H-energy flow@(inlet)= 0 ??

Roland R · December 20, 2012, 10:06

Hello,

If I define Total Pressure at inlet then I can check the energy imbalance and the H energy flow at inlet/outlet in the solver. But I define mass flow rate at the inlet then the h energy flow is constant zero at inlet, and it changes at the outlet. Therefore I can not check the energy imbalance because it changes between 100 and -100. I dont understand it. While will be zero the h energy flow in this case?

Regards
Roland

evcelica · December 27, 2012, 21:17

What is your reference enthalpy? If you have not defined it perhaps it uses zero at your inlet conditions?

Roland R · January 2, 2013, 04:59

The reference enthalpy is 0 [J/kg] in the material model. Do you think about this?

Roland

ghorrocks · January 2, 2013, 06:15

I do not think the problem is related to the refernce enthalpy (but I could be wrong).

The error comes about more often when you have one boundary, with all the rest walls. As flow goes in and out the boundary the imbalances go from +100% to -100% as any small numerical error results in a 100% imbalance error.

The solution for this is to ignore imbalances in this case. They are not a good represntation of convergence. In fact they never converge.

Roland R · January 2, 2013, 10:25

Quote:

Originally Posted by ghorrocks

The error comes about more often when you have one boundary, with all the rest walls.

OK, but I have two boundaries. I have one inlet boundary and one outlet boundary. If I define Total pressure inlet boundary and Static Pressure outlet boundary then the convergence is ok.

If I define Mass flow rate for inlet and Static Pressure for outlet then the H-Energy flow is zero at inlet and it fluctuates between 0,8 and -0,8 at outlet therefore the energy imbalance can not converge. But the flow direction is ok because the mass imbalance is < 0,2%.

So the energy flow will be zero in case of mass flow rate boundary condition. How can it be?

Roland

ghorrocks · January 3, 2013, 18:59

Are you getting reverse flow at either or both boundaries?

Roland R · January 4, 2013, 05:34

Based on result there is not reverse flow. MassFlow_at_inlet:850g/s and massFlow_at_outlet: -850g/s. Direction of velocity vectors is correct.

I reduced the mass flow from 850g/s to 20g/s, but the result doesn't changed.

I tried to change the boundaries: Inlet: total pressure, outlet: mass flow rate. The result is same. H-Energy flow at inlet: 0.

What can be the problem?

Roland

ghorrocks · January 5, 2013, 05:11

If either boundary has no flow then any flow on the other boundary will be a 100% imbalance. I suspect this is where the imbalance is coming from.

Roland R · January 7, 2013, 08:02

OK, it can be understood. But if the mass flow is positiv at inlet, and value of the air temperature is ~ 300K, then the energy flow should be positiv at inlet, shouldn't it?

Possa · January 7, 2013, 13:23

Hello Roland,

H energy through a boundary is the heat flux. For a fluid is defined as "Energy=mass_flux*(Enthalpy (T,P)-Enthalpy (Tref,Pref))".
If the fluid is composed by different components then the ideal mixture properties theory is adopted.
So if you have H energy = 0 at an inlet boundary then the mass flow = 0 or (Enthalpy (T,P)-Enthalpy (Tref,Pref)) = 0.

Something that can occur sometimes is an error in the energy balance due to the boundary conditions. Then the problem is converged but the energy balance is not (like Glenn stated). An example of this happens when you have a solid moving through a boundary but the user forgets to check the box for "advection through boundary" in the boundary conditions.

So I suggest you revise your boundary conditions. You can simply estimate the H energy through each boundary manually and then find what CFX is not considering.

I hope that helps you,

Regards,
Possa

December 20, 2012, 10:06	H-energy flow@(inlet)= 0 ??	#1
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	Hello, If I define Total Pressure at inlet then I can check the energy imbalance and the H energy flow at inlet/outlet in the solver. But I define mass flow rate at the inlet then the h energy flow is constant zero at inlet, and it changes at the outlet. Therefore I can not check the energy imbalance because it changes between 100 and -100. I dont understand it. While will be zero the h energy flow in this case? Regards Roland

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December 27, 2012, 21:17		#2
evcelica Senior Member Erik Join Date: Feb 2011 Location: Earth (Land portion) Posts: 1,188 Rep Power: 23	What is your reference enthalpy? If you have not defined it perhaps it uses zero at your inlet conditions?

January 2, 2013, 04:59		#3
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	The reference enthalpy is 0 [J/kg] in the material model. Do you think about this? Roland

January 2, 2013, 06:15		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	I do not think the problem is related to the refernce enthalpy (but I could be wrong). The error comes about more often when you have one boundary, with all the rest walls. As flow goes in and out the boundary the imbalances go from +100% to -100% as any small numerical error results in a 100% imbalance error. The solution for this is to ignore imbalances in this case. They are not a good represntation of convergence. In fact they never converge.

January 3, 2013, 18:59		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	Are you getting reverse flow at either or both boundaries?

January 4, 2013, 05:34		#7
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	Based on result there is not reverse flow. MassFlow_at_inlet:850g/s and massFlow_at_outlet: -850g/s. Direction of velocity vectors is correct. I reduced the mass flow from 850g/s to 20g/s, but the result doesn't changed. I tried to change the boundaries: Inlet: total pressure, outlet: mass flow rate. The result is same. H-Energy flow at inlet: 0. What can be the problem? Roland

January 5, 2013, 05:11		#8
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,872 Rep Power: 144	If either boundary has no flow then any flow on the other boundary will be a 100% imbalance. I suspect this is where the imbalance is coming from.

January 7, 2013, 08:02		#9
Roland R Senior Member Roland Rakos Join Date: Mar 2009 Posts: 131 Rep Power: 17	OK, it can be understood. But if the mass flow is positiv at inlet, and value of the air temperature is ~ 300K, then the energy flow should be positiv at inlet, shouldn't it?

January 7, 2013, 13:23		#10
Possa New Member Talita Possamai Join Date: Sep 2012 Posts: 23 Rep Power: 14	Hello Roland, H energy through a boundary is the heat flux. For a fluid is defined as "Energy=mass_flux*(Enthalpy (T,P)-Enthalpy (Tref,Pref))". If the fluid is composed by different components then the ideal mixture properties theory is adopted. So if you have H energy = 0 at an inlet boundary then the mass flow = 0 or (Enthalpy (T,P)-Enthalpy (Tref,Pref)) = 0. Something that can occur sometimes is an error in the energy balance due to the boundary conditions. Then the problem is converged but the energy balance is not (like Glenn stated). An example of this happens when you have a solid moving through a boundary but the user forgets to check the box for "advection through boundary" in the boundary conditions. So I suggest you revise your boundary conditions. You can simply estimate the H energy through each boundary manually and then find what CFX is not considering. I hope that helps you, Regards, Possa