Identical oscillating heat pipes (OHP) charged separately with water and acetone were observed thermally and visually at varying condenser temperatures and heat inputs. Neutron radiography allowed visualization of hydrogen rich liquid water and acetone within the copper OHP. Four identical 6 turn OHPs were constructed; two as open loop and two close loop. One set of open and close loop OHP was charged with acetone and an open and close loop OHP were charged with water at fill ratios of 50%. The OHPs were also instrumented with 24 thermocouples, heated with a strip heater and cooled with heavy water. Both thermal and visual data were collected simultaneously. The experiments show that at low heat flux and condenser temperature, the acetone OHP performed better than the water OHP. Using neutron radiography, the acetone OHP was seen to have higher fluid velocity than the water OHP. The fluid flow pattern was also more consistent throughout the entire close loop acetone OHP including full circulation of the fluid. The close loop water OHP never fully circulates through the turn connecting the two sides. Lastly, neutron radiography shows that the open loop OHPs not only have less fluid motion at the side turns, the interior turns also have reduced fluid motion compared to the close loop OHPs.

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