Mass flow distribution along a pipe system 2

[pegus]

Good day All,

I was performing a CFD analysis to a pipe system when I came up with this doubt.

I have a piping configuration that starts with 2 pipe lines(inlets)(they have different diameters, as it is shown in the attached image) and merge into one pipe(Outlet). The data that I know is the mass flow rate at the outlet and static pressure at the inlets.

As boundaries conditions, I am setting the known mass flow rate in the outlet, and I want to set total pressure in the inlets (Total pressure = Static pressure + Dynamic pressure). To obtain the total pressure for each pipe, I need to know the velocity for each pipe line that merges in the outlet pipe. To calculate the velocity for each pipe, I need to know the mass flow that is running in each pipe line. Is there any method to find the mass distribution for each pipe line for this case? Does the reduction in one of the pipe lines affect the mass flow distribution?

Thanks in advance!

P.P.

 

[MikeHalloran]

I compute a Cv for each branch by estimating the pressure drop at an arbitrary flow, then combining them to get units of gpm/sqrt(psid).

Then use the Cv s to compute a psid for each parallel branch with one branch set to Q1, and the other branch set to Qt - Q1, then goal seek on Q1 until the psid's are equal.

That works to a first approximation in a reasonable time for networks that are always full of liquid.

Of course, there is no mechanism to force the real network to behave like your model does, so don't expect miracles.



Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

You can't know "dynamic pressure" unless you know velocity. Set the static pressure. Don't call it dynamic, or total.

With the known static pressures at inlets and at the branch connection point, mass flow can be found by iteratively assuming flow rates in each branch until the head loss in each branch meets the pressure boundary conditions.

 

[LittleInch]

pegus,

If your diagram is accurate for lengths and pipe sizes, then you should be fairly close to 50% in each branch as a start point then iterate as noted above.

As BI says, you have two known pressures - start point and the pressure at the outlet.

This looks like quite a short system and things which can normally be ignored might have a bigger impact such as entry losses, swirl etc.

diameter has a huge impact on flow. All other things being the same ( pressure drop, of flow rate, the other aspects can change by (D1/d2)^5.

Is this gas or liquid? Gas might be even harder as it could set up a pressure wave or start to pulse.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[pegus]

Thanks for your responses,

LittleInch, what is running in this pipe lines is gas (air at high temperature(+1000F) and pressure (+300psia)).

Regards!

P.P.

 

[pegus]

MikeHalloran,

Can you please explain a little more detailed the method that you are suggesting.?

Thanks in advance!

P.P.

 

[chicopee]

Attached is the graphing method that I would use. I am assuming density of gas to be constant. Other equations for pressure drop in gas lines can be used instead of the proposed Darcy Weishbach equation. Since such graphing procedure is normally involving flow rates, mass rates can be easily calculated and directly plotted instead. This procedure is the quickest way by hand to find the values that you seek.