Natural convection is an important phenomenon in liquid metal blankets where the breeder velocity is minimized to avoid large magnetohydrodynamic (MHD) pressure losses, as it is the case for the Water-Cooled Lithium Lead (WCLL) concept. Movement of electrically conducting fluids in the presence of magnetic fields causes several fundamental changes in the flow structure. Among these, one should mention the formation of thin boundary layers, turbulence dampening, and convective cells alignment with the field direction. The resulting peculiar regime, called ¿magnetoconvection¿, has important consequences on the heat and mass transport phenomena happening in the blanket. This research proposal is concerned with "extreme" magnetoconvective conditions, which are established when strong magnetic fields and large temperature gradients interact, as it is often the case in fusion reactors. An extensive numerical campaign is proposed to investigate the extreme magnetoconvective regimes ensuing in a horizontally heated shallow cavity, which is representative of the WCLL elementary cell. This activity is expected to produce results that are going to drive the component design and will gain insights in the fundamental phenomena existing in this poorly characterized configuration. As a secondary objective, the numerical campaign will prove useful to support the preliminary design of a test section, which is planned to experimentally study the same phenomena treated by the simulations. The test section is planned to be realized in the new DIAEE laboratories in Via Salaria in the next years.
Only in the past 20 years the international scientific community has started to investigate systematically the complex phenomena appearing when strong magnetic field and large temperature gradient coexist at the same time. In light of this, there is a relative paucity of publications in the literature addressing extreme magnetoconvection and none at all considering the important practical case of a shallow cavity with internally refrigerated obstacles. This research proposal aims to contribute to the discussion by investigating this configuration with direct numerical simulation tools, thus producing accurate and detailed numerical results that can directly be exploited to guide the development of both WCLL ITER TBM and blanket.
Among the expected outcomes, there is the characterization of the heat transfer regime at Ha = O(10^4) and Gr = O(10^10) to assess the maintenance of the structural materials below the critical temperature T = 550 °C in normal operative conditions. Preliminary results have hinted to the fact that a slight decrease in the maximum temperature should be expected between a purely conductive and magnetoconvective regime [1]. This study will provide a more accurate and quantitative answer on this aspect, thus allowing to define the component safety margins more clearly.
Preliminary unsteady studies performed with ANSYS FLUENT have highlighted that stable Q2D magnetoconvective cells are established in the mixing vane with a negligible contribution from the forced convection [1]. In this work, it is aimed to further investigate the purely magnetoconvective problem in order to identify possible instabilities that could lead to high-amplitude/low-frequency temperature fluctuations and associated thermal stresses. In particular, large-scale convective cells, as the one identified in [1], could be susceptible to shear instability triggered at the external or cooling pipe wall. To the best of our knowledge, it would the first time that magnetoconvective instabilities will be investigated in a similar configuration.
Steady and unsteady characteristic magnetoconvective regimes are going to be identified by performing a parametric analysis on Ha and Gr. The outcome would eventually be the realization of a regime map like the one presented in [2] for the case of Rayleigh-Bénard magnetoconvection. This result could be used to have a first definition of the transition regions in this configuration and as a base to define an experimental test matrix in order to verify the findings of the numerical analysis and further explore the involved phenomena.
As mentioned, the proposed numerical simulations are going to be designed to provide useful information for the realization of a test section dedicated to the investigation of extreme magnetoconvection in WCLL-like geometries. If numerical results are scarce for even prototypical extreme magnetoconvection cases, experimental works addressing this kind of problem are even more rare. In this framework, this activity must be regarded as the first step in the realization of an experimental campaign which is going to explore extreme magnetoconvection in a shallow cavity with internal obstacles. Even if outside of the scope of this proposal, the realization of a first-of-a-kind test section like the one proposed has the potential to gather invaluable data for the development of liquid metal blankets. Several UDV sensors arranged to provide a comprehensive measurement of convective structures, as demonstrated in [2] and currently foreseen, have the potential to produce further insights in the characteristic heat transfer regime expected in the blanket normal operation. Results produced by this activity will determine if the convective structures expected in extreme conditions could be amenable to be measured by the proposed method.
Finally, it should be noted that experimental activities concerning liquid metal MHD are currently foreseen as part of the scientific collaboration between EU and Russian Federation proposed for the next EU 7-year framework program (FP9). Even if still in its early stages, it is likely that part of the collaboration will be focused on the realization of a WCLL mock-up at the NIEEFA Efremov in St. Petersburg. If funded, the proposed numerical study will allow the Nuclear Engineering group to start accruing know-how which will, in turn, make much more easier to assume a leadership role in EU-RU FP9 and secure external funding for the foreseen test section. Moreover, the latter could be uniquely posed to support the mock-up campaign by providing preliminary information about temperature distribution, convective structures, and possible sources of instability.
[1] Urgorri, F. R. et al., (2020) Support studies for WCLL BB in 2019, EFDA_D_2PCN47.
[2] Vogt, T., et al. (2018). Transition between quasi-two-dimensional and three-dimensional Rayleigh-Bénard convection in a horizontal magnetic field. Physical Review Fluids, 3(1), 013503.