Experimental and analytical study of a loop heat pipe at a positive elevation using neutron radiography

• A loop heat pipe was studied experimentally using neutron radiography.
• A gravity-controlled operating mode at a positive elevation was presented.
• An analytical model was formulated to predict steady state performance.
• Analytical model and experimental data showed agreement at a positive elevation.
• The mechanism explains the unique steady state trend at a positive elevation.

An experimental and analytical study has been conducted of a loop heat pipe's steady state operating conditions at a positive elevation, which refers to when the condenser is higher than the evaporator. A unique trend of the steady state operating temperature as a function of evaporator heat load at a positive elevation was observed in the experimental data. A gravity-assisted operating theory was proposed and explained in detail. In addition, the proposed hypothesis was validated by neutron radiography, a non-destructive visualization tool. When the LHP is operated at a positive elevation, it can operate in the capillary-controlled mode, which means the system is driven by pressure gain from both surface tension and liquid head, or in the gravity-controlled mode, which means the system is driven only by the pressure gain from the liquid head. A pressure–temperature diagram illustrating the thermodynamic states of the circulating fluid was presented when the system is operating in a gravity-controlled mode. Experimental temperature data were presented for a loop heat pipe operating at 25.4, 76.2, and 127.0 mm positive elevations. Lastly, predicting results from an analytical model with the newly added features at a positive elevation were compared with the experimental results obtained at a 76.2 mm positive elevation. The model prediction and the experimental data agree well, which means the operating mechanisms were understood and captured in the model. This is the first study of a loop heat pipe focusing on a positive elevation, which unveils the unique temperature trend at low heat load operating conditions.