Laminar Flow Combustion

RayhanWMU · March 26, 2024, 18:51

I am trying to re-do a paper work where the following set ups are used for getting the methane air combustion.
Axisymmetric.
1. Viscous Model: Laminar
2. Energy equation: on
3. Radiation: DO
4. Species Transport: Volumetric, Stiff Chemistry, Full multicomponent diffusion, GRI Mech 3.0 imported for transport and thermal data
5. velocity inlet: Fuel - 3m/s, 1 mole fraction CH4 at species
air - 2m/s, 0.23 Oxygen & 0.77 Nitrogen mole fraction at species. Both 298 k temperature
6. Pressure Outlet
7. 1000 iteration and residual chosen 1*10^-6

OUTPUT: The reaction happened just at the inlet part and some temperature changes in the inlet and outlet part of the 2D flow region. Again velocity is found on those small part near to the inlet and outlet but the rest of the chamber's fluid's velocity remain zero. Why the flow and reactions are not happened in the middle part of the fluid region?

I will appreciate all of your contribution. If you want I can share the paper with you.

bigfootedrockmidget · March 27, 2024, 17:38

Which solver? And which paper? Steady-state?

Usually, you start such a simulation first without any chemical reactions, only flow and transport of species. This is fast and you can then check that at least this part of the simulation works as expected.
Then, you ignite a specific part of the domain by setting the temperature to a high value, like 1500K. You have to carefully choose this region to make sure the flame does not blow off or flash back.

(Gas) Radiation and multicomponent diffusion is usually not important in methane-air flames, so you can switch it off in this phase because they are also time-consuming. Once it has converged a bit, you can activate these models.

Also, if this is Fluent, then 1000 iterations and R=1e-6 is not enough. You probably need 100,000 iterations and R=1e-10. It might be more than that when you include radiation. Other solvers might be worse.

RayhanWMU · April 1, 2024, 14:14

I have tried according to the way you were asked. First I tried with air. Then I use CH4-air, and then chemkin GRI 3.0. Even when I am applying the volumetric reactions and Full multicomponent diffustion. It all works and showed convergence.

But when I go to see the contour and velocity vector, I found that the solution is not correct because the flow does not go from inlet to outlet fully. In the middle of the 2D domain the velocity magnitude is shown zero. Some velocity is found at the region close to inlet and outlet. The rest of the part has no flow.

bigfootedrockmidget · April 4, 2024, 02:56

How is the statement 'it all works' compatible with your results?
When you simulate without combustion, only air flow, what does the result look like then? If you place a monitor in the middle of your domain and observe pressure, velocity and species mass fractions, do you observe that all variables have reached their steady states?

March 26, 2024, 18:51	Laminar Flow Combustion	#1
RayhanWMU New Member Asif Mahmud Rayhan Join Date: Mar 2024 Posts: 2 Rep Power: 0	I am trying to re-do a paper work where the following set ups are used for getting the methane air combustion. Axisymmetric. 1. Viscous Model: Laminar 2. Energy equation: on 3. Radiation: DO 4. Species Transport: Volumetric, Stiff Chemistry, Full multicomponent diffusion, GRI Mech 3.0 imported for transport and thermal data 5. velocity inlet: Fuel - 3m/s, 1 mole fraction CH4 at species air - 2m/s, 0.23 Oxygen & 0.77 Nitrogen mole fraction at species. Both 298 k temperature 6. Pressure Outlet 7. 1000 iteration and residual chosen 1*10^-6 OUTPUT: The reaction happened just at the inlet part and some temperature changes in the inlet and outlet part of the 2D flow region. Again velocity is found on those small part near to the inlet and outlet but the rest of the chamber's fluid's velocity remain zero. Why the flow and reactions are not happened in the middle part of the fluid region? I will appreciate all of your contribution. If you want I can share the paper with you.

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March 27, 2024, 17:38		#2
bigfootedrockmidget Senior Member bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 676 Rep Power: 21	Which solver? And which paper? Steady-state? Usually, you start such a simulation first without any chemical reactions, only flow and transport of species. This is fast and you can then check that at least this part of the simulation works as expected. Then, you ignite a specific part of the domain by setting the temperature to a high value, like 1500K. You have to carefully choose this region to make sure the flame does not blow off or flash back. (Gas) Radiation and multicomponent diffusion is usually not important in methane-air flames, so you can switch it off in this phase because they are also time-consuming. Once it has converged a bit, you can activate these models. Also, if this is Fluent, then 1000 iterations and R=1e-6 is not enough. You probably need 100,000 iterations and R=1e-10. It might be more than that when you include radiation. Other solvers might be worse.

April 4, 2024, 02:56		#4
bigfootedrockmidget Senior Member bigfoot Join Date: Dec 2011 Location: Netherlands Posts: 676 Rep Power: 21	How is the statement 'it all works' compatible with your results? When you simulate without combustion, only air flow, what does the result look like then? If you place a monitor in the middle of your domain and observe pressure, velocity and species mass fractions, do you observe that all variables have reached their steady states?