## 5 Results and Discussion
### Channel-Wise Frequency Analysis
It has to be stressed that since the color and Pw scales of the provided scalograms and power spectra are adjusted for comparison to match the ones at the highest power input, the values seem to be generally null. However, by observing, in Figure 12, a representative scalogram of Ch 4, Figure 10, 50 W, having an adjusted color scale around zero, it can be noted that the graph presents non-null power areas. Nonetheless, such non-null, low-amplitude regions account for residual noise in the input heat flux data rather than actual fluid oscillations. The same conclusion can be drawn for the other scalograms at 50 W, horizontal orientation, thus confirming the absence of fluid motion in the considered channels for the analyzed heat load. Such a remark agrees with what was observed in the previous literature [30,38,39], where operating FPPHPs in a horizontal orientation led to delayed start-ups at higher power inputs due to a lack of pumping forces assisted by gravity effects. For \(Q\) = 150 W, the scalograms of Figure 10 exhibit medium-power stripes which persist with continuity during almost the whole acquisition in the considered channels, except for some higher-frequency spots that are especially evident for Ch4. High \(P_{w}\) are clearly noticeable from the power spectra of Figure 11 (\(Q\) = 150 W), confirming the presence of regular fluid oscillations with dominant frequencies around 0.9 Hz. At 250 W, the scalograms of Figure 10 show high-power spots which replicate at almost the same frequency over time, denoting the time regularity of the strong fluid oscillations occurring in such operating conditions. Such vigorous fluid oscillations in every FPPHP channel are also clear from the power spectra high peaks of Figure 11 (\(Q\) = 250 W), with dominant frequencies around 1.2 Hz. It is worth noting that, contrarily to the vertical BHM orientation, the peaks of \(P_{w}\) span a much narrower range of frequencies for every channel in the horizontal orientation for medium/high power inputs. This might be due to the fact that the buoyancy forces acting on the fluid in BHM increase the complexity of the device's operation and, consequently, of its oscillatory behavior. In fact, in the vertical BHM, a transition between slug-plug and semi-annular flow may occur, especially due to the squared section of the present FPPHP channels [16,40,41], thus increasing the chaoticity of the thermofluidic wall-to-fluid interactions. On the contrary, in the horizontal orientation, the flow pattern could probably be only slug-plug even for high power inputs to the  
Figure 9: Power spectra of the wall-to-fluid heat fluxes for the fixed axial coordinate \(z\) = 0.02 m related to every FPPHP channel at different heat loads to the evaporator (vertical BHM). evaporator, i.e., high inertial forces on the fluid driven by damped oscillations of the vapor bubbles, thus resulting in a similar oscillatory behavior between every FPPHP branch. In fact, a slug-plug flow is more likely to occur in a horizontal orientation due to absence of buoyancy forces which tend to deform the liquid-vapor interfaces. For instance, [PERSON] et al. [42] observed a slug-plug flow during microgravity experiments on a closed-loop FPPHP with channel dimensions of 2.5 \(\times\) 2.5 m\({}^{2}\) and FC-72 as a working fluid. Of note, weightlessness acts similarly to the horizontal orientation in terms of the negligible effects of buoyancy forces on the fluid stream, as proven in [43]. In [42], when a transition from hyper-gravity to microgravity occurred, the flow pattern varied from annular to slug-plug, confirming the higher stability of fluid menisci and overall fluid flow observed during horizontal operation in the present investigation. Figure 11: Power spectra of the wall-to-fluid heat fluxes for the fixed axial coordinate \(z=0.02\) m related to every FPPHP channel at different heat loads to the evaporator (horizontal orientation).