The performance characteristics of a commercially available rotary heat exchanger or were determined using CFD. A rotary air pre-heater of a thermal power plant (fuelled by natural gas), situated in Ramin, Iran, was numerically modelled with full scale dimensions using 3D approach. This model was then validated against the experimentally measured values. After a satisfactory validation of the CFD model, a design modification was applied to the model by modelling the heat exchanger with four ducts, with hot and cold fluid ducts arranged alternatively. This was done to improve the heat the heat transfer from hot gases to cold air. The medium of heat exchange (core) is modelled as porous media, since the core in the actual equipment has many number of fine passages arranged in complex manner, which would make the modelling difficult. The model was subjected to same boundary conditions as the validation model. The temperature of the outlets and effectiveness of both the models were compared and was found out that the 4-ducts model is around 12% more effective than the base model.