Thermal performance of concentrically arranged two interconnected single-loop pulsating heat pipes

Pulsating heat pipes (PHPs) have garnered significant attention due to their complex thermo-hydrodynamic behavior and their broad applicability in heat transfer. This study focuses on enhancing the two-phase flow startup characteristics of PHPs under concentrated heat loads. A novel concentric-loop PHP configuration was investigated, consisting of two single-turn loops arranged concentrically. The heat pipe has an interconnection between the loops, with the heater positioned on the opposite side to utilize both heating and flow-path asymmetry. The performance was evaluated using pure water as the working fluid, with varying filling ratios, heat loads, and inclination angles. The setup was fabricated with the pure copper capillary tube loops with bend radii of 10 mm and 16 mm, an adiabatic section length of 165 mm, and a condenser section length of 40 mm. The condenser was cooled using a fluid at 20 °$C$ with a flow rate of 20 kg/h. Thermal performance was assessed through measured temperature and thermal resistance under transient conditions. The results showed successful startup at a minimal heat load of 5 W, with the evaporator reaching a lower temperature of 40 °$C$, leading to a rapid temperature drop and early establishment of a pseudo-steady state. The evaporator was capable of handling a heat flux of up to 22.5 W/cm² in vertical bottom heat mode with a 70% filling ratio. These findings suggest that this new PHP configuration is highly effective for versatile heat transfer applications, including thermal energy storage, power electronics, and electronic systems.