Abstract: A rotor assembly comprises a rotor having a plurality of blades and an axis about which the rotor rotates, each blade being spaced from the axis of the rotor such that there is a gap between the axis and an inner edge of each blade through which fluid can flow, and inner and outer edges of each blade lying in and defining a blade plane and each blade being offset from the axis of the rotor such that the axis does not lie in the blade plane; a casing partially surrounding the rotor, the casing having a first opening to permit a fluid flow into or out of the casing in a direction generally perpendicular to the rotor axis; and a second opening at an axial end of the casing to permit a fluid flow into or out of the casing in a direction generally parallel to the rotor axis.