20 research outputs found
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Development and extended operation of a high power radiation loaded heat pipe
A high temperature, high power molybdenum-lithium heat pipe has been fabricated and tested at 1500 K for 1700 hours with radiant heat rejection. Power throughput during the test was approximately 14 kW, corresponding to an axial flux density of 11 kW/cm/sup 2/ for the 1.59 cm diameter heat pipe. Radial flux density was 70 W/cm/sup 2/ over an evaporator length of 40.0 cm. Condenser length was approximately 150 cm with radiant heat rejection from the condenser to a coaxial water cooled radiation calorimeter. A plasma sprayed, high emissivity coating was used on the condenser surface to increase the radiant heat rejection during the tests. The heat pipe was operated for 514 hours at steady state conditions before being damaged during a planned shutdown for test equipment maintenance. The damage was repaired and the initial 1000 hour test period completed without further incident. After physical examination of the heat pipe at 1000 hours the test was resumed and the heat pipe operated at the same conditions for an additional 700 hours before conclusion of this test phase
Plant functional and taxonomic diversity in European grasslands along climatic gradients
Aim: European grassland communities are highly diverse, but patterns and drivers of their continental-scale diversity remain elusive. This study analyses taxonomic and functional richness in European grasslands along continental-scale temperature and precipitation gradients.
Location: Europe.
Methods: We quantified functional and taxonomic richness of 55,748 vegetation plots. Six plant traits, related to resource acquisition and conservation, were analysed to describe plant community functional composition. Using a null-model approach we derived functional richness effect sizes that indicate higher or lower diversity than expected given the taxonomic richness. We assessed the variation in absolute functional and taxonomic richness and in functional richness effect sizes along gradients of minimum temperature, temperature range, annual precipitation, and precipitation seasonality using a multiple general additive modelling approach.
Results: Functional and taxonomic richness was high at intermediate minimum temperatures and wide temperature ranges. Functional and taxonomic richness was low in correspondence with low minimum temperatures or narrow temperature ranges. Functional richness increased and taxonomic richness decreased at higher minimum temperatures and wide annual temperature ranges. Both functional and taxonomic richness decreased with increasing precipitation seasonality and showed a small increase at intermediate annual precipitation. Overall, effect sizes of functional richness were small. However, effect sizes indicated trait divergence at extremely low minimum temperatures and at low annual precipitation with extreme precipitation seasonality.
Conclusions: Functional and taxonomic richness of European grassland communities vary considerably over temperature and precipitation gradients. Overall, they follow similar patterns over the climate gradients, except at high minimum temperatures and wide temperature ranges, where functional richness increases and taxonomic richness decreases. This contrasting pattern may trigger new ideas for studies that target specific hypotheses focused on community assembly processes. And though effect sizes were small, they indicate that it may be important to consider climate seasonality in plant diversity studies
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High temperature heat pipes for waste heat recovery
Operation of heat pipes in air at temperatures above 1200/sup 0/K has been accomplished using SiC as a shell material and a chemical vapor deposit (CVD) tungsten inner liner for protection of the ceramic from the sodium working fluid. The CVD tungsten has been used as a distribution wick for the gravity assisted heat pipe through the development of a columnar tungsten surface structure, achieved by control of the metal vapor deposition rate. Wick performance has been demonstrated in tests at approximately 2 kW throughput with a 19-mm-i.d. SiC heat pipe. Operation of ceramic heat pipes in repeated start cycle tests has demonstrated their ability to withstand temperature rise rates of greater than 1.2 K/s
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Development of high-temperature liquid-metal heat pipes for isothermal irradiation assemblies
