Visual Observation of Oscillating Heat Pipes Using Neutron Radiography

Abstract

Qualitative observation of flow patterns in water and nanofluid oscillating heat pipes was conducted at various heat inputs and condenser temperatures. Images of the liquid flow within the copper tubing were first captured at 30 frames per second using neutron radiography. Neutron radiography allows direct observation of a fluid position because liquid water is hydrogen rich and opaque while water vapor (because it is much less dense) and the other materials in the oscillating heat pipes are transparent. Flow visualization was conducted on an 8-turn water oscillating heat pipe, an 8-turn nanofluid oscillating heat pipe, and a 12-turn nanofluid oscillating heat pipe. The water oscillating heat pipe was filled with high performance liquid chromatography grade water. The 12-turn nanofluid oscillating heat pipe was filled with 1% by volume (35.0 g-ml -1 ) diamond nanoparticles in high performance liquid chromatography water and the 8-turn oscillating heat pipe contained 0.016% by volume (0.5 mg ml -1 ) diamond nanoparticles high performance liquid chromatography water. The diamond nanoparticles were 5 to 50 nm in diameter. All oscillating heat pipes were charged at a filling ratio of 50%. Visual observation shows for all heat pipes that at low heat inputs, fluid oscillation is very random and intermittent. Increasing the heat input causes a steady flow pattern to appear. For all tested oscillating heat pipes, increased heat load or operating temperature resulted in an increased fluid velocity. Also, nucleation was never observed in the tested oscillating heat pipes.