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Old   April 23, 2013, 03:44
Exclamation Combustion problem
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Alok Deshpande
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Hi All,

I am solving a flameless combustion problem with the following BC's,

V_air = 7.53 m/s, T_air = 955 K
V_ch4 = 24.34 m/s, T_ch4 = 300 K

Flue gases recirculates in the combustion chamber and exits from the exit located above the air/fuel inlet, thus giving finite turbulence-chemistry interaction. I am using Eddy Dissipation Concept (EDC) under Species Transport model.

Now the problem is, while solving the problem in unsteady case, combustion does not occur (as the peak temperature after 15-20 sec is still of order 957 K). Do I need to provide any ignition method?

Such problem occurred when I was simulating a benchmark problem on combustion. Using the Species Transport Model, steady state simulations showed the combustion (but overpredicted the temperatures) but unsteady simulations did not show any combustion.

Please somebody tell me what might be going wrong.. I need to submit my dissertation in 10-15 days..

Regards,
Alok
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Old   April 23, 2013, 08:09
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James Willie
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Hi,
What was your time step size in the simulation? Plus i would suggest you do a steady run and then switch from it to unsteady. You surely have to ignite the mixture to get the flame. It does not affect the converged solution since the fuel you add is little and it burns out in a short time. About the time step u have to ensure that the time step u are choosing is less than or of the order of the chemical time scale. U can estimate it by taking the smallest cell in your grid and using the mean flow velocity in it that cell and ratio of the cell size to the mean flow velocity would give you an idea of the time step size u can use. Jimmy
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Old   April 23, 2013, 08:41
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Alok Deshpande
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Hi Jimmy,

Thank you for your suggestion. I will check the timestep. Flow and chemistry timescales are similar in this case. I will choose the timestep accordingly.

Can I choose the ignition location and time arbitrarily? What could be the energy of ignition, default would do?

Thanks

Alok
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