• experimental analysis
• heat pipe
• numerical modeling
• operating limit
• Two-phase closed thermosyphon

Two-phase devices are key elements of any thermal control management. Since HPs and thermosyphons are reliable, passive devices that offer low thermal resistance, they can be used in a wide range of applications. TPCTs also have received far less attention compared wicked HP, likely due to use of wicked HP in the electronic cooling sector. This study could answer some critical questions in such two phase devices: i) what are the most effective parameters on the heat transfer behavior and operating limitations of HPs and TPCTs? ii) what is a general guideline for using HPs and TPCTs concerning operating conditions and design variables? Iii) are TPCTs reliable to replace with HPs from a performance perspective for innovative thermal energy systems of present day? A mathematical model for heat and mass transfer of the HP and TPCT is presented to define its performances under steady state and transient operations, respectively. A two-dimensional thermal-fluid model for HPs is presented, including the wall, and both liquid and vapor flows based on the physical and material properties of the pipe, wick and working fluid. The model can be used for design applications and further applied for predicting heat transfer limitation of HPs, in particular capillary limitation. The development of a numerical model is also presented for TPCTs with the purpose of determining the transient characteristics of a TPCT. The model includes the heat transfer through the wall, vapor core, liquid pool and the falling condensate film. The complete two-dimensional conservation equations for mass, momentum, and energy are solved using finite volume scheme for the vapor flow and pipe wall. The model is presented to predict optimal filling ratio of TPCTs and also to predict dryout limitation. Both HP and TPCT models are validated by comparison with existing experimental data in the literature and also in the current study. Basing on the objective of the present study, a specific experimental setup has been designed, at the thermal laboratory of the University of Pisa, aimed to determine the operating condition with major details and measuring the maximum heat transport rate, the overall thermal resistance and heat transfer coefficients. This study presents experiments of the TPCT and HP with same dimension, material, and manufacturing process and partially filled with water. The results indicated several important aspects for applying heat pipe and thermosyphon at same operating condition and the published results maybe serve as a reference for engineers when they try to incorporate relatively large diameter HPs into their thermal solutions.