[THA SEANGHAI]

Helllo! I am really struggling with using ANSYS FLUENT, I input x-velocity of air from inlet and I want to input mass flux or molar flux of a species on a boundary , but there are only some options such as mass fraction, mole fraction or we can write expression in the new verson of fluent that we can input molar concentration (kmol/m³), but there is no the option for the mass flux(kg/m²s) or molar flux(kmol/m²s). How can I do? Or do i need to write UDF? I never learn code programming before, so UDF is really difficult for me.Please help me! I do not know how to do now! Thank you!

[AlexanderZ]

in Fluent there is mass-flow-inlet boundary condition
Unit is kg/s
If you want to define mass-flow-inlet as mass flux(kg/m²s) you need to multiply your mass flux value on area of inlet

kg/m²s * m² = kg/s

to get area you may do:
press initialize
reports-> surface integrals -> report type: area | select your inlet -> compute

[THA SEANGHAI]

Thank for your reply, but my purpose is just to input the mass flux of a species without velocity, if I chose the mass flow inlet BC, it will include velocity. I knew that mass flux depends on velocity, and it also depends on the diffusion coefficient of a species. In my case mass flux of a species= -Diffusion coefficient*concentration gradient. I mean that I want my species spreading out to the whole domain by diffusion coefficient, not by velocity. Thank you! Do you have any idea that I can input the mass flux of a species on a boundary without velocity? I will appreciate your help!

[AlexanderZ]

had never made it before, but you may try:
1. Once species model is ON, you may specify mass source [kg/m3 s] and concentration
2. You may create very thin volume (but take care about mesh quality) , so you will get almost flux, not volumetric source -> [kg/m2 s]


I don't know other ways, but definitely there should be

[THA SEANGHAI]

Thank again for your reply! Yes, I already input the concentration once the species model is on. I cannot input the concentration directly, because there is only two options that we can input in the species, they are mass fraction and mole fraction, however in the New version starting from ANSYS 18.0 up, there is an expression of mass fraction that we can write the amount of concentration instead of mass fraction only, because we know the relation between mass fraction and concentration that mass fraction=((molar weight of the component*molar concentration)/(mixture density)).
But one thing that I am really curious about is that: when we input the concentration on a boundary, so will ANSYS FLUENT calculate the mass flux automatically by computing the concentration from the equation that ANSYS has? If so, I do not need to write mass flux on the boundary, I just input the concentration and then calculate mass flux from the equation. Do you have any idea? Thank you!

[THA SEANGHAI]

Originally Posted by AlexanderZ

had never made it before, but you may try:
1. Once species model is ON, you may specify mass source [kg/m3 s] and concentration
2. You may create very thin volume (but take care about mesh quality) , so you will get almost flux, not volumetric source -> [kg/m2 s]


I don't know other ways, but definitely there should be

Hello! @Alexander! Thank you very much! I have just understood your idea, now I have been giving the mass source to the cell next to my boundary, but it is very thin volumetric cell, so it is almost area source [kg/m²s], but in order to input mass source, I saw the material name, when I select mixture, species transport is still on, and that is fine, but when I select only the chemical species that I want to add, not mixture, then i realized that species transport is off. What is this problem?
Another one problem that I realized is that even though I add mass source or not, the results of my species are still the same, is there something wrong in my program or not? Note that: to add source to the cell, I just input the constant value, not UDF, so because of being no UDF that make the results not effected? Thank you! I will always appreciate your help!