## 1 Introduction
Fluid oscillations became stronger at high heat loads both in terms of power and frequency, and every channel exhibited similar oscillatory behavior. While most of the fluid oscillation frequency investigations have dealt with tubular PHPs, i.e., PHPs constituted by a pipe arranged in a serpentine manner, some researchers have focused on the thermofluidic behavior of other configurations, such as flat-plate PHPs, i.e., PHPs with generally rectangular/squared channels that are directly machined on metallic/polymeric plates [15]. In particular, FPPHPs exhibit a rather different thermal response with respect to tubular layouts. First, the conduction through their solid domain promotesheat dissipation at the evaporator, even though transverse interactions between adjacent channels may hamper the device's operation [16]. Secondly, the non-circular section of the channels may modify the overall thermofluidic behavior. Due to the manufacturing complexity of inserting pressure probes inside the fluid stream, the fluid-dynamic behavior of FPPHPs is commonly studied by means of direct fluid visualizations. [PERSON] et al. [17] investigated stopovers of a seven-turn FPPHP under micro-gravity conditions through high-speed visualization. By means of a liquid-vapor interfaces tracking technique, previously introduced by [PERSON] et al. [18], the menisci displacement was assessed and both fluid velocity and fluid acceleration were evaluated. [PERSON] and [PERSON] [19] highlighted that starting from the dried condition, the phase of liquid plugs oscillations is not independent in every channel, and the menisci displacement propagates in a waveform. Due to the spring effect of the vapor bubbles, the fluid motion is sequentially transmitted from one branch to another along the PHP. Moreover, they showed that the oscillation frequency is influenced by the heat flux and heat energy is accumulated in the solid walls. From a direct visualization of a horizontally oriented FPPHP, [PERSON] and [PERSON] [20] found that the nature of the working fluid significantly affects the oscillating motion of liquid slugs. With water, no oscillations were observed, whereas large oscillation amplitudes and velocities (up to 5 cm/s) were reached with ethanol under same operating conditions. On the other hand, [PERSON] et al. [21] found maximum velocities around 0.6 and 1 m/s, and frequencies from 2.5 up to 4 Hz, for heat powers of 80 W and 200 W, respectively, in their vertically oriented FPPHP with horizontal channels and filled with ethanol. From a comparison between a multi-turn, tubular PHP and a FPPHP having transparent inserts, [PERSON] et al. [9] observed that oscillation amplitudes increased with the heat load to the evaporator in both configurations, although the tubular PHP exhibited much larger amplitudes. The absolute measured fluid velocities were about 8 to 10 times higher than those inside the FPPHP. The tubular PHP denoted more vigorous, large-amplitude oscillations with respect to the flat-plate layout. [PERSON] and [PERSON] [22] proposed a mass-spring-damper model for the fluid displacement description in FPHPs during dry-out phases and until the large amplitude oscillations mode. The model was validated by coupling high-speed videos with fluid temperature/pressure measurements on a ten-turn micro FPPHP, thus highlighting optimal conditions for normal device operation in a horizontal orientation. [PERSON] and [PERSON] [23] designed and experimentally validated, with rather good accuracy, a mass-spring-damper model that considered not only volume variations in slugs and plugs, but also mass variations due to evaporation/condensation phenomena. Such a model has also been proven to perform better than the usual ones where mass variations are generally neglected. Although FPPHPs with transparent windows have been extensively studied in terms of flow characteristics, to the authors' knowledge, a significant amount of data regarding fluid oscillations in fully metallic devices is missing in the literature.