Often, Chemical Engineers need to compute the RTD of their Continuous 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 user-defined scalars. A method with user-defined scalars (UDS) is outlined here. Approach: 1. Solve for single-phase steady state flow field with inlets and outlets. 2. Switch 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. 4. Change UDS diffusivity to zero or reasonable values. For water-water system, this is quite low ~ 1e-10 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 area-av. UDS value at the outlet. Plot, print/ write to file. 7. Turn off all equations except the UDS equation. Run for the required 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.