Uses Spray Cooling with Flow Pulsing to Prevent the Unnecessary Boiling of Cryogenic Propellant Film in Microgravity and Reduced Gravity Environments
This storage tank and propellant transfer apparatus conserves valuable cryogenic propellants in reduced gravity and microgravity environments. The extension of human space exploration is NASA’s primary challenge for the new millennium, and having an effective, sufficient, and reliable supply of cryogenic propellant fluids is integral to this mission. Storing and maintaining rocket propellant as a liquid at cryogenic temperatures enables lighter fuel tanks and denser energy storage. However, transfer of such propellant into empty fuel tanks, e.g., during refueling of a mission beyond Earth’s orbit, results in fuel losses as the propellant boils off in contact with the relatively hot tank walls. The losses can only end when the wall temperature equilibrates with the propellant, a process known as chill-down. However, chill-down is inefficient in low gravity, impeded by vapor film formation between the solid tank walls and the liquid propellant. The vapor film reduces heat transfer, resulting in “film boiling.”
Researchers at the University of Florida have developed a storage tank and propellant transfer line apparatus for quickly escaping the film boiling regime using a combination of a polymer coating on the tank wall and spray flow pulsing. This storage tank facilitates a shorter chill-down process and conserves rocket fuel.
Application
Cryogenic propellant storage tank and transfer lines for efficient thermal management in microgravity and reduced gravity environments
Advantages
- Targets the propellant transfer line and storage tank chill-down steps of cryogenic propellant transfer, improving refueling chill-down efficiency
- Applies a thin polymer coating to the tank wall together with flow pulsing, enhancing the chill-down process and reaching the 50% thermal efficiency goal
- Sprays the cryogenic propellant onto the tank walls in pulses, repeatedly achieving rapid heat transfer upon first contact
Technology
In space, propellant transfer includes transfer line chill-down and storage tank chill-down. The processes cause the cryogenic propellant to boil off due to the high temperatures of the transfer line and receiver tank, making a more efficient chill-down process necessary. The primary reason for inefficient chill-down is the insulating vapor film that forms between the cold cryogenic propellant and the tank wall, drastically slowing the heat transfer. This redesigned storage tank combats the film boiling regime with two strategies. First, it contains a thin polymer coating on the tank wall that drives initial faster cooling of the wall surface at the cost of later slower interior cooling. However, surface cooling alone is enough to break out of the film boiling regime, so this strategy shortens the inefficiency period. Second, because heat transfer from the wall is strongest upon first contact with the wall, then decays, this apparatus deploys cryogenic propellant to the wall in repeated pulses, taking advantage of the most efficient moments of the heat transfer process repeatedly.
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