A heat transfer analysis of phase change processes, combining boiling and condensation has been applied to a closed heat pipe. Experimental tests and numerical modeling have been carried out on U-type horizontal smooth heat pipe using R134a as working fluid. The pipe consists of a 180 mm evaporator, 280 mm adiabatic section and 180 mm long condenser. The heat pipe is fabricated from aluminum alloy and has a cross section of 25.5 mm and an outer radius of 33.6 mm. Tests have been conducted using a filling ratio of 50% and exposing the evaporator to different power inputs, ranging from 24 to 80 W. A numerical model has been developed with OpenFOAM to assess heat transfer processes during boiling and condensation. One of the most important challenges tackled with the numerical model is to consider both the coupled effect of boiling and condensation in a closed system. The numerical work has been validated by experimental results. It can be concluded that the model adequately predicts the phase changes characteristics at various operating conditions. In addition, a parametric analysis performed with numerical model is used to investigate the effect of independent parameters like heat input, filling ratio, diameter of tube and type of refrigerant on the equivalent thermal resistance, which is identified as heat pipe performance. (C) 2019 Elsevier Ltd. All rights reserved.