboration with two universities in Germany. It will provide educational opportunities and exposure to international research environment to one graduate and several undergraduate students.
