## 5 Results and Discussion
### Channel-Wise Frequency Analysis  
The wavelet method was extended to the heat flux signals referred to all the FPPHP branches, thus assessing channel-wise fluid oscillation frequencies at varying heat loads and orientations. In Figure 8, the scalograms referred to three meaningful channels, namely channels 4, 8 and 12 (Ch4, Ch8 and Ch12, respectively), which are shown for the vertical BHM at power inputs equal to 50 W, 150 W and 250 W. Specifically, the three channels only were chosen to achieve a good trade-off between a clear spatial overview of the global oscillatory behavior in the studied device and a better readability of the discussion. The  
Figure 7: Wall-to-fluid heat flux signals over time related to channel 9 and \(Q=250\) W for three different axial coordinates and corresponding power spectra; vertical BHM (**a**,**b**) and horizontal orientation (**c**,**d**). power spectra related to every FPPHP channel are further presented for a comprehensive description of the oscillatory behavior in Figure 9 for the three considered heat loads. Note that magnitude scalograms in Figure 8 and power spectra in Figure 9 present, respectively, the same scale for better comparison. In Figure 8, \(Q\) = 50 W, the scalogram of Ch4 exhibits a medium-power area during the first seconds of acquisitions, suggesting the presence of weak oscillations. On the contrary, Ch8 and Ch12 do not present any significant power spots except for some low-power areas, thus highlighting a near absence of fluid motion. Such evidence is confirmed by the power spectra of Figure 9 (\(Q\) = 50 W), which are flat for most of the channels, i.e., no fluid oscillations can be perceived in almost the whole device. As the power input to the evaporator increases to 150 W, the scalograms of Figure 8, referred to the vertical BHM, present more persistent medium-power areas over time; additionally, all the power spectra of Figure 9 (\(Q\) = 150 W) exhibit perceivable peaks, suggesting that the fluid oscillates in every branch with dominant frequencies mainly below 1 Hz. For the maximum provided power input (\(Q\) = 250 W), the scalograms of Figure 8 highlight many high-power spots, denoting strong fluid oscillations over time. Nevertheless, the power spots are referred to different frequencies, suggesting that the oscillatory behavior is not stable over the observation window. The presence of strong oscillations described by high-power spots is confirmed by the power spectra of Figure 9 (Q = 250 W), which assume high peaks of \(P_{w}\). Their absolute maxima are furthermore scattered within a wide range of frequencies between 0.6 and 1.6 Hz, thus denoting greatly different dominant fluid oscillation frequencies from channel to channel. The observed progressive increase in fluid motion in every channel confirms the typical flow regimes in PHPs [35; 36; 37]: at low heat loads, the device operation is mainly intermittent, presenting fluid oscillations in few channels; meanwhile, at high heat loads, every branch presents strong oscillations, although they are not persistent over the observation window. Figure 8: Wavelet scalograms for channels 4 (Ch4), 8 (Ch8) and 12 (Ch12) of the wall-to-fluid heat fluxes for the fixed axial coordinate \(z\) = 0.02 m at different heat loads to the evaporator (vertical BHM). Again, the scalograms and power spectra scale are adjusted to match the one at the highest heat load to the evaporator (\(Q\) = 250 W) to allow for a better and more straightforward comparison between different power inputs. In Figure 10 at \(Q\) = 50 W, the scalograms do not present any significant power spots, denoting either the absence of fluid motion or very weak and intermittent oscillations. The device inactivity over the observation window is additionally highlighted by a complete absence of peaks in the power spectra of Figure 11 (\(Q\) = 50 W).