Duct CFM: Velocity (mass flow average) vs Mass flow * Density

Witchking782 · January 27, 2016, 14:57

Hi, I'm trying to calculate CFM through a fan interface in a simple duct. I have created a report of mass flow and I have density which I can use to calculate CFM. However, when I use mass flow average and generate velocity and then use the area of the fan interface to calculate CFM. They give very different results. Can someone explain which one is more accurate?

fluid23 · January 28, 2016, 17:36

If you have incompressible (or nearly incompressible) then dividing mass flow by density is probably your best bet. Using a mass flow averaged velocity essentially biases your average velocity against itself.

To be 100% correct you would want to create a new unit for volume flow rate (CFM). Then define a field function that is the product of area magnitude and velocity (normal to your interface). Then you can get volume flow rate by doing a surface integral of the field function with your interface as your input part.

Pay attention to the dimensions of the field function, there is a trap there waiting to trip you up if you aren't paying attention.

January 27, 2016, 14:57	*Duct CFM: Velocity (mass flow average) vs Mass flow Density**	#1
Witchking782 New Member Join Date: Feb 2010 Posts: 9 Rep Power: 16	Hi, I'm trying to calculate CFM through a fan interface in a simple duct. I have created a report of mass flow and I have density which I can use to calculate CFM. However, when I use mass flow average and generate velocity and then use the area of the fan interface to calculate CFM. They give very different results. Can someone explain which one is more accurate?

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January 28, 2016, 17:36		#2
fluid23 Senior Member Matt Join Date: Aug 2014 Posts: 947 Rep Power: 18	If you have incompressible (or nearly incompressible) then dividing mass flow by density is probably your best bet. Using a mass flow averaged velocity essentially biases your average velocity against itself. To be 100% correct you would want to create a new unit for volume flow rate (CFM). Then define a field function that is the product of area magnitude and velocity (normal to your interface). Then you can get volume flow rate by doing a surface integral of the field function with your interface as your input part. Pay attention to the dimensions of the field function, there is a trap there waiting to trip you up if you aren't paying attention.