Liquid-gas separation - a big challenge in space
Scientists from the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen are successfully testing phase separation equipment on DLR's 33rd parabolic flight and expanding the fluid mechanics basis for future space exploration.
On Earth, we can rely on the fact that gravity has everything under control: Separating a lighter gas from a heavier liquid can be done easily and reliably thanks to hydrostatic pressure. The gas floats upwards and the liquid automatically accumulates at the bottom. A process that we can observe every day in a full petrol tank in a car, where the fuel is directed bubble-free to the engine.
This physical regularity is shaken when we look into space. The environmental conditions there are mainly determined by weightlessness. There is no top and bottom anymore and so the hydrostatic pressure remains ineffective. Nevertheless, it is essential for satellites, space probes or astronautic exploration missions to ensure phase separation of gas and liquid on board space vehicles: Propulsion systems must be supplied with gas and bubble-free fuel, life-support systems must separate a gaseous from a liquid phase, systems for regulating the heat balance must separate a vapor phase from a liquid phase, and even systems that might one day be operated on Moon or Mars to produce or convert raw materials must be designed to ensure phase separation.
The ZARM scientists developed an apparatus that makes phase separation possible under space conditions and expands the basic knowledge on handling liquids in space. The test fluid was selected to resemble the properties of rocket propellants used in space flight, while at the same time being completely harmless to humans and suitable for use in a parabolic flight.
Experiment setup and findings
The test setup consists of a rectangular, about five millimeter wide channel as a liquid line, which is open to the ambient air on one side along a ten centimeter long measuring section. The open measuring section is covered solely by a very fine-meshed metal sieve with pores of only 14 thousandths of a millimeter, which can be described in abstract terms as a porous medium. At the top of the sieve there is a measuring tube, ten centimeters long and five millimeters wide, which is also filled with liquid. In the experiment under weightlessness, it can be observed how the liquid is sucked out of the measuring tube through the metal sieve into the flow channel without ambient air entering the liquid phase as bubbles over the entire open measuring section.
Here the capillary force, the bubble breakthrough pressure and the properties of the porous medium interact: The capillary force - the property of liquids to spread in gaps and tubes - first ensures that the liquid spreads into the fine pores of the entire metal sieve and saturates it with liquid.
The metal sieve functions like a membrane that allows liquid to pass through but blocks gas. The prerequisite for this, however, is that the bubble penetration pressure does not exceed the bubble point, i.e. the point at which the ambient air is drawn through the metal sieve into the flow channel and bubbles form in the liquid stream. The bubble penetration pressure is directly dependent on the size of the pores of the sieve - the smaller the pores, the greater the pressure can be before gas penetrates the liquid stream.
The parabolic flight campaign
The apparatus, developed by ZARM scientists, was successfully tested during the 33rd parabolic flight campaign of the German Aerospace Center (DLR) Space Administration from 12 to 14 March 2019 and proved its full functionality for liquid-gas separation. On three consecutive flight days, the Airbus 310 ZERO G flew over the Atlantic with more than 90 individual parabolas from Bordeaux in France, enabling the scientists to fly in weightlessness for 22 seconds each - an experimental environment that posed a challenge not only for the test apparatus, but also for the scientists who were on board and had to carry out the experiment in the air. The human body reacts immediately to the phases of reduced gravitation (weightlessness) when the plane goes into free fall, and also reacts to the phases of increased gravitation that occur at the beginning and end of the parabolas. Therefore, it was all the more important for the ZARM crew to prepare well for the parabolic flights. This included defining a clear division of tasks within the four-man team, practicing the steps to control the experiment, monitoring the implementation of the previously defined test parameters and ensuring the technical readiness of the experiment setup and data collection.
Relevance of the results
The ZARM scientists' research approach is dedicated to application-oriented basic research and serves to achieve good measurability and observability of fluid mechanical processes. The chosen experimental setup is therefore not the representation of a concrete technical design of a component for space vehicles. With the results of the "Investigations on liquid-gas separation using porous media with compensated gravity", however, a quantitative database is created and made available which allows the transfer of the findings to concrete applications in space exploration and thus forms a basis for innovative concepts of liquid pipelines. In addition to the data obtained from the parabolic flight campaign, the research team also has access to a series of data from experiments previously carried out in the Bremen Drop Tower at ZARM. The project is funded by DLR Space Administration.
Contact person for questions regarding the research project:
Prof. Dr. Michael Dreyer
Contact person for general press inquiries and photo material:
Tel: +49 421 218‐57817
Information from DLR about the parabolic flight: