Mass Flow Inlet

AZ1621 · July 5, 2015, 12:17

Hello,

I am running a transient simulation of a air flowing through a wind tunnel test section with a bluff body inside of it. I am using the ideal gas law setting to calculate the air density and I am trying to set the mass flow at my inlet. I want my inlet velocity to be 175 ft/s, the inlet temperature to be 700F and the inlet pressure to be 14.7 psi. I used the ideal gas law to calculate my density and then used the density with my velocity and my cross section area (18in^2) to get a mass flow of 0.7484 lb/s. Unfortunately when I use this mass flow as my inlet mass flow boundary condition, the inlet velocity I am getting is ~13.3 ft/s and nowhere near the 175 ft/s I am trying to achieve. I have done the math several times so I feel like that isn't the issue. Any thoughts on how to fix this or what would be causing this?

Thanks for the help!

Steffen595 · July 5, 2015, 21:10

metric would be easier to comprehend...
Pressure is gauge pressure?
Do you need temperature and all that, maybe just be lazy and define the gas with the density you want?
Check the units, maybe you put K in instead F?

AZ1621 · July 5, 2015, 21:36

Thanks for the reply, in metric the inlet I need would be 53.34 m/s velocity, 644.261 K inlet temp. and pressure of 101325 Pa. I have the operating pressure set to 101325 for the model so the gauge inlet pressure is 0 Pa. I am trying to model my simulation as compressible flow so I can compare to experimental data so I can't just set my gas to a constant density( which would enable the use of a velocity inlet). Furthermore my model is calculating the gas density correctly when I generate density contours (my density should be 0.54799 kg/m^3 or 0.0342 lb/ft^3) so it can't be an issue with the pressure or temperature. It has to be something with the mass flow calculation.

Steffen595 · July 6, 2015, 02:06

try with a static density, see if it gets the mass flow right. It should. I am doing R134a and use velocity inlet and its not off by factor 10.
Check if you set the domain tight. Check in post, what the density is and mass flow in/out.
Are your boundaries ok, ok does it put walls in, check if mass flow in is same as mass flow out.
setup-General-reference values, reference pressure would be 0 (gauge). set density there.

AZ1621 · July 6, 2015, 20:35

I tried with a static density and received the same result. In post the density is calculated correctly and I believe my domain is setup correctly. Perhaps it has something to do with what it thinks the area of my inlet is? Any idea how to check that? I can see that my domain is the proper width so I don't see how the inlet could be the wrong area but if my mass calculated mass flow is correct then fluent thinks that my inlet is 0.10156 m^2 instead of 0.01161288 m^2.

I just checked my inlet length in design modeler and it is the correct length( 6" or 0.1524 m). Do you know how fluent calculates inlet area for mass flow rate when faced with a 2d simulation and a line for an inlet? I think the issue is there I'm just not sure how to troubleshoot that.

AZ1621 · July 6, 2015, 20:54

I just solved my issue. It was the way fluent calculates mass flow inlet area in a 2D model.

solution link for those interested:
http://www.cfd-online.com/Forums/flu...s-flow-2d.html

July 5, 2015, 12:17	Mass Flow Inlet	#1
AZ1621 New Member Join Date: Jun 2015 Posts: 7 Rep Power: 11	Hello, I am running a transient simulation of a air flowing through a wind tunnel test section with a bluff body inside of it. I am using the ideal gas law setting to calculate the air density and I am trying to set the mass flow at my inlet. I want my inlet velocity to be 175 ft/s, the inlet temperature to be 700F and the inlet pressure to be 14.7 psi. I used the ideal gas law to calculate my density and then used the density with my velocity and my cross section area (18in^2) to get a mass flow of 0.7484 lb/s. Unfortunately when I use this mass flow as my inlet mass flow boundary condition, the inlet velocity I am getting is ~13.3 ft/s and nowhere near the 175 ft/s I am trying to achieve. I have done the math several times so I feel like that isn't the issue. Any thoughts on how to fix this or what would be causing this? Thanks for the help!

July 6, 2015, 02:06		#4
Steffen595 Senior Member Join Date: May 2012 Location: Melbourne Posts: 161 Rep Power: 14	try with a static density, see if it gets the mass flow right. It should. I am doing R134a and use velocity inlet and its not off by factor 10. Check if you set the domain tight. Check in post, what the density is and mass flow in/out. Are your boundaries ok, ok does it put walls in, check if mass flow in is same as mass flow out. setup-General-reference values, reference pressure would be 0 (gauge). set density there. Last edited by Steffen595; July 6, 2015 at 08:23.

July 6, 2015, 20:54		#6
AZ1621 New Member Join Date: Jun 2015 Posts: 7 Rep Power: 11	I just solved my issue. It was the way fluent calculates mass flow inlet area in a 2D model. solution link for those interested: http://www.cfd-online.com/Forums/flu...s-flow-2d.html spx likes this.

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July 5, 2015, 21:10		#2
Steffen595 Senior Member Join Date: May 2012 Location: Melbourne Posts: 161 Rep Power: 14	metric would be easier to comprehend... Pressure is gauge pressure? Do you need temperature and all that, maybe just be lazy and define the gas with the density you want? Check the units, maybe you put K in instead F?

July 5, 2015, 21:36		#3
AZ1621 New Member Join Date: Jun 2015 Posts: 7 Rep Power: 11	Thanks for the reply, in metric the inlet I need would be 53.34 m/s velocity, 644.261 K inlet temp. and pressure of 101325 Pa. I have the operating pressure set to 101325 for the model so the gauge inlet pressure is 0 Pa. I am trying to model my simulation as compressible flow so I can compare to experimental data so I can't just set my gas to a constant density( which would enable the use of a velocity inlet). Furthermore my model is calculating the gas density correctly when I generate density contours (my density should be 0.54799 kg/m^3 or 0.0342 lb/ft^3) so it can't be an issue with the pressure or temperature. It has to be something with the mass flow calculation.

July 6, 2015, 20:35		#5
AZ1621 New Member Join Date: Jun 2015 Posts: 7 Rep Power: 11	I tried with a static density and received the same result. In post the density is calculated correctly and I believe my domain is setup correctly. Perhaps it has something to do with what it thinks the area of my inlet is? Any idea how to check that? I can see that my domain is the proper width so I don't see how the inlet could be the wrong area but if my mass calculated mass flow is correct then fluent thinks that my inlet is 0.10156 m^2 instead of 0.01161288 m^2. I just checked my inlet length in design modeler and it is the correct length( 6" or 0.1524 m). Do you know how fluent calculates inlet area for mass flow rate when faced with a 2d simulation and a line for an inlet? I think the issue is there I'm just not sure how to troubleshoot that.