## 1 Introduction
[PERSON] and [PERSON] [21] similarly assessed the better heat transfer performance of non-uniform geometries due to more predominant liquid backflows to the evaporator. [PERSON] et al. [22] speculated on the heat transfer modes in an eleven-turn micro FPPHP partially filled with FC-72 by image recognition of the flow patterns. Under a high thermal conductivity condition, the device exhibited large amplitude and high frequencies fluid oscillations, with longer and thinner liquid films that promoted latent heat transfer from the heated to the cooled section. [PERSON] et al. [23] highlighted the disadvantages of flat-plate geometries with respect to tubular layouts in terms of thermal performance mainly due to transverse conduction by directly visualizing fluid motion and evaporator dry-outs. Additionally, [PERSON] and [PERSON] [24] suggested a guideline for the working fluid selection in FPPHPs based on the identification of condensation and evaporation/nucleate boiling processes by means of optical acquisitions. Although these visualization experiments are extremely useful for the description of FPPHPs operation, they do not usually provide meaningful data for a complete interpretation of the local heat transfer behavior. Specifically, a proper estimation of thermofluidic interactions in FPPHPs is generally missing in the literature, undermining a full understanding of the heat transfer modes governing the FPPHPs physics. Such a complex behavior emphasizes the need for evaluation of heat transfer quantities, such as heat fluxes locally exchanged between the working fluid and the device walls. To the authors' knowledge, the only experimental work dealing with a wall-to-fluid heat flux estimation in FPPHPs is the one performed by [PERSON] et al. [25]. Here, the temperature referred to the wall-fluid interface was acquired in a five-turn micro FPPHP by means of infrared measurements on a multilayer substrate. Space-time wall-to-fluid heat flux maps were therefore evaluated by adopting the method introduced in [26], and the device areas dominated by sensible/latent heat transfer were identified by optical visualization of liquid films. However, such a technique may be only employed when thermographic acquisitions on dedicated substratescan be achieved, and it cannot be extended to the study of FPPHPs having fully opaque walls, e.g., metallic devices. Recently, [PERSON] et al. [27] proposed a method for the evaluation of the wall-to-fluid heat flux in the adiabatic section of a multi-turn tubular PHP made of aluminum. Space-time wall temperature maps were recorded by means of infrared thermography. The heat fluxes exchanged between the working fluid and the wall were estimated by the inverse heat conduction problem (IHCP) resolution approach, using filtered temperature distributions as input parameters. The aim of the present work is to develop and thoroughly validate a novel post-processing method, inspired by the one presented in [27], for the wall-to-fluid heat flux quantification and heat transfer behavior description in metallic FPPHPs, where neither optical nor thermographic direct visualizations of the wall-fluid interface or fluid flow can be obtained. The adiabatic section of an eight-turn FPPHP made of copper is analyzed at varying orientations and heat loads to the evaporator by means of common techniques, i.e., evaluation of equivalent thermal resistance and thermography. The acquired raw temperature distributions, referred to two spatial dimensions and time, are filtered by a three-dimensional Gaussian kernel and used as inputs for the IHCP resolution approach, allowing the evaluation of wall-to-fluid heat fluxes exchanged within the considered zone. The resulting heat flux distributions are statistically reduced to provide quantitative pieces of information for the device working regimes detection, as well as for the device global performance interpretation. Furthermore, considerations on the effects of conduction between the evaporator and the condenser are given.