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VII Programmatic Recommendations The design of successful HEDS systems and components, and decisions concerning alternative designs, will necessarily be done by the application of computational analysis codes that have been carefully developed and validated and that are based on physically correct mathematical models. This is true of all modern system engineering practice and will be especially true for the elaborate systems needed by HEDS. These codes will be needed at all scales, including those of the smallest components, but in many cases there is insufficient information available for the development of reliable computational models. Moreover, the data are often costly to obtain at the gravity levels of interest. For this reason, special attention must be directed toward the design of the experi- mental test matrices required for model (i.e., code) validation. All these things being true, these codes and their underlying mechanistic models, even more so than the codes for aircraft engine analysis and design, for example, must be physically rather than empirically based. This is because of the need for fidelity in variable-gravity environments and because they must be robust and flexible enough to credibly evaluate innovative concepts. NASA research must supply the information needed to build these models and codes, especially that needed to delineate the ranges of the applicable scaling laws. To ensure progress in developing the desired computational analysis and design capabilities, such research must clearly in part be directed research based on an evolving understanding of the programmatic needs. This will be a significant task, requiring patience, commitment, and careful peer review. In this context, NASA should investigate the possibility of consulting the U.S. Department of Energy-Naval Reactors program for help in designing research programs that can develop multivariate, physically based, multiphase computational models. In the following paragraphs, suggestions are made concerning goals, research planning, and activities of NASA in support of gravity-related research for HEDS. A number of these recommendations were contained in the phase I report (NRC, 1997) and are reflected again here, in this more extensive study. It was thought then, and is still believed, that in view of the long time scale needed for the evolution of basic scientific concepts into practical applications, the suggested research programs will require a sustained commitment on the part of NASA to understand gravity-related issues. 185
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186 MICROGRAVITY RESEARCH A RESEARCH APPROACH FOR THE DEVELOPMENT OF MULTIPHASE FLOW AND HEAT TRANSFER TECHNOLOGY The purpose of this section is to recommend that NASA give consideration to adopting an integrated approach for the conduct of research on multiphase flow and heat transfer technology for HEDS applications. As pointed out in earlier discussions of multiphase and heat transfer flow issues, the reliable operation of multiphase systems and processes in reduced-gravity environments is likely to be required if the HEDS program is to be successful. In particular, phase-change systems are likely to be necessary for power production, propulsion, and life support. Unfortunately, relatively little is currently known about the effect of gravity on multiphase systems and processes. Nevertheless, it is known that the impact can be large (e.g., the flow regimes will be strongly affected). It should be stressed that the empirical approaches that have been used for the design of multiphase systems on Earth will not be reliable for space applications. Moreover, it will not be possible to parametrically test in space the required full-scale multiphase systems and subsystems until a suitable design is achieved. Rather, the approach recommended in this report is that a reliable, physically based analytical model be developed and qualified against appropriate terrestrial and microgravity data. The resultant computational model could then be used to analyze and optimize final designs and to compare the relative merits of various alternate designs (e.g., Rankine vs. Brayton cycle power plants). Significantly, these analytical models could be used to develop new design features and to analyze, and support, separate effects experiments and the scale-up of those systems and subsystems that cannot be tested at full scale in space. Indeed, it is vital that NASA have the capability to scale up data from small- scale tests if the HEDS research and development program is to be timely and cost-effective. As is described in Chapter IV, the most detailed and advanced mechanistic models of this type that are currently available for the analysis of multiphase flow and heat transfer are multidimensional, multifield, two-fluid computational fluid dynamics (CFD) models. This type of model has been developed by the U.S. DOE-Naval Reactors program and used by the Navy for numerous applications on Earth. Similar models are being developed in France (for the Atomic Energy Commission of France) under the FASTNET program (Bestion et al., 1999), which is focused on nuclear reactor applications. Fortunately, it appears that these models can be extended to applications involving reduced-gravity environments (Alajbegovic et al., 1999; Lahey and Drew, 2000~. In order to carry out the various activities that are needed, as described above, to develop a reliable multidi- mensional, multifield, two-fluid CFD model for HEDS use, a directed program of experimental and analytical research will be required. The range of specific terrestrial and microgravity data needed to successfully develop and test the model is unlikely to be obtained in a reasonable time frame without a coordinated, focused effort on the part of NASA. In particular, careful attention must be paid to understanding and modeling such important phenomena as flow regimes (including flow-regime transitions), phase distribution, phase separation, multiphase turbulence, Marangoni forces, boiling/condensing heat transfer, multiphase pressure drop, static and dynamic instabilities, and condensation-induced loads in reduced-gravity environments. As part of this effort, the dimen- sionless parameters that