In the present study, thermodynamic characterization and thermal optimization of pulsating heat pipes (PHPs) operating in a circulation mode are conducted experimentally and analytically. A series of experiments is performed to identify the thermodynamic state of the vapor plugs inside the PHPs. Two-turn closed-loop PHPs are made of glass capillary tubes with inner diameters of 1.2, 1.7, and 2.2 mm. Temperature, internal pressure, and high-speed flow visualization data are obtained for PHPs at various input powers in a vertical orientation inside which the flow in the steady state exhibits a circulating motion with a sporadic pulsating motion. It is experimentally found that when a PHP is operating in a circulation mode, the vapor plugs in the PHP exist in a saturation state and the saturation pressure of each vapor plug is nearly identical. This physically means the phase changes can be treated to occur at a constant saturation temperature corresponding to the system pressure along the entire tube. On the basis of the experimental findings, a one-dimensional numerical model is developed to predict the thermal performance of PHPs operating in a circulation mode. The numerical results are shown to be in close agreement with the experimental data within an error of 7%. Finally, thermal optimization is conducted using the proposed model. The number of turns (N-turn) is optimized under the given three-dimensional space (200 x 190 x 6 mm(3)) for various lengths of the condenser section (20, 40, and 60 mm). According to the numerical results, there is an optimum number of turns at which the thermal performance is maximized in the case of PHPs with condenser length of 20 mm, while the thermal performance of PHPs with condenser length of 40 and 60 mm is monotonically enhanced as the number of turns increases. The optimum thermal performance results from the combined effect of the mass flux of circulating flow and the condensation area. (C) 2017 Elsevier Ltd. All rights reserved.