From the different cross flow turbine concepts developed in the last years, the reported performance is between 20% and 35%. The use of a channelling device allows obtaining a variable confinement of the flow which enables higher water velocities in the proximity of the rotor's blades, increasing the delivered power. The main objective of this paper is to compare, by means of a CFD code, the performance of a cross flow water turbine to that equipped with a channelling device. The chosen test case corresponds to the three bladed turbine currently under investigation in the LEGI hydrodynamic tunnel. This turbine, operating in a confined flow configuration is calculated using a two-dimensional approach. The effect of the channelling device is examined at different tip speed ratios. Different turbine positions in the device are also considered. It is shown that the device increases strongly the power delivered by the turbine. It also reduces the torque fluctuations during one revolution. Although the use of a channelling device increases cavitation risk, this phenomenon is not considered in the present study.