Weightless research: BTU team investigates convection currents in free fall

They have a week to thoroughly test their experiment after the long transport from Cottbus to Bordeaux, because on September 17, the A310 ZERO-G of the French company Novespace will take off with a total of eleven experiments for research in free fall. Then everything should work perfectly.

From 17-19 September 2024, Prof. Dr.-Ing. Christoph Egbers, Head of the Chair of Aerodynamics and Fluid Mechanics, and a team led by Dr. Vasyl Motuz will fly tests with the current cylinder gap experiment and carry out investigations into thermoelectric convection under zero gravity conditions.

Even though they can draw on many years of experience from a total of 14 years of participation in parabolic flight campaigns, Prof. Christoph Egbers knows: "The tension is very high every time and almost bursting just before departure to France. Preparations for a parabolic flight take around six months and require the full attention of the entire team, which in Cottbus consists of eleven people." Project manager Dr. Vasyl Motus adds: "We build everything ourselves, down to the smallest detail. As we are experimenting with lasers, liquids and high voltage, everything has to be safe and work flawlessly. In addition, we are in close contact with our French colleagues from Novespace for all adjustments and changes to the experimental setup. Everything follows a strict protocol."

The challenge this year is to adapt the experimental setup for each flight. The researchers are investigating the influence of a thermoelectrohydrodynamic force field on heat and mass transfer in three differentiated experimental setups. This requires a high level of concentration and extremely precise work under the stringent safety requirements of the German Space Agency of the DLR.

The data collected in the current project "Dielectrophoretically induced convection (DEPIK)- PFK 2" is intended to provide important information for the development and optimization of heat exchangers, such as those that can be used for satellites. The focus here is onthermal convection in a dielectric fluid within a confined cavity under the influence of an electric force field. Unlike in the laboratory, under microgravity conditions the force field generated by the high electrical voltage is the only mechanism that causes flows to develop in the cylindrical or flat gap.

The Cottbus team has been conducting research in this field for more than 14 years, in close cooperation with French colleagues. Dr. Antoine Meyer from the University of Le Havre in France will also be taking part in this parabolic flight and the team consisting of Dr. Vasyl Motuz, MSc. Matthias Strangfeld, MSc. Yaraslau Sliavin, BSc. Raj Tark Bista.

The parabolic flight campaigns are carried out with a specially equipped aircraft. In order to achieve weightlessness, the pilots fly a special maneuver: they bring the aircraft onto a parabolic flight path. The aircraft climbs steeply upwards from horizontal flight, then reduces the thrust of the turbines and "falls" upwards due to the residual thrust and then back down again after reaching the peak of the parabola, so that weightlessness prevails for a period of around 22 seconds. The scientists use this time for their experiments. After each parabola, the aircraft is brought back into horizontal flight. Depending on the weather conditions, the flights are carried out either along the French west coast or in southern France on the Mediterranean.

The German Space Agency's parabolic flight campaigns take place once or twice a year and usually consist of three flight days with around four flight hours each, during which 31 parabolas are flown. In total, a flight campaign provides around 35 minutes of weightlessness - alternating between normal and double gravity acceleration. The flights are divided into six sets, which are used to verify the experiments.

About the experiment

This time, the parabolic flight experiment contains two different geometries: A plate-slit geometry with liquid in between and a cylinder-slit geometry. High voltage and a temperature gradient can be applied in both. The cylinder gap is created between two vertical tubes placed one inside the other and is bounded at the top and bottom by the top and bottom plates. The gap is filled with an electrically non-conductive (dielectric) oil whose viscosity is in the range of water, for example.

The inner tube is heated and the outer tube is cooled from the outside, so that a temperature difference is imposed. Due to the buoyancy on the ground, this initially leads to the formation of a single convection cell in the gap (base flow), which covers the entire test space. If the temperature difference is increased, this amplification of the thermal drive leads to instabilities. The basic flow takes on new forms. In comparison to the basic flow or pure heat conduction with the fluid at rest, the heat transport between the inner and outer tube is then increased. If a force field in the form of an applied alternating voltage now acts on this system, the inhomogeneous electrical force field and the temperature dependence of the permittivity of the liquid lead to an electrohydro-dynamic force effect. Under earth conditions, this artificial force field disturbs the stability of the buoyancy flow and can increase heat transport.

However, under microgravity conditions, such as those that occur in parabolic flight, the "Archimedean" buoyancy flow becomes negligibly small. The force field created by the high voltage is then solely responsible for the development of flows in the cylinder gap, which can take on a variety of forms, including turbulent flows. These forms of flow - and therefore also the heat transfer between the inner and outer tube - can be controlled by the level of electrical voltage and can also be easily switched on and off. The flow field is visualized and characterized using a novel combination of two optical measurement techniques, shadowgraph and PIV measurement technology. The researchers use additional sensors to measure the flow-induced change in heat transfer between the inner and outer tube.

Expert contact

Prof. Dr.-Ing. Christoph Egbers, Brandenburg University of Technology Cottbus-Senftenberg (BTU), Chair of Aerodynamics and Fluid Mechanics, Tel.: +49 355 69 4868, egbers(at)b-tu.de, www.b-tu.de/fg-aerodynamik-stroemungslehre/

Further information on the parabolic flights:https://www.dlr.de/de/ar/themen-missionen/weltraumforschung/forschung-unter-weltraumbedingungen/forschungsplattformen/parabelflug