Experimental study on a three-dimensional pulsating heat pipe with tandem tapered nozzles

Optimization in the geometry structure of the pulsating heat pipe (PHP) may provide an effective method to improve the heat dissipation capacity. In this study, tandem tapered nozzles are proposed to promote the uni-directional flow in a three-dimensional (3D) closed-loop PHP (CLPHP). For comparison, two six-turns CLPHPs comprised of 12 glass tubes with a 2 mm inner diameter (ID) and 13 silicone hoses with a 2.5 mm ID were examined to study the effect of tapered nozzles. Specifically, one (ordinary) had an equal glass tube ID, while the other one (asymmetrical) adopted glass tubes that had one tapered nozzle end formed by burning and melting the end from 2 mm into 0.95 mm ID. A series of experiments were performed to investigate the effects of filling ratios (ethanol) and heating powers on the thermal resistance, start-up time, and flow patterns. Results showed that the thermal resistance was reduced by 29.5% after the optimization, and the unidirectional flow was significantly promoted. Moreover, the dry-out phenomenon that happened in the ordinary CLPHP at the 40% filling ratio was also prevented in the asymmetrical one. Finally, we explain the variation trend of start-up time from the driving force theory. The present application of the tandem tapered nozzles in the 3D CLPHP provides an effective, reliable, and easy-to-implement method for improving the heat transfer performance, which leads to the direction for the future design of the PHP.