[shbn]

Hello everyone,

I am modeling hydrogen storage in a tank with adsorbent materials using Ansys 2023R1.

In my setup, the inlet boundary condition is initially a mass flow inlet, which automatically switches to a pressure inlet after 1 minute through a scheme command. However, Fluent crashes (floating point exception) when the set pressure at the inlet is slightly higher than the tank pressure at that time. It only accepts set pressures equal to or lower than the tank pressure.

While the simulation continues to run, I observe severe reversed flow (40%-80%) at the pressure inlet boundary right after the switch. I have tried reducing the time step size from 0.1s to 0.01s and lowering relaxation factors, but the issue persists. I suspect the pressure gradient caused by the heat of adsorption is affecting the solver’s ability to handle the new boundary conditions.

I appreciate your advice and insights on this.

Thank you.

[MKuhn]

The change in the boundary conditions probably causes a strong change in pressure. Check the pressure at the mass fow inlet shortly before the change, in this way you can check whether your reality is represented correctly at all. Reverse flow is not an error and can occur when the pressure at the outlet face is not homogeneous. Maybe the the "target mass flow" function of the pressure inlet helps to avoid the switching between the two boundary types.

[shbn]

Thank you, MKuhn, for insightful comments.

I created a report file to monitor inlet pressure so that the set pressure is as accurate as I needed. Probably, I need to avoid switching boundary conditions by adjusting the mass flow inlet stepwise to meet the capped pressure.

[MKuhn]

I believe that the mass flow inlet distributes the mass flow evenly over the inlet surface. Maybe also the pressure is constant over the inlet surface in case of the mass flow inlet. This is not the case with the pressure inlet, where pressure differences can occur over the surface, which can then lead to a reverse flow.

[shbn]

You are absolutely right about the pressure inlet as I observed the reversed flows fluctuating significantly between 40% and 80% of the faces on the inlet boundary over time.

Do you think adjusting the mass flow rate to control the inlet pressure through a scheme command is a viable approach, or is there a better way to handle this?

[MKuhn]

I don't know your case in detail, but modeling the entrance a little further away (with an additional pipe for example) would be a solution. The problem should also disappear with higher mass flows. How high are the flow velocities at your inlet?

[shbn]

My case involves only hydrogen flow into a tank with no outlet. I fully agree that using a higher mass flow rate could help create stronger momentum to overcome the pressure differences in the flow direction.

Since the issue seems to be related to numerical instabilities and slow convergence, I'll try using the low-Mach number flow setting to see if it helps.