Hydrogen Fast Fill Modeling and Optimization of Cylinders Lined With Phase Change Material

This work is a continuation of a previous study (IMECE2019-11449) which sought to explore the feasibility and means of successfully modeling the hydrogen fast filling process of cylinders lined with phase change material (PCM) entirely in CFD software. The first focus of this work was the address the simplistic approach of how the liner temperature was modelled in the previous study. Previously, the entire liner was assigned a single temperature which was obtained and updated through the lumped heat capacity method. This meant that the hotter gas at the end of the cylinder opposite the inlet was in contact with a liner at a temperature lower than could realistically be expected. This was remedied by splitting the liner into four sections. Two sections were used for the curved portions at each end of the cylinder, and the straight wall section was split into two. Each section had its temperature independently calculated through the lumped heat capacity method. A temperature difference on the order of a few degrees was observed between the different sections of the liner prior to latent heating beginning. The mass averaged temperature of the hydrogen inside the cylinder obtained with the sectioned wall case matched that obtained with the single wall temperature almost exactly, less than a degree difference.

Despite the unexpected findings of the overall hydrogen temperature not changing much when the wall is split into sections, this approach was still taken with all the cases completed in this study. The liner could be split into a greater amount of sections than four, but this was considered unnecessary due to the findings regarding the overall hydrogen temperature. Typically, only 3 or 4 sections were used to model the temperature gradient along the wall or liner. The effect of gravity on the filling process was also explored based on the orientation of the cylinder. This required completing three-dimensional simulations to accurately simulate buoyancy driven flow in horizontally mounted cylinders. The positioning of the PCM liner and novel methods of inducing a flow of hot gas past the liner to increase the heat transfer rate to the liner were also explored and modelled.

All the simulations were completed with ANSYS Fluent 2019 R1 without the use of additional software to handle the heat transfer involving the PCM.  All simulations were completed with the coupled pressure-based solver and K-Omega SST turbulence model. The gas properties were obtained from tables generated from NIST properties (REFPROP) available within ANSYS Fluent to limit the amount of error in the accumulated mass within the cylinder due to inaccurate gas properties. The initial conditions for the gas and liner temperatures were 298 K and 100 bar for the gas pressure. A constant mass flow rate of 0.02174 kg/s at a temperature of 273 K were used as the initial conditions for the inlet. The filling process was completed once the gas pressure inside the cylinder reached 700 bar.