Liquid metal magnetohydrodynamic mixed convection flow in a rectangular channel with volumetric heating and internally aligned cooling pipes

Liquid metals containing lithium are candidate working fluids for implementation in technological demonstrators and first-of-a-kind nuclear fusion power plants. Used as tritium breeder and carrier in breeding blankets, liquid metals are tasked with producing enough tritium to ensure the machine fuel self-sufficiency and are characterized by excellent breeding potential and the possibility to perform in-line extraction and composition control. In some breeding blanket concepts, the intense volumetric heating produced by nuclear breeding reactions in the liquid metal is extracted by a secondary coolant, like water or helium, to minimize the liquid metal velocity and magnetohydrodynamic (MHD) effects arising due to its motion in presence of the intense magnetic fields necessary for the machine operation. This study numerically investigates the MHD flow expected in a liquid metal breeding blanket that is internally refrigerated by a large number of cooling pipes aligned with the streamwise direction. In particular, this work address the effect of the interaction between buoyancy, inertial and electromagnetic forces in a prototypical configuration for an upward flow characterized by both intense uniform volumetric heating and static applied magnetic field. Forced and mixed convection MHD cases are simulated to predict the flow pattern and estimate the thermal–hydraulic performance in terms of pressure drop and heat transfer in reactor-relevant conditions. The pressure drop is found to be slightly higher (20%) for a mixed convection MHD flow compared with a forced convection case at strong magnetic field intensity due to the additional dissipation caused by buoyancy-aiding and buoyancy-opposing flow regions. Large recirculation bubbles are observed close to the pipes suggesting a potential risk in terms of enhanced tritium permeation toward the coolant. Heat transfer is mostly dominated by conduction, while advection plays only a minor role. The governing parameter Ri/N is found to be extremely important to preserve the thermal–hydraulic performance of the system when performing simulations at a lower magnetic field intensity to reduce computational cost.