Research paperVisualization of He II boiling process under the microgravity condition for 4.7 s by using a drop tower experiment
Introduction
Superfluid helium (He II) is often used as a coolant in space-related applications. Thus, the knowledge regarding heat transfer occurring in He II in a microgravity environment is highly sought after. However, the experimental research on heat transfer in He II under microgravity conditions is infrequent [1], [2]. Especially, He II boiling experiments are limited because parabolic flight experiments require both high safety and substantial funding [2]. Our research group has investigated the boiling of He II in a microgravity environment by using a drop tower.
This is also interesting from the physical point of view, because the heat transfer mechanism of boiling under microgravity is associated with zero subcooling. It is well known that He II heat transfer accompanied by boiling depends on the subcooling caused by the presence of a hydrostatic pressure head [3].
Several microgravity experiments have been previously conducted using the drop tower at the Hokkaido center of the National Institute of Advanced Industrial Science and Technology (AIST), Japan [4], [5], [6]. The AIST drop tower can produce microgravity conditions for only approximately 1.3 s. During such a short period, the boiling process does not seem to reach a steady state [4], [5], [6]. In the present study, the microgravity experiments were conducted using the drop tower located at ZARM in Bremen, Germany. The ZARM drop tower can produce a microgravity environment for approximately 4.7 s. The experimental study was aimed at registering the He II process up to steady-state conditions.
Section snippets
Experimental setup
The present experiments were conducted using the drop tower of ZARM. The entire experimental set-up, including driving batteries, was set in an air tight capsule, which was dropped in a 122 m high vacuum tube, under a pressure not exceeding 50 Pa. After approximately 4.7 s from the start of the free fall, the capsule was stopped in a shock absorber tank filled with vast polystyrene foam beads. The peak deceleration reached approximately 45g (1g is the gravitational acceleration on the Earth’s
Visualization results
Fig. 3(a)–(d) shows typical visualization results of the vapor bubble growth in He II under microgravity conditions. In the initial phase, the bubble oscillated because of the wire length; the wire is not an ideal point of heat source. The bubble always formed a perfect sphere, and its size measurements yielded very similar results along both optical axes. However, the bubble center did not overlap with the mid-point of the heater wire. This may be because of the non-uniformity of the heater’s
Conclusion
The visualization experiment of He II boiling under microgravity conditions was successfully conducted using a 122 m high drop tower located in ZARM, Bremen, Germany. The process of bubble growth was registered up to the point of reaching the steady state. This process can be roughly described using the equations based on the kinetic theory in the case of both He II and He I. However, this method could not be applied to predict the behavior of bubble formation at the vicinity of the lambda
Acknowledgements
This study was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (B) 25289300, the grant by JAXA and by European Space Agency HRE/UP/2016-01/AO.
The operation of the present experiments was supported by the team at Zentrum für Angewaudte Raumfahrtte chnologie und Mikrogravitatation (ZARM) FAB mbH. We would especially like to thank Dr. Thorben Konemann, Mr. Jan Siemer and Mr. Thorsten Lutz. In addition we thank Mr. Shuichi Goto (Jecc Torisha) for his help in the development of the
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