Study on flow structure transition in thermocapillary convection under parallel gas flow

Crystals materials exhibit excellent physical properties, making them viable for use in modern science and technology development. Among the methods available for the preparation of high-quality crystals, the floating zone method, driven mainly by surface tension and thermocapillary convection, has been commonly used to prepare monocrystal silicon. However, thermocapillary affects some key crystal characteristics such as the quality of the crystallization and may require the incorporation of other methods to eliminate. Previous researches have revealed that thermocapillary conception is greatly affected by the influence of the velocity, direction of parallel gas flow. Nevertheless, most of the studies on the effects of parallel gas flow on the thermocapillary convention are mainly focused on the effects of parallel gas flow on the particle velocity and free surface. As such, a detailed understanding of the effects of parallel gas flow on the flow structure in the non-isothermal liquid bridge is highly desirable.

In an effort to address the above challenge, at Northeastern University: Professor Ruquan Liang, Xiaochen Jin (Ph.D. Student), Dr. Shuo Zhang in collaboration with Professor Shuo Yang from Shenyang Institute of Engineering and Dr. Jianhui Shi from Linyi University investigated the influence of direction, temperature and velocity of parallel gas flow on the flow structure in the non-isothermal liquid bridge. In this approach, the authors utilized a particle image velocimetry (PIV) technique coupled with a 10 cSt silicone oil as the working fluid. Their main objective was to gain a better understanding of the underlying mechanism of the effects of parallel gas flow on the flow pattern inside the non-isothermal liquid bridge. Their work is currently published in the journal, Experimental Thermal and Fluid Science.

Results showed that when the parallel gas flow entered from the upper disc, the thermocapillary convection was initially suppressed but later enhanced when the velocity of the parallel gas flow increased to 3 m/s. Meanwhile, an increase in the parallel gas flow velocity from 0 to 2 m/s resulted in the formation of a larger blank below the central axis of the flowing liquid as well as the migration of the newly formed sickled-shaped convective vortices towards the fee surface. However, the flow field returned to its initial state upon a further increase in the velocity. On the other hand, when the parallel gas flow entered from the lower disc, the blank areas below the central axis gradually increase with the increase in the parallel flow velocity. As such, the thermocapillary convection was weakened because the vortexes were closer to the free surface. Furthermore, considering the significant effect of the flow structure inside the liquid bridge on the temperature of the parallel gas flow, thermocapillary could be inhibited in two ways: reducing the maximum temperature difference between the free surface of the liquid bridge and the parallel gas flow and introducing the parallel gas flow from the lower disc.

In a nutshell, the study investigated the flow structure transition in thermocapillary convection under the influence of the parallel gas flow. Based on the results, thermocapillary convection can be significantly weakened when the parallel gas flow enters from the lower disc. Altogether, the study presented an in-depth understanding of the mechanism of the effects of parallel gas flow on the flow pattern inside the non-isothermal liquid bridge and can be of great significance in developing effective methods for the preparation of high-quality crystals.