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Specifying a Flow's Mass Flow Rate and Inlet Pressure

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Old   February 27, 2015, 18:36
Default Specifying a Flow's Mass Flow Rate and Inlet Pressure
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Jonathan
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As the title says, is this possible? I am modelling a pressure safety valve. I know the set pressure and the mass flow rate.

If I only set the mass flow rate (120,000 lb/hr) at the PSV inlet, the inlet pressure is only about 70 psig, whereas the actual PSV's set pressure is 550 psig.

If I try to ramp the PSV inlet all the way up to 550 psig, the mass flow rate goes far too high.

Any ideas on how to model this? My current setup has the PSV surrounded by an air domain, the pipe interior region and PSV inlet (the actual valve is not modelled, is just an open pipe) is methane. The vent outlets (scarfed tee shape) are an interface between the methane region and surrounding air region.

-Cheers
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Old   March 1, 2015, 03:34
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Gajendra Gulgulia
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you can try stagnation inlet where you can set the inlet pressure as your inlet bc, and using the stagnation equations of ideal gas you can derive the density of your fluid
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Old   March 1, 2015, 03:51
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you can try stagnation inlet where you can set the inlet pressure as your inlet bc, and using the stagnation equations of ideal gas you can derive the density of your fluid
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Old   March 2, 2015, 11:53
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I think my problem may be that the inlet pressure to the PSV itself is 550 psi, but the PSV outlet to the vent piping (what I'm interested in) is likely at a lower pressure. I am working on finding out if this understanding is correct.
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Old   March 2, 2015, 13:54
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I don't think you can specify both a pressure and mass flow rate. They would contradict eachother. The mass flow BC, as I understand it, adjusts the pressure to achieve the desired flow rate. Specifying a pressure on top of this would seem to cause a conflict.

As mentioned, your best bet is probably a stagnation inlet. (Make sure that you enter this as a gauge pressure, not absolute. That screwed me up the first time.) The flow rate through your system will then be set by a combination of geometry and back pressure.

Also, I have to ask... Why Jayhawk? You a KU grad?
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Old   March 3, 2015, 18:02
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Thanks for the replies. I ended up keeping the inlet as just a mass flow since that was the more "important" parameter. I know what the mass flow should be, while I'm not so sure on what the pressure of the gas coming out of the PSV into the vent exhaust piping should be. So I figure it is better to specify the quantity I do know.

I almost went to KU but ended up going elsewhere. I'm a KU bball fan though! (Though that's been more difficult this year.)
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Old   March 3, 2015, 18:13
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That seems reasonable. Ususally flow rate is a better parameter to set if it is known a priori, but life isn't always so easy. I typically use stagnation inlet for something like bleed air line where we know a pressure but the flow rate is really a product of the system as a whole.

All is not lost either, it is March and there are still some (although not many) who give KU a shot at a #1 seed. Kentucky just takes all the fun out of it.
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Old   March 4, 2015, 15:57
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Correction, I mispoke about the nature of the mass flow inlet. It sets velocity, not pressure. Not sure where I got that from. You might be able to specify both, if they are in reasonable agreement. I would maybe check the steve portal or tech support on this one.
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Old   March 19, 2015, 14:25
Default mass flow inlet specifying with pressure
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I am working on the problem where i have mass flow inlet as 390kg/s at 42 bars. Since I am not able to specify both the conditions at the inlet.
I tried with only specifying mass flow inlet and the velocities are coming very high.
I think am not specifying conditions right.

I gave mass flow inlet with mentioned value
reference pressure as 0 pa
also initialized everything with 0 values since solving steady state
and other end as pressure outlet
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Old   March 19, 2015, 16:09
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that is a very large mass flow and pressure. are you sure about those values? how big is your duct?
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Old   March 19, 2015, 17:55
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Yeah I know about that but certainly these are my boundary values. And my geometry contains channels and pipes in series, in all the length will be around 2m with prior diameter 5 cm.
Also I want to know how can we input the water with 42 bars. One thing I did is I changed the density and viscosity based on the pressure (42 bars).
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Old   March 19, 2015, 18:47
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In pipe flows your pressure and geometry determine your mass flow. That's basic fluid dynamics. If you set a mass flow in cfd the pressure at the boundary condition is going to have to float to meet that target. I'm still not convinced that you can or should specify both. If you set mass flow only and run to convergence how off is your pressure at the boundary condition? I

FYi, when you specify a pressure Inlet it is asking for gauge pressure not absolute. That has screwed me up before. You probably knew that already though...
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Old   March 19, 2015, 18:51
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You are pushing over 300 kg/s through a 5 cm pipe? Are you choking your flow? What fluid are you using? Molten lead? Can you do a hand calc and tell me what your average velocity would be? These numbers just don't make any sense to me. That doesn't mean they are wrong, I just want to understand your application.
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Old   March 20, 2015, 09:14
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Try using a velocity inlet. You can set pressure and temperature there, it may not be 100% accurate on your mass flow rate but should get you pretty close.
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Old   March 20, 2015, 09:52
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Hi,

I agree with Matt saying that you cannot specify both velocity (or mass flow rate) and pressure for an inlet condition. the code calculates one based on the other and geometry.
But for specific cases if you want the some amount of fluid to flow at a given pressure, the workaround for that is you apply pressure outlet at and specify higher pressure there.
One drawback is, you need to do some trial and error to figure out the correct value at outlet for the required value at the inlet.

Hope this helps!!!
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Old   March 20, 2015, 16:57
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Matt,
I am working for a problem where I am simulating very high heat flux but initially I am doing cold flow analysis and not dealing with temperatures right now.
Matt I am confused with boundary conditions and let you know after cross checking... Till now

Attempt:
I want to know, changing references pressure to 42 bars and providing mass flow inlet and specifying reference pressure point at the inlet and other boundary condition as pressure outlet at gauge 0 Pa...is this right physics?? If not what type of physics will have such boundary conditions??
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Old   March 20, 2015, 17:03
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reference pressure is whatever you would subtract from absolute pressure to achieve your gauge pressures. (in most cases atmospheric pressure). definitely not 42bar.

I think you would be better off using a velocity inlet like I suggested. You should know mass flow, pressure and temp. The velocity inlet will let you specify all pressure and temp (which gives density). Velocity * Density * Area is mass flow, you should know the area of your inlet. It won't vary velocity to meet your mass flow though, so it may not be 100% accurate, but should get you within 1-2%. What will really determine how close you get are boundary layer losses.
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