Global and local performances of a tubular micro-pulsating heat pipe: experimental investigation

Heat exchanger optimization is mandatory in almost any industrial application. Thanks to their performances, the Pulsating Heat Pipes (PHPs) are a very interesting application. Micro-PHPs, which are defined as PHPs with a tube that has a hydraulic diameter < 500 mu m, have shown big advantages in terms of their ability to dissipate high heat fluxes, their reduced size, and their low weight. However, the great majority of the works that investigate the thermal behavior of micro-PHPs only deal with the average performance of the system, usually represented in terms of global thermal resistance of the device. Our study aims to begin to fill this lack by investigating the local thermal behavior of a typical multi-turn micro-PHP. A micro-PHP characterized by seven turns and realized with a stainless-steel pipe was investigated. It was positioned in a vertical position, with the evaporator at the bottom, and it was partially loaded with HFC-134a. The studied micro-PHP is tubular, while almost the totality of the micro-PHPs investigated to date are constituted by microchannels engraved in silicon-based wafer, and they present a great potential in terms of three-axis flexibility compared to the flat micro-PHPs that are usually investigated. To highlight the different thermal functioning of each turn, an infrared camera was used to acquire the local temperature distributions on the wall of the PHP condenser. It was found that the best performance was reached for a filling ratio of 46% and for a heat input ranging between 1.9-3.7 W. To thoroughly study the pulsating behavior of the proposed PHP, the dominant frequencies were investigated by performing a wavelet analysis. The results allow the identification of different flow regimes, such as start-up, non-persistent oscillating flow (0.05-0.6 Hz; Q(net) < 2.3 W), and quasi-periodic oscillating flow (0.6-1.5 Hz; Q(net) = 2.8-4.7 W). Eventually, the results highlight that the approach proposed herein can provide worthy evidence about the fluid motion inside the PHP, thereby allowing to overcome the limits introduced by the adoption of transparent materials for the direct flow visualization or by the invasive insertion of pressure sensors, particularly in devices with such small dimensions.