摘要:

Two‐phase flow is an excellent alternative to the conventional single‐phase system in transporting large amount of thermal energy at a uniform temperature regardless of variation in the heat loads. In addition, two‐phase flows exist in a wide range of applications and enabling technologies in space. These include material processing and ceramics at ultra high temperatures, bioreactors and life‐support systems, storage and transport of cryogenics, tank filling and fluid management, and in the design of many cold plate assemblies where heating or cooling takes place at the instrument/utility interface. Reliable design of such systems and many others require a thorough understanding of the mechanics of the two‐phase flow under microgravity conditions. Of most interest are the phase distribution in a conduit (the flow patterns/regimes), the pressure drop, and the heat‐transfer rates at different liquid and gas flow rates. Recent progress in the study of gas‐liquid flows at microgravity conditions will be reviewed and discussed in this paper. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47236

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Forest fires that have endangered remote US Air Force sites equipped with radioisotope thermoelectric generators (RTGs) has prompted the assessment of power generating systems as substitutes for RTGs in small scale (10–120 watt) applications. A team of scientists and engineers of the US Air Forces’ Wright Laboratory conductd an assessment of electrical power technologies for use by the Air Force in remote, harsh environments. The surprisingly high logistics costs of operating fossil fuel generators resulted in the extension of the assessment to non‐RTG sites. The candidate power sources must operate unattended for long periods at a high level of operational reliability. Selection of the optimum power generation technology is complicated and heavily driven by the severe operating environment and compounded by the remoteness of the location. It is these site‐related characteristics, more than any other, that drive the selection of a safe and economical power source for Arctic applications. A number of proven power generation technologies were evaluated. The assessment concluded that RGTs are clearly the safest, most reliable, and most economical approach to supplying electrical power for remote, difficult to assess locations. The assessment also indicated that the logistics costs associated with combustion driven generator systems could be substantially reduced through the use of conversion technologies which have been previously developed for space power applications. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47114

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Terrestrial flame fired thermionics took a great leap forward in the earlier 1980’s with the development of reliable long‐lived hot shells. These results were presented by Goodale (1981). The hot shell protects the fractory emitter from oxidizing in the combustion environment. In earlier efforts with supralloys emitters it was found that superalloys were poor thermionic emitters since they operated at too low a temperature for practical and economical use as discussed by Huffman (1978). With the development of Chemical Vapor Deposited (CVD) silicon carbide and CVD tungsten, it became possible to fabricate long‐lived thermionic converters. These results were shown by Goodale (1980). Further improvements were achieved with the use of oxygen additives on the electrodes. These developments made thermionics attractive for topping a power plant or as the energy conversion part of a cogeneration plant as described by Miskolczy (1982) and Goodale (1983). The feasibility of a thermonic steam boiler and a thermionic topped gas turbine plant become a possibility, as shown by Miskolczy (1980). © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47230

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A sealed isotopic heat source (IHS) with a nominal thermal inventory of 60 watts is being developed by the U.S. Department of Energy (DOE) for use in remote terrestrial applications that require isotopic power for electrical power generation. Emphasis is on use in radioisotope thermoelectric generators (RTGs) and dynamic cycle power units. The selected IHS design incorporates technologies developed for prior space and terrestrial IHSs to minimize development cost and span time. A General Purpose Heat Source (GPHS) Fueled Clad (FC), comprised of a plutonium‐238 enriched pressed‐plutonia pellet contained within a vented iridium clad, is the source for thermal energy. The GPHS FC technology was developed by DOE for use in space RTGs. The GPHS FC is, in turn, enclosed within a three‐layer cladding system similar to that developed by DOE for earlier terrestrial heat sources. The cladding system provides for retention of the helium gas generated by the decay of the isotopic fuel and containment of the isotopic fuel under normal operating and accident conditions. Test hardware is currently being fabricated and safety demonstration testing is scheduled to be completed in early 1995. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47083

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

An electric power supply, small enough to be man‐portable, is being developed for remote, terrestrial applications. This system is designed for an operating lifetime of five years without maintenance or refueling. A small Radioisotope Stirling Generator (RSG) has been developed. The energy source of the generator is a 60 watt plutonium‐238 fuel clad used in the General Purpose Heat Sources (GPHS) developed for space applications. A free piston Stirling ENgine drives a linear alternator to convert the heat to power. The system weighs about 7.5 kg and produces 11 watts AC power with a conversion efficiency of 18.5%. Two engine models have been designed, fabricated, and tested to data: (a) a development model instrumented to confirm and test parameters, and (b) an electrically heated model with an electrical heater equipped power input leads. Critical components have been tested for 10,000 to 20,000 hours. One complete generator has been operating for over 11,000 hours. Radioisotope heated prototypes are expected to be fabricated and tested in late 1995. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47092

