STARCD - inlet boundaries definition in combustion problem
 

Hi!
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?
Thanks,
Paolo

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

Or use the command rsmo.

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.

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

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?
Thanks

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.

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

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?
Thanks

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?