Learning Forum

[Weiqiang Liu]

Hi all,

I am doing a diesel particulate filter simulation with fluent. I can not get reasonable temperature distribution during soot combustion. I think it's because of my velocity distribution is not right. I put the Y-velocity contour in literature below:

I noticed the negative Y-velocity which is filtration velocity is very obvious. However, I can never get similar filtration velocity contour with given viscous resistance coefficients in literature. The author gave 2e12 viscous resistance for porous wall and 4e13 viscous resistance for soot cake. With this recommended values, I can never get similar Y-velocity contour. Instead, Y-velocity or filtration velocity is almost zero in the whole domain for my simulation.

I am wondering is something wrong with literature or something wrong with my model?

Best regards

Weiqiang 

[Rob]

Please can you replot with node values off and also show the pressure field and mesh resolution.  Were the coefficients in the paper per metre or scaled to the media thickness?

[Weiqiang Liu]

Hi,

below is my Y-velocity contour with node values off. You can see the Y-velocity in the whole domain is almost uniform. I can never get that obvious negative Y velocity at end of the inlet channel.

below is my pressure contour. You can see the pressure drop seems reasonable.

The coefficients in paper were not scaled

Best.

Weiqiang

[DrAmine]

Can please plot vectors sometime better than contours. Moreover What do you mean with porous wall and soit cake.? Both are defined as porous zones right? Screenshot of porous settings for both. What dies happen if you switch the reaction part?

[Weiqiang Liu]

Hi Amine,

Yes, both soot cake and porous wall are defined as porous zones. I put the screenshot of porous settings below;

viscous resistance coefficient and porosity are defined by UDF. By switching reaction part, do you mean if I turn off all source terms arising from reaction?

Best.

Weiqiang

[DrAmine]

Yes switching off the reaction part.  Are the viscous and inertial coefficients from the paper you want to reproduce?

Can you post the vector plots on the plane corresponding to the velocity contours you ahve already shared.

[Weiqiang Liu]

Hi Amine,

Yes, switching off reaction gives me same velocity contour. I debugged my code in VS and it returned me just the same viscous coefficients from the paper. I put my Y-velocity vector plot below:

you can see the filtration velocity in porous zone is very small. However, in the paper, the author got very obvious negative Y-velocity aka filtration velocity. Remember I told you that when I increase the reaction rate 2.2 times than original rate like the author did, temperature went extremely high and divergence happened. I found the reason for divergence is also because of my velocity contour. Because convection is the only heat dissipation mechanism in my model. When negative Y velocity is too small, then heat would accumulates in the corner and makes temperature extremely high.

Because I tried to use a default viscous coefficient in fluent which is much smaller than literature. Then divergence never happens. Therefore, my only problem now is to figure out how to get reasonable velocity contour.

Best regards.

Weiqiang

[Rob]

I'm assuming that the vectors at the other (inlet) end of the filter look much the same but with the higher velocity in the upper part. 

[Weiqiang Liu]

Hi,

I think my problem is the macro of defining viscous resistance coefficient.

Best.

Weiqiang

[vkumar12]

Weiqiang

I saw your post, I dont want to side track your question but from the screenshot, i think you can help me out for a problem about porous zone. I am very new to CFD so please keep that in mind. 

I want to create a UDF for viscous resistances and porosity. 

I have a cylindrical porous zone (3d geometry)  .  Since it is cylinder, I am using conical coordinate with cone angle of zero. The viscous resistance is dependent on the cell temperature of porous zone. and also Porosity is dependent on temperature. 

I have the equation of those but I am not sure what macro to use for these. If you can write me an udf example of viscous resistance of dir 1 and udf for porosity , it would be a great help for me. 

Say viscous resistance (dir1 ) = C_T(c,t)*exp^0.007*C_T(c,t)  // just for example. and same goes for porosity 

and porosity = 0.1*C_T(c,t)

Thank you very much for your help

 

[Weiqiang Liu]

I am sorry. I can not write UDF for you. But there are a lot of sample UDFs in fluent UDF manual for your case. You can check it.