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-   -   Setting both Pressure and Mass Flow at a HeaderInlet (http://www.cfd-online.com/Forums/star-ccm/91294-setting-both-pressure-mass-flow-headerinlet.html)

 MentalVacancy August 5, 2011 20:04

Setting both Pressure and Mass Flow at a HeaderInlet

Hello,

I am trying to model a header with one inlet and five outlets. Essentially, I wish to define a mass flow rate and pressure at the inlet so as to obtain the resulting pressures and flow rates at the five outlet boundaries.

Does anyone have any insights into how to go about doing this? The available boundary condition settings in STAR do not seem to allow me to set the problem up this way.

For example, do I need to combine the five outlet surfaces into one part and use a mass flow inlet boundary to specify total pressure, then use derived parts for the pressure readings?

 abdul099 August 7, 2011 16:09

It is not possible to define pressure and mass flow at the same boundary. You might use the target mass flow option, but it does only adjust the pressure until the target mass flow is reached.

Further it's not possible to get the pressures at the five outlets (assuming they are different) when you don't have any information about the split ratios (how much of the mass flow exits at which outlet). And it should be easy to understand why: The pressure depends on your inlet pressure and the pressure drop in every flow path of your domain. The pressure drop depends on the mass flow through every flow path. And the mass flow depends on the pressure drop in every flow path and the outlet pressure. So the cat is chasing it's own tail. There are too many variables.

You HAVE to fix at least one variable: Either outlet pressure or outlet mass flow or the split ratios (e.g. 10% at the first outlet, 30% at the second outlet etc.).

 MentalVacancy August 8, 2011 01:37

I have managed to implement something similar to what I was going for. I combined all five outlets as a single boundary, set this boundary as a mass flow inlet, then set the total outflow I desired. This allowed me to set a pressure outlet as my inlet.

I then created a number of constrained planes to allow me to output the outlet pressures (since the outlets were no longer selectable separately.).

It does give me a solution with varied outlet flows which, when combined, equal my total mass flow. The pressure seems to behave oddly, however. It seems as if the inlet pressure is close to (rho*g*h) lower than what I set it to, something I did not see in other configurations. Might the treatment of hydrostatic pressure have changed?

 rwryne August 8, 2011 08:31

Quote:
 Originally Posted by MentalVacancy (Post 319101) Hello, I am trying to model a header with one inlet and five outlets. Essentially, I wish to define a mass flow rate and pressure at the inlet so as to obtain the resulting pressures and flow rates at the five outlet boundaries. Does anyone have any insights into how to go about doing this? The available boundary condition settings in STAR do not seem to allow me to set the problem up this way. For example, do I need to combine the five outlet surfaces into one part and use a mass flow inlet boundary to specify total pressure, then use derived parts for the pressure readings? Thanks in advance.
Set the boundary to a Pressure Outlet (it says outlet, but it can act as an inlet). Then under its options, you can enable a "Target Mass Flow" option.

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