characterize the relevant fundamental multiphase phenomena should also be cataloged and the various distinct operational regimes should be identified. COORDINATION OF RESEARCH AND DESIGN Considerable difficulty in the generation of this report arose from NASA's intra-agency organization, in the sense that outstanding research efforts in some centers were either poorly communicated to groups at other centers or duplicated the efforts of those other centers. The committee felt that such difficulties were symptomatic of a counterproductive "territoriality" that has been allowed by NASA senior management to develop and even to accelerate. Such organizational roadblocks will obviously be detrimental to the implementation of many of this committee's central research recommendations. The NASA office responsible for microgravity research (presum- ably the Microgravity Research Division) should diligently inform NASA at large about the issues of reduced gravity that are foreseen for space hardware design, so that these issues enter design thinking at the conceptual stage rather than as afterthoughts. Conversely, the microgravity research community should be kept apprised of design issues whose resolution requires the understanding that would be gained through phenomenonological
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PROGRAMMATIC RECOMMENDATIONS 187 research. The peer review process is crucial for ensuring the quality of supported research, but that does not preclude NASA from taking a leadership role in encouraging research on programmatic needs. NASA should encourage the blending of conceptual design and phenomenological research. Often, phenom- ena depend strongly on boundary conditions or on the conceptual design of devices, and in such cases it becomes crucial to involve basic researchers in the development process in a fruitful way. For example, the subject of fundamental aerodynamics has been developed in close association with aircraft wing design. To accomplish this melding of research and design is not easy; it requires attentive and sophisticated management. Equally, communication and coordination among basic researchers and system designers and users will be essential for HEDS success and should be actively encouraged. For example, problems of bearing and seal integrity in low or variable gravity suggest a need for novel bearing concepts, which no doubt would raise significant fundamental research issues. Also, new concepts for flow control devices to replace conventional valves should be encouraged and pursued through research, in order to provide long-term reliability and to assure the desired control of dynamic effects in distribution systems. An especially significant example of the need to connect component development with research concerns swirling-flow devices intended to provide the centrifugal force needed for phase separations. While the basic principle of desired operation is understood, device-specific issues of great subtlety will determine success or failure. Therefore, a variety of configurational concepts should be explored, computationally to the extent fea- sible. Also, novel concepts for phase separation and mixing by wave processes, oscillations, or flow deflection should be explored through appropriate research activities. It is further recommended that the process of gathering and exchanging information relevant to research directions in support of HEDS missions be strengthened by holding workshops and study groups attended by both mission technologists and microgravity scientists. The August 1997 workshop at NASA's Lewis Research Center in Cleveland was just such an endeavor; experts presented a well-organized, ambitious program on the various activities that are focused on Martian exploration. The program used carefully prepared presentations and work- shop discussion panels to exchange very useful information. It was a learning experience for the participants and brought them up to date in a way that could not have been easily done by other means. The committee recom- mends that workshops of this kind be an ongoing feature of NASA's gravity-related research program. In particular, workshops should be held, at least biannually, at which technology issues are presented to the scientific community and relevant microgravity research is presented to the engineering community. Such workshops should culminate in two types of recommendations: the first type would involve assessment of the applicability of current research results to the technological issues pertaining to current plans for space exploration; the second type would focus on definition of future directions for microgravity research in support of the plans for space exploration. The presentations on both science and technology, along with the recommendations developed, should be published as a workshop proceedings. The recommendations for the directions of microgravity research should be reflected in NASA Research Announcements. It should be emphasized that the goals of HEDS require the development of complex systems that depend on knowledge from traditionally distinct fields such as biology, fluid physics, and materials science. Such systems are needed to address life-support needs and in situ resource recovery efforts. Therefore, it is suggested, as it was in the phase I report (NRC, 1997), that NASA might profitably initiate and support selected interdisciplinary research projects. The problem of human bone loss in reduced gravity is clearly an interdisciplinary one and one that must be of great concern to the microgravity research community. While the dimensions of this problem are uncertain, its undeniable importance justifies encouragement and support of appropriate research by the microgravity and other relevant divisions in NASA. Specific research on the physical effects of spacecraft rotation is recommended in Chapter VI. Beyond that, novel concepts short of spacecraft rotation that might hold promise for countering the effects of reduced gravity simultaneously for humans and technical devices should be encouraged and pursued. Furthermore, fluid-mechanical expertise in the microgravity research community may help to explain fundamental bone-modeling processes, or at least to interpret such processes in relation to proposed countermeasures.
