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Paolo October 21, 2009 07:17

STARCD - inlet boundaries definition in combustion problem
I have a problem in defining a fuel consisting of a mixture of gases (CH4,CO,N2,CO2,H2), which enter in the combustion chamber each characterized by its own mass fraction.
The fuel enters from a single entry.
The solution gives me some problems:
- incorrect mass fractions for single gases
- incorrect values for pressure
How can i define the inlet boundary values for this fuel consisting of a mixture of gases, in order to introduce the gases mass fraction?

Pauli October 21, 2009 11:26

Define Boundaries->Scalar-Boundaries. Pick the inlet boundary region, pick the scalar, enter the mass fraction value & hit apply.

Or use the command rsmo.

Paolo October 21, 2009 12:05

Thanks Pauli,
i already tried to define inlet fuel boundary as you have suggested..
The simulation startes without problems but results are bad:
  • the mass fractions in the inlet fuel tube (before combustione zone) are wrong;
  • temperature goes to absurd values like 1e+05;
  • combustion occurrs in a strange zone: theinlet pipe of the fuel
I think the problem is in defining density of a mixture: in the inlet boundary i define the density of the entire mixture, calculated from the mass fraction and density of single gases which form the syngas.

Pauli October 21, 2009 12:42

Are all your scalars "active" with temperatures dependent properties?

Paolo October 22, 2009 04:47

Hello Pauli,
thanks so much for your help.
Yes, all my scalars are active with temperatures dependent properties.
But i think i have found the problem in defining boundary conditions - scalar boundaries: i must understand how to define the three variables "MIX_FRAC-FUEL_1", "MIX_FRAC-FUEL_2" and "MIX_FRAC-FUEL_3". These are three ‘passive’ scalars representing the mixture fractions of the leading reactants.
I have defined a 3-step reaction:
CH4 + 1.5O2 -> CO + 2H2O
CO + 0.5O2 -> CO2
H2 + 0.5O2 -> H2O
The composition (in terms of mass fractions) of the syngas is:
CH4 = 0.21
CO = 0.29
N2 = 0.05
CO2 = 0.38
H2 = 0.07
How can i introduce the values of MIX_FRAC-FUEL variables values?
And, in relation to this, how do i introduce the scalars mass fractions?

Pauli October 22, 2009 18:06

The mass fractions you provided add to 1. Where is the O2? Is it coming from a separate inlet?

As you stated, the passive scalars are the mass fractions of the leading reactants for each reaction equation. Since your inlet has no O2, I believe they all get set to 1.

Paolo October 23, 2009 04:01

Hi Pauli!
The O2 is coming from a separate inlet.
So the passive scalars represeant the mass fractions of leading reactants for each reaction equation in relation to oxidant mass fraction.. So if the oxidant enters from a separate inlet, the scalar value is set to 1, right?
I try to apply your suggestion.
Thank you so much.

Paolo October 23, 2009 07:03

Do i also define CO2 as a reagent?
(when i define the three reactions, i put CO2[R] because if a product is transported into the solution domain from an external source, it also should be specified as a reactant)
Is it correct?

Paolo October 23, 2009 12:00

Hi Pauli,
i tried to set passive scalars values to 1, but the solution diverges.. Can you explain the meaning of variables MIX_FRAC-FUEL? If the leading reactant (CH4) mass fraction of reaction 1 is 0.21, do i set the MIX_FRAC-FUEL_1 variable to 1.0 or to 0.21?

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