Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Microgravity Research Opportunities for the 1990s: Chapter 3 Microgravity Research Opportunities for the 1990s PART IIâSCIENTIFIC ISSUES 3 Fluid Mechanics and Transport Phenomena INTRODUCTION The field of fluid mechanics and transport phenomena plays a dual role in the microgravity research program. It stands as a distinct disciplinary area but also appears as a theme running through other microgravity disciplines. Presentations in other chapters of this report reflect the role of the fluids and transport discipline as providing supportive science for other microgravity disciplines. REPORT MENU Many fluid mechanics and transport phenomena (heat and mass transfer) NOTICE are influenced directly by gravity. Such phenomena are also vital elements in a MEMBERSHIP wide range of physical, chemical, and biological systems, many of which are PREFACE important in both terrestrial- and space-based technologies. The main EXECUTIVE SUMMARY characteristics of a low-gravity environment in relation to fluid mechanics and PART I transport phenomena are the diminished importance of buoyancy-driven flows CHAPTER 1 and sedimentation, and the relative increase in importance of other forces, such CHAPTER 2 as surface tension at fluid-fluid interfaces. These effects have important PART II consequences not only for the conduct of research in other microgravity CHAPTER 3 CHAPTER 4 disciplines but also in the development of many mission-enabling technologies. CHAPTER 5 The fluid mechanics and transport phenomena area of the microgravity research CHAPTER 6 program should have both fundamental and applied objectives: CHAPTER 7 PART III Fundamental problems. A key objective for fundamentally oriented CHAPTER 8 research should be the identification and description of new phenomena that may APPENDIX A influence transport and other applications in microgravity due to the change in APPENDIX B parameter range from the ground-based norm.1 A second general objective for fundamental research should be to focus on problems in which a low-gravity environment can contribute to the fundamental understanding of observed phenomena by providing a unique experimental window. file:///C|/SSB_old_web/mgoppch3.htm (1 of 9) [6/18/2004 11:16:38 AM]
Microgravity Research Opportunities for the 1990s: Chapter 3 Applications. Transport problems are relevant to the other disciplines in the microgravity program and to mission-enabling technologies, including, for example, understanding how altered transport phenomena in microgravity influence the operation or adaptation of biological systems to space, understanding how combustion processes change, or understanding how materials processing is modified.2 Many mission-enabling technologies involve transport phenomena, but predictive models for low-gravity performance and operation are frequently inadequate. A knowledge base should be established for the efficient design, development, and operation of such technologies. STATUS A few crude demonstrations, consisting primarily of the unusual effects of surface tension on fluid behavior in microgravity, were performed aboard Skylab.3 These represented the first fluid experiments in a low-gravity environment. It should be noted that despite the completion of more than 50 space shuttle flights to date, relatively little time was allotted to microgravity research on fluid dynamics and transport phenomena. Six fluid experiments by European scientists were performed in 1983 aboard Spacelab-1. Five of those concerned the effects of capillarity on fluids: isothermal meniscus between two plates of unequal radii; free surface oscillations in drops; liquid-column stability with rotation, vibration, and stretching; axisymmetric free surface behavior due to forced disturbances in partly filled containers; and kinetics of the spreading of a tethered drop. The sixth experiment was designed to study thermocapillary flows in a liquid bridge. Some of these experiments were repeated on the D-1 mission in 1985, along with seven other experiments-two dealing with thermocapillary flows in rectangular containers; two with thermocapillary migration of bubbles and drops; one with diffusocapillary convection in a rectangular container; and two with separation of fluid phases (one in a liquid column and one with bubbles in a reacting liquid). The first U.S. fluid experiments were performed aboard Spacelab-3 in 1985. One was a geophysical fluid flow cell (i.e., a rotating hemispherical shell of fluid with an electric field to simulate radial gravity forces) and another was concerned with the dynamics of rotating and oscillating drops in acoustic fields. The third, which was not primarily a fluid experiment, studied the growth from solution of triglycine sulfate crystals by means of holograms and indicated the reduction, during crystal growth, of buoyancy-driven convection. It was repeated on the IML-1 mission in 1992. The evaluation of Spacelab-1 fluid experiments noted a number of problems, including power limitations, limitations on time available for experiments, an insufficient theoretical basis, shortcomings in equipment performance, and the poor quality of data transfer to the ground.4 Reviews of the experimental results of the D-1 mission5 also indicated numerous problems in file:///C|/SSB_old_web/mgoppch3.htm (2 of 9) [6/18/2004 11:16:38 AM]
