Flame Dissapears

Prateep Chatterjee

I'm using the segregated solver to simulate a laminar flat flame in a straight tube. With SIMPLE for pressure-velocity coupling, ideal gas properties and single step chemistry. I'm using a velocity inlet BC and a pressure outlet BC (at atmospheric pressure). The inlet velocity is low at ~3 cm/s. The time step selected is of the order of 10^-7. I can see a wrinkled flame forming after 2/3 time steps (total number of iterations per time step is 2000). The pressure distribution and the velocity profiles look to be okay. Within 2/3 more time steps the solution messes up and I get complete flow reversal. I'm using 0.1 as the relaxation factor for both energy and density, which I increase to 0.2 after 2/3 time steps. For the pressure and velocity relaxation factors I use 0.6 & 0.4 (sum total of 1, as SIMPLE requires). The flame (shown by the high reaction rate zone) dissapears but the reactants-products interface remains. The temperatures remain high of the order of 2500 K.

1. Should I slowly increase the energy and density relaxation factors ? Increasing them abruptly to 1 makes the solution diverge.

2. Should there be any change made to the pressure and velocity relaxation factors ?

3. Grid adaptation in the high reaction rate zones might be another way out.

I must mention here that I'm running an adiabatic case. Does this result in the dissapearance ?

Any suggestions would be of help. I should also mention that the coupled solver failed to give me any kind of a solution with fast divergence and high temperatures (of the order of 5000 K !).

Thanks