## 2 Experimental Apparatus  
The studied device is an eight-turn FPPHP (overall dimensions: 80 \(\times\) 200 \(\times\) 4 mm) entirely made of copper and with 3 \(\times\) 3 mm\({}^{2}\) squared channels (Figure 1a). One side of the device is coated with a black high-emissivity paint (\(\varepsilon\) = 0.92) to allow thermographic measurements on the outer wall temperature within the adiabatic section. This first plate was covered and brazed with tin to a second copper plate having the same dimensions and 0.5 mm of thickness. The reader is referred to [30] for the description of the manufacturing process. The opaque paint emissivity was evaluated by performing dedicated IR acquisitions on a coated isothermal surface (same opaque paint, at controlled temperature and monitored through thermocouples). The emissivity parameter set in the IR camera software was regulated until a match between the temperature measured by thermocouples and the one by the infrared camera was reached in the range of absolute uncertainty of the camera (\(\pm\)1 \({}^{\circ}\)C). Since the emissive layer had a thickness lower than 100 \(\upmu\)m, its thermal resistance was considered negligible, thus reasonably assuming that the temperature measured on top of the coating corresponded to the one of the device's surfaces. To guarantee full accessibility of the adiabatic section in terms of thermographic measurements, such a portion was not covered with insulating material. Power is provided to the evaporator zone by a wire electrical heater (Thermocoax(r) Type NcAc15, 1 mm external diameter), connected to an electrical power supply (ELC(r) ALR3220, \(\pm\)10 mV). The condenser section is cooled by a loop connected to a copper cold plate (width: 100 mm, length: 80 mm), directly brazed on the rear side of the FPPHP (Figure 1a). The loop temperature is controlled by a cryostat (Huber(r) CC240 wl). Two pipes are brazed on the top and bottom sides of the device, allowing the filling and vacuuming of the system, respectively. The FPPHP was filled with a water/ethanol mixture (20 wt.% of ethanol) with a filling ratio of 50% (at ambient temperature). Specifically, such a working fluid was chosen since it led, as proven in [30]--for the same FPPHP and among a selection of various fluids (pure water, pure ethanol, water aqueous mixtures with 5% of butan-1-ol, 5% of butan-2-ol, 16% and 20% of ethanol, 28% of methanol and 0.5% of surfactants--Tween(r) 20 and Tween(r) 40)--to better thermal performances of the device at different tilt angles and working conditions, making the adopted water/ethanol aqueous mixture suitable for robust and repeatable fluid oscillation analyses. Part of the adiabatic section was monitored by means of a high-speed Medium Weave InfraRed camera (MWIR, FLIR(r) SC5500, 320 \(\times\) 256 pixels resolution). In Table 1, the uncertainties of the adopted experimental equipment are listed. The reference for the channels' numeration is reported in Figure 1b, together with the heat transfer sections, i.e., evaporator (red box, 40 mm long) and condenser (blue box, 80 mm long), and the area framed by the camera (green box, 40 mm long). The reference for the axial direction z is also shown. The analysis was performed for two device orientations, namely the vertical bottom heated mode (BHM) and the horizontal orientation (Figure 2), and different heat loads \(Q\) applied to the evaporator, spanning from 50 to 250 W. During the tests, IR acquisitions, each lasting 20 s, were performed with a sampling frequency of 50 Hz in the so-called pseudo-steady state operation of the device, \begin{table}
\begin{tabular}{l l l} \hline \hline
**Experimental Equipment** & **Uncertainty** & **Operating Range** \\ \hline Power supply & \(\pm\)2 W & 50–250 W \\ \hline Cryostat & \(\pm\)1 \({}^{\circ}\)C & 20 \({}^{\circ}\)C \\ \hline \multirow{2}{*}{MWIR camera} & Absolute: \(\pm\)1 \({}^{\circ}\)C & 3–5 \(\upmu\)m \\  & Sensitivity: \(<\)25 mK & 5 \({}^{\circ}\)C–300 \({}^{\circ}\)C \\ \hline \hline \end{tabular}
\end{table}