**1. Introduction**

The lifespan and reliability of a wide range of electronic components and electromechanical assemblies are often compromised by the poor performance of the thermal control system (TCS). Cooling capacity, weight, and cost requirements are becoming very challenging in high-density PCBs, microprocessors, photovoltaic solar arrays, and actuators, not only limiting the expected performance [1] but also creating safety issues, as in EV battery systems [2]. On the other hand, energy consumption for cooling purposes has critically increased in recent years. Data centers consume 200 TWh each year worldwide [3], where 38% (76 TWh) is estimated to go toward cooling processes. There are a wide variety of available cooling processes for electronics. The most common methods based on two-phase flow are flow boiling [4–10], pool boiling [11–14], and impinging jets [15–18].

Pulsating heat pipes (PHPs) can play a leading role in reducing cooling costs due to their resulting equivalent thermal conductivity that is several times higher than that of pure

**Citation:** Loyola-Fuentes, J.; Pietrasanta, L.; Marengo, M.; Coletti, F. Machine Learning Algorithms for Flow Pattern Classification in Pulsating Heat Pipes. *Energies* **2022**, *15*, 1970. https://doi.org/10.3390/ en15061970

Academic Editors: Luis Hernández-Callejo, Sergio Nesmachnow, Sara Gallardo Saavedra and Dmitry Eskin

Received: 9 February 2022 Accepted: 4 March 2022 Published: 8 March 2022

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copper [19]. Furthermore, no pumping power is required for the circulation of the working fluid. This results in a sensible reduction in complexity, volume, and weight of the TCS. The PHP is a thermally driven heat transfer device patented in the 1990s [20,21], which has seen a growing research interest since then. It is simpler in its construction and more cost-effective, compared to other similar heat transfer devices (e.g., heat pipes, loop heat pipes). It is composed either of a tube bent in several turns or of two plates welded with a serpentine-like path milled on one of the surfaces. Once filled and sealed, a working fluid resides in the PHP as an alternation of liquid slugs and vapor plugs due to the dominant effect of capillary forces with respect to buoyancy. When heat is applied to the evaporator zone, the fluid motion inside the tube is activated, and the pressure fluctuations drive a self-excited [22] oscillating motion of liquid plugs and vapor bubbles, also identified as oscillating Taylor flow. This condition significantly enhances the heat transfer [23] by exploiting both sensible and latent heat.
