Changing inlet mass fractions of CH4 and O2
 

I'm changing the inlet mass fractions boundary condition for methane and air from 0.5 to 0.75 equivalence ratio, keeping the solution from the 0.5 run case as the initial solution for the 0.75 case (to save time). I see a patch near the inlet to my combustor which shows correct mass fractions of the species. this patch is very close to the inlet and then the mass fraction of, say, methane increases by around 10% till the flame region is reached.

why is there a non-physical increase in the mass fraction of methane? I'm using single-step chemistry.

thanks

Re: Changing inlet mass fractions of CH4 and O2
 

Hello,

are you sure your solution is converged? Try to turn of the residual-convergence-checks and let it iterate further. See What happens.

hopefully it's only this...

Laika, still orbiting

Re: Changing inlet mass fractions of CH4 and O2
 

on the basis of the previous solution for the other equivalence ratio case, 20 iterations per time step have been chosen. I'm reasonably sure that every time step I do get a converged solution - apart from the mass fractions, all other variables show contour distributions which are physical.

Re: Changing inlet mass fractions of CH4 and O2
 

Aha, this is a transient calculation.

Do you really want to model the transient effects? If so, do you really want to model the discontinuity in your inlet condition (you suddenly jump from 0.75 to 0.5 mass fraction)?

Transient calculations are not hapy with such jumps.

If the jump is not of concern, converg the solution in a steady state, and switch to transient afterwards. If you really want to model the transition of one inlet regime to an other, implement it more physical. Use a UDF to gradually change the inlet mass fraction.

greetings,

Laika, still orbiting

Re: Changing inlet mass fractions of CH4 and O2
 

Laika:

Thanks for the suggestions. I used the steady-state solver to converge the solution and then switched to the unsteady solver. The unsteady solver did change the solution quite a bit (especially the heat release from the flame), but now I don't get a screwed up mass fraction distribution.

thanks a ton for waking me up from the deep slumber! :) I was obviously not looking forward to getting a simple solution to my problem.

prateep