Bubble coalescence and break-up in confined oscillating two-phase flows under microgravity conditions

Passive two-phase heat transfer systems, such as pulsating heat pipes, are a promising thermal management solution for the rapidly growing space sector. The strong coupling between thermal and hydraulic phenomena complicated the development of reliable off-the-shelf components. This was due to the difficulties in establishing and maintaining the desired slug-plug flow pattern in a wider region sufficiently far from the annular flow, which can lead to evaporator dry-out. In this work, inertial effects on the flow pattern are investigated under adiabatic conditions to define what limits the operability range of a device. The fluid motion characteristics of pulsating heat pipes have been mechanically induced to observe conditions that lead to the break-up or coalescence of bubbles. The authors were granted access to the ESA drop tower microgravity platform to explore surface tension dominant flows and higher hydraulic diameters compared to the ground capillary diameters. A range of fluids (FC-72 and ethanol) and diameters (2.5 mm to 8 mm) has been explored, along with combinations of oscillating fluid flow, analysing high-speed images and estimating velocity and acceleration. Both the Reynolds number, in combination with the modified Bond number, have been investigated, plotting the operating points on a flow map, showing a very clear limit between break-up and coalescence when the velocity and acceleration signs are considered. This new conceptual flow map can be used to further improve existing modelling tools for pulsating heat pipes operating under reduced gravity. Furthermore, images demonstrating the break-up and coalescence may be aid to CFD scientists in verifying their results.