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how to avoid residuals oscillating?

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Old   June 2, 2020, 02:17
Default how to avoid residuals oscillating?
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Hi all,

I am doing a methane combustion simulation with fluent. The mechanism is imported in chemkin format. I just followed all settings with literature.

My problem is: The general trends are very similar with literature results. However, the species profiles are not very reasonable. I suspect it's because of my case is not converged yet.

I check the flux report of mass and energy and they are all balanced. however, the residuals are not all below 1e-6 and they are oscillating with small magnitude.

Can anybody who have combustion simulation experience give me some suggestions on how to get full converged results.

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Weiqiang
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Old   June 2, 2020, 09:36
Default Residuals
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1e-6 is already too small. Even if residuals are below 1e-4, it's more than good enough until the values of species fractions are themselves of that order.
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Old   June 2, 2020, 10:24
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Quote:
Originally Posted by vinerm View Post
1e-6 is already too small. Even if residuals are below 1e-4, it's more than good enough until the values of species fractions are themselves of that order.
Hi Vinerm,

In my case, the mole fraction of combustion products starts from 2% from inlet and increases to 16% at outlet.

However, in literature the combustion products increases from 0 to 16%, which is more reasonable I think.

also mole fraction of fuel at inlet is smaller than the value I set. they should be the same I think.

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Old   June 2, 2020, 10:45
Default Mole Fractions
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When the post processing is done, even at the inlet boundary, values are plotted based on the values at the centers of the adjacent cells. Therefore, what you are looking at is actually the solution at the center and those values extrapolated to the inlet.
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Old   June 2, 2020, 10:57
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Quote:
Originally Posted by vinerm View Post
When the post processing is done, even at the inlet boundary, values are plotted based on the values at the centers of the adjacent cells. Therefore, what you are looking at is actually the solution at the center and those values extrapolated to the inlet.
Hi Vinerm,

Do you mean the profile or even contour are node values even if I selected node values in post processing? Because I selected node value when I export data files.

If all values are nodes values instead of boundary value, then how can the author draw species profile starting from 0?

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Old   June 2, 2020, 11:12
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Node values are not even boundary values, those are node values that could be lying on the boundary. If you specify 0 mass (or mole) fraction for a specie at the inlet, it would be strange to have a rather high mass fraction within such as small distance as half-cell thickness. Reactions take time and flow can travel quite some distance before products are produced. But it all depends on the reaction model being used, flow speed, and flow structure; a highly diffusion controlled system can have high mole fraction of a product close to the inlet.
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Old   June 2, 2020, 11:23
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Quote:
Originally Posted by vinerm View Post
Node values are not even boundary values, those are node values that could be lying on the boundary. If you specify 0 mass (or mole) fraction for a specie at the inlet, it would be strange to have a rather high mass fraction within such as small distance as half-cell thickness. Reactions take time and flow can travel quite some distance before products are produced. But it all depends on the reaction model being used, flow speed, and flow structure; a highly diffusion controlled system can have high mole fraction of a product close to the inlet.
Hi Vinerm,

In my simulation, chemical reactions are very fast. All fuels are consumed in a very short distance. Can it be a highly diffusion controlled system?

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Old   June 2, 2020, 11:35
Default Diffusion Controlled
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To look at whether a system is diffusion controlled or not, you need to compare diffusion and convection time-scales. Diffusion time scale is given by diffusion coefficients, such as, thermal diffusivity or mass diffusivity and convection time scale is given by ratio of domain length scale and flow velocity. If diffusion time-scale is much larger than convection, which it usually is, then it is not diffusion controlled. However, if diffusion is much faster, i.e., its time-scale is smaller, then system becomes diffusion controlled. This is possible in laminar or highly recirculating flows.
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Old   June 2, 2020, 12:53
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Quote:
Originally Posted by vinerm View Post
To look at whether a system is diffusion controlled or not, you need to compare diffusion and convection time-scales. Diffusion time scale is given by diffusion coefficients, such as, thermal diffusivity or mass diffusivity and convection time scale is given by ratio of domain length scale and flow velocity. If diffusion time-scale is much larger than convection, which it usually is, then it is not diffusion controlled. However, if diffusion is much faster, i.e., its time-scale is smaller, then system becomes diffusion controlled. This is possible in laminar or highly recirculating flows.
Hi, Vinerm,

Thanks very much for your patience. It enlightened me a lot.

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