A single loop pulsating heat pipe in varying gravity conditions: Experimental results and numerical simulations

Pipes may have broad applications in electronic cooling for ground and space applications. In order to better understand the complex physical phenomena occurring in the device, a Single Loop Pulsating Heat Pipe (SLPHP) has been characterized during the 66th ESA Parabolic Flight Campaign. As simplified PHP geometry, it is utilized in order to analyze the thermo-fluid dynamic behavior at different gravity levels, and hence to provide new evidence regarding the PHP basic working principles. The SLPHP with 2mm inner diameter has been tested in bottom heated mode, varying physical parameters, such as the working fluid (FC-72 and ethanol), the heat power input (from 1W to 24W) and the gravity level: microgravity (0.01g), Earth gravity level (1g) and hyper-gravity (1.8g). Two transparent tubes connect the heated and the cooled sections, allowing local fluid flow visualization. The device is equipped with two accurate pressure transducers, located at the ends of one of the transparent sections (i.e. just after the evaporator zone and before the condenser zone) measuring the fluid pressure. Three heating elements, controlled independently, provide heat to the device in different locations at the evaporator, allowing different heating configurations. In microgravity, the selection of the fluid is an important parameter for such a simplified geometry: for the same heating power input, a slug/plug flow motion is detectable only when the device is filled up with FC-72. As a result, when the SLPHP is gravity assisted, specific heating distributions establish a circulation of the two-phase flow in a preferential direction enhancing the overall heat transfer performance of the device. A set of tridimensional maps, derived from semiempirical correlations usually adopted in literature to estimate the critical diameter at different gravity levels (i.e. through the so-called Garimella and Weber criterion), are drawn for the different fluids tested, liquid velocities, fluid temperatures and critical diameter values. Such set of maps are utilized in order to compare the flow velocity observed experimentally in hyper-gravity, microgravity and Earth gravity level with the critical diameter value calculated from such semi-empirical correlations. Additionally, an enhanced Volume of Fluid (VOF) model is utilised to simulate an imposed slugplug flow within a straight 2mm inner diameter channel, replicating the same global conditions with selected runs from the experimental measurements. The aim is to quantify the various underpinned mechanisms and give deeper insight to the experimental findings.