## 6 Conclusions  
The presented study concluded the analysis on the global and local thermal behavior of a fully metallic flat-plate pulsating heat pipe, first discussed in [29]. The aim of the present work was to design a novel approach for the study of the inner fluid dynamics in FFPHPs without taking advantage of any direct fluid visualizations, which represent most of the experimental studies on flow behavior in such heat transfer systems. The wall-to-fluid heat fluxes, evaluated within the adiabatic section of the device by means of thermographic acquisitions on its outer surface, were processed by the wavelet method to estimate dominant fluid oscillation frequencies at varying heat loads and orientations. A validation procedure on synthetic data was carried out to assess the reliability of the estimation procedure. Scalograms (i.e., time-frequency representation of the wavelet transform) were introduced to give useful references for the interpretation of fluid motion in terms of time evolution of the oscillatory phenomena. The characteristic frequencies of the system were furthermore assessed in every device branch by means of power spectra of the considered signals, thus providing a channel-wise description of the fluid motion. The K-means clustering method was used for peak identification in scalograms; two quantitative parameters, namely the average continuity time of oscillations \(t_{\mu\
u}\) and the average activity time of channels, were consequently proposed for the description of the device oscillatory behavior over time. The average \(f_{d,av}\) and standard deviation \(f_{d,std}\) of channel-wise dominant frequencies were therefore evaluated for the whole device to give an overview on the global oscillatory modes. Finally, thermal interactions between adjacent channels were estimated through the \(Q_{ratio}\) parameter and explained in terms of fluid oscillation frequencies. The following remarks can be drawn from the present study:  
* The higher average activity of channels found in the vertical bottom heated mode confirms its better heat transfer capability with respect to the horizontal orientation, even though values of \(t_{e,av}\) equal to 23 and 27% of the total observation window at average and high heat loads, respectively, highlight a lower continuity of fluid oscillations in the vertical operation;
* The generally greater continuity of oscillations in the horizontal orientation is also established by the \(f_{d,std}\) values, which suggest more similar dominant fluid oscillation frequencies among the device branches especially at high heat loads, where \(f_{d,std}\) is set below 0.05 Hz;
* For the vertical BHM orientation, \(f_{d,av}\) ranges from 0.78 Hz up to 1 Hz during the device's full activation, while the horizontal orientation exhibits values of \(f_{d,av}\) that increase from a minimum of about 0.84 Hz to a maximum of 1.25 Hz;
* The heat transferred by conduction between adjacent channels is higher in the horizontal orientation, where \(Q_{ratio}\) ranges from about 60% at low heat loads to 15% at high heat loads;
* Similar oscillatory behaviors in adjacent channels increase the magnitude of transverse thermal interactions. In conclusion, the presented results quantified the oscillatory behavior of the investigated device. The working regimes assessed in [29] of the analysis were confirmed by the data, thus giving a deeper insight into the heat transfer modes of the device without adopting any transparent sections for direct fluid visualizations. In view of the results, the adopted technique provided a good insight into the inner fluid dynamics and the overall thermal behavior of the studied device, in accordance with what was observed in previous experimental works relying on different experimental approaches.