## 5 Results and Discussion
### Local Heat-Transfer  
To present a deep overview of the device heat transfer behavior, the wall-to-fluid heat flux \(q\) was evaluated for every study case by adopting the post-processing procedure described in Section 3. It has to be reminded that \(q\) is not here referred to the whole wall-fluid interface, but only to the median portion of thin walls in each channel (Figure 3a). However, despite such an evaluated thermal quantity it is limited to the thermofluidic interactions within a rather small part of the device, it is believed to describe the FPPHP thermal behavior in terms of fluid oscillations and working regimes. In fact, the low thickness of such a portion guarantees higher wall temperature variations, directly linked to the inner thermo-dynamics. In Figure 8, the raw temperature acquired by means of thermography is shown (Figure 8a), together with the corresponding assessed wall-to-fluid heat flux distribution (Figure 8b) for a single test case (horizontal orientation and \(Q\) = 50 W) and for channel 8 (central channel, see reference of Figure 1a). Both quantities are additionally plotted in Figures 9 and 10 for \(Q\) = 100 W and \(Q\) = 200 W, respectively, and for the same orientation and reference channel. According to the adopted reference system (Figure 3), the wall-to-fluid heat flux is positive when it is exchanged by the working fluid to the device wall, i.e., the wall heats up. Hence, fluid motion can be perceived and described in most of the cases as follows:  
* When the wall-to-fluid heat flux significantly increases over time, hot fluid may flow from the evaporator to the condenser, thus warming up the wall;
* When the wall-to-fluid heat flux significantly decreases over time, flow reversal phenomena may occur, where fluid colder than the wall flows back from the condenser to the evaporator. During the intermittent flow (Figure 8), the temperature distribution does not exhibit substantial variations over time except for the first seconds, in which few temperature fluctuations can be perceived. Similarly, the wall-to-fluid heat flux (Figure 8b) does not vary over the whole observation window but only the first few seconds, confirming an almost constant absence of fluid motion. As long as the power input to the evaporator increases, variations of both temperature and heat flux become more and more persistent over time, verifying the onset of a device full activation. The oscillatory behavior of the studied FPPHP is thus clearly noticeable from the alternation of peaks, either positive or negative, in the wall-to-fluid heat flux distribution of Figures 9b and 10b: straight vertical peaks suggest that the working fluid flows almost instantaneously, i.e., at high velocity, through the analyzed channel for both considered heat loads. It has to be pointed out that the amplitude of \(q\) presents non-negligible variations along the z coordinate. In particular, the wall-to-fluid heat flux exhibits, in all the reported study cases, an increasing trend from the evaporator (z < 0 m), where it seems to assume lower values, to the condenser (z > 0.04 m), where it seems to assume higher values instead. Such variations are especially notable at low and intermediate power inputs (Figures 8 and 9), while, for high power inputs (Figure 10), amplitudes of the wall-to-fluid heat flux are more uniform along space. To better outline heat flux variations along space, the values of \(q\), averaged over time (\(q_{\textit{air}}\)), are shown in Figure 11 as a function of the axial coordinate \(z\) for three FPPPHP branches, namely channels 3, 10 and 16, and the representative test case of Figure 9 (horizontal, 100 W). Specifically, two outer channels and one middle channel were chosen as meaningful branches. Here, the wall-to-fluid heat flux clearly exhibits an increasing trend with \(z\) in all the considered channels, thus verifying the presence of significant space variations in the heat flux distributions.