The problem is that I have a cylinderical vessel with two inlets (say inlet1 and inlet2) and an outlet. My supervisor wants me to find the mean residence time of the vessel, and the way he does it in Fluent is that when the fluid from inlet1 carrying species 1 enters the vessel it transforms itself into species 3 at a constant rate, inlet2 is carrying species2. So that the longer the time species1 stays in the reactor the more concentration of species3 it is going to produce. Species 2 is inert. So you end up so a plot of mass fractions of each of the species, in a sense showing areas of the vessel where species1 (from inlet1) is stagnant and where it is not mixing well.

apparently I am meant to use some source terms but being new to CFX I am not sure how the subroutine works or how I am meant to alter it. I was wondering if your expertise could shed somelight on my situation, I think the subroutine is called USRSRC

I would greatly appreciate any help

(1). The other simple way to find out how the fluid particle from the inlet-1 behave in the container is to use the graphic particle trajectory tracing. (2). At the inlet-1 you can create an array of fluid particles at the grid points (or cells), and then trace their trajectories as they move into the container. It is a simple and effective way to evaluate the flow field and the design. (2).For the use of specific code and subroutines, you need to read the user's guide or talk to the vendor's support engineers. They should be able to give you the correct answer.