The methods used to receive liquid hydrogen into a container impact the pressure and temperature response during a filling process. Tests conducted with tanks that are not vented during filling as shown above provide insights into which methods are best used for various applications [1].
Bottom Filling
The left top and left bottom panels of the above graphic plots the pressure and fluid temperature responses from a nonvented liquid hydrogen filling from the bottom of a tank using a dip tube diffuser. The general trends would be similar for an open discharge pipe in the same configuration.
Tank pressure slowly rises as the liquid level increases and compresses the ullage gas. The ullage gas remains thermally stratified with gas temperatures slowly rising due to compression. As the liquid level rises, it submerges each temperature sensor causing a sudden drop to liquid hydrogen temperature at that location.
This method of filling works well for tanks that will be partially filled and/or stored for extended periods of time. Liquid hydrogen is conserved by not unnecessarily cooling the container walls above the liquid level.
Upward Discharge Filling
Injecting liquid hydrogen upward from the bottom of a container as shown in the middle panels above actively mixes the liquid in the tank. This mixing enables sufficient condensation at the liquid-vapor interface to keep the pressure near saturation condition until relatively high fill levels are reached.
If the discharge jet has sufficient initial momentum to reach the upper portion of the tank, then the fluid and wall temperatures will be rapidly cooled. For lower velocity injection, the temperatures in the gas and wall will behave similar to the bottom filling configurations, while the liquid remains relatively close to saturation due to constant mixing.
For applications where a well mixed liquid and controlled tank pressure during filling are needed, this filling method works well. Ground vehicles, aircraft, and marine vessels are examples where these characteristics may be desirable.
Top Spay Filling
A top spray liquid hydrogen injection configuration is shown in the top and bottom right panels above. The resulting droplets drive the tank pressure and ullage gas toward saturation due to the mass transfer occurring between each droplet and the hydrogen vapor.
Design of the spray cone angle will dictate how much of the tank wall is cooled by the incoming droplets. The location and number of spray nozzles are also important considerations if this method is used.
Since top spray filling directly controls the ullage gas conditions, this configuration works well for applications where tank pressure control is a driver during filling. A spray nozzle can also be used in an internal mixing system to drop tank pressure during storage and mitigate the need to vent.
[1] "Hydrogen No-Vent Fill Testing..." Moran, Nyland, and Driscoll, NASA TM-105273, 1991.
