ZARM experiment on sounding rocket
On 13 February 2012 at 10:30 CET, the sounding rocket maser 12 was launched from Esrange Space Center near the North Swedish town Kiruna for its six-minute flight in the service of microgravity research. On board was an experiment operated by the Center of Applied Space Technology and Microgravity (ZARM), which examines the processes occurring in case of spontaneous heating of a liquid. The results will provide a first clue to the question of what happens in the fuel tank of a spacecraft when exposed to a strong heat source.
SOURCE 2 (Sounding Rocket Experiment Compere) is the second in a series of experiments in which scientists observe the behavior of a test fluid in a container that has extremely different temperature zones. In a very simplified form we are familiar with this phenomenon from boiling water in a saucepan. At the hottest point of the vessel we can see that bubbles are formed as soon as the water begins to boil. The bubbles rise from the liquid as hot steam, part of which steam hits the cooler lid of the saucepan and is condensed again into wat
The same evaporation and condensation effects were now observed under microgravity conditions. The central unit of the experiment set-up is a glass cylinder that was heated to about 130° C at the top and cooled to about 35° C at the bottom. After the start of the MASER-12 rocket, the cylinder was filled with cool liquid (hydrofluoroether) and hot vapor (150° C) of the same substance. The video recording shows that where the liquid touches the hot walls of the test cell, bubbles are formed. Due to the lack of gravity these bubbles do not rise, but are carried along with the prevailing currents.
Of particular interest is the evaluation of the pressure conditions during the experiment. Because of the evaporation of the liquid on the hot wall, the pressure rises inside the closed cylinder. The condensation of the hot gas at the cooler liquid on the other hand, results in a decrease of pressure. Surprisingly, the ZARM team observed that the influence of the condensation was stronger than the effect of evaporation - resulting in a drop instead of an increase of pressure. These observations provide an important clue concerning the question of how a fuel tank in space will react to a short term partial overheating and whether - or how long - the interaction between evaporation and condensation prevents the development of overpressure. The evaluation of the entire data of the three-minute test period is expected to last several months. The ZARM scientists are positive to gain further important insights essential for the design of future fuel rocket engines that are operated with cryogenic propellants, such as liquid hydrogen or oxygen.
In May 2008 six valuable minutes under weightlessness could already be used for the first SOURCE experiment on the MASER-11 rocket. Both experiments are included in the "Microgravity Application Program" of the the European Space Agency (ESA). SOURCE 2 was carried out in collaboration with Air Liquide, Grenoble, Toulouse IMFT and EADS Astrium Bremen. The hardware and the flight were funded by the ESA and the ZARM research team by the German Aerospace Center (DLR).
The experiment consists of a transparent quartz glass test cell which has a square ground area of 80 mm x 80 mm. The height is 120 mm and the bore diameter is 60 mm. Solid, liquid and vapor temperatures are measured by thermocouple groups positioned at specific regions. The wall temperature is measured in the region of the expected contact line. Liquid temperatures are measured shortly below and along the expected liquid-vapor interface. The vapor temperature is measured in the region of the rotational axis shortly above the liquid-vapor interface. The test fluid is a perfectly wetting HFE-7000 with a zero degree static contact angle. Data of the saturation curve, provided by the manufacturer, are well known.
In the beginning of the experiment the test cell is evacuated and preheated. At steady state the top interface has a temperature of 130 °C, whilst the bottom part, connected to a cooling plate, is set to 35° C. The temperature evolution within the quartz cell in vertical direction yields a temperature gradient of 1 K/mm in the region of the expected contact line. Automatic filling with cold liquid started at 65 s after lift-off and continues for 25 s. Microgravity is achieved at 75 s. At the same time the test cell was pressurized for 10 seconds with hot vapor up to 0.21 MPa. The final desired position of the free surface was adjusted at 112 s by manual filling with a reduced volume flow rate of 3 ml/s. Afterwards the pressure was adjusted to 0.2 MPa at 126 s. The wall superheat temperature was set to 20 K at the expected contact line and nucleate boiling could be observed.
Since the influence of condensation at the free surface is obviously stronger than evaporation effects in the vicinity of the wall the decreasing pressure had to be corrected again at 143 s up to 0.25 MPa. This way nucleate boiling could be suppressed and an oscillating and fluctuating but stable free surface could be observed until the beginning of the second experiment phase at 180 s.
Contact for technical questions:
Prof. Dr. Michael Dreyer
Head of working group "Fluid Dynamics and Multiphase Flow"
michael.dreyer @ zarm.uni-bremen.de
0421 218 - 57866
Contact for general press inquiries:
Head of Communications
birgit.kinkeldey @ zarm.uni-bremen.de
0421 218 - 57755