Design of a Radiator System for Lunar Applications

Photo of Andrew King, Tyler Mann, Casey Zak Students: Andrew King, Tyler Mann, Casey Zak

Sponsor: NASA

Date: Spring 2010

Requirements:
The radiator must reject a maximum heat load of 10 kW when the lunar outpost is occupied and a minimum heat load of 1 kW when it is unoccupied. The vessel temperature must be maintained at 21° C. The radiator's footprint is constrained to a 3 meter diameter circle on the roof of the lunar outpost. Since the radiator will be used in a space application, mass is an important consideration. Our mass per square meter of radiative area should be less than or equal to 5.85 kg/m2. Consideration should also be given to the extreme environmental conditions seen on the moon. The radiator will be subjected to varying solar radiation in the range from 0 to 1392 W/m². Additionally, the temperatures of the surroundings will be extremely low (the moon at 70 K and deep space at 0 K). Lastly, the gravitational field on the moon is much less than that of the Earth (1.63 m/s² versus 9.81 m/s²). There are two internal constraints for the design. First, there is an entrainment constraint. Entrainment arises if the momentum flux of the vapor is high enough that the uprising vapor prohibits the thin liquid film from traveling down the walls. This can cause a sudden drying out of the evaporator, which leads to a decrease in efficiency. Second, there is a limitation caused by the boiling of the working fluid. If bubbles form in the evaporator, hot spots can be formed that obstruct the liquid flow. The entrainment limitation is a limit on the axial heat flux, whereas the boiling limitation is a limit on the radial heat flux in the evaporator.

Problem:
As participants in the Texas Space Grant Consortium (TSGC) Design Challenge, the design team was tasked with designing a radiator system that could thermally regulate a permanent research habitat on the Moon. This habitat will be located on the rim of Shackleton crater near the lunar South Pole and will experience long alternating periods of darkness and direct sunlight. The radiator system must reject varying heat loads caused by crew metabolic processes and waste heat from electrical equipment. The design team was required to use variable-conductance reflux boilers (VCRBs) as the main heat exchangers in the radiator system. VCRBs utilize fluid phase change and buoyant forces to move heat away from the habitat. Varying heat loads are managed by modulating the volume of a non-condensable gas (NCG) contained in the top of the boilers; as the volume expands the effective length of the boiler decreases and the heat transferred also decreases.

Solution:
The proposed design consists of a circular array of reflux boilers that fits the circular space available on the roof of the habitat. The boilers are made of aluminum and contain ammonia as a working fluid and helium as an NCG. The array is encased in a supportive aluminum honeycomb matrix and faced with specially coated aluminum sheets. The reflux boilers carry the heat upward where it is transferred to the face sheets and radiated out to space. Reservoirs installed on top of each boiler contain the NCG and facilitate modulation of the gas volume. Varying the temperature of the NCG varies the volume it occupies. Additionally, a working reflux boiler was constructed and used to conduct physical experiments to test the viability of the design. Although the results of the testing were unclear, IR images indicated that the boiler was operational.

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