A great journey of impressive achievements and a significant presence of Greece in the new era of space exploration has reached its destination with the successful completion of the operation of the RUBI experimental device on the International Space Station, for which the Dynamics of Multiphase Systems group of the Department of Chemistry of the Aristotle University of Thessaloniki worked.
The historic journey of the Greek scientists who collaborated in a multinational team with their colleagues from Universities and Research Institutions of Germany, France, Italy, USA, Russia and Japan, started after almost ten years of preparation. After 350 days in space, it has to present the account of more than 4,500 experiments, which, as they will now find practical application in hundreds of technological fields, are expected to make people's lives easier in many ways.
The RUBI (Reference MUltiscale experiment for Boiling Investigation) was switched off on Saturday 23 January 2021. The Dynamics of Multiphase Systems group of the Department of Chemistry of the Aristotle University of Thessaloniki, led by Professor Thodoris Karapantsios, who is the national representative to the ESA in the field of Space Exploration and Human Missions, was responsible for the real-time operation and execution of experiments from the Remote Telemetry Station at the Department of Chemistry of the Aristotle University of Thessaloniki.
«The successful completion of the space experiment is a moment of emotion and pride for our university family and for the Greek research community as a whole. It is a great honour for the country and the Greek public university to have Greek scientists participating in such emblematic projects, the results of which are relevant to the progress of humanity», said the rector of the Aristotle University of Thessaloniki, Prof. Nikos Papaioannou, adding that «the preparation of doctoral theses by students of the Aristotle University of Thessaloniki in projects related to the conquest of space, constitutes a legacy and a guarantee for greater success in the future and even more dynamic participation of our teams in future missions and major international collaborations».
«What was tested in the 4500 experiments»
The experiments on the RUBI apparatus were concerned with the study of the boiling of coolants in conditions of zero gravity. In particular, boiling under the influence of external forces, such as the forces generated during the flow of liquids inside refrigerant tubes and during the application of strong electric fields, was studied. These forces influence the separation of the vapour bubbles from the boiling surface and therefore determine the boiling efficiency. In conditions of zero gravity, the effect of buoyancy is negated and thus boiling phenomena could be studied in great detail and at different size scales.
The mission took place in two phases. The first phase lasted from July 2019 to March 2020 and the second phase from October 2020 to January 2021. «During the mission more than 4500 experiments were performed -1500 different combinations of experimental parameters over 3 iterations. In all experiments we received telemetry data at the Aristotle University of Thessaloniki via satellite, which concerned the temperature field and the development of steam bubbles during boiling,» Karapantsios explained to APE-MPA.
He said that his team has collected a huge amount of experimental data at various boiling conditions which, he added, «will contribute to a better understanding of the heat transfer mechanisms during boiling and lead to more efficient industrial devices».
«We are now in the process of analysing the measurements in cooperation with all partners. During the ten years of preparation and execution of the RUBI mission, experiments were carried out in our laboratory in a boiling cell with the same geometry as the cell in the International Space Station to make comparisons between measurements in the Earth's gravity field and in microgravity conditions,» the professor added.
It is worth noting that in the context of these experiments, three doctoral theses (M. Vlachou, I. Rios-Lopez, C. Argyropoulos), two postgraduate theses (V. Papadopoulou, M. Bilia) and two postdoctoral researchers (S. Eugenides, O. Economidou) were completed at the Department of Chemistry of the Aristotle University of Thessaloniki.
«Applications of boiling»
The boiling process is found in many technological fields, such as in the energy production and conversion industries, in environmental applications, in food and chemical industries and in space applications. The most important and widespread application of boiling is for the efficient cooling of thermal loads, for example for the cooling of large electronic systems. Boiling allows the rapid transfer of large thermal loads, using a small heat exchange surface (small device size) and with small temperature differences as the driving force.
Despite the wide variety of applications, the boiling process is still not fully understood today. The mathematical correlations that exist in the literature to describe the physical phenomena governing boiling are largely empirical and are therefore valid for limited operating conditions and only when used within the range of parameters for which they were developed. Understanding the mechanisms of heat transfer during boiling can only be achieved by careful study of boiling at different size scales both by experiments and theoretical modelling. For example, reliable data are needed concerning both the growth and detachment of single bubbles and arrays of multiple bubbles for different thermal loads and fluid flow rates.
«The aim of the RUBI mission»
The aim of the experiments on the RUBI device was to investigate key boiling phenomena at different size scales and thereby contribute to a better understanding of the heat transfer mechanisms during boiling and lead to new data that will contribute to the evaluation of theoretical models. Boiling experiments performed in a gravity-free environment allowed the observation of phenomena that in Earth's gravity conditions are distorted by the presence of buoyancy (e.g., microlayer effects between bubbles and the hot wall, Marangoni interfacial flow).
Smaro Avramidou
