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September 27, 2022, 04:41 |
Pressure Drop
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#1 |
New Member
Join Date: Jan 2020
Location: Germany
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Hey guys,
I'm trying to calculate pressure drop in a pipe when air is flowing through. The pipe has four straight parts and three pipe elbows (sketch in picture). I checked my simulation results and compared them with calculations from some Online Calculators from the internet. There are huge differences especially in the pipe elbows (picture). Do you have any ideas where these differences come from? Assumptions: INLET BOUNDARY CONDITIONS: FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END HEAT TRANSFER: Option = Static Temperature Static Temperature = 375 [C] END MASS AND MOMENTUM: Mass Flow Rate = 435 [kg s^-1] Mass Flow Rate Area = As Specified Option = Mass Flow Rate END OUTLET BOUNDARY CONDITIONS: FLOW DIRECTION: Option = Normal to Boundary Condition END FLOW REGIME: Option = Subsonic END HEAT TRANSFER: Opening Temperature = 375 [C] Option = Opening Temperature END MASS AND MOMENTUM: Option = Opening Pressure and Direction Relative Pressure = 0 [Pa] END DOMAIN MODELS: BUOYANCY MODEL: Option = Non Buoyant END DOMAIN MOTION: Option = Stationary END MESH DEFORMATION: Option = None END REFERENCE PRESSURE: Reference Pressure = 1.9 [bar] END END FLUID DEFINITION: Fluid 1 Material = Air Ideal Gas Option = Material Library MORPHOLOGY: Option = Continuous Fluid END END FLUID MODELS: COMBUSTION MODEL: Option = None END HEAT TRANSFER MODEL: Option = Thermal Energy END THERMAL RADIATION MODEL: Option = None END TURBULENCE MODEL: Option = k epsilon END TURBULENT WALL FUNCTIONS: Option = Scalable END Thanks Irie |
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September 27, 2022, 05:44 |
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#2 |
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Gert-Jan
Join Date: Oct 2012
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What is the diameter of the pipe?
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September 27, 2022, 06:59 |
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#3 |
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2250 mm and the elbow bend radius 3360 mm
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September 27, 2022, 07:21 |
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#4 |
Senior Member
Gert-Jan
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According to your settings, you have a density of 1 kg/m3. This means that your inlet velocity exceeds 100m/s. Is that what you expect?
If so, then you are studying an uncommon problem. Is it North Stream I or II? Nevertheless, then I would use Total Energy since you are reaching Mach=0.3. |
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September 27, 2022, 07:37 |
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#5 |
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"Is it North Stream I or II?"
No but I will try Total Energy. Velo is correct. Why do you call it uncommon problem? |
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September 27, 2022, 07:50 |
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#6 |
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Gert-Jan
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- Transporting 420 kg/s of air at 1.9 bar and 375°C, is not somehting I encounter very often. Therefore I was checking.
- I don't think the online tools cover these operating conditions. - I guess you now use uniform inlet conditions? I wonder if that is correct with 100 m/s. Don't think so. Better add additional geometrical info upstream the inlet. - Also I wonder how the velocity looks like. |
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September 27, 2022, 08:05 |
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#7 |
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"- I guess you now use uniform inlet conditions? I wonder if that is correct with 100 m/s. Don't think so. Better add additional geometrical info upstream the inlet." - Could you explain a little bit more? What do you mean by "add additional geometrical info upstream the inlet"
"- Also I wonder how the velocity looks like." - picture |
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September 27, 2022, 09:28 |
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#8 |
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Gert-Jan
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- Let me reformulate..... What is present upstream the inlet? More straight duct? Or a piece of equipment where the hot air is generated. A furnace, a DeNOx installation.....
- Did you perform a mesh sensitivity check? - Remember, total pressure is a better indicator for pressure drop than static pressure. |
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September 27, 2022, 09:50 |
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#9 |
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"- Let me reformulate..... What is present upstream the inlet? More straight duct? Or a piece of equipment where the hot air is generated. A furnace, a DeNOx installation....." - unfortunately no infos about that...
"- Did you perform a mesh sensitivity check?" - I did "- Remember, total pressure is a better indicator for pressure drop than static pressure." - I used (areaAve(Total Pressure)@Inlet - areaAve(Total Pressure)@Outlet) I thought that the problem should be scalable until the influence of the wall is getting bigger. Thats why I ask u of uncommon problem.. |
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September 28, 2022, 15:25 |
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#10 |
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Erik
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In my experience hand calcs for elbows always show much higher pressure drops than a CFD result. My theory is that in CFD you have a perfect smooth bent elbow with constant cross section. The hand formulas are likely empirical and the result of some threaded elbows with expansions and contractions, or ovaling of the tube form the bend, or with internal weld protrusion at the elbows. The difference is real world elbows vs a perfect curved tube.
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September 29, 2022, 04:18 |
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#11 |
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Thanks for your replies.
@evcelia: That makes total sense. I didn't expect such a huge difference but maybe that is the answer. |
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