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Proper BCs for internal fan

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Old   June 25, 2010, 21:18
Default Proper BCs for internal fan
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Hi everyone,

I am trying to setup and internal fan in CFX and I would like to get your opinion.

I am specifying an CFX inlet (fan outlet) with static pressure BC, where
P(fan_outlet)=P(fan_inlet)+dP,
and where dP is a function of mass flow rate at fan inlet according to fan curve.

The BC on fan inlet is CFX outlet BC with mass flow rate condition, where mass flow rate (fan inlet) is equal to mass flow rate (fan outlet).

It seems like these conditions work, but I would like to get your opinion if this is a good way to setup an internal fan.

Any suggestions and comments are appreciated.
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Old   June 26, 2010, 07:31
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You can also do this where you get the pressure on either side of the fan and apply the mass flow rate from the fan curve. Same fan curve, just looking at it a different way - but this way will probably be more stable.
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Old   June 28, 2010, 13:48
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I think you are right. The pressure condition does seem less stable. I will try the mass flow rate BC instead. Thank you!

Update: I tried setting Mass flow rate (MFR) on both sides of the fan using the Fan curve and pressure difference across the fan, and it turns out this BC is unstable and leads to a divergence within 3-5 iterations.

Update 2: Setting MFR_in=MFR_out on fan outlet and pressure BC (according to fan curve and MFR) on fan inlet resulted in oscillation of residuals (no convergence), so even though it is more stable, it does not converge.

Update 3: The poor convergence (oscillations) was resolved by weakening the average pressure condition on fan inlet side: P_weak(average)=0.5*P_current+0.5*(P_fan_outlet-dP(fan curve))
The fan outlet condition is still mass flow rate: MFR_outlet=MFR_inlet.
I will experiment with coefficients in the weak pressure BC, but as long as there no oscillations of residuals, I believe the BCs work fine.

Last edited by serezhkin; June 29, 2010 at 20:47.
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Old   July 28, 2010, 10:04
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