Cape Canaveral, 25 July 2019: A falcon 9 rocket carrying a Dragon spacecraft takes off for the International Space Station (ISS) at 18:01 local time. Alongside several NASA experiments, the spacecraft is carrying the boiling experiment Rubi (Reference mUltiscale Boiling Investigation) onboard. It was developed by scientists at the Institute for Technical Thermodynamics (TTD) at TU Darmstadt in cooperation with international partners under the umbrella of the European Space Agency (ESA) and was shrunk to the size of a shoe box for its use in outer space. Rubi has now been successfully sending measurement data back to Earth for six months. It is designed to provide insights into which physical processes of boiling, or more precisely the transition phase where a liquid turns into vapour, can be influenced and in what way.
Almost everyone is familiar with this phenomenon from working in the kitchen: When a kettle heats up water for a cup of tea, it firstly starts to simmer: Single bubbles filled with vapour start to form initially at the bottom and then in increasing numbers, rising up to the surface. At 100 degrees Celsius, the water is at full boil and would completely evaporate if the kettle was not switched off. The advantage of the boiling process is that it uses the liquid and also the gaseous phase of a fluid and a lot of energy is transferred during this transition phase. “Boiling is one of the most efficient processes available for transferring energy in the form of heat”, explains Axel Sielaff, a scientist at the TTD. It is used, for example, to cool high-performance electronics. Using the measurements taken by Rubi, Sielaff and his team now want to develop more precise models for the heat transfer process.
They are conducting research at the interface between mechanical engineering and physics and specialise in everything to do with boiling and evaporation. The current focus of their work are vapour bubbles. “We want to understand the physical phenomenon associated with the creation of vapour bubbles better than has been possible up to now”, says the expert. Sielaff boots up the PC in his office at the Lichtwiese Campus and selects one particular experiment from numerous cryptic files. A black-and-white film starts up on the screen. The extremely sharp image shows how a bubble is “ignited” in a controlled manner on a heated surface, slowly grows in size and then comes to a standstill.
Experiments that are only possible in a weightless environment
What is just a fascinating recording for a layperson, is a source of valuable and detailed information for the experts in thermodynamics. They can analyse the measurement data on the physical properties of the boiling experiments to find out, amongst other things, how much heat can be transferred to a particular point on the surface of the heater, how much energy is expended to create a bubble in the first place, how the temperature in the liquid develops and what influence different combinations of parameters have on the geometry of the bubbles and overall amount of heat transferred.
These are experiments that are only possible with this level of precision in a weightless environment. This is because processes that happen extremely fast on Earth occur in slow motion on the space station. For example, vapour bubbles form at one single boiling point in a household kettle or in a research laboratory on Earth at a frequency of around 100 per second. In the best case scenario, this figure can be reduced down to zero in space according to Sielaff. This means that the entire development phase for every single vapour bubble can be observed from all possible perspectives and under different framework conditions. What's more, vapour bubbles are only tiny on Earth. In weightlessness, they can reach a diameter of up to ten millimetres.
Making an experiment such as Rubi suitable for use on the ISS and ensuring it runs reliably is no mean feat. During the interview with Sielaff, a technical problem is currently being resolved in Belgium at the User Support and Operations Centre (B-USOC), which controls and monitors Rubi from Earth. Yet Sielaff remains calm. 550 of the 850 experiments that were planned have already been successfully completed at this point in time. This is already considered a triumph. Once this type of experiment has left Earth, the scientists are no longer able to directly intervene in the experiments. After the spacecraft successfully docked with the ISS, Rubi was also connected up manually to the European research module Columbus by the ESA astronaut Luca Parmitano. Control over the experiments was then taken over remotely by the experts at the B-USOC.
Parabolic flights for optimization
“The most difficult issue when developing this type of application for use in space is that it is not possible to test the absence of gravity on Earth.” For this reason, Sielaff and his team also optimised their experiment in parabolic flights. In order to achieve weightlessness without simply flying into space, this type of flight on a specially equipped Airbus is used to generate alternating phases of 2G and zero gravity – a strenuous workout that the research group completed once a year by flying from Bordeaux out over the Atlantic or Mediterranean. The experimental parameters here are subject to much greater variations than on the ISS. However, the researchers are able to directly influence the experiments on the plane, test different liquids and also quickly change the parameters.
The measurement data recorded during the parabolic flights will be compared with the data generated on the ISS. Sielaff estimates that around 15 to 16 terabytes of data, including around 15 million images, have been transmitted back to Earth from the space station for the Rubi experiment alone since the start of the measurements – firstly via satellite to the base station responsible for the Columbus module in Oberpfaffenhofen and then on to the B-USOC that is responsible for Rubi, where the data that was scrambled in space is recompiled into one file and then sent to the scientists on the core Rubi team in Darmstadt, Pisa and Toulouse for analysis.
Once the measurement phase has been concluded, the evaluation phase begins. The results could be groundbreaking not only for enabling more environmentally friendly cooling and heating of equipment and facilities on Earth, such as computers, data centres, air conditioning systems, batteries or power plants, but also for optimising the thermal regulation systems used in satellites or spacecraft according to Axel. “We want to use our basic research for the development of products that are not just safer and more compact but most importantly also more efficient.”
The boiling experiment Rubi (Reference mUltiscale Boiling Investigation) is part of the Fluid Science Laboratory (FSL) in the European ISS research module Columbus. A heater that was developed and built in Darmstadt heats up a coolant. A laser “ignites” single bubbles at a defined location. A high-speed camera captures the entire development of this bubble, while an infrared camera measures the heated region. Rubi also contains a pump to generate a shear flow, meaning that the liquid continuously flows over the heated surface from one side. High voltage can also be generated in the experimental space using an electrode to investigate the influence of an electric field on the development of the bubbles. The experiments are saved on the ISS but can also be followed live from Earth.
Under the coordination of Prof. Dr.-Ing. Peter Stephan, Head of the Institute for Technical Thermodynamics (TTD) at TU Darmstadt, 14 universities and research institutions from Europe, Russia, Japan and the USA are participating in Rubi, including the Department of Energy, Process and System Engineering at the University of PISA, the “Institut de Mécanique des Fluides de Toulouse” (IMFT) and the Multiphase Dynamics Group at the Aristotle University of Thessaloniki. The experiment was financed by the European Space Agency (ESA), jointly developed with Airbus Defence & Space and is controlled by the Belgian User Support and Operations Centre (B-USOC). In order to share and jointly optimise the evaluation methods developed as part of Rubi, the TTD initiated the programme “Code exchAnge for Rubi Analysis Tools” (CARAT) which can be accessed by all research partners. Rubi is part of the Thermo-Fluids & Interfaces Profile Area at TU Darmstadt.