Numerical investigation on the start-up and heat transfer performance of dual-diameter pulsating heat pipes

The pulsating heat pipe stands out as a promising heat transfer device in various applications. Its start-up and heat transfer performance are however crucial to understand. In this study, three single-loop dual-diameter pulsating heat pipes (DPHPs) are proposed by changing both channel diameters of a uniform pulsating heat pipe (UPHP). The diameter difference is 1.0 mm, 2.0 mm, and 3.0 mm, respectively. The filling ratios range from 30 % to 70 %, and the inclination angles range from 0° to 90°. The effects of diameter differences, filling ratios, and inclination angles on the start-up and heat transfer performance are analyzed through two-dimensional transient numerical simulations. Results show that the diameter difference significantly affects the start-up and heat transfer performance of PHPs. Among the proposed diameter differences, 2.0 mm is the appropriate case for obtaining the best thermal performance. The start-up time and thermal resistance reduce by 19.1 % and 34.5 %, respectively, compared to a UPHP. In the start-up stage of DPHPs, two growth patterns of vapor bubbles are discovered, namely directed growth and swift amassing growth. They are the primary reason for the earlier start-up of DPHPs compared to UPHPs. Moreover, the start-up and heat transfer performance of DPHPs exhibit lower sensitivity to inclination angles, and the sensitivity reduces with the increasing diameter differences. However, the DPHP with a larger diameter difference (3.0 mm) causes the start-up temperature to rise. The optimum filling ratio is dependent on heating powers, 30 % and 50 % are the optimum case for the heating power of 30 W and 90 W, respectively. The present study aims to provide a reference for the design of DPHPs.