The effects of operational conditions on the mixing behavior of a rotary energy recovery device have been systematically investigated through the combined methods of computational fluid dynamics and validating experiments in this paper. The sliding mesh technique and species transport equations were applied in the simulation process. An innovative parameter of inflow length was defined to express the moving distance of a mixing section in rotor ducts. A theoretical formula between the inflow length and operational conditions was first established on the basis of the mass balance and computational fluid dynamics analysis. Simulation results revealed that the mixing has a polynomial relation with dimensionless inflow length, which was in good agreement with the experimental results. The obtained formulas between mixing and dimensionless flow length provide a simple way to calculate and predict the mixing behavior of the device, which will be beneficial to design and operate the rotary energy recovery device in a lower mixing level.