Tiangong-2 is going to heaven! See in advance what experiments scientists will do.

2016 is destined to be an extraordinary year, not only because it is the first year of China’s 13th Five-Year Plan, but also because it is about to become a turning point in the development of space science in China. The reason for this is that China will launch Tiangong-2 Space Laboratory in mid-September to build the first real space laboratory in China and lay the foundation for the establishment of China’s space station.

As the "successor" of "Tiangong-1", Tiangong-2 will carry out an unprecedented 14 experiments, making it the "busiest" space laboratory in China’s space history. Then, what scientific experimental projects are carried by Tiangong-2, which is about to be "employed"? What is the value of the scientific problems that scientists have painstakingly explored? For the "Thermal Capillary Convection Space Experiment Project", we invited the deputy chief designer of Tiangong-2 and researcher Duan Li from the Institute of Mechanics of Chinese Academy of Sciences to reveal the incredible scientific experiment on Tiangong-2.

Strange phenomenon of thermocapillary convection

Under the special condition of complete weightlessness of space environment, the thermal capillary flow driven by surface tension becomes the main form of natural convection, and it is also the main factor affecting the heat and mass transport process of space fluid. So, what kind of wonderful physical process is thermal capillary phenomenon? Researcher Duan Li explained that thermal capillary convection is a kind of thermal convection phenomenon related to fluid surface or interface. As we all know, there is an interaction force between molecules on the interface of fluid, that is, surface tension, and with the change of temperature, the surface tension will change accordingly. Therefore, when the temperature distribution on the fluid interface is uneven, the surface tension will be different at different positions, thus forming the phenomenon of driving fluid flow, which is thermocapillary convection.

In addition, the phenomenon of thermal capillary convection is widely used in practical industrial production, especially in the process of high-quality crystal growth. However, scientists’ understanding of thermal capillary flow is still very limited today. Therefore, the study of thermal capillary convection is helpful for human beings to better explore and apply space.

Exquisite "liquid bridge" experimental device

As a typical capillary flow system, liquid bridge is also a model of floating zone method, which is an important method for crystal growth, and it is also the main experimental device of Tiangong-2 thermal capillary convection space experiment project.

Fig. 2 is a schematic diagram of the liquid bridge model. The liquid is located between the upper and lower disks, and the upper and lower disks are heated at different temperatures to form a temperature difference, thus forming a thermal capillary convection on the surface of the liquid. Duan Li said that the experimental equipment can complete operations such as liquid injection, bridge pulling, bridge cleaning, temperature control, temperature and image acquisition, packaging and transmission through control. Among them, silicone oil, the fluid medium used in the experiment, is stored in the liquid cylinder. During the space experiment, liquid injection and bridge pulling are realized by high-precision PI motor traction, and finally the process of bridge construction is completed. The parameters of height-diameter ratio and volume ratio of the liquid bridge are controlled by bridge pulling and liquid injection motor. At the same time, Duan Li specially introduced that the bridge cleaning system is designed and installed in this equipment. Once the bridge is broken during the space experiment, the bridge can be rebuilt by scrubbing the bridge column with the motor-driven bridge cleaning system, which is also one of the important features of the experimental device of this project.

Duan Li said that during the whole experiment, images will be collected by two CCD image sensors located on the liquid bridge, one on the left, one on the right, one on the close and one on the far, and the shape of the liquid bridge will be observed to judge whether the volume of the liquid bridge changes or not. In the process of space experiment, scientists will simultaneously complete the two-way transmission of data downloading and instruction uploading through control software. They can not only conduct experiments by inputting programs in advance, but also inject instructions for scientific operation according to the actual needs of experiments, thus realizing the interaction between heaven and earth.

Essential ground experiment

In addition to the experimental device attached to Tiangong-2, the same experimental device was set in advance on the ground, so what is the necessary reason for setting up the ground experiment? Researcher Duan Li explained that in view of the fact that there are few opportunities and high costs for space experiments, and the relationship between thermal capillary convection and its influencing factors can not be accurately described by simple formulas, it is necessary for ground experiments to provide a scientific and reasonable reference range for the smooth progress of space experiments and lay a foundation for the smooth progress of space experiments.

In the ground environment, scientists built a large-scale liquid bridge contrast experimental platform, using the same multi-channel temperature acquisition device as the space experiment. In addition, in order to observe the flow mechanism of the liquid bridge more comprehensively, sapphire and infrared bridge columns are processed according to the experimental requirements, and the velocity field and temperature field of the liquid bridge flow are observed by particle image velocimetry and infrared thermal imager respectively. Through ground experiments, scientists have accumulated a lot of experimental data to "warm up" the smooth development of space experiments: First, the volume ratio effect proposed by Academician Hu Wenrui was verified for the first time in the ground large-scale liquid bridge experiment, and the influence of crystal geometric parameters on critical conditions was measured; Secondly, the temperature field at the interface of liquid bridge is measured, and the convection mode conversion is obtained: a relatively complete flow field mode conversion process is obtained; Thirdly, the velocity field of liquid bridge is observed, and the flow field structure and two transitions are obtained; Fourthly, the instability, bifurcation and transition process of fluid flow system and its oscillation behavior are analyzed and studied. These accurate data will become an important support in the space experiment of Tiangong-2.

Tiangong-1 is memorable, and Tiangong-2 Space Laboratory, which took over the post this summer, will create more firsts. On this basis, China will build a permanent space station around 2020 and fully operate it in 2022. Recalling that the International Space Station project rejected China, now China said: "We welcome international cooperation." With the increasing strength of science and technology, China is making a magnificent turn from a "big" country to a "strong" country in space.

The article was produced by China, a popular science student, and was authorized to be published in the column "Learning Science with Xiaobian" of CCTV. Please indicate the source for reprinting.

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