## 6 Conclusions  
A metallic FPPHP with a total of 16 channels (8 turns in the evaporator zone) was experimentally studied in vertical BHM and horizontal orientation to assess its local thermal behavior at heat loads ranging from 50 to 250 W. The adopted operating fluid was a water-ethanol mixture, with volumetric filling ratio of 50%. The evaporator/condenser  
Figure 16: 80 th percentile of the absolute value of \(q\) as a function of the heat load to the evaporator for the two considered orientations. temperature was monitored by thermocouples, thus allowing device characterization by evaluating its equivalent thermal resistance. The external wall temperature in the adiabatic zone was acquired by means of an IR camera. The resulting time-space temperature maps, effectively pre-processed by a three-dimensional Gaussian filtered, were used as inputs for the IHCP resolution approach for the evaluation of the heat flux exchanged between the working fluid and the thin walls of the device. The method was validated by adopting synthetic data. The wall-to-fluid heat fluxes were first statistically reduced to quantify, by means of two dedicated coefficients, namely \(cvt\) and \(cvs\), their time and space variations from channel to channel. The analysis of these coefficients supplied a quantitative tool for the identification and description of the device working regimes. In addition, statistical heat flux amplitudes \(q_{80}\) were presented to achieve a comprehensive overview of the local wall-to-fluid thermal interactions. In general, as expected, the wall-to-fluid heat flux distributions exhibit a significant variation over both time and space in every channel. The most meaningful and quantitative results of the current study are summarized as follows:  
* The maximum estimation error for the adopted inverse approach was assessed equal to about 15% of the wall-to-fluid interactions occurring in the real set-up;
* In the intermittent flow (\(Q<100\) W), \(cvt_{av}\) and \(cvt_{std}\) assume values up to around 75% and 30% for both orientations, respectively, denoting a generally discontinuous FPPHP operation. On the contrary, the two coefficients stabilize at about 70% and 8% during the device full activation (\(Q>100\) W), thus denoting comparable fluid oscillatory behaviors at high heat loads, which result in similar device global performances;
* \(cvs_{av}\) decreases for both orientations within the intermittent flow to settle at around 43% and 50%, respectively, during the full activation, suggesting that the wall-to-fluid interactions are more uniformly distributed along the device axis with the onset of stable and full fluid oscillations;
* \(q_{80}\) reaches a maximum of 7100 W/m\({}^{2}\) in the horizontal orientation at \(Q=250\) W, while it increases up to 2200 W/m\({}^{2}\) in the vertical BHM. In conclusion, the presented non-intrusive approach, coupled with a statistical reduction of the available data, has been proven to provide quantitative pieces of information regarding the wall-to-fluid thermal interactions and heat transfer modes of FPPHPs. The identification of the two-phase flow regimes can be carried out without the need of a transparent insert, a transparent plate or a multilayer sheet for IR analysis, and even from thermographic maps acquired on a metallic, opaque geometry. The evaluated local heat transfer quantities can be extensively used for the design of these heat transfer devices, as well as for the benchmarking of existing numerical models. Conceptualization, L.P. and F.B. ; methodology, L.P., F.B. and L.C. ; software, L.P.; validation, L.P. and M.S. ; formal analysis, L.P. and F.B. ; investigation, L.P., M.S., V.A. and C.R. ; resources, F.B. and V.A. ; data curation, L.P. and L.C. ; writing--original draft paper, L.P.; writing--review and editing, F.B., L.C., V.A. and S.R. ; visualization, L.P.; supervision, F.B., V.A. and S.R. ; project administration, F.B., V.A. and C.R. ; funding acquisition, F.B., V.A. and S.R.