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,
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.