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188 MICROGRAVITY RESEARCH MICROGRAVITY RESEARCH AND THE INTERNATIONAL SPACE STATION It is expected that the International Space Station will provide a unique platform for conducting long-duration microgravity scientific research and assessing the efficiency and long-term suitability of many of the technical systems important to HEDS. Unlike previous space shuttle experiments, which by necessity were confined to component-level tests and typically operated for only a few days, the semipermanent operating status of the International Space Station will permit testing of integrated hardware systems over time intervals that are long enough to address critical HEDS system operation requirements. By incorporating hardware systems that are designed for extended, autonomous operation, critical HEDS technology issues can be addressed, including: (1) conduct of multiphase flow and heat transfer experiments in support of computational model development; (2) identification of fundamental channel or system instabilities that are either produced by or modified by microgravity and the subsequent establishment of appropriate system modifications or control strategies to avoid or control those instabilities; (3) development of instrumentation and control systems that can tolerate gradual system degradation and yet respond automatically and appropriately to operational upsets, distinguishing between instrumentation degradation or failures and actual system failures, while maximizing safety, survivability, and long-term hardware reliability; and (4) development of the necessary autonomous operational principles that will be required to automatically shut down and "safe" complicated processing hardware units during serious system upsets, to serve purposes similar to what is accomplished by methodologies employed currently for spacecraft computer systems that experience unexpected upsets when they are far from Earth. An example of this type of opportunity is the development and extended operation of an autonomous water electrolysis system for the International Space Station that can convert wastewater into oxygen for life support (and hydrogen exhaust). Development of that system as an advanced, redundant life-support system for the International Space Station will mean the simultaneous evolution of a key ISRUi processing module that can be used for primary life support on crowed missions to Mars and for producing oxygen and hydrogen feed gas (for the production of other chemicals) on the surface of Mars or in the polar regions of the Moon, and for similar HEDS water electrolysis opportunities on other microgravity surfaces such as asteroids and icy comet cores. The extended operation of systems of this type in the microgravity environment of the International Space Station is a unique opportunity to subject future HEDS technology to realistic tests prior to actual deployment. The foregoing comments appeared in the phase I report (NRC, 1997) and are reiterated here with the added thought that the Space Station will be an especially important facility for the multiphase flow research program urged earlier in this chapter. PEER REVIEW FOR REDUCED-GRAVITY RESEARCH The NASA Research Announcement and peer review mechanisms have been of great benefit to the productiv- ity and quality of NASA's gravity-related research, as pointed out in the phase I report (NRC, 1997~. These mechanisms should be maintained as the areas of science and engineering affecting HEDS technologies are further developed, ensuring the pursuit of truly scientific objectives while enhancing the HEDS enterprise. REFERENCES Alajbegovic, A., D.A. Drew, and R.T. Lahey, Jr. 1999. An analysis of phase distribution and turbulence in dispersed particle/liquid flows. Chem. Eng. Commun. 174:85-133. Bestion, D., J.-P. Clement, J.-M. Delhaye, P. Dumaz, J. Gamier, D. Grand, E. Herview, D. Lebaigue, H. Lemonnier, C. Lhuillier, J.-R. Pages, I. Toumi, and M. Villand. 1999. FASTNET: A proposal for a ten-year effort in thermal-hydraulic research. Multiphase Science and Technology 11:79-145. Lahey, R.T., Jr., and D.A. Drew. 2000. The analysis of two-phase flow and heat transfer using a multidimensional, four-field two-fluid model. Proceedings of the Ninth International Topical Meeting on Nuclear Reactor Thermal Hydraulics (NURETH-9), October 3-8, 1999. Nuclear Engineering and Design, in press. National Research Council (NRC), Space Studies Board. 1997. An Initial Review of Microgravity Research in Support of Human Exploration and Development in Space. Washington, D.C.: National Academy Press. In situ resource utilization.
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