# Compressible flow, no data at the outlet

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 July 12, 2010, 05:49 Compressible flow, no data at the outlet #1 New Member   Mireia Ayop Join Date: Apr 2010 Posts: 12 Rep Power: 8 I’m a student and I would want to analyze a part of a pressure regulation chain. To do this I consider just a plenum with a pipe for the inlet and a pipe for the outlet. The latter narrows at the end to impose a mass flow rate of 5 mg/s at the outlet whereas the mass flow rate at the inlet is around 3000 mg/s. What I want to obtain is the difference in pressure between a point near the outlet and a point in the middle of the plenum. For the inlet I use the “mass-flow-inlet” boundary condition as I can specify the mass flow rate, total temperature and pressure at this point. The problem comes when I have to enter the conditions at the outlet: I’m working with a compressible flow (Xenon at almost 3 bar and 70 degrees) so Fluent doesn’t allow me to use the ‘outflow’ boundary condition. If I use the “pressure-outlet” condition I’m obliged to impose a pressure there that I don’t know and that is what I would like to obtain from the analysis. My attempts have been the following ones: 1) To lengthen the outlet pipe. Then the outlet is far from my point of interest and the pressure I impose there is not exactly the pressure I want to obtain. Anyway this is not a good solution since the results depend on the pressure at outlet. 2) To use the target-mass-flow-rate option as the only thing I know for sure at the outlet is the mass flow rate. This option permits specifying a target mass flow rate there. Fluent calculates a mean static pressure as a first value, then it calculates the corresponding mass flow rate and using the difference between the obtained mass flow rate and the target one it adjusts the static pressure. The problem is that, from the results, it seems that Fluent just imposes this static pressure at the outlet and the rest of the fluid does not perceive this change. Is there any way to avoid imposing a pressure at the outlet when working with a compressible flow? When I initialize a solution in fluent from the inlet conditions (you can choose between inlet, all zones, outlet …) it propagates the flow conditions at the inlet along all the model and when It arrives to the outlet it imposes the conditions you specified there?

 July 28, 2010, 05:22 #2 New Member   Mireia Ayop Join Date: Apr 2010 Posts: 12 Rep Power: 8 the use of a 'pressure-far-field' condition would be correct?

 September 1, 2015, 07:28 no data at the outlet for the compressible flow #3 Member   sonu Join Date: Jul 2013 Location: delhi Posts: 91 Rep Power: 5 Hello, I am trying to simulate the flow in the air transfer pipe. At the inlet i have static pressure, temperature, mass flow rate. so from this i checked that the flow is compressible. at the inlet i have applied the mass flow inlet boundary condition but at the outlet i don't have any data so what boundary condition should i applied at the outlet ? Thanks & Regards, Sonu.

September 1, 2015, 16:25
#4
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Lucky Tran
Join Date: Apr 2011
Location: Orlando, FL USA
Posts: 1,306
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Quote:
 Originally Posted by yash.aesi Hello, I am trying to simulate the flow in the air transfer pipe. At the inlet i have static pressure, temperature, mass flow rate. so from this i checked that the flow is compressible. at the inlet i have applied the mass flow inlet boundary condition but at the outlet i don't have any data so what boundary condition should i applied at the outlet ? Thanks & Regards, Sonu.
You must known something about the outlet or you don't even have a problem to begin with. Is the massflow at the outlet equal to the inlet or some other known value? If so, you should try a pressure outlet with targeted mass flow rate.

 September 2, 2015, 06:58 #5 Member   sonu Join Date: Jul 2013 Location: delhi Posts: 91 Rep Power: 5 Hello Lucky, i already tried this by using the pressure inlet(known) and mass flow outlet BC(as mass flow remains constant). but then the problem is that after simulation velocity fields are coming wrong. for more clarity : at inlet i have mass flow = 0.1867 kg/s, area = 0.001256 m2, pressure(static)= 159 kPa, temperature(static) = 163 Degree Celsius. these are the given conditions. so if i use the compressible flow relations then my velocity at the inlet should have to come around 126 m/s but after running with above BC its coming around 47 m/s. so any idea about it ??? Thanks & Regards, Sonu.

 September 3, 2015, 01:51 #6 Senior Member   Lucky Tran Join Date: Apr 2011 Location: Orlando, FL USA Posts: 1,306 Rep Power: 20 In Fluent, pressures are specified in gauge pressures. The default operating pressure is 101.325 kPa so make sure you are specifying stagnation gauge pressures with respect to 101.325 kPa. For the pressure inlet, you have to specify the stagnation pressure and stagnation temperature and not the static pressure & static temperature. Are these correctly specified? The total temperature is not a big deal, the total pressure is more likely a problem.

September 3, 2015, 02:10
#7
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sonu
Join Date: Jul 2013
Location: delhi
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Hello,

yes i applied the total pressure and total temperature only which i calculated by using the isentropic relations( as shown in the attached image)
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 Tags compressible, target-mass-flow-rate

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