Pulsating heat pipes: Critical review on different experimental techniques

Pulsating heat pipes are modern two-phase passive heat transfer devices which have been achieving resounding interest within the scientific community due to their great flexibility, heat transfer capability and reliability. The operation of such systems is based on thermally driven oscillations of the working fluid through the adiabatic section between a heating zone, named evaporator, and a heat sink, named condenser. Capillary effects, coupled with phase-change phenomena and consequent pressure fluctuations, ensure self-sustained fluid motion without the need of any auxiliary electrical equipment (pumps). Despite the number of experimental works on pulsating heat pipes has exponentially grown during the last decade, a full understanding of their working behaviour is still far from being achieved. Hence, no established design techniques are nowadays available, undermining a large-scale application of these devices from an industrial standpoint. Such a strong limitation is not only due to the complex physics underlying these heat transfer systems, but also to the investigation procedures generally employed, which might not be able to comprehensively disclose unclear aspects concerning the pulsating heat pipes operation. The present review work aims at critically examine typical experimental techniques by pondering strengths and weaknesses of the approaches proposed so far. The literature analysis highlights a general lack of experimental standardization, which could prevent the identification of meaningful figures of merit for the quantitative pulsating heat pipes description and for a satisfactory prediction of heat transfer performance under different working conditions.