Operation of nitrogen pulsating heat pipes: Analysis of orientations and turn numbers

A pulsating heat pipe is promising for the cryogenic cooling of power electronics and is required to operate in various orientations. In this paper, simultaneous visual and thermal experiments are conducted on flat-plate pulsating heat pipes with a hydraulic diameter of 0.691 mm. The nitrogen pulsating heat pipes of 5-turn, 10-turn and 20-turn are measured at inclination angles from 0° to 90°. With the filling ratios of 40 %–54 %, the gravity dependence of the operation is investigated using coupled analysis of heat transfer and two-phase flow. Small gravity component along the channel restricts the liquid reflux and deteriorates the heat transfer, and the thermal conductivity can decrease below 0.3 times. High turn numbers improve the heat transfer and reduce the gravitational influence, with the highest thermal conductivity of 3674 W/(m·K) achieved for 20-turn vertically. Since nitrogen has small saturated pressure gradients, only the 20-turn operates horizontally. The operational mechanism is analyzed considering the interfacial characteristics. The two-phase flow instability is thermally driven and generates local pressure unbalance. The evaporation momentum force originates from the rapid expansion in the confined channels, which drives the pulsation. Moreover, the failure at high heat loads and large filling ratios is caused by the suppression of flow instability. It is discussed considering the flow stiffness of a two-phase flow system, which depends on the compressibility of the bubbles.