Experimental Study on the Heat Transfer Characteristics of Large Diameter High Temperature Heat Pipes at Negative Inclination Angles

Negative inclination angle can cause performance degradation for freezing start and decrease the capillary force, leading to the failure or burning of large diameter high temperature heat pipes (LDHTHPs) in the application process. In this paper, we studied the start-up behaviors and temperature uniformity of the prepared LDHTHP at negative inclination angles and the relevant influencing factors. The results showed that the LDHTHP could start normally and reached a steady state in the inclination angle range of −45°. The temperature difference between the evaporating section and the condensation section at −45° was lower than that of the positive inclination angle, leading to a more sufficient start-up. Furthermore, more working liquid would aggregate at the condensation section at −45° obviously increasing the vapor volume and temperature at the condensation section. The condensation section start-up time, start-up performance, gravity resistance, and the average temperature would be reduced with the increase of the heating power and condensation area when the LDHTHP was at an inclination of −45°. And the axial temperature uniformity and equivalent thermal conductivity of the LDHTHP tended to decrease when the condensation area increased. Further, the LDHTHP was mainly limited by the capillary limit at negative inclinations. However, the overall temperature uniformity and equivalent thermal conductivity of the LDHTHP wouldn't be changed significantly when the heating power increased. In consequence, the LDHTHP showed excellent anti-gravity characteristics.