Microgravity Research Opportunities for the 1990s: Chapter 3 equipment performance, high g-jitter accelerations, and diagnostics. Thus, many of the results were essentially qualitative and descriptive. The USML-1 mission, which was launched in June 1992, was the first microgravity flight dedicated solely to U.S. scientists. Thirty-one investigations were included in the payload, with two of the three major facilities for fluid research. The first, the Surface Tension-Driven Convection Experiment (STDCE), was dedicated to a single investigator; the second, the Drop Physics Module (DPM), involved three investigators. A Space Acceleration Measurement System (SAMS) was also included to enable the acceleration environment aboard the shuttle to be described accurately. The STDCE6-14 conducted in an interactive mode on the USML-1 mission was the first thermocapillary flow experiment with state-of-the-art diagnostic techniques to yield quantitative flow field and temperature data over a wide range of conditions with two different heating modes and various initial interface shapes, which are unique to the microgravity environment. The nature and extent of large Marangoni number steady thermocapillary flows in a cylindrical chamber were determined. The g-jitter due to thruster firings and other spacecraft activities did not affect the experiments. No flow oscillations were observed even for Marangoni numbers larger than 10, which indicates that the Marangoni number alone is not sufficient to specify the onset of oscillations. The STDCE was designed to determine quantitatively the nature and extent of flows (velocity and temperature distributions) driven in a cylindrical container by temperature gradients along the liquid free surface imposed by different thermal signatures due both to various CO2 laser heat fluxes and to various temperature differences between a heated center post and the cooled wall. The behavior of drops and shells subject to rotational torques by acoustical fields was studied in the DPM. Another experiment involved determining the surface properties of liquid drops in the presence of surfactants and also the coalescence of droplets with surfactants. The third investigation in the DPM measured the liquid-liquid interfacial tension of a compound drop. A glovebox facility was also aboard that mission. Thirteen qualitative experiments that were developed relatively inexpensively in a short period of time were accommodated in this way. Six of these were related to fluid research: two were on thermocapillary flows (with one generalizing the results of the STDCE), three dealt with capillarity (with two related to the DPM experiments), and one was concerned with the aggregation of fine particles in air. All of the detailed data have not, as yet, been completely analyzed by the investigators, but the results reported15 are interesting and encouraging. RESEARCH AREAS file:///C|/SSB_old_web/mgoppch3.htm (3 of 9) [6/18/2004 11:16:38 AM]
Microgravity Research Opportunities for the 1990s: Chapter 3 Priority should be given to the study of phenomena that are unique to the low-gravity environment and to those that are critical to space mission-enabling technologies. Fundamental Research Among the basic topics that may be studied with special advantage in the low-gravity environment are the following: Surface tension gradient-driven flows and capillary effects. The effects of flow phenomena driven by surface tension forces are frequently masked by competing gravitational forces in the terrestrial environment but become significant or dominant in reduced gravity. This provides an opportunity for a unique experimental window that can be used to study surface tension-driven convection phenomena, as well as the microscale physics of the interfacial zone (e.g., surface or interfacial rheology in systems with surfactants or the dynamics of the contact line on a solid boundary), all of which remain poorly understood. Fluid interfaces with surface tension variations are also common for many spaceflight-enabling technologies (e.g., boiling, heat transfer, combustion, welding, brazing, or soldering). Processes that rely on gravity-driven separations in the terrestrial environment, such as the removal of gas bubbles from a liquid, must be accomplished by other means in microgravity environments (e.g., by thermocapillary-driven migration via nonuniform temperature fields). Frequently, however, additional study will be necessary to enable practical implementation of these processes. Multiphase flows. Many important phenomena involve multiphase flows or two-fluid systems (e.g., bubbles, drops). In ground-based studies, gravity imposes a specific orientation on multiphase fluids, structures, and phase organizations (e.g., gas-liquid, liquid-solid). Furthermore, the systems are generally too complex for fundamental predictions and simulations. Thus, gravity cannot be separated from the observed phenomena in ground-based studies. In a reduced-gravity environment, the gravitational phase orientation is greatly diminished, and it is expected that multiphase flows and associated transport phenomena will become significantly altered. Low-gravity experiments might not only contribute a foundation for space-enabling technologies, as discussed below, but also contribute to an understanding of the essential physics of terrestrial multiphase flows. Diffusive transport processes. Under normal gravitational conditions, multicomponent fluids experience various modes of thermosolutal convection (depending on the relative orientations of temperature and concentration gradients with each other and with the gravity vector). In addition, there are other effects due to important convection phenomena that derive from the differences in diffusivities for heat and mass. Such phenomena occur in many materials and industrial processes (e.g., macrosegregation). With reduced-buoyancy convection, there is a significant opportunity to obtain a better fundamental understanding of other complex temperature- and concentration gradient-induced interactions, such as Soret and Dufour effects, which are usually overshadowed under terrestrial conditions. file:///C|/SSB_old_web/mgoppch3.htm (4 of 9) [6/18/2004 11:16:38 AM]
