Microgravity experiments for a better understanding and modelling of boiling phenomena

Abstract

Boiling is of significant importance to many technical processes and applications on earth and in space. It is a complex process since it involves various physical parameters which govern the heat and mass transfers. Despite the numerous correlations that exist, the prediction of boiling heat transfer remains difficult. For better modeling, it is therefore important to improve our understanding of local physical phenomena, such bubble growth and detachment of isolated bubbles. The continuous detachment of bubble due to buoyancy during boiling in terrestrial gravity condition complicates the investigation. Microgravity experiments are therefore relevant for observing larger bubbles on longer time scales. It is the objective of the Multiscale Boiling experiment RUBI designed for more than a decade by various European teams, under the framework of ESA (European Space Agency) Project BOILING. RUBI was operated on the International Space Station from 2019 to 2021. Experiments have been performed in pool boiling and in shear flow, both with and without the presence of electric field. The fluid used is a refrigerant N-perfluorohexane. Bubbles are nucleated on an artificial cavity located on a surface heated with heat flux from 0.5 to 1.5W/cm2. Different liquid subcooling up to 10°C and flow rates are investigated. High-speed Black and White camera records the images of the bubble growth and departure. The temperature field at the surface of the heated wall is measured by an infrared camera. We analyze the bubble growth, its departure from the nucleation site and its detachment from the wall. The bubble diameter evolves as the root mean square of the time, characteristic of a growth controlled by thermal diffusion. The bubble departure diameter by sliding along the wall is found to be well predicted by a balance between the drag force exerted by the flow and the capillary force acting at the contact line. After its departure, the bubbles slide along the heated wall and they detach from the wall after a coalescence mechanism which is known as coalescence-induced jumping bubble, similar to this observed for droplets in condensation.

Registration

To register, email Melania Torok, Executive Assistant at chemeng.ea@ucl.ac.uk