This paper describes the development of high-temperature heat pipes and their operating performance using liquid metal working fluids to provide high heat transfer assemblies for in-pile testing of UO/sub 2/ fuel. The fuel assembly consists of thin UO/sub 2/ wafers sandwiched between molybdenum discs, and is one of the components of the space nuclear reactor electrical power plant currently under development. The intended operation of the heat pipes is to control the temperature of the UO/sub 2/ irradiation experiment in the Experimental Breeder Reactor (EBR-II). This application involves vertical operation in a gravity-assist mode, with the evaporator end down. Heat pipe construction and preparation techniques are described. Laboratory tests were made and the performance characteristics determined. Test results are compared with calculated heat transfer limits
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Transient performance investigation of a space power system heat pipe
Start-up, shut-down, and peak power tests have been conducted with a molybdenum-lithium heat pipe at temperatures to 1500 K. The heat pipe was radiation coupled to a water cooled calorimeter for the tests with rf induction heating used for the input to the evaporator region. Maximum power throughput in the tests was 36.8 kw corresponding to a power density of 23 kw/cm/sup 2/ for the 1.4 cm diameter vapor space of the annular wick heat pipe. The corresponding evaporator flux density was approximately 150 w/cm/sup 2/ over an evaporator length of 40 cm at peak power. Condenser length for the tests was approximately 3.0 m. A variable geometry radiation shield was used to vary the load on the heat pipe during the tests. Results of the tests showed that liquid depletion in the evaporator region of the heat pipe could occur in shut-down and prevent restart of the heat pipe. Changes in surface emissivity of the heat pipe condenser surface were shown to affect the shut-down and re-start limits. 12 figs
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High-temperature, deployable, membrane heat-pipe radiator element; demonstration and status
Performance tests of a high-temperature, deployable, membrane heat pipe have been conducted. This system is intended for use in thermal rejection systems for space nuclear power plants. Because current developmental programs for space nuclear power require heat rejection systems in the 2 to 100-megawatt range, development of lightweight, large-area, heat rejection radiators with operating temperatures of greater than 600 K is being investigated. Heat-pipe radiators are potentially the lightest-weight closed-loop systems available in this power and temperature range. Current state-of-the-art radiator designs provide a specific mass in the range of 5 to 20 kg/m{sup 2}. Membrane heat-pipe designs using alkali metals as the working fluids and metal foil for containment, offer the potential for a specific mass of about 1.8 kg/m{sup 2} and a mass-to-power ratio of approximately 0.04 kg/kW at 1000 K. Because the membrane heat pipes are flexible, the radiator may be rolled up for compact storage and shielding between operating periods. Passive deployment is achieved by the internal pressure developed as the working fluid is brought to operating temperature and thus requires no linkages, actuators, or special-purpose elements. Upon deployment, the high-temperature radiator unrolls, like a party favor, to a fully extended position. 5 refs
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Transient heat pipe investigations for space power systems
A 4-meter long, high temperature, high power, molybdenum-lithium heat pipe has been fabricated and tested in transient and steady state operation at temperatures to 1500 K. Maximum power throughput during the tests was approximately 37 kW/cm/sup 2/ for the 1.4 cm diameter vapor space of the annular wick heat pipe. The evaporator flux density for the tests was 150.0 W/cm/sup 2/ over a length of 40 cm. Condenser length was approximately 3.0 m with radiant heat rejection from the condenser to a coaxial, water cooled radiation calorimeter. A variable radiation shield, controllable from the outside of the vacuum enclosure, was used to vary the load on the heat pipe during the tests. 1 ref., 9 figs
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Heat pipe technology development for high temperature space radiator applications
Technology requirements for heat pipe radiators, potentially among the lightest weight systems for space power applications, include flexible elements, and improved specific radiator performance(kg/kW). For these applications a flexible heat pipe capable of continuous operation through an angle of 180/sup 0/ has been demonstrated. The effect of bend angle on the heat pipe temperature distribution is reviewed. An analysis of lightweight membrane heat pipe radiators that use surface tension forces for fluid containment has been conducted. The design analysis of these lightweight heat pipes is described and a potential application in heat rejection systems for space nuclear power plants outlined