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Puzzling about the "diameter" in Eulerian Model

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Old   October 19, 2009, 08:38
Default Puzzling about the "diameter" in Eulerian Model
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yuanyuan
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Dear friends,

I am running a simulation for a bubble column. Eulerian Model is adopted into my simulations, where the parameter "diameter of the bubble" need to be specified into the material property. With the different simulations, I am confused by the parameter. My questions are below:

1. Does it stand for the real bubble size in experiments?

2. I think that the bubble size should be a result from the eulerian model, it should be displayed after the simulations, doesn't it? Why we need to specify it before similations?

3. I could get different gas holdups (which is an important parameter for bubble column) with the different "diameters of the bubbles". Furthurmore, it takes a longer time for a simulation coming to a nearly stable result with a smaller "diameter of bubble" . How big should be it?

4. I also have performed simulations with the PBM model. However, I found that a initialization for the bubble size is needed. If I initialized the bubble size with 0.5mm, after calculations, I could find that the maximal number density of the partical diameter is 0.5mm; if I initialized it with 1mm, after calculations, the maximal number density of it is 1mm. Does it indicate that we have to have a prior knowledge about the "diameters of bubbles"?

Thank you very much for explaining,

yuanyuan
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Old   October 20, 2009, 21:52
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Is there somebody that could help me for the questions above? Thanks again

Quote:
Originally Posted by yuanyuan View Post
Dear friends,

I am running a simulation for a bubble column. Eulerian Model is adopted into my simulations, where the parameter "diameter of the bubble" need to be specified into the material property. With the different simulations, I am confused by the parameter. My questions are below:

1. Does it stand for the real bubble size in experiments?

2. I think that the bubble size should be a result from the eulerian model, it should be displayed after the simulations, doesn't it? Why we need to specify it before similations?

3. I could get different gas holdups (which is an important parameter for bubble column) with the different "diameters of the bubbles". Furthurmore, it takes a longer time for a simulation coming to a nearly stable result with a smaller "diameter of bubble" . How big should be it?

4. I also have performed simulations with the PBM model. However, I found that a initialization for the bubble size is needed. If I initialized the bubble size with 0.5mm, after calculations, I could find that the maximal number density of the partical diameter is 0.5mm; if I initialized it with 1mm, after calculations, the maximal number density of it is 1mm. Does it indicate that we have to have a prior knowledge about the "diameters of bubbles"?

Thank you very much for explaining,

yuanyuan
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Old   October 22, 2009, 08:38
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JP
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You are right. Eulerian model requires that you know the diameter of the drops / bubbles upfront. It stands for the real bubble size.
You can specify
(1) constant bubble diameter or
(2) The Sauter-mean diameter or
(3) the diameter distribution using a UDF

To model change in diameters due to coalescence, breakup etc, you need "Population Balance Model" which is an add-on module to Fluent. Check if you have it.

Regarding convergence, check your grid size as compared to the bubble diameter. If your diameter is small compared to the grid size, you may run into issues.
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