Pressure Outflow BC in reactive flow

MFiX
#1

Hello, MFiX community.

I’ve been trying to simulate reactive flow and I started with the CH4 combustion following the adiabatic flame and the silane pyrolysis codes as references. I produced a case with an inlet mixture of CH4 and a synthetic air (O2 and N2). I also gave the right conditions for combustion, considering the flow to be laminar to start. The .mfx and usr_rates files follow attached.
usr_rates.f (2.4 KB) methane_comb.mfx (22.2 KB)

My question is about the Pressure outflow BC. At the beginning the flow is quite stable, however, when the mass fractions of the products reach the outlet it seems to me that “the gas is not getting out”, and there is a lot of recirculation, the flow gets disturbed so does the flame region. A video with CO2 mass fraction is attached below:

In this case, I know the mass fractions in the output, but in another situation, I might not know as in gasification simulation so I let the mass fraction field in blank.

outflow

I tried different configurations, increasing the length in x and y, the velocity and the inlet length, but the result is quite similar. Am I missing something or is this how it should be? Is this BC disturbing the flow? If so how could I change it?

Thanks in advance.

1 Like
#2

Usually, there is back-flow when PO boundary is used, and thus cause oscillations. You may need to further increase the length in the x and y-direction to reduce the wall and end effect. Also, try to increase the grid resolution in the near inlet region.

#3

Thank you for your answer. I tried these different configurations and it worked. But what about some simulation of equipment, for example, whose geometry is fixed. Is there some other way to avoid this backflow or is it part of the physics going on in there?

#4

I think there is no general answer for this. When the geometry is fixed, if you still see the back-flow after apply those tricks, it is part of the physics going on in there.

2 Likes
#5

Thank you for your answer.