Fluent Pre exponential factor units

Ed_PA · March 2, 2018, 17:31

I am modeling the reduction of iron ore particle with hydrogen at high temperature
Fe3O4 + 4H2 = 3Fe + 4H2O
I have the gas, particles and temperature working in my model. I am trying to get the chemical reaction working. The formula that agrees bets with test data is
(-LN(1-X)) =8.65x10^6 x exp(-193,000,000/RT) x [pH2 - (pH2O/K)] x t
or instantaneous
dX/dt = 8.65x10^6 x exp(-193,000,000/RT) x [pH2 - (pH2O/K)] x (1-X)

p is atm
t is seconds
for a given case [pH2 - (pH2O/K)] = .18 - when I get this case working I'll work towards a UDF

Activation energy = 193,000,000 J/kgmol
Can someone tell me how to convert the pre-exponential factor to the proper units? It appears to currently be (atm second)^-1 and Fluent wants
kg/(m^3 s) (or mol/(m^3 s)?)
When I hunt and peck the pre-exponential factor that works in Fluent is between .3 and 1 but I haven't found a unit conversion that comes anywhere close to that number.

obscureed · March 6, 2018, 04:51

Hi Ed_PA,

Please could you be specific about which reaction model you are using? For example, is it the Kinetic/Diffusion Surface Reaction Rate Model of combusting DPM particles? or Multiple Surface Reactions? What DPM injection type? -- combusting? multicomponent?

Also, it is worth thinking ahead to what type of UDF you might want to develop. (The answer might be that the only way available is to take complete control via DEFINE_DPM_LAW, and the conclusion might be that this is not worth the effort, so then your thinking has to be directed to how to set up multiple reactions to achieve what you want.)

Ed

Ed_PA · March 6, 2018, 08:26

We are using multiple surface reactions, surface injection and multi-component combustion

obscureed · March 7, 2018, 11:24

Hi Ed_PA,

OK, I've forgotten which model is which, and I'm afraid I don't have time to chase this for you.

One of the troubles is that I cannot remember a particle surface reaction that is given a simple Arrhenius rate with a pre-exponential factor and an activation energy. I *do* remember chasing the rather obscure "diffusion rate constant", which appears in the Theory Guide as symbol C1,r. There are quite a lot of unexplained assumptions packed into that constant, including an assumption that the temperature dependence of the gas reactant's diffusivity is (F*T^1.75), known as Fuller's Law. In case this is not relevant, I'll save myself the trouble of typing it all. If you have access to the ANSYS Customer Portal, look for solution 2037866.

Can anyone else help?
Good luck!
Ed

March 2, 2018, 17:31	Fluent Pre exponential factor units	#1
Ed_PA New Member Ed Join Date: Mar 2018 Posts: 3 Rep Power: 8	I am modeling the reduction of iron ore particle with hydrogen at high temperature Fe3O4 + 4H2 = 3Fe + 4H2O I have the gas, particles and temperature working in my model. I am trying to get the chemical reaction working. The formula that agrees bets with test data is (-LN(1-X)) =8.65x10^6 x exp(-193,000,000/RT) x [pH2 - (pH2O/K)] x t or instantaneous dX/dt = 8.65x10^6 x exp(-193,000,000/RT) x [pH2 - (pH2O/K)] x (1-X) p is atm t is seconds for a given case [pH2 - (pH2O/K)] = .18 - when I get this case working I'll work towards a UDF Activation energy = 193,000,000 J/kgmol Can someone tell me how to convert the pre-exponential factor to the proper units? It appears to currently be (atm second)^-1 and Fluent wants kg/(m^3 s) (or mol/(m^3 s)?) When I hunt and peck the pre-exponential factor that works in Fluent is between .3 and 1 but I haven't found a unit conversion that comes anywhere close to that number.

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March 6, 2018, 04:51		#2
obscureed Senior Member Join Date: Sep 2017 Posts: 246 Rep Power: 11	Hi Ed_PA, Please could you be specific about which reaction model you are using? For example, is it the Kinetic/Diffusion Surface Reaction Rate Model of combusting DPM particles? or Multiple Surface Reactions? What DPM injection type? -- combusting? multicomponent? Also, it is worth thinking ahead to what type of UDF you might want to develop. (The answer might be that the only way available is to take complete control via DEFINE_DPM_LAW, and the conclusion might be that this is not worth the effort, so then your thinking has to be directed to how to set up multiple reactions to achieve what you want.) Ed

March 6, 2018, 08:26		#3
Ed_PA New Member Ed Join Date: Mar 2018 Posts: 3 Rep Power: 8	We are using multiple surface reactions, surface injection and multi-component combustion

March 7, 2018, 11:24		#4
obscureed Senior Member Join Date: Sep 2017 Posts: 246 Rep Power: 11	Hi Ed_PA, OK, I've forgotten which model is which, and I'm afraid I don't have time to chase this for you. One of the troubles is that I cannot remember a particle surface reaction that is given a simple Arrhenius rate with a pre-exponential factor and an activation energy. I do remember chasing the rather obscure "diffusion rate constant", which appears in the Theory Guide as symbol C1,r. There are quite a lot of unexplained assumptions packed into that constant, including an assumption that the temperature dependence of the gas reactant's diffusivity is (F*T^1.75), known as Fuller's Law. In case this is not relevant, I'll save myself the trouble of typing it all. If you have access to the ANSYS Customer Portal, look for solution 2037866. Can anyone else help? Good luck! Ed