*2.1. Experimental Setup*

The experimental campaign was conducted on a simplified passive heat transfer loop under varying gravity level and power inputs; ethanol and FC-72 were selected as the working fluid, mainly due to their significant differences in surface tension, density, and latent heat of vaporization. The main fluid properties are listed in Table 2. The varying gravity level was obtained via access to the ESA parabolic flight microgravity platform [45]. The main controlled and observed experimental parameters selected for the setup are detailed in Table 3.


**Table 2.** Main properties of the two working fluids at 20 ◦C.


**Table 3.** Experimental matrix with controlled parameters and parameters observed.

The device can be defined as a hybrid pulsating heat pipe/closed loop thermosyphon depending on the working fluid used and on the gravity level. The setup is equipped with wall-side thermocouples, glass tubes for high-speed shadowgraph visualization of the flow pattern, and pressure transducers, and the power input is supplied via three heaters coiled around three sections of the evaporator. The temperature at the condenser is kept constant with an external cooling loop. A detailed description of the experimental apparatus is provided in [29] and a rendering of the experimental setup is depicted in Figure 2. As discussed in [29], the threshold between confined and unconfined flow is conventionally defined through Bo or Co numbers. In both cases, the limit between stratified or unstratified displacement of the fluid (observable if the tube is in a horizontal position), is a function of the diameter, the surface tension, the density of the phases, and the gravity acceleration. When the gravity acceleration is reduced, the confinement conditions (or unstratified displacement of the fluid) are easy to reach. This is due to a change in the hierarchy of the forces acting on the fluid, where capillarity becomes dominant over gravitational forces. The opposite behavior is observed under hyper gravity conditions.

**Figure 2.** Rendering of the single-loop PHP with position of sensors and camera [45]. Reproduced with permissions.
