Learning Forum

[Young Duk Lee]

HI!

i am modeling ammonia dissociation reaction in the SOFC module. for that purpose I have defined the reaction rate and the mechanism.

at the inlet i have given NH3 to be 0.99 mole fraction but when i simulate and i measure the mole fraction of ammonia at the inlet its is only 0.4... 

1. can someone explain why is  it like this

2. can i give an upwind estimation at the inlet boundary condition so the boundary conditions remains unchanged?

Thankyou. 

[Rob]

Can you recheck on the inlet surface with node values off (if using contours) and local scale? Also double check you've got mole fraction on the inlet and not mass fraction. 

[Young Duk Lee]

Hi!

Thank you for your reply. please see the image below. at the inlet the mole fraction of ammonia is 0.99 while after simulation the mole fraction of ammonia at the inlet is only 0.42. I have double checked numerous times and the same result comes. 

second the under-relaxation factor for all species is also 1. I also checked using report definitions as area weighted average for the mole fraction of ammonia at the inlet and it gives the same result of 0.42. so the last thing I could think of was if the inlet could be estimated as upwind boundary condition. 

[Rob]

That's odd. How many iterations have you done? How well resolved is the mesh in that region?

[Young Duk Lee]

i went upto 100 iterations and the mesh is quite fine compared to the reported studies. 

i think it is probably because the reaction produces twice the gaseous volume so there is a back pressure on the inlet. does it makes sense?

[Rob]

With a mass flow inlet I'd expect the values to be respected. How fast is the reaction? 

[Young Duk Lee]

Thankyou for your reply. 

I am not sure about the values of the reaction rate. but I am giving the equation:

r=4e15 exp(-196200/RT) /1000 

divided by thousand only for units balance as the fluent units are in kg.mol/(m3.s) and the equation is for g.mol/(m3.s) basis

with this the maximum reaction rate is 225.6 kg.mol/(m3.s).

I must inform that I extended the inlet before the reaction site and the molar fractions improved significantly. now I am getting measured inlet mole fraction of 0.93 with the given input of 0.99. 

 

[Rob]

Ah, kgmol = kmol  it's an older form of the unit courtesy of the US and some combustion history within Fluent. 

OK, good. It sounds like the model is doing something that's messing with the reports. If you check the mass flow of ammonia in both cases I assume it's the same? 

Just a comment, I saw you'd got 5 listed species, what is the 6th (bulk) material? Fluent solves for N-1 species so (for example) if I was solving a mixture of NH3, N2, O2, H2 & H20 I'd look for the bulk material & only see 4 of the 5 on the boundary condition settings, the 5th is (1-the rest).