Microgravity Research Opportunities for the 1990s: Chapter 3 Colloidal phenomena. Colloidal suspensions are representative of a larger class of systems in which surface forces play a critical role. In many instances, however, understanding the consequences of these forces is complicated by competing gravitational effects. Although gravitational effects can be minimized in the terrestrial environment by density matching, this frequently may change the chemical environment or require introduction of materials different from those of direct interest. Thus, the microgravity environment provides an opportunity for experimental studies of phenomena such as order- disorder transitions in dispersions of hard spheres or electrohydrodynamic effects such as drop deformation in electric fields, which can strengthen the fundamental understanding of those physicochemical interactions and their macroscopic manifestations in complex fluids. The above topics are also important to biotechnology, combustion, and materials science. Some of them are discussed in subsequent sections that deal in detail with those areas. Beyond these specific discipline-based areas, microgravity offers the possibility for experimental studies of fundamental fluid physics phenomena that, in the terrestrial environment, cannot be done or can be done only for limited parameter ranges. One example that has received significant study already is the dynamics of individual bubbles or drops in acoustic fields. Technology Development One of NASA's missions is to "provide technology for present and future civil space missions and provide a base of research and technology capabilities to serve all National space goals."16 Many of the technologies required for NASA programs involve transport phenomena, including power systems; thermal management systems; spacecraft fire hazard management; cryogenic engines; fluid systems tankage; physical and chemical life-support systems; and user- support subsystems such as refrigerators. More specifically, multiphase flow phenomena, which are highly gravity dependent, are central to heat and mass transfer in all systems, and gas-liquid contacting for air purification and energy generation also have a major role in chemical processes such as catalysis and ore beneficiation techniques. Fundamental studies of such phenomena will contribute to better process design for the microgravity environment. Safe and efficient heat transfer design for reduced-gravity applications is currently not possible because fundamental knowledge of convection processes at very low Reynolds number, and of boiling and condensation in a low-gravity environment, is lacking. NASA's long-term mission to explore the universe is also clearly dependent on exploiting the behavior of materials and processes in low gravity. For example, the mining, winning, and processing of extraterrestrial materials will file:///C|/SSB_old_web/mgoppch3.htm (5 of 9) [6/18/2004 11:16:38 AM]
Microgravity Research Opportunities for the 1990s: Chapter 3 be essential for extensive exploration of the solar system. Since it would be costly to transport Earth-based materials to the Moon and other planets, in situ materials must be considered. Current processing techniques will not suffice for obtaining materials under the unusual conditions encountered on other, nonterrestrial, bodies. Technologies to enable processes such as fluidized-bed hydrogenation, electrowinning, and vapor-phase pyrolysis will eventually be needed, and all involve transport phenomena that are affected by gravity. The technologies for life- and operations-support systems, such as power generation and storage, water purification, oxygen production, and fuel and fluid storage and management, also involve gravity-dependent transport phenomena. Thus, if reliable, cost-effective, and efficient technologies are to be developed to enable NASA's mission to be successful, the fundamental understanding of the effects of gravity on many transport phenomena is essential. The committee believes that the following topics, in addition to being of intrinsic scientific interest, are critical to provide the knowledge base required to design effective and reliable space-based systems and facilities in which fluid processes are involved: Convective processes at low Reynolds number. Buoyancy affects the transport of heat and mass when the Reynolds number becomes small enough in the terrestrial flow of fluids with density variations. Investigation of low Reynolds number flows with density variations, at reduced gravity levels, can reveal the altered nature of the transport phenomena. Transport processes with a phase transition. Aspects of condensation, evaporation, and boiling are influenced by both gravity and interfacial forces. How such phenomena differ in a low-gravity environment, with interfacial forces dominant, requires further study. Complex materials. The structure and transport of complex materials are often influenced by gravitational effects; for example, multiphase flows in porous media, flows of powders and granular materials, the motion and stability of foams, and the sedimentation of colloidal dispersions. Research is necessary to understand the behavior of such materials in a microgravity environment. This research may also lead to a better understanding of the behavior of complex materials at 1 g. Materials processing. Buoyancy, sedimentation, and interfacial phenomena influence such important processing methods as fluidized-bed hydrogenation, electrolysis and electrowinning, and vapor-phase pyrolysis. Their features in microgravity should be investigated. Physical processes in life- and operating-support systems. Some of the effects indicated above apply to such processes as power generation and storage, water purification, oxygen production, and fuel and fluid storage and management. file:///C|/SSB_old_web/mgoppch3.htm (6 of 9) [6/18/2004 11:16:38 AM]
