## 1 Introduction
This represents an extreme limitation for the deep understanding of FPPHPs heat transfer modes. On one hand, designing post-processing techniques for only direct visualizations precludes the possibility of extending the investigations to every possible device. On the other hand, the conductive phenomena largely occurring in FPPHPs might be influenced by the different thermal properties of transparent inserts, thus preventing robust comparisons with fully metallic devices. Moreover, such a lack of insight into fluid oscillation frequencies undermines the validation and improvement of existing numerical and theoretical models on FPPHPs. For instance, [PERSON] et al. [24] designed a three-dimensional thermal conduction-based model for pseudo-steady state operation to predict the FPPHP thermal resistance. The effect of inner fluid oscillations has been considered negligible due to the high uniformity of wall temperature usually observed in flat layouts. However, such an assumption may not be always true, especially when the device undergoes strong transient behavior in terms of intermittent operation or the occurrence of local hot spots [25]. In [23], since the investigation was conducted for micro-geometries, the resulting fluid oscillation frequencies were found to be much higher than those observed in regular, metallic layouts, thus hampering the capability of the adopted mass-spring-damper model to predict the oscillatory behavior in metallic FPPHPs. For the sake of completeness, it has to be stressed that some works have dealt with the direct observation of fluid motion inside fully metallic FPPHPs through neutron radiography [26; 27; 28]. However, such an experimental method is not easily implementable, especially due to the extremely high cost and complexity of instrumentations and peripheral facilities. Hence, the present work proposes a novel experimental approach for the evaluation of dominant fluid oscillation frequencies in fully metallic FPPHPs by means of infrared (IR) thermography. The aim is to provide a straightforward and effective technique for the estimation of fluid oscillation frequencies in fully opaque flat-plate layouts (i.e., where direct fluid visualizations cannot be carried out) to obtain interesting pieces of data regarding the inner fluid dynamics and to enlarge the overall literature dataset. The procedure is designed as an extension of the one presented in [13] for a metallic tubular PHP, as well as a natural evolution of the experimental work described in [29] on the same device under the same working conditions. Here, the authors thoroughly explained a novel method for the evaluation of wall-to-fluid heat fluxes along the adiabatic section of an eight-turn FPPHP made of copper, starting from IR acquisitions on the outer wall surface at varying heat loads to the evaporator and tilt angles. In the current analysis, the evaluated wall-to-fluid heat flux distributions are used as inputs for the wavelet method to quantify dominant fluid oscillation frequencies in every branch of the device. The time evolution of oscillatory behavior is further quantified by such a non-intrusive technique, which does not rely on transparent windows nor on fluid pressure/temperature sensors. A statistical reduction of the results provides a complete description of the overall FPPHP oscillatory behavior, in accordance with the global and local heat transfer characteristics highlighted in [29]. Finally, a link between transverse interactions and fluid oscillations is presented to provide an overview on the thermo-fluid dynamics of the studied device. To the authors' knowledge, this represents one of the first attempts of experimentally quantifying fluid oscillatory features in metallic FPPHPs through infrared observations. The newly proposed method, based on the reduction in local heat fluxes for the estimation of fluid oscillation frequencies in FPPHPs, provides a good degree of insight into the oscillatory behavior of the investigated devices without any direct fluid visualization.