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umesh August 30, 2003 04:13

fixed conc. B C

1. I want to specify conc. of oxygen as constant at the inlet of a vessel. i.e. C_YI(c,t,0)= Constant at mass flow inlet.

As this is a constant value and not a flux..i cant specify this in BC panel.. I may have to use some UDF. How can i do this?

Andrew Garrard August 30, 2003 10:05

Re: fixed conc. B C
I don't think that you need a UDF to do this. If you enabel sepecies in Fluent 6 and go to your inlet boundary condtion panel then there is a "Specify Mass Fraction" for the n-1 species. Here you can specify the mixture components. If you want to make a UDF to put here in the case that you do not want a fixed mass fraction then I believe you require a DEFINE_PROFILE(name, thread, i) macro, of which there is a good explanation in the the manual.

umesh September 1, 2003 11:50

Re: fixed conc. B C
Dear Andrew, thanks for your response

My problem is that.. If i specify the mass fraction in the Inlet BC as you suggested, then IT IS THE MASS FRACTION FOR THE FEED TO THE VESSEL

I just want a Constant Conc. of Oxygen at the Inlet of the Vessel. I dont need a FLUX of oxygen at inlet.

I had written a source term to specify the consumption of oxygen in the vessel as:

DEFINE_SOURCE(oxy_source, c, t, dS, eqn)



return kac*( Coxy_eqm - C_YI(c,t,0)*1000/32 ) - k*C_YI(c,t,3)*1000/180;


How can i modify this UDF or write another UDF to specify C_YI(c,t,0) = constant; at the inlet of vessel

Greg Perkins September 1, 2003 17:45

Re: fixed conc. B C
Sounds strange to me.

At an inlet - you DON'T want a FLUX of Oxygen???

Then why do you want it to be an inlet. Specify it as a wall with constant mass fraction and temperature to give you the appropriate concentration - if that is what you really want!

Are you sure this boundary condition is appropriate?


umesh September 2, 2003 01:00

Re: fixed conc. B C
Dear Greg, Thanks for your response,

I will have to give some details for my case,

I am modelling a Biofiltration process, which involves the flow of fluid through a porous bed,

Inside the bed, various biodegradation is involved which needs the supply of oxygen.

This oxygen is available through natural convection only..i.e. at the inlet of the cylinderical bed, Coxygen = Constant.

this oxygen then diffuses to the bed by conc. gradient.

so, If i specify the mass fraction of oxygen in Mass-flow inlet, it takes the oxygen flux,,depending on the flow rate of fluid.

It is somewhat tricky case...but I am sure some option must be available for that..

Any suggestion is welcome.


Greg Perkins September 2, 2003 20:24

Re: fixed conc. B C
Sounds to me you don't have a mass flow inlet as such but that you have a wall with a fixed mass fraction (concentration if you prefer but note the link through say the equation of state ie temperature and pressure).

Have you tried specifying a wall and fixed oxygen fraction??

I've simulated some natural convection problems using a simialr approach (not quite the same since I had a porous wall through which gas was supplied and wrote some udfs to handle the mass flux). But it turns out to be very similar for low mass flux application sin which case the natural convection dominates.

Do you have a fixed temperature at the wall/inlet where your oxyegn is? If so then I'd try a wall bc and see what happens.


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