# STARCD - inlet boundaries definition in combustion problem

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 October 21, 2009, 07:17 STARCD - inlet boundaries definition in combustion problem #1 New Member   Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 7 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, 11:26 #2 Senior Member   Pauli Join Date: Mar 2009 Posts: 181 Rep Power: 8 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, 12:05 #3 New Member   Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 7 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.

 October 21, 2009, 12:42 #4 Senior Member   Pauli Join Date: Mar 2009 Posts: 181 Rep Power: 8 Are all your scalars "active" with temperatures dependent properties?

 October 22, 2009, 04:47 #5 New Member   Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 7 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 Senior Member   Pauli Join Date: Mar 2009 Posts: 181 Rep Power: 8 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 New Member   Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 7 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 New Member   Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 7 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 New Member   Paolo Join Date: Oct 2009 Posts: 7 Rep Power: 7 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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