Thermal instability of a Closed Loop Pulsating Heat Pipe: Combined effect of orientation and filling ratio

A multi-turn Closed Loop Pulsating Heat Pipe (CLPHP) made of copper (I.D. 1.1mm, O.D. 2mm), filled with FC-72 is experimentally investigated. The study focuses in particular on the combined effect of the inclination angle (gravity) and the filling ratio at different heat input levels on the device operation stability and the thermal performances. Results show that this CLPHP is very much sensitive to the gravity head and that the vertical operation is affected by unstable operation at high heat input levels. On the other hand the CLPHP in the horizontal position is less efficient, but it does not undergo any performance drop with respect to the heat input level until the maximum heat input level is reached. The repeatability analysis, performed for the horizontal and vertical operation only, reveals that the higher discrepancies occur when the PHP operates in vertical position at the start-up heat input levels while the instability occurring at high heat loads is repeatable and does not depend on the previous heat power input level. This behavior is confirmed at different filling ratios, and the optimal one is 0.5.