[JWYeoh]

Hi,

I am running a bioreactor multi-component single phase simulation to study the concentration gradient distribution of various components such as nutrients etc.
Those components are solved by species transport equations. The simulation time starts at t=0 until t=10 h. The substrate is added at t = 5 h. The source term described under Expression includes how the substrate is being converted into product. When setting the Initialization of the Mass fraction for the substrate, I have tried to use the 'if' function such as (if t == 5 [h], 1E-3, 0) or (if t>= 5 [h], 1E-3, 0). The results (transient data) from both settings turned out to be a flat line at zero while other components with a constant value initialization were solved properly.

How should I approach this issue?

Thank you.

[LuckyTran]

Why do you use any if statements based on time in the initialization? Initialization happens only once and at t=0, and your initial mass fraction for the substrate is 0 everywhere.


You should be having a time-dependent source term, which is not initialization.

[JWYeoh]

Originally Posted by LuckyTran

Why do you use any if statements based on time in the initialization? Initialization happens only once and at t=0, and your initial mass fraction for the substrate is 0 everywhere.


You should be having a time-dependent source term, which is not initialization.

Thank you for your prompt response.

I do have a source term under expression to describe how the substrate will change overtime, which is expressed as something like this:
Substrate = -Kmax*(Substrate.mf/(Substrate.mf+ Ksub)),

I would expect this source term function to act like a differential equation.

The initial mass fraction of substrate at t=0 would be zero and the substrate of mass fraction (1E-3) is only being added into medium at t=5 [h] to kick start the conversion process into product. So I suppose the substrate.mf should be set to 1E-3 at t=5 [h].

I wonder if there is way to assign some values to substrate.mf directly under expression?

But yes, I will try to figure out a way to use time-dependent source term. Thank you for your suggestions.

[LuckyTran]

Seems to be a language barrier. Ok now I see what you are trying to do. You want to set substrate.mf to a different value at a certain time not create an added source of the substrate.


The trick is to run the case until t=5. Then rerun the case starting at t=5 but using the correct initial value for the substrate. Is that what you are already doing?

[JWYeoh]

Originally Posted by LuckyTran

Seems to be a language barrier. Ok now I see what you are trying to do. You want to set substrate.mf to a different value at a certain time not create an added source of the substrate.


The trick is to run the case until t=5. Then rerun the case starting at t=5 but using the correct initial value for the substrate. Is that what you are already doing?

I have tried the time-dependent source term by adding in an additional equation so that it can reach my desired concentration (1e-3) at t=5. The case is working.

Thank you so much for your advice.

I have also run the suggested newest trick with the substrate initialization as 0 in the first case, and 1E-3 as second case using setting "continuous from previous case" .res file, but I still got a straight line at 0 for the substrate mass fraction, whereas other components are solved properly (the mass fraction results continued from case 1 results).