# Question:Considerations about the evaporation in VOF

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 January 20, 2011, 04:31 Question:Considerations about the evaporation in VOF #1 Member   Dokeun, Hwang Join Date: Apr 2010 Posts: 71 Rep Power: 7 Hello, dear experts!~ Now I'm trying to simulate evapration of water on which very hot air flows(T eq 4000K, M eq 3~4). I found some questions from those who applied UDF for evaporation. They were using some macros such as "DEFINE_SOURCE" or "DEFINE_MASS_TRANSFER". I'm not so sure but "evaporation and condensation" mechanism for mass trasfer is provided by fluent for VOF model. Q1 Is there any special reason that I have to use UDF to calculate the mass transfer -evaporation in my case- for my simulation? As I think, for the case of evaporation, that model(Phases>Interaction>mass) is provided from fluent for VOF and it doesn't require to use UDF for that. Q2 As evaporation accures, heat energy absorbed from any phase should be considered due to latent heat. But, according to the fluent manual, VOF model doesn't offer energy exchange between phases. Is it reasonable to apply energy transfer by using UDF? And how? Just one thread had something to do with my question but it was not understandable... Q3 From some references, surface tension and heat of vaporization(latent enthalpy) are function of temperature. They have critical temperatures in which those properties go to zero. These would affect the shape of liquid water surface and energy transfer between the phases. Can they be ignored? If not, how can i realize those phenomenon in fluent? Q4 Before I try to use VOF model. I applied DPM model for water evaportation and get meaningful result in its own way. But it took too much time for the calculation. I wonder how shorter time will be taken for the same problem with Euler-Euler method like VOF in comparingn with DPM. I'll wait for your help with hope Any comment would be a great help to me

 January 20, 2011, 06:25 #2 New Member     Khalid Baker Join Date: Mar 2009 Posts: 19 Rep Power: 8 1) If you see there are two option available on the menu of define>phases>interaction>mass which they user defined and population balance if you not want to use udf u can select the second one. 2) I think you can apply energy transfer by using UDF. In my simulation i turn energy equation on with vof from define>model>energy it work with vof. 3) Yes you can ignore it by setting there values as zeros in define>material 4) I am not familiar with DPM & Euler-Euler Models so I am not sure about your last question. k.baker

 January 20, 2011, 15:05 #3 New Member   Mehdi Famouri Join Date: Jan 2011 Posts: 12 Rep Power: 6 Hi every body, I'm using VOF to model two-Phase flow in a heat pipe. Here is my questions: FLUENT\Phases\Interactions\Mass 1- Am I supposed to define more than one mechanisms? one for and another one 2- About mechanism which has a Edit option to define a criterion based on both Temperature and Pressure. Anyway, my question is that, after clicking Edit button, it'd ask you about Evaporation and Condensation Frequency. What's that? FYI: Thanks in Advance,

January 20, 2011, 21:49
Thank you!
#4
Member

Dokeun, Hwang
Join Date: Apr 2010
Posts: 71
Rep Power: 7
Dear k.baker

I really appreciate your answers! they are very helpful for me.

And I realized that "evaporation-condensation" is not available to VOF from ur answer and user's guide.

Thanks again~!

Quote:
 Originally Posted by k.baker 1) If you see there are two option available on the menu of define>phases>interaction>mass which they user defined and population balance if you not want to use udf u can select the second one. 2) I think you can apply energy transfer by using UDF. In my simulation i turn energy equation on with vof from define>model>energy it work with vof. 3) Yes you can ignore it by setting there values as zeros in define>material 4) I am not familiar with DPM & Euler-Euler Models so I am not sure about your last question. k.baker

January 20, 2011, 22:08
mass transfer in VOF
#5
Member

Dokeun, Hwang
Join Date: Apr 2010
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Rep Power: 7
Hello, mehdi8619!

I'm not sure perfectly but i hope you it would be some help for you

A1. From some mass transfer relative UDF questions i had found in this forum, they used +,- sign to define the direction of mass transfer in their UDF according to the Temp condition only.

There woud be some expansion of consideration(actually it's question). Evaporation

가. can be occure under the boiling point and

나. pressure, velocity of gas can affect that phenomen too.

I just guess for those issues that if temperature is high enough and pressure is moderate(actually i have no idea about the pressure) then those can be ignored

A2. According to my thanks message for k.baker. "evaporation-condensation" mechanism should not be applied to VOF. You can fine that from user's guide 26.2.8. So you have to use UDF or other mechanism so called "population balance".

I hope it would be a tips for you

Quote:
 Originally Posted by mehdi8619 Hi every body, 1- Am I supposed to define more than one mechanisms? one for and another one 2- About mechanism which has a Edit option to define a criterion based on both Temperature and Pressure. Anyway, my question is that, after clicking Edit button, it'd ask you about Evaporation and Condensation Frequency. What's that?

