Value of diffusion coefficient in scalar transport foam

Hello everyone,

I am modelling a continuous stirred tank reactor, with water, which has been simulated at steady state using simpleFoam. after the steady state is achieved, I wanted to calculate the RTD curve for this and the mean residence time for the same. I tried doing so with the help of scalarTransportFoam. Now i want to simulate it such that diffusion is does not contribute towards the motion of the tracer, therefore I set the value of diffusion coefficient to zero, but the results that i get is wrong. I can say that the result is wrong because the inlet concentration is one, and the tracer injection type is step input of 1, therefore after some time the outlet concentration must be 1 but it is much lower than that. I would like to know why is the max outlet concentration lower than the inlet concentration only when i set the dT to zero, else the max outlet tracer conc is 1 for all other values.

Thanks and regards,

Harsh

OpenFOAM 18-02-21, 10:36 a.m. Harsh235

Hi Harsh,

In the RTD plot, the concentration on the vertical axis is not the actual concentration of the tracer, but an area intergral of concentration at the outlet. If you were to plot the concentration at one point/cell at the outlet face, then yes, the plot would look like the one you expected, i.e. a step-function.
As for the diffusion term, I can say that a non-zero diffussion coefficent will result in a change residence time, but at the outlet, the area intergral will ulimately be the same as the one for 0 diffusion, for a given geometry/mesh.

Regards,
Ashley

18-02-21, 6:44 p.m. ashleymelvin

Hello Ashley,

Actually the concentration on the vertical axis is already divided by the area of the outlet. As for other non zero diffusion setups, the maximum value for outlet concentration reaches 1 , so therefore, the problem persists only when I set the value of diffusion coefficient to 0.

Best regards,

Harsh

19-02-21, 11:57 p.m. Harsh235

Hi Harsh,

Could you share the concentration (T) contour at the outlet at any time t > 40? A screenshot would suffice

Regards,
Ashley

20-02-21, 4:47 p.m. ashleymelvin

Hello Ashley,

The outlet concentration contour looks a bit wierd. I am sharing the screenshot for time = 70 s. As I cannot paste an image for some reason, I am sharing the drive link for the same.

Best regards,

Harsh

20-02-21, 5:05 p.m. Harsh235

Hi Harsh,

From the contour, it's clear that the concentration is not uniform throughout the outlet. That should account for your concentration in the RTD plot not reaching 1.

The reason why DT > 0 leading to T→1 could be that additional diffusion aiding a stronger movement of tracer, compared to the DT = 0 where only the velocity  field is responsible for tracer movement.

I'm sure this requires some more reading.

Regards,
Ashley

22-02-21, 7:50 p.m. ashleymelvin

Hello Ashley,

I was of the opinion that the concentration will not be uniform throughout the outlet. The integral value will be one. I am not sure if this is correct. Correct me if this is wrong.

Best regards,

Harsh

23-02-21, 1:24 p.m. Harsh235

Hi Harsh,

I don't think the concentration T, is the conserved quantity in the convection equation (the governing equation that the scalarTransportFoam solves with DT=0). The T value at the outlet is, in some way, dependent on the velocity field. In most RTD plots, the reason why the T at the outlet stagnates to 1 is because the flow is mosly oriented from inlet to outlet. Could you share the velocity contour for which the RTD plots were obtained?

Regards,
Ashley

03-03-21, 9:06 p.m. ashleymelvin

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