CFD Online Discussion Forums (https://www.cfd-online.com/Forums/)
-   FLUENT (https://www.cfd-online.com/Forums/fluent/)

 as April 1, 2005 21:09

Udf about the distribution of the diameter

My problem is about writing UDF program. I want use Mixture model to simulate two phase flow. I want to write a UDF about the distribution of the diameter of solid particles In my flow ,about 10 percent(10%) of the solid particles is 20 micron. about 90 percent(90%£© of the solid particles is 50 micron¡£

 as April 1, 2005 21:10

Re: Udf about the distribution of the diameter

Can anyone help me about the UDF .Thank you

 ap April 2, 2005 14:55

Re: Udf about the distribution of the diameter

I'm not sure it's possible to do this. Check the manual.

Regards, ap

 as April 2, 2005 20:10

Re: Udf about the distribution of the diameter

Thank you for you response !I can not find any example in the manual ,I want to know if the udf can solve this problem

 pUl| April 3, 2005 00:44

Re: Udf about the distribution of the diameter

I do not know if the following makes any sense at all, but I do know that fluent lets you do this without any errors or warnings which makes it even dangerous to use :)

Define the 'Primary phase' as usual. Next define 2 'Secondary phases'. Let both the secondary phases represent the same dispersed material. Define different diameters for both the secondary phases. Under "Interaction" disable any interaction between either secondary phase (i.e. drag). Carry on with the problem definition as usual. Once the solution has converged, define a "Custom field function" that equals (1 - volume fraction of the continuous phase) which will now give you the volume fraction of the dispersed phase. One of the things to think about is defining inlet velocities and volume fraction guesses for the dispersed phase. Whatever you do, you need to ensure that the total mass flow rate of the dispersed phase matches with what you would have had, if you introduced a mono-dispersed dispersed phase. This would seem like simulating a size distribution without breakup or coalescence, but I stll have my doubts. On second thoughts, it does seem to make no sense, so please use at your own risk!

 ap April 3, 2005 09:15

Re: Udf about the distribution of the diameter

Yes, this is a tecnique used to simulate a size distribution for example in gas-solid flows.

In this case it seems ok, but if you have a spread size distribution it's not convenient because you have to use too many secondary phases, increasing the computational time too much.

One question: why do you have to disable the interaction between secondary phases? You should just choose the proper drag correlation.

Regards, ap

 pUl| April 3, 2005 15:33

Re: Udf about the distribution of the diameter

Wow ap, thanks for your reply. I never knew whether this method was accepted for use.

The reason for disabling the interaction between the two secondary phases (same dispersed material) is that I find it rather unreasonable to believe that two gas bubbles for instance can have a 'drag' kind of interaction between each other when the possibilities of coalescence seem much more possible.

Perhaps you could shed more light on this.

Best Regards,