To decarbonize the mobility sector, hydrogen is a relevant solution that continues to develop, particularly for heavy transport (trucks, trains, etc.). The speed at which hydrogen gas tanks in vehicles powered by fuel cells can be filled is a commercial and industrial challenge for manufacturers and operators of hydrogen refilling stations (HRS).
For safety reasons, the heating kinetics of the gases due to their compression during the filling phase must be controlled so that the temperatures in the tanks remain at acceptable levels with regards to the mechanical strength of the structures. Hydrogen tanks are usually designed for 350 or 700 bar, making this challenge even more complex than for natural gas tanks. In addition, for operating constraints reasons, the duration of this filling phase must also be optimized.
The filling speed, the associated heating of the compressed gases and the cooling of the gases by the cold filling jet are therefore key points of the issue. The pre-cooling of the injected hydrogen (usually between -40°C and 0°C) is necessary for reducing as much as possible the filling duration. Nevertheless, this aspect has an important economic impact on the cost of the HRS, and better understanding what is the optimal pre-cooled temperature is also a key aspect.
Manufacturers and operators of hydrogen filling stations have simplified models (0D for the gas volume and 1D for the wall layers of the tank) whose set of physical parameters is based on manufacturer data or feedback. These models can still be improved because they do not reproduce all cases of interest, in particular cases for which the temperature field is not homogeneous. In this scenario, the difference between the mean temperature and the minimum and maximum temperatures should be available to complete the safety assessment.
A 3D CFD model theoretically allows to take into account all the physical processes involved. More specifically, it provides the local dynamic/thermal fields, thus highlighting possible stratification phenomenon during the filling phase of the tank.
The CFD model is based on a RANS approach with a two-equations model including buoyancy effects. A real gas (RK) state of the law is used, well adapted for high pressure compressible gas. Thermal fluid / solid coupling allows to consider thermal transfers between the hydrogen flow, and the liner / wrapper structure layers.
Sensitivity studies are carried out especially on turbulence models (k- standard and Realizable), boundary conditions for the hydrogen filling (imposed pressure or flow rate) and the nozzle shape (mono or bi-nozzles).
Overall, results show that after a stage of progressive and homogeneous increase of the hydrogen temperature and pressure during the filling, thermal fluctuations appear systematically with the development of (high pressure) flow vortices in the tank, eventually enhancing stratification processes.