Liquid lithium is expected to be the promising coolant for the next generation of space nuclear reactor power sources, and pipe structures have critical impacts on the flow and heat transfer characteristics, whereas related studies are still incomplete. In this work, a steady-state non-isothermal heat transfer model with modified turbulent Prandtl number is developed for the turbulent flow of liquid lithium in elbows with different bends. The results demonstrate that liquid lithium has the best performance in a 90o bend in the Pe range of 320-955, which has the highest heat transfer performance per unit pressure drop and exhibits the lowest entropy generation. Due to the difference in flow velocity between the inner and outer sides of the pipe, a high-temperature region exists in the inner-wall area from the pipe bend and migrates from the wall side to the center area in the exit area. When decreasing the flow velocity and increasing the wall heat flow density, a more significant increase in the percentage of the high-temperature region is demonstrated on the inner wall side of the pipe in the exit area of the bend. Under a given total length of L, the inlet length H with the H/L ratio of 0.025 has the minimum entropy generation within the unit pressure drop, and is the best U-tube structure. This work provides comprehensive investigations on flow and heat transfer characteristics of liquid lithium in bends and U-tubes and inspires practical applications.

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