# Area weighted or mass weighted average

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 January 31, 2011, 07:13 area weighted or mass weighted average #21 New Member   Amit Chauhan Join Date: May 2010 Location: Chennai, India Posts: 18 Rep Power: 14 thank you sir... thank you very much.

May 3, 2013, 17:10
#22
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Ehsan
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Quote:
 Originally Posted by jola Area-averaging and mass-averaging will give different results and what you should use depends on what variable you are interested in. For conserved variables like total-pressure you should use mass-averaging, whereas for other variables, like static-pressure, you should use area-averaging. Check out some of the refences mentioned. To make things even more complex another type of averaging often used is flux-averaging. With flux-averaging all gradients are mixed out and a flux-averaged value is the value you would get if you measure at the outlet of an infinently long friction-free pipe put at the boundary. In turbomachinery applications flux-averaging is common, and sometimes you do not flux-average everything, you do flux-averaging circumferentially to mix-out wakes, but you do not do flux-averaging radially because you do not want to mix-out the end-wall boundary layers.
hello
what do you mean by a conserved variable exactly?
is a scalar transport variable conserved?I have mixed up with a conserved variable and a conserved equation.
thank you.
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March 4, 2014, 06:19
#23
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Quote:
 Originally Posted by jola Area-averaging and mass-averaging will give different results and what you should use depends on what variable you are interested in. For conserved variables like total-pressure you should use mass-averaging, whereas for other variables, like static-pressure, you should use area-averaging. Check out some of the refences mentioned. To make things even more complex another type of averaging often used is flux-averaging. With flux-averaging all gradients are mixed out and a flux-averaged value is the value you would get if you measure at the outlet of an infinently long friction-free pipe put at the boundary. In turbomachinery applications flux-averaging is common, and sometimes you do not flux-average everything, you do flux-averaging circumferentially to mix-out wakes, but you do not do flux-averaging radially because you do not want to mix-out the end-wall boundary layers.
Dear Sir, I have in my project mixing chamber CO2 with air (with different temperature ) but mass fraction for CO2 very small when try to find static temperature or total temp. by mass - averaging different if I find it by area - averaging, and my project incompressible flow
I interested to find temperature.

 March 4, 2014, 13:37 #24 New Member   adnan Join Date: Nov 2013 Location: Germany Posts: 6 Rep Power: 11 Dear friends, I have in my project mixing chamber CO2 with air (with different temperature ) but mass fraction for CO2 very small when try to find static temperature or total temp. by mass - averaging different if I find it by area - averaging, and my project incompressible flow I interested to find temperature. Thanks in advance

July 8, 2015, 07:15
#25
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Shane
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Quote:
 Originally Posted by Jan Rusås ;31045 Regarding the average methode for pressure, you can find that from the energy equation. It should actually be volume flow averaged, but if the density is constant you can use mass flow average. Only if the velocity is near constant, same mass flow at each position, will result in same value for mass flow and area average.
The energy equation is a function of Velocity across the surface area. Should one calculated the Pressure and velocity average over the Surface Area for energy & heat transfer calculations ?

April 26, 2018, 02:12
#26
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Quote:
 Originally Posted by jola Area-averaging and mass-averaging will give different results and what you should use depends on what variable you are interested in. For conserved variables like total-pressure you should use mass-averaging, whereas for other variables, like static-pressure, you should use area-averaging. Check out some of the refences mentioned. To make things even more complex another type of averaging often used is flux-averaging. With flux-averaging all gradients are mixed out and a flux-averaged value is the value you would get if you measure at the outlet of an infinently long friction-free pipe put at the boundary. In turbomachinery applications flux-averaging is common, and sometimes you do not flux-average everything, you do flux-averaging circumferentially to mix-out wakes, but you do not do flux-averaging radially because you do not want to mix-out the end-wall boundary layers.
Dear Jola

Actually, I want to use mass-averaging in calculating bulk tempertature. But, my supervisor ask me to use basic equation in calculating bulk temperature in pipe. I see some papers use mass averaging to calculate bulk temperature. Why they do not use basic equation to calculate bulk temperature?

