Experimental investigation of effects of extremely low pumping speed on isothermal performance and filling ratio of sintered copper heat pipe

In the present research, extremely low pumping speed and special charging method are employed to investigate the effects of pumping speed on the filling ratio and isothermal performance of heat pipe. Results show that the saturation degree of porous wick is about 92% and water loss during vacuuming is significantly decreased when pumping speed is decreased to 0.1 m3 h−1. In addition, excessive extension of evacuation time cannot decrease NCG level further but increase water loss. NCG level after first vacuuming is high, and the temperature difference (ΔT) along heat pipe cooling section is larger than 7 °C. But, ΔT can be significantly decreased when secondary degassing is conducted. Increasing heating temperature and duration both obviously decrease NCG level and increase isothermal performance. The maximum ΔT of the heat pipe can be decreased to 0.5 °C when secondary degassing was conducted at 140 °C for 100 s. Moreover, the decrease in shrunken section length decreases the isothermal performance and 40 mm is enough in the present study. Finally, the water loss of whole fabrication process can be decreased to ~ 0.015 g leading to approximately 1 mass% water loss in heat pipe.