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-   -   -What udf macro for boiling - (http://www.cfd-online.com/Forums/fluent/32814-what-udf-macro-boiling.html)

 thomas January 7, 2004 18:24

-What udf macro for boiling -

Greeting everyone, I want to simulate an impinging jet with boiling. I need to write the UDF to compute the boiling effect on the heat transfer. My fluent version is 6.2 so i am using the Euler-Euler model with energy equation. The wall BC is T = 600 K. The thing i need to compute by udf is the heat flux at the wall. For boiling there are 3 heat fluxes coming up: Convective_flux + Quenching_FLux + Vaporization_Flux.

What macro should i use to compute these flux at the wall ? Here are some ideas:

1 - I thought about defining the quenching and vaporization flux using a Define_heat_flux (with the cid[0] variable). - Using that way ?, Does the convective flux will be computed by default in Fluent or do i need to integrate it in the Define_heat_flux macro ? - What will be the Total flux report in the report_flux_panel ?

2 - I have read in some fluent ugm presentation that these 3 fluxes should be set in a define_source and be be put in the energy equation. - How can I just use a define_source just at the wall ? - How can I hook this define source in the energy equation.

I hope someone is gonna bring me some clarification. Thanks by advance. Thomas

 andrew January 14, 2004 23:50

Re: -What udf macro for boiling -

Thomas, I am also interested in studying impinging jets with boiling. There is a tutorial available from the fluent users website (www.fluentusers.com that allows boiling to be included in the VOF model. I am currently investigating this for impinging jets with boiling.

I am interested in how how you plan to do this with the euler-euler model, especially the ugm presentations you mention.

regards andrew

 thomas January 15, 2004 05:11

Re: -What udf macro for boiling -

Hi andrew, Glad to hear that I am not the only one to work on that subject ;). I am almost done with it though. All the work done on it will be posted on a website next week and I won't miss to post the address. First I decrease the difficulty by simulating a LAMINAR impinging jet with the euler-euler solver. It is difficult for me to summarize here what I did for boiling cause there is a lot to say. I set the inpinging wall as a velocity inlet and inject bubbles (T = Tsat)in the jet -> this is to represent the nucleation of bubbles. I am working under nucleate boiling not film boiling. Then to tackle the energy part, boiling has 3 flux components: Convective, quenching, evaporation. These fluxes exchange energy with the wall only in the first range of cells after the wall. As I have replace the wall by a velocity inlet BC i cannot use in my UDF a DEFINE_HEAT_FLUX or a DEFINE_PROFILE. Therefore I implemented these fluxes by a source term in the energy equation only in the first range of cell after the wall. This work include a lot of assumption and is specific to an impinging jet but it is a beginnin. I think i will also put the UDF i wrote, I am of course open to any remarks or critics !!!!

Cheers thomas.

 andrew January 15, 2004 21:24

Re: -What udf macro for boiling -

It would like to see you website when its finished.

I am also hoping to look at nucleate boiling, though i was intending to do it within the VOF framework. I have worked out a possible method for including nucleation, though i havent implemented my ideas yet.

I have a webpage that sort of outlines what i am doing, though havent been able to update it as they've disabled ftp access (hopefully i will update it sometime soon), its at

http://www.student.curtin.edu.au/~09709500/

cheers andrew

 thomas January 16, 2004 05:16

Re: -What udf macro for boiling -

Hi, The problem with inpinging jet with boiling is you have 2 different phenomena to simulate at a different scale. The jet is macro phenomena which fits with the VOF model and nucleate boiling is a micro phenomena which only fits the mixture, DPM, Euler-euler solver (a very very thin mesh could allow you to apply the VOF model but it is obviously out of questions). Is there a possible way to couple all that in a VOF model without using a too thin mesh ?

thomas

 andrew January 19, 2004 03:14

Re: -What udf macro for boiling -

Hi again thomas, The way i was planning to implement nucleate boiling was to provide a simpl(ish) model to provide a kickstart to the boling model i mentioned earlier. this model would be applied to the cells adjacent to the wall at which boiling can occur.

first the cells where the bulk fluid pressure is less than the saturation temperature are assumed to be cells where nucleation is able to take place (in addition there should be only the liquid phase present). second define the number(and size) of nucleation sites per cell (ie at the surface). third determine a rate of bubble growth (during the inertial phase). from this information mass transfer & energy source terms can be determined and included in the continuity and energy eqns. For cells that have both phases present the existing boiling model (previous post) is used.

This method (which i'm not sure will work) makes a number of assumptions that i need to convince myself are valid (or otherwise).

I still need to compare this method against the euler-euler method, as well. If there are any (possibly obvious?) errors that i may have missed, please feel free to point them out. ;-), any input would be valuable.

cheers andrew

 thomas January 19, 2004 08:31

Re: -What udf macro for boiling -

Hi, I am using the exact same model. I found it in number of papers and class notes. Could you tell me more about the assumptions you are talking about ?

thomas

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