Setting up units for Convection-Diffusion

I am trying to set up a simple 2D convection-diffusion problem where oxygen diffuses from a small cirlce into a larger circle. The larger circle (which represents brain tissue) is consuming oxygen at a certain rate.

My boundary condition for O2 concentration in the small circle is in mL / L. What should be my units for the reaction rate, R, in the large circle? I understand that the Toolbox generally does not assume units, but it surely has to assume a unit of time? Should I use mL / L / second, mL / L / minute, etc.?

Noah wrote

I understand that the Toolbox generally does not assume units, but it surely has to assume a unit of time?

Hi, thanks for trying the toolbox.

How and in what unit you interpret d/dt is entirely up to you (and what other units you have chosen). But the general recommendation is to stick to a unit system (for example SI) to make it simple. For the convection-diffusion physics mode you would for example typically use the following SI units:

  Concentration, c           [mol/m3]
  Diffusion coefficient, D   [m2/s]
  Reaction rate, R           [mol/(m3/s)]
  Velocities u, v, w         [m/s]
  Boundary flux, N           [mol/(m2/s)]

But you could just as well substitute the time unit s to minutes if you like, as long as it is consistent.