## 1 Introduction  
The self-sustained motion of the working fluid greatly affects the overall thermal behavior of passive two-phase heat transfer devices, such as heat pipes (HPs) and pulsating heat pipes (PHPs). In fact, in PHPs, fluid oscillations are responsible for efficient heat dissipation at the heated section (evaporator) [1]. Specifically, in such passive systems, the fluid flow is promoted by temperature differences between the evaporator and the cooled section (condenser) rather than external auxiliary equipment (such as pumps). Since the fluid is mainly saturated, thermal instabilities result in pressure differences which act as driving forces for its self-motion [2]. It is thus evident that such devices are governed by an extremely complex interplay between thermodynamic and fluid-dynamic behavior. The employment of HPs and PHPs has been found to be very attractive for many thermal management applications requiring a drastic reduction in electrical energy consumption [3]. Nevertheless, one of the main drawbacks of passive devices is represented by the need for a constant and self-induced backflow of liquid from the condenser to the evaporator, i.e., a continuously supplied evaporation process. While in HPs a persistent liquid flow to the evaporator is ensured by properly designed wick structures, such a phenomenon is only supported by non-equilibrium conditions in PHPs, thus largely increasing the complexity of their working behavior [4]. For this reason, the thermo-fluid dynamics underlying the oscillatory modes in PHPs are not yet completely understood despite the intensive experimental efforts of the last decades. In general, fluid oscillations are described in terms of amplitude, i.e., fluid displacement and frequency. However, a proper quantification of frequency in PHPs is hampered by the non-periodicity of fluid oscillations [5]. Consequently, all the oscillation frequencies experienced by the flow pattern during the PHPs operation are usually represented by a single frequency, named dominant (or characteristic) oscillation frequency, defined as a peak in the power spectrum of a given signal [6]. Generally, the dominant fluid oscillation frequency is evaluated by means of Fourier transform [7; 8; 9; 10], short-time Fourier transform [11] or time-strip technique [12] on wall temperature measurements or optical acquisitions on the fluid flow. [PERSON] et al. [6] suggested the use of the wavelet method for a reliable time-frequency localization of dominant fluid oscillation frequencies, which is not always obtainable from the Fourier or short-time Fourier transforms. In their analysis, the fluid pressure signals, measured at two different points of a multi-turn PHP channel and sampled at 200 Hz, were proven to be suitable for the fluid oscillations assessment. However, the intrusive insertion of pressure transducers inside the fluid stream is generally hard to achieve from a manufacturing standpoint. To deal with this issue, [PERSON] et al. [13] replicated the analysis presented in [6] on the same device under the same working conditions by adopting instead wall-to-fluid heat flux signals as inputs for the wavelet method, evaluated from thermographic acquisitions, i.e., non-intrusive measurements, on the outer wall surface. The outcomes denoted that the local wall-to-fluid thermal interactions allow for a similar evaluation of dominant fluid oscillation frequencies to that provided by pressure signals. In addition, the experimental approach reliably quantified the oscillatory behavior in every device branch, thus representing an effective solution for the study of PHPs when fluid pressure measurements in multiple branches cannot be achieved. [PERSON] et al. [14] performed a similar analysis to characterize the oscillatory behavior in a tubular micro-PHP tested in a vertical position. The IR acquisitions were performed within the condenser section of the device, dissipating heat to the environment by radiation and free convection with air.