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plz help me T_T humidity > 100`%

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Old   November 9, 2010, 09:01
Default plz help me T_T humidity > 100`%
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Kitipon
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I saw others have question about humidity exceed but there is no one answer this.

My problem is water injection to the air.I use species and DPM with polynomial vapor pressure for the water (L) but for the result, humidity exceed 100%

please help me,if any one know.Perhaps,I can't pass my degree if I can't solve this.

Thank
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Old   November 9, 2010, 16:37
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Roman Gobitz-Pfeifer
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Hello padthai,

I'm facing exactly the same problem. What I found right now written in the theory guide chapter 15.4.2 is that there could be some problems due to the vaporization temperature.

"15.4.2 Droplet Vaporization (Law 2)
Law 2 is applied to predict the vaporization from a discrete phase droplet. Law 2 is initiated when the temperature of the droplet reaches the vaporization temperature, Tvap, and continues until the droplet reaches the boiling point, Tbp, or until the dropletís
volatile fraction is completely consumed:
.....
Note that once vaporization is initiated (by the droplet reaching this threshold temperature), it will continue to vaporize even if the droplet temperature falls below Tvap...."

I think the main problem is that the droplet continues to evaporate although the vapor pressure is higher than the saturation pressure. Now I'm thinking of solving this problem via udf, but right now I couldn't handle this problem too.

I'm very interested in solving this problem and I'll give you all my support for this case. If you don't habe access to the user manuals let me know I'll send it to you.
If you found the answer for our problem please let me know, I'll also post me results.

cheers
wikie
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Old   November 10, 2010, 13:14
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Roman Gobitz-Pfeifer
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Hi padthai,

I think we should try to write a UDF which is switching between law 1 and 2 for Heat and Mass Exchange of discrete particles.
law 1: Inert Heating or Cooling if the humidity is 100% or more
law 2: Droplet Vaporization if the humidity is less than 100%
These laws are described in chapter "15.4 Laws for Heat and Mass Exchange" of the theory guide.
We should use DEFINE_DPM_SWITCH for switching between these two laws. This DEFINE macro is described in chapter "2.5.13 DEFINE DPM SWITCH" of the udf guide.
Do you have acces to these guides? Otherwise I'll send it to you via email.

cheers
wikie
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Old   November 10, 2010, 22:31
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Hi,Wikie

I have seen that UDF and agreed with your point but when I interpreted it in the Fluent,there is an error occured (line5: parse error).I am trying to fix it.

Could you interpret that UDF? Is it good?

Thank
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Old   November 11, 2010, 14:54
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Hi padthai,

instead of interpreting your UDF you should try to compile it. I've also had problems to interprete some functions, but all function are working if you compile them. You can find a manual here: http://www.cfd-online.com/Wiki/Fluen...at_is_wrong.3F
If you're having any questions feel free to ask, I needed one day even with this guidance to compile UDFs, but now it is working fine :-)

cheers
wikie
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Old   November 15, 2010, 08:22
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Hi Wikie,

Sorry,I have to bother you again.

After I compiled the law,I selected the function "dpm_switch::libudf"from switching drop-down list in custom law of injection dialog box .But, when I simulated,there is an error massage occured.It showed that

FLUENT received fatal signal (ACCESS_VIOLATION)
1. Note exact events leading to error.
2. Save case/data under new name.
3. Exit program and restart to continue.
4. Report error to your distributor.
Error Object: #f

Anyways,I simulated with steady state in both phase

Do I have something wrong?

How about your work with this UDF? Is it good?

If you know,could you tell me how to solve this problem?

Thank
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Old   November 15, 2010, 17:00
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HI padthai,

could you please post your UDF. I think there is a mistake in your c-code.
This is my code I'm working on right now:

#include "udf.h"
#include "dpm.h"

DEFINE_DPM_SWITCH(dpm_switch,p,coupled)
{
cell_t c = P_CELL(p);
Thread *t = P_CELL_THREAD(p);

if(C_YI(c,t,0) < 0.083) /*x_H2O_sat(C_T(c,t)))*/
P_CURRENT_LAW(p) = DPM_LAW_VAPORIZATION;

else
{
P_CURRENT_LAW(p) = DPM_LAW_INITIAL_INERT_HEATING;
}
}


As you can see I'm switching the law if the h2o mass fraction is higher than 0.083. This is just a test version. I'm simulating in 2D and I'm still having some problems. The solution didn't converge but the mass fraction is never higher than 0.083. In the final version I wanna switch if the humidity is higher than 100%, but I couldn't find the right command for this case.

cheers
wikie
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Old   November 16, 2010, 00:34
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Hi Wikie,

Very thank for your helps,I've seen my mistake in UDF manual.I forgot to open some law.

