STARCD - inlet boundaries definition in combustion problem

Paolo · October 21, 2009, 07:17

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

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?
Thanks

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

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?
Thanks

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?

October 21, 2009, 07:17	STARCD - inlet boundaries definition in combustion problem	#1
Paolo New Member Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 16	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

October 21, 2009, 12:05		#3
Paolo New Member Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 16	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.

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October 21, 2009, 11:26		#2
Pauli Senior Member Pauli Join Date: Mar 2009 Posts: 189 Rep Power: 17	Define Boundaries->Scalar-Boundaries. Pick the inlet boundary region, pick the scalar, enter the mass fraction value & hit apply. Or use the command rsmo.

October 21, 2009, 12:42		#4
Pauli Senior Member Pauli Join Date: Mar 2009 Posts: 189 Rep Power: 17	Are all your scalars "active" with temperatures dependent properties?

October 22, 2009, 04:47		#5
Paolo New Member Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 16	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

October 22, 2009, 18:06		#6
Pauli Senior Member Pauli Join Date: Mar 2009 Posts: 189 Rep Power: 17	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.

October 23, 2009, 04:01		#7
Paolo New Member Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 16	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		#8
Paolo New Member Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 16	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

October 23, 2009, 12:00		#9
Paolo New Member Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 16	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?