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NASDA has so far conducted many microgravity experiments by means of sounding rockets and Space shuttle. After the microgravity experiment planning processes for those experiments, we concluded that in order to fully utilize the microgravity environment of space we need to have a better knowledge base on subjects such as thermophysical properties of the samples, fluid behaviors in the experiments, and so on. Needless to say, fluid physics should play a central role in microgravity science, since microgravity affects mainly the state of fluid and vapor. To obtain accurate information from an experiment, we should know various thermophysical properties such as temperature dependence of viscosity, surface tension, density, and so on, determined by the specific experiment purposes. We also should know about related material science such as crystal growth and so on, if the experiment deals with material processing under microgravity. In order to optimize the experimental parameters to accomplish its experiment objectives, we must develop experimental technology such as design of specimen container, measurement of temperature, etc. A successful microgravity experiment requires much prior knowledge, as mentioned above, therefore we should accumulate sufficient knowledge base to accomplish the experiment objectives, which can be done by the so-called team research method consisting of many knowledgeable researchers.
For that reason the Space Utilization Research Program (SURP) was established in order to do team research in collaboration with researchers from outside NASDA. The role of SURP in microgravity science is defined to perform systematic research for demonstrating the potential of microgravity utilization by producing SHOWCASE and to promote microgravity utilization as the frontier in Microgravity Science Research.
Much effort has been made to try to understand the effectiveness of microgravity utilization so far, but only limited systematic results have been derived from microgravity experiments. NASDA has sought appropriate microgravity research themes related to the above and selected the following subjects; thermocapillary flow in fluid physics, high quality semiconductor production in materials science, and diffusion behavior modeling in fundamental physics. The present report is written for the thermocapillary flow project.
Convective motion induced by local variations of surface tension along a liquid free surface is called Marangoni convection. Although such variations can be caused by differences in temperature or composition, our main interest here is temperature-induced convection, often called thermocapillary convection. In a terrestrial environment, Marangoni convection is usually overshadowed by buoyancy-driven flow. In the reduced gravity environment of space, however, buoyancy is greatly reduced and Marangoni convection could become very important. In such applications as crystal growth from melts and two-phase flow with heat transfer, Marangoni flow is known to play an important role. For that reason much attention has been given in recent years to Marangoni convection. Since the floating-zone crystal growth process is considered to be a promising method of obtaining high quality crystals in microgravity, Marangoni convection in floating-zone melts is an important subject.
Much work has been done on Marangoni convection in the past, both experimentally and theoretically. Most of the experimental investigations were conducted in normal gravity but some results from microgravity experiments are now available, as summarized in Table 1. High Prandtl number (Pr) fluids have been used in most experiments. Although Pr of crystal melts is generally smaller than unity, much less experimental information is available for low Pr fluids due to some experimental difficulties. The transport phenomena in the floating-zone melt have been simulated by many investigators in the so-called liquid bridge configuration, in which a liquid column is suspended vertically between two differentially heated rods.
One important feature of Marangoni convection in the liquid bridge configuration is a transition from steady to oscillatory flow, as illustrated in Figure 1. Since oscillations have significant implications to crystal growth, it is important to understand how and when the transition occurs. Despite the fact that much research has been conducted on the oscillation phenomenon in the past twenty years, the cause of oscillations is not yet fully understood. For example, recent experiments under microgravity conditions showed that the Marangoni number corresponding to the onset of oscillations is dependent on the size of the liquid bridge as seen in Figure 2, suggesting that the Marangoni number alone cannot determine the conditions of the transition. The subject is obviously very complex and requires extensive experimental, numerical, and theoretical efforts to solve it. That is the main motivation behind the present Marangoni Convection Modeling Research. The major objectives of the project are: (1) to investigate oscillatory Marangoni convection in the liquid bridge configuration experimentally, numerically, as well as theoretically, and (2) to determine the cause of oscillations and construct a physical model to delineate it.
Figure 1 Transition Phenomena of Thermocapillary Flow
Figure 2 Dependence of Critical Marangoni Number on the size of the liquid bridge
The present project deals with low, medium, and high Prandtl number fluids. Each Pr range has unique features. In the high Pr range (Pr larger than about 10), although many experimental data can be found in literature, no accurate stability analyses nor numerical simulations of the oscillation phenomenon are currently available. One important aspect is that dynamic free surface deformation, albeit small, may play an important role in the oscillation mechanism. Therefore, more experiments and analyses are necessary to understand the oscillation phenomenon in this Pr range. The phase relationship between the dynamic free surface deformation and surface temperature oscillations in the hot corner has been successfully determined by the simultaneous measurements of both in this year, so our understanding of a role of dynamic free surface deformation in the oscillation mechanism has increased appreciably. Some important features of oscillatory flows are also being investigated experimentally and numerically. In the low Pr range (Pr less than about 0.1), the flow is known to become at first three-dimensional due to a hydrodynamic instability and then becomes oscillatory (see Figure 1), according to theoretical and numerical analyses. The result needs to be proven experimentally. We have successfully overcome a difficult problem of oxidation at the free surface of low Pr fluid, molten tin, thus enabling us to obtain reliable experimental data. The experimental apparatus has unique features for obtaining and sustaining a clean free surface of the fluid and for detecting surface temperature oscillations. Numerical simulations of oscillations in low Pr fluid have given us important information and the predictions will be compared with the experimental results. In the medium Pr range (Pr about unity), it is known theoretically that the flow becomes oscillatory, which needs to be confirmed experimentally. We have been able to reduce evaporation from the free surface of medium Pr fluids near room temperature, but its effect is still important. However, meaningful results have been obtained concerning oscillatory thermocapillary flow in conjunction with linear stability analysis of medium Pr fluid including evaporation.
A Marangoni Convection Modeling Research group has been formed under the sponsorship of NASDA to study the various aspects of Marangoni convection described above. As reported in this document, the group has made significant progresses in the past year. Based on the results from these investigations, we decided to conduct microgravity experiments to further our research. The planned microgravity experiment is to verify the physical model, which was proposed by our lead scientist, Prof. Kamotani, to describe the onset of oscillatory thermocapillary flow. To apply the Microgravity Science Research International Announcement of Opportunity (IAO) 2000, a proposal was prepared as a result of deep discussions and hard efforts with all of the members, and was submitted. The proposal document is attached in Appendix. We hope that this proposal could be one of references as a guide when researchers desire to apply to microgravity announcement of opportunities. The results from the present project will be very important to provide scientific knowledge whether or not the experiment were conducted in space or on the ground.
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