Microgravity Research Opportunities for the 1990s: Chapter 3 SUMMARY The modifications in fluid flows and in heat and mass transfer processes that occur in the transition between the terrestrial environment and microgravity underlie modifications that occur in many biological processes, materials behavior, combustion, and a number of mission-enabling technologies. These changes result from the diminished significance of buoyancy for gravity-driven motions and the relatively greater importance of other forces. The microgravity environment also provides unique conditions for experimental studies that can contribute new understanding of fluid flow and transport processes that are significant primarily to Earth-based applications. For these reasons, a research program on fluid physics, aimed at primarily fundamental studies of fluid mechanics and transport phenomena that are partly or completely masked at 1 g, has been under way for the past several decades, and the committee recommends that this program be continued. Among the topics that this program should cover are surface tension-driven flows and capillary effects; multiphase heat transfer and fluid flows; and the behavior of complex fluids such as colloids, granular media, and foams, whose dynamical behavior can be strongly influenced by gravity. In addition, some emphasis should be placed on microgravity transport processes that will play a role in the design and operation of future space facilities, such as low Reynolds number convection, boiling, evaporation or condensation, or methodologies that may be required for materials processing, such as fluidized-bed hydrogenation, electrowinning, or vapor-phase pyrolysis. It is further recommended that steps be taken to enhance interaction between the research community and the engineering community that will design, build, and operate future space facilities to help prioritize studies related to "enabling technologies." Similarly, steps should be taken to foster interdisciplinary research involving fluid dynamics and transport processes in other areas such as the life sciences, materials processing, and combustion. REFERENCES 1. Ostrach, S. 1982. Low-gravity fluid flows. Ann. Rev. Fluid Mech., 14:313-345. 2. Ostrach, S. 1988. Industrial processes influenced by gravity. NASA Contractor Report 182140. 3. Ostrach, S. 1974. Skylab science demonstrations. Proceedings of the ESRO Symposium on Processing and Manufacturing in Space. file:///C|/SSB_old_web/mgoppch3.htm (7 of 9) [6/18/2004 11:16:38 AM]
Microgravity Research Opportunities for the 1990s: Chapter 3 4. European Space Agency. 1985. Assessment of the Results of Fluid Science and Materials Science Experiments Conducted During the Spacelab-1 Mission. 5. Natur Wissenschaften. 1986. D-l Erste Ergebnisse der deutschen Spacelab Mission. Vol. 7, June. 6. Ostrach, S., Y. Kamotani, and A. Pline. 1993. USML-1 Surface Tension Driven Convection Experiment: Preliminary analysis of the data. In International Symposium on Microgravity Science and Applications, Beijing, China. 7. Kamotani, Y. 1993. Results from the Surface Tension Driven Convection Experiment. Presented at Gordon Research Conference. 8. Pline, A., T. Jacobson, Y. Kamotani, and S. Ostrach. 1993. Surface Tension Driven Convection Experiment. AIAA Paper 93-4312. Presented at AIAA Space Programs and Technologies Conference, Huntsville, Ala. 9. Kamotani, Y., S. Ostrach, and A. Pline. 1993. A thermocapillary convection experiment. 1993 ASME Winter Annual Meeting, New Orleans, La. 10. Kamotani, Y., S. Ostrach, and A. Pline. 1994a. Some results from the Surface Tension Driven Convection Experiment aboard USML-1 Spacelab. 1994 AIAA Aerospace Sciences Meeting, Reno, Nev. 11. Kamotani, Y., S. Ostrach, and A. Pline. 1994b. Some velocity field results from the Thermocapillary Flow Experiment aboard USML-1 Spacelab. 1994 COSPAR Meeting, Hamburg, Germany. 12. Kamotani, Y., S. Ostrach, and A. Pline. 1994c. Analysis of velocity data taken in Surface Tension Driven Convection Experiment in microgravity. Physics of Fluids, 6:11. 13. Kamotani, Y., S. Ostrach, and A. Pline. 1994d. Summary of results from the Surface Tension Driven Convection Experiment in microgravity. 45th International Astronautical Congress, Jerusalem, Israel, October 9-14. 14. Kamotani, Y., S. Ostrach, and A. Pline. 1995. A thermocapillary flow experiment in microgravity. Journal of Heat Transfer, in press. 15. Ramachandran, N. 1994. Joint Launch + One Year Science Review of USML-1 and USMP-1 with the Microgravity Measurement Group. NASA Conference Publication 3272. 16. Space Systems and Technology Advisory Committee. 1991. Advanced Technology for America's Future in Space: A Review of NASA's file:///C|/SSB_old_web/mgoppch3.htm (8 of 9) [6/18/2004 11:16:38 AM]
Microgravity Research Opportunities for the 1990s: Chapter 3 Integrated Technology Plan for the Civil Space Program. Preliminary draft. Last update 4/12/00 at 4:08 pm Site managed by Anne Simmons, Space Studies Board The National Academies Current Projects Publications Directories Search Site Map Feedback file:///C|/SSB_old_web/mgoppch3.htm (9 of 9) [6/18/2004 11:16:38 AM]