I would like to know the reference of your statement written in which book or paper. Can you tell me your references?

Best Regards

Teguh
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 April 26, 2018, 09:10 #27 Administrator     Jonas Larsson Join Date: Jan 2009 Location: Gothenburg, Sweden Posts: 792 Rep Power: 10 The averaging discussed here is how to average over a whole inlet or outlet boundary in a CFD simulations. Your problem of "calculating bulk temperature" sounds like a different problem. Do you have a CFD simulation available? Do you want to average over a known boundary and what property to you want to average? What will the averaged result be used for?

April 29, 2018, 21:02
#28
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Lucky
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Quote:
 Originally Posted by teguhtf I see some papers use mass averaging to calculate bulk temperature. Why they do not use basic equation to calculate bulk temperature?
Most of the time, the bulk temperature at any plane is the mass-averaged temperature at the plane. What else would you use if not the definition of bulk temperature? What do you mean by basic equation?

So when is the bulk temperature not the mass averaged temperature? When there are variable specific heats or you consider the bulk temperature as the temperature associated with the bulk enthalpy.

April 29, 2018, 22:40
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Quote:
 Originally Posted by jola The averaging discussed here is how to average over a whole inlet or outlet boundary in a CFD simulations. Your problem of "calculating bulk temperature" sounds like a different problem. Do you have a CFD simulation available? Do you want to average over a known boundary and what property to you want to average? What will the averaged result be used for?
Dear Jola

I am sorry maybe my understanding is wrong. I think the bulk temperature is similar to mean temperature so I ask about it. Yes, I have CFD simulation. I need the bulk temperature in inlet and outlet to calculate the Heat transfer coefficient and also Nusselt number. Actually for the property (density, viscosity, conductivity and heat capacity) is constant.

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April 29, 2018, 22:50
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Quote:
 Originally Posted by LuckyTran Most of the time, the bulk temperature at any plane is the mass-averaged temperature at the plane. What else would you use if not the definition of bulk temperature? What do you mean by basic equation? So when is the bulk temperature not the mass averaged temperature? When there are variable specific heats or you consider the bulk temperature as the temperature associated with the bulk enthalpy.
Dear LuckyTran

I use the attached equation (It is taken from Fundamental of Heat and Mass Transfer Incropera). Actually, I have tried to calculate the bulk temperature by using that equation and mass-weighted average temperature. But there is a difference in each result. I don't know which one is the correct.

Attached Images
 Bulk Temperature.jpg (4.2 KB, 31 views)
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 April 30, 2018, 03:16 #31 Senior Member   Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,112 Rep Power: 60 The rho u sitting in the integral is what makes it a mass average integral. So you see that is a mass weighted average integral of Cp*T. Obviously the integral of rho*U is the mass flow rate mdot. So if Cp is a constant and you can bring it out of the integral (which is implied in this formula by comparing with the denominator), then the bulk temperature is the mass averaged temperature. If cp was not constant, then there would be an integral of rho*U*cp in the denominator or something equivalent. Maybe you missed that you are staring at the formula for the mass-average? The reason it makes sense to call this integral a mass-weigted average (vs the more precise mass-flowrate weighted average) is that there's no mass on a plane, only mass-flux or mass-flows.

April 30, 2018, 05:12
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Quote:
 Originally Posted by LuckyTran The rho u sitting in the integral is what makes it a mass average integral. So you see that is a mass weighted average integral of Cp*T. Obviously the integral of rho*U is the mass flow rate mdot. So if Cp is a constant and you can bring it out of the integral (which is implied in this formula by comparing with the denominator), then the bulk temperature is the mass averaged temperature. If cp was not constant, then there would be an integral of rho*U*cp in the denominator or something equivalent. Maybe you missed that you are staring at the formula for the mass-average? The reason it makes sense to call this integral a mass-weigted average (vs the more precise mass-flowrate weighted average) is that there's no mass on a plane, only mass-flux or mass-flows.
Dear Lucky Tran

Thank you for your explanation. Now, I have a clear understanding of the equations. Thank you so much.

Thank you

best regards
Teguh
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 Tags area weighted average, mass weighted average