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-   -   New at Species Transport- Please Help.. (http://www.cfd-online.com/Forums/fluent/30175-new-species-transport-please-help.html)

Julie August 15, 2002 04:09

New at Species Transport- Please Help..
 
Hi,

I'm doing a modelling of removal of ammonia gas caused by soiled bedsheet in a ventilated room.

Ive read the manuals but still not certain on certain things hope someone can enlighten me. First of all, my model is a room with beds, hence I specify one of the top face of the bed as "velocity_inlet". Also, there is no chemical reaction involved.

After enable "species transport" the next step is to select the materials used. In my case, is my material still a mixture? Because ammonia gas comes out of one bed only and not from the air inlets, is my material "mixture" or "fluids"?

If my material is mixture, then perhaps it is air and ammonia gas, however since there is no mixture as "ammonia-air" so I need to create a new mixture template.

when I try to edit the species,under species panel,do I put the species as ammonia on top and air at the bottom, or do I input ammonia on top, and break up the air into its consitutents, that is, followed by CO2,O2,N2 (where CO2+o2+n2 represents the species consituents of "air" itself)?

Beacuse when I try inputtin ammonia and air, my BC for all the inlets do not show the word "air" but only NH3.

Also, since the bed is soiled, for the inlet of the bed, is it ok to put 0m/s as the velocity?

Please help! Thanks

Shankar August 15, 2002 16:10

Re: New at Species Transport- Please Help..
 
You always the specify the bulk constituent second in the list. And if you have N components in a mixture and you want to specify the mass fraction say for a "velocity inlet", you will have to specify the vlaues for N-1 components only, since mass fractions add up to 1.

Julie August 16, 2002 01:08

Re: New at Species Transport- Please Help..
 
Thanks Shankar!

But for the soiled bedsheet where it will be the inlet for species of mass fraction =1 for ammonia gas, is there just a thing as velocity=0?

since the soiled bedsheet is just left there and exposed to the air surrounding it, I really do not think that there is any velocity for the ammonia gas that evolves frm the bedsheet. But does velocity=0 makes sense?

walter schwarz August 16, 2002 09:14

Re: New at Species Transport- Please Help..
 
its probably best to build your model with a thin fluid region adjacent to the soiled bed surface eliminating the use of a velocity inlet. then set-up a volumetric source for the amomonia gas in that region ... don't forget to also put the same kg/s-m^3 for the mass source in addition to the ammonia mass source.

Julie August 16, 2002 09:44

Re: New at Species Transport- Please Help..
 
Hi,

sorry that I may not understand your answer fully because I am relatively a begineer in Fluent.

From what I understand from your solution is that, you ask mi to create another thin fluid volume above the soiled bedsheet. Thus, in gambit, this means that I will have to create another volume on top of the bed, then spilt this volume from the main volume of the room, then from the continuum, I will specify this thin volume to be a fluid volume.

Then after exporting into Fluent, under define/boundary conditions i will put input the fluid material in this thin volume the "ammonia-vapour" ?

So is it still a species transport modelling? Sorry but I do not understand what is a volumetric source and I also dun quite get what you mean by "put the same kg/s-m^3 for the mass source in addition to the ammonia mass source".


walter schwarz August 16, 2002 10:10

Re: New at Species Transport- Please Help..
 
>From what I understand from your solution is that, you
:ask mi to create another thin fluid volume above the
:soiled bedsheet. Thus, in gambit, this means that I
:will have to create another volume on top of the bed,
:then spilt this volume from the main volume of the
:room, then from the continuum, I will specify this
:thin volume to be a fluid volume.

yes, that is correct

>Then after exporting into Fluent, under
:define/boundary conditions i will put input the fluid
:material in this thin volume the "ammonia-vapour" ?

no, the fluid material is your mixture

>So is it still a species transport modelling?

yes

>Sorry but I do not understand what is a volumetric
:source and I also dun quite get what you mean by
:"put the same kg/s-m^3 for the mass source in addition
:to the ammonia mass source".

from section 6.27.1 of the fluent user guide ...

"If you have more than one species, you can specify mass sources for each individual species. There will be a total Mass source term as well as a source term listed explicitly for each species (e.g., h2, o2) except the last one you defined. If the total of all species mass sources (including the last one) is 0, then you should specify a value of 0 for the Mass source, and also specify the values of the non-zero individual species mass sources. Since you cannot specify the mass source for the last species explicitly, FLUENT will compute it by subtracting the sum of all other species mass sources from the specified total Mass source.

For example, if the mass source for hydrogen in a hydrogen-air mixture is 0.01, the mass source for oxygen is 0.02, and the mass source for nitrogen (the last species) is 0.015, you will specify a value of 0.01 in the h2 field, a value of 0.02 in the o2 field, and a value of 0.045 in the Mass field. This concept also applies within each cell if you use user-defined functions for species mass sources.

The units for the species mass sources are kg/m 3-s."

Julie August 17, 2002 07:04

Re: New at Species Transport- Please Help..
 
Hi

ok, Ive understood what you mean by "put the same kg/s-m^3 for the mass source in addition to the ammonia mass source".

But here comes another question. You were saying that puttin a volumetric source is a way of avoiding having to put a velocity value in velocity_inlet.

However, its the same thing because in mass source, it is in terms of kg/m^3sec, in that case, it is a mass flow rate. That is, if I'm putting say 0.001 value for mass flow rate for both the sources and the species, I am saying that in one sec, there is constant 0.001 kg/m^3 of species entering the domain...

This is means that, if I run my simulation under transient for 60sec, there will be a continuous flow of species from the source.

However, I don't think the soiled bed evolves species in the same mass flow rate continuously. What I want is something like, the source is left there with a certain kg/m^3 of species in the volume, and as time passes, the species is going to Diffuse and be carried away by the recirculating air surrounding it... such that as time passes, there will be less and less ammonia gas in that area.

Is there a way of doing it?

walter schwarz August 19, 2002 08:29

Re: New at Species Transport- Please Help..
 
yes, either patch an ammonia mass fraction in the fluid volume adjacent to the bed as the initial ammonia mass fraction _or_ write a user-defined function (UDF) to control the amount of ammonia released as a function of time.

Julie August 20, 2002 02:40

Re: New at Species Transport- Please Help..
 
Ok thanks Walter, I will try that.

Julie August 21, 2002 03:54

Re: Walter PLease HELP!!!!
 
Hi,

I tried to do by patchin method. But there is no units for the input of ammonia gas in the Patching Panel???What is the unit by Fluent???

walter schwarz August 23, 2002 11:56

Re: Walter PLease HELP!!!!
 
mass fraction


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