Understanding the effects of oscillation and bifurcation in thermocapillary convection is important for the study of a strong nonlinear flow in space. Due to limited space resources and conditions, the experimental payload should be small in size, light in weight, and possess anti-vibration abilities. Space technology breakthroughs, such as performing liquid surface maintenance and the liquid injection without bubbles, can further enhance the technical capacity of microgravity experiments in fluid physics.
Observing the convection transition, temperature oscillation, and the surface deformation of a liquid requires the use of thermocouples an infrared thermal camera, and a displacement sensor. Demonstrating the procedure will be Wang Jia, Wu Di, and Hu Liang, technicians from my laboratory. Begin by constructing a copper annular liquid pool with an inner radius of four millimeters in diameter and an outer radius of 20 millimeters in diameter and a height of 12 millimeters.
Use a 20-millimeter-diameter polysulfone plate as the bottom of the liquid pool, and drill a small, two-millimeter-diameter hole six millimeters away from the center of the plate as the liquid injection hole. Add sharp, 45-degree-angle corners on the inner and outer side walls, and apply anti-creeping liquid to the inner and outer walls to a height greater than 12 millimeters. Next, select an appropriate low-viscosity silicone oil as the working fluid, and heat the liquid to 60 degrees Celsius.
To discharge any gas from the oil, apply less than 150 pascals of pressure for six hours, followed by vacuuming of the liquid storage system until the pressure reaches just below 200 pascals. Then, relieve the valve to allow the silicone oil to fill in the vacuumed cylinder without gas. To set up the injection system for the working liquid, select a step motor to drive the injection and suction of liquid, and apply a solenoid valve to control the on/off switch of the injection system.
Use a universal joint to connect the step motor to the liquid cylinder, and use a four-millimeter-outer-diameter pipe to successively connect the liquid cylinder, solenoid valve, and injection hole. To establish the measurement system, place six thermocouples inside the liquid pool to measure the temperatures at different points, as illustrated in the figure. Place an infrared camera directly above the liquid surface, and rotate the lens to adjust the focus and to collect the temperature field information on the liquid-free surface.
Adjust the displacement sensor to measure the displacement of a specific point of interest on the liquid surface, and use the CCD camera to focus on the liquid surface. Then, record the change of the free surface. To begin the experiment, start the experiment control software, and turn on the power button.
To perform the liquid injection, apply 12 volts to the solenoid valve to open the valve. Next, turn on the motor button to start the motor at a step of 2.059 millimeters to inject 10, 305 milliliters of silicone oil into the liquid pool. When all of the oil has been delivered, turn off the solenoid valve power to close the solenoid valve.
To perform linear heating, set the heating target temperature to 50 degrees Celsius, the cooling target temperature to 15 degrees Celsius, and the heating rate to 0.5 degrees Celsius per minute. For data collection, set the sampling frequency of the infrared imager to 7.5 hertz, the thermocouple frequency and the displacement sensor to 20 hertz, and the CCD frequency to 24 hertz. When all of the parameters have been set, click the data collecting system button, and monitor the temperature, displacement, and other information of interest in the computer software.
At the end of the analysis, turn off the power. These experimental model and measurement methods were integrated in this payload on the SJ-10 satellite. 23 microgravity experiments on surface wave thermocapillary convection are finished.
In these infrared thermal images of the temperature distributions on a liquid-free surface in thermocapillary convection, a variety of oscillatory flow patterns can be observed, including radial oscillations and clockwise and counterclockwise circumferential rotations. In this representative experiment, the temperatures inside the fluid increased linearly with the temperature difference increase, with the temperature field fluctuating periodically once the temperature difference exceeded a certain threshold, indicating that the thermocapillary convection developed from a steady state to an oscillatory state. In addition, the amplitude of the oscillatory temperature grew as the flow field evolved, as indicated in this spectrum analysis, showing that the critical oscillation frequency was 0.064 hertz.
Although the buoyancy convection of the small-scale ground system was weakened, the flow was still a coupling of thermocapillary and buoyancy convections, with different results observed in space experiment results, compared results obtained in ground experiments. By comparing a large number of deformation data for the liquid-free surface measured by the displacement sensor and the temperature data of the fluid measured by the thermocouples, it was also observed that the surface deformation and the temperature field in the fluid began to oscillate at the same time and at the same frequency. These two key technologies, the maintenance of a fluid surface and the injection of liquids without bubble formation, play essential roles in experiment space research.
We hope that the present work can provide a scientific basis and technical support for viewers interested in attempting these techniques.
