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Old   December 14, 2011, 06:06
Default İnlet boundary condition
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Jack
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Dear all,

I am trying to simulate the turbulence flow through a pipe. For the inlet condition, I know the mass flow rate, total pressure and temperature. I have a problem with chosing the appropriate boundary condition for the inlet.

If I choose pressure inlet and put the values for pressure and temperature that I already know, I get a mass flow rate that is significantly different than the real mass flow(almost 6 times higher) rate that I already know.

If I choose mass flow inlet condition and put the values for mass flow inlet, temperature and pupersonic/initial gauge pressure(I put a value slightly lower than the total pressure at the inlet) that I already know, I get a stupid pressure distribution that has nothing to do with real values. The real value of the total pressure at the inlet is 170 bar, however, it comes up as 1-2 bar when I use mass flow boudnary condition.

So, what kind of boundary condition would you offer?

THANKS.
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Old   December 22, 2011, 06:32
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The topic is still active.
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Old   December 22, 2011, 13:44
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This is quite simple, using pressure inlet and pressure outlet. In pressure outlet select the target mass flow and enter the value. Pressure inlet uses static pressure input as initial value, fast convergence could be obtained if you enter static pressure close to final expected value.
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Old   December 22, 2011, 14:06
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that is right but the thing is there is only one inlet but more than one outlets. thus, I do not know the exact mass flow rate for each outlet, I know the total which is surely equal to the inlet. that's why "target mass flow rate" is not that useful.

then if I choose pressure inlet&outlet boundary conditions, I need to specify the pressure at the inlet and outlet. the inlet is fine, however I can not specify the outlet pressure because that is what I am seeking for.

what would you dofor this ??

Thanks.
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Old   December 22, 2011, 19:44
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I am also seeking the solution to the same type of problem
Kindly someone help
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Old   December 22, 2011, 20:25
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Are you using Fluent? have you tried this, mass flow at inlet, outflow at outlet, set operating pressure 170 bar.
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Old   December 23, 2011, 04:07
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Quote:
Originally Posted by aggie View Post
Dear all,

If I choose mass flow inlet condition and put the values for mass flow inlet, temperature and pupersonic/initial gauge pressure(I put a value slightly lower than the total pressure at the inlet) that I already know, I get a stupid pressure distribution that has nothing to do with real values. The real value of the total pressure at the inlet is 170 bar, however, it comes up as 1-2 bar when I use mass flow boudnary condition.

So, what kind of boundary condition would you offer?

THANKS.
Mass flow inlet and outflow will work for incompressible flow. I think the results you got is correct. Pressure based solver always solves for change in pressure and not for absolute pressure. You need to interpret the results as delta pressure with respect to reference value.
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Old   December 23, 2011, 05:09
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Thank you all for your comments.

k_k , that was my initial idea. However, when I did this the results are completely confusing. Mass flow inlet is 12500 kg/hr=3.45 kg/s and the inlet pressure is 170 bar. This flow goes through many loops and bends, as a result I clearly expect a pressure drop. However, when use mass flow inlet and outflow boudnary conditions and set the operating pressure to 170 bar, the total pressure loss through the pipe is only 40-50 Pa. 40 Pa is nothing compared to 170 bar. so I believe there is soemthing wrong with mass flow inlet and outflow conditions for this kind of problem.

duri, what do you think about my above comments?
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Old   December 23, 2011, 05:13
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Just to put it in a more clear explanation, the conditions are as follows:

İnlet conditions:
m=3.5 kg/s
P=170 bar (gauge pressure)
T= 350 C

exit conditions:
m= the flow is seperated into two ways a few meters after the inlet and the pressure drop through two ways are possibly different so the exact mass flow rate for each pipe may not be the half of inlet flow rate. one can be slightly higher than the other. so the mass flwo rate at the exits are not SURELY known but can be estimated.
P= not known. the main thing that I should find.
T= not known. another important thing I should find.

what would you guys use as boudnary conditions?


mroe information= the working fluid is actually water liquid-water vapor mixture. However, I will do the simulations for only water liquid, only water vapor and vapor-water mixture.
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Old   December 23, 2011, 05:22
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Direct comparison of 170bar and 40kpa is not a good idea. Better convert the results to some non dimensional numbers (k-factor for pipe flows) before jumping in to conclusions. If you have any results to validate that helps a lot. Some free tools are also available in internet to estimate k-factor for standard pipe shapes.
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Old   December 23, 2011, 05:28
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that should be 40 Pa, not 40 kPa. by mistake I wrote kPa.

duri, I think 170 bar and 40Pa can directly be compared. whatever the reason is, the pressure drop should be much higher. but anyways I will give a try do do that k-factor thing.
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inlet boundary condition, mass flow inlet, pressure inlet, pressure outlet, velocity inlet

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