My thesis has to be ok in 20 days! Please explain to me ignition modelling for Local source (Steady calculation) My mesh is from Centaur and I have to define ignition cells first of all. During CFD calculation it seems not to react!!! Please help me! Thanks. Thanks

Lorenzo,

I'm not going to comment your 20 days ). (very logical sentence, isn't it?)

I suppose, you want to use so called EDBR model. Supposing furter on, that an initial condition, you have is fresh mixture or air (do not know if it is premixed or not) throughout the domain.

Now, as the scalars are progressing, poor them -they only mix together not knowing they should react. It is because of the definition of the model. Try it yourself with a calculator and some border values to see when you get a source and when the model is dumb.

So what you do is to increase the temperature or you set a mixture fraction (concentration) to a fixed value of burnt products. You specify a type of cells, and iterations to apply this setting on. Appearently prior to the ignition setting, you have to set the cell type (fluid) to your favourite ignition color. Choose cells exposed to a stream of fresh reactants, so the ignition can propagate further on its own. Choose something like 10-100 cells. Set the iterations to apply the ignition not at the beginning, but to something like 50-100 iter. These settings really depends on your geometry and mesh. You have to tune it.

You'll find more info also in a user guide and methodology pdfs.

hope this helps.

matej

Thank you Matej! I'll try it. I'll have an initial field restart from non-reacting field. Probably I can start ignition at first iteration, or not? Two questions: 1)I have a little pipe where pure CH4 enters: do I have to set mixture fraction 1 in this inlet? 2) What is the right value of Fraction in ignition setting for Methane-Air combustion?

Thanks for everything.

ad 0) yes, I thing you may try to start iginition from the very beginning if you restart, however it may not work ;o)

ad 1) yes. mixture fraction is defined to be 1 in fuel stream and 0 in a oxidiser stream

ad 2) the stoichiometric mixture fraction for methane-air is ~0.055. I'd go for something around this value.

Don't forget to set chemico-thermal enthalpy calculation! Don't forget to set underelaxation parameters for scalars and enthalpy following advice in a manual.

matej

You are a friend! Manual says that under-relaxation factors must be identical for enthalpy (it is temperature or not?) and scalars. Where can I set the under-relaxation factor for enthalpy ? In Equation Behaviour -> Primary variables or somewhere else?

The under-relaxation factor for enthalpy is called for some obscure reason temperature underrelax. factor. The code is calculating enthalpy equation. Anyway the tutorial says to set it to 0.7 and set the residual tolerance to 0.001 to make the calcualtion more precise. se the same for scalars.

if you have problems with convergence, try to tune underrelax. factors for temp.and scalars and density. try a double precision run.

matej