## 1 Introduction  
During the last decades, the extensive employment of micro-electronics in almost every engineering process has been leading to critical thermal issues related to high heat power densities in micro-processor chips [1]. In fact, high operating temperatures reduce the performance and long-term reliability of electronics, eventually causing fatal failure. Among other thermal management systems, two-phase passive heat transfer devices represent remarkable solutions for the cooling of electronics due to their capability to operate without the need for any external energy sources [2]. In particular, pulsating heat pipes (PHPs) are receiving growing academic and industrial interest thanks to some fascinating features, such as good flexibility, high reliability and low manufacturing cost for both ground and space applications [3, 4]. Depending on their geometry, PHPs can be usually classified as tubular PHPs or flat-plate PHPs (FPHPs) [5]. While the former devices have a better flexibility and generally higher thermal performances, the latter present higher compactness, usually coupled with a better capability of spreading and dissipating high-heat flux rates by conduction through their solid structure [6, 7]. For this reason, FPHPs might be successfully employed in those applications requiring good thermal contact on narrow surfaces, such as in the micro-electronics field. It has to be stressed that, when compared to other passive two-phase heat transfer solutions having flat layouts, e.g., flat-heat pipes, FPPHPs do not present any porous structure for the liquid backflow to the evaporator section [8; 9; 10]. On one side, this represents a critical aspect for the FPPHPs performance since the fluid motion cannot be controlled by the capillary effect of the wick, resulting in generally lower reliability. Nonetheless, the absence of a porous medium significantly decreases the manufacturing cost in terms of raw materials and wick design/optimization. The attractive features of good thermal spreading capability and low fabrication cost have highlighted the industrial relevance of FPPHPs as valuable thermal management solutions worth investigation. Although a great experimental effort has been devoted to the study of FPPHPs under many different working conditions [11], their working principles are still not fully clarified. The experimental analysis on FPPHPs has been mainly focused on global characterizations, for example, on the evaluation of equivalent thermal resistances for varying influencing parameters, such as inner diameter, working fluid, number of turns, operating temperature and tilt angle [12; 13; 14; 15; 16; 17]. Other works attempted a more precise description of the internal FPPHP operation by means of direct fluid visualizations. In fact, FPPHPs are suitable for simultaneous visualizations of the fluid flow in every channel by utilizing transparent windows, i.e., by using sheets of glass or polymeric materials [18], with a rather low manufacturing effort. Such direct visualizations lead to interesting thermo-fluid dynamics considerations, which give a deeper insight into the heat transfer behavior of FPPHPs in terms of bubble generations, liquid-vapor interface evolution and liquid/vapor location inside either the evaporator or the condenser sections. The number of visualization experiments on FPPHPs has grown exponentially in the last years, and some of the most meaningful ones for the present analysis are reported here. [PERSON] et al. [19] showed that two-phase bubble flows in square channels tend to enhance the heat transport, compared to pure liquid flows. [PERSON] et al. [20] suggested that non-uniform device geometries are beneficial for heat dissipation since the condensation processes, studied by means of direct visualizations, seem to be more efficient due to higher condensation rate and bubble activity in the condenser zone.