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October 21, 2009, 07:17 
STARCD  inlet boundaries definition in combustion problem

#1 
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Paolo
Join Date: Oct 2009
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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 
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Pauli
Join Date: Mar 2009
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Define Boundaries>ScalarBoundaries. 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 
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Paolo
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Thanks Pauli,
i already tried to define inlet fuel boundary as you have suggested.. The simulation startes without problems but results are bad:


October 21, 2009, 12:42 

#4 
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Pauli
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Are all your scalars "active" with temperatures dependent properties?


October 22, 2009, 04:47 

#5 
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Paolo
Join Date: Oct 2009
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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_FRACFUEL_1", "MIX_FRACFUEL_2" and "MIX_FRACFUEL_3". These are three ‘passive’ scalars representing the mixture fractions of the leading reactants. I have defined a 3step 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_FRACFUEL variables values? And, in relation to this, how do i introduce the scalars mass fractions? Thanks 

October 22, 2009, 18:06 

#6 
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Pauli
Join Date: Mar 2009
Posts: 183
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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 
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Paolo
Join Date: Oct 2009
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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 
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Paolo
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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 
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Paolo
Join Date: Oct 2009
Posts: 7
Rep Power: 8 
Hi Pauli,
i tried to set passive scalars values to 1, but the solution diverges.. Can you explain the meaning of variables MIX_FRACFUEL? If the leading reactant (CH4) mass fraction of reaction 1 is 0.21, do i set the MIX_FRACFUEL_1 variable to 1.0 or to 0.21? 

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