Innovations in pulsating heat pipes: From origins to future perspectives

Since the early 1990s, the pulsating heat pipe (PHP) has emerged as one of the most innovative, effective and potentially more convenient passive two-phase heat transfer systems, thanks to its good performance, versatility, and construction simplicity. On the other hand, the PHP is characterized by complex thermohydraulic behaviour that still presents a true challenge to designers, which has led to significant interest by a growing number of researchers. The technological readiness level (TLR) of this technology is quite broad depending on the application: for instance, the industrial community is starting to consider the PHP as a reliable solution for electronic cooling in ground conditions, while implementations in the cryogenic temperature range and in space environments is also being extensively explored. This vision paper aims at shedding light on the current knowledge and prediction capability of PHP numerical models, on unsolved phenomenological issues, on the current technological challenges and the future perspectives of this fascinating heat transfer device. Specifically, after a general introduction and a brief overview of the current knowledge and the open issues of PHPs, special focus is devoted to the following topics: flat-plate PHP assessments; advancements in PHP modelling and simulation; flow stabilization techniques; non-conventional fluids subdivided into fluid mixtures, self-rewetting fluids, nanofluids; cryogenic applications, space applications, and finally the newest frontiers of flexible PHPs. Each section is accompanied by a brief roadmap providing directions for future research based on key challenges, which are also gathered and summarized in the final outlook section.