A series of experiments was performed to investigate the effect of a dual-diameter tube on the flow and heat transfer characteristics of single-turn pulsating heat pipes (PHPs). Various types of PHPs were made of glass capillary tubes with various inner diameters. Using thermometry and high-speed photography, experiments to quantitatively observe operational characteristics of the PHPs were performed with varying input power and inclination angle. The results show that circulating flow promoted by a dual-diameter tube reduces thermal resistance of the PHP by as much as 45%. Based on experimental observations, a simplified model was developed to predict thermal performance of PHPs with circulating flow, and the results based on the model matched well with experimental data to within the error of 15%. The data calculated by the proposed model show that there is an optimum range of diameter difference where the thermal performance enhancement is maximized. The optimal range of the dimensionless diameter difference is found to be 0.25 < Delta D/D-avg < 0.4. (C) 2014 Elsevier Ltd. All rights reserved.