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Old   August 25, 2005, 07:45
Default UDF: species source term
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Hi! I am modelling pyrolysis process. I created for that an UDF, which satisfactorily works for the simulation of the wood evolution. I have however some problems of simulating the gas species evolution. My source terms are too low and thus species mass fraction are far from experiments. Has anyone some experience of the species source terms and could show an example?

Thanks. Luc
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Old   August 26, 2005, 12:08
Default Re: UDF: species source term
Allan Walsh
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Not sure what you mean by the source terms are too low. You can set the mass fraction of volatiles in the "combusting particle" panel (should be on a dry wood basis). Volatiles are 70% to 80% of the dry wood mass fraction.

I create a psuedospecies - wood volatiles - which is my source from volatilization of the wood particle. The psuedospecies then decomposes into H2, H2O, CH4, CO, CO2, etc., with weak dependence on kinetics.

The molecular weight and heat of formation of the psuedospecies has to be determined from the original wood elemental composition and the amount of char formed.

Takes a few weeks to a month to set this up and work out the balances. If you don't have the time to spend on it, I would recommend just using the built-in Fluent formulation and forgot about UDFs.
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Old   August 30, 2005, 11:50
Default Re: UDF: species source term
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Thank you Allan for the advice.

actually i am modelling a whole bed of wood. The use of Discrete phase model is too computer-time consuming as I have lots of particles. I use thus UDMI to store the mass of wood, char, and liquid water contained in each cell of the bed. The behaviour of these quantities is oki. My problem is to connect these quantities to a mass source term (the gas and tar produced by wood volatilization). By the way i keep the equivalent of your "psuedospecies - wood volatiles" to simplify (no decomposition into H2, H2O, CH4, CO, CO2, etc. i use for that empirical data from experiments) which seems very interesting.

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