January 21, 2011, 00:57
#6
New Member

Mehdi Famouri
Join Date: Jan 2011
Posts: 12
Rep Power: 6
Quote:
 Originally Posted by dokeun Hello, mehdi8619! I'm not sure perfectly but i hope you it would be some help for you A1. From some mass transfer relative UDF questions i had found in this forum, they used +,- sign to define the direction of mass transfer in their UDF according to the Temp condition only. There woud be some expansion of consideration(actually it's question). Evaporation 가. can be occure under the boiling point and 나. pressure, velocity of gas can affect that phenomen too. I just guess for those issues that if temperature is high enough and pressure is moderate(actually i have no idea about the pressure) then those can be ignored A2. According to my thanks message for k.baker. "evaporation-condensation" mechanism should not be applied to VOF. You can fine that from user's guide 26.2.8. So you have to use UDF or other mechanism so called "population balance". I hope it would be a tips for you
Thanks for your help.

A1- Based on the phase change diagram for the material we use, we can defined the pressure and temperature at which phase-change would happen (the Mechanism I draw a picture for you, has this options)

A2-But I couldn't find this part (26.2.8) in my User Guide. I'm using <Revision: 12.0.16 for the ANSYS Release Version 12.0.1> and there is no such thing you are talking about. Please make yourself clear a bit more, which user guide are you talking about?

Thanks in Advance,

January 21, 2011, 01:42
#7
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Dokeun, Hwang
Join Date: Apr 2010
Posts: 71
Rep Power: 7
Quote:
 Originally Posted by mehdi8619 Thanks for your help. A1- Based on the phase change diagram for the material we use, we can defined the pressure and temperature at which phase-change would happen (the Mechanism I draw a picture for you, has this options) A2-But I couldn't find this part (26.2.8) in my User Guide. I'm using and there is no such thing you are talking about. Please make yourself clear a bit more, which user guide are you talking about? Thanks in Advance,
A1. Oh really?! I couldn't find that option on your picture but i'll find anyway later. thanks~

A2. Then you can find like this
Modeling Multiphase Flows>Steps for Using a Multiphase Model>Including Mass Transfer Effects>some explanations for the mass transfer mechanims
Or you can find same explanation in theory guide 16.7.5 Evaporation-Condensation Model
These are saying "evaporation-condensation" model is for mixture and eulerian multiphase models.

January 21, 2011, 12:04
#8
New Member

Mehdi Famouri
Join Date: Jan 2011
Posts: 12
Rep Power: 6
Quote:
 Originally Posted by dokeun A1. Oh really?! I couldn't find that option on your picture but i'll find anyway later. thanks~ A2. Then you can find like this Modeling Multiphase Flows>Steps for Using a Multiphase Model>Including Mass Transfer Effects>some explanations for the mass transfer mechanims Or you can find same explanation in theory guide 16.7.5 Evaporation-Condensation Model These are saying "evaporation-condensation" model is for mixture and eulerian multiphase models.
Thanks for your attention and reply,

A1- In my picture, at the last Box, if you change the Saturation Temperature from Constant to the Polynomial, then the Edit button would appear through which you can defined any polynomial equation (T vs P) to address the phase change diagram

A2- I didn't get. If you mean I should Google that way, I did and I ended up with nothing. If you search this way and you find sth, let me know.

Yours,

 January 25, 2011, 20:50 #9 Member   Dokeun, Hwang Join Date: Apr 2010 Posts: 71 Rep Power: 7 Dear mehdi8619 A1. Thank you A2. Although we can choose evaporation-condensation mechanism when using VOF, from the user's guide (find the red box below), there was no mention about VOF but mixture, euler. I think you've already read this. Do you have any other idea about this?

January 25, 2011, 22:52
#10
New Member

Mehdi Famouri
Join Date: Jan 2011
Posts: 12
Rep Power: 6
Quote:
 Originally Posted by dokeun Dear mehdi8619 A1. Thank you A2. Although we can choose evaporation-condensation mechanism when using VOF, from the user's guide (find the red box below), there was no mention about VOF but mixture, euler. I think you've already read this. Do you have any other idea about this?
Thanks for your reply. Actually, I didn't have the manual to read. I'd really apprecite it if you could send me this Manual (famoor62@yahoo.com).

Back to the discussion, I should say that using VOF and Evaporation-Condensation is wrong as it's stated that this mechanism is not available for VOF.

Best Regards,

 February 24, 2011, 21:47 VOF with evap&cond mechanism and UDF for energy source term #11 Member   Dokeun, Hwang Join Date: Apr 2010 Posts: 71 Rep Power: 7 Usually Fluent hides undesirable options with some messages from dialogue box when we choose a certain option. In this point of view, the fact that I can choose evaporation & condensation mechanism with the VOF multiphase model may imply that I can use this mechanism for my case. So now I'm attempting to calculate with this option but I'm not sure this mechanism include energy source term for the latent effect within the domain. If I have to calculate the term with UDF, what can I begin with? Do anyone tell me about my curiosity? Thanks in advance.

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