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-   -   Evaporation model in ansys Fluent 12.1 (http://www.cfd-online.com/Forums/fluent-udf/78846-evaporation-model-ansys-fluent-12-1-a.html)

oldisbest August 3, 2010 23:27

Evaporation model in ansys Fluent 12.1
 
Hi, every one,

I have to simulate a gas-liquid flow in multi-port evaporator used for automotive HVAC system.

AS I found the latest ansys Fluent 12.1 has introduced a new mass transfer model named "Evaporation/condensation model", I set up my problem as follow:

Viscous model - Mixture, with consideration on slip velocity.
Bubble diameter - for Refrigerant 134a, about 0.1mm
Surface tension effect - constant, 0.01 N/m
Drag function - Default, S-N model
Mass transfer effect - liquid to gas, saturation temperature =8 deg C ,Frequence of Evaporation = 0.1(default)

Boundaries:
Inlet - velocity (Mass fraction of gas phase is about 10%)
Wall in Core area - Constant heat flux
Outlet - Pressure
----------------------------

As I read the User guide, I found the new evaporation model is theoretically the same to UDF for mass transfer, while the default coefficient is still 0.1.

But the energy conservation is not considered in the new model.
That's because when evaporation happened, the quantity of latent heat needed is not known.

How can input heat be balanced by the evaporation?

-----------------------------
Since there is no heat transfer in the head pipes(tanks), I decide to set the inlet flow temperature to 7.99 deg C, which ensure no evaporation happened there.

Then I run the simulation, and get solution converged.
----------------------------

In my results, the volume fraction seems ok, what most confused me is that inlet mass flow rate not equal to outlet one (a big difference, about 50%).

---------------------------
So, what's my question are:
1. when use the new evaparation model in Ansys FLUENT 12.1, how to consider the energy conservation.
2. Why the mass not balanced between inlet and outlet while continuity equation is converged well? Where does the over mass come from?
3. Why the default coeff. is 0.1?
That mean when local temp. is higher than saturation temp., only 10% liquid flow will evaporate, is it true?

Your comments and advices are welcomed.
Thanks in advance.

oldisbest August 8, 2010 22:33

No body here has the same problem with me?

May be I have to use UDF in simulation.

The Evaporation/Condensation model in Ansys Fluent 12.1 seems to be dissapointing.:o

JohannVV September 27, 2010 14:19

Hi, unfortunately I cant help you with you problem. But in the near future may be.
Could you tell me which chapter of the user guide contains the condensation model?

domino October 18, 2010 15:51

Hi,
I have a similar problem. I don't understand this evaporation-condensation model completely either, but here is what I have learned:
- the model is temp-driven. No temp difference, no mass transfer.
- the latent heat is given in enthalpy of the "gas phase"; putting 0 for enthalpy of the liquid phase; use mass-transfer liq to gas (if you are interested in the evaporation, if you are interested in condensation, probably, you have to define it differently).
- 0.1 is default and the fine tunning parameter - higher means more evaporation or condensation.
--> in my case, I ve notived the mixture temp is higher than temp saturation. Shouldn't it be equal?? something in the energy/enthalpy balance between phases is not clear for me.

KamilSobczak December 13, 2010 07:00

So evaporation/ condensation isn't properly resolved?
 
So this model is unproper?

I conclude that mass transfer is calculated with assumption to temperature but my question is that:
- do I need define two mass transfer effects liquid->gas and also gas->liquid or only one of them is enough
- could energy source described by multiplication of mass released and heat latent be incorporated to energy conservation equation

Maybe we could write some udf together for calculation of this?

domino December 13, 2010 11:23

I would not say the model is unproper. There are fine-tunning parameters. It means, if you don`t have experimental data to figure out those parameters, you have to consider the calculated void fraction distribution as qualitative results. The temperature distribution is probably an approximation - check it.
As I understand, you just need to define the mass transfer is one direction (boiling or condesation?).
The energy source IS described by multiplication of mass released and heat latent and IS incorporated to energy conservation equation.
Heat latent is the enthalpy I said you define in the material properties.
I don`t know what you could improve by writting UDF???

Valpress September 30, 2011 10:19

Hello evreone,

one comment abou that. Id like to simulate water evaporation with Mixture Elerian model. i can do that but evaporation starst when the saturation temp reaches certain value. if i set ie. 273.15K and temp of air at inlet is 300K then evaporation mass transfer starts at 273.15K

Does anyone know how to set saturation temp. For example if i want to evaporate water and ambient air temp is 340K. Because when i set sat. temp as 373K (100C) there is no evaporation. In real life water evaporates even at normal room temperature (i.e. 20C)....ok it evaporates very slowly but with higher temp. higher evaporation. The question how to set it in fluent 13.0??

oldisbest April 14, 2013 22:30

Re
 
1 Attachment(s)
Hi, everyone,

Since my position in company was changed, the simulation on evaporation model was interrupted and became a pending problem.

As some friends are asking me to provide the tutorial and case for their study,
I have uploaded an example of 2-phase pipe flow with the link:
http://dl.vmall.com/c0v5t52aff

I am sorry for late coming back and reply.:o

mariam.sara April 16, 2013 01:22

The link not work i try download the file but i failed may you upload it in other site?

Quote:

Originally Posted by oldisbest (Post 420543)
Hi, everyone,

Since my position in company was changed, the simulation on evaporation model was interrupted and became a pending problem.

As some friends are asking me to provide the tutorial and case for their study,
I have uploaded an example of 2-phase pipe flow with the link:
http://dl.vmall.com/c0v5t52aff

I am sorry for late coming back and reply.:o


TinyBlack June 13, 2013 22:44

Quote:

Originally Posted by oldisbest (Post 420543)
Hi, everyone,

Since my position in company was changed, the simulation on evaporation model was interrupted and became a pending problem.

As some friends are asking me to provide the tutorial and case for their study,
I have uploaded an example of 2-phase pipe flow with the link:
http://dl.vmall.com/c0v5t52aff

I am sorry for late coming back and reply.:o


Hi, Xu.
I realise that you upload your files on Huawei netdisk. Are you live in China?
Could you give me you email address so that I can exchange ideas with you ?


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