Hey there @rajasekhar1234
In a passive scalar transport simulation, the passive scalar “moves” within the medium via convection and diffusion. The diffusion coefficient indicates how easy it is for the solute to spread in the solvent via diffusion effects.
Thus this coefficient is given for a binary of substances, for example helium in air, CO2 in water and so on. There are formulas to calculate the diffusion coefficient, like in this paper, but they’re mostly empirical and depend on a lot of parameters. Diffusion is heavily enhanced by temperature.
In this specific simulation, reading the project description, it’s CO acting as solute and air acting as solvent. I would assume the coefficients came from this table and the exact value was interpolated to account for the exhaust gas temperature (but I’m just guessing here). If you are intending to run this simulation, choosing values from this table should yield pretty much the same results, as convection effects should be much more pronounced than diffusion.