hi

i am studying diffussion of one species in static fluid . the domain is 2d rectangular in which one wall is maintained at const( mass fraction=1) concentration and other 3 walls are maintained at zero concentration.(no inlet and outlet).

now i need to calculate mass flux from the wall.(i.e flux due to the term Dab*dc/dx).

can any one explain me how to calculate or write udf to obtain the flux in fluent .

thanks in advance

Here is a trick if you do not use the temperature equation, I will suggest to you to use the energy equation to model your convection-diffusion equation. You have to enter the diffusivity at the place of the heat conductivity, and set the heat capacity to one.... then you can use the FLUENT ready options to get the flux through the wall....instead of getting the heat flux you will get the mass flux......

Be very careffull to mesh correctly the diffusif boundary layer, which has to be much much smaller than the hydrodynamic boundary layer...(usually!)

I hope you did understand....

hi Kharicha,

in latter stages i will be using temperature equation so i will face diffucility in calculating flux

OK, you have to write an UDF Profile where you fix the concentration to zero, but where you also calculate the flux....

for that you need the value of concentration at the boundary (known) and at the adjacent cell.

assume (f, thread) is the boudary cell, you get the adjacent cell (cell,t) by writting:

cell = F_C0(f,thread);

t = THREAD_T0(thread);

then you get the position and the value of the concentration...this is what you need for defining the flux...