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

A study of the modulation capabilities of the Cs‐Ba tacitron with different grid designs was made. Results showed the ignition grid potential to be independent of grid design at a modulation frequency, fg, of 1 kHz. At fg=22 kHz, discharge ignition with a 26% transparent grid was harder to achieve than at fg=1 kHz. The grid design greatly affects not only the extinguishing potential, but also the forward voltage drop. Increasing the grid transparency and/or aperture size makes it harder to extinguish the discharge (increases ‖ Vg− ‖), but lowers the forward voltage drop across the tacitron. Increasing fgalso increases ‖ Vg− ‖, but lowers the forward voltage drop across the tacitron. Increasing fgalso increases ‖ Vg− ‖ and lowers the range of Cs pressure for stable current modulation. For a TOPAZ‐II type space nuclear reactor power system (SNPS), the lowest electrical power consumption by Cs‐Ba tacitrons inverters, employing 34% transparent, 0.5 mm diameter holed grid and operating at IC=10 A or 20 A and fg=1 kHz, is ∼6.5% of the total system output power. This electrical power consumption includes that of the grid power supply and the power loss in the tacitron during discharge. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47217

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

Experimental measurements are performed in a Cs‐Ba diode to investigate the validity of the Maxwell‐Boltzman energy distribution for electrons and the homogeneity of the plasma parameters throughout the discharge volume. Results show that the energy distribution is Maxwellian at high discharge current, but deviates from a Maxwell distribution at lower currents. Also, the ion densities and electron temperatures near the emitter and the collector are significantly different. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47207

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This research activity attempts to develop an ultra‐thin capacitor using TiO2as a dielectric film for high‐temperature microelectronics in commerical or military applications. There has been an increasing demand for a high dielectric constant insulator for the replacement of SiO2. TiO2film is promising because it has a high dielectric constant with a sufficiently high breakdown voltage. In spite of continuous efforts toward diamond and SiC film for capacitor applications, it has been difficult to provide sufficient reliability toward LSI fabrication due to recrystallization of the dielectric film which results in a reduction of dielectric breakdown strength. Defect density of the dielectric film significantly increases as the thickness of the film is reduced to sevevral hundred angstroms. Application of the ultra‐thin capacitor to a switch‐load resistor memory cell makes it possible to reduce the memory cell area to one‐third that of a conventional memory cell. Techniques, such as thermal oxidation of sputtered titanium and evaporation of Ti3O5are used to generate TiO2thin film. The quality of the film, electrical properties such as breakdown voltage, dissipation factor, and equivalent series resistance have been evaluated. The prototypical, ultra‐thin, TiO2capacitors are demonstrated. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47212

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

HPTAM, a two‐dimensional Heat Pipe Transient Analysis Model, is developed to model the operation of heat pipes in space environment. This paper describes the approach used in HPTAM for modeling heat and mass transfers in the wick of a heat pipe during the startup from a frozen state. HPTAM uses the Brinkman‐Forchheimer‐extended Darcy flow equations to predict the flow of liquid in the fully‐thawed porous wick, and a modified enthalpy method to calculate the progression of freezing and thawing of the working fluid in the wick. Recommended methodologies for calculating the properties of wire‐screened wicks (such as permeability and effective thermal conductivity) are given, based on analysis of experimental data. The continuum conservation equations for heat and mass transfers in the wick are successfully benchmarked using experimental data for solid‐liquid phase change in porous media in the presence of natural convection. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47213

出版商:AIP    

年代:1995

数据来源: AIP

摘要:

This report discusses development and proof‐of‐concept testing of a new lightweight carbon‐carbon (C‐C) space radiator heat pipe developed under the NASA Civil Spqce Technology Initiative (CSTI) High Capacity Power Program. The heat pipe was filled with potassium working fluid and tested for 11 hours, including startup from ambient temperature with the working fluid initially in the frozen state to near 700 K condenser temperature. Steady‐state heat pipe input power during testing was facility limited to about 300 watts, representing about 50% of the design input power. Post test inspection showed the heat pipe to be in excellent condition after eight thermal cycles from ambient to steady‐state operating temperature. Potential applications, ranging from small spacecraft heat rejection to aircraft and terrestrial uses, are discussed. © 1995American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.47214

出版商:AIP    

年代:1995

数据来源: AIP