Effects of frontal and sagittal inclination angles on the thermal performance of a single-loop pulsating heat pipe

In contrast to multi-turn pulsating heat pipes, the thermal characteristics of single-loop pulsating heat pipes (SLPHPs) exhibit an extremely strong dependence on orientation. While the effects of the frontal inclination angle (FIA) have been extensively studied, the role of the sagittal inclination angle (SIA) remains largely unexamined. This work systematically investigates the impact of FIAs and SIAs, varied from 0° to 90° in 15° increments, on the thermal performance of a SLPHP across heat inputs of 10–70 W. Through simultaneous flow visualization and thermal resistance measurements, it was determined that at a 30 W heat input an SIA of 60° yields optimal thermal performance by establishing a stable unidirectional circulation. Although dry-out did not occur at SIA = 0°, thermal performance was substantially inferior to that at the optimal angle. For the FIA, the optimum was also observed at 60°, attributed to the combined effects of gravity and bubble-nose shape on flow velocity. Moreover, in contrast to the SIA case, FIA = 0° resulted in dry-out. A comparative analysis of all orientations confirmed that SIA = 60° provides the lowest thermal resistance, followed by FIA = 60°. These findings provide new insights into the orientation-dependent behavior of SLPHPs and offer practical guidance for their application.