Simulation of blowdown through vent
 

Hi. I am relatively new to FLUENT and using this software as part of my undergraduate thesis. My task involves simulating the blowdown/depressurisation of a closed section of pipeline through a vent pipe located somewhere at the center of the pipeline section. While the actual problem involves a pipe length of several kilometers, i am simulating a 350meter 3D section of the pipe for start.

Here is what ive done so far:

GAMBIT Meshing
1. created 350 meter long pipe with 0.9meter diameter in sections, volume meshed with cooper scheme
2. for center section containing vent (vent diameter=0.3m, length=5m), i individually meshed the pipe wall faces with quad (pave) and then meshed the volume with TGRID scheme
3. Set all walls and main pipe ends as WALL, pipe volumes as FLUID, and vent pipe outlet as PRESSURE OUTLET

FLUENT Setup
1. Solver--> coupled, explicit, unsteady
2. Viscous---> k epsilon standard (all default)
3. Material---> methane density method=ideal gas
4. Boundary conditions---> i left the pressure outlet as 0KPag since this is the required final pressure. Pipe volume material set as methane
5. Operating conditions--->101350Kpa
7. Initialize---> All zones initial pressure 80Bar gauge
8. Iterate--> time step=0.05, 15 iterations per time step

I also set surface monitor on the outlet face to monitor the mass flow rate out. The estimated time to depressurize a short segment like this is around a minute or so, with the mass flow rate quickly decreasing after initial start. However, I find the mass flow rate remaining very high for a prolonged period and the pressure remaining high in the main pipe(for this setup, i get a relatively steady mass flowrate of -900kg/s constant, while the entire pipe segment is estimated to have only 12000kgs of methane!). It does not seem to match the behavior i have researched on from experiments etc. I suspect there is something wrong with my setup. Can anyone advice?

Any help guys? This is quite urgent :(

Hi,
Can you please get back with boundary conditions.
Are the velocities exceeding 0.3 mach. Why are you choosing coupled solver?

Thanks for the feedback. The flow velocity is expected to approach M=1. I have switched to the segregated solver and used a smaller timestep of 0.0001 sec to initialize, then increasing it to 0.001 secs. The simulation now runs satisfactorily but it takes very long of course due to the small timesteps (20000 iter for 20 sec). In the future, i'm supposed to run a full scale blowdown scenario that may last up to an hour. Any advice on how i can speed up the calculation?