Innovative Experimental and Numerical Analysis of Pulsating Heat Pipes Phenomena and Operation

The aim of this thesis is to document all the activities undertaken during three years of PhD research.Two primary objectives have driven the research activity: first, the development of an innovative experimental apparatus, the \textit{Smart Loop}. This apparatus provides a platform for a comprehensive and systematic study of the operational limits of the Pulsating Heat Pipe. The second goal to use of the experimental data to perform a multi-parametric validation of one of the most sophisticated numerical models designed for simulating the Pulsating Heat Pipe. The first year was devoted to defining requirements and designing the experimental apparatus. Modularity and visual accessibility were the most influential design drivers in the development of the test cell. The requirement for modularity arose from the need to create a test cell that could reproduce different Pulsating Heat Pipes topologies and geometries, thereby mimicking the operation of various devices. The need of visual accessibility derives from the the aim to correlate macro-scale phenomena (i.e., overall performance and behavior) to the micro-scale physical phenomena involving the local fluid physical phenomena (evaporation, velocity, flow pattern etc.). Additionally, innovative measurement systems were developed and characterized. The first part of the second year was dedicated to the test cell assembly and thermodynamic characterization confirming that the thermo-hydraulic behavior of such modular test cell was typical of a Pulsating Heat Pipe. The second part of the year was dedicated to a high-speed high-precision data acquisition system development and finally to the experimental campaign. The first part of the experimental campaign was dedicated to the analysis of the effect of the number of heated sections and condenser temperature on the global thermal performance and on the startup performance. It appeared that the increase of the condenser temperature and number of turns lowers the overall thermal resistance; moreover, with the increase of condenser temperature the device is able to operate even at low number of turns. On the other hand, the increase of condenser temperature is observed to influence the initial liquid phase distribution which, in turn, influences the startup times. Finally, the analysis of the local fluid thermodynamic state in the adiabatic section and condenser section has been performed. The fluid in the adiabatic section is mainly saturated while in the condenser is mainly subcooled. Moreover, the presence of metastable subcooled vapor and superheated liquid has been observed. The third year was dedicated to the validation of the simulation tool (CASCO) developed at the Atomic Energy Commission (CEA, Paris Saclay). CASCO has been configured to reproduce the geometry, topology, material properties as well as initial and boundary conditions of two distinct experimental apparati. The model was able to reproduce the startup behavior of a PHP tested in microgravity conditions during a parabolic flight campaign. The simulated temperatures, pressures and local fluid characteristics (i.e., liquid plugs length, velocity and temperature distribution) agree with the experiment. The data collected with the \textit{Smart Loop} has been used to validate CASCO on the steady-state operation. The overall thermal resistance has been closely captured as well as the local pressures and fluid temperature trends.