With the goal of decreasing the environmental impact of aircraft, ducted propellers emerge as an efficient propulsive alternative. Ducts are able to increase the thrust to power ratio of a propeller system by both producing thrust and/or lowering tip losses of propellers. In this thesis, RANS CFD simulations were used to analyse the possible impact of modifying a propeller duct shape from a circular to a square geometry. Initially, the two duct designs and the propeller were studied as isolated cases, in order to characterise their aerodynamic performance. In the installed simulations, the propeller was first modelled as an actuator disk, and afterwards with a full blade model. The results indicate two main disadvantages of square ducts. Square duct corners were found to be prone to separation, and to contribute towards the generation of strong vortices. This research can be important for the future study of unconventional ducted propellers, for example with applications to distributed propulsion concepts.