For my case,I just coppy the code from example 12.5.13 of UDF manual (I think you have seen this it's quite long.So I don't post in this).This code is used for "the case where the humidity is greater than 1. "Condensation is computed by applying a simple mass transfer calculation.Otherwise, one of ANSYS FLUENT’s standard laws for Vaporization or Inert Heating are applied, depending on the particle mass. The UDF requires one UDML and needs a species called h2o to compute the local humidity"



Anyway,I don't know how to define UDML so I can't use DEFINE ON DEMAND function.I'm trying to fix it.


Thank
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Old   November 18, 2010, 05:18
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Hi Wikie,

Sorry,I have to bother you again.

I'm very doubtful about laws of heat and mass transfer for droplet.

As describe in theory guide,"Law 2: droplet Vaporization is initiated when the temperature of the droplet reaches the vaporization temperature",so when my droplet injection temperature more than vaporization temperature (i.e.,my droplet temp = 308 and vapor temp = 288) evaporation should be occured and only this law should be used,is it right?
but when I closed Law 1:inert heating from custom law(open only law2),that I think law1 is only important when the temperature of the droplet is belower than the vaporization temperature ,
why does not evaporation occure? it seems that I have to open both law for evaporation.



Thank

Last edited by padthai; November 18, 2010 at 06:33.
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Old   November 21, 2010, 13:45
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hi padthai,

sorry for my late reply but I was very busy the last days. I'll have a look on your problem tomorrow.

cheers
wikie
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Old   January 20, 2011, 13:03
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Hi padthai,

I'm sorry for my late reply, but I was very busy the last month and couldn't work on this project. Did you solve your problems? Please let my know if you found an answer, cause I'm still facing this prablem.

Cheers,
Wikie
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Old   January 20, 2011, 14:21
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Hi padthai,

I hope I'm not too late for your problem, but I found a solution for our problem. Here is my UDF. In this case you have to calculate the saturation mass fraction of H2O in dependency of the cell temperature.
I hope you can understand the UDF. If it's not clear, please feel free to ask.

Cheers,
Wikie

#include "udf.h"
#include "dpm.h"

DEFINE_DPM_SWITCH(dpm_switch,p,coupled)
{
cell_t c = P_CELL(p);
Thread *t = P_CELL_THREAD(p);
real T = C_T(c,t); //getting Temperatur of cell
real x = -82.11224736 + 0.783683395*T - 0.002501591*T*T + 2.67227E-06*T*T*T; // calculation for x_H2O_sat

if(C_YI(c,t,0) <= x)
P_CURRENT_LAW(p) = DPM_LAW_VAPORIZATION;

else

P_CURRENT_LAW(p) = DPM_LAW_INITIAL_INERT_HEATING;

}
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Old   January 24, 2011, 06:14
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Hi Wikie,

Very thank for your helps.

I saw the reason for humidity exceeded in my problem at last time

since I forgot to open gravity acceleration.However,when I increased

the flow rate of water injected,the humidity was exceeded again so at

first I have simulated with low water flow rate instead.

By the way,I'm still interested to solve this problem.I have tried to use

UDF from example 12.5.13 of UDF manual but it didn't work because

droplets were not evaporated when their diameter more than 1e-6m.I

don't know why but it seemed like when I closed Inert Heating from

custom law and simulated by using Droplet Vaporization raw only.

Therfore,I thought we should open both law for evaporation(Droplet

Vaporization + Inert Heating )

I also tried your code but I can't interpeted it (There is error message occured : line6: P_CELL :undeclared variable)

cheer
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Old   January 24, 2011, 06:26
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Roman Gobitz-Pfeifer
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Hi padthai,

you have to compile this code, it's not possible to interprete it.

Cheers,
Wikie
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Old   August 20, 2013, 09:03
Default udf for vaporization and condensation
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Hi Padthai and Wikie
Firstly thanks for ur useful comments.Exactly I also have this problem.
In my case I have species(air+water vapor) and also water liquid as DPM.
I need an udf for mass transfer. Did you solve it ?
Thanks for attention and ur time in advance.
afarnoud
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