Cryogenic Boil-off Recovery - Cool Down, Condensation and Recovery of Vent Gases

Photo of Chris Berg (Team Leader), Matthias Lang, Mark Peltier Students: Chris Berg (Team Leader), Matthias Lang, Mark Peltier

Sponsor: NASA

Date: Fall 2010

Requirements:
Since a passive cooling was required, the system must operate with minimum energy consumption and without active refrigeration or compression cycles. If needed, the design can include a two-phase splitter to filter condensed liquid from leftover boil-off gas. The materials chosen must be space-rated. Design of a cryogen pump and accounting for piping losses were not required solutions to those problems already exist. Based on the boiling point of liquid oxygen, the cryogrn must remain below 90K in order to remain in a liquid state. The deep space environment is defined as 3K and 0kPa. Heat leakage into the tanks ranges from 7.0 to 43 Watts/day and the boil-off rate is 0.016%/day.

Problem:
NASA's future plans include long-term missions such as lunar colonization and a roundtrip to Mars. At current technology levels, heat infiltration into fuel and oxidizer tanks causes boil-off of the cryogens stored into space. In order to maintain safe tank pressures, the gases are vented into space, which creates the need to carry additional cryogen mass. Current cryocoolers consume a lot of energy and add extra mass to the mission. This project focused on analyzing a passive cooling solution that utilizes the deep space environment to its advantage in cooling the cryogens.

Solution:
The focal point of the team's design was the design of the radiator. A MATLAB program was created to optimize the radiator parameters and determine the required length for the given ranges of heat leak. The radiator parameters include the width, length, and height of the radiator, and the pipe diameter as well as the number of pipes needed in the heat exchanger. Several subprograms to compute Reynolds, Nusselt, and Prandtl Numbers, and a property calculator, were incorporated in the optimization and simulation programs. A 2-D finite-difference algorithm was developed to solve the complex heat transfer in the radiator, including convection, conduction, and radiation. The figures show a contour plot of the inlet temperatures and a 3-D temperature plot. The team determined that the implementation of a passive cooling system that constantly circulates the liquid cryogen is a feasible solution under certain conditions. The trade-off in mass savings for additional fuels to the mass of the radiator will be achieved for missions lasting over 260 days, i.e. a roundtrip to Mars.

Images related to the project:

Photo related to Cryogenic Boil-off Recovery - Cool Down, Condensation and Recovery of Vent Gases project
Photo related to Cryogenic Boil-off Recovery - Cool Down, Condensation and Recovery of Vent Gases project
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