Neutron Imaging of Lithium (Li) Coolants inside High Temperature Niobium (Nb) Heat Pipes

Lithium (Li) behavior inside a high temperature Nb-Li leading edge heat pipe was successfully imaged under induction heating operation via neutron imaging. Startup and cool-down operations gave visual confirmation of bulk Li movement using both gravity assisted and inverted operating orientations. The pipe was imaged during an operation cycle from ambient conditions, heated to a steady state temperature of 908.8 ⁰C, and allowed to cool below 200^°C. The experiment was performed at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, and at the Oak Ridge National Laboratory (ORNL) in Oak Ridge, Tennessee.

Tomographic images of the heat pipe sample at ambient conditions were taken at NIST using three 5s exposures combined for each step, with a step angle of 0.18^° resulting in 1001 projections. The reconstructed images gave insight into workmanship and Li wetting characteristics of the heat pipe. The cross sectional images allowed for visualization of the sample’s inside mesh wicking structure and Li distribution, giving understanding into structural workmanship that is critical for the pipe’s working efficiency. The reconstructions were used to create a 3-D rendering of the heat pipe and the inner lying Li distribution.

Comparative heat transfer experiments were conducted for a heat pipe containing non-condensable gases (NCGs) and another sample that was free of NCGs, using a threaded stainless steel water jacket. The heat pipe containing no NCGs resulted in double the cooling heat flux for the same induction heating input power, with a maximum of 440 W being removed from its condenser at an input of 1800 W. The result of the experiment was used in conjunction with visual observation of the pipes under heating loads to confirm the negative impact of NCGs on a heat pipe.

Future recommendations are to more realistically simulate hypersonic leading edge heating loads using localized heating instead induction heating. A butane-powered portable torch can be considered to localize heating to the leading edge and allow for higher test temperatures. The setup would be done in open air, allowing for combustion but would require a silicide anti-oxidation coating for the tested heat pipe surfaces.