Learning Forum

[Gaétan]

Hello everyone,

I am trying to determine the time required for a 50-liter tank, initially at atmospheric pressure, to reach its nominal pressure of $a$, knowing that it is pressurized by a compressor supplying air at 12.5⋅10^5 Pa through a circular inlet with a diameter of 12 mm.

I attempted to model this problem in Fluent, but the results I obtained are not satisfactory: the pressure evolution in the tank shows a linear behavior over time until it stabilizes at 12.5⋅10^5 Pa; the airflow rate at the inlet remains constant and then drops to zero once pressure equilibrium is reached. This result is physically inconsistent.

I expected to observe a significant increase in pressure during the initial moments, followed by a progressive decrease in the slope of the pressure evolution until reaching an asymptote at pressure equilibrium. Similarly, the airflow rate should decrease over time in the same way.

Model setup:

• General: 3D / Pressure-based / Transient
• Models: Energy (On) / SST k-omega
• Material: Air (ideal gas for density; other properties are constant)
• Operating Pressure : 10^5 Pa
• Inlet: Pressure-inlet (Gauge Total Pressure: 11.5⋅10^5 Pa)
• Wall: No slip / Adiabatic
• Methods: Coupled / Distance-based / Least Squares Cell-Based / All second-order
• Time step: $-3\; s$

I considered using the density-based solver since the flow is highly compressible, but it requires a very small time step (1⋅10^−6 s), while the simulation time spans several tens or even hundreds of seconds.

Do you have any solutions or ideas to correct the physical behavior of the results?

Thank you in advance for your responses.

Best regards,
Gaétan

[Rob]

You shouldn't need the density based solver. How does the convergence look?