How to compute Residence Time Distribution (RTD) for stirred tanks with inlet and outlet?
Often, Chemical Engineers need to compute the RTD of their Continous stirred tank reactors (CSTRs). FLUENT's DPM model is challenging to use in these systems due to difficulty in getting statistically meaningful number of particles at the outlet. A second method is to introduce passive tracer material either with species or userdefined scalars. A method with userdefined scalars is outlined here. Approach: 1. Solve for singlephase steady state flow field with inlets and outlets. 2. CHange to unsteady solver 3. Introduce a UDS with mass flux as convection term and default unsteady term. The UDS represents a passive tracer that is used to determine RTD experimentally. 4. Change UDS diffusivity to zero or reasonable values. For waterwater system, this is quite low ~ 1e10 m2/s. 5. For pulse input, patch a known amount of UDS near the inlet; for step input, make UDS value = 1 at inlet. 6. Turn on surface monitor of areaav. UDS value at the outlet. Plot, print/ write to file. 7. Turn off all equations except the UDS equation. Run for needed flow_time. The UDS conc. at the outlet as a function of time can be used to extract the residence time distribution. If you use step input of tracer(UDS value = 1 at inlet), the outlet UDS profile when normalized (Coutlet/Cinlet) is called F curve which is a cumulative residence time distribution. If you introduce a pulse, the normalized response is called C Curve which is the RTD function. 

