Heat transfer and liquid slug flow characteristics of plate pulsating heat pipes with series and parallel channels

Series-channel and parallel-channel configurations are two fundamental channel topologies in pulsating heat pipe (PHP) design. However, their thermo-hydrodynamic differences remain insufficiently understood. In this study, two copper flat-plate PHPs with identical dimensions but different channel configurations, namely a parallel-channel PHP (P-PHP) and a series-channel PHP (S-PHP), were experimentally investigated under various filling ratios and heating powers. The results show that both liquid-slug velocity and length vary continuously during forward motion. As liquid slugs travel from the evaporator to the condenser, their velocity first increases and then decreases, reaching a maximum within the 80–100 mm region of the adiabatic section. Meanwhile, slug length exhibits a similar increase-decrease trend, with the maximum value occurring near the adiabatic-condenser transition. Increasing heating power promotes liquid-slug migration toward the condenser and increases the populations of both forward- and reverse-moving slugs. The thermal performance of the PHPs is strongly correlated with the liquid-slug inventory in the evaporator section, where a higher liquid inventory leads to lower thermal resistance. A transition filling-ratio region between 40% and 50% was identified. At low FRs (30–40%), the P-PHP outperforms the S-PHP, whereas the opposite trend is observed at high FRs (50–60%). These findings demonstrate the important role of channel topology in governing liquid-slug behavior and thermal performance, providing useful insights into the design and optimization of high-performance PHPs.