Technical devices are becoming smaller and more powerful – both in applications on earth or in space flight. If these devices have to be cooled, the heat flow per area to be dissipated also increases. The cooling medium can be single-phase (without phase change) or two-phase (with phase change). The advantage of the phase change process is the fact, the a comparatively large amount of heat (in the form of the enthalpy needed for the phase change) can be dissipated at only small temperature differences. This results in a significantly lower temperature rise of the cooling medium for the same amount of heat to be dissipated than with heat transfer by a singlephase cooling process.
The phase transition between liquid and gaseous phase can be divided into three regimes: free convection, nucleate boiling and film boiling. Nucleate boiling has the most favorable combination of heat transfer efficiency and moderate temperature differences. Therefore it is aimed for to exploit it in processes such as cooling of electrical components in wind turbines and charging stations or in space flight.
In order to get an idea of how complex and fast the nucleate boiling process generally is, one only needs to look at the boiling process of a cooking pot with water on the stove. Currently only empirical correlations exist to describe the hydro- and thermodynamics of nucleate boiling. Almost all of these correlations have been determined under Earth's gravity and lose their validity for microgravity or weightlessness. Consequently, a deeper understanding of the mechanisms of heat and mass transport during nucleate boiling is aimed for. Microgravity experiments are particularly valuable for this purpose, as the processes there are much slower and can therefore be studied more easily.
On a macroscopic level, experiments provide informative findings which can be transferred to empirical correlations, for example. On the smallest space and time level, however, the processes during nucleate boiling proceed too fast to be captured with today's experimental methods. This is where numerical analysis comes into play: By implementing the governing equations and expending enough computational effort, these processes can be observed and analysed on a scale of size and time that is not accessible experimentally. However, numerical analysis only works in combination with experiments. The results of numerical analysis must always be validated against very specific experiments, so that the results can be trusted.
For this purpose, a benchmark experiment called “Reference Multiscale Boiling Investigation” (“RUBI”) was developed under the leadership of ESA. In this experiment, individual bubbles are experimentally investigated in a test cell on the ISS under various external conditions such as laminar shear flow or an electric field. Based on the data obtained in this way, the institute's own solver is expanded and adapted. This solver was developed in 2011 with the open source toolbox OpenFOAM and has been continuously improved since then.
The solver combines high-resolution models at the 3-phase contact line with macroscopic models for evaporation at the phase boundary. As of 2020, it is applicable to nucleate boiling in resting fluid and under laminar shear flow. In the future it shall be extended with the ability to model the effects of an electric field on the contour and movement of one or more bubbles. The imposition of an electric field is one promising way of replacing the non- or barely-existent volume force of gravity in space. This allows to influence the transport of mass, momentum and energy.
Summarized the main aim of the work is as follows: With the numerical analysis of the nucleate boiling process in different conditions a deeper understanding shall be reached. This can contribute to making a small step away from empirical correlations to universally valid relations.
