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This paper describes modeling and test results for a two Loop Heat Pipes (LHP) with a condenser designed to be used in a deployable spacecraft radiator system. These units were provided by the two major commercial US vendors of LHPs. The condenser used is a freeze-tolerant, parallel-flow design based on the International Space Station (ISS) deployable radiator design. This condenser allows the deployable radiator to be utilized without freeze-protection heaters.

This paper describes modeling results for the on-orbit thermal response of spacecraft propellant used for orbit stationkeeping. The specific objectives of the work are to improve analytical models of a transient thermal Propellant Gauging System (PGS) in use on communications satellites. This simple method uses tank wall heaters and temperature sensors and a transient energy balance to obtain periodic measurements of remaining propellant fill. The fill levels must be related to the on-orbit sensor measurements through a thermal model of the system. In the past, relatively simplified representations have been used due to the complex liquid/gas geometry when in a weightless state. In the current work, advanced modeling tools are utilized to allow a more detailed representation of the actual liquid/gas geometry. Model results are presented for a range of fill levels later in life to assess PGS measurement resolution.

In recent years, loop heat pipe (LHP) technology has transitioned from a developmental technology to one that is flight ready. The LHP is considered to be more robust than capillary pumped loops (CPL) because the LHP does not require any preconditioning of the system prior to application of the heat load, nor does its performance become unstable in the presence of two-phase fluid in the core of the evaporator. However, both devices have a lower limit on input power: below a certain power, the system may not start properly. The LHP becomes especially susceptible to these low power start-ups following diode operation, intentional shut-down, or very cold conditions. These limits are affected by the presence of adverse tilt, mass on the evaporator, and noncondensible gas in the working fluid.

During 1995, we tested instruments and attempted a seed-to-seed experiment with Super-Dwarf wheat in the Russian Space Station Mir. Utah instrumentation included four IR gas analyzers (CO2 and H2O vapor, calculate photosynthesis, respiration, and transpiration) and sensors for air and leaf (IR) temperatures, O2, pressure, and substrate moisture (16 probes). Shortly after planting on August 14, three of six fluorescent lamp sets failed; another failed later. Plastic bags, necessary to measure gas exchange, were removed. Hence, gases were measured only in the cabin atmosphere. Other failures led to manual watering, control of lights, and data transmission. The 57 plants were sampled five times plus final harvest at 90 d. Samples and some equipment (including hard drives) were returned to earth on STS-74 (Nov. 20). Plants were disoriented and completely vegetative. Maintaining substrate moisture was challenging, but the moisture probes functioned well.

Heat pulse moisture sensors have been used widely to determine moisture levels in substrates. Existing theoretical models of the sensors neglect temperature distribution inside of the probe. This leads to inaccurate solution for small time approximation. An improved model of a heat pulse moisture sensor, which takes into account thermal conductivity of the probe material, is discussed in this paper. Solution for the system of two differential equations, with corresponding initial and boundary conditions, are found. The solutions depend on three dimensionless parameters, two time scales calculated for the probe and the substrate correspondingly, and criteria Bio. Analytical expressions for small time approximations were obtained. The solution includes what is known as “contact resistance” between the probe and substrate. Contact resistance is responsible for data scattering during calibration of heat pulse moisture sensors.

A model to calculate effective thermal properties (heat conductivity and temperature diffusivity) of substrates is proposed. Obtained theoretical results are in good agreement with experimental data on the effective heat conductivity of the substrate Balkanin. The contributions of vapor diffusion to the effective thermal conductivity of a substrate is analyzed by comparing the heat conductivity at temperatures between 15-45°C. The results of the effective thermal properties studies are used to evaluate the results obtained from the Greenhouse-2 plant growth experiment on the Mir Space Station. The temperature variations in the range of 25-35°C observed during the Greenhouse-2 experiment significantly affected the effective heat conductivity of the substrate used in this experiment. The importance of correct interpretation of the data on moisture levels obtained by the heat pulse moisture sensors used in the Greenhouse-2 experiment is shown.

A NASA Learjet was used to produce a low-gravity environment for two series of nucleate pool boiling experiments. Surface-temperature and heat-flux measurements and high-speed microphotography of bubble phenomena were made on 18 prepared boiling surfaces. The surfaces were polished copper disks, 25.4 mm and 19.1 mm in diameter, with variable artificial nucleation site densities from 0.2 to 32 sites/cm2. Both 1-g and low-g data were obtained for comparison. In every case, the boiling heat-transfer coefficient increased significantly to a new steady value for the duration of the low-gravity period. Rapid movement of the surfaces of the large vapor masses that were observed is indicative of considerable turbulent liquid motion, apparently induced by the bubble growth and coalescence. In no case was a decreased heat-transfer coefficient observed, which would be indicative of film boiling.

This paper employs a simplified model of an air conditioning and delivery system, and a lighting source to calculate the energy and volume costs associated with growing plants in a closed environment. Some empirical data, drawn from the testing of the Engineering Development Unit of the CELSS Test Facility have been incorporated in the model. The analysis of the results suggests that operating requirements imposed upon a similar system need to be carefully examined in terms of their energy and volume costs, as these may be substantial, particularly under Space Flight conditions.

Knowledge of capillary migration of liquids in granular beds in microgravity is essential for the development of a substrate based nutrient delivery system for the growth of plants in space. This problem is also interesting from the theoretical as well as the practical point of view. The purpose of this study was to model capillary water propagation through a granular bed in microgravity. In our ground experiments, water propagation is driven primarily by capillary force. Data for spherical particle sizes in the range from 0.46 to 2 mm have been obtained. It was shown that the velocity of water propagation is very sensitive to particle size. Theoretical consideration is also provided. Actual space flight experiments are planned for the future to confirm our results.