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Specified mass-flow rate in an outlet

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Old   August 14, 2003, 17:43
Default Specified mass-flow rate in an outlet
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Biga
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Hy there!

I'm currently trying to implement a boundary condition into a 3D Finite Volume method. This bc should handle an outlet in which the user know the mass-flow rate. The bc actually modifies the exit static pressure in each iteration in order to reach the prescribed mass-flow rate. I have a doubt, however: What's the equation that relates mass-flow rate and the pressure? Does any one now it? I know some commercial codes do it, but I couldn't have access to the proper formulation. Does anyone have any idea?

Thanx a lot in advance. Biga
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Old   August 15, 2003, 04:40
Default Re: Specified mass-flow rate in an outlet
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Tom (the other one)
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The equation you are looking for is the Navier-Stokes equation, relating velocity to pressure and therfore massflow to pressure.
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Old   August 15, 2003, 22:02
Default Re: Specified mass-flow rate in an outlet
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Jonas Holdeman
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In general the pressure gradient is given by irrotational part of (-u.grad(u) + 1/Re*del^2(u)) + conservative forces, which cannot be expressed in terms of flow.

For the special case of fully-developed internal flow, a relation of the form you described does exist. It is of the form: grad(p)=c_p/Re*phi, where phi is the net internal flow, Re the Reynolds number, and c_p is known as the "Poiseuille constant". c_p depends only on the flow geometry, and is 3 for flow in a 2D duct.
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Old   August 17, 2003, 21:02
Default Re: Specified mass-flow rate in an outlet
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Biga
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Thanks, Jonas

That's more "implementable"... =D However, that gradient would make it a bit harder... :\

I've been looking in the Fluent UDFs and found one equation, which is delta p = MFR_desired * delta MFR / (rho * area^2), where MFR = mass flow rate. Do you where this equation comes from???

I've tried the implementaion with a pressure "driver" of my own, written as: delta p = p * delta MFR / MFR_desired, and it seems to work, what makes me think that I don't really need a physical equation but only a driver to the desired pressure.

Weel, thanks again anyway, Biga
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