Modeling of Chill Down in Cryogenic Transfer Lines

A numerical model to predict chill down in cryogenic transfer lines has been developed. Three chill down cases using hydrogen as the working fluid are solved: 1) a simplified model amenable to analytical solution, 2) a realistic model of superheated vapor flow, and 3) a realistic model of initially subcooled liquid flow. The first case compares a numerical model with an analytical solution with very good agreement between the two. Additionally, the analytical solution provides a convenient way to look at parametric effects on the chill down. The second and third cases are numerical models which provide temperature histories of the fluid and solid tube wall during chill down as well as several other quantities of interest such as pressure and mass flow rate. Of great interest is the ability to predict accurate values of chill down time (the time required to achieve steady-state cryogenic flow). The models predict that a 26 in. long, 3/16 in. ID aluminum tube has a shorter chill down time (approx. equal to 100 sec) and uses less hydrogen with superheated vapor flow than with initially subcooled liquid flow (greater than 200 sec for chill down)