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Old   February 25, 2002, 05:28
Default combustion model
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Hennie van der Westhuizen
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I'm trying to introduce the Magnussen combustion model to my SI engine model but at the time of combustion nothing happens. I cannot find clear instructions on how to set the various parameters for this model in the manuals. My parameters are currently:

ufactor: 0.5

mixture fraction: 1

no conditional enthalpy

tscale adjust: standard

reaction: C8H18 + 12.5O2 -> 8CO2 + 9H2O

rate eqn: no products

Amix: 4

Bmix: 0.5

reactant parameters: 02 fixed fraction 0.233

leading reactants: C8H18

products: CO2,

H2O ignition fraction: 0.05

time: iteration

knock model: off

Ignition starts 10 deg BTDC and continues till the last time step.All the scalars have been defined and their concentrations in the cylinder set to:

C8H18: 0.0665

O2: 0.1960

N2: 0.7375

When running the simulation, the fluid temperature in the combustion chamber gets up to 600K due to compression but no chemical reaction takes place.Is there any other parameters I need to set for the model to work? I would appreciate any suggestions.
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Old   February 25, 2002, 12:36
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Daniel
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My speculation is that since the products switch in the Magnussen model indicates that the reaction rate is dependent on products concentration as well as the reactants, and in your case there is no CO2 in the chamber before reaction occurs, this may cause the reaction rate to be zero.
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Old   February 25, 2002, 14:03
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Richard
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One thing which could be wrong is you say "mixture fraction: 1", whereas the mixture fraction should agree with the fuel concentration (at boundaries and initial conditions).
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Old   February 26, 2002, 03:09
Default Re: combustion model
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Hennie van der Westhuizen
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Thanks guys for the response. I looked at the concentrations of the scalars in the chamber and it seems that from iteration 1 all combustion products are already formed and all fuel burned. This happens irrespective of what I set the initial scalar concentration to and despite the setting of ignition occurence at iteration 170(10 deg btdc). No heat is released either.
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Old   February 26, 2002, 07:24
Default Re: combustion model
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Daniel
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Well, the magnussen model does not require ignition at all, so that is no problem. How did you conclude that no heat is released in your model ?
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Old   February 26, 2002, 07:50
Default Re: combustion model
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Hennie van der Westhuizen
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The RVB concentration was not set. Setting this value to 1 caused my model to work as this is the scalar determining the concentration of the mixture in the background fluid. Combustion now starts at the first iteration i.e. as you said, no ignition is required. How do I controll the time of ignition in this model?
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Old   February 26, 2002, 08:13
Default Re: combustion model
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Daniel
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That is a bit of a problem with the magnussen model, since this is a model of the "mixed is burnt" type. I guess you could just run your model the required number of time steps without combustion, then stop it, turn combustion on and restart the analysis. Expect some problems (convergence, etc...) as you restart.

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Old   February 27, 2002, 03:10
Default Re: combustion model
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Hennie van der Westhuizen
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Thanks Daniel, I switched over to the standard eddy break up model, which allows control over combustion time. This seems to work quite wel.
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