Multi-parametrical investigation on the start-up of nitrogen pulsating heat pipes for space applications

Abstract

The successful startup of a pulsating heat pipe is crucial for ensuring its stable operation both on ground-based applications and, even more in space conditions. Indeed, under microgravity conditions, the startup behavior of the pulsating heat pipe differs significantly from that observed under normal gravity. Consequently, it is vital to conduct comprehensive research into the startup mechanisms of the pulsating heat pipe under zero-gravity conditions. A systematic analysis was conducted on how initial vapor–liquid distribution, filling ratio, assembly parameter, and thermal load influence the startup characteristics of nitrogen PHPs by ground-based horizontal experiments. The results showed that initial vapor–liquid distribution was key in determining startup success. Precooling and filling in the horizontal orientation inevitably led to startup failure. In contrast, performing precooling and filling in the vertical orientation or modifying the initial vapor–liquid distribution by heating the condenser facilitated the startup of the pulsating heat pipe in the horizontal orientation. With a 20% filling ratio, the PHPs could successfully start up and operate across a broad thermal load range, ensuring efficient heat transfer. With a 40% filling ratio, periodic flow stagnation tended to occur inside the PHPs. With a 60% filling ratio, the PHPs struggled to start successfully. Even when startup occurred, its operational thermal load range for efficient heat transfer was limited. Moreover, increasing the number of turns has helped to minimize fluctuations in the evaporator temperature and PHP pressure during operation. A double-loop configuration has also greatly enhanced the temperature uniformity of the evaporator. Among the three configurations, the single-loop 12-turn PHP exhibited the optimal heat transfer performance at a 20% filling ratio.