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

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Old   December 20, 2012, 10:06
Default H-energy flow@(inlet)= 0 ??
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Roland Rakos
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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?

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Old   December 27, 2012, 21:17
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Erik
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What is your reference enthalpy? If you have not defined it perhaps it uses zero at your inlet conditions?
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Old   January 2, 2013, 04:59
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Roland Rakos
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The reference enthalpy is 0 [J/kg] in the material model. Do you think about this?

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Old   January 2, 2013, 06:15
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Glenn Horrocks
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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.
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Old   January 2, 2013, 10:25
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Roland Rakos
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Quote:
Originally Posted by ghorrocks View Post
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?

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Old   January 3, 2013, 18:59
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Glenn Horrocks
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Are you getting reverse flow at either or both boundaries?
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Old   January 4, 2013, 05:34
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Roland Rakos
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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
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Old   January 5, 2013, 05:11
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Glenn Horrocks
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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.
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Old   January 7, 2013, 08:02
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Roland Rakos
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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?
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Old   January 7, 2013, 13:23
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Talita